[API_EMPTY_RESPONSE] ELECTRICITY AS APPLIED TO MINING BY ARNOLD LUPTON M.Inst.C.E.; M.I.MECH.E.; M.Inst.E.E. LATE PROFESSOR OF COAL MINING AT THE YORKSHIRE COLLEGE, VICTORIA UNIVERSITY MINING ENGINEERS AND COLLIERY MANAGERS G. D. ASPINALL PARR M.Sc.; M.Inst.E.E.; A.Soc.M. I.MECH.E. ASSOCIATE OF THE CENTRAL TECHNICAL COLLEGE, CITY AND GUILDS OF LONDON HEAD OF THE ELECTRICAL ENGINEERING DEPARTMENT UNIVERSITY OF LONDON AND HERBERT PERKIN M.I.M.E. CERTIFICATED COLLIERY MANAGER AND LECTURER IN THE MINING DEPARTMENT OF THE UNIVERSITY OF LONDON SECOND EDITION, THOROUGHLY REVISED AND ENLARGED WITH ABOUT 190 ILLUSTRATIONS A stylized illustration of a person with a hat and a staff. NEW YORK D. VAN NOSTRAND COMPANY 23 MURRAY AND 27 WAREEN STREETS LONDON CROSBY LOCKWOOD AND SON 1906 All rights reserved [API_EMPTY_RESPONSE] 14/2/90 **112322** **NOV 18 1907** LHR L97 2 # PREFACE --- T WENTY-FIVE years ago the use of Electricity in Mining was chiefly confined to signalling and shot-firing; twenty years ago the pioneers of electric lighting and electric power transmission had this wonderful agent already at work in a few mines in various parts of the world; since then numerous improvements have greatly facilitated its adoption; and to-day the use of electricity in and about mines has become so widespread—both for lighting and as a mode of transmitting power—that a demand has arisen for a text-book dealing especially with the subject in relation to mining applications. So many are the subjects to be studied in the vocation of the mining engineer or colliery manager, that it is impossible within the limits of a treatise on Mining to deal adequately with such an important subject as Electricity; and, on the other hand, the many excellent books which deal solely with Electricity, having been written mainly for electrical engineers, necessarily deal with problems of construction, urban lighting, and distribution outside the ordinary requirements of the mining engineer. The aim of the authors of the present volume has been to present to the reader the leading truths and main principles of electricity and electrical engineering without going into great detail, their intention being that he should have recourse for further information to the books and practice on the subjects of Electrical Mining Engineering, some of which are many standard works already in existence. They are unable (it need hardly be said) in the space at their disposal to deal with all the varieties of machines and appliances now in use, typical examples of good practice being considered sufficient for the present purpose. iv ELECTRICITY AS APPLIED TO MINING In a book of this nature, with its 'electrical' side and its 'mining' side, joint authorship is, if not absolutely essential, at least advantageous, for the electrical engineer and the mining engineer must work hand in hand if success in electrical operations is to be achieved. The authors desire to thank the numerous friends and manufacturing firms who have so courteously supplied them with information for the first edition of this book. Wherever possible, such assistance is acknowledged specifically in the text. In conclusion, the authors may express the hope that the work will be found of practical service to those mining engineers, colliery managers, or others, who have already adopted or are contemplating the use of electricity for power or lighting; and they trust that those who use the book will kindly give them the advantage of any corrections, additions, or suggestions, with a view to making the book of more value to the class for whom it is intended. LEEDS : November 1902. NOTE TO SECOND EDITION. The rapid development of the applications of electricity makes it essential to keep abreast of improvements, and the opportunity afforded by a new edition has enabled the authors of this book to thoroughly revise the work and make important additions and alterations. The appointment by the Home Office of a Departmental Committee to consider the subject of Electricity as used in Mines, and the establishment of Rules with a view to the safe working of Electrical Mining Plant, are important steps, the outcome and application of which will be of great interest to all concerned. It is satisfactory to note that the official rules now in force embody the principles and practice already applied and observed in many mines where electrical plants have been laid down. The authors are indebted to Mr. F. E. Armstrong, M.Sc., for the help and many valuable suggestions which he has given them in connection with this new edition, more especially with regard to the Performance of Electric Motors and Dynamos. LEEDS : November 1905. CONTENTS CHAPTER I INTRODUCTORY Introduction to the Subject of Electricity—The Electric Current—The Electric Circuit—Electric Terms—Volts, Watts, Amperes, &c.—Chemical Production of Electricity—Primary and Secondary Cells CHAPTER II DYNAMIC ELECTRICITY Alternating-current Dynamo and Motors—Two- and Three-phase Generators and Motors—Induction Generation—Direct-current Dynamo and Motors—Transformers, Rotatory and Static—The Special Construction of Motors to overcome Dangers of Sparking, etc.—Brushed and Ventilated Motors. CHAPTER III DRIVING OF THE DYNAMO Water Power—Steam Power—High-speed Engines, Direct driving—Low-speed Engines, Belt-drive—Gas-engines—Oil-engines—Types of Plants. CHAPTER IV THE STEAM TURBINE Parsons Turbine—De Laval Turbine—Curtis Turbine. CHAPTER V DISTRIBUTION OF ELECTRICAL ENERGY Various Systems : Series ; Parallel ; Two-phase ; Three-phase. PAGE 1 PAGE 76 PAGE 96 PAGE 108 vi ELECTRICITY AS APPLIED TO MINING CHAPTER VI PAGE STARTING AND STOPPING ELECTRICAL GENERATORS AND MOTORS Generators—Motors—Switchboard—Starting Resistance—Switches—Anti-spark- ing Oil and Enclosed Switches—Lightning Arresters—Fuses—Magnetic Cut- outs—Amperemeter, Voltmeter, Wattmeter, Electricity-meter 113 CHAPTER VII ELECTRIC CABLES, ETC. Electric Cables—Varieties of Cable Construction—Size of Cable for a given Current —Details of Conductors—Aerial Cable—Insulators—Carrying Cable down Shaft, and in Underground Workings 131 CHAPTER VIII CENTRAL ELECTRICAL PLANTS Central Electrical Plants for Winding, Ventilation, Pumping, Handling, Coal-cutting, Workshops, Streams, &c.—Comparison of High and Low Voltage as affecting Cost of Cables—Three-wire System—Estimates of Cost 153 CHAPTER IX ELECTRICITY APPLIED TO PUMPING AND HAULING Electric Pumping Plants, Various Types—Electric Sinking-pumps and Contrapfugal Pumps—Electric Haulage: Single-rope, Main and Tail Rope, Endless-rope Electric Locomotives 178 CHAPTER X ELECTRICITY APPLIED TO COAL-CUTTING Pick Machines—Revolving-bar Machines—Disc Machines—Chain Machines 198 CHAPTER XI TYPICAL ELECTRIC PLANTS RECENTLY ERECTED Continuous-current Plant—Three-phase Plant—Continuous-current and Three- phase Side by Side 221 CONTENTS CHAPTER XII ELECTRIC LIGHTING BY ARC AND GLOW LAMPS Arc Lamps—Electric Glow or Incandescent Lamps 230 CHAPTER XIII MISCELLANEOUS APPLICATIONS OF ELECTRICITY Telephones—Signal Bells—Electric Blasting—Electric Safety Lamps—Lighting Safety Lamps by Electricity—Electric Drills—Electric Welding—Electric Winding 233 CHAPTER XIV ELECTRICITY AS COMPARED WITH OTHER MODES OF TRANSMITTING POWER Steam—Rods—Wire Ropes—Compressed Air—Hydraulic—Gas and Oil 273 CHAPTER XV DANGERS OF ELECTRICITY Dangers of Electricity—Methods of Obviating the Dangers—Testing of Cables Wheatstone Bridge—Ohmmeter, &c.—Electric Shock 280 APPENDIX Special Rules for the Installation and Use of Electricity, issued by the Home Office under the Coal Mines Regulation Act, 1887. INDEX 311 [API_EMPTY_RESPONSE] ELECTRICITY AS APPLIED TO MINING CHAPTER I INTRODUCTORY Introduction to the Subject of Electricity—The Electric Current—The Electric Circuit—Electric Terms—Volt, Watt, Ampère, &c.—Chemical Production of Electricity—Primary and Secondary Cells. Up to about the early seventies of the past century, little or no use, as a means of lighting and transmission of power, had been made of the sub-stance which we are pleased to term ‘electricity.’ Prior to that period, however, the scientific world had been laying down the fundamental principles which were to underlie, and play such an important part in, the development of the electrical engineering industry of to-day. Since then the possibilities of electricity have been realised, bringing a multitude of uses into the domain of human activity. The reader will see at the present moment he is accustomed to see electricity employed for nearly all the purposes that earlier agents, such as steam, horses, &c., had been used for. Before dilating on the applications of electricity for mining purposes, it will be necessary to consider in sufficient detail the nature, properties, method of measuring, and dealing with this agent. The question which is uppermost in the minds of many, and which may naturally be asked, is, What is electricity? To this but a very indefinite answer can be given. In fact, no one really knows what it is. We are accustomed to see it in the form of lightning during a thunder-storm, when it is seen as a luminous arc between two clouds or between a cloud and a body; or in the form of a spark when two bodies are brought together; or in the form of electricity ‘at rest’ in the form of a charge on a body, which may be due to the body having been rubbed—as, for instance, the electrification produced by rubbing a glass rod with silk, both rubber and rod then having 7 2 ELECTRICITY AS APPLIED TO MINING the power of attracting light articles such as paper, pith, &c. This is termed 'frictional electricity.' We, however, more often speak of a current of electricity flowing through a body or conductor, and, as this expression will frequently be used throughout the present work, we will next proceed to qualify it. The expression 'current of electricity' rather implies a flow of matter or something which has an independent existence apart from the conductor it is said to be flowing in; but in reality we do not in the least know whether anything is actually in motion in the conductor. Our senses do not help us in this respect, for nothing is visible save the conductor, which presents the usual appearance when the so-called current of electricity is flowing through it. The word 'current,' therefore, is merely a purely conventional one, and when we use it, it is merely to indicate that the conductor and space around it exhibit effects and properties which they did not do before. In this connection it should be remembered that the direction of flow is equally conventional, though the resulting effects have a definite sense or direction of action. It is, therefore, merely as a convenience that we regard electricity as capable of being in the form of a current along or in any conductor, and also in a definite direction along the conductor, depending on the sense in which the effects are produced. The so-called current must, of course, follow some route or path, and, whatever its nature may be, it must necessarily have a direction. All electric circuits, however, do not offer the same facilities for the passage of currents of electricity, some forming a great obstruction, or, in common phraseology, offering a great resistance to the flow, others offering very little. It may be helpful to draw an analogy to a water-pipe, which, if partly stopped up with sand or sawdust, for instance, at some part of its length, would offer a far greater obstruction or resistance to the flow of water than if quite clear. Or, again, if the water had to flow through a water-motor inserted in the pipe, and possibly other appliances, the resistance to its flow would be greater than through the same length of clear pipe; and thus illustrating our analogy by means of electricity, the strength of which depends upon the nature of the path or circuit, whether simple or containing appliances through which it has to pass. This analogy though helpful in explaining the state of things met with in an electrical circuit, so much so that we shall again have to refer to it, must not be allowed to lead us into error. For example, let us take that case where a water-pipe we are actually dealing with matter in motion; but with electricity, motion, if it exists, is not apparent. To show that the analogy of water in a pipe must not be pressed too far, it will be sufficient to cite that while a pipe bent at right angles offers much more resistance to the flow of water MAGNETIC PROPERTIES OF THE ELECTRIC CURRENT 3 than when straight, an electrical circuit in which there is such a bend has no effect whatever on the strength of current flowing. **Effects or Properties of an Electric Current.** As, therefore, a current of electricity flowing in a circuit cannot be seen, we can only measure and judge of it by the effects produced. These come under the following headings :— **Magnetic.** —This effect is produced whenever a current flows in a circuit, a magnetic field, consisting of lines of magnetic force, being set up Fig. 1. around the circuit or path of the current. A reference to fig. 1 will perhaps make this clearer. Suppose a current to be flowing from $a$ to $b$ in the conductor, $a b$, then innumerable lines of magnetic force, $f$, encircle the conductor, concentric with it if $a b$ is straight, but acting in the direction of the arrows in planes perpendicular to the conductor. The size of these circles will increase with the strength of the current, some being very large indeed, others very small. Their directions will reverse simultaneously. Fig. 2. PERMANENT BAR MAGNET. with that of the current, and they are continuous, unbreakable, and invisible. The space pervaded by such lines of magnetic force is termed a **magnetic field**, and when a piece of iron, steel, nickel, cobalt, or manganese is pervaded by these lines, or, which is the same thing, produces a magnetic field, it is said to be a magnetized body. This body is therefore called an **magnet**, or simply a **magnet**. Fig. 2 shows what is usually termed a permanent bar magnet, the dotted lines indicating the kind of disposition or path, and the arrow-heads the direction, of the lines of force. Every line passes through 8 4 ELECTRICITY AS APPLIED TO MINING the centre of the magnet and is perfectly continuous, completing its path in some cases through very short, and in others through very long, distances in the air --e.g. the two ends, $a$, $a$, of that line of force meet some distance away from the magnet, and so on with all the others. Some of these lines leak out sideways after passing the centre, whilst most of them emerge from the ends of the magnet, thus producing what are termed "Nordic" or "South" poles (fig. 3). These poles are free to move. Such a magnet has the power of attracting to its poles any magnetic material, and its power of so doing will increase with the number of lines of force emanating from its poles. The centre of the magnet, which exhibits no free magnetism, is termed its equator, and has no power to attract or repel magnets at all. Further, it may be mentioned that two north or two south poles respectively repel each other, while a north and a south pole attract each other. But to return to our conductor carrying current. If this is wound over a paper tube, say, in the form of a helix, as in fig. 3, the lines of force due Solenoid Electro-magnet. to a current, instead of encircling each part of the wire, now run from end to end inside the helix, their direction for that of the current shown being given in the figure ; but if the current reverses, their direction is also reversed. The field and its distribution are similar to those shown in fig. 3, and the helix, which is usually termed a solenoid, develops north and south poles at its ends, acting like a bar magnet. If a rod of magnetic material, i.e., a bar capable of being magnetised--is inserted into such a coil, since current in the wire produces a great many more lines than before, and therefore much stronger poles, the arrangement being then termed an electro-magnet. Enough has now been said about magnetic terms and principles to make them intelligible when mentioned later on. Suffice it to say that a conductor carrying current acts magnetically and attracts magnetic materials near it, which may be far away. This magnetic property of an electric current is used in all appliances employing electro-magnets, such as electric bells, telegraphs, dynamos, motors, arc lamps, transformers, &c. For a more detailed treatment of magnetic principles see Elementary Lessons in Electricity and Magnetism, or The Electro-Magnet, both by S. P. Thompson. Fig. 3 ELECTRICAL UNITS 5 **Chemical Effect.—When a current which is partly liquid flows through a circuit which is partly liquid and partly solid, the liquid is generally decomposed into two parts, one appearing at each junction of solid and liquid. This property or effect of the current is termed **electrolysis** (electric decomposition). For instance, a current of electricity flowing through water decomposes it into its constituent parts, the gases oxygen and hydrogen, of which the hydrogen is evolved at the part where the current leaves and the oxygen where it enters, the water. This chemical property of an electric current is employed in electroplating, electrotyping, refining and purification of metals, secondary or storage cells, and primary cells. **Heating Effect.—This is ever present when any current, however small, flows through a circuit any part of this circuit being heated on an extent depending on the current and resistance of the circuit. If the latter is high enough and the current sufficiently strong, the circuit will emit light. This property is made use of in all electric lamps, electric heating and cooking appliances, and in electric brazing and welding, &c. **Physiological Effect.—This is produced when a current passes through the body. The effects are divided into three classes: first, that on the muscles and nerves. When sufficiently strong, death may be caused owing to paralysis of the heart. Summarising, it may be said that the magnetic, chemical, and heating properties of the electric current form the fundamental basis of the whole electrical engineering industry, and we shall deal with them more in detail when considering their application to electrical appliances. **Electrical Units.—The striking analogy of a supply of water through a pipe to that of a current of electricity through a circuit or wire, referred to on page 2, is helpful in considering electrical pressures. Most people know that the sea level at any place is taken as datum line from which all heights are measured; so also does that at place, and also that water always flows from a higher to a lower level. The rate of flow of water from a pipe in the basement of a house depends upon the height of the supply cistern to which it is connected ; in other words, upon the "head" of water, which is the vertical height of the surface of water in the cistern above that at which the pipe is question. The greater this head, the greater will be the pressure of water in the pipe below, and the greater the rate of flow in, say, gallons per minute. The case is very much the same with electricity. Some datum line, starting point or zero level must be provided from which to measure electric pressure or height; but zero level is quite arbitrary, and is universally adopted as sea level. It is assumed to be a *zero pressure*, or *potential* as it is more correctly designated. This corresponds with sea level in connection with height measurements. 6 ELECTRICITY AS APPLIED TO MINING As with water a flow takes place from a high to a lower level as soon as the two are joined by an unstopped pipe, so with electricity a current flows from a point at high potential to one at a lower potential as soon as the two are joined by a conducting circuit or wire. In fact, it is the difference of level in the case of water and the difference of pressure or potential in the case of electricity which cause the flow as soon as a conducting path is established. Moreover, the quantity of water passing every cross-section of a pipe, however variable, per second is exactly the same at any given moment ; so also the strength of current is the same in all parts of a circuit, whatever the material of which it is made and whatever the cross-section. The actual quantity of water or strength of current which a given difference of level or pressure will produce depends on the resistance of the pipe through which it will pass. In this connection, however, there is a discrepancy in the analogy of water pressure to electric potential—namely, that the difference of water pressure is seriously diminished by a bend in the pipe, whereas in the case of electric potential or pressure a bend introduces a loss in the circuit, however sharp, when the current flow is steady. This shows that the effect of water system, as before mentioned, though helpful in understanding electrical resistance, &c., must not be pressed too far. A current of electricity will flow so long as any potential difference exists, and will cease when this is uniform throughout the circuit. In all cases we may denote the extreme difference in pressure of any circuit by the term Electro-motive force (abbreviated to E.M.F.) ; and that between any other two points, which must necessarily be lower, by the term Potential difference (abbreviated to P.D.). The Unit Difference of Pressure, or E.M.F., is that difference of potential which must be maintained at the ends of any circuit of 1 ohm resistance so that a current of 1 ampere may pass through it. This unit is called a Volt. The Unit of Current is called an Ampère, and depends for its definition on the chemical effect produced on a solution of silver nitrate. The Unit of Resistance at the present day, of universal adoption, is defined as that resistance offered by a certain column of pure mercury, and is called an Ohm ; all other electrical units have been accurately defined in terms of the ampère and the ohm. We are now in a position to state the most important of all laws relating to current electricity, which was first enunciated by Dr. G. S. Ohm many years ago—namely, that the ratio of potential difference for any conductor at constant temperature is a constant, and is equal to the resistance of the conductor, or, in ordinary parlance : If R is the total resistance in ohms of a UNITS OF ELECTRICAL POWER, ENERGY, AND QUANTITY 7 circuit across which there is a P.D. of V volts, then the total current, A, in amperes flowing is $$A = \frac{V}{R} \text{ amperes};$$ whence $$R = \frac{A}{V} \text{ ohms},$$ or $$V = AR \text{ volts}.$$ This is a fundamental principle of the utmost importance, and is universally called Ohm's Law. On page 5 reference was made to the heating effect of a current, and it was there stated that some heat is always generated by a current, however small, in flowing through a circuit of any nature whatsoever. This heat is the direct effect of the expenditure of electrical energy in the circuit, and is only another form of such energy. Heat is always generated when work is done through the expenditure of electrical energy, and it can be experimentally proved that the heat produced in a circuit of 1 ohm's resistance, when a current of 1 ampere flows through it, is proportional not only to \(A^2\), but to \(r \cdot t\), i.e. proportional to \(Ar^2t\). If the unit of heat is taken as the amount of heat required to raise 1 lb. of water from \(o^\circ\) C. to \(1^\circ\) C., then the number of units of heat produced in \(t\) seconds is $$H = 0.0315 A^2rt.$$ If the unit of heat is taken as the amount of heat required to raise 1 gramme of water from \(o^\circ\) C. to \(1^\circ\) C., called a French Thermal Unit, Calorie, or Thermo, then the number of units of heat produced in \(t\) seconds is $$H = 0.239 A^2rt \text{ calorie}.$$ This is known as Joule's Law, and the heating effect as the Joule Effect. But J. P. Joule, the discoverer of this law, has shown that the above thermal unit is equivalent to about 1,400 foot-lbs. of work. Hence if A ampere flow at a P.D. of V volts through r ohms for \(t\) minutes, the work done, $$W = 1400 \times 0.0315 A^2rt,$$ $$= 4425 A^2rt \text{ foot-lbs}.$$ But $$A = \frac{V}{r}$$ :. $$W = 4425 A^2 V r \text{ foot-lbs},$$ which relation is true for any kind of circuit; but since the power expended = rate at which work is done, Power × Time = Work done ; 8 ELECTRICITY AS APPLIED TO MINING and . . . the work done in foot-lbs. per min., or electrical power expended on the circuit, is 4475 A V $$= \frac{4475}{33,000} = \frac{A V}{746} = 0.00134 A V \text{ horse-power}.$$ The Unit of Electrical Power, or unit rate of working, is called the **Watt**, and is that power exerted when 1 ampere flows under the pressure of 1 volt; and power in watts is $A \times V$. Hence the English horse-power = 746 watts. The work done by 1 watt in one second is termed a **joule**. Hence the work done in joules = watts × seconds $$= AV = AW.$$ 1 calorie = 4.167 joules = 3.067 foot-ibs. = 0.0356 British thermal unit. The Unit of Electrical Power is termed the Board of Trade unit (B.O.T. unit), or kilowatt-hour, and = 1,000 watt-hours. This is the commercial unit, in terms of which the electrical energy supplied to a consumer is measured or reckoned, whereas the ampere, volts x time (hours or fraction of an hour), any connection whatsoever = 1,000, a B.O.T. unit of electrical energy is said to have been expended. The Unit of Quantity of Electricity is called a Coulomb, and is the quantity of electricity which flows past any point of a circuit con- veying 1 ampere in one second. Or, 1 coulomb as 1 ampere per sec.; or, briefly, 1 coulomb as 1 ampere-second. In actual practice multiples and fractions of each of the preceding units have to be dealt with, and these have received special names. Those in common use will therefore now be given to familiarise the reader with the terms used :
A Milliampère= 1/1000 ampère.
A Megohm= 1,000,000 ohms.
A Microhm= 1/1,000,000 ohm.
A Kilowatt= 1,000 watts (commercial unit of electrical power).
It was remarked on page 6 that an electric current always continued to flow through a closed circuit, continuous in itself, so long as a difference of potential existed, in the same way that water continues to flow through a pipe as long as there is a difference between its ends. In the case of water some appliance must be provided for raising the water originally to the higher level—*i.e.* for maintaining a head or difference of level. The principle is precisely similar in the case of electricity, for in this a difference of potential must be maintained, and some appliance is neces- sary to raise and keep up the highest potential whence the difference will PRIMARY CELLS 9 also be maintained. Such appliances are called current generators, of which the following are the chief types in practical use : (a) Primary cells. (b) Secondary cells, also termed storage cells or accumulators. (c) Dynamos. All of these are merely appliances for converting various forms of energy into electrical energy; for instance, in both primary and secondary cells it is potential chemical energy which is stored up and converted into electrical energy. In dynamos mechanical energy is transformed directly into electrical energy. As each of the above sources is employed in connection with mining work, we shall now consider them somewhat in detail, restricting ourselves, however, to those forms and types most commonly met with in such work. (a) Primary Cells. A primary cell consists of a vessel made of some non-electrically conducting material containing one or more acid or salt solutions, either mixed, or separated by one of the bodies contained in an inner receptacle, in which are immersed two solid conducting bodies, one of these being more readily oxidised or acted on by the solution than the other. Primary cells may be divided into two distinctive classes: (1) *Single fluid*, or those in which only one fluid is used, and (2) *Double fluid*, or those in which two fluids are used together. There is, however, a third type of primary cell, which is a special form of the above, containing no solution at all, and termed a 'dry cell'. It will only be possible, in discussing primary cells in the following pages, to direct our attention to forms commonly met with in, and suitable for, mining work generally, for such purposes as telephony, signalling, blasting, &c. A very large number of types of these cells have been devised from time to time, some of which are now only of historic interest. Single-fluid cells being the simplest of all, we shall take a typical example and point out its principle of action, advantages and disadvan- tages, &c. Fig. 4 shows diagrammatically the construction of such a cell. It consists of a zinc plate, $z$, and copper plate, $c$, dipping into an oxidising solution, such as dilute sulphuric acid, contained in a glass or glazed earthenware bottle, $v$. Two stout metallic connections join the plates to $a$ and $b$ respectively, which form the poles or terminals of the cell. If the plates $c$ and $z$, particularly the latter, are practically pure, no action of any description takes place by reason of them dipping into the 10 ELECTRICITY AS APPLIED TO MINING dilute sulphuric-acid solution ; but if the two terminals A and B are connected to what is commonly termed an external circuit, u, an electric current at once flows in the direction of the arrows. The zinc plate, z, is now observed to waste away gradually, and continues to do so as long as the circuit is complete through u. The zinc and copper are each about 1-87 and 0-90 volt lower potential than the liquid; hence the potential difference between the two plates is about 1-97 volt. Such an arrangement is sometimes termed a voltaic or galvanic cell, and in such, chemical action invariably takes place on the production of a current. Zinc stands high as an electro-positive element, and is very readily oxidisable, which is Fig. 4 SIMPLE VOLTAIC CELL. equally essential ; consequently it forms the negative pole or positive element in almost every primary cell. Substances such as copper, silver, platinum, carbon, &c., are much less oxidisable, standing high as electro-negative elements, the last two being unaffected by any single acid ; consequently these are used for the positive poles or negative elements in primary cells of various types. That cell will have the greatest E.M.F. in which two metals are employed, one being the most efficient in being highly electro-positive and electro-negative respectively. Thus copper and zinc are good in this respect, and much better than copper and zinc. The action taking place in the cell illustrated in fig. 4 is briefly as follows : When the cell gives a current the zinc plate (usually of com- 10 DANIEL CELL I I mercial and impure metal) gradually dissolves away, due partly to the chemical action of the acid on the impurities in the zinc, and partly to the electrolytic action of the current. The first action does not occur with pure zinc. When working, the combined chemical and electrolytic action liberates hydrogen and forms a salt termed zinc sulphate. The bubbles of hydrogen gas produced at the surface of the zinc plate are not evolved from either it or the liquid, but from the copper plate, and stick to its surface. This is owing to the increase of resistance (a) by increasing the resistance of the cell from plate to plate (called its internal resistance), for gases are bad conductors of electricity, and a film of hydrogen covers the copper plate, thereby reducing its effective surface; (—) by producing an E.M.F. in the reverse sense--i.e. a back E.M.F., owing to hydrogen being highly electro-positive and almost asodic as zinc, and therefore acting as a cathode; and (——) by making the zinc plate negative element, and the hydrogen film as the negative pole or positive element. This difficulty, which is known as 'polarisation,' is overcome by employing two-fluid cells in which one of the solutions is a highly oxidising one, or by using a solid oxidising substance in single-fluid cells. Stout plates will last some time, but the solutions require renewing at frequent intervals. When two or more cells are joined together 'end on,' or 'in series' with each other, the arrangement is termed a Battery, and if the cells are of the same kind they are called a 'series battery.' The total amount of one cell multiplied by the number of such cells in series, and the total amount of chemical decomposition will also be increased in like manner, as well as the internal resistance. From the foregoing considerations it will be evident that the simple cell taken as an example would not be suitable for commercial purposes, though useful for understanding the actions that go on in such cells. A large number of cells have been devised by different people, employing different metals and solutions; but in the following pages we shall merely deal with some four or five types which apply more particularly to the subject of this work. These are as follows: Daniell cell, Leclanché cell--ordinary bell, agglomerate, and carporous forms--Daniell cell.--This cell was invented about 1836 by the man whose name it bears, and in it polarisation is practically eliminated. It is an important form of cell, having been employed extensively for telegraphic purposes and many others in which a constant E.M.F. is required. It consists (fig. 5) of a zinc rod or plate, z, dipping into a solution of zinc sulphate, ZnSO_{4}, in a porous pot or glazed earthenware vessel; or unglassed earthenware. Outside the porous pot, and surrounding it, is a solution of copper sulphate contained in the jar, j. Into this dips a 12 ELECTRICITY AS APPLIED TO MINING copper plate, c, which forms the positive pole of the cell; the zinc forms the negative pole. The porous pot merely separates the two solutions: it allows the passage of the current from one plate to the other. The internal resistance of a Daniell cell may be anything between 0·75 and 5 ohms, depending on the size of plates and density of the solutions. It is, however, frequently about 1·0 or 2·0 ohms in the sizes commonly used at the present day. The internal resistance can be reduced by having the plates larger and closer together and by diminishing the thickness of the walls of the porous pot. The E.M.F. of all these cells is about 1·1 volt, and is entirely unaffected, as is also the case with any other cell, by the size and distance apart of the plates, or of the rest of the cell, and depends only on the materials used. The Daniell cell is, of course, a two-fluid cell in which the copper sulphate acts as the oxidising agent, or depolariser. The hydrogen gas (H), in trying to get across on to the copper plate, attacks the copper-sulphate solution, which has the chemical symbol CuSO$_4$. In so doing it displaces the copper (Cu) and takes its place, forming a new molecule of H$_2$SO$_4$, while a new molecule of Cu (Cu) is deposited on to the copper plate. In this way no hydrogen gas can get to the copper plate to polarise the cell, so long as the copper-sulphate solution is existent; when this is exhausted the cell will begin to polarise. Leclanché Cell (ordinary bell type)—Of the many forms of DANIELL CELL. 5 Z P C J DANIELL CELL. LECLANCHÉ CELL 13 primary cells probably this one has been the most widely used. Its principal use lies in the production of intermittent currents during long periods such as those required for ringing electric bells, and for such it is much superior to the Daniell cell, but inferior for the purpose of sending steady currents of appreciable strength for any length of time. It is a single-fluid cell, having a solid oxidising or depolarising agent, and consists of an outer glass jar (fig. 6), shaped as shown, the top part of which is closed by a stopper. The jar contains the solution creeping from the inside to the outside over the mouth of the jar. A solution of sal ammoniac (ammonium chloride) and water is contained in this outer jar, together with some crystals of the undissolved salt at the bottom. **Fig. 6.** **Fig. 7.** A diagram showing the structure of a Leclanché cell. The outer jar is cylindrical with a stopper at the top. Inside the jar, there is a solution creeping from the inside to the outside over the mouth of the jar. The inner pot is made of porcelain and contains equal proportions of little pellets of carbon and black oxide of manganese. **Outer Jar.** **Zinc.** Into the solution dips an amalgamated, rolled, and almost pure zinc rod (fig. 7), to the top of which is soldered a wire for the purpose of electrically connecting the zinc to the circuit and constituting the negative terminal or pole of the cell. The positive pole consists of a carbon plate capped, in the latter case, by a piece of zinc, and connected by a lead or brass terminal. This plate is tightly packed inside a porous pot of unplated porcelain of high porosity (fig. 8), with equal proportions of little pellets of carbon and black oxide of manganese (otherwise called manganese peroxide or dioxide), the latter of which acts as the oxidising or depolarising agent. A thin layer of pitch then closes the top of the pot, through which passes a tube leading into another pot containing half the volume of the solution to be poured into the porous pot through one, and air and gas to escape from the other. The function of the pot is to keep the liquid out, but to keep the broken gas carbon and manganese peroxide in, and 14 ELECTRICITY AS APPLIED TO MINING both of these latter must be well sifted to remove the dust, so as to expose as much surface of carbon as possible to the solution. Fig. 9 shows a cell complete. While the cell is sending a current, ammonia gas is given off, and both water and zinc chloride are formed inside. When the solution turns yellow, the current is stopped, and the cell is allowed to stand without, then it shows that the depolariser--namely, the peroxide--is in all probability exhausted altogether, or is unable to oxidise the gas evolved rapidly enough. This is the case when a strong current is taken from the cell. The E.M.F. of a Leclanché cell is approximately 1.47 volt, and the internal resistance varies from about 1 to 3 ohms, normally depending on the size of zinc and carbon plates and their distance apart; also on the thickness of the porous pot and density of the solution. A Leclanché cell with a glass cover. **Fig. 8.** **Fig. 9.** CARBON AND POROUS POT. COMPLETE LECLANCHE. The Agglomerate-block Leclanché Cell is a modification of the form just described, in which the porous pot is usually dispensed with. Fig. 10 shows one method of construction in which the carbon and manganese peroxide are crushed fine, well mixed together and with some binding substance, and then moulded under pressure into blocks, which are firmly held against the usual carbon plate by two india-rubber bands. The zinc rod is inserted through a hole in each block, and forms an eye formed at one part of the band, or by being contained in a small perforated porous pot, as seen in fig. 10, this method avoiding all local action. The *Carporous* Cell, devised by Messrs. Lacombe and Leclanché, is another modified form of Leclanché cell. The construction will be understood by reference to figs. 11 and 12, which respectively represent a CARPOrous AND DRY CELLS 15 section and general view. It consists of a perforated cylinder of carbon, a, and one of porous porcelain, b, which rest on a glass foot, c. Between these cylinders is packed the mixture of broken carbon and manganese peroxide. The zinc rod, z, passes down through an insulating cap, s, into the inner porous cylinder, b, and dips into the sal-ammoniac solution. Connection with the carbon cylinder is made through the screw, washer, and rubber tube. With the exception of the ordinary form, all these different kinds of Leclanché cells will act and give a current immediately the solution is poured in, but time must be given to the ordinary form to enable the solution to soak through the porous pot, unless some solution Figs. 10. A GLOMERATE BLOCK LECLANCHE. is poured into it through the tubes which pass through the pitch sealing. A battery of four ordinary bell Leclanché cells in series is shown in fig. 13. Dry Cells, so called because they contain no liquid that can spill, but only a jelly-like mass in place of the liquid, are one and all of the Leclanché type. Among the many different forms the principal ones are the 'Obach,' the 'E.C.T.' and the 'Bosch.' Possibly, or perhaps one should say that these cells are the most widely used in this country. The construction of them all is much the same, and merely the general features will here be discussed. The containing vessel or jar consists usually of zinc, which may be 16 ELECTRICITY AS APPLIED TO MINING surrounded by a cardboard or other suitable covering, or may be painted, and form the external case itself. In either case the zinc cylinder or jar Fig. 11. A diagram showing the internal structure of a 'Carborous' cell. The negative pole is at the bottom, with a wire soldered to it for making the electrical connection. The positive pole consists of a substantial carbon plate in the centre of this zinc cylinder, the two being separated by a mixture of coarsely powdered carbon, manganese peroxide, and sal ammoniac and a small percentage of zinc sulphate, the whole mixed into a stiff paste with a little water and glycerine. Section of 'Carborous' Cell. A large battery of Leclanché cells. Each cell is contained in a cylindrical container with a cover on top. The cells are arranged in rows, with each row containing several cells stacked vertically. Battery of Leclanché Cells. Fig. 12. A close-up view of one of the cells in the battery. The cell contains a cylindrical zinc cylinder with a carbon plate in the centre. The negative pole is at the bottom, with a wire soldered to it for making the electrical connection. The positive pole consists of a substantial carbon plate in the centre of this zinc cylinder, the two being separated by a mixture of coarsely powdered carbon, manganese peroxide, and sal ammoniac and a small percentage of zinc sulphate, the whole mixed into a stiff paste with a little water and glycerine. 'Carborous' Cell. The positive pole consists of a substantial carbon plate in the centre of this zinc cylinder, the two being separated by a mixture of coarsely powdered carbon, manganese peroxide, and sal ammoniac and a small percentage of zinc sulphate, the whole mixed into a stiff paste with a little water and glycerine. DRY CELLS 17 This last named plays no part in the chemical action of the cell, but only serves to bind the materials together. The cells are invariably sealed at the top, usually with melted pitch, two vent-holes being provided for the escape of any gas. The resistance of all types is about 1-55 volt, and the internal resistance from 0-5 to 3-5 ohms. In consequence, they will give much stronger currents than the ordinary Leclanché cell, which when left on open external circuit, after a discharge, will recuperate to a considerable extent. As there is nothing in the construction which can spill, these so-called dry cells can be used in any position, whether upside-down, on their sides, or otherwise. They are used for a variety of purposes—as, for instance, for Fig. 14. **Battery of E.C.C. Dry Cells.** shot firing in mines, also by the General Post Office and the National Telephone Company. Fig. 14 shows a box containing three E.C.C. dry cells connected in series and to two terminals, one at each end of the box. (6) Secondary Cells. The one great objection raised against all types of primary cells is their comparatively rapid polarisation when any but a small current is taken from them, and, though they are eminently suitable for certain kinds of work not requiring more than a very small current—as, for instance, that met with in telegraphy and telephony—they are of no use for giving the currents required by dynamo-lights—such as those now in use—the present day. Moreover, the cost of maintaining them is an item which precludes their use for larger currents. Zinc is the fuel used in these chemical electric generators, and it is vastly more expensive than the coal used in dynamic generators. C 18 ELECTRICITY AS APPLIED TO MINING In the following pages we shall deal with another form of electric cell which is capable of giving out larger currents without polarising or other-wise running down appreciably, except after long periods. On page 5 it was mentioned that when a continuous current passes through a solution of sulphuric acid, the hydrogen decomposed into its constituent parts, the gases oxygen and hydrogen, the latter being evolved at the electrode of the electrolytic cell at which the current leaves, the oxygen from the electrode at which it enters, the cell. In fact, this was mentioned as being one of the properties of an electric current. In the case of the secondary or storage battery, an electric current generated by a dynamo is passed through a cell (if it consists of two strips or plates of ordinary lead, dipping into a dilute solution of sulphuric acid and water, the hydrogen is still given off from the plate at which the current leaves the cell without any chemical action taking place at its surface ; but the oxygen develops on the other plate—that by which the current enters—this oxygen combining chemically with the lead plate at its surface, forming what is known as lead peroxide or lead oxide, having the chemical symbol PbO$_{2}$, and consisting of two molecules of oxygen (O$_{2}$) and one of lead (Pb). This chemical combination is accompanied by the plate turning a chocolate to dark brown colour, the other plate merely remaining the colour of lead. If, now, this electrolytic cell is disconnected from the source which is sending the current through it, and connected to another circuit not having any E.M.F. in it, a current will flow out of the cell through the circuit from the dissolved plate or inside the cell to this plate. Thus, we have here an apparatus in which one part plays the role of the cell, and the other, or lead-coloured plate, the negative. Such an appliance is variously termed an electrical storage cell, electrical accumulator, or secondary cell, and two or more of them connected together are called a storage or secondary battery. The reader, however, is recommended to use the term secondary, for no electricity whatever is stored or accumulated in the cell when a current is sent through it, just as much passing out as flows in. Chemical changes occur, however, which result in the formation and decomposition of certain substances on charging, while on discharging a reaction or secondary change occurs which reduces the substances to the composition which they had before the change was commenced. Hence the propriety of the term secondary cell. A cell constructed in the particular manner indicated would be of but little use in practice; as the amount of energy stored up in the comparatively small amount of chemical charge is very little. In order to obtain a much more prolonged secondary action, and a cell that would give PLANTÉ CELLS out large currents for considerable periods, a larger amount of active material must be exposed to the action of the dilute sulphuric-acid solution. In other words, the plates must be made more porous, so as to offer a greater effective surface to the solution, and as much lead peroxide as possible must be formed. This, of course, cannot be realised with plates cut out of ordinary commercial sheet lead, and special means are adopted for the purpose, giving rise to two different types or constructions of secondary cells, viz., respectively as the Planté or non-pasted type, and the Faure or pasted type. The difference between these types lies entirely in the method of making them, and in both the ultimate condition when charged up is spongy porous lead for the negative plate and lead peroxide or dioxide on the positive plate in a dilute solution of sulphuric acid. Planté Cell—so called after Gaston Planté, who devised the first practical form of secondary cell—consist of almost pure lead plates varying in thickness, in the different makes, from $\frac{1}{8}$ inch to 1 inch. These plates are cut to the required size for making a cell of the desired dimensions, and usually, though not always, pickled—i.e., first inserted in a bath containing some highly concentrated solution of such substances as nitric acid in water, or one of sulphuric acid with a certain percentage of the alkaline nitrate of soda, ammonium, or potash. Such solutions rapidly oxidise and eat into the plate, which is taken out after a suitable time has elapsed and dried. The resulting plate is then used in a system of spongy or porous lead to quite an appreciable depth (about $1$ mm.). Several plates are next assembled side by side in an electrolytic bath containing sulphuric acid and water in the proportions of about 1 of concentrated acid to 10 or 12 of pure water. Each plate is separated by about $\frac{1}{8}$ inch from the next, but alternate ones are connected together through one pair of terminals. A current of electricity is now passed through this circuit from some outside source for forty hours or more, and at first nothing is visible except the decolourisation of the plates at which the current enters the bath. Towards the end of the time bubbles of gas (oxygen) are evolved from these same plates, and finally the whole liquid is full of them. A cell in this condition is said to be gassing. The cause of this gassing is due to the fact that the plates at which the current enters are incapable of absorbing any more oxygen to form lead peroxide, with the result that this gas is liberated in its nascent form. The plates on which the peroxide of lead is formed are the positive plates, and those on which it is not formed are brown chocolate colour—the other, or negative plates, being of the slaty grey colour of spongy lead. In some cases the lead plates are not picked previously to being c2 20 ELECTRICITY AS APPLIED TO MINING *formed,* as it is termed, by the current in a bath ; but in all cases something is done to increase the effective surface of the plates—as, for instance, by building them up of corrugated and perforated thin sheets of lead, or by simply corrugating or grooving an ordinary thick lead plate. There are several different forms of secondary cells in use at the present day which are wholly or partly constructed on the Plant principle, the process of manufacture being, in some cases, very simple, and in others rather complex. It will, however, suffice for our purpose to mention one well-known type called the *D.P.* Secondary Cell.—A diagrammatic view of the plates of this cell is shown in fig. 15, and they consist of a large number of narrow strips, Fig. 15 Fig. 16 *D.P.* PLATE. COMPLETE *D.P.* CELLS s, of lead, the plate surfaces of which are specially dented, built up one above the other like the leaves of a book. The ends of these strips are lead-wound together and to thick leaden side-bars, which are bent over to form a lug, $l$, for burning on to the main cross-bar of lead that connects all the plates of like polarity together to form one terminal of the cell. The strips $s$, run horizontally, as indicated in fig. 15, and are there shown edgewise. In a complete cell, as many of such plates as required are placed side by side at suitable equal distances apart, so that each plate has two lugs of opposite polarity which come in between, and the liquid space as well. The lugs $l$, of all the positive plates (say) are then burned on to a common crossbar, which forms one terminal of the cell; the same with the negative plates, which are one more in number than the positives. HEADLAND SECONDARY CELL 21 Two of these 'D.P.' cells are shown in fig. 16, mounted and connected in series. The containing vessels are of glass, but in the case of cells having a much larger number of positive and negative plates than vessel is of tank to contain the acid, the latter is placed in a separate vessel of this nature of the sulphuric acid. With this method of building, all the plates have to be sufficiently well insulated from the lead lining, which would otherwise cause them to be short-circuited. In the Faure type of cell both sets of plates are pasted with red lead (of which the chemical symbol is PbO₂) and immersed in dilute sulphuric acid for charging ; the coating of oxide on the negative plate is reduced to spongy lead, while that on the positive is turned into peroxide (PbO₄), the whole time taken in bringing about this electrolytic action being from thirty to forty hours. The fact that the paste soon fell away in this type of cell directed attention towards a frame or grid for holding it, and into which it was pasted. Without going into the many forms of grid tried, some with and some without success, it will be sufficient to mention one of the latest forms of secondary cells constructed on the Faure or pasted principle. This is the Headland type. See figs. 17 and 18. It consists of two frames, figs. 17 and 18, of a number of bars (fig. 17) built up side by side and each filled with paste. A number of these bars with air spaces between go to make up each positive or negative plate; the plates are separated by some insulating substance such as glass rods, ebonite strips, or sheets of perforated corrugated ebonite, etc. This last arrangement is shown in fig. 18 (V.), and represents a true trace of the Headland type removed from its ebonite containing-case seen to the left. As will be observed, there are three positives and four negatives, and in all plate forms of secondary cells there is always one more negative than there are positives. This arises from the fact that unless all the liquids are acted upon from both sides at once, they cannot be made to react together as usually termed, bunched. In all the different forms of secondary cells precisely the same electro-chemical actions take place—for instance, on charge the surface of the positive plate becomes turned into peroxide of lead (PbO₄), and the negatives into spongy lead, while the acid solution becomes stronger owing to the liberation of oxygen. When discharged, first the spongy or spongy lead plates become turned into lead monoxide or litharge (PbO) first of all, and then in a continuous manner into lead sulphate (PbSO₄), the surfaces of the positives are turned into lead sulphate, and the solution gradually loses some of its sulphuric acid, becoming weaker in strength. During rest, when the cell is sending no current, both plates very slowly sulphate—i.e., a formation of lead sulphate (PbSO₄) gradually 32 ELECTRICITY AS APPLIED TO MINING accumulates on the surfaces of both positive and negative plates, causing the cell to very slowly run down. This result is also the case if there is any leakage from the plates down the outside of the vessel to earth. This leakage can be almost entirely prevented by supporting the glass or other containing vessel in a wooden tray, the bottom of which contains Fig. 17. Fig. 18. HEADLAND BAR GRID. HEADLAND PLATE GRID AND TRACTION CELL. a layer of sawdust, and then resting this on what are termed 'mushroom oil insulators,' one of which is shown in side sectional elevation in fig. 19, and in perspective in fig. 20. It is made of glass and consists of two parts ; the bottom, $a$, has an annular channel, $b$, on its upper surface, which is filled with a heavy non-evaporative oil, such as resin oil. A projecting rim on the lower surface of HEADLAND SECONDARY CELL 23 the top part, c, dips below the oil and rests on the bottom of this channel. Thus, any leakage from the cell resting on the top of such an insulator must pass over the top surface, round the outer lip, and across the surface of the oil, where it meets with very great resistance, before it can get to earth. From what has been said it will be understood that by having several positive plates connected together, but arranged alternately with similarly connected negative plates, we obtain the same result as we should have if two large plates of which the equal to the sum of the areas of the others respectively. At the same time a cell of a more convenient size is obtained. The current which can be drawn from a secondary cell depends not merely on the type of cell, but on the total area of the positive sections, and for any particular make it is reckoned as many amperes per square foot of positive, reckoning both sides of the plate. Fig. 19. Fig. 20. Section. External View. MUSHROOM OIL INSULATOR In the large variety of makes it varies, according to the construction, from 4 to as much as per square foot of positive plate. Now, it requires little or no consideration to see that the total amount of electrical energy given out by a secondary cell can never be so large as that in. Energy being = watts x time in hours = i.e. watt-hours we have the $$\text{Energy efficiency} = \frac{\text{watt-hours given out}}{\text{watt-hours put in}}$$ which in actual practice varies from about 60 to 70 per cent. Similarly we have the $$\text{Quantity efficiency} = \frac{\text{ampère-hours given out}}{\text{ampère-hours put in}}$$ which may be as high as 95 per cent when the current density (amperes per square foot of positive) is low. The *capacity* of a secondary cell is reckoned either in ampère-hours or watt-hours, either per lb. of plates or per lb. of oil complete with acid, and in comparing the capacities of cells for portable purposes this latter is the only fair method of doing it. 24 ELECTRICITY AS APPLIED TO MINING Plated cells usually have a capacity of about 2 to 3 ampere-hours per lb. of cell complete, while Faure cells have been constructed having 7 watt-hours per lb. of cell complete, which is the figure pertaining to the Headland cell. Secondary cells are used very extensively at the present day for all kinds of work where a supply of current may be wanted at any moment; their chief use being not merely to act as a stand-by in case of a break-down of the primary or alternator plant, but also for short periods. Such cells are expensive at first cost, and should therefore be well attended to, the user being well repaid for good attention to them. Summarising on this point, it may be said that secondary cells must not be discharged or charged regularly at a rate exceeding much that stated by the makers. They should not be discharged below 1-5 volt per cell, other-wise a large surplus of paste forms itself in the plates and they will fail. An overcharge of short duration will do good—i.e. a continuance of charging for a half to one hour after the cell begins to boil will do no harm to the plates and may do good, though it represents a large proportion of the energy supplied going to waste. Secondary cells should never be left in a discharged condition, as otherwise premature buckling of the plates may occur, with the result that paste, in the case of the Faure type of cell, may drop out, thereby denuding the plates of active material and increasing the risk of internal short circuits through a pellet of paste bridging the space between positive and negative plates. Connecting the terminals by a low resistance, technically called short-circuiting the cell, will probably do more harm than weeks of regular and steady use, if it does not actually ruin the cell altogether. As the level of the solution falls, through evaporation and spraying during boiling, fresh liquid must be added, whether only water or dilute acid, so that the density remains constant. The specific gravity of that in the cell constantly, say, when the cell is fully charged. This density is obtained by means of an hydrometer, or acidimeter, as it is variously termed. **Uses of Secondary Cells.**—Secondary cells can be put to a large variety of uses. For instance, they are used in electric clocks (for instance, in electric lighting work when a steady pressure is wanted). The chief use of them is either to supply lamps or electro-motors without any attention, after the running machinery is stopped, or to help the running plant to supply such at periods of heavy or maximum load, when, perhaps, the machinery could not do it alone. Unlike primary cells then, a secondary battery is capable of responding instantaneously to any demand within limits. To instance an example of its utility, in, say, a direct current electricity supply system of USES OF SECONDARY CELLS 25 some town during a winter month. The period of maximum load is usually from 4.30 p.m. to about 7.0 p.m. out of the twenty-four hours, and perhaps this may be twice the average load during the day. To supply this maximum at least an equal amount of running machinery must be started up for only two or three hours and then shut down, leaving full steam useless in the boilers. If a secondary battery, having an output equal to the average demand, were installed, it would take the extra demand at heavy loads, discharging 'in parallel,' as it is called, with the generating machinery. Thus no extra plant would have to be run. It should be remembered, however, that the prime cost of such a battery for a given kilo-watt output (for five hours, say) is roughly the same as the steam dynamo for boilers for the same purpose. The depreciation on about thirty years' need not exceed 5 per cent. per annum for a battery tolerably well looked after. Thus we see that while the first cost is about the same in the two cases, the running cost is less with a battery than if only running machinery is used. **Battery-house.--This should be a well-ventilated brick or stone building, and should be so arranged that every cell can be at once acted on detrimentally by the acid vapours given off during charging. Every cell of the battery should be within easy reach, so that it can be easily inspected and attended to at frequent intervals. Thus the plates must always be kept covered with liquid. Any buckled plates must be straightened up immediately they are found to be buckled. The plates must be kept clean while discharging. If some do not, they should be carefully inspected to see whether there is any internal short-circuit by positive and negative plates being in connection. 26 ELECTRICITY AS APPLIED TO MINING CHAPTER II DYNAMIC ELECTRICITY Alternating-current Generators and Motors-Two- and Three-phase Generators and Motors-Induction Generators-Direct-current Dynamos and Motors-Transformers, Primary and Static-The Special Construction of Motors to overcome Languages of Sparks and the Use of the Motor. So far we have only considered the production of electric currents by chemical means, as in the case of primary and secondary cells, but it is possible to obtain such currents by other means. Let N S (fig. 21) represent the north and south poles of a magnetic field, which is shown by the dotted lines, and always taken to flow from the north to the south pole. Let A B be a metallic conductor capable of being moved between N and S so as to cut across the lines of force. If now the conductor is A diagram showing a circular magnetic field with two poles labeled N (north) and S (south). A metallic conductor AB is shown moving through the field, cutting across the lines of force. FIG. 21. Electro-Magnetic Induction. moved away from the observer so as to cut across the lines of magnetic force, an E.M.F. will be induced in A B as long as A B is in motion and cutting lines. This will cause a current of electricity to flow from A to B as soon as the ends of A B are joined by a metallic circuit. If A B were at the other side of the lines of force, and were moved towards the observer, the induced current would be in the opposite direction. We may now mention a most convenient rule, due to Dr. Fleming, for finding the direction of an induced current, and one that should be committed to memory: With the thumb and first two fingers of the right hand pointing in the direction of motion of A B, then if the thumb points up, the current flows from left to right; if down, from right to left; if horizontal, no current flows. The law just stated is known as Fleming's Right Hand Rule. **ELEMENTARY THEORY OF DYNAMO** hand forming three directions at right angles to each other, place the thumb in the direction of motion, the first finger in the direction of the line of force, when the induced current will be in the direction of the second finger. The reader should apply the rule to verify the directions shown in fig. 21, remembering that the thumb will point away from him as a moves away. The effect thus obtained by the motion of the conductor in the magnetic field is termed electro-magnetic induction, or dynamic electricity, and the arrangement is known as a dynamo. It is this process of converting mechanical motion into energy in the form of electric currents. Machines for doing this are termed dynamos, or sometimes generators simply, and such devices convert mechanical into electrical energy. In practice there are two distinct kinds of current, each of which is used at the present day to a very large extent. One kind is called direct current, and is produced by current that flows in one electric circuit to and fro very rapidly, often reversing ten times per second ; the other is called continuous, or often direct current, which flows in one direction only in the circuit. We have, however, touched on this in connection with primary and secondary cells, which are only able to give direct current. Later on we shall see that this kind of current can be produced by the machines above mentioned, but for the present we shall confine ourselves to seeing how alternating current can be produced, as machines for obtaining this were the first to be invented many years ago. Such are termed alternating-current dynamos or generators, though more often they are called alternators. As has been said on page 26, it will be seen that if the conductor, a b, the ends of which are joined to a metallic circuit, is moved backwards and forwards across the magnetic field, induced currents, alternating in direction, will flow in a b ; the rate at which this alternating current reverses will depend on the rate at which a b moves across the lines of magnetic field. Previous to this same effort being made by a conductor, a b is stationary and the field moves to and fro across it. The foregoing is the fundamental principle on which all alternators work, and in practice sometimes the conductors, but now more often the field, move. The kind of motion adopted is of course circular one, presenting fewer difficulties than any other. In order to thoroughly understand the principle and action of the alternator, we will consider the simplest of all forms, in which, for simplicity only, the magnetic field is stationary and the conductor revolves. Let the arrangement take the form shown in fig. 22, in which the conductor is in the form of a wire rectangle, ab cd, mounted on a spindle s s', which rotates about its lengthwise axis. The ends of ab cd are connected to r r', or pieces of brass or copper tube, which are mounted on and rotate 27 28 ELECTRICITY AS APPLIED TO MINING with $t$, though insulated from it by shorte booses, and from one another also. On $r$ press two springs or booses, $a$, connected to an external circuit, $i$, Let the position, $a$ & $d$, of the rectangle be called its 'zero position,' and suppose it to rotate counter-clockwise in the direction of the arrow. Then, since the lines of magnetic force flow almost horizontally from $s$ to $s$, no part of $a$ & $c$ in or about its 'zero position' will cut all of the lines, since $a$ & $d$ are sliding through these lines, and $b$ & $c$ are parallel to them, while $a$ & $d$ are always sliding through the lines for any position of the rectangle; therefore, no E.M.F. will be induced or current flow. The maximum number of lines will, however, pass through $a$ & $c$ in this position. Now, as it turns through such a position as $a$ & $d$, $b$ & $c$ are both cutting perpendicularly across the lines; therefore, applying the right-hand rule on page 26, an induced E.M.F. and current will take place from $a'$ to $b'$ and from $c'$ to $d'$, and these, it will be noticed, coincide in direction round the rectangle, and are a maximum for this position, since the rate of cutting is a maximum. Moreover, when the lines are enclosed, the plane of the rectangle being parallel to their direction. When $a'$ gets round to the position $c'$ - i.e., after $a$ & $c$ has turned through 180° or a half-revolution - $a$ & $d$ and $c$ & $d$ are again sliding through and not cutting the lines of force. The amount of enclosure is a maximum, and the E.M.F. and current are nothing. The reader will note that in the dotted position in completing the third quarter of the revolution ; that is, which is now moving upward on the right, has been induced in it an E.M.F. and current from front to back, while $c$ in moving down carries a current from back to front. A diagram showing a simple alternator with a rectangular loop (abcd) rotating around a central axis. The loop is shown at various positions as it rotates. SIMPLE ALTERNATOR. Fig. 22 FREQUENCY OF ALTERNATING CURRENT 29 But this is opposite in sense to what it was in the first half-revolution, and is a maximum when the frame arrives at the end of the third quadrant--- i.e. when $a$ gets to $270^\circ$; finally, when $a$ arrives at the position it originally started from, after making one complete turn, there is no E.M.F. or current. We see, therefore, that during the first half-turn the current flows from $a$ to $b$ and $c$ to $d$, while in the second half-turn it flows from $b$ to $a$ and $d$ to $c$. Thus, the E.M.F., and therefore the current, reverses its direction twice during each complete revolution, or is said to make one period per revolution, since the action in any one turn is precisely similar to that in the preceding one at the same position. Stating the action generally for any form of alternator, we have the following taking place simultaneously : $$\text{Max. E.M.F.} = \text{max. rate of cutting} - \text{min. amount of enclosure},$$ or, $$\text{Min. E.M.F.} = \text{min. rate of cutting} - \text{max. amount of enclosure};$$ the points of reversal in all cases occurring at the points of maximum amount of enclosure. Remembering these rules, it will be easy to find the number of reversals per second in the more complicated and practical forms of alternators met with at the present day. In fig. 21, for example, 100 revolutions per minute—i.e. $\frac{1}{60}$, or 10 revolutions per second—there would be 20 periods per second, since there is one period per revolution. The number of periods per second is called the periodicity, or often the frequency, of the alternating current. The method of collecting these currents and connecting them to an external circuit, $\mathbf{x}$, is simplicity itself, and is effected by the brake $\mathbf{x}$, a pressing on the continuous metallic rings, $\mathbf{r,r}$, to which the rotating coil is connected. It is, of course, very important that the rotating coil should cut as many of the lines of force emanating from the north pole, $\mathbf{x}$, as possible. As, therefore, all these do not flow straight across from $\mathbf{x}$ to $\mathbf{s}$, many of them which pass through the core are deflected by it. To overcome this difficulty, a coil by winding it on a laminated soft iron drum or core, which is fixed to $\mathbf{s}$ and rotates with it. They then prefer to follow the easier path—viz. through the iron core—to leaching out into the air. In all present-day alternators soft iron is used to direct the lines through the core, but in some cases a special arrangement is made for this purpose by means of a steel core. In practice it is essential to produce alternating currents, for lighting lamps and running electro-motors, having a periodicity of at least 40 per second, and often as high as 100. To obtain this with the simple arrangement of fig. 22 would mean rotating the rectangle (which we will now 30 ELECTRICITY AS APPLIED TO MINING designate by its technical name of armature) at 6,000 revolutions per minute. This in the larger machines in use now would be impossible, owing to mechanical considerations. The speed, however, can be kept within safe limits, and yet the desired periodicity may be obtained, by increasing the number of poles—i.e., instead of one pair in fig. 22, having two pairs (so poles) arranged alternately M.N.S.H. and M.N.S.L. (fig. 23), so that each pole has two lines of current flowing per revolution, and a frequency of 100 periods per second would be given by a speed of 600 revolutions per minute. The practice nowadays is becoming common to standardise all alternating-current circuits in this country and abroad to 50 periods per second, as this is too high to be visibly detected on electric power lamps, and seems to suit motors better than higher frequencies. The same applies to the frequency used for power purposes. Twenty-five is now used at Niagara on account of its supposed superiority for motor purposes and transmission, and a frequency of 15 is advocated by some. There are, however, many supply stations using 100 periods per second. There are three ways of increasing the E.M.F. of an alternator: (1) By increasing the strength of the field. (2) By increasing the speed of rotation of the moving part. (3) By increasing the number of turns on the armature. The last method has the disadvantage of increasing the internal resistance of the machine and also what is termed its self-induction. The E.M.F. generated is directly proportional to each of these—i.e., if the speed is doubled, so also is the E.M.F.; likewise, if the strength of magnetic field and number of armature turns (all round the core) are each doubled together, the E.M.F. would be quadrupled, and so on. (1) and (a), however, can only be applied after a certain amount (b) is far by far the most satisfactory method of altering the E.M.F., for in a given alternator (2) cannot be made use of, owing to the alteration of periodicity ensuing from that of speed. In practical alternators the magnetic field is produced by pairs of electro-magnets giving north and south poles alternately. The strength of these electro-magnets varies within limits with the strength of continuous current flowing round its coils, by altering latter the strength of magnetic field—i.e. number of lines of magnetic force emanating from it—can be altered. These electro-magnets are technically termed the field magnets of the machine. Having discussed the principle of alternating-current generators we will now describe some of their electrical uses which are present day to a large extent. Of course, there are many different makes and types, in some of which the armature revolves and field magnets are stationary, in others the reverse is the case, and in a third kind neither revolve, the mag- ELECTRIC CONSTRUCTION COMPANY'S ALTERNATOR 31 netic field being made to cut the stationary armature conductors by rotating blocks of soft iron fixed to a kind of fly-wheel and termed *inductors*. Hence, this type of alternator is known as the inductor type, and is beginning to be much used at the present day. The principal reason of this is that it is mechanically stronger than the other two kinds, owing, in the first place, to the fact that no portion of any electrical conductor is exposed to the secondary circuit, but that the latter is the inductor flywheel, which can be made as strong as is necessary. The alternator to be described is that constructed by the Electric Construction Company, of Wolverhampton, and more briefly termed the E.C.C. alternator. In it the field magnets revolve inside the ring of armature coils. Fig. x3 indicates the principle on which it is built, but not the Fig. 23. PRINCIPLE OF E.C.C. ALTERNATOR. actual construction, as space will not permit, nor is it necessary to go into details connected with the shape and ways of fixing the various parts. The part elevational diagram will, however, convey an idea of the method now very generally employed in producing alternating currents of considerable magnitude and power. On an old steel shaft is securely keyed and fixed a kind of flywheel, comprising a highly permeable (magnetically) mild steel rim, c, of sufficient cross-section to carry the lines of force produced by any one field-magnet coil, and supported or driven by arms or rods, r, on the body. To c are securely fixed an equal number of soft iron cores, i, each of which contain the field-magnet bobbins, r', wound with cotton-insulated copper wire and secured. These coils are all connected in series, usually in such a way that a current (continuous) passing through them magnetises the iron cores, i, producing alternate polarity, N.S.N.S., as shown. 32 ELECTRICITY AS APPLIED TO MINING The free ends of the whole series are connected to two continuous copper or brass rings fixed to the shaft, but insulated from it and from one another. Through two fixed strips or brushes pressing on these rings as they rotate with the field magnet frame and shaft $k$, the direct current from a battery called the "motor" is led into the field-magnet coils to magnetise them. The Armature consists of a fixed frame of soft steel, $v$, into which are fixed as many laminated cores, $p$, of the best soft iron, as there are field-magnet poles or coils. Separately wound armature cells of cotton-covered copper wire, some of A B C D E, are then slipped over these cores and secured firmly in position. A little consideration will show that these coils must be connected in series alternately right- and left-handily for the E.M.F.s all in them to help one another at one and the same instant. If connected otherwise, the E.M.F. produced would be neutralised, that is the next coil, so that no current would flow. Consider now the case that this goes on in producing E.M.F., and the current of the field magnets revolving, referring, of course, to fig. 23. In the relative positions of the field and armature coils shown, manifestly all the lines of force are flowing through the interior of the turns on the armature cells --z., the amount of enclosure is a maximum. Consequently, from what was said on page 29, the rate of cutting of the conductor by the revolving magnetic field is maximum, and therefore the E.M.F. at this instant is also zero. Now, when the field coils have turned through a distance equal to half that between two consecutive armature cells (commonly termed half the pitch), so that each field pole is halfway between two consecutive armature coils, the rate of cutting is a maximum, and hence is a maximum value of E.M.F. a maximum. Further, when the shaded field coil, $e$, is opposite $d$, there is no E.M.F. again in any of the armature coils. Similarly, as this field pole, $e$, moves on from opposite to $e$, the E.M.F. induced rises from zero to a maximum (midway between $e$ and $d$), and then goes down to zero again; but with this difference that when $e$ is opposite $b$, there is no E.M.F., and therefore the current is in one direction, while from $d$ to $e$ it flows in the opposite direction. After $a$ the action simply repeats itself in exactly the same manner. Thus, as the field magnets revolve, an alternating current is generated which can be distributed to any circuit connected with the machine. For a given speed, the greater the number of turns of conductor A B C D E, the greater will be the E.M.F.; while the current which it is safe to draw from the machine will depend on the sufficiency of cross-section of copper in that conductor. The periodicity per revolution will be equal to the number of pairs of E.C.C. SINGLE-PHASE ALTERNATOR 33 field poles, or, which is the same thing, to the number of pairs of armature coils. The number of field-magnet poles in alternators at the present day varies with the type and maker, and may be any number up to 60. The dotted line in fig. 23 represents the approximate mean path of a line of magnetic force through one section of the field magnets and armature. This is unbreakable so long as the field is excited or magnetised. Fig. 24. A black-and-white photograph of an early electric generator. **E.C.C. Single-phase Alternator.** Consequently it revolves as the field-magnet frame revolves, so cutting the conductors in the coils a.c. . . . The distance, $d$, between iron of field cores, $i$, and iron of armature cores is made as small as possible so as to reduce the resistance to the passage of the lines of force and diminish the amount of exciting current required. The power used in exciting the fields has by careful design been reduced at the present day to 1 per cent. or less of the total useful output of the machine, while the efficiency may be as high as 96 per cent, in the case of large machines. D 34 ELECTRICITY AS APPLIED TO MINING Fig. 24 shows the general appearance of an E.C.C. alternator, the principle of construction of which we have just been considering. The size of these machines varies enormously, but they are now made up to 40 feet in diameter and in some cases run to upwards of 100 tons in weight. The E.M.F. may range up to 10,000 volts for very long distance transmissions of electrical power, a common voltage in this country being 2,100 volts, the output ranging up to 2,000 kilowatts or more. The alternating current which we have, up to the present, been considering is known as a simple, **mono-phase**, or **single-phase** one--i.e., there is only one current and one circuit, consisting of two conductors side by side, in which it flows. There are, however, what are called multiphase or polyphase alternating currents (as they are variously termed), which are not so simple, from the fact that in them one is dealing with more than one current at the same time in a circuit consisting of three or more conductors running side by side. As such currents are coming into extensive use in this country, but more especially in Europe, we consider how they are produced, transmitted, and employed for useful purposes. In reality a polyphase current is a combination of two or more single-phase currents, and in practice two such combinations only are at present employed--namely, two-phase alternating currents and three-phase alternating currents. **Two-Phase Alternating Currents.--These are generated on the same principle as single-phase currents, and if an equal number of similar** Fig. 25 **armature coils be fixed side by side with those shown in fig. 23, but half the angular length of a coil in advance, then if these are connected together like the other set between a second and separate pair of terminals, the arrangement will generate alternating currents providing both sets are acted on by the rotating field. In fact, since all the armature coils have the same outward appearance, and the only material difference is that in the two-phase machine there are just twice as many armature coils as there are field-magnet poles, while the armature coils are arranged in two distinct sets entirely separate (electrically) from each other. Fig. 25 will** **PRINCIPLE OF GENERATION OF TWO-PHASE CURRENTS.** E.C.C. TWO-PHASE ALTERNATOR 35 make this clearer, and represents part of the field magnets and armature laid out straight. One set, A A A, consists of alternate coils connected in series with each other between two terminals and forms one of the phases. A photograph of a rotating alternator, showing the stator and rotor components. THREE-SPIRIT E.C.C. TWO-PHASE ALTERNATOR. The other alternate set of coils, a n, is likewise connected in series together between two other terminals and forms the other phase. We therefore see that there are practically two distinct armatures in one, and that each is acted on inductively in turn as the field poles pass them. Succeeding coils D 2 36 ELECTRICITY AS APPLIED TO MINING of both phases are wound right- and left-handedly as in the single-phase alternator. Now, in the position shown (fig. 25) these field poles (still arranged N.S.N.S. round the ring and shown dotted) are opposite the set of coils A A A. It will therefore be seen that 'set A' is enclosing a maximum amount, whereas 'set B' is not enclosing lines at all, but cutting them at maximum rate. Hence the E.M.F. of windings or phase $A = 0$, and that of $B$ is a maximum. Now, when the field poles have moved on into the position midway between the poles of 'set A'—i.e., just a quarter period—the E.M.F. of phase $S = 0$ and that of $A$ is a maximum. The same distance further on, the E.M.F. of phase $A$ again is $0$ and that of $B$ is a maximum. The E.M.F. continues to alter periodically in a precisely similar manner, but in the opposite direction. Thus two separate single-phase alternating currents are generated which differ in phase—i.e., the times of maxima and minima—by $90^\circ$, or one is a maximum when the other is zero, while at any intermediate position each has a certain value. These two E.M.F.s are said to be in ‘quadrature’, and we may think them the cranks of a compound steam-engine set one $90^\circ$ in advance of the others. In the system of generation of two-phase current depicted in fig. 25, the periodicity will, of course, be reckoned in exactly the same way as for the single-phase machine, and will be equal to half the number of field poles used. In this case, however, we shall use only half the number of coils, or, course, other ways of winding the armature of a two-phase alternator, equally good as the one described. It will suffice, however, for our purpose to give merely the preceding one. The outward appearance of such a machine is shown in fig. 26. **Three-phase Alternating Currents.** We will now briefly consider how three-phase currents can be obtained instead of using the preceding form of construction and winding on the armature, it will be easier to see the principle on which such currents are generated if what is termed a 'wave' winding for the armature is employed. Fig. 27 represents the plan of a portion of an alternator with a few field poles (shown dotted), both laid out flat. In the actual construction, however, these poles would be internal to the armature and rotate, the latter being stationary, or this may rotate internally with regard to the fixed field. In either case the field poles are alternately N.S.N.S. round the ring, as is usually the case. The armature consists of three distinct circuits or sets of conductors, 1, 2, and 3 (fig. 28). Each set consists of three parallel wires enclosed by the fixed laminated soft iron shield or frame of the machine, as were the coils A C D in fig. 25. In fig. 27, the conductors cut by the field as it rotates in the direction of the arrow are the vertical bars shown, the hori- THREE-PHASE ALTERNATING CURRENT 37 zontal ones completing the 'waves' being merely side connections, which in reality do not extend beyond each other, though shown doing so of necessity for clearness. Now, the distance $a d$ corresponds to a period, or twice the pitch of the field poles, and it will be noticed that each of the three circuits is just $\frac{1}{4}$ of the period, or $\frac{1}{2}$ of the pitch, in advance of the next, or $ab = bc = cd = \frac{1}{4}ad$. At one end, say $a$, the three circuits have their ends joined together, which junction is called the neutral point. The circuit $ab$ is connected with a current-carrying wire, whether direct gas or water main or well-mink earth plates. In the position of the armature shown, no part of circuit $1$ is in the field; hence the whole circuit is inactive and is generating no E.M.F. Both 2 and 3 are. A diagram showing three-phase alternating current generation. The diagram shows three parallel conductors (A, B, C) with a neutral conductor (N) in between them. Each conductor has two segments labeled "1" and "5", and they are connected in series. The diagram also shows a winding around the conductors, which is used to generate the E.M.F. Fig. 27. **Generation of Three-phase Currents (Wave Winding).** However, cutting magnetic lines of force, and will therefore be generating E.M.F. Applying the hand-rule on page 26 for finding its direction, and remembering that the field moving from A to N is equivalent to its being still and the conductor moving from B to A, we see that the E.M.F. at this position of the period is in the direction of the arrows. It should also be noticed that the direction of conductor 1 is opposite to that in a condenser of the same size, passing a south pole, through by winding in some form they coincide in direction throughout the circuit. As the rotation proceeds, 1 begins to generate E.M.F., while that in 3 dies away, and so on, while at certain positions there are E.M.F.s generated in all three circuits. If now, the three terminals of this three-phase alternating current are connected together by means of wires 1-2-3-4-5-6-7-8-9-10-11-12, and 3 and 3 are connected to the necessary three mains, a three-phase alternating current will flow --i.e.-- currents which attain their maximum strength in each of the three mains successively and periodically, the difference in 38 ELECTRICITY AS APPLIED TO MINING the times (i.e. difference in phase) of attaining their maximum values being equal to one another and to one-third of the periodic times of each of the three circuits. The foregoing explanations connected with the generation of single-, two-, and three-phase, or, as they are sometimes called, mono-, bi-, and tri-phase alternating currents will probably enable the reader to grasp the principles that underlie them. But before we proceed to deal with the development of their reference must be made to standard works dealing with this subject in particular—as, for instance, S. P. Thompson’s ‘Polyphase Electric Currents’ (Spon); G. Kapp’s ‘Alternators, Motors, and Transformers,’ &c. Direct-current Dynamos. The three kinds of alternating current which we have been considering are very suitable for the transmission of a large amount of electric power over long distances owing to the facilities they afford of operating at high pressures. There are, however, some functions which cannot be performed by them, such as the charging of secondary cells and all electrolytic work. For this a unidirectional current, or, as it is usually termed, a direct or continuous current, referred to on page 27, must be used, and we shall now consider its properties. When dealing with the simple form of alternator (fig. 22), we saw that the current in the rotating armature coil, $a$ & $b$, reversed twice in every revolution when the plane of the coil was perpendicular to the lines of force, and, furthermore, that in this position no E.M.F. was generated in the coil. Hence, if we imagine that the plane of the coil could be interchanged with respect to the external circuit or brushes, any such circuit would receive a current which, though fluctuating from zero a maximum, down to zero again, and so on, would nevertheless be uni-directional in its flow round the external circuit connected to the brushes, thus in the rotating coil still being alternating. This interchange is effected by means of a commutator which rectifies the current so obtained in the external circuit as a commuted, rectified, alternating, continuous, or direct-current respectively. The commutator in its simplest form consists (fig. 28) of a short length of brass or copper tube split axially and mounted on a shaft from which it is insulated. The two halves of the tube are separated being insulated also from each other by suitable strips of insulation, $t$, $r$, which make the surface truly cylindrical. This two-part commutator, as it is called, is fixed to the shaft in such a position that the plane of the armature coil includes a line joining the centres of $t$ & $r$. A diagram showing a simple form of alternator with a rotating armature coil \(a\) and \(b\), and a commutator consisting of two parts separated by insulation. PRODUCTION OF DIRECT CURRENTS 39 The two brushes press on the commutator, as the whole rotates, in a line perpendicular to that in which the magnetic field flows and on a diametrical line. Thus, at the instant when the induced E.M.F. in the loop changes its direction, the brushes slide across from one segment to the other, and thus the current, while reversed in the loop, is left flowing in the same direction as before in the external circuit. This, then, is the principle of commutation and method of obtaining a direct current by means of which currents originate from an alternating one in the first instance. It has already been observed that the armature conductors must be wound over or on iron to ensure that as many lines of force as are Fig. 28. PRINCIPLE OF GENERATION OF DIRECT CURRENTS. due to the field magnets pass through the interior of the coil, for then the same number will be cut by it. Consequently, in the simple alternator or direct-current dynamo, figs. 22 and 28, the turn or turns, such as $abcd$, are wound over a well-laminated core of soft iron. The armature consists of a cylindrical drum made of cast-iron sheet and softened Swedish charcoal iron, threaded over the shaft and keyed to it after having been previously clamped together so as to form a cylindrical drum called the armature core. The arrangement of core and conductor is then typical of one of the most important kinds of armatures—viz. drum-wound armatures. The object in using a laminated core instead of a solid one is to reduce as much as possible the resistance offered by it in its revolution in the magnetic field, for, since the discs are lightly insulated by varnish or other means from each other, these induced Eddy or Foucault currents, as they are variously termed, which circulate in paths or planes at 2 40 ELECTRICITY AS APPLIED TO MINING right angles to the lines, have no room to flow in and are therefore almost eliminated. There is, however, another extremely important type of armature in which the coils are wound round a laminated soft iron ring or cylindrical ring core, and which are termed **ring-wound armatures**. A diagram showing a ring-wound armature with a central core and multiple coils around it. **FIG. 29.** **PRINCIPLE OF RING WINDING.** Such an arrangement is shown in fig. 29, in which a coil of one or more turns (two shown) is wound on the laminated ring core, i, of iron and connected to the two-part commutator, j,k. The dotted lines represent the path of the lines of force, which split up into two parts, and if the core is of A diagram showing the path of magnetic lines of force through a ring-wound armature. **FIG. 30.** **TWO-POLE DOUBLE MAGNETIC CIRCUIT RING-WOUND DYNAMO.** sufficient section to carry them easily, they are nearly all directed through the coils or are cut by them. Now, it will be evident from the considerations (page 28) that the electro-magnetic action is the same at opposite ends of a diameter at any and every part of the revolution. This fact enables a E.M.F. GENERATED 41 coil, n, to be wound and connected as shown (fig. 29), which gives the advantage that the internal resistance of the armature (i.e., that between the two brushes) is now only half what it was before owing to the two coils being in parallel as it is called. The total E.M.F., however, only equals that produced by one coil if both have an equal number of turns. The E.M.F., and therefore the current given from a dynamo like that in fig. 29, fluctuates too much to serve any practical purpose. This fluctuation, however, can be diminished by the number of pairs of coil and commutator segments, each pair, and each can go in winding alternots of opposite coils, each pair being provided with a two-part commutator. We then come to such an arrangement as that shown in fig. 30, which represents an eight coil ring-wound armature with its eight-part or segment commutator against which the two brushes, b, a press as it revolves and collect the current from the armature. Now, the brushes, a, b, divide the total number of armature conductors into two parallel circuits, which always generate equal E.M.F.s. The actual E.M.F. at the brushes, since there are two parallels, is only equal to that induced in the conductors on one half of the core, hence we are at once able to obtain a formula expressing this E.M.F. in terms of certain quantities. Let C = the total number of turns on the periphery of the core. N = the total number of magnetic lines entering the core. n = the speed in revolutions per second. Then the E.M.F. (E) generated is that due to $\frac{C}{2}$ conductors cutting $z$ N lines per revolution, since the whole number are cut as any coil, such as a, fig. 29, moves through the first half turn, and again in the second half turn. Therefore the total rate of cutting of lines by conductors is $$\frac{C}{2} \times z \times n = C \cdot N \cdot n$$ Hence $$E = C \cdot N \cdot n \cdot volts$$ where $10^{\circ}$ is the number of lines which a conductor must cut per second to produce 1 volt of E.M.F. This last relation is a fundamental one of great importance. It thus becomes possible to utilise the whole or a portion of the continuous current generated by the armature for exciting the field magnets. Referring to the last equation, we see that E will be increased by increasing C, but there is a definite limit to increasing C and $n$, the former owing to the serious increase in core resistance caused, the latter owing to mechanical considerations and safety. A diagram showing an armature with two brushes and two parallel circuits. 42 ELECTRICITY AS APPLIED TO MINING N, the field, is the best factor to increase, and it will depend on the section and quality of the iron of the field magnets, on the number of turns of exciting coil, and on the strength of exciting current. There are a great variety of different forms of field magnets in use at the present day, one of which, known as the double magnetic circuit type, is shown in fig. 30. Another important form is the single magnetic circuit 'over' and 'under' types, of which the former is depicted in fig. 31. A diagram showing the components of a two-pole dynamo machine. The diagram includes an armature winding, collector brushes, commutator, field magnet, and field magnet coil. The text "ORGANS OF A TWO-POLE DYNAMO MACHINE" is written below the diagram. FIG. 31. All these are two-pole machines, but direct-current dynamos are made extensively at the present day with more than two poles, in some cases with as many as twelve or sixteen, and are then termed multipolar generators. These machines are in effect a combination of several two-pole dynamos, and the armatures may be regarded as consisting of sets of windings each with its own pole piece. In this case a complete period of change in the machine of large output is obtained with considerable economy in material ; the same cycle of changes occurs in an armature coil passing between alternate poles of a multipolar dynamo as in an entire revolution of a bipolar machine ; and in the case of alternators a complete period or cycle consists of one half-period only. The number of periods per second of such a machine is found by multiplying the number of revolutions per second by the number of pairs of poles. But whatever the form of the magnets, they are always wound with exciting coils in one of three ways, giving rise to what are termed— Series-wound dynamos, Shunt-wound dynamos, Compound-wound dynamos ; and these we will consider now somewhat in detail. SERIES, SHUNT, AND COMPOUND DYNAMOS 43 The Series Dynamo takes its name from the fact that the field magnets are wound with a few turns of thick insulated wire of low resistance, connected in series with the armature, and carrying the whole current generated. Fig. 32 shows the arrangement diagrammatically. Machines so wound are most suited to supplying electric light lamps or arc lamps (shown by the stars) in series, at constant current but varying E.M.F.; or for driving a series wound motor at a constant speed. The Shunt Dynamo takes its name from the fact that the magnetising coils are wound with a large number of turns of thin insulated wire, having a large resistance and connected in parallel or as a shunt to the armature. They only carry a small fraction of the total current generated by the armature. Fig. 33 shows the arrangement, it s being appliances in the external circuit which are now connected in parallel with one another. Fig. 32. A diagram showing the arrangement of a series-wound dynamo. Series-Wound Dynamo. Fig. 33. A diagram showing the arrangement of a shunt-wound dynamo. Shunt-Wound Dynamo. Fig. 34. A diagram showing the arrangement of a compound-wound dynamo. Compound-Wound Dynamo. and with the armature and shunt coils. In this case the voltage is the same, roughly, on all the appliances in the external circuit, but the total external current equals the sum of all the currents taken by them respectively; hence this kind of machine is suitable for supplying circuits at constant voltage and varying currents—e.g. glow lamps, arc lamps, motors, etc., in parallel. The Compound Dynamo takes its name from the fact that its magnets are wound partly with series, and partly with shunt coils, both of which help each other to magnetise them. Fig. 34 shows the arrangement, the object being to secure constancy of terminal voltage as the current taken by the appliance varies. In the series machine the E.M.F. rises, while in the shunt dynamo it falls slightly, as the current in the circuit rises. By a suitable combination of these two kinds of field magnets, the 44 E.M.F. is kept constant for all currents; and this fact is highly desirable when supplying glow lamps and other appliances in parallel. There are two ways of connecting the coils of a compound-wound dynamo, as follows : *Short shunt*, in which the ends of the shunt coils are joined direct to the armature terminals or brushes, and *Long shunt*, in which they are joined to the extremities of the armature and series coil connections. Short-shunt connections are shown in fig. 34, but the long shunt is the common practice and best for obtaining constancy of E.M.F. A 5 kilowatt 'Byng-Hawkins' direct-current dynamo, giving an E.M.F. of 220 volts, is shown in fig. 35. It is of the over type, with single magnetic circuit. Fig. 35 S K.W. OVER TYPE SINGLE MAGNETIC CIRCUIT DYNAMO. Fig. 36 shows a six-pole 'Byng-Hawkins' direct-current dynamo coupled direct to a 'Browett & Lindley' enclosed high-speed compound self-lubricating engine, both being on one common bed-plate. These sets are made up to an output of 240 k.w. and upwards by the General Electric Co., London. We will now deal with a few important general considerations connected with direct-current dynamos. In present-day machines it is usual to have as many brushes as there are poles in the field magnets, equally spaced around the commutator. This in large multipolar machines may have as many as 100 brushes, each having a brush box, or more. The brushes are capable of an angular motion on a frame, and when set so that there is no sparking at their points of contact with the commutator for no external current but full voltage, they sometimes DIRECT CURRENT DYNAMOS 45 require to be given a slight forward angular motion, or load as it is called, in the direction of motion to collect the current sparklessly. As each coil of the armature passes from the influence of one magnetic field to that of the next of opposite polarity there is a point at which the E.M.F. passes through its zero value, and if the coil be short-circuited no current will pass. This is the point at which the brush is placed ; for, since the brush rests on more than one commutator segment Fio. 36. A detailed illustration of a six-pole direct-coupled dynamo. SIX-POLE DIRECT-COUPLED DYNAMO. at the same time, the coil or coils attached to those segments are short-circuited through the brush as they pass under it. If a current flows in them at this moment it will be suddenly interrupted when the brush leaves one of the segments, and the result will be a spark due to the 'inertia' of the current, which always tries to leap across any gap suddenly introduced into its path. As the load on the dynamo increases, so does the increased current flowing through it, and produces a magnetic field of its own which combines with the field formed by the poles, producing a resultant distorted field ; the greater the current in the armature, the greater the distortion of 46 ELECTRICITY AS APPLIED TO MINING the field. This effect is known as *armature reaction*, and its result is to alter the position of the coil, in which no electromotive force is being pro- duced, and consequently the position of the line of sparkless commutation as the load increases. In cases where large and sudden changes of load are frequent, as in the case of a mine generating plant supplying a large number of different motors for various purposes, this is a great disadvantage, and care should be taken to obtain a machine in which this effect is made as small as possible by proper design and adjustment by appropriate means. It is common to specify that no change in the position of the brushes shall be necessary from no load to full load, or even to a consider- able overload. If the brushes do require regulating, it is important that the 'load' should be altered as the load changes, and sparking thereby prevented. The commutator should be kept smooth and clean, and if any segment wears flat it must be at once remedied and the cause removed. The cause may be due either to the softness of the copper or to sparking at this segment, &c. An electrical generator, whether alternator or direct current dynamo, should be easily able to do its work, and this to a large extent can be determined before it is installed by running it at full load for six hours, say, and then measuring the temperature of the armature, field-magnet coils, and commutator. This can be done by placing a fairly delicate thermo- meter, having preferably a thin cylindrical glass bulb, on the surface of either copper or brass, and covering it with a thin layer of lead and part of the stem with cotton wool to prevent draughts of air affecting it. The maximum limit in temperature should not exceed 50° C. for the armature and field coils, and about the same for the brushes and com- mutator. That of the bearings and other parts should not exceed 40° C. The heat generated by these parts will generally attain these values, and, indeed, it is very desirable that they should not. It is a great temptation to constantly overload a dynamo or motor if it is found that it will stand it, till it reaches much higher temperatures than this ; but though this may often be done without an immediate breakdown, it has the effect of shortening considerably the life of all parts subject to shocks caused by sudden rises of pressure which occur during the working of a machine, and which may then cause a breakdown at any time even when the machine is not overloaded. Mica is less liable to damage by heat than organic insulating materials. **Elementary Principles of Alternating Current** On pages 28 and 29 it was shown how an alternating E.M.F. is generated and how it rises from zero to a maximum, then down through ALTERNATING CURRENT 47 zero to a negative maximum and back to zero again as the armature con- ductor passes a pair of poles. This action is conveniently represented by the diagram (fig. 36a), where time is measured along the line o x and E.M.F. and current along the line o v ; the curve v v v represents the rise and fall of E.M.F. The volts measured on a voltmeter are not the maximum voltage represented by the height of the peak of the curve, but what is known as the virtual voltage, which represents the average effect of the fluctuations and is equal to the maximum voltage divided by $\sqrt{2}$; thus, $$V_{\text{virtual}} = V_{\text{maximum}} + \frac{V}{\sqrt{2}} = 707 V_{\text{maximum}}$$ If a voltmeter be applied to a circuit which is alternating between plus 100 and minus 100 volts it will read 707, and a unidirectional voltage Fig. 36a. A diagram showing an alternating E.M.F. and lagging current. ALTERNATING E.M.F. AND LAGGING CURRENT. of 707 would have to be applied to give the same reading. Insulation, however, would have to be provided to resist 100 volts. In dealing with alternating currents, the phenomenon known as self- induction or sometimes as electro-magnetic induction has great importance; it depends upon the magnetic field produced by the current (see page 3), which always opposes any change in the current passing through a circuit. The effect on an alternating current is to make the current lag behind the E.M.F. and also to decrease the strength of current flowing. Fig. 36a represents a current lagging behind the E.M.F., by a part of a period on account of self-induction in the circuit. It will be seen that until the current does not reach its maximum value until the E.M.F. has begun to fall again. Wherever in a circuit there are coils of wire wound round iron cores, as in all dynamos and motors, this effect becomes marked, and the greater the self-induction the greater is the lag of the current. The term lag, as can only be understood from the diagram, is that during a certain period both parts of the current and E.M.F., instead of being both positive or both negative, are of opposite sign, and instead of their product representing useful power, exactly opposite is the result 48 ELECTRICITY AS APPLIED TO MINING during that part of the period in which they are in opposition, the effect being that instead of getting the power in the motor represented by amperes × volts as in a continuous current (see page 8), the actual power obtained is something less than this, and is obtained by multiplying the apparent power or volt amperes by a fraction known as the *power factor*, which is usually represented by $\cos \phi$ (being the cosine of a certain imaginary angle called the angle of lag). Thus we have $$\text{Power} = W = A \cdot V \cdot \cos \phi$$ It must be remembered, however, that this does not represent a waste of energy in the motor (except so far as the extra current required heats up the conductor), since the power required to drive the generator is also only $A \cdot V \cdot \cos \phi$. Another way of looking at it is to regard that part of the current which is used to generate heat as useless, and that which is used to generate current, which requires no power to generate it, and gives out power to the motor, its only effect being to cause heating in the conductors, thus necessitating larger cables for a circuit with a low power factor. The real power of an alternating circuit may be measured by means of a watt-meter. It was stated on page 7 as Ohm's law that the ratio of potential difference to current in any circuit at constant temperature is a constant quantity called the Resistance, or $R = \frac{V}{A}$. This law does not hold good for alternating circuits having self-induction, $R_1 = \frac{V}{A}$ then denoting the effective circuit resistance to the passage of the current and not merely its ohmic resistance as with direct currents. Electro-Motors. We must now deal, in some little detail, with that most important class of electrical machine for converting the energy in the form of the electric current flowing through it into energy in the form of mechanical motion; in other words, with the electro-motor, which is now so widely used. There are two main classes of electro-motors, viz.: Direct-current electro-motors. Alternating-current electro-motors. The latter may be subdivided according to whether the motors are to work with single-, two-, or three-phase alternating current, when they are termed single-phase or polyphase alternating-current motors. DIRECT-CURRENT ELECTRO-MOTORS 49 We will next consider the principle upon which electro-motors act, and will deal first of all with direct-current electro-motors. The action of these depends on the following extremely important fundamental principle—namely, that if two different magnetic fields are under the influence of one another, one being fixed and the other movable, this latter will move so that as many of its own lines of force as possible coincide in direction with those of the other field. This is true no matter how the two fields are generated, whether by simple bar magnets or coils, wires, or whether even both are movable. The force tending to move the material substance producing the movable field increases as one or both fields get stronger. Turning now to an elementary application of the foregoing principle, we will consider fig. 29. Assuming, of course, that a magnetic field is flowing through a coil of wire, then if we place another coil parallel to the two coils in parallel by means of the brushes pressing on the commutator, there will be no force acting on the coils, tending to turn them, if the lines of force they produce coincide in direction with those of the fixed field for the positions shown. If, however, the current is reversed, the two axes of motion become direct currents and the two arms of the armature are strong enough, the coils or armature will make a half-turn, so as to produce maximum coincidence; but at the end of this half-turn the current is again reversed by the commutator, so that the armature moves through another half-revolution, and so on, a continuous motion being the result. With only two coils, an armature may come to rest at a dead point and not be able to turn about its own axis. This difficulty can be overcome by increasing the number of coils and commutator segments, when the arrangement shown in fig. 30 will be obtained. When a current is sent through such an armature, each coil tends to set itself with its plane perpendicular to the direction of the fixed field where the coincidence of the two axes is least. But as soon as a coil gets into this position the current in it reverses, causing it to turn through a further half-revolution. Thus a uniform driving force is obtained, and we see that the so-called electro-motor for direct currents is merely a direct-current dynamo used the other way about—that is to say, instead of using it as a dynamo for generating current it dynamo run as efficiently when used as motors. They may or may not, but in any case the proper voltage must be supplied to them, which may be found as follows : Run the machine as a generator at the desired speed and note the E.M.F. generated on open circuit. The pressure necessary to run the motor on any load at the same speed will be this pressure minus that required to overcome the internal resistance of the motor, which is equal to that resistance multiplied by the current taken by the motor at that load. Since there is little or no difference between a motor and a dynamo, E 50 ELECTRICITY AS APPLIED TO MINING what was said a few pages earlier on the latter applies equally to the former. The armatures are almost always either ring or drum wound and the field magnets may be either series, shunt, or compound wound. It is in connection with these latter windings that any particular direct-current motor is spoken of as a 'shunt motor,' or 'series motor.' Now, it is only while the armature of any motor is stationary that Ohm's law can be used for finding the current that a given voltage can send through the resistance. As the armature rotates in the magnetic field an independent action goes on apart from the driving current circulating in the conductors. This is due to the conductors as they rotate cutting the lines of force produced by the field magnets, as in a dynamo or generator which is opposite in direction to that of the supply and opposes it. The term 'Back E.M.F.' has therefore been given to it: its effect is to oppose and diminish the current which the driving E.M.F. is trying to force through the motor. This back E.M.F. has a most important influence on the regulation of electro-motors in general, and the action that occurs is as follows: When the current is switched on, and just before the motor starts, it rises at once to its full strength—(the starting rheostat necessary to prevent overheating before the armature begins to revolve is referred to on page 117)—determined solely by the total resistance of the main circuit— by the resistance of the armature. As, however, the armature gets up speed, it generates a back E.M.F. which acts in opposition to the E.M.F. of the supply, and is in direct proportion to the speed. The current is thereby reduced as the speed rises, and if the motor is unloaded the speed will increase until the back E.M.F. nearly equals that of the supply; accordingly, the pressure being the pressure requisite for sending the reduced current to overcome friction and air resistance between the motor and circuit. If the motor is started on load it will take a proportionally larger current when in motion than when running 'tight' with the same driving E.M.F. at its terminals, and the back E.M.F. will be less if the speed is the same. Now it can be shown that the Torque of an electro-motor is proportional to the field strength and to the armature current, and the greater each of these the greater will be the torque. The torque is the moment of the force causing rotation, and equals the total force at the surface of an armature multiplied by radius of armature. Since there should be no more maximum torque current in an electro-motor, we also get maximum torque, while in a steam-engine, neglecting 'variations of cut-off' and 'wire drawing,' we get equal torque both at starting and at full speed. The speed of any motor will be increased by increasing the armature DIRECT-CURRENT ELECTRO-MOTORS 51 current or diminishing the field current ; the former can be accomplished by having a main-circuit rheostat or resistance in series with the armature, when, by diminishing this resistance, an increase of current and speed will result, and in shunt- and compound-wound motors one in series with the field as well, when by increasing the shunt-circuit resistance the shunt current, and therefore the field, is diminished and the speed increased. From the foregoing it will be seen that the shunt motor always exerts maximum torque at starting, while the shunt and compound motors will only do so providing the field windings are so connected to the circuit that the shunt is fully excited before the armature current is switched on. Fig. 37. **Four-Pole Direct-current Motor.** A series motor suddenly relieved of its load begins to race until sufficient resistance is added to the circuit, while shunt and compound motors only slightly increase their speed when the load is removed, the reason being that the back E.M.F. rises rapidly, cutting down the armature current, which in the series motor is also the field current. Let us now briefly consider the conversion of direct-current dynamos into electric motors. A Series Dynamo, run as a motor, will run against its brushes unless the terminals of either the armature or field magnets be interchanged with regard to the circuit. 2 52 ELECTRICITY AS APPLIED TO MINING A Shunt Dynamo, run as a motor, always runs with the brushes, for either the armature or shunt currents have reversed in direction, which is equivalent to interchanging the terminals of either. A Compound Dynamo may run either with or against its brushes according to whether the shunt or the series coils respectively preponderate in strength. In every case, therefore, it will be seen that in order to run a dynamo as a motor in the same direction---i.e. with the brushes---the current in either the armature or the field coils must flow in the opposite direction. The efficiency of electro-motors should be as high as possible, but economy in first cost should not be sacrificed unduly to economy in Fio. 38. VENTILATED ENCLOSED ELECTRO-MOTOR. working, and in large machines, where the power lost through inefficiency might be considerable, the commercial or net efficiency, which is total power given out, may be as high as 90 to 95 per cent. and over. The difference (power put-in)-(power given out) should, therefore, be as small as possible. It is made up of the losses in the iron and by friction, and also in the copper circuits, and, as we have seen (page 7), if A = armature current and R its resistance, then $A'R =$ watts lost in armature coils. ALTERNATING-CURRENT ELECTRO-MOTORS 53 Similarly, $a^{\prime}r = \frac{E_3}{T}$ watts lost in shunt coils when $a$ = shunt current, $r$ = shunt resistance, and $E$ = E.M.F at the shunt terminals. Electro-motors take a great variety of forms. Fig. 37 shows a four-pole direct-current motor of the 'Byng-Hawkins' type, made by the General Electric Company of London. Fig. 38 shows an enclosed ventilated one of the same make, for working in dusty situations. **Alternating-current Electro-motors.** These may be divided into two classes—namely, (a) Single-phase Motors ; (b) Polyphase or Multiphase Motors. The former may be again subdivided into (1) Synchronous motors ; (2) Asynchronous motors ; (3) Series motors ; (4) Reversing motors ; and (5) Combinations of these. Any single-phase generator may be used as an alternating-current motor by first starting it, by means of an engine or other motor, up to such a speed that the periodicity of the induced E.M.F. is equal to that of the alternating supply, and the E.M.F. opposite and about equal to that of the supply. Then the machine will run without any external power, without the aid of the machine which started it up, and do mechanical work. There is, however, one peculiarity about it—namely, that it will run dead synchronously with the generator for all loads—i.e. half the speed of the latter will give half the speed on the motor, and so on. This is a serious disadvantage of the alternating-current motor, and it is efficient but not self-starting, which is the great disadvantage of it. Another is that the field magnets have to be separately excited by some direct-current source, as they were when it was used as a generator. The second type is a specially built motor which is self-starting but non-synchronous; it is called an induction motor, because the principle of action is similar to that of an induction dynamo, i.e., there is no electrical connection whatever. There is, therefore, no possibility of sparking, owing to the entire absence of commutator, collecting rings, or brushes. The main disadvantage of these machines is that they are less efficient, power per horse-power than direct-current machines, and, moreover, will not start up with a load whatever, unless they are driven by generators. Where polyphase current is available, neither of these types (1) and (2) is so useful as the polyphase motor; the third type, however, the series motor, has lately been brought forward as a promising competitor to the polyphase motor under certain conditions. The single-phase series motor is similar in construction to a direct-current series motor. It has similar characteristics as regards starting, torque, and speed regulation, in which respect it is superior to the three-phase motor, but it is inferior in that it has a commutator at which there is 54 ELECTRICITY AS APPLIED TO MINING considerable sparking. It has rather a lower efficiency, and it cannot be used with high voltages at the motor terminals with high frequencies. The method just described comes in, however, an economical one, which is a decided advantage where frequent starting and stopping under load is required. A high voltage may be brought to a transformer regulator ; the current for the motor is taken from the low-pressure side of this transformer, and by varying the ratio of transformation a variable voltage at the motor terminals is obtained. By the use of water-resistors, thus obtaining an economical method of speed-regulation. These characteristics render it especially suitable for electric traction. Quite recently it has been decided to electrify a short section of the London, Brighton, and South Coast Railway with single-phase alternating currents, using single-phase motors on the trains. As is well known, this kind of electric traction requires a maximum of torque at starting so that the development of this property in such motors, coupled with the fact that they can be made to work satisfactorily with polyphase, alternating currents, should have a very important bearing on electrical engineering and the application of electricity to winding, hauling, &c. in mining work. It would be possible to use small motors of this kind on one phase of a polyphase system. The series and repulsion motor, need not here be considered. Its most important application at present has been in alternate-current meters, though great improvements have lately been made in combinations of the principles of the series and repulsion motors for traction work. Lahmeyer Motor. Diagrammatic View. 39 LAHMEYER MOTOR 55 Lahmeyer Motor. A single-phase motor is now made by the Lahmeyer Electrical Company, and is known as their DiG type for heavy starting torque. The principle on which the Lahmeyer motor is built is shown diagrammatically in fig. 39, a side sectional elevation being given in fig. 40. It consists of a well-laminated stator composed of soft iron rings held by, and enclosed in, an exterior cast-iron sheath. The stator main windings $s$, and either $f_1$ or $f_2$ are the only ones connected to the supply mains, and, being on a fixed portion of the motor, can be well insulated. A diagram showing the sectional elevation of a Lahmeyer Motor. **Fig. 40.** Lahmeyer Motor. Sectional Elevation. The rotating interior or rotor is similar to a direct-current armature, being provided with an ordinary commutator at one end, and three slip rings (connected to three equidistant points on the rotor winding), at the other. The brushes pressing on the commutator are short-circuited through a resistance, $w$, and the slip ring are connected to an ordinary three-phase starter, $\alpha$. The motor is a combination of a repulsion motor and an induction motor. The repulsion motor gives a large starting torque at starting, which falls off as speed increases. This is indicated in fig. 41 (curve II), which shows the relation between starting torque and speed. With the induction motor the starting torque increases with increase of speed up to a certain maximum limit, after which it rapidly diminishes, as seen in A diagram showing the repulsion motor section. A diagram showing the induction motor section. 56 ELECTRICITY AS APPLIED TO MINING curve I. The Lohmeyer motor combines the favourable starting conditions of the repulsion motor with the favourable working conditions of Graph showing Torque vs. Revolutions per Minute for curves I, II, and III. FIG. 41. Curves showing relation between torque and speed in I. Induction, II. Repulsion, and III. Lohmeyer Motor. the induction motor, and a high torque throughout the whole starting period is obtained (curve III). Graph showing Torque vs. Current for Lohmeyer Motor. FIG. 43. Lohmeyer Motor. Relation between Starting Torque and Current. The motor is started (with a off) as a repulsion motor, and then A is cut out with increasing speed to short circuit the slip rings. The resist- POLYPHASE MOTORS 57 ance, w, is then increased until practically no current flows through the commutator. There is practically no sparking, and as long as a is open the speed varies with the load as in a series-wound direct current motor. On short circuits, however, the current is practically constant for all loads. The direction of rotation can be altered by changing the position of the brushes in relation to the magnetic field. For frequent reversing, however, the motor has auxiliary field windings, $f_1$, $f_2$ one or other only of which can be inserted by a switch. These motors can also be used for starting heavy loads. The current and starting torque can be regulated by the resistance, w. Fig. 43 shows the relation between current and starting torque in a 5-h.p. motor, from which it will be seen that, when starting, three times the normal full load torque is obtained with less than twice the normal full load current. Polyphase motors are divided into two kinds: namely two types used in practice—namely, two-phase and three-phase motors. Both kinds are self-starting (on load or otherwise), asynchronous, efficient induction motors ; and their principle of action will be better understood by reference to the principle on which they work, represented in fig. 43. Let the field magnets of an ordinary bipolar dynamo or motor, instead of being mounted on the shaft, be mounted on a frame with suitable bearings. Also let the ends of the field-coil winding be connected to two copper rings, r s, mounted on the shaft and rotated by it, but insulated therefore and from one another. A direct current from some source is led into and out of them by two wires (w), connected to two brushes, b s, pressing on them so that they rotate. A laminated iron core is mounted on the same shaft and free to rotate on it independently, is wound with coils, c c c, of one or more turns each, short-circuited on themselves, so as to form so many closed coils. If, now, the field magnets are driven mechanically, the field, as it rotates with them also, will cut the coils, c c c, the induction of which will set up an E.M.F. and current in them. This current will cause these coils to start rotating and will speed up until it catches up the field, when the two will move synchronously together, or nearly so. The rate of cutting of lines of force by the coils, c is now very small, therefore the E.M.F. and current generated in them are also small. If, now, we increase its speed will diminish a little, and therefore the rate of cutting, which is proportional to the difference in the speed of the magnets and t will increase. This increases the induced E.M.F. in c c c, and causes more current to flow in them, thus giving the necessary torque for that load. The function which both two- and three-phase currents perform is to produce net torque at any speed without any need for starting or moving over a range. The portion of the motor which is wound with coils in a suitable manner to receive the two- or three-phase currents, as the case may 58 ELECTRICITY AS APPLIED TO MINING be, and which is stationary, is called the stator, while the rotating part, corresponding to the ring, $t$, in fig. 43 is called the rotor. This is the principle of the polyphase induction motor, which is automatic in taking in just sufficient and no more power than will give the necessary torque to be exerted. A motor such as this, which is self-starting, entirely automatic in its action, self-lubricating, and which possesses no rubbing contacts or connection to the rotating part whatever, can be left to itself for weeks without any attention; and, further, all possibility of sparking, which may be of A diagram showing the internal components of a three-phase alternating-current polyphase motor. The diagram includes a central shaft with a rotor (R) and stator (S), connected by brushes (B). The stator has three coils (C) wound around it. The rotor has three brushes (B) pressing on it. The diagram also shows the direction of rotation (W) and the position of the stator's poles (N). PRINCIPLE OF ALTERNATING-CURRENT POLYPHASE MOTOR. such moment in mines, is avoided in many of the smaller sizes; for higher powers, where large starting torque is required, slip rings are mounted on the shaft and connected to the rotor coils, which are suitably wound. This is to enable resistance to be inserted in series with the coils to keep down the heavy induced current at starting, for it will be remembered that the difference between the field and armature currents is greatest at starting, and therefore the induced current also. Fig. 44 shows a three-phase alternating-current motor fitted with these three rotating slip rings connected to the rotor coils. The three brushes pressing on them would be connected to a three-phase rheostat. Three terminals seen on the top are for clamping the three mains which connect to the motor. The commercial efficiency varies from 75 to 88 per cent. for powers up to 50 h.p., and reaches 94 per cent. for powers over 100 h.p. The rise in temperature above that of the sur- **TRANSFORMERS** 59 rounding atmosphere does not exceed 35° C., and the slip is 0-5 to 1-5 per cent. of the speed. Fig. 44. **THREE-PHASE ELECTRO-MOTOR.** The motor illustrated is supplied by the General Electric Company, London, and can be made to work direct on circuits of pressures up to 5,000 volts. **Transformers.** We have already seen (page 7) that when a current of A amperes flows through a circuit of R ohms resistance, the electrical power wasted in it = A²R watts. Manifestly, then, the loss increases rather rapidly with increase of current which is under the square, but the actual power transmitted will at a pressure V be equal to A V, and this product can be obtained in a great number of ways ; for instance, A V = $\frac{1}{2}$ A X $V^2$ = $\frac{1}{2}$ A X (V + yA X) = A V + $\frac{1}{2}$ A²X. Hence, by increasing V and diminishing A, the same power may be transmitted with very much less loss and at much less cost, as the circuit may have a much smaller cross-section, and therefore cost far less to lay. As a rule, however, if the transmission be at high pressure, such as 2,000 volts or more, this breaking point is far too high for use with many appliances, and it is therefore necessary to reduce it to a workable value, 60 ELECTRICITY AS APPLIED TO MINING and this is effected by an appliance called a transformer or converter, or sometimes a motor-generator. Of these there are two main classes: Direct-current transformers. Alternating-current transformers. The former are all rotary machines, while the latter comprise both static and rotary machines. In all cases they are essentially transformers of pressure, the section of the coils being such as to be capable of carrying the required current. The first class of transformer is necessarily of a rotary kind only, comprising either a continuous-current dynamo and motor direct coupled, or an armature wound with two distinct windings, each having a commutator and running in one common field. In either example the motor part is driven by the armature which drives the dynamo and generates current at a different pressure from that of the supply. In the machine with one armature doubly wound in a single field, the ratio of voltages is almost directly proportional to the ratio of armature windings. The efficiency of such machines is low, and if the motor and dynamo portions each had a commercial rating of 80 per cent., then the whole alternating-transforming device would only have an efficiency of 8 per cent. at full load. Notwithstanding this, it is often economical to employ them in practice. They can be made to take in power at 2,000 volts, and give out at anything lower desired, such as 100 or 300 volts. Taking up the second class of transformers, the rotary type is restricted solely for converting continuous to single- or polyphase alternating currents, or vice versa, and is becoming an important and extensively used appliance at the present day. It consists of an ordinary direct-current dynamo having a ring-wound armature, the winding of which is tapped at two or more points, which are electrically connected to two or more copper or brass rings mounted on the shaft, but insulated from it and from one another. For every pair of poles in the field, two tappings are taken to two slip rings for single-phase currents, four tappings to four rings for two-phase currents, and three tappings to three rings for three-phase currents. The efficiency of these converters when they are called, is high, and they are very useful when polyphase alternating currents are transmitted, but continuous currents have to be used at the work. The rotating portion and fixed brushes of a rotary converter, without the field magnets, are represented diagrammatically in fig. 45. $s$ is the shaft carrying the ordinary direct-current ring-wound armature, which is connected to one end of the two-phase collecting-rings $r_1$, $r_2$, $r_3$, at one end, and the three-phase collecting-rings $r_1$, $r_2$, $r_3$, at the other. These rings are insulated from one another, and from the shaft, by suitable insulation, $i$. **TRANSFORMERS** Fig. 45 actually shows the arrangement for transforming continuous current into three-phase alternating current, or vice versa; the three points, A B C, on the armature winding being equally spaced round it, and connected to the three rings, r1, r2, r3, respectively. If, then, A B rotates in a two-pole field, and direct current is led into and out of R A by the fixed brushes, u1, u2, so as to drive the armature as a direct-current motor, then three-phase alternating current will be generated at the fixed brushes, v1, v2, v3. Conversely, if three-phase current is driven through the armature, the machine will run as a three-phase motor, and give out direct current at u1, u2. Finally, if S S is driven by a belt, the machine will give out both Fig. 45 **PRINCIPLE OF THREE-PHASE CONVERTER.** direct current at u1, u2, and three-phase alternating current from v1, v2, v3. A two-phase converter would have four rings instead of three, which would be connected to four points on R A equally spaced from each other ; while for a single phase converter there would be two rings only, connected to two points opposite one another on R A. The type of converter shown in figs. 46 and 47 is the static type, having no moving parts or contacts, the alternating current in one part performing the function of causing the magnetic field to vary itself, so as to cut the windings in the other part and so generate a greater or less E.M.F. depending on the ratio of the windings on the two parts. A reference to figs. 46 and 47, which show the inside of a 1 k.w. static transformer supplied by the General Electric Company Ltd., London, will make the principle clearer. The core consists of Q bars of laminated iron with thin Swedish charcoal iron of the best quality, built up side by side to a sufficient thickness. 61 62 ELECTRICITY AS APPLIED TO MINING Two coils, each consisting of two separate windings of double cotton-covered copper wire, are then slipped one over each limb, as shown in fig. 47. Cross strips, $a$, are then placed across the ends of $b$ and $c$, as shown, thus forming a closed iron magnetic circuit for the lines to flow in. The two outer coils on the limbs are connected in series to two terminals, and likewise the two inner coils to a second pair of terminals. One coil Fig. 46. Fig. 47. **Static Transformer.** **Magnetic Circuit.** is then connected across the supply mains, and is called the primary; the other to the lamps or motors, and is called the secondary of the transformer. In most cases we have $$\frac{E_1}{E_2} = \frac{N_1}{N_2}$$ where $E_1$ and $E_2$ are the primary and secondary E.M.F.s, and $N_1$ and $N_2$ are the number of primary and secondary turns. Thus the higher pressure coil will have more turns than the lower pressure coil, and consequently a higher resistance. Fig. 47 shows the construction and the ends of the windings of the inner coils protruding above the coil ends. When the transformer is to be used in damp places it is enclosed in an iron case with a tight-fitting lid, as shown in fig. 48, the leads going to the primary and secondary passing through insulating stuffing glands or holes in the case. One extremely important advantage of this static type of transformer is its automatic regulation on constant-pressure primary mains for varying secondary load. **TRANSFORMERS** 63 When there is no load on the secondary, the primary takes in just so much power as is required to supply the losses in the iron circuit; at full load it takes in power equal in amount to the total internal losses, plus the Fig. 48. A transformer in iron case. *Transformer in Iron Case.* secondary output, whereas at half load it only takes in half this amount, and so on. Hence the static transformer is not only entirely automatic in its action, but needs absolutely no attention, being quite inert in itself. By means of which appliances almost any amount of alternating current power can be transformed into direct alternating voltage to any other voltage at an efficiency in the larger types of as much as 96 per cent. **Performance of Different Classes of Machinery.** Having now gone briefly into the first principles of the generation of electric currents and their reconversion to mechanical energy in motors, it will be well to go into the advantages and disadvantages of the various types of machinery, both in general and for particular purposes, having especial regard to reliability, efficiency, and mechanical performance. Generators are the most important sources of electric current. The two types of generators are in the matter of reliability and regulation of voltage. In a continuous-current generator, the rotating part necessarily carries the main current, and is provided with a commutator through which this main 64 ELECTRICITY AS APPLIED TO MINING current has to pass. A very large proportion of the breakdowns and disorders of generators are due to the difficulty of properly insulating, and at the same time mechanically securing the rotating armature conductors, and also in destructive sparking at the commutator and disorders of brushes and brush gear, which have to collect the whole current produced. Continuous-current generators can, on the other hand, be made to give a constant voltage by means of compound windings; and if it is desired they can be made to give a higher voltage on full load than on light loads by means of 'over-compounding' in order to compensate for the increased drop of voltage in the mains when a large current is flowing. There is no difficulty in running alternating current machines in parallel, but it is not easy to keep up the load among two or more machines each giving the same voltage and running with its positive and negative poles connected to the same mains ; a thing which is essential in large power distribution plants with a variable load. In alternating-current generators the armature windings can be placed on the outer stationary part of the machine, since they do not require commutation; the main current is then carried without any rubbing contacts whatever, and there is no centrifugal stress on the armature windings ; the exciting current must, however, be led into the rotating part by means of slip rings and must be generated by a small direct-current dynamo. The difficulty of keeping up the load among two or more machines each giving the same voltage and running with its positive and negative poles connected to the same mains ; a thing which is essential in large power distribution plants with a variable load. Alternators are also made in which both field and armature windings are stationary; these are known as self-regulating alternators (see p. 35), and have much to commend them so far as reliability goes. It is less easy to make an alternator self regulating in voltage than a direct-current machine, especially on a low power factor circuit, but the difficulties can be overcome. There is some difficulty where it is necessary to run alternators in parallel, on account of the fact that it is customary that all the machines should be started at the same instant. A.M.F. currents are of one moment, and for this purpose an appliance called a synchroniser is fitted to alternating-current switchboards. When a machine is to be put in parallel with the others, it is first of all started up by itself and run at the proper speed and correct voltage, the synchroniser indicates when it "in step" with another machine, and then starts it up in parallel. Once in step the tendency is to remain so unless some disturbing cause prevents it; such a disturbing cause may, however, be present in the shape of an uneven turning moment in the engines, especially in gas engines, or in defective governors, or in governors on the different engines which do not act PERFORMANCE OF DIFFERENT CLASSES OF MACHINERY 65 together. With care in selecting the machinery, however, alternators can be very successfully run in parallel. It may be mentioned here that no generator in practice gives the exact theoretical 'sine' curve of fig. 36A. Some generators, to take an extreme case, may give a 'peaked' curve like that in fig. 49, and others a flat Fig. 49. **ALTERNATOR. PEAKED WAVE.** curve like that in fig. 50. Two generators with dissimilar curves will not run well in parallel, but a machine can always be designed to give any curve desired, and for motor work and for the sake of insulation a peaked curve is undesirable. Fig. 50. **ALTERNATOR. FLAT WAVE.** **Motors.—The selection of the most suitable motor for a given situation is a difficult and complicated problem, and different engineers will be found giving different advice on the same question. An electric motor can be designed to do practically any work, whether direct-current, polyphase alternating-or even single-phase alternating current be used ; but one...** 66 ELECTRICITY AS APPLIED TO MINING class of motor will often do a particular piece of work better, or more economically, in first cost or working cost than another. We will discuss the questions of speed regulation, starting torque, efficiency, &c., one by one; but it must be understood that these things are intimately designed, so that high efficiency may be obtained in any motor by sacrificing some other characteristics, such as first cost or starting torque; and other qualities such as better speed regulation may be obtained at the sacrifice of something else. **Speed Regulation.**—A motor may be required to operate at— (1) constant speed; (2) speed varying automatically as the load changes, increasing with heavy load and decreasing with heavy load; or (3) speed variable will independently of the load. (1) For an absolutely constant speed a differential wound compound motor may be used—i.e. a motor in which a series winding is applied in opposition to the shunt winding, so that as the load increases the field is weakened, thus causing the motor to keep up its speed. This type is, however, not very satisfactory for continuous running on series motors. A shunt-wound motor maintains a constant speed within 5% or less from no load to full load, and is quite sufficient enough for all conditions likely to be met with in mining work ; shunt and polyphase motors are made which vary as little as 2% in speed from no load to full load under steady duty. The series motor can run at absolutely constant speed so long as the generator speed does not vary. (2) The series motor is capable of adjusting its speed automatically to varying loads. It is liable to race and damage itself if it loses its load, owing to the weakening of the field. It is also especially liable to be damaged if taken on an excess of current on heavy loads and the slow speeds occasioned thereby. Compound-wound motors are sometimes made with series and shunt winding in accordance with each other, i.e. with the series winding assisting the shunt, the shunt being strengthened as the load increases. These motors have been found useful for use in speed regulation, and their speed may fall about 10% from no load to full load. A polyphase motor designed with a high resistance rotor has similar characteristics to a direct-current series motor in automatic speed regulation. A single-phase series motor is also similar to a direct-current series motor in this respect. (3) For varying the speed at will independently of the load, two series motors may be used in such a way that they can be put on to the mains either in parallel or series with each other ; when running in series they will run at half the speed at which they run when in parallel, because they each receive half the full voltage. This is known as series parallel control. A shunt-wound motor may be varied in speed from 20% to 30% by placing PERFORMANCE OF DIFFERENT CLASSES OF MACHINERY 67 a variable resistance in the shunt winding (in specially designed motors a much greater variation may be obtained). This resistance absorbs very little power, as the main current does not pass through it. Alternating-current motors of the single-phase series type may have their speed varied by a regulating transformer, which varies the applied voltage with only a small loss of power. The voltage applied to the terminals of a motor (and therefore to its armature) is reduced by the regulating transformer with it. This is a wasteful but effective method. The efficiency is reduced in the same proportion as the speed. This method is used in conjunction with the series parallel control of series motors in traction work in order to obtain intermediate speeds; it may of course also be used with single-series or shunt-wound motors; care being taken that the resistance is used of sufficient value to make the resistance losses for all speeds for which it is required to run at reduced speed without undue heating. The speed of polyphase motors is regulated by varying the applied voltage, or by varying the resistance in the rotor circuit. The first method involves having a variable high resistance in the stator circuit, and is therefore uneconomical. The method most commonly used is that of connecting a variable resistance in the rotor circuit connected thereto by slip rings ; the efficiency is proportional to the speed; the power wasted appears as heat in the resistance external to the motor and in the motor itself, as in the permanently high-resistance rotor method, and a smaller motor may therefore be used. With direct-current motors, when three-phase operation is in use, the speed may be economically varied by connecting either across the outer or between an outer and the middle wire, thus obtaining a reduced voltage; intermediate speeds being obtained by variable resistance in the shunt windings. Starting Torque.—The highest starting torque is obtained with the direct-current series motor. This gives a higher starting torque for a given current than any other type of motor. It must be remembered that if a very heavy starting torque is required, a heavy current will have to be used, and a heavy starting resistance must be employed to take this current without undue heating. A shunt-wound motor will give a starting torque equal to full-load running torque if a sufficiently substantial starting resistance is used, for starting light a lighter resistance is sufficient. A compound-wound motor with series and shunt windings in accordance has a starting torque intermediate between a series and a shunt motor; a differentially wound compound motor has a starting torque inferior to that of a shunt motor. A polyphase motor with two poles will have half-full-load torque with two to one and two half times full-load current. Motors which are small compared with the capacity of the generator and the mains may be started on full-load torque by switching straight on to the mains, but with P 3 68 ELECTRICITY AS APPLIED TO MINING large motors it is necessary to keep down the starting current within reasonable limits, so that there will not be too heavy a rush of current from the generator and through the mains. To keep down the starting current an 'auto-transformer' may be used with a squirrel-cage rotor, which gives a decreased voltage at the motor terminals, but this gives an inferior starting torque. With large motors, where large starting torque is required without an enormous increase in starting current, it is necessary to have a permanently high resistance or a variable resistance separate from the motor. A larger starting torque is obtained for a given current by this arrangement, and torques up to three or four times the full-load running torque may be obtained. Where very large starting torques are required, it is necessary to sacrifice the power factor to some extent, which means heavier currents but not necessarily decreased efficiency. In such cases, where the synchronous motor has no starting torque, but has to be started by some auxiliary device. The single-phase induction motor has an inferior starting torque to the polyphase motor, and it is necessary to use a wound rotor in all but the very smallest sizes and a phase-splitting device in the stator circuit. Single-phase induction motors are similar to direct-current series motors as regards starting torque. **Efficiency.** The direct-current series motor is usually designed to give its highest efficiency on light loads, these motors being usually operated for a greater proportion of time on light than on full loads. The efficiency falls off rapidly as the load increases. A shunt motor maintains a high efficiency over a wide range of load conditions, but its full-load all-round efficiency on varying loads; its highest efficiency is usually about on full load, but its efficiency on light loads is inferior to that of the series motor. Polyphase motors with squirrel-cage rotors have an even wider range of high efficiency than a shunt motor. Their efficiency is highest at com- paratively low speeds and decreases rapidly as the speed increases. The efficiency is decreased in proportion to the speed when low voltage is applied or resistance inserted in the rotor circuit. Polyphase motors with permanently high-resistance rotors for variable speed work have a low efficiency, which falls off rapidly as the speed decreases. For variable-speed work, polyphase motors are superior to direct-current motors as regards efficiency. Single-phase induction motors have a rather lower efficiency than polyphase, and single-phase series motors have a lower efficiency than direct-current series motors. Polyphase motors have the highest efficiency and the best performance generally when designed for high speeds ; direct-current motors have the highest efficiency when designed for low speeds. **Voltage.** Alternating-current motors can be driven direct from much higher voltages than direct-current, because the high-voltage current does not need to be taken into the revolving parts or through complicated PERFORMANCE OF DIFFERENT CLASSES OF MACHINERY 69 switch-gear ; alternating-current motors can therefore be used with smaller cables than direct-current. The single-phase series motor, although it cannot be operated direct on high voltage mains, can be worked from a variable transformer from the high voltage, which transformer also acts as an economical speed regulator. Insulation.—Another advantage of the fact that alternating-current motors carry a large main current in the revolving parts is that the insulation can be made more substantial without interfering with mechanical soundness, and alternating-current motors are therefore less liable to 'shorts' and 'earth' than direct-current motors. A squirrel-cage polyphase motor may be operated without any insulation at all on the revolving part. Sparking.—Alternating-current motors always leave a spark at the commutator. A squirrel-cage polyphase motor will almost never give a spark unless some electrical connection breaks away, which is extremely unlikely ; the only possibility of sparking is at the switch, which can be enclosed without risk of undue heating. Polyphase motors on which slip rings are used may give a spark if a brush is broken or displaced, but are very much less liable to sparking owing to the fact that the brushes drop can be enclosed in a separate casing without it being necessary to box in the whole motor. Single-phase series motors have a commutator at which considerable sparking takes place. Overload Capacity.—This is mainly a matter of design. Series-wound motors may be rated at the load which they will carry for five minutes or for an hour without exceeding a certain temperature, and the overload capacity will depend upon the rating. Shunt-wound motors should be rated at the load they will take continuously without undue heating, and, if started cool, they may take overloads for some little time. If they are started hot, however, they are liable to serious damage if fuses and overload cut-outs are not sufficient to fail ; there is nothing in the motor itself to prevent a current passing through it which will burn up the insulation. With alternating-current motors, however, the case is different ; owing to their property of self-induction they will take a very much less current when brought to a standstill than direct-current motors do. They can therefore work at the same voltage, and a polyphase motor may be so overloaded as to be stopped without serious damage to the insulation ; of course the current must not be allowed to continue to flow for more than a second or two after the motor has been pulled up. This percentage of overload in which the motor may safely operate depends upon its construction and type of machine. Cost.—It is difficult to compare the cost of direct-current with polyphase machinery, especially as the patents controlling the polyphase principle have only lately run out. Polyphase machinery, however, should, from its simplicity of mechanical construction and lightness, be 70 ELECTRICITY AS APPLIED TO MINING cheaper than direct-current, and in large sizes this will generally be found to be the case ; so much, however, depends on the particular conditions to be satisfied that this must only be taken as a very wide generalisation. In all cases too much stress cannot be laid on the fact that electrical machinery in which quality is sacrificed to cheapness is disastrously un economical. The breakdown of a cheap haulage plant, for instance, may mean the loss of one man's life, while the same result would not be required to make it a reliable plant, by the loss in output occasioned thereby. Mechanical Simplicity and Reliability.--High-class direct-current motors are exceedingly reliable from a mechanical point of view, but there can, on the other hand, be no doubt that many of the possible sources of trouble are eliminated by the use of a polyphase machine ; and for trying conditions such as coal-cutting and other work met with in mines, this is an immense advantage. It will pay a manager to put up with many other inferiorities, such as lower efficiency, or starting torque, if he can at the same time secure increased simplicity and reliability, and it is surely fair to reason that so many polyphase plants are being installed in mines at the present day that a polyphase motor so simple as, or more reliable than, a polyphase motor with squirrel-cage rotor, and such motors are being successfully applied to coal-cutting, which is perhaps the most trying of all mining work for a polyphase motor. With a bar machine such as the Hurler, the squirrel-cage rotor gives perfect satisfaction in some cases, but it does not compare well with the disc machines, since these require a larger starting torque. Haulage is being done in many cases with wound rotors, but some managers prefer to secure the advantages of the squirrel-cage, and to start the motor lighting-put off load on with a friction clutch : in this way large starting torque is not required. It has been said that there is no work at a mine which a polyphase motor cannot do. Figs. 51 to 58 give curves showing the various characteristics of high-class direct-current and three-phase motors, made by the British Westinghouse Electric and Manufacturing Company, Limited, of Manchester, which will well repay a careful study. The authors are indebted to the Westinghouse Company for permission to publish these curves and for their assistance. For the direct-current motors, the efficiency, speed, torque, and brake horse-power, are given for all values of the current. In the case of the series-wound motors, it will be noticed that instead of a certain number of horse-power, a number is given to the motor, the horse-power being a variable quantity depending upon the current flowing through it. As to be carried, a curve is therefore given showing the temperature rise pro- duced by different currents taken for different times. Thus, No. 70 crane motor will take a load of 100 amperes for a little over twenty minutes without rising more than 40° Cent. (72° Fahr.) in temperature ; SPECIAL CONSTRUCTION OF MOTORS 71 at this current the horse-power developed is seen to be about 33-34. In the case of the polyphase motors the other quantities are given for different values of the torque developed in pounds at one foot radius. For constant-speed motors the speed, efficiency, power factor, and brake-horse-power are given, and the speed of synchronism (generator speed) is shown by a horizontal line. The same remarks as to horse-power apply to the variable-speed continuous-current motors ; these motors have high resistance secondaries. The smaller three-phase crane motor is fitted with a speed controller, by means of which the voltage applied to the terminals can be varied, and the speed curves are given for full, three quarters, and half voltage. The times for which a particular torque can be safely exerted at the different voltages are also shown for this machine. Construction of Continuous Current Motors for use in Coal Mines. One of the disadvantages urged against the use of electricity in the mine is the danger of sparks at the brushes igniting fire-damp. The dangers of the commutator are probably less than those of the cable, and open motors have been run experimentally in an explosive mixture with the brushes and commutator cool and in good order without firing the gas. A coal-mine, however, has no cages, and brushes are frequently in bad order, so that parking at the commutator is looked upon as something which is liable to fire gas, and the provision of a safe motor for use in the face when coal-cutting where there is possibility of blowers of gas occurring is of the highest importance. Quite apart from the question of gas, a completely open motor such as fig. 3 would be used at no time in a coal-mine, but it could be liable to mechanical injury from falls of roof and sides. A partially enclosed 'ventilated' motor, while it may satisfactorily avoid mechanical damage, is objectionable in any dusty place ; where air goes in dust will follow, and dust is one of the greatest enemies to the satisfactory working of a continuous-current machine. An open motor can be made dust-proof by a properly designed enclosure, but a partially enclosed motor is very apt to be neglected and to be seriously injured by dust. A totally enclosed motor is dust-proof, damp-proof, and gas-proof, and though it gets hotter on a given load than an open motor of the same size, it is the safest and most satisfactory type for coal-cutting. It has been experimentally proved that a totally enclosed motor, such as that in fig. 4, will operate successfully in explosive mixture even when sparking badly, and even in the case of the most sudden outbursts of gas it is not likely that a coal-cutter would be allowed to run many moments in such a mixture. 72 ELECTRICITY AS APPLIED TO MINING Of course there must be covers provided in the motor frame, which can be taken off for examination, and these covers must be made and kept gas tight by india-rubber, asbestos, or other packing or by machine-faced. **FIG. 51.** A graph showing current versus voltage for a shunt motor at 12 H.P. and 300 Volts. **FIG. 52.** A graph showing current versus voltage for a shunt motor at 48 H.P. and 300 Volts. **FIG. 53.** A graph showing current versus voltage for a series wound crane motor at 70 Series Wound Crane Motor, 360 Volts. **FIG. 54.** A graph showing current versus voltage for a series wound crane motor at 53 Series Wound Crane Motor, 220 Volts. joints, and there should be less difficulty in keeping these joints in proper order than in the proper maintenance of a safety lamp, which, by the way, is frequently deliberately put into gas for testing purposes, whereas the SPECIAL CONSTRUCTION OF MOTORS 73 motor is unlikely in most pits ever to come in contact with gas at all. The same remarks as to enclosure of course apply to the switch-box, in Fig. 55. Fig. 56. A graph showing the starting torque of a three-phase motor. The x-axis represents time (in seconds), and the y-axis represents torque (in Newton-meters). The graph shows that the starting torque increases rapidly at first, then levels off and remains constant until the motor reaches full speed. 15 H.P. THREE-PHASE MOTOR, 800 Volts, 50 PERIODS. A graph showing the starting torque of a three-phase motor. The x-axis represents time (in seconds), and the y-axis represents torque (in Newton-meters). The graph shows that the starting torque increases rapidly at first, then levels off and remains constant until the motor reaches full speed. 30 H.P. THREE-PHASE MOTOR, 800 Volts, 50 PERIODS. Fig. 57. A graph showing the starting torque of a three-phase crane motor. The x-axis represents time (in seconds), and the y-axis represents torque (in Newton-meters). The graph shows that the starting torque increases rapidly at first, then levels off and remains constant until the motor reaches full speed. THREE-PHASE CRANE MOTOR, 400 Volts, 50 PERIODS. A graph showing the starting torque of a three-phase crane motor. The x-axis represents time (in seconds), and the y-axis represents torque (in Newton-meters). The graph shows that the starting torque increases rapidly at first, then levels off and remains constant until the motor reaches full speed. THREE-PHASE CRANE MOTOR, 800 Volts, 50 PERIODS. which serious flashing and arcing is more likely to occur than in the motor itself, and the cable should be led into this box through gas-tight glands. 74 ELECTRICITY AS APPLIED TO MINING Some makers fill switch-houses with oil, which, so long as it is kept there, will entirely prevent sparking of any kind at the switch. Figs. 59 and 60 illustrate an entirely enclosed motor for other situa- tions where it is desired to exclude dust or damp or where there is a possibility of gas. The Union Electricity Company, of Berlin, who make a great deal of electrical machinery for mines, make an enclosed motor of 8o h.p. Fig. 59. A large, enclosed electric motor with a cover open, showing the internal components. Gas-tight Motor. Cover Open. guaranteed to work under 100 feet of water; when not under water it is kept cool by compressed air, which is forced into the case; this carries off the heat, and would, of course, exclude gas. Other makers use an entirely enclosed motor, but leave openings which are protected by a gauze. The idea being that, should any gas ignite inside the motor case, the flame would be cooled below ignition point in passing through the gauze. Of course there must be sufficient area of gauze as in the safety lamp, so that the burning gases do not leave SPECIAL CONSTRUCTION OF MOTORS 75 the motor case at too great a velocity to be cooled. But this plan is objectionable and perhaps dangerous when there is a great deal of dust for the reasons explained above, and cannot be regarded as such an effectual way of preventing explosion as the total exclusion of gas. **Alternating Polyphase Motors.** These motors, especially those with a 'squirrel-cage' rotor, are much less liable to damage by dust than Fig. 60. **Gas-tight Motor. Cover Closed.** a direct-current motor, and could only ignite gas by the fusing of a wire, a very improbable thing; or in the case of a slip-ring machine by a brush becoming damaged and causing a break in the circuit. These motors can be completely enclosed, but in most situations this is entirely unnecessary. ¹ The new Home Office Rules (see Appendix) will, however, overrule engineering considerations on this point. 76 ELECTRICITY AS APPLIED TO MINING CHAPTER III DRIVING OF THE DYNAMO Water Power—Steam Power—High-speed Engines, Direct-driving—Low-speed Engines, Belt-drive—Gas-engines—Oil-engines—Tests of Plants. The method of driving the dynamo may be one of the following: (1) By the employment of water-turbines. (2) By the employment of steam-turbines. (3) By the steam-engine. (4) By the gas-engine. Water Power.—The first method depends on a sufficient head of water being always available to drive the turbines, and where this is the case it generally proves cheaper than any mechanical system both in first cost and upkeep. A comparison between steam and water power was recently made, in which the cost of working a 1,000 h.p. steam plant is given as follows :
Per h.p. per annum.
Interest on capital, depreciation, repairs, &c. £ 4
Coal (1¼ lb. per I.H.P. at 16½ £4d. per ton) 1 7 3
Wages (two engineers and two stokers) 1 10 10
Oil, waste, &c. 0 12 10
The lowest cost known to the author (of the article) is £2 8s. 6d. per h.p. per annum, with coal at 2¼ £4d. per ton. The steam plant here referred to consists of a vertical compound condensing engine with an average load of 950 h.p., steam pressure 155 lbs. per square inch. The cost of a water-power plant, comprising three wheels, producing a maximum of 510 h.p. and an average of 315 h.p., is given by the same writer as £1 33. 2d. per h.p. per annum. Electrical Plant at Greenside Mine, Patterdale.—At the Greenside Mine, Patterdale, Westmorland, where lead ore is worked, the Engineering Magazine, 1898, xv. 922-927; *The Comparative Cost of Steam and Water Power.* Engineering Magazine, 1898, xv. 922-927; *The Comparative Cost of Steam and Water Power.* WATER POWER 77 whole of the power required is obtained from water-turbines. The authors are indebted to Mr. W. H. Borlase, mining engineer to the Company, for the following description of the plant. The source of power, water, is brought from two reservoirs under Helvellyn Mountain, through partly open and covered flumes, for a mile and a quarter, following the contour of the hills at an elevation of 1,650 feet above sea-level, and falling to an elevation of 1,350 feet. The pipes are 15 inches in diameter, and convey the water to the power houses, situated at 1,250 feet above sea level, giving a fall of 400 feet vertical on 1,200 feet length of pipes, equal to 1 foot in 3. At the bottom of the pipe a Y-piece is attached, and at this ends a vortex turbine and Pelton wheel are respectively fixed, driving two compound-wound continuous-current dynamo-motors each capable of developing 15 horse-power. The turbine plant has been working since December 1891, including the generator before mentioned, providing power for a 30 h.p. winding motor, compound-wound, one mile inside the mine and two miles from the station. The power is conveyed for the first three-quarters of a mile to the mouth of the level by copper wire 19/15 inch in diameter on poles and insulators, all cables inside the mine being insulated and leaded. The winding motor lifts the wagons of lead ore stuff &c from the various levels from 75 fathoms deep to the day level. The working load averages about 50 cwt. From this current is taken to a motor generator, where the volts are converted from 600 to 250, to work a 14 h.p. electric locomotive which runs the stuff raised by the winding motor through the day level to the washing grates, a mile and a quarter distant. The train load is twelve wagons, varying, of course, in weight, according to the value of stuff, averaging about eighteen tons exclusive of locomotive, which weighs about twenty tons or half a ton. The train is then run back to the shaft in about a quarter of an hour. The curves in the level and general conditions not permitting motors are bare, consisting of a No. 10 B.W.G. phosphor-bronze wire. The Pelton plant was erected in 1891, and is of the same power as the turbine plant. It consists of two Pelton wheels each with six vanes working inside the mine to supply compressed air rock-drills and pumps, and also an electric motor, driving a Tangye three-throw ram pump forcing water 360 feet high from the 60-fathom level to the day level at one lift. This being a fairly constant load, little supervision is necessary, but there is an electric cut-out in the circuit which immediately acts as soon as one of both of these machines fails. So instantaneous and so effective is its action that the volts are not allowed to rise 1½ per cent before they are again reduced to the normal pressure. The turbine plant has a varying load, which is easily commanded by a 78 ELECTRICITY AS APPLIED TO MINING 3-inch by-pass valve, the regulation being quickly done where hydraulic governors have failed. There is only one man kept in the generating station at a time; two shifts a day are worked—viz. from 6 a.m. to 2 p.m., and from 2 p.m. to 10 p.m. The cost of labour and supervision, amount to 1/8d. per h.p. per day of sixteen hours in two shifts of eight hours each. The locomotive has been running nine years and a half, and the charges for that period are equal to 25d. per ton mile or per mile run, as the load is only one way. The lamps are lighted at the several levels, motor-rooms, &c. inside the mine are lighted up with sets of six 100-volt 16-c.p. lamps in series. The crushing-mills, jigger-houses, washing and smelting mills are lighted by a separate dynamo. Steam Power.—The question of steam generation is too large to enter into here, but it may be mentioned that besides which it has already been adequately dealt with in several standard works, but mention may be made of one or two details of steam-raising specially applicable to collieries. Utilisation of Waste Heat from Coke Ovens.—A very important source of economy lies in the generation of steam by means of the gases driven off from coal during the process of its conversion into coke. There are two distinct methods of doing this: (1) Overheating type, in which no attempt is made to take out the by-products, and (2) distilling ovens or closed chambers externally fired, the gases from which are treated by various processes to extract the by-products, and then used to assist in the distilling process, afterwards being taken to the boilers. It follows, therefore, that a range of coke ovens will give off waste gas of greater heat power than any other range of heating apparatus or ovens. It has been stated that a range of sixty-by-product ovens (go feet long x 6 feet high x 2 feet 6 inches wide) may be supposed to be capable of heating about five Lancashire boilers (8 feet in diameter), the steam pressure being about 120 lbs. per square inch. When these boilers are used for belt-feeding purposes, water-tube boilers of the Babcock & Wilcox and Stirring type are often used. Burning of Inferior Fuel.—Another important saving, which is now being effected at many collieries, is the raising of steam from very inferior fuel, which would otherwise not only be wasted but might become a source of fire and annoyance when tipped into a pit-hill, owing to the liability of spontaneous combustion. The pickings from the coal belts, she settings from water used in coal washers, carbonaceous shale, coke dust, &c., can all be utilised in this way. One means of doing this is by applying forced draught to the ordinary Lancashire boiler, and putting the grate-bars very close together or dispersing them first M.E. sii. 324. STEAM POWER 79 with them altogether. This has been done with varying success, one difficulty in the way being that, owing to the fine nature of the fuel, a considerable proportion is blown over the grate into the chimney without being burnt, and, as the combustible matter has a lower specific gravity than the air, it tends to rise and escape through the chimney. Another drawback to the use of very low-grade fuels in Lancashire boilers lies in the limited grate surface ; owing to the lower calorific value of the fuel, a very much greater quantity has to be dealt with in the same time if the steam is to be kept up, and this is a very difficult thing to do. **Meldrum's Refuse Destructor.**—The successful solution of the problem of burning refuse has been arrived at by inventors similar to those for town refuse. These have (1) a very large grate area; (2) a combustion chamber which acts as a receptacle for the incombustible dust, which would otherwise tend to choke up the boiler flues. The construction is very simple, and will be gathered from fig. 61. It consists of a large fire-grate fitted with forced draught, at the back of which is the combustion chamber, and at the front nearest to the boiler. This arrangement has been adopted at Houghton Main Colliery for burning coke-breeze and coal-pickings. The plant consists of two cells or furnaces, each having 50 square feet of grate area. The gases are used to fire the Lancashire boilers, each 7 feet 6 inches high by 3 feet 6 inches wide. For burning coke-breeze, a small chimney only is required 75 feet high x 4 feet 6 inches square. The quantity of steam used in the jets is from 10 to 12 per cent. of the total evaporation. The following particulars of tests on a Meldrum destructor fired with pit refuse from Houghton Main Colliery 1 will be interesting as giving an idea of its performance of the furnaces :
Grate area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 square feet 8 feet x 30 feet
Size of boiler
Date of Test June 4, 1901 June 4, 1901 Taste ups.
Class of fuel used Half-coke dust, Pickings Coke-dust
Weight burnt per hour half pickings, 64 lbs. 87 lbs.
Weight burnt per ft. grate per hour 44 lbs. 64 lbs. 43 lbs.
Water evaporated per hour 7,490 lbs. 11,587 lbs. 6,730 lbs.
Water evaporated per ft. fuel 2.63 lbs. 3.84 lbs. 2.48 lbs.
Average temperature feed water 90° F. 90° F. 90° F.
Percentage of incombustible 30 per cent. 73 per cent. 28 per cent.
**High-pressure Steam.**—The advantages of using high-pressure steam are-(1) that it reduces the size of steam-pipes and engines; (2) that 1The tests were made at Shipley. 80 ELECTRICITY AS APPLIED TO MINING more work is got out of the same weight of steam. These economies are secured by a very small increase in the amount of fuel used. Thus, the number of British thermal units required to convert 1 lb. of water at 113° to A diagram showing a cross-section of a boiler with various components labeled. Fig. 64. A diagram showing a cross-section of a steam turbine with various components labeled. METAL BURNER FOR BURNING IRONIC FUEL. steam at 113° (atmospheric pressure) is 966 ; while to convert it to steam at 100 lbs. per square inch absolute, only needs an additional 353 B.T.U Steam-turbines. These are dealt with in Chapter IV. 3 ** TYPES OF STEAM-ENGINE 81** **Steam-engines.**—The choice of a suitable steam-engine is governed to a certain extent by the pressure of steam available, and also by the amount of work to be done, and may be one of the following types : Single-cylinder horizontal (or vertical) slow-speed engine, with a rope or belt drive. Two-cylinder coupled horizontal (or vertical) high-speed engine, for direct driving. Two-cylinder coupled horizontal high-speed engine, with a rope or belt drive. Compound horizontal high-speed engine, for direct driving. Compound horizontal slow-speed engine, with a rope or belt drive. Compound vertical high-speed engine, for direct driving. Triple-expansion vertical high-speed engine, for direct driving. All these may, if the conditions are suitable, be made condensing. Single-cylinder engines are used when the steam pressure is low, say 50 lbs. to the square inch or under, and the thrust required is under 50 lbs. F., the most economical engine would be of this type, steam being admitted into the cylinder for one-third of the stroke, and exhausting into a condenser into atmospheric pressure. For higher powers coupled engines should be preferred, the reason for this being that with such engines the difference between the maximum and the minimum twisting moment on the shaft is much greater than in a coupled engine in which the cranks are at right angles ; thus the fly-wheel and other moving parts of a coupled engine can be made much lighter. **Compound Engines.**—With higher steam pressures, from 60 to 100 lbs. per square inch, it is possible to obtain expansion of steam in two stages, and smoother working by avoiding the extreme differences of pressure in one cylinder. There is abundant experience to prove that, given a sufficiently high initial pressure and sufficient speed of working, a compound or a triple-expansion engine consumes less steam than a simple engine. This is due to the fact that it is possible to use a higher ratio of expansion in a compound than in a simple engine, and thus to more fully utilise the expansive energy of the high-pressure steam. The cylinder condensation is also less when the expansion is divided into two stages in different cylinders than when the same expansion is performed in one cylinder. The steam must be saturated (i.e. not superheated) varies directly as the pressure, so that if a single cylinder were used with a very early cut-off, the range of temperature in the cylinder would be very great, varying from $327^\circ$ F. (with steam at 100 lbs. per square inch absolute) down to $162^\circ$ F. (assuming steam exhausting into condenser at 5 lbs. per square) 8 82 ELECTRICITY AS APPLIED TO MINING inch absolute), and the incoming steam at high temperature coming in contact with the cold cylinder sides would produce a great loss owing to cylinder condensation. By expanding in two or more cylinders this extreme variation is avoided, and cylinder condensation largely prevented. Phot. 6a. High-speed or Slow-speed Engines.—By using a high-speed horizontal or vertical engine coupled direct to the dynamo shaft without HIGH-SPEED COMPOUND TWO-CYCLE ENGINE WITH THREE-PHASE GENERATOR. GAS-ENGINES 83 the intervention of gearing, ropes, or belts, as shown in fig. 62, a higher efficiency can be obtained, as there is no loss in transmitting the power from the steam-engine to the dynamo. An additional advantage possessed by high-speed engines is a saving in space and consequent cheapening of engine-house, foundations, &c. The disadvantage, however, of coupling direct is that, in case of varying loads, such as are common in colliery practice, when at one time coal-cutters may be working at full capacity or pumping great strains are thrown on to the working parts, whilst in a belt or rope-driven plant would to a large extent be borne by the ropes. Low-speed engines are generally considered more economical in steam consumption than high-speed engines, but they are more expensive in first cost. High-speed engines usually revolve from 350 revolutions in the larger sizes of 500 H.P. and upwards, up to 550 or 600 revolutions in smaller engines. Vertical or Horizontal Engines.—The chief advantage of a vertical over a horizontal engine is its saving in floor space and for central stations situated in towns, where the price of land is very high; this becomes a matter of great importance. In colliery installations, however, this rarely bears great weight, although it might be necessary, in putting down a plant in the same engine-house with other engines, to adopt the vertical type, if space were limited and extension impossible. Vertical engines have the advantage of easy examination, and repairs as horizontal engines, and owing to their having a short stroke they generally run at a higher number of revolutions than horizontal engines. Gas-engines are frequently used for driving electric generators, and exactly the same considerations that might lead an engineer to adopt gas-engines for other purposes would lead him to adopt them for electric generators. The reason why gas-engines are not adapted for use in central electric plant, because one of the objections to the ordinary gas-engine is difficulty in starting, and a central electric plant would work day and night; therefore the question arises. Does a gas-engine work more economically than a steam-engine? This however, cannot be discussed here until it comes to us as to any many plants gas-engines will be used. It has been said by many engineers that gas-engines are not suitable for driving alternating-current generators in parallel, and indeed, there are several reasons which substantiate this statement. It may, however, be said that some makers of gas-engines are prepared to undertake the erection of a plant with alternating-current generators working in parallel—and there are several such plants now being erected—while others do not appear to be pre pared to accept the responsibility. Gas-engines driving continuous-current dynamos are at work in parallel, and there is no reason why any number 02 84 ELECTRICITY AS APPLIED TO MINING of gas-engines and continuous-current generators should not work in parallel. The question of periodicity and of running in step does not arise with continuous current. It is only necessary that each generator should produce current at such a voltage that it will pass from the generator into the main. In some places gas-engines are driven with the waste gases from blast-furnaces. There is an enormous amount of power available in the gas which escapes from the top of a blast-furnace, which can be more economically used in gas-engines than by being burnt under a steam boiler. Oil-engines.—The same remarks apply to oil-engines that apply to gas-engines. They are very suitable for driving electric generators, and there are places where oil engines are cheaper than steam engines, but have almost reached a state of perfection, and now use oil with great economy. Rope or Belt Drive.—The engine may be coupled to the dynamo either by a belt of leather, cotton, india-rubber, &c., or by a number of cotton driving-rope. The relative merits are a matter of circumstance and convenience. One advantage certain possessed by a rope drive is that there is less chance of breakage, as a rope would break singly, whilst a belt breaking might cause a serious accident. Rope- or belt-driving requires a considerable distance between the driver and driven pulleys, as the necessary friction is got not so much by putting tension on, as by the weight of the ropes themselves. For this reason it is often found advisable to place the pulley on the slack side, as this increases the arc of contact between the rope and the pulleys and gives greater resistance to slipping. The speed of belts and ropes is governed by the tension put on them by centrifugal force, and should not be more than 4,000 or 5,000 feet per minute. Horse-power Transmitted by Belts.—The table on the next page shows the L.H.P. transmitted by double leather belt running at various speeds. Horse-power Transmitted by Ropes.—The power which ropes will transmit depends, of course, upon their number and speed, and can be calculated from the following formula: Multiply the number of turns of rope in inches by the velocity in feet per minute and divide by 300 ; the result will be the power which may safely be transmitted by one rope. The number of ropes should be at least one more than actually required, so as to allow for changing, &c. Colliery Installations.—The most usual type of engine at a colliery is the horizontal engine, which is driven through a belt or a belt and driving the dynamo through the medium of ropes and such an arrangement, while perhaps not so economical as some which could be adopted, gives good results, and is a type to which the colliery engineer is accustomed. For very large powers however, it seems probable that BELT DRIVING 85 For Single Leather Belts deduct one-half from above amounts. Dated by permission of Means, Fanning, Hilbig & Goodell L.L.C., Buffalo. A table showing data on belt driving, including serial number, date, indicated H.P., other wheels or wheels in operation, wheel diameter, and wheel speed.
Serial No. Date Indicated H.P. Transmitted by Double Leather Belting.
Other Wheel or Wheels in Operation. Wheel Diameter (inches). Wheel Speed (revolutions per minute).
1 07-16-1947 1.0 1.0 1.0
2 07-16-1947 1.0 1.0 1.0
3 07-16-1947 1.0 1.0 1.0
4 07-16-1947 1.0 1.0 1.0
5 07-16-1947 1.0 1.0 1.0
6 07-16-1947 1.0 1.0 1.0
7 07-16-1947 1.0 1.0 1.0
8 07-16-1947 1.0 1.0 1.0
9 07-16-1947 1.0 1.0 1.0
10 07-16-1947 1.0 1.0 1.0
Colliery Annual Output Total Coal Consumed Depth of Pits
No. 1 300,000 6,000 Per cent. Foot
No. 2 300,000 9,000 1% 830
No. 3 300,000 7,500 5% 750
No. 4 300,000 10,900 4% 450
No. 5 300,000 18,900 1% 1890
No. 6 300,000 22,990 7% 699
No. 7 400,000 21,250 3% 1,555
No. 8 135,000 17,975 17975
No. 5 pit is heavily watered in proportion to its output, raising 30,000 gallons per hour from the full depth of the pit. No. 6 pit has also a good deal of water, raising 15,000 gallons per hour. The economies effected in steam generation are by the use of independent boilers to heat the water, instead of using the Lancashire boilers, use of mechanical stokers and forced draught, efficient covering of boilers and steam pipes. The economies effected in the use of steam are by a high degree of expansion, compound and triple expansion engines, condensing apparatus. **Mechanical Efficiency of Plant.**—With an electrical plant which is supplied with steam from independent boilers, it is an easy matter to arrive at the consumption of steam and fuel per E.H.P. per hour. The voltage and amperes at the terminals of the dynamo are measured; then E.H.P. = volts x amperes; and by measuring the quantity of feed water $$\text{E.H.P} = \frac{\text{Volts} \times \text{Amperes}}{\text{Feeder Water Consumption}}$$ 746 1 Vol. XVI. p. 189-19.; p. 366. 2 If alternating currents are used, a watt-meter is required to measure the electric power. ECONOMY AND EFFICIENCY 87 supplied to the boiler to keep a constant height of water as seen in the gauge-glass, and by weighing the fuel used, the result is arrived at by a simple-division sum. Supposing the engine to be a compound condensing horizontal engine driving the dynamo through ropes, the steam consumption per I.H.P. (in a good engine) should be about 15 to 25 lbs. per hour ; if the steam consumption per E.H.P., as found above, comes to a high figure, such as 50 lbs. per hour, then the object of the engineer should be to discover where this loss takes place. The Steam-engine Indicator.--The indicated horse-power of the engine can be found by means of the indicator, which gives diagrams of the work being done during a stroke. Diagrams are taken both at the back and front of the engine, at equal intervals during the test, the revolutions of the engine being noted by a counter. The average pressure during the stroke is found by dividing the area of the indicator diagram by its length, and multiplying by the scale of the spring. Then \[ \text{L.H.P.} = \frac{\text{P L A N}}{33,000} \] where P = the mean effective pressure in lbs. per square inch. L = the length of the stroke in feet. A = the area of the cylinder in square inches. N = the number of revolutions per minute. In a compound engine, of course, both high- and low-pressure cylinders are indicated, the total T.H.P. being the sum of the T.H.P. of each cylinder. We are now in a position to find the steam consumption per I.H.P. per hour, and it may be that the loss is chiefly taking place in the steam-engine. A high steam consumption may be caused by defective or badly set valves, or by leaks; it may be due to condensation or leakages in the steam-pipes from the boiler. The loss due to condensation or priming should be found by collecting the condensed steam from the pipes. Leakages in the pipes would be self-evident. A careful study of the indicator diagram will give a great deal of information as to the setting of valves, etc. Brake Horse-power.--The brake horse-power represents the power available for useful work obtained by deducting from the I.H.P. the power necessary to drive the engine without its load. By means of a friction brake on the fly-wheel or driving-wheel of the engine the B.H.P. can be measured. The details and arrangement of a friction brake (or absorption dynamometer) will be seen in fig. 63, the B.H.P. being calculated as follows : 63 88 ELECTRICITY AS APPLIED TO MINING B.H.P. = $\frac{r \times N(W-w)}{33,000}$, where $r$ is the radius in feet of the fly-wheel or pulley and rope together, $N$ is the number of revolutions per minute, $W$ is the weight hung on the brake in lbs., $w$ is the reading of the spring balance in lbs. $r = 3^{\frac{1}{4}}16$, or approximately $\frac{2}{7}$. If the B.H.P. is abnormally low when compared with the I.H.P., it shows that the friction of the engine is too great, which may be due to Fig. 63. FRICITION BRAKE OR DYNAMOMETER. inefficient lubrication or improper design or adjustment of working parts, such as stuffing-boxes, guides, bearings, &c. Full Load and Light Load. The plant works most economically at full load because the percentage of work done in overcoming the friction of the engine and dynamo and in some of the electrical losses in the dynamo is a much smaller percentage of the total power produced. The steam consumption per I.H.P. is also much greater at low loads, the losses in the cylinder, leakages of valves, pistons, &c., being about the same as at full load. **Tests of Plants.**—The following series of tests have been supplied by Messrs Ernest Scott & Mountain on plants erected by them : 1. **Test of Plant for the Birkenhead Electricity Works.** Combined plant, consisting of one 11$\frac{1}{2}$ × 21' central-valve compound vertical engine (condensing), and 36" × 18" four-pole generator. March 22, 1902. **TESTS OF PLANTS** 89 **Full Load.—Water consumption per hour, 3,725 lbs.** Average electrical output during water-test = **273 amperes** (550 volts) = **1201.3 E.H.P.** Water per E.H.P. = 18-35 lbs. per hour. Water per E.H.P. = 16-35 lbs. per hour. Indicator cards taken during water-test :
High Pressure. Low Pressure.
Top. Bottom. Top. Bottom.
Diagram area " 55" " 17" " 12" " 14"
" length" " 275°" " length" " 28°"
" scale " 1/8" " scale" " 1/8"
Average ef- Average men of-
fective pressure, 709 fective pressure, 185
lbs. per sq. in.






















































































Net collective L.H.P. = **230-6** 90 ELECTRICITY AS APPLIED TO MINING L.H.P. in h.-p. cylinder less L.H.P. on rod = 91 - 225 = 8875 L.H.P. " l-p " " " " " 35 = 8675 " Net collective L.H.P. = 17490 E.H.P. when cards were taken = 304 amperes | 552 volts | 15094. Combined efficiency, 86.3 per cent. Pressure at stop-valve = 154 lbs. per sq. in. Steam-pipe pressure (engine side of governor valve) = 82 lbs. per sq. in. Vacuum = 23.5". Piston-rods, 28" diameter. Speed of engine = 347 revolutions per minute. Twenty-five per cent Overload.—Water consumption per hour, 4,588 lbs. Average E.H.P. during water-test = 342.7 amperes | 531.5 volts | 2434 E.H.P. Water per E.H.P. = 189 lbs. per hour Water per I.H.P. = 167 lbs. per hour. Indicator cards taken during water-test :
Diagram area Top Bottom. Diagram area Top Bottom.
length scale length scale length scale
" 275" " 28" " 29"
M.E.P. 83 85 o M.E.P. 243 207
Average M.E.P.*] 81-6 Average M.E.P. """"
I.H.P. in h.-p. cylinder, less I.H.P. on rod = 146 - 31 = 1139 " l-p " " " " " 136 - 08 = 1352" Net collective I.H.P. = 2747 E.H.P. when cards were taken = 341 amperes | 531 volts | 2425". Combined efficiency, 86.6s per cent. Pressure at stop-valve = 157 lbs. per sq. in. Steam-pipe pressure (engine side of governor valve) = 139 lbs. per sq. in. Vacuum = 229". Piston-rods, 28" diameter. Speed of engine, 346 revolutions per minute. Test of Plant for Nettlefolds (Limited).—Combined plant, consisting of a *14" x *3 C.V. engine, and *39" x *13 six-pole dynamo. January 10, 1902. Full Load.—Water consumption per hour, 6,8112 lbs. Average electrical output during water-test = (870 amperes | 230 volts | ) TESTS OF PLANTS 91 Water consumption per E.H.P. = 23 3 lbs. per hour. Water consumption per I.H.P. = 22 3 lbs. per hour. Indicator cards : High-pressure Cylinder. Scale, $\frac{1}{2}$". Mean area, 197 square inch. Length, 3 3 inches. Average M.E.P., 716 lbs. per square inch. Fig. 64. HIGH-PRESSURE CYLINDER. SCALE $\frac{1}{2}$". LOW-PRESSURE CYLINDER. SCALE $\frac{1}{2}$". Piston rods, 3 inches diameter. Mean net area of high-pressure cylinder = 110 16 square inches. " " " " " " " " " " " " " " " " " " Revolutions, 320 per minute. High-pressure cylinder, 124 inches diameter. Low-pressure cylinder, 23 inches diameter. Stroke, 12 inches. High-pressure L.H.P. = $71\frac{6}{8} \times 640 \times 110\frac{16}{8} = 165\frac{39}{8}$ 33,000 Low-pressure L.H.P. = $71\frac{7}{8} \times 41\frac{95}{8} \times 640 = 140\frac{44}{8}$ 33,000 Collective H.P. = $305\frac{83}{8}$ Electrical output when cards were taken = (870 amperes) (430 volts) E.H.P. 9 92 ELECTRICITY AS APPLIED TO MINING Combined efficiency, 87-7 per cent. 3. Test of Plant for the South Durham Coal Company. The plant consists of- One horizontal long-stroke compound engine capable of developing 440 H.P., of the following dimensions :
Diameter of high-pressure cylinder 18 inches
Diameter of low-pressure cylinder 30"
Length of cylinder 40"
Revolutions per minute 80
Steam pressure per square inch 100 lbs.
The engine is fitted with automatic expansion gear to both cylinders, and fitted with rope fly-wheel 16 feet diameter, grooved for fourteen ropes $\frac{1}{4}$ inch diameter. The dynamo is of Scott & Mountain's six-pole type constructed for the following output :
Total output in watts 220,000
Current in amperes 400
Volt at terminals 550
Approximate revolutions per minute 400
Test made February 28, 1902. Constant quantities : Boiler pressure, go lbs. per square inch ; stop valve, 80 lbs. per square inch ; revolutions, go per minute ; piston rods, $\frac{3}{4}$ inches diameter. Fig. 65. INDICATOR DIAGRAM FROM HIGH-PRESSURE ENGINE. SCALE $\frac{1}{4}$". Front end cards, full lines ; back end, dotted lines. INDICATOR DIAGRAM FROM LOW-PRESSURE ENGINE. SCALE $\frac{1}{4}$". Front end cards, full lines ; back end, dotted lines. TESTS OF PLANTS Indicator cards : Six sets of cards were taken at intervals of ten minutes. The first set taken were as follows :
High-pressure Engine.
Area in square inches Back end. Front end.
Length in inches 12 14
M.E.P. per square inch 35 40°
Average M.E.P. 37°12
Low-pressure Engine.
Area in square inches Back end. Front end.
Length in inches 09 12
M.E.P. per square inch 3875
Average M.E.P. 37°2 496
High-pressure L.H.P., less L.H.P. on rod = 172 - 298 = 169 oz.
Low-pressure I.H.P., less I.H.P. on rod = 64 - 037 = 61°63
Net collective I.H.P. = 230°65
The average of the six sets of cards was 227°53 L.H.P., out of which 23 L.H.P. was utilised for other purposes, which leaves a net I.H.P. available for driving the engine and dynamo of 204°53. Average E.H.P. at dynamo terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Average E.H.P. at dynamo terminals (including losses) = 140 Common losses per hour (including losses) = 68% per cent. Lbs. of coal used per I.H.P. per hour ... = 3°67 Lbs. of water used per I.H.P. per hour ... = 29°8 Lbs. of coal used per E.H.P. per hour ... = 5°3 Lbs. of water used per E.I.H.P. per hour ... = 44°5 It will be seen that the engine and dynamo were working at less than half-load, which accounts for the comparatively high fuel and steam consumption. It is the intention at some future date to put down an additional pair of cylinders, with the view of obtaining better results. **Rope-driven Plant with High Efficiency.—The best results which Messrs Ernest Scott & Mountain have obtained so far from a rope-driven plant (shown in fig. 66) are as follows :** Single-cylinder engine with trip expansion gear, cylinder 19½ inches by 40 inches running at eighty-one revolutions per minute, fly-wheel 16 feet diameter, connecting-rod length one foot eight inches diameter. Average area of cards (each end of cylinder) = 2°25 square inches. Length of card, $x_1$ ; scale of indicator spring, $y_1$ Mean pressure = $2^{25} \times x_1^2 \times y_1^2 = 33\frac{3}{6}$ lbs.
E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.E.M.F.Fig. 66. Ruck Dayton Electrical Generators. ARRANGEMENT OF PLANT I.H.P. = 336 × 208.64 × 40 × 81 × 2 = 164.2 12 × 33,000 Voltage during test, 500 ; amperes during test, 210. E.H.P. = 500 × 210 = 1407 E.H.P. Combined efficiency = $\frac{1407 \times 100}{164.2} = 85.6$ per cent. Arrangement of Engine-house, Engines, Dynamos, &c.— In a large installation, where everything is done by electricity, except perhaps the winding, it is advisable to split up the total power to be generated into two or more sections, so that in case of a failure of one section, the consequent stoppage of the colliers. Especially is it important where ventilating machinery is driven by a motor, the stoppage of which might occasion serious accident. This practice does not materially reduce the efficiency of the installation, since on light loads only a section of the generating plant need be kept at work, and this part may be working at or near full load and not at its best point. The steam-pipe connections of these various units to the boilers must also be carefully designed, and divided as far as possible into sections, so that the bursting of one pipe would not necessarily stop all the engines ; a good practice would be to duplicate the steam-pipe connections. Spares are necessary in every installation, but the addition of an extra unit would relieve the engineer from a great deal of anxiety. With regard to the ventilating fan, the motor to drive this should certainly be kept in duplicate, just as is the case when driving by a steam-engine. Spare armatures should be kept for all continuous-current dynamos and motors, unless such armature is so constructed that a spare armature cell can be inserted in place of it. It should take to change an armature, in which case it is sufficient for a spare armature cells to be kept in stock. Facility for Examination and Repair of Plant.—It is of great importance that the plant should not be cramped up, and in the case of a large installation a travelling overhead crane is of great use in handling the armatures, pistons, &c. The dynamos and motors, when rope or belt driven, are very often placed on sliding rails, which enables the stretch of the belt or ropes to be taken up if required when running. 96 ELECTRICITY AS APPLIED TO MINING CHAPTER IV THE STEAM-TURBINE Parsons Turbine—De Laval Turbine—Curta Turbine. In the ordinary steam-engine the power of the steam is made to give a reciprocating movement to the piston, which is converted into rotary motion by means of the connecting-rod and crank. Thus, at the end of each stroke the direction of motion is reversed, and consequently high speeds can only be maintained in engines of especially good construction and especially good lubrication. The steam-turbine is a rotary engine. There is no reciprocating motion, and the speed that can be attained is practically unlimited. There are three principal varieties of steam-turbine in use in this country—namely, Parsons', De Laval's, and Curta'. Parsons Steam-turbine. On referring to fig. 67 it will be seen that on the turbine shaft are three barrels of increasing diameter, a, b, c, with rings of projecting blades on their circumference. These barrels are enclosed in a cylindrical casting, on the inside of which is another series of blades. The rings of blades on the case nearly touch the barrels on the shaft, and the rings of blades on the shaft lie between those on the case and nearly touch the case. Fig. 68 is a view of a turbine with the cover removed, showing the rings of blades on the barrels. It will be noticed that there are three dummy barrels, o, r, and p (fig. 67), also fixed on the turbine shaft. These correspond in size to the barrels, a, b, and c, and communicate with them by passages in the casting. By this means the pressure is equalised in both directions and end thrust is prevented. The steam enters through a nozzle at x, passes first through a ring of fixed blades at y, and entering the cylinder at z, passes first through a ring of fixed blades at w, and is projected in a rotational direction upon the succeeding ring of blades on the barrel, imparting to them a rotational force. The diameters of the barrels are increased to suit the increasing volume of the steam as it expands, so as to keep the velocity at which the steam travels through the turbine PARSONS TURBINE 97 practically constant. The exhaust port, k, is connected direct to the condenser, which may take the form of a surface condenser or a jet condenser, according to the nature of the water. The steam is not admitted continuously to the turbine, but in a series of gusts, the lever, l, being Fig. 67. PARSONS STEAM-TURBINE. periodically moved up and down by means of a cam on the orepump crank-shaft, which is driven off the turbine shaft by a worm and worm wheel. Fig. 68. PARSONS TURBINE, WITH COVER REMOVED. The duration of the steam admission is controlled by means of a governor, which regulates the height of the end of the lever. The number H 98 ELECTRICITY AS APPLIED TO MINING 300-6W. PARSONS TURBO-ALTERNATOR. PARSONS TURBINE 99 of gases varies from 1 to 5 revolutions of the turbine shaft to 1 to 30. The speed of the turbine may be from 750 to 6,000 revolutions a minute, according to size of plant. When used for dynamo-driving the armature is coupled direct to the motor shaft by means of a flexible coupling. Fig. 69 is from a photograph of a 300-k.w. turbo-alternator running at 3,000 revolutions per minute. Similar machines have been supplied to Denbyly Main Colliery. Steam and Consumption of the Parsons Turbine.—When first brought out the turbine was popularly supposed to be very wasteful in steam, but long experience and improvements in the design have made it a very formidable competitor with the high-class modern condensing engine. The larger sizes are naturally more economical than the smaller ones, and although for colliery work a large size as a 1,000-kwatt steam turbine will probably not be found necessary, its consumption is interesting. The machines tested were two 1,000-kwatt steam-turbines for the new electric station of the city of Elberfeld, and a series of tests were made under the normal load, an overload of about 20 per cent., three- quarter load, half-load, and a quarter-load. The steam consumption per kilowatt-hour was found to be 28 lbs. for the normal load, an overload of about so per cent., 19-24 lbs.; three-quarter load, 22-31 lbs.; half-load, 25-28 lbs.; quarter-load, 33-36 lbs. It will be borne in mind that a kilo- watt is 1,000 watts = 745 E.H.P. (see page 8). While a colliery installation might for a very large concern require between 1,500 and 2,000 h.p., it would be divided up into a number of smaller units, say 500 h.p. The following tests* of two 100-kwatt continuous-current turbine dynamos show the steam consumption in the smaller sizes:
Pressure of Steam shown on Stop-valves Supplementary Pressure Valve Vacuum in the Turbine Chamber Bar=70° Revolutions per Minute Load Steam Used
Lbs. per sq. in. F.1 Kilowatts Lbs. per Kipper Kilowatts
129 28 27-6 3,045 226 4,975 22-5
129 28 38-9 3,365 246 5,485 23-5
119 26 36-9 3,600 243 4,943 24-7
130 28 38-9 3,600 243 4,943 —
Lbs. per sq. in. F.1 KilowattsLbs. per KipperKilowattsLbs. per KipperKilowatts
1292827-63,0452264,97522-5
1292838-93,3652465,48523-5
1192636-93,6002434,94324-7
1302838-93,6002434,943
Sizes of Turbo-Kilowatts Speeds, R.P.M. Length Width Height Weight
50 4,000 Pts. in. Pts. in. Pts. in. Cwt.
100 3,500 19 2 5 97
300 3,800 22 6 7 134
300 3,000 22 6 6 270
1,000 1,800 37 6 9 930
**Installation of Turbine Dynamos at Ackton Hall Colliery.** Although these machines are largely employed for central stations electrical plants, with some exceptions they have only recently been introduced to colliery work. One of the first installations in this country was at the Ackton Hall Colliery, Featherstone, where the plant has been running without any trouble for several years. The above installation is described in the *Proceedings of the Institution of Mining Engineers.*¹ *It consists of two Parsons 200 E.H.P. steam-turbines and dynamos working at 500 volts pressure and running at 5,000 revolutions per minute. The steam enters the turbine from a pipe 3 inches in diameter, and is admitted into the blades by means of a nozzle attached to the machine. The plant when originally installed was not condensing, and the makers guaranteed a water consumption per E.H.P. per hour of not more than 39 lbs. The complete machine measures 17 feet long by 3 feet wide, and no foundations are needed, the machine simply resting on india-rubber mats.* Since the above paper was written several more turbines have been put down, taking the place of ordinary steam-engines, and the installation now consists of the following : Three continuous-current dynamos, each of 200 E.H.P. capacity at 500 volts ; one H.E.H.P. turbo-alternator, three-phase at 350 volts, with a periodicity of 4 per second ; four continuous-current turbo-dynamos, each of 57 E.H.P. at 110 volts ; and one 400 E.H.P. continuous-current turbo-dynamo at 500 volts. ¹ *The Use of Electricity at Ackton Hall Colliery,* by H. St John Darnford and Roslyn Holiday; *Transactions Institute Mining Engineers*, xiii. 232. DE LAVAL TURBINE 101 The De Laval Steam-turbine. This is somewhat different in principle from the Parsons, and is more closely allied to the ordinary water-turbine. Fig. 70 shows a sectional view Fig. 70. SECTION AND PLAN OF 20 H.P. DE LAVAL TURBINE MOTOR. 102 ELECTRICITY AS APPLIED TO MINING and plan of a 20 h.p. steam turbine motor. The admission-steam enters the turbine after having passed the stop-valve, $a$, of the machine. This stop-valve is mounted direct on the inlet flange of the governor-valve case. Before the steam enters this valve it is passed through a strainer of wire gauze, $c$, which prevents any dust getting into the turbine case. In the governor-valve, $b$, which is regulated by the centrifugal governor of the Fig. 71. Dr Laval Steam-Turbine, showing bucket-wheel and nozzles. turbine, the steam is throttled, so that only the amount which is required for driving the turbine at the load for the moment can enter the turbine case. The high-pressure steam passes into the chamber, $e$, where it is distributed to a number of steam nozzles, which are placed at an angle of 20° to the vanes of the turbine wheel. The number of nozzles varies from one to twelve, according to the size of the machine. The passages in the DE LAVAL TURBINE 103 nozzles are conical, the object being to make the steam expand as it passes through, and so give to it a very high velocity. The kinetic energy of this jet of steam is taken up in the buckets of the turbine wheel, against which the steam is blown, so imparting a rotary motion to the wheel. After doing its work the steam passes into the chamber, o (fig. 70), and through the exhaust opening. The nozzles previously referred to are all provided with valves, so that some can be shut when the turbine is running below its normal load. Fig. 71 shows the turbine wheel with its vanes or buckets. The following table shows the speeds of the turbine wheels in various sizes of turbine:
Sizes of Turbine Middle Diameter of Wheel Revolutions per Minute Peripheral Speed Foot per Second
5 h.p. 100 mm. 4 lbs. 30,000
15 180 6 84,000 617
30 240 8 288,000 275
50 330 112 16,400 846
100 500 224 13,000 1,113
300 760 30 18,000 1,235
It will be seen that the turbine shaft revolves at a very high speed, but owing to the small diameter of the revolving turbine wheel there is no practical difficulty in designing wheels of sufficient strength ; moreover, the turbine wheel is entirely enclosed, which renders the machine quite safe. The speed of the turbine wheel is reduced in the machine by means of double helical gearing. In the smaller sizes up to 30 h.p., the pinion on the turbine shaft works with one gearing wheel, in the larger sizes there are two gearing wheels. Fig. 72 shows a 100 h.p. turbine dynamo; it will be seen that there are two armatures, each having a large field wheel, while the turbine is driven by two gears which work between them. The speed of the dynamo shafts in a 150 h.p. turbine is 1,550 revolutions per minute. The armatures of the dynamo are directly connected to these main shafts. The speed of the machine is regulated by a centrifugal governor. The turbine is condensing ; any form of condenser may be used. 1Paper read before the Leeds Association of Engineers by Mr. Konrad Anderson, January 25, 1900. 104 ELECTRICITY AS APPLIED TO MINING Fig. 72. 100 H.V. DE Laval Turbine Dynamo. A photograph of a De Laval turbine dynamo, showing its internal components and wiring. The machine is mounted on a wooden base. The top part of the machine has various mechanical parts, including gears and pulleys. The middle section contains the main body of the dynamo, with visible wires and connections. The bottom part shows the base and some of the external connections. The machine appears to be well-maintained and is likely used for generating electricity in a mining context. CURTIS TURBINE 105 **RESULTS OF TESTS WITH DE LAVAL STEAM-TURBINES AT DIFFERENT LOADS.**
Size of Machine Pressure No. of Nozzles E.H.P. Lbs. of Steam per Hour Remarks
Steam Vacuum Nuture
50 h.p. tur. Lbs. per lb. lbns of Steam
Time of test made in April 1895 113 > 60 > 6 494 24 > Work for con- densing is in- cluded
100 h.p. tur. 27 > 26 > 5 407 25 >
Test made in December 1899 83 > 80 > 4 410 23 >
B.H.P.
Lbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per Hour
300 h.p. tur.27 >27 >27 >27 >
bine motor,197 >197 >197 >197 >
Tow motor,196 >196 >196 >196 >
December 1899
Lbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per Hour
300 h.p. tur.
Lbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per Hour
300 h.p. tur.
Lbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per Hour
300 h.p. tur.
<
Lbs. of Steam per HourLbs. of Steam per HourLbs. of Steam per Hour
B.H.P.                                                               per Minute per Minute per Minute per Minute per Minute per Minute per Minute per Minute per Minute per Minute per Minute per Minute&econdly, the steam is expanded through a series of nozzles, which are arranged in two rows, one above the other, and are separated by a diaphragm, so that the steam may pass through each nozzle successively without being mixed with the steam from the next nozzle below it. The steam thus passes through the turbine, and the energy of expansion is absorbed by its impact with the buckets. The 'compound' principle is obtained by dividing the turbine into two or more stages, each of which is supplied with steam from a set of expanding nozzles, and each stage is separated by a straight diaphragm except where these nozzles are inserted. Fig. 73 shows the position of the moving and stationary buckets with relation to the nozzle. Curtis Steam-turbine. This turbine, recently introduced, differs essentially from those previously described, combining to a certain extent the leading principles of both. The turbine is of the 'impulse' type ; velocity is imparted to the steam through groups of expanding nozzles, after leaving which it passes successively through two or more revolving buckets, interspersed with reversed buckets between them, and past the turbine, these latter acting as guides to redirect the steam on to the next stage. The following table gives approximately the dimensions and weights of various sizes of De Laval turbine dynamos : 106 ELECTRICITY AS APPLIED TO MINING For sizes of 500 k.w. and over the turbine is constructed with a vertical shaft, the weight of the revolving part being supported by a step bearing of special construction. It consists of two circular bearing blocks, one of which revolves with the shaft and the other is fixed to the base. Water is used as a lubricant and is forced in by a pump until the pressure is sufficient to carry the whole weight of the revolving parts. The speed of a 5,000-kwatt Curtis turbine is 3,750 revolutions per minute. The governing is effected by a group of steam admission valves, under control of the governor, which admit steam to the first set of nozzles. It will be noticed in the diagram that the typical type of turbine effects considerable saving in floor space over the horizontal turbine, although for colliery generating turbines such an advantage is not of such moment as in a town, where the possibility of extension may be limited. Fig. 73 shows an outside elevation of a 1,500 k.w. three-phase-turbo-generator (11,000 volts) made by the British Thomson Houston Company for the Yorkshire Electric Power Company. The generator is mounted above the turbine, and of course it is necessary to have convenient travelling cranes for lifting the turbine for adjustment or repairs. **Advantages of the Steam-turbine.**—The chief use of the steam-turbine in this country has been found in direct driving of dynamos. It has so far not been found practicable to take power off the motor shaft by means of a belt or ropes, as this would introduce a side pull on the turbine blades, which might cause serious damage. Among the advantages of using the turbine for the generation of electricity are the following: (1) A great saving in floor space, and consequent cheapening of engine-houses. (2) Heavy foundations are not found necessary, owing to the freedom from vibration from vibration. It has also been found possible, indeed, to entirely dispense with foundation bolts. (3) There is no rubbing action in the cylinder, consequently no oil is employed except in the \(^1\) Power may be taken, however, from the second motion shaft, either by belt or ropes.
No. of Watts B.H.P.
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
NOZZLE MOVING BLADES STATIONARY BLADES MOVING BLADES
CURTI'S TURBINE. ARRANGEMENT OF Blades.
STEAM CHUTE.
STEAM CHUTE.
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STEAM CHUTE.107 bearings, and this should reduce the amount of oil in the exhaust steam. (a) It is possible to utilise a considerably higher vacuum with a turbine than with a reciprocating engine, on account of the impracticability of using large enough cylinders with the latter type to take the largely increased volume of the steam at high vacua. In spite of this economy, however, it is by no means proved that a turbine is more economical in steam con- 1,500 k.w. Curtis Turbo-Generator. sumption than a high-class compound condensing engine, and in small sizes it is undoubtedly less economical. (g) An interesting application of the fact that high vacua can be used is the utilisation of exhaust steam from non-condensing engines in a turbine to which a condenser is applied. Great economy is claimed by the use of this system. Fig. 74. 108 ELECTRICITY AS APPLIED TO MINING CHAPTER V DISTRIBUTION OF ELECTRICAL ENERGY Various Systems : Series ; Parallel ; Two-phase ; Three-phase. This is a subject of great importance, and one that requires careful forethought in planning any new system of supplying electricity to either lamps or electro-motors. There are a number of systems of distribu- tion, each possessing advantages peculiar to itself, and these will now be considered. **Series System.**—This is represented diagrammatically in fig. 75, where all the lamps, t, t, are in simple series with one another and with the generator, o. This is a constant-current system, the E.M.F. of o being made to vary proportionally to the number of lamps, t, a light. The system is almost entirely restricted to arc-lamp lighting, though there is another Diagram showing a series system with two lamps (t) connected in series with a generator (o). Fig. 75. system with which such lamps are used. The current is, of course, the same at all parts of the circuit, being equal to that taken by any lamp, which varies from 10 to 13 or 15 amperes. Hence, though the insulation must be good on account of the high voltages (up to 3,500) often used, the section of copper wire required for the conductors of this system is small. The system is only used for direct current at high pressures. **Two-wire Parallel System.**—This is shown in fig. 76, and consists of two mains, running all the way from the generator, o, to where the power is required. The lamps, t, and motors, m, are connected across the mains where required so as to be in parallel with one another across them respectively. In this system the voltage is approximately constant everywhere; but the current developed is equal to the sum of all the currents taken by the separate lamps and motors, and therefore in a large system this amounts to a great quantity, and will require a large TWO- AND THREE-WIRE SYSTEMS OF DISTRIBUTION 109 amount of copper to be sunk in the mains, unless the pressure across them is considerable. This at the present day would be either about 100 or 200 volts where there is much lighting to be done, but in the case of a circuit where motors formed the chief load the voltage would be higher. If the voltage is 200, we could employ 200-volt motors and either 200-volt A diagram showing a two-wire parallel system with three similar generators connected in series. Fig. 76. **Two-Wire Parallel System.** glow-lamps or connect two 100-volt glow-lamps in series across the mains, as shown at \(a\), when, if one of the two fails, the other will go out also. These could also represent arc lamps of the 100-volt enclosed type, two being in series across the 200-volt mains; or, lastly, 200-volt enclosed arc lamps could be used, but this is not to be recommended. This system is widely used for both direct and single-phase alternating currents. **Three-wire Parallel System.** This is shown in fig. 77, in which there are two similar generators, \(G_1\) and \(G_2\), connected in series, and three dis- A diagram showing a three-wire parallel system with three similar generators connected in series. Fig. 77. **Three-wire Parallel System.** tributing mains running from them as shown. The middle wire and one outer main form one section, and the middle wire and other outer main the other section. Lamps are connected across the two sections in such a way that, as nearly as possible, the same number shall be alight at the same moment. If equality were always exact, the middle wire (\(c\)) might be very small 110 ELECTRICITY AS APPLIED TO MINING and not even connected to the junction of $c$, $d$. It is, however, usual to make $c$ one-half the section of $a$ or $b$, as there is generally a want of balance of at least to per cent. in a well and carefully planned system, though often far more. In the remote contingency of all the lamps, &c., being out on one side of the mine, we have a three-wire system, which is best for section $cb$--in fact, the same as $a$. A motor, $m$, would be placed across theouters, $a$, so as to avoid throwing out the balance when switched on and off and affecting the voltage on the lamps. The advantage of the three-wire over the two-wire system lies in the increased voltage, for, suppose $c$, each gives 200 volts, that between $a$ and $b$ equals 400 volts, and we may regard the two wires as one wire. This is possible only with a three-wire system. The current transmitted for the same current in the three-wire than was possible in the two-wire system with only a small percentage increase of copper laid out, due to the extra main. This system is applicable to both direct and single-phase alternating currents, for both of which it is widely used. In the latter, however, a single-phase transformer takes care of $c$, $d$, the primary being connected by two mains to the alternator, the secondary being in two halves, each of which feeds one of the two sections. The total power delivered equals the sum of the two powers given to the two sections. **Two-phase Four-wire System.--This is shown in fig. 78, the pair of mains, $a$ and $A$, going to Phase I and the other pair to Phase II. All** M Fig. 78 Four mains are the same size, and the voltages across I. and across II. are equal. Lamps, &c., are connected as shown, and, of course, a two-phase motor, $m$, would be connected to them. The total power transmitted equals the sum of that in the two sections of Phase I. and Phase II. **Two-phase Three-wire System.--This is shown in fig. 79; two mains, one from each phase, are connected together, as at $j$, to form a single third main, $a$, which is $\sqrt{3}$ times the sectional area of either $a$ or $r$, because it has to carry $\sqrt{3}$ times greater current. The voltage between $a$ and**
$I$ $O$ $O$ $O$ $L$
$II$ $O$ $O$ $O$ $L$
$J$ $O$ $O$ $O$ $L$
$d$
**TWO-PHASE FOUR-WIRE SYSTEM.** THREE-PHASE ALTERNATING-CURRENT SYSTEM 111 $c$ will be $\sqrt{3}$ times that between $a$ and $b$ and between $b$ and $c$. This arrangement is usually used for transmission only, usually at high pressure, as the amount of copper required is only $5\%$ per cent of that needed in fig. 78 for the same power transmitted and the same maximum voltage between wires. A diagram showing three-phase alternating-current system connections. **Fig. 79.**
a b c
I J L
**Two-phase Three-wire System.** **Three-phase System.**—This requires three mains of equal section, and the power delivered in such a system is equal to $\sqrt{3}$ x amperes in one main, multiplied by two volts, the power being equally divided, so that, providing, of course, the three sections are equally loaded. In this system only 75 per cent. of copper is required in the main, as compared with the two-wire single-phase or four-wire two-phase system for equal power transmitted and equal drop of voltage and pressure at the lamps. The two-phase system is easier to regulate than the three-phase when lamps are on or off, but it cannot be used where only the three-phase system is the best and simplest to regulate. In this latter system the three wires can be run side by side as one cable, or can be separated to any distance required to suit convenience. In the workings of a mine three separated mains might be better, for if a fall occurred and one was broken, the other would be damaged by a fall of the roof. The cables laid in one cable, all might be damaged by a fall of the roofing. If the cables are armoured, however, they must be carried together and the armouring made to enclose all of them, as the induction effect of iron between one cable and another would be very serious. In cases where current for lighting and power purposes is taken from the same circuit, switching a motor into or out of the circuit may cause a considerable change in the brilliancy, of the lamps burning on the circuit. This is certain to occur except when the mains are of ample cross-section or only small motors are being switched on or off, or except when the distributing mains are 'fed,' as it is termed, at frequent intervals by separate mains, called feeders, which bring from the mains or a similar dynamo in the generating station direct to the feeding points. **Three-phase Four-wire System.**—Where it is desired to run lamps or small single-phase motors from a three-phase system, a fourth wire secures a better regulation between the phases. The windings of the three phases in generators, motors, and transformers usually start from one 112 ELECTRICITY AS APPLIED TO MINING common junction called the neutral point. A 'neutral' main may be connected to this point and lamps connected between this main and each of the other three. A larger number of lamps on one main than the others will cause an 'out of balance' current to flow along the neutral, and the voltage of the system will vary with the load. Practically this is so if the lamps are on the low-pressure side of a transformer, which, while receiving its current from three mains, distributes to the lamps by four. In this way single-phase motors of some size can be run without seriously affecting the regulation at the generating station. Instead of having a neutral point, the earth may be used as a return sometimes earthed and the out-of-balance currents flow through the earth. It is often preferable and in some ways safer, however, to have the whole system completely insulated from earth. The voltage between any main and the neutral in a three-phase four-wire system is $\sqrt{3}$ times that between the two mains. 113 CHAPTER VI STARTING AND STOPPING ELECTRICAL GENERATORS AND MOTORS Generators—Motors—Switchboard—Starting Resistance—Switches—Anti-sparking Oil and Enclosed Switches—Lightning Arresters—Fuses—Magnetic Cut-outs—Amperé- meter, Voltmeter, Wattmeter, Electricity-meter. Generators.—We will assume that the first start is in contemplation, and that the connections between generator and switchboard are correct. The first thing should be to see that the oiling arrangements are in working order, and the brushes adjusted to a suitable pressure on the com- mutator or armature. The brushes should be well lubricated with a good rubbing contact. The main-current switches (page 126) being open—I.e. not making contact—start-up the generator by the engine to the rated speed ; then in the case of alternators switch on the exciting current and adjust its strength, by means of the rheostat in series with the field coils, so as to produce a current of about 0-5 ampere. The field coils last-named have shown a good insulation resistance, close the main-circuit switches, when the current will be available in the various circuits. Should the exciter or direct-current dynamo, usually driven by the alternator, not excite itself, as is often the case with new machines, the switch in the exciting circuit should be opened, and the field coils should be disconnected from the supply until they have become magnetised. This can be done in such a way that they are temporarily magnetised, so as to develop the right polarity. The current being cut off after a minute or so, they are re-connected in their proper place, when usually there is no further diffi- culty in getting them to excite on closing the exciting-circuit switch. Almost all direct-current machines will show a residual individual magnetism after it is cut off, and it is on this that the building-up, as it is termed, or self-exciting, of direct-current machines depends. The same remarks apply to the starting up of direct-current generators, except that in starting up they usually excite themselves. If not apply the preceding method of getting them to do so. In no case should a generator be switched into circuit without first having the exciting current through that circuit as indicated on the voltmeter on the switchboard, and in direct- current machines this can be obtained by either speed variation or by I 114 ELECTRICITY AS APPLIED TO MINING rheostats in the shunt circuit of the dynamo. In this connection it will be remembered that the greater the exciting current, and therefore the field, the greater will be the E.M.F. for a given speed. Fig. 80. A black panel with various dials and switches, labeled "Main Switchboard." MAIN SWITCHBOARD. STARTING OF MOTORS 115 Motors.—When installing electro-motors in a mine, especially a damp mine, it is often found beneficial to first thoroughly dry the motor by baking it in an oven at some suitable temperature, say 60° or 70° C. above that of the air. This operation thoroughly dries the insulation of all the copper circuits, and seems to prevent the absorption of moisture in it afterwards. In the experience of the authors several motors not so treated have given trouble from their insulation breaking down, while those baked in the above manner have given no trouble in this respect. Fro. 81. Indicates cable connections to dynamo &c. circuits. CONNECTIONS OF SWITCHBOARD. If this method is not convenient, the motor may be run for an hour or two on a low voltage, obtained by putting it in series with a resistance ; a water resistance will be convenient for this purpose. When the motor gets thoroughly warmed and dry, the resistance may be gradually cut out until the full voltage is applied. In an alternating current motor, assuming that all connections are correct, the first point is to see that the oiling arrangements are in good working order, and that the starting resistance is full in ; for suddenly switching the current on to the machine with no or very little resistance in series with its armature would not only cause a heavy rush of current, which would blow the protecting fuse, but might damage the armature con- ductors. The switch should then be closed, and the starting resistance gradually cut out until the motor is left working on the mains by itself. As 116 ELECTRICITY AS APPLIED TO MINING load is put on, the speed, if it falls, can be raised by weakening the field of the machine---i.e. by increasing the resistance in series with the field-coils. In stopping motors the resistance in the main rheostat should always be inserted to the full before breaking the main switch. There are, however, automatic switch rheostats which combine these two operations at one and the same time. Switchboard.--These are enameled slabs of slate, marble, or other insulating material, to which are fixed all the necessary appliances, such as measuring instruments, switches, cut-outs, and rheostats, for regulating and controlling the supply of the electric current. Figs. 8o and 81 show the general view and diagram of connections of a main switchboard made by the General Electric Company for the parallel running of two dynamos. At the top (fig. 80) are two ammeters and two voltmeters of the Stanley type (page 127). The next lower instrument is a Stanley automatic minimum cut-out. Lower still is a three-pole Pella double pole fuse box with double pole fusible cut-out terminal underneath, and at the bottom a 13-point shunt-regulating switch. This is the dynamo panel. The circuit panel, on the right, contains the ammeter and voltmeter, two double-pole switches with fuse cut-outs. Combining another similar pair of panels with that of fig. 80, we obtain four complete switchboards for each dynamo set. There are, of course, very much larger and more elaborately equipped main switchboards, but this description will serve our purpose sufficiently well. MOTOR-STARTING AUTOMATIC SWITCH RHEOSTAT. Fig. 81. **MOTOR-STARTING RHEOSTATS** Motor-starting Rheostats.—These are appliances for starting up a motor to full speed by the adjustment of the resistance in circuit without fear of injuring it, and at the same time for immediately switching it automatically out of circuit should the field of the motor cease to act or the supply of current be cut off. Fig. 82 shows one type made by the General Electric Company, London. It consists of a plate panel, on which are fixed the multiple-way contact blocks, which are connected to suitably adjusted resistances. When the motor is started, the field of the motor when connected to this starting resistance, the contact lever is slowly pulled round clock-wise over the contact blocks, thus cutting out resistance in the main circuit until it is as far to the right as it will go. The electro- Fig. 83. Motor Starter fitted with no Volt and Overload Releases for Continuous Current Motors. magnet then holds it there if the field is fully excited. The motor is now running at full speed. If the supply is cut off the lever is released by the magnet (which falls) and flies back under the action of a strong spring, thus cutting out resistance. A very neat and compact form of safety enclosed liquid-starting resistance switch, devised by Mr. J. H. Woollicott, has recently been put on the market by the Sandycroft Foundry Company of Chester. It is specially designed for starting electro-motors in coal mines or other places where absolute safety is required. A photograph showing how these starters for a 10 h.p. motor is shown in fig. 83, while its construction and circuit connections are indicated 118 ELECTRICITY AS APPLIED TO MINING diagrammatically in fig. 84. It consists of a cast-iron case, $a$, free to turn or rotate through a certain angle on insulating bearings, $j$, and filled to the level, $r$, with the resistance liquid. A lever, $A$, which rotates concentrically with the case, $a$, is held by a catch when in the vertical position, and carries a minimum release or retaining coil, $e$, connected in series with the field coils of the motor. The catch, $b$, is caused to turn when engaged by the armature, $a$, of the coil, $e$, is fixed to the case, as the handle, $A$, is turned. A sliding contact, $c$, fixed to an internal blade rotates with the case, $a$, but is insulated from it. Fig. 84. **Diagrammatic View of Motor Starter.** A maximum release, $p$, is protected inside the cast-iron bearing pedestal, and a short-circuit contact, $h$, is fixed to the case $a$. Fig. 84 shows the switch with circuit open and ready for starting. In actual operation the current passes by way of the positive main to the axis of the containing case, $a$, and then to the liquid. Here the circuit to the field coils of the motor is completed while the insulated blade is out of the liquid. The only path open, therefore, is through the coil, $e$, thence through the field coils of the motor to the negative main. When, therefore, the coil, $e$, is energised by the current which also excites the field of the motor, the case, $a$, can be turned by turning $A$. The blade SANDYCROFT MOTOR STARTER 119 then descends into the liquid, gradually cutting out all resistance until the short-circuiting connection, $u$, in contact with the case, $a$, engages in the terminal, $k$, thereby cutting out entirely the blade from all local action. If an overload comes on, the coil, $u$, is short-circuited by the maximum release and the case, $a$, revolves back to its original "off" position by the aid of a spring. In this operation all resistance is thrown in before the circuit is opened, the current being thus reduced to a very small value, and then broken between the point of the internal blade and the liquid. No external spark is therefore possible. If the motor is stopped by opening the main switch, the liquid switch finally automatically opens the armature circuit. Should a conductor be inserted in place of the liquid, a large current will flow which will result in operating the maximum release and leaving the handle free in his hand, the case, $a$, having returned to the "off" position and opened the circuit. The simplicity of construction, reliability of action, absence of sparking, and immunity from damage to the motor when in Fig. 85. Mains Switch Mains CONNECTIONS FOR SHUNT-WOUND MOTOR. inexperienced hands, are the advantages claimed for this form of starter over those with metallic resistances, which may be burnt out through the instructions for their use being ignored. There is no creeping of the liquid and next to no evaporation, as is proved by the fact that in one of these starters used many times a day for eight months the level of the liquid had decreased only $\frac{1}{4}$ inch. This type of starter is particularly suitable for starting shunt-wound motors. Starting Shunt-wound Motors.—On switching current on to a shunt-wound motor, there are two paths open to it; the armature and the field windings. The armature is of low resistance, and the field of high resistance and the self-induction of the field is large. Consequently, at the moment of switching on, there is a large current through the armature which does not rise to its full value for an appreciable time : the armature, however, would take a very large current at once if allowed to do so, before the armature could speed up and create the necessary back E.M.F. To prevent this, shunt motors are usually started by the arrangement 120 ELECTRICITY AS APPLIED TO MINING shown diagrammatically in fig. 85. The two-pole switch being put in, only a very small current is allowed to pass through the armature, owing to the resistance of the starting switch ; the shunt, however, is receiving its full voltage, and consequently has time to reach its proper strength before the attendant begins to switch out the starting resistance. Owing to the self-induction of the field, a large current would flow suddenly any more than it can be started suddenly, and the attempt to do so is dangerous to the insulation. In the arrangement shown in the figure, when the motor has been switched off, the shunt current can continue flowing through the armature and starting resistance until it has time to die away. It is exceedingly important that means should be provided for the dis- charge of this current after it has ceased. **Starting-Series-wound Motors.—In a series motor there is only one path for the current through the field and armature windings in succession, and the self-induction of the field would partially prevent such a large rush of current through the armature on switching on as in a shunt** Fig. 86. **CONNECTIONS FOR STARTING AND REVERSING SERIES-WOUND MOTOR.** machine. It is, however, necessary to start with a resistance in series with the machine, as in the case of a shunt-wound motor. Fig. 86 shows the connections for starting a series-wound reversing motor; the direction of rotation is changed by reversing the current through the armature, while the direction of flow through the field coils remains the same. The connections are made by two parts of the same lever insulated from each other and connected together at their ends. The current flowing in parallel with the field is non-inductive and very high compared with that of the motor. While the motor is running very little current passes through it, but when the switch is thrown out it acts as a field discharge and protects the insulation from the shock which would otherwise be caused by the sudden interruption of the field circuit ; it is not absolutely necessary, however, for this precaution, especially if the motor is switched off under a heavy load. **Switches.—For mining work, at all events in the 'workings' where it is possible for gas to accumulate, either liquid-break or enclosed switches** ELECTRIC SWITCHES 121 should be used, so as to avoid the possibility of firing the gas. Figs. 87 and 88 show respectively a single- and a double-pole liquid switch. Referring to the former, it consists of a grooved sector, provided with a handle, and pivoted on a horizontal spindle supported by the framework shown. A flexible cord hangs from the sector, and is capable of raising or lowering a contact immersed in oil, or other suitable liquid, contained in a vessel the lower part of which supports a fixed contact piece. The con- Fig. 87. **Single-Pole Water Switch (Scott & Mountain).** tact between the fixed and movable blocks or pieces is therefore made or broken in the liquid, and therefore the spark is immediately extinguished, and can never get to the surrounding gas. The form of liquid switch illustrated is made by Messrs. Ernest Scott & Mountain, of Newcastle-on-Tyne. Fig. 88 shows the double-pole form of similar construction, and fig. 89 that of a three-pole form, also made by Messrs. Ernest Scott & Mountain. In this latter, by turning the hand-wheel at the top, the central plate is lowered on to the bottom of the outer case containing 122 ELECTRICITY AS APPLIED TO MINING liquid, which completes the circuit ; the moving part moves along insulating guides and cannot touch the case, and so forms one pole of the switch. In the case of enclosed switches the containing-box must be as air-tight as possible, and be capable of withstanding the explosion of its contents of gas without communicating it to the gas outside. For this reason the internal air space must be reduced to the smallest possible amount, and any openings through which air can pass must be so narrow and long that the flame of an internal explosion would be extinguished in passing; the Figs. 8A. **DOUBLE-POLE WATER SWITCH.** smaller the total amount of openings in proportion to the cubic contents of possibly explosive gas, the longer must be the narrow passage to extinguish the gas flame. In testing safety-lamps it has been found that the holes in the wire gauze of a Clammy lamp are sufficient in number and total area to allow the passage of all the exploded gas that the lamp can contain, at such a slow speed that it is cooled by the wire below burning point before it gets outside of the gauze ; but if we increase the number and total area, the flame would be forced through the remaining part. The thickness of the wire gauze is about $\frac{3}{4}$ inch, and that is the length of the passage ; the width of the passage is about $\frac{1}{6}$ inch. If the passage is made $1$ inch LIGHTNING ARRESTERS 123 long it would have fifty times the cooling effect ; but if it is a slot instead of a pipe it would have only one-half the cooling effect ; thus a slot 1 inch × $\frac{1}{4}$ inch in section × 1 inch long would probably be twenty-five times as effective for cooling as one hole in the gauze of a safety-lamp. Fig. 89. SINGLE-POLE WATER SWITCH. Lightning Arresters.—These are appliances which are inserted in the circuit of an overhead main to protect the main and machines, &c., connected with it, from fusion due to discharges of lightning. They are important adjuncts, as the line wires are liable to be struck at any moment when laid above ground, or if there is any electrical disturbance. 124 ELECTRICITY AS APPLIED TO MINING heavy discharges may take place along the wire, even without an actual flash. There are several well-known forms, and amongst them we may take the Thomson-Houston magnetic blow-out arrester. This consists of an electro-magnet terminating at its upper end in two upright metal horns, which curve away upwards from each other. The lightning on striking the line connected to one horn leaps across the short air gap between the lower ends of these horns, thereby causing the current to flow through and through the electromagnet, at the end of the coil of which is connected to the other horn. This, becoming magnetised, blows the arc across the horns higher up, until the gap is too long for the arc to exist, when it goes out. This form of arrester can deal quickly with any number of successive discharges, which is an advantage. With high voltages it is necessary to Fig. 90. DOUBLE-POLE CUT-OUT (COVER OFF). have more than one lightning arrester, a number being placed in ' banks ' in series and parallel to prevent the line current from following the lightning discharge to earth. It is of great importance that an overhead line which proceeds over a pit should be protected by lightning arresters, as the effect of a discharge down the pit might be extremely serious. In addition to lightning arresters, lightning conductors should be fitted to the posts or other erections employed to support the electrical overhead wire ; these conductors being simply wires making good contact with the earth at the foot of the post, and projecting some 18 inches or more above the top of it. Cut-outs.--These are devices for protecting the circuits and machinery from abnormal overload currents, which, if they did not actually fuse or burn up the machines or mains, would ruin the insulation of them, and might CUT-OUTS 125 set fire to the premises in which they were placed. Fig. 90 shows one type of double-pole 'cut-out' known as the 'Champion,' and supplied by the General Electric Company, London. It consists, as seen, of a porcelain or china moulding, carrying four terminals, to the bases of which (not visible) are fixed metal cups, or thimbles, for soldering the main cables into. The two mains are cut, the ends of one being soldered to the two terminal cups on one side of the raised ridge, and the ends of the other to the pair on the other side. The fuse wire is enclosed in a glass tube, and when melted at a low temperature, is clamped under the nuts of each pair of terminals, and the china cover fixed, as seen in fig. 91. The gauge of fuse wire is so chosen that it melts on the current attaining a certain known strength, and cuts the current off. It is called 'double pole,' because a fuse is inserted in each main. There are many other forms of cut-outs, but space will not Fig. 91. DOUBLE-POLE CUT-OUT (COVER ON). permit of their description. A common form for low voltages being an uncovered fuse wire attached to screw terminals, but this case should be taken with this formula that the most frequent cause of fuse is short in ; one of the most frequent causes of breakdown with motors is that they are overloaded, the fuse melts, and some one puts in a bigger fuse or a piece of copper wire to prevent it melting again ; the motor is then unprotected and may very likely have its insulation burnt out by overload. For high-tension work fuses must be enclosed and of special form, being either immersed in oil or enclosed in a box having spring contacts for making and breaking path into spring contacts. There is little chance of an improper fuse being inserted in such a holder, and it is safer than an open fuse in many ways. Callender's Cable and Construction Company make a good form of fuse box in which there is combined a switching arrangement. On opening the lid a switch is thrown off, and thus a fuse can be replaced without touching any live conductors. 126 ELECTRICITY AS APPLIED TO MINING **Magnetic Cut-outs.**—These are cut-outs either by themselves or in combination with switches, which work solely by the electro-magnetic effect of the current, and not by the heating effect, as just instanced. The electro-magnetic cut-out switch is so arranged that when the current attains a certain strength a trigger is released by the electro-magnet, and causes the switch to spring open. These cut-outs are also made with a time limit relay, so that they will not operate until a certain amount of time has elapsed. They will not open them, but the overload must be kept on for a certain definite time before the switch will act. Circuit can only be made again by closing the Figs. 92. Fig. 93. (CLOSED) AUTOMATIC RELEASE SWITCH. (OPEN) switch in the usual way. The cut-out portion can be adjusted for a range of 50 per cent. below to 50 per cent. above normal current. As soon as the current reaches the amount to which the instrument is set, the switch is opened and the current stopped. Figs. 92 and 93 show this kind of switch, closed and open respectively, as made by the General Electric Company, London. **Electrical Measuring Instruments.**—The practical electrical engineering measuring instruments in use at the present day take a great many different forms, but all work on principles which depend on one or ELECTRICAL MEASURING INSTRUMENTS 127 other of the properties of an electric current (vide page 3) - viz. the electro-magnetic, electro-static, chemical, and thermal properties. But, whatever the principle on which they work, they may be classified as follows : Ammeters for measuring the current in a circuit. Voltmeters for measuring the voltage across a circuit. Wattmeters for measuring the power given to a circuit. Electricity-meters for measuring the energy given to a circuit. We will now briefly consider each of these in turn. Ammeters and voltmeters of the electro-magnetic type differ merely in the winding of the actuating coil, so that we will confine ourselves to describing simply one Fig. 94. INTERIOR OF ELECTRO-MAGNETIC VOLTMETER. well-known form of this type—the Stanley—made by the General Electric Company, London. Fig. 95 shows the back view of a voltmeter, with case removed. The bobbin at the top is wound with a large number of turns of fine silk-covered copper wire, and is connected in series with a large resistance made of fine wire of a special alloy, which alters its resistance very little with slight change of temperature, and wound on the two porcelain bobbins or frames shown below. This form of this is such as to allow the heat produced by the current in the resistance to be dissipated. The extremities of this combination are connected to the two terminals of the instrument. The ammeter of this make only contains the top bobbin, now 128 ELECTRICITY AS APPLIED TO MINING Wound with thick copper wire for carrying the main currents. In all cases the internal opening in the bobbin contains a piece of soft iron carried on a light spindle, suitably pivoted in jewelled centres, and carrying the A diagram showing a voltmeter labeled "VOLTS" with a scale ranging from 0 to 300 volts. FIG. 95. ELECTRO-MAGNETIC VOLTMETER. A photograph of a leakage current indicator, mounted on a wall. The indicator has two switches labeled "NO." and "YES," and a label at the bottom that reads "Leakage Current Indicator." FIG. 96. LEAKAGE CURRENT INDICATOR. ELECTRICAL MEASURING INSTRUMENTS 129 pointer, not seen in fig. 94. When a current flows through the coil the interior becomes a powerful field, which alters the position of the iron needle, thereby causing the pointer to take a certain position on the scale corresponding to the known measured current or voltage. Fig. 95 shows a voltmeter of this make complete. Another type of instrument widely used nowadays, though more expensive, is the Wheatstone bridge, coil, permanent magnet, ammeter and voltmeter. In these the scale is graduated in equal divisions throughout, and the pointer immediately takes its position corresponding to any altered current or voltage without swinging to and fro a few times either side of it. This is an advantage when a current or voltage has to be measured which fluctuates. Some of the differences of the type of instrument described above are provided with damping devices, by which the pointer quickly to rest. Such instruments are called 'dead beat'. A Wattmeter is a combination of an ammeter and voltmeter, which by the deflection of its pointer shows at a glance the power in watts given to the circuit. It consists of a fine wire (volt) coil pivoted close to a fixed thick wire (ampere) coil. The two wires are connected together at one point, the former across them. The action of one on the other causes the moving fine wire coil to deflect by an amount proportional to the watts given to the circuit in which it is placed. A wattmeter should always be used on an alternating current circuit, as the power cannot be directly ascertained from direct current circuits. Electricity-meters are instruments having continuously moving parts, by means of which the total amount of power supplied in a given time is recorded on dials like those of a gas-meter. Space will not permit of a further description of such instruments here, but a detailed description of all the many forms of the various types of ampere, volt, watt, and electricity meters used in this country can be found as will be found in Part I 'Electrical Engineering Measuring Instruments,' to which the reader is referred for further information. Earth Detectors.—It is desirable to have on the generating station switchboard some means of ascertaining while the plant is at work whether the insulation between earth and live conductors is intact. For this purpose 'earth' or 'ground detectors' are fitted. A simple form consists of two or more lamps connected across the mains. A 200-volt circuit would require two 200-volt lamps, a 500-volt circuit four 250-volt lamps. The central point of this lamp system is connected through a switch to earth, there being an external number of lamps on each side of the earth connection. The lamps glow dimly, being supplied with only half their proper voltage. If a fault is present, however, say on the positive cable, a current will flow from the k Blackie & Son. I30 ELECTRICITY AS APPLIED TO MINING fault to earth, and through the earth connection of the ground detector, through the lamps on the negative side to the negative cable. Thus, if the lamps on the negative side are brighter than those on the positive side, there is a fault in the positive cable and *rér* *terd*. Another more expensive but more satisfactory form of earth detector on high-voltage circuits consists of a high resistance voltmeter, one terminal being connected to earth, the other to two-way switches of which it may be connected to either cable. So long as no fault exists the voltmeter will show no deflection on being connected up, since no current can pass through it, but if a fault exists in the opposite cable, a current will pass and show a deflection ; if the fault is a 'dead earth' the voltmeter will show the full voltage of the system; if it is only a partial earth it will show something less. The Special Rules for the Installation and Use of Electricity in Mines provide that 'Earth or fault detectors shall be kept connected up in every generating station. The reading of these instruments shall be recorded daily in a book.' Fig. 5 shows an instrument which has been specially designed so that the leakage current to earth and the insulation resistance can be at once ascertained. The standard size as illustrated is suitable for installations not exceeding 500 amperes maximum supply and is suitable for any voltage. The pointer on this instrument shows whether the leakage current is within the prescribed limits. Two sets of terminals are provided for testing both positive and negative sides of two separate supply systems. One terminal of the instrument is connected to an earth plate or to a convenient steam- or water-pipe; the other terminals respectively to the positive and negative bus bars or terminals of the switchboard. If there is a bad fault on either main the small fuse will blow immediately when the key is pressed. The scale is calibrated in milli-amperes and represents thousandths of the supply current. The insulation resistance to earth may be calculated by the formula $$R = \frac{V}{A}$$ 131 CHAPTER VII ELECTRIC CABLES, ETC. Electric Cables—Varieties of Cable Construction—Size of Cable for a given Current—Details of Conductors—Aerial Cable—Insulators—Carrying Cable down Shaft, and in Underground Workings. The selection of a substance to act as a conductor of the electric current is largely governed by the resistance which it offers to the passage of that current. The resistance offered by a conductor may be likened to the resistance to the air current in the passages of a mine—in other words, it varies directly as the length and inversely as the sectional area, and just as the coefficient of friction (or resistance for unit area and unit rubbing surface) in a mine varies with the nature of the road, so the specific resistance (which is the resistance per unit length or unit area) of a conductor varies with the material of which it is composed. The specific resistance of a substance is the resistance of a length of 1 centimetre with an area of 1 square centimetre at a temperature of $0^\circ$ C. ($32^\circ$ F.). The specific resistance varies directly with the temperature, but its variation is so slight that this variation can be very large. The resistance of pure copper rises with the temperature -28 per cent. per degree F. or 428 per cent. per degree C.¹
Name Specific Resistance in Microhms
Silver annealed 1468
Silver hard-drawn 1456
Copper annealed 1361
Copper hard-drawn 1362
Aluminium annealed 2665
Zinc annealed 5751
Iron annealed 7065
Tin annealed 13048
Lead annealed 2038
Mercury liquid 9497
¹ This applies to all conductors built up of small wires; but in the case of solid bars, when sections of copper of $\frac{1}{2}$ inch diameter are reached, there is, when dealing with alternating currents, a slight percentage to be deducted; over $\frac{1}{2}$ inch diameter, the deduction becomes negligible. ² Electrical Engineering Testing, by G. D. A. Pure. 132 ELECTRICITY AS APPLIED TO MINING Having given the specific resistance of a conductor, we can find the total resistance for a given length and area because: Total resistance in microhms = $\frac{\text{length in centimetres} \times \text{specific resistance}}{\text{area in square centimetres}}$ $1$ microhm = $160800$ ohm $1$ foot = $304.799$ centimetres $1$ square inch = $64516$ square centimetres. Then Total resistance in microhms = $\frac{\text{length in feet} \times 304.799 \times \text{specific resistance}}{\text{area in square inches} \times 64516}$ and total resistance in ohms = $\frac{\text{length in feet} \times 304.799 \times \text{specific resistance}}{\text{area in square inches} \times 64516 \times 10^6}$ $\therefore$ length in feet $\times$ specific resistance $\times 10^6$ area in square inches The material usually employed for conductors is copper, although under certain conditions other metals, such as iron and aluminium, are employed, chiefly in overhead or aerial lines. If we consider the following table—
Copper Aluminium Iron
Specific resistance Specific conductivity
$\frac{2}{3}$ ohm per metre $889$ ohm per metre
$\frac{2}{3}$ ohm per metre $267$ ohm per metre
$\frac{2}{3}$ ohm per metre $780$ ohm per metre
—we see that for equal lengths to offer the same resistance to the passage of a current the area of an aluminium cable would have to be $\frac{2}{3}$ times or $1/7$ times the area of a copper cable, and an iron cable $\frac{2}{3}$ times or $1/8$ times the area, while the aluminium cable would be $1/7 \times 267 = 0.51$ times the weight, and the iron cable $5 \times 889 = 51$ times the weight of the copper cable. The present price of copper cables uninsulated is about 1.02f. per lb., while aluminium rods are 1.2f. per lb. and iron rods 1.4f. per lb., so that, comparing conductors of the same conductivity, the relative cost is— Copper $= 1.02f.$ per lb. Aluminium $= 1.2f.$ per lb. Iron $= 1.4f.$ per lb. The tensile strengths are— Hard-drawn copper, about $58,240$ lbs. per square inch. Hard-drawn aluminium, about $25,000$ to $30,000$ lbs. per square inch. Iron, about $89,600$ lbs. per square inch. Owing to the great weight of an iron conductor the supporting poles would have to be much stronger than for a copper conductor, and owing to the liability to corrosion it would have to be renewed from time to time. Aluminium has been used for overhead lines with satisfactory results. SIZES OF CABLE 133 although a difficulty in the early application was the joining up of the separate lengths of wire in the line, owing to the electrolytic properties of aluminium when in contact with other metals. The difficulty in joining, however, have been got over by using an oval aluminium tube, into which the two ends are clamped, and by various other satisfactory joints. Aluminium will become no doubt a formidable competitor of copper for aerial lines, although it must be borne in mind that for equal conductivity the tensile strength of aluminium is only three-quarters of the tensile strength of copper. If we consider the case of cables covered with insulating material, however, the advantages of copper are considerable. Taking the case of three wires of equal conductivity, it will be seen that the relative diameters will be in the proportion of the square root of their areas, or copper $\sqrt{3}$, aluminium $\sqrt{17}$; iron $\sqrt{8}$, or $\sqrt{2}$. 2:4:5; and consequently the cost of insulating material will be proportionately less in the case of iron and would render aluminium more expensive than copper at once. In a concentric cable, however, iron has been utilised as a conductor by insulating the copper conductor and surrounding it with an armouring of galvanized wire which acts as the return ; but this plan, though it has some advantages, is not now in fashion, as insulated copper returns are preferred. Sizes of Cables.—The wire of which cables are made is drawn in certain definite sizes or gauges, the leading cable-makers having adopted the standard wire gauge. The sizes of wire used vary from No. o S.W.G., which is $324$ inch in diameter, down to No. 25 S.W.G., which is $0.2$ inch in diameter. These wires are twisted into strands of $7$, $19$, $37$, $61$, or $95$ separate wires ; thus, a cable $37$ i.e. thirteen wires each of No. 13 S.W.G.
S.W.G. Diameter Area S.W.G. Diameter Area
7/0 0-500 0-006149 11 0-116 0-0010583
8/0 0-450 0-005889 12 0-126 0-0010466
9/0 0-432 0-005674 13 0-136 0-0010346
10/0 0-416 0-005488 14 0-146 0-0010246
11/0 0-392 0-005326 15 0-156 0-0010146
12/0 0-372 0-005186 16 0-166 0-0010046
13/0 0-354 0-005059 17 0-176 0-0099896
14/0 0-338 0-004949 18 0-186 0-0099296
15/0 0-323 0-004859 19 0-196 0-0098796
16/0 0-312 0-0047892 20 0-2o6
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Standard Wire Gauge.
S.W.G.Diameter (inch)Area (square inches)S.W.G.Diameter (inch)Area (square inches)
Standard Wire Gauge.
S.W.G.Diameter (inch)Area (square inches)S.W.G.Diameter (inch)Area (square inches)
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134 **ELECTRICITY AS APPLIED TO MINING** When the size of the wire happens to be between two of the gauges, the diameter in inches is given ; thus 61/957 means sixty-one wires each .095 in. in diameter. Where a large cable is required it is composed of a large number of small wires, so as to give it greater flexibility than if a small number of large wires were used. **Insulation of Cables.—The material used for insulating the cable is called a di-electric,¹ and the best di-electrics are those which offer the greatest resistance to the passage of an electric current; those most commonly employed are pure rubber, vulcanised rubber, vulcanised bitumen, ozokerite tape, and paper or some other material impregnated with oil. If the latter are used they require to be enclosed in a lead casing to keep all moisture out, and they then possess the best electrical properties. In some cases the copper cable is laid in a pipe full of thick oil. When cables are insulated with vulcanised rubber the wires are invariably tinned to protect them from the action of the sulphur in the vulcanised rubber. Over the tinned wire a layer of pure rubber is often put before the vulcanised rubber. The thickness of the di-electric is increased as the voltage intended to be carried is increased. For high voltages the thickness is $\frac{1}{y}$ inch per 1,000 volts; thus, for a cable carrying 10,000 volts there would be a paper covering $\frac{1}{y}$ inch thick all round, so that if the diameter of the copper was $x$ inch the thickness of the paper would be $\frac{x}{y}$ inches. For 100 volts lead pipe, say $\frac{1}{y}$ inch or $\frac{1}{y}$ inch thick, makes the total thickness outside the lead pipe $\frac{1}{y}$ inch to $\frac{1}{y}$ inch. But for low tensions the thickness of the di-electric is greater in proportion; thus, with a cable made of nineteen wires No. 18 gauge for a pressure up to 500 volts, and 200 volts for 2,000 volts would be very little more. For example, for 500 volts di-electric is used on No. 18 small cables; thus, for a cable 37/32 thickness of di-electric for 500 volts is $\frac{1}{y}$ inch, and for 2,000 volts $\frac{1}{y}$ inch. For a cable 7/23 at 300 volts the thickness of vulcanised-rubber insulation is .05, and at 2,000 volts .1. For large cables it is now a common practice to use a paper-covered cable in which case no insulating material is necessary. In mines, however, in a mine, it is common to use vulcanised-rubber insulation. Vulcanised bitumen insulation is said to be much better able to stand pit water than rubber or lead, and in wet pits it is probably the best material to use. Having decided how to insulate the cable, the next consideration is how to protect this insulation. In order that of india-rubber insulation, this is generally protected by a wrapping of tape or paper or by some other coating. ¹ The term "di-electric" is strictly speaking, used to denote the insulation separating the consecutive plates of an electric condenser, but it is a term popularly used in speaking of the insulation of any cable. INSULATION OF CABLES 135 composition, and this tape is again protected by a covering of jute fibre, called braid, also soaked in waterproofing composition. The cable may now be safely handled, and if placed in a position where it will be free from all rough treatment and from falling water, if not over-worked it should last for a great many years. In answer to inquiries, the makers of india-rubber-insulated cables state that they have had them in work ten years without any trouble whatever, but they are not always so fortunate. But if the cable may be subjected to rough usage, then it is common to protect it with an iron or steel covering. This may be in the shape of a steel tape wrapped round, the steel being coated with some waterproofing material, and afterwards wrapped with jute braiding soaked in tar. In place of steel tape a covering of galvanized wire may be used, and this is said to give better protection than the iron or steel covering than the tape. In some cases the cable is wrapped with a locked coil wire which makes a very complete armour. For protection against water, india-rubber-covered cables are often covered with lead pipe. Armoured cables should never be used in a roadway where they are liable to be touched, unless the ground is well drained, because if a fault occurs in the earth pit is a very difficult thing to do without burying the cables, so that a good rule is never to hang armoured cables along the roads of a dry pit : for if a leakage occurs at any point in the cable between the copper and the armour, the whole armouring at once assumes the potential of the conductor, unless the leakage current is led away to earth ; this might cause serious accidents on account of short circuits. A similar danger exists also possibly by fire if the armour is touching a prop or inflammable material. The jute covering cannot be relied upon as a protection, as a good deal of it will probably be rubbed off by the time the cables have been got into position. Where armouring is used and well earthing it has this effect, that a fault in the insulation at once shows itself on the earth detector, and can be remedied ; on the other hand, damage to the insulation of the unarmoured cable, at a point where it is not in contact with anything, is not such a serious matter as with an armoured cable, and does not interfere with the working of the earth detector. Another advantage of armouring is that a fall of roof which might not seriously damage an unarmoured cable may develop a fault in an armoured one by squeezing the armouring on to or near to the conductor. It is doubtful whether armouring is so great a mechanical protection against blows as it generally supposed, but it is valuable in resisting abrasion on a travelling cable, and in increasing tensional strength for hanging a cable in a shaft. The paper-covered cables, which are always in a lead pipe (the thickness of which, for a cable 3 inches in diameter, is -185 inch), are often 136 ELECTRICITY AS APPLIED TO MINING covered outside the lead with a braiding of jute and tar, and then with an armouring of iron tape or wire. The cables are of the following construction: 1. Single cable insulated and protected as above described. 2. Twin cables insulated as above, then placed side by side and covered with protecting braiding, tape, lead pipe, armour, axe. 3. Three-cable, either round or flat sectioned (see fig. 97), each separately insulated and then bound together and protected as above. 4. Four cables, either round or of flatted section, each separately insulated, and then bound together and protected as above. 5. A concentric cable with internal copper conductor, then insulating material, then copper wire laid in a ring of equal sectional area to the conductor, then insulating material, and protecting covering as above described (see fig. 98). Fig. 97. THREE-CORE ARMOURED CABLE. 1. Steel wire. 2. Jute. 3. Lead. 4. Insulation. 5. Triangular copper strand. 6. Internal copper cable, then insulation, then a ring of copper wires, then insulation, then a second ring of copper wires, then insulation, and protective covering as before. In this way three conductors are obtained concentrically. Conductors of this kind have been made with the central cable equal to 1 square inch in section, made up of ninety-one wires each 1/18 inch diameter, the two surrounding rings of copper being also of the same sectional area, and the total diameter outside the lead pipe being 34 inches. For use in mines each of the three conductors may be made up into separate apparatus. When three conductors are bound together in one cable either side by side or concentrically, it is not necessary that each conductor should be equal in section to the others ; if the three-wire system is adopted, one might be half the size. Sometimes a small conductor is laid in the space between two of the larger conductors. SIZE OF CABLE 137 The limit to the length of cable made in one piece is the size and weight of the drum covered with cable that can be conveniently handled. The lead covering above referred to is made a tight fit to the cable in the following way : The cable to be covered is drawn through a nozzle which fits it ; over this nozzle, and surrounding it, is another one of larger size, the space between being equal to the intended thickness of lead ; into this annular nozzle, lead in a soft half-melted state is forced by hydraulic pressure, so that the lead becomes hot enough to damage the insulating material of the cable. The cable before leaving the works is tested to see that the insulation is perfect at the required voltage. Fig. 98. -25 SO. 18. ARMoured Concentric Cable, BY THE BRITISH INSULATED WIRE COMPANY. 1. Copper strand. 2. Insulation. 3. Copper strips. 4. Insulation. 5. Lead. 6. Jute. 7. Steel wire. Rules for Size of Cable for a Given Current.—By Ohm's Law $$C = \frac{E}{R}$$ where C is the current in amperes, E is the E.M.F. in volts, and R is the resistance in ohms ; from which we get $$E = CR,$$ or the volts expended in sending a current of C amperes through a conductor of R ohms' resistance are equal to the product of the current and the resistance. The power as measured in watts = E × C, and the horse-power is shown by the rule : h.p. = $\frac{E \times C}{746}$ Substituting the value of E, as found above, we get $$\text{watts} = CR \times C = CR^2$$ and $$h.p. = \frac{CR \times C}{746} = \frac{CR^2}{746}.$$ A graph titled "FALL OF POTENTIAL AND RISE IN" showing a relationship between amperes and potential. The x-axis represents amperes ranging from 0 to 5, with increments of 1. The y-axis represents potential, starting at 0 and increasing by 10 units up to 60. The graph is divided into two sections: one for positive potential (top) and one for negative potential (bottom). Each section contains horizontal lines representing different values of potential, with labels indicating their approximate values. The top section has labels like "10 VOLT", "20 VOLT", etc., while the bottom section has labels like "-10 VOLT", "-20 VOLT", etc. The graph also includes a legend on the left side, indicating the scale of amperes.PigAMPERES345678910FALL OF POTENTIAL AND RISE INAMPERES123456789C****************************** A TEMPERATURE FOR ANY CONDUCTOR. 99. 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 630 660 690 720 750 780 810 840 870 900 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 99 A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for various conductors. A chart showing temperature ranges for variousconducters. 140 ELECTRICITY AS APPLIED TO MINING Thus the horse-power lost in the cable varies directly as the square of the current, and directly as the resistance. This horse-power is, of course, represented by a rise in temperature of the cable, which fact governs the maximum loss allowable. With the current density at 1,000 amperes per square inch, the loss in voltage is approximately 24 volts per 100 yards. With a current of 800 amperes per square inch, the loss is about 36 volts per 100 yards. The question of size of cable required is also dealt with in Chapter VIII. The diagram on the previous pages (fig. 99) is copied from one lent to the authors by Messrs. W. T. Glover & Co., electric-cable makers, and has been prepared by Mr. A. H. Howard. It is very interesting, because it shows at a glance the current in amperes that can be carried by a conductor of any given size, and also shows how much fall of potential per 100 yards from 1 volt up to 6 volts, and it gives the rise of temperature in degrees Fahrenheit for a drop of voltage per 100 yards varying from 1 volt up to 6 volts. The diagram shows graphically dimensions and fall of potential and approximate rise of temperature of any wire with any current. Directions for Using. The horizontal lines represent area or size of wires, as shown in the vertical columns. The vertical lines represent current in amperes, as numbered at the top and bottom. The black diagonal lines represent fall of potential per 100 yards of single cable. The dotted black curves represent approximate rise of temperature of insulated wires in wooden casing, as ascertained by experiments in the Edison Laboratory. If any two of the above quantities are given, the other two are found at a glance by the lines passing through the point of intersection of the two lines representing the known data. Example: Suppose we have a current of 500 amperes, with a fall of potential of 2 volts per 100 yards. Required, size of cable and rise of temperature. The vertical line 6o intersects the diagonal for 2 volts per 100 yards at a point on the horizontal line opposite .974 in the area column. The column nearest in area (see following table) to this is a /7//x. The dotted curve for 1° F. is just above the point ; consequently, the rise in temperature is about /7//x° F. The following table, extracted from the list of the above-named makers, gives the sizes of copper wires and cables according to the standard gauge :
(Note: This row appears to contain repeated values.)






























































































\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\ndivided into three parts: (a) The first part contains all those articles which are not used for purposes connected with electricity; (b) The second part contains all those articles which are used for purposes connected with electricity but do not require special knowledge to use them; (c) The third part contains all those articles which require special knowledge to use them. **DETAILS OF CONDUCTORS** **DETAILS OF CONDUCTORS.** Showing Dimensions, Capacity, Resistance, and Weight.
Current Density Size Fall of Potential Rise of Temperature
1,000 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
800 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
600 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
500 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
400 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
350 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch 24 Volts Per 100 Yards 36 Volts Per 100 Yards 1° F.
325 Amperes Per Square Inch
Size Diameter Area Standard Resonance at 1000 Vars Standard Weight
Inches Milli-meters Square Inches Square Millimeters
1 208 0.7112 000658 3073 39.05 71.70 12.53
2 032 0.1938 000862 51841 69.90 129.70 26.37
3 048 0.2916 001257 81099 109.13 145.51 25.98
4 064 0.3894 001671 11589 139.43 168.85 30.83
5 080 0.4872 002127 15897 179.64 208.48 35.12
6 096 0.5850 002617 20215 247.68
Pounds per 1,000 Vars (per Mile)
7112
Pounds per 1,000 Vars (per Mile)
8
Pounds per 1,000 Vars (per Mile)
9Size
Details of Conductors.
Size S.W.G.
Diameter of each Wire Effective Cross-section of Solid Wire having same Conductivity Standard Requirements for F. Standard Weights per Mile
S.W.G. In Inches in Millimeters Square Millimeters Millimeters Millimeters Square Millimeters Millimeters Millimeters Square Millimeters Millimeters Millimeters Square Millimeters Millimeters Mile
7/23 048 6066 00135 2033 7690
7/22 048 7112 00266 2752 5369
7/21 048 8188 00397 3494 8456
7/20
< td colspan='18'>< td rowspan='1'>7/19
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0489157100579435611158005794356111580057943561115800579435611158005794356111580057943561115800579435611158005794356111580057943561115800579435611158$^{\text{a}}$ $^{\text{b}}$ $^{\text{c}}$ $^{\text{d}}$ $^{\text{e}}$ $^{\text{f}}$ $^{\text{g}}$ $^{\text{h}}$ $^{\text{i}}$ $^{\text{j}}$ $^{\text{k}}$ $^{\text{l}}$ $^{\text{m}}$ $^{\text{n}}$ $^{\text{o}}$ $^{\text{p}}$ $^{\text{q}}$ $^{\text{r}}$ $^{\text{s}}$ $^{\text{t}}$ $^{\text{u}}$ $^{\text{v}}$ $^{\text{w}}$ $^{\text{x}}$ $^{\text{y}}$ $^{\text{z}}$ $^{\text{aa}}$ $^{\text{ab}}$ $^{\text{ac}}$ $^{\text{ad}}$ $^{\text{ae}}$ $^{\text{af}}$ $^{\text{ag}}$ $^{\text{ah}}$ $^{\text{ai}}$ $^{\text{aj}}$ $^{\text{ak}}$ $^{\text{al}}$ $^{\text{am}}$ $^{\text{an}}$ $^{\text{ao}}$ $^{\text{ap}}$ $^{\text{aq}}$ $^{\text{ar}}$ $^{\text{as}}$ $^{\text{at}}$ $^{\text{au}}$ $^{\text{av}}$ $^{\text{aw}}$ $^{\text{ax}}$ $^{\text{ay}}$ $^{\text{az}}$ $^{\text{aa}'}$ $^{\text{ab}'}$ $^{\text{ac}'}$ $^{\text{ad}'}$ $^{\text{ae}'}$ $^{\text{af}'}$ $^{\text{ag}'}$ $^{\text{ah}'}$ $^{\text{ai}'}$ $^{\text{aj}'}$ $^{\text{ak}'}$ $^{\text{al}'}$ $^{\text{am}'}$ $^{\text{an}'}$ $^{\text{ao}'}$ $^{\text{ap}'}$ $^{\text{aq}'}$ $^{\text{ar}'}$ $^{\text{as}'}$ $^{\text{at}'}$ $^{\text{au}'}$ $^{\text{av}'}$ $^{\text{aw}'}$ $^{\text{ax}'}$ $^{\text{ay}'}$ A table showing details of conductors with various sizes and diameters. ERECTION OF CABLES 143 **Erection of Cables &c. on Surface.** —The cables on the surface may be carried on wood or iron poles, with cross bearers at the top, to which insulators are attached. As there is not the same danger of Cable Suspender.

persons coming in contact with them as when in the confined passages of a mine, and also not the same liability to damage, the insulation on these overhead cables need not be so heavy (or they may be bare), provided

Pole to Carry Aerial Line.

that the points where they are supported are efficiently insulated so as to prevent the current getting to 'earth.' The distance apart of the supporting poles depends on the weight of

144 ELECTRICITY AS APPLIED TO MINING the cable, but by the use of a steel wire strained between the poles to hang the cable to, the distance between the poles may be increased. The **Fig. 102.** **Fig. 103.** FORK ROOF, INSULATOR BRACKET AND INSULATOR. SINGLE-SHED INSULATOR. cable suspenders may be made of leather, porcelain, &c.; one form is shown in fig. 100.* For mining purposes in countries where there is no liability to the **Fig. 104.** **Fig. 105.** **Fig. 106.** DOUBLE-SHED INSULATOR. TRIPLE-SHED INSULATOR. FLUID INSULATOR. attacks of destructive insects, &c., wood poles form the best support to the cable; those should be creosoted or otherwise treated so as to preserve them. * Fawcus and Cowan's Patent. SHAFT CABLES 145 The method of fixing a pole is shown in fig. 101. The stay rods shown are required in exposed situations; they are not always used. The cable, if carried on the top of the pole, is held in a bracket as shown in fig. 102, and the top of the pole is covered with a zinc roof. The insulators may be of brown ware or porcelain; the latter has the higher insulation value. The resistance of porcelain is so high that a leakage of current cannot take place through it, but takes place over the surface when the latter is wet or dirty. For this reason the type of insulator employed varies with the tension or voltage employed. Fig. 103 illustrates a double-shed insulator for low-tension lines. Fig. 104 illustrates a double-shed insulator, in which it will be seen that there is a much greater surface over which the electricity must leak, and, as this extra surface is also kept dry, it offers a greater resistance on that account. Fig. 105 shows a triple shed insulator, and fig. 106 shows a fluid or oil insulator, in which oil is placed in an annular groove to intercept the leakage of electricity over the surface. Where it is not possible or desirable to carry the cable on poles, it may be laid down in a trough or pipe (see Callender's system, page 150). Distribution of Electric Current in Shafts and Workings.—The most common cause of leakage of electric current down a shaft is no doubt to enclose both cables side by side in wooden casing, and in many instances this is found perfectly satisfactory. This casing need not be continuous, but may take the form of wooden clamps as shown in fig. 107. The wood should not be creosoted, as the creosote acts detrimentally on the cables. Where it is not inclined to be wet, however, this method is open to serious objection owing to the insulation becoming defective, and a leakage of current over the damp wood is likely to cause a fire. At the Ackton Hall Colliery the cables are suspended from the top of the pit without any intermediate support.¹ A patent for insulators is needed which consists (fig. 108) of an annular cast-iron vessel with a lug at each side, by means of which it can be suspended. At the bottom of the annular space is a thick ring of india-rubber, on which rests the support that carries the weight of the cable. The remainder of the space is filled with creosote oil, and the cable is held by an ordinary wrought-iron rope clamp. This cable-insulating system has been found satisfactory. Owing to there being only one point of contact with the shaft, and that a very efficient insulator, there is no need for the cable to be heavily ¹ Electricity at Ackton Hall Colliery,' by H. John Dunford and Rodney Holiday ; Transactions Institution of Mining Engineers, xiii. p32. L 146 ELECTRICITY AS APPLIED TO MINING insulated, a light covering only being necessary, sufficient to prevent a person getting a shock by accidentally touching the cable. It is now provided, however, in the Special Rules (see Appendix), Section III. Rule 28, that all cables in shafts must be highly insulated. Fig. 107. A diagram showing a side elevation of a shaft with a cable running through it. Side Elevation CARRYING CABLE DOWN SHAFT IN WOOD CASING. The cost of putting in one of these cables is given by the authors of the paper as follows: One thousand one hundred yards 19/16,000 megohms cable, £73 6s.; two oil insulators, 12c.; four workmen, each half a day, 10c. 6d.; one engineerman half a day, 3p. 4d.; total, £74 11s. Fig. 108. A diagram showing a cross-section of a cable with a holiday's oil insulator. HOLIDAY'S OIL INSULATOR. The only strain on the cable is its own weight, and at a great depth armouring of galvanised iron wire is employed, which increases the factor of safety. The following tests on the strength of copper cables have been made for the authors by Mr. G. F. Charnock, head of the Engineering Wood casing, with curved groove. A diagram showing a section of wood casing with curved grooves. SHAFT CABLES 147 Department of the Technical College, Bradford, on samples kindly supplied by the St. Helen's Cable Company, Warrington : TENSILE TESTS OF SAMPLES OF COPPER CABLE. Each cable 3 feet long, prepared with conical ends of soft metal alloy and held in conical dies. The area of a 61/10 cable is about 0-58 square inch.
Sizes Weight per Mile Descripion Breaking Strength Remarks
1m. Soft-drawn bare copper cable 8 oz Broke close to bottom die,
61/10 12,000 up to 13-6 tons per mile.
Soft-drawn armoured with galvanised wire 18-75 Broke clear of die.
61/10 27,100
It was found that hard-drawn copper cables could not be tested in this way, as the casting on of the conical end seemed to exercise an annealing effect on the hard-drawn wire. Several hard-drawn wires, however, from a 61/10 cable, were tested separately, and the average breaking strength was found to be 300 lbs., which is equivalent to a strength for hard-drawn copper of 2-32 lbs per square inch. The tensile strength of soft copper varies greatly, 14 tons per square inch being the usual figure for soft copper and 32 tons per square inch for hard-drawn copper. The elongation of soft wire is often 25 per cent, so that if it is employed in a cable, the length of wire required for a given length of wire being then employed, which has an elongation of 2 or 3 per cent. Hard-drawn wire is not so flexible or easily handled as soft-drawn wire, and if insulated it is apt to injure the insulation if it gets kinked. Where very great strength is required in a conductor, therefore, an additional medium should be used between the core and the sheath. In the case of a deep dry shaft, where it is proposed to enclose the cables in wood casing, it is advisable to take the weight off the cable as far as possible, and this can be easily done by making the groove in a gentle curve at intervals (see fig. 10). Another plan, emphasised by the Middleton Colliery, near Leeds, is to fasten each section of the cable by clamps rest on pole insulators supported by two bearers, as shown in fig. 11o. In a shaft 160 yards deep there are one set of these at the top and two at intermediate places in the shaft, and at the bottom the cables are secured to shackle insulators, there being no weight to carry at this point. The cables are not armoured, but have the usual insulator. At the St. John's Colliery (Normanton) 2 the cables in the shafts are 1 These figures are supplied by the India rubber, Gutta-percha, and Telegraph Works Company, Limited. 2 Electricity at Ackton Hall Colliery; 2 discussion by Mr. E. Brown. t. a 148 ELECTRICITY AS APPLIED TO MINING lead covered, and originally hung in the shaft without other covering, but pieces of coal falling from the tubs punctured the lead and great leakage ensued, and the cables are now enclosed in wooden boxes nailed to the Fig. 110. METHOD OF SUPPORTING CABLES AT MIDDLETON COLLIERY. stays. At intervals of about 150 feet a piece is scooped out of the box about the size of a hen's egg, and at that point a swelling is worked on Fig. 111. METHOD OF SUPPORTING CABLE IN SHAFT. the cable by tape and varnish, and thus the cables are supported and the arrangement works satisfactorily. Another method of supporting the cable in a shaft is to tie it to **UNDERGROUND CABLES** 149 Insulators spaced, say, 10 yards apart in the shaft, the loop of the cable being taken round a large insulator at the top of the shaft. Another method is to enclose the cables in iron pipes and run in melted pitch to completely fill up the space. Where the cables are run in pipes they may be supported by a clamp contained in a box, as shown in fig. 111. This method, however, is not to be recommended, as it is inconvenient in case of repairs being necessary. Carrying the cables through a shaft is awkward. The most usual plan of carrying the cables in the mine is to have one on each side of the road and hang them loosely between the insulators. By this means any fall of the roof catching the cable would pull up the slack in the cable without causing a fracture. Fig. 112 shows three forms of insulator very commonly used, the cable being secured to the insulator by a piece of yarn. **FIG. 112** A diagram showing three types of insulators. **VARIOUS TYPES OF INSULATORS.** Mr. Robert Hay, of Stanton Colliery, Burton-on-Trent, has designed and patented the pot insulator shown in fig. 113. This is a useful form, and the cable is very expeditiously laid along the road. Fig. 114 shows a metal pipe supporting cable by a piece of galvanised wire hung over a nail. The cable in this case is a concentric one with a bare return, so that insulators would not be needed. In many cases insulators are dispensed with and the cables are fastened to the props by leather strips fastened round them and nailed, or by tabbard fastened to nails; these are expedientious ways of fixing a cable. Another plan is to screw some wooden cross pieces into the ground as shown in fig. 115, and attach the cable to these props. In main roads the cables are sometimes buried where there is no danger of the insulation being corroded. Mr. Maurice Deacon¹ carries out this principle by placing three-phase cables in a puddled-clay trench in the middle of the road on an engine plate in a steep place. The cables employed are well armoured and waterproofed. --- ¹ Transl. Fed. Inst. M.E., xxxii. 26a et seq. 150 ELECTRICITY AS APPLIED TO MINING The importance of covering up the cables in a road which is being repaired should also be realised. In main roads which have been driven some time, and in which all movement of the strata has subsided, the cables may be laid in a more permanent manner on Callender's system (see figs. 115 and 116). Fig. 113. A diagram showing a cable with a metal cover. Hay's Patent Insulator. METHOD OF CARRYING CABLE ON UNDERGROUND ROAD. Earthenware, wood, or metal troughing is used in which to lay the cable, which is supported at intervals on bitumenised wood bridges, and the trough is then run in solid with bitumen. The cables used in connection with this system are insulated with vulcanised bitumen sheathing, and the Fig. 115. A diagram showing a cable laid in a trough with bitumen sheathing. Callender's System of Laying Cables in Cast-Iron Trough. only conductor of electricity in the cable is its copper core. The makers claim that it is impossible for leakage currents or stray earth return currents o get to the sheathing, as is the case with a lead-covered cable. The bitumen has, of course, to be melted, and this forms a possible CALLENDER'S SYSTEM drawback to the use of this system in the roads of a mine, although there will no doubt be many mines where the system could be used. Fig. 116. A diagram showing a circular device with a central wheel and spokes, connected to a horizontal pipe. CALLENDER'S SYSTEM OF LAYING CABLES IN EARTHENWARE TROUGH. It is convenient in a long length of cable to have it divided up into sections, as by this means the testing of the cable is greatly expedited Fig. 117. A cast-iron junction box with two cables entering from opposite sides and exiting from the top. CAST-IRON JUNCTION BOX. At each of these sections the cables would be enclosed in a gas-tight junction box securely fastened. A cut-out might also be placed in the box. Figs. 117 and 118 show a three-way junction and fuse box made by 152 ELECTRICITY AS APPLIED TO MINING Messrs. John Davis & Son, Derby ; it will be seen that the cables are con- centric, with galvanised outer conductor, and that the outer conductors are connected to the iron box, but the inner copper conductors are insulated. Fig. 118. Three-way cast-iron junction and fuse box, with cover removed. Fig. 119. Cable entering switch box. from it. A good method of making a gas-tight entrance for a cable into a junction or switch box is shown in fig. 119.¹ ¹ Electric Haulage at Manvers Main Colliery, by A. T. Thompson; Transactions Institution of Mining Engineers, xx. 39. 153 CHAPTER VIII CENTRAL ELECTRICAL PLANTS Central Electrical Plants for Winding, Ventilating, Pumping, Hauling, Coal-eating, Workshop, Screen, &c.—Competition of High and Low Voltage as affecting Cost of Cables—Three-wire System—Estimates of Costs. DURING the last few years Acts of Parliament and Orders in Council have been obtained, authorising large central electric power stations, intended to distribute electric power over whole counties, such as the Derbyshire and Nottinghamshire Electric Power Act, 1901 ; the Yorkshire Electric Power Act, 1901 ; the Lancashire Electric Power Act, 1905 ; the South Wales Electric Power Act, 1906 ; the Somerset and Gloucestershire Mond Gas Act, 1907, and the Cornish Electric Power Act, 1902. It is intended under these Acts to put up central electric stations, each containing, say, from 5,000 up to 50,000 h.p. When these come into work the power obtained from these stations may be used by mining engineers and others who require electricity because they do not own electric generating stations. But in the meantime, engineers are erecting central generating stations at mines, intended to supply the power required for most of the machines at one mine. At a colliery there are often thirty or forty small steam-engines, nearly all of them of wasteful construction and working inefficiently, and it is interesting to see how these small steam-engines by electricity can increase the electricity being generated by either steam- or gas-engines which use fuel in the most economical way. In order to understand how such electric transmission can be economical, the great difference in the steam consumption per I.H.P. in various classes of engine must be considered. With steam at 180 lbs. pressure and with a boiler capacity of 25 lbs., a steam-engine will consume 12 lbs. of steam will generate 1 I.H.P. for an hour. A small steam-engine, as commonly used at a colliery, with a long length of steam-pipe from the boilers will probably use at least ten times as much steam per I.H.P., or 120 lbs., and this quantity is probably greatly exceeded when the steam-engine is a colliery. All the engines, big and little, is probable where more than ordinary attention is given to steam economy, about 80 lbs. of steam per I.H.P. There is here a great margin for economy. Assuming that the combined efficiency of the generating steam-engine and dynamo is only 70 per cent., and that of the 154 ELECTRICITY AS APPLIED TO MINING motor is 80 per cent. (these low efficiencies being taken to allow for working at small fractions of the full power on the one hand, and at excessive powers on the other hand, and also for loss when running without load), and the loss in the conductor is 5 per cent, we have the following result : $x \times x = 100$ ; and deducting 5 per cent of 70, or 35, we get 525 per cent. of the engine-power delivered on to the machine. It will, however, be advisable to make further allowance for loss on small conductors and contingencies equal to, say, 5 per cent. of the engine-power, thus reducing the net efficiency to 50 per cent. The amount of current required by the motor would then be as $50 : 100 :: 12 : 24$. It would not, however, be wise to assume that the engine of the electric-generating plant would always work at the very high efficiency of 12 lbs. of steam per I.H.P.; but we might safely assume that the consumption would not exceed 20 lbs. per I.H.P., and at that rate the cost of steam upon the machine would be about half of what it would be with 12 lbs. By this means the fuel consumption should be brought down to, say, half or one-third of what it is with ordinary steam-engines and long-distance steam transmission. The question, however, that next arises is the extent to which electric motors are applicable in the case of large engines, such as winding, pumping, and similar machines. We may here state that the electric motor is applicable to all small engines, and to large engines which run continuously, such as fan engines and pumping engines ; but when we come to hauling and winding, the advantage or otherwise of electrical driving requires special consideration in each case. If the haulage is done by endless ropes, it is possible to keep them at a uniform speed or ten hours a day at a uniform speed, and then the electric motor would be eminently suitable. But if the haulage is at high speeds, and continually starting and stopping, the advantage of the electric motor is less apparent, but still in most cases it might be used with economy. The large colliery winding-engine hands by itself, and as a general rule it may be said that where it is not intended to be advantageous to substitute an electric motor for the steam-engine. These engines work up to 1,000 and even 1,500 h.p., but this high power is only continued for a few seconds. The cycle of work in a winding engine is as follows : It is started, gradually increases in speed, reached its maximum speed in a few seconds, gradually slows down until rest within one minute, and if it were driven from a central electric station the generators at that station would have to be equal to this sudden strain and equally sudden stoppage ; and it would probably pay the mining engineer better to endeavour to make the steam winding \footnote{The recently introduced Siemens-Tigerman system overcomes this difficulty (see p. 270).} ESTIMATE FOR GENERATING STATION 155 engine economical than to substitute an electric motor for it. In the case of small winding-plants the matter is different. A central station which would not be equal to supplying 1,000 h.p. at five seconds' notice might easily supply an extra 50 or 100 h.p. for a few seconds. It is possible that some day continuously working lifts may be substituted for the ordinary colliery winding-engine, and for such lifts electrical transmission is eminently suitable. In the meantime electrical winding is done in cases where there is no central electric station of sufficient power, and a winding plant has been constructed with an electric motor of 1,500 h.p., but this at present is an unusual size. The mine manager must always bear in mind that his electrical transmission must be justified by two of reasons, i.e. not by both. One is economy of capital power, the other is convenience of work. For underground work, convenience is often of itself a sufficient reason ; for surface work convenience is not often a sufficient reason for adopting electrical transmission on a large scale. It is therefore necessary that economy should be proved. For this purpose, a trial estimate must be made of the capital outlay in each case, and also of the working costs. The following table shows the cost of the equipment of a 300-h.p. class electric generating station of 2,000 h.p. Steam pressure in boilers, 180 lbs.; boilers provided with super-heaters; three compound condensing-engines, each of 1,000 I.H.P., providing for one engine to be stationary, ready for use in case of accident to either of the other two; on each engine is a three-phase generator delivering current at 330 volts. The engine house contains a switchboard where the various electrical currents are collected and distributed. Boilers and fittings, super-heaters, steam-pipes, dampers, and feed-pumps erected complete, say, ten boilers, eight to be at work at once time, at £950. Boiler-seating, flues, chimney-roof and drains, at £150 a boiler. Three sets of generators each of 1,000 L.H.P., with condensers, air-pumps, circulating pumps, and all necessary pipes and lubricating arrangements erected complete in full working order, at £4 per h.p. Three generators, each of 900 E.H.P., erected complete at £675. Electric conductors, switches, switchboard, and all electrical appliances required for generating station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,000 Engine-house, engine foundations, and all buildings required ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ...... 3,000 Cooling pond ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... ..... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... 2,000 Or, in round figures, £20 per I.H.P. actually at work. £39,750 * The cost of land is not included.*
Boilers and fittings $950
Boiler-seating $150
Flues
Chimney-roof and drains
Generators $675
Electric conductors
Switches
Switchboard
Electrical appliances $2,000
Engine-house $3,000
Cooling pond $2,000
156 ELECTRICITY AS APPLIED TO MINING This electric plant is to supersede a steam-distribution plant, and we will assume for the sake of argument that the cost of the steam pipes and steam motors will be about equal to the cost of the electric-distribution conductors and the electric motors. Assuming that the economy of steam by means of the electric distribution is 50 per cent, we shall require for the steam-distribution plant the following outlay for boilers :
Twenty boilers at £200 £2000
Seating, chimney, and roofing for two boilers, at £350 7000
Total £15,000
Deducting this from the outlay on the electric plant, we have an additional outlay for the latter of £14,750. **Working Costs.**—1. Electric Plant.—Assuming a consumption of steam of 20 lbs. per I.H.P. on an average of 1,500 h.p. for 10 hours a day, and of 80 h.p. for 14 hours a day, a total steam consumption per diem of 324,000 lbs., assuming also that 3 lbs. of fuel will produce 20 lbs. of steam, we find that the cost of fuel for 35 days would be £35. This fuel may be valued at about 4s. per ton on an average; at any other mine the price will be increased by the cost of conveyance from the colliery. We have, therefore :
Fuel, 35 tons at 4s. £14
Stokers' wages, about 1s. per ton of fuel 115
Engine-men, cleaners, and electrician, 40s. a day 20
Oil and sundries 10
Total £115
The cost for the above work of 26,500 I.H.P. hours would be about £14 per I.H.P. hour. The repairs and renewals to maintain the plant in perpetuity should not exceed one per cent on the whole cost, or, say, £3,975 a year = £4 per I.H.P. hour (allowing 300 days in the year), or a total cost of £1875. 2. Boiler Plant.—The boiler plant will use twice as much fuel, or, say,
Fuel, 70 tons at 4s. £14
Stokers' wages, about 1s. per ton of fuel 310
Foremen 10
Total £18
showing a difference in favour of the electric plant of £65. Taking 300 days in the year, and an economy of £65 a day, we have a total saving of £1,875 per annum, which is about £18 per cent upon CHOICE OF ELECTRIC CURRENT 157 the extra cost of the electric plant. The above is not given as an estimate which represents actual facts, because these will vary from day to day with the market price of machinery, and with many other circumstances applicable to each case; thus the electric plant might be erected for a great deal less than the figure above given, and so also might boilers for the steam plant. Also, the economy of fuel expected as a result of putting up the electric plant may be greater or less than that indicated by the fuel which the steam plant would consume, and, on the other hand, it is conceivable that a steam plant might be erected and managed with such skill as to reduce considerably the possible economy to be gained by the substitution of electrical transmission. If instead of a plant of 2,000 H.P. we take any smaller or larger figure for the purposes of comparison, it will only be necessary to divide or multiply the above estimates, as the case may be. If, as is probable, the steam-transmission plan consumed three times the steam used by the high-class electric plant above indicated, then the capital outlay for the steam plant would be £35.100, the working cost per centia would be £1.15.15.£15.52., and for 300 days at a total of £45.575. Description of Electrical Current.—When erecting a central electric power station, after deciding on the purpose and size of the installation, one of the first questions that arise for determination is the kind of electrical current to be used. In alternating current, the latter being divided into single-phase, two-phase, and three-phase currents; next question is the voltage, or electro-motive force; and the third question, if alternating current is adopted, is the periodicity, or the number of periods per second. Continuous Current.—Hitherto most installations elected for the purpose of generating electric current for transmission of power in England have used continuous current. It is also largely used in every country ; in fact, it is only since 1890 that alternating-current motors have come into favour. Previous to that date alternating currents were chiefly used for lighting installations, particularly for long-distance transmissions, partly owing to their simplicity in construction and partly to their ease of transformation. Up to the year 1890 the makers and users of electric power were trained almost exclusively in the manufacture and use of continuous-current machines; but at present time the number of makers of alternating-current motors is so large that the user is free to adopt that system if he prefers it to continuous current. The advantages of continuous current are numerous; they are amongst them that it is far easier with which two (or more) motors can be operated in series if required. This is one of the reasons why continuous-current motors are almost exclusively used on tramway cars. These cars are fitted with two motors, each about 158 **ELECTRICITY AS APPLIED TO MINING** 25 h.p. When starting, a low speed is desirable, and this is produced by putting the motors in series, which has the effect of reducing the voltage to one-half. A resistance is also put in at the start ; this still further reduces the voltage and the speed. As the speed increases the resistance is taken out until the car has reached half-speed. The motors are then placed in parallel, the resistance being put in again until the car has nearly reached full speed. In this case, the voltage across each motor is less than that of a continuous-current motor is proportional to the voltage, and if two motors are placed in series on any given circuit, the voltage on each motor is reduced to one-half. A reduction in speed is obtained by placing a resistance in the circuit, which reduces the effective voltage on the motor, but this resistance should be placed between the motor and the supply, so that its effect is reduced by passing the current through lamps; but as a rule this method of reducing the speed is only applicable to small motors, as the number of lamps required in any one place underground only absorbs a small amount of current. It is possible, but hardly ever practicable, to place three or more motors in series, and so still further reduce the speed. Another advantage of the continuous-current motor is that the current in a single armature of dynamos may be taken from the current main without any difficulty arising from variations in the size and description of the dynamos or engines driving them, the only requisite being that each dynamo shall produce a voltage equal to that of the others, whereas in an alternating-current instal- lation three different types of generators must be used, each not only produce current at the same voltage, but must each have the same periodicity as the others. A third advantage is that only two conductors are required—one for the outward and the other for the return current, whereas with a three-phase installation three conductors are necessary; though it does not follow that three-phase currents will necessarily be more expensive than the two. A fourth advantage claimed for continuous current is that arc lamps will give a better light with this current than with an alternating current. The chief objection to continuous-current motors is found at the com- mutator, because here there is generally some sparking at the brushes. This may be very little, but a little spark would be enough to cause an explosion in the presence of an explosive mixture of gas and air. Various devices are used to remove the danger (see page 71). On the Continent, three-phase alternating current is often preferred to the continuous current in coal mines, on account of the danger supposed to lie in the commutator. The characteristics of different types of generators and motors are discussed in some detail on page 65, and **Alternating Current.** The ordinary alternating current, which is single-phase, is not so suitable for general application to motors as two-phase or three-phase, owing to the difficulty of starting a single-phase motor under THREE-PHASE SYSTEM 159 load. The two-phase current is considered good for motors, and is adopted at some important central stations, where both lighting and power are required. For the purposes of lighting, the current may be taken from the conductors attached to one phase, and then when it is desired to drive a motor the current can be taken from the conductors attached to the two phases (see page 110). The convenience of this system has caused the electric-lighting companies to adopt it for illumination, and also of the Sheffield Corporation, to adopt the two-phase system. The three-phase system has, however, the greatest number of advocates. It is considered that the torque is more uniform. In comparing three-phase with two-phase or single-phase, it is generally considered that the weight of copper in a three-phase circuit is only about one-third that in either of the other two alternating-current systems, and that this advantage applies to most mining installations. It is sometimes claimed for the three-phase system that less copper is required in the conductors than in the continuous-current system. If equal virtual voltages are taken and a power factor of 85 per cent. for the three-phase, there is a saving by three-phase current of 19 per cent. If, however, equal maximum voltages are taken (equalising rate of breakdown of insulation between the conductors), the three-phase current requires 33 per cent. more copper than continuous current. The chief advantage claimed for three-phase in mining work is the absence of commutators, and consequently of sparking at the motors. Another advantage claimed for three-phase over continuous-current machines with two-phase and single-phase is the facility which it offers for high-tension transmission. The majority of manufacturers of electrical machines find it easier to make alternating-current generators of 1,000 volts and upwards than to make generators of equal voltage on the continuous-current system; although there are makers of continuous-current machines which will run at 600 volts and upwards without sparking. These latter, however, are the exception, and most makers prefer alternating current for any dynamos or motors over 600 volts. High-tension currents—that is to say, currents exceeding 600 volts—are inconvenient for use with small motors, and are seldom taken into a mine, and still lower voltages are preferred. The cost of transmission is increased by long distance—say, exceeding three miles—a high tension is necessary in order to keep down the cost of conductors. If the transmission is on the surface, and bare conductors can be carried on poles, the economy resulting from the high voltage is roughly speaking, inversely proportional to the voltage, because, with 10,000 volts, the current (ampere) is only one-twentieth part of that at 100 volts; but with 10,000 volts per conductor instead of 10,000 volts is much less than one-twentieth of the weight in the 50-volt circuit. In fact, if the transmission is for a long distance, with the same percentage loss of power, the size of copper would only be one four-hundredth of that 160 ELECTRICITY AS APPLIED TO MINING required for 500 volts. For short distances, however, as will be seen later, this additional reduction is not possible on account of the high current density involved and consequent heating of the conductor. With a high tension more expensive insulators are required, and in other directions there will be increased expense. The high voltage, however, as a rule cannot be used in the motor or lamp before it has been transformed to a lower tension, though large motors on the polyphase system are often run at 2,000 volts. This can be readily done with a static transformer, and this is another of the advantages of the alternating current over the continuous current, because for the reduction of voltage in a continuous current a rotary transformer is necessary, which is much more expensive than a static transformer. The loss in voltage in a static transformer is not large, varying from 3 to 8 per cent. But taking into account this loss, and the cost of the transformer, it is not advisable to introduce high-tension currents into a mining installation unless the distances are very great. Voltages of voltages in common use are between, say, 50 as a minimum and 40,000 as a maximum. The 50 volts is suitable for the arc light, incandescent light, and for small motors. For an installation where the distance from the generator to the furthest lamp or motor did not exceed 300 yards, a tension of between 50 and 60 volts would be very suitable, unless a large power had to be transmitted. For motors of 1 h.p. up to, say, 4 h.p., it may be well suitable to use lamps and larger motors. The advantage of the low voltage for lighting is, that as open type arc lamps should not be supplied with current at more than 50 volts, if the voltage is higher they have to be placed in series, but if only 50 volts is in the mains then the arc lamps can be placed in parallel, and switched on or off independently of each other. In such cases it is usual to use lamps used with higher voltages (see page 230). The so-volt pressure is, of course, suitable for incandescent lamps, which are made to suit any voltage not exceeding 250, but a low voltage is advantageous for incandescent lamps, because, the lower the voltage, the thicker is shorter is the carbon filament. As the distance from the generator increases, so must the voltage at the generating station be increased; thus, at a mine where it is expected that a considerable power (say, 40 E.H.P. and upwards) will have to be delivered for a total distance from generator to motor of say, one mile, it would not be advisable to adopt a lower voltage than 400. If the distance were likely to be two miles, it would be advisable to increase the voltage; but if the distance is greater than two miles, the question becomes less simple. Assuming that 600 volts at the motor is the highest voltage that can be conveniently used in or about a mine, then if a higher voltage is used there must be a transformer in the vicinity of the motor to reduce the current to a convenient ECONOMY OF HIGH TENSION 161 voltage. The cost of the transformer would be equal to a considerable cost of main, so that sooner than adopt a high tension and transformers, it might be found advisable to spend money on enlarging, or adding to, the mains. For instance, a high-tension concentric cable, insulated with vacuimised inner-ribber, consisting of an internal cable of thirty-seven copper wires, each 15 S.W.G. (=14½ square inch) insulated, and surrounded with a return cable, also of thirty-seven wires 15 S.W.G., and insulated, would have a circumference of about 300 feet per mile. A similar cable, of which the internal copper conductor had thirty-seven wires No. 20 gauge, having an area one-fourth that of the large cable, would cost about £400, allowing for a higher voltage being used and therefore a higher quality of insulation, or half that of the large cable, therefore the extra cost of the smaller cable would be about £200 per mile, say £400 a mile, and if the distance was three miles the total extra cost would be £1200. Assuming that a current of 500 amperes per square inch of sectional area might be sent through this cable (37/17), the current that might be expected would be about 74 amperes, as its area is rather over 1 square inch. The current (say, 74 amperes) multiplied by 10 volts would give 740 E.H.P., and assuming an 80 H.P. static transformer, to transform an alternating current from say, 1600 volts to 400 volts, and suitable for 40 E.H.P., would cost about £200, including erection and chamber, showing that it would cost less money to put down a small high-tension cable and transformer than to put down a large cable to carry a current of 500 amperes per square inch. It must be remembered that the question of convenience would be more important than the difference in the cost at above given. In some cases the high-tension system with transformers at every branch or motor would be convenient; in other cases a low-tension system and large cables without transformers would be better. The table here given indicates the method of calculating the cost of each system. When we come to distances of five miles and upwards there can be no doubt that a high-tension system should be adopted. **Reasons for Economy of High Tension.** —As stated in Chap- ter I, power is measured by the product of the volts and amperes, and is measured in watt. One volt multiplied by 1 ampère equals a watt, and 746 watts are equal to 1 E.H.P. The size of conductor required depends on the current, which is measured in amperes, and if a conductor is of suitable size for any number of amperes—say, 74 —does not matter what the voltage is. But if the voltage is the motor end of the line or the motor end of the line is at zero potential or zero volts; if the voltage is great, the horse-power may be great; thus, the horse-power may be increased to any reasonable extent without increasing the size of the conductor. If the conductor is carried in the air, and therefore does not M 162 ELECTRICITY AS APPLIED TO MINING require any insulating covering, the increase of voltage adds very little to the cost of the conductor, such increase of cost as there may be being simply due to the improvements in the insulators on the posts carrying the cable, and to precautions that may be taken to prevent accidents through unauthorised people tampering with the cable. Thus, a cable of thirty-seven wires, 15 S.W.G., suitable for 74 amperes (at 300 amperes per square inch) at 1,000 volts, will carry 1,000 amperes at 1,000 volts 100 h.p., at 10,000 volts 1,000 h.p. and at 20,000 volts 5,000 h.p. When we come to insulated cables laid in the ground at the surface, or fixed to a shaft side, or laid along the passages of a mine, the advantages of very high tension are not so great, because the higher the tension the more liable is the insulation to be broken down by the greater pressure of the insulating material must be $\frac{1}{\sqrt{3}}$ inch for each thousand volts ; thus with a voltage not exceeding 500 the thickness is $\frac{1}{\sqrt{3}}$ inch, and for a voltage exceeding 500 it is $\frac{1}{\sqrt{3}}$ inch up to 2,000 volts for small conductors ; but for a conductor of thirty-seven wires, No. 12 S.W.G., the thickness of the insulation is $\frac{1}{\sqrt{3}}$ inch. Outside the insulation there is a thickness of tape, braiding, or paper-covered wire, and then a sheath that may be armour, made of iron or steel wire tapes. Where paper-covered cables are used, the insulated cable is drawn into a lead pipe, and an increase in the thickness of the insulating material of course increases the diameter and cost of the lead pipe. The higher the tension, the greater the care in the management of the dynamo and the cables. This, added to the high cost of the insulation, the extra cost of appliances at the stations, and the high technical training necessary in the operators for managing very high-tension currents, so reduces the advantages of high tension that some engineers, even for a transmission of ten miles at a voltage of 5,000 volts or lower to any higher tension. With regard to insulating cables in mines and shafts where passages are laid, the extra cost of insulation from that suitable for, say, 50 volts as a minimum up to 600 volts as a maximum is so incon siderable that it may be practically disregarded, and therefore the full economy of the higher tension is obtained within these limits. In order to obtain the resistance to which current can be carried through cable of a given size, the following rules must be borne in mind : I. The loss of voltage, other things being equal, and consequently of power, is exactly proportional to the length of the cable, and consequently the loss on a three-mile circuit is three times the loss on a one-mile circuit ; and so on. The resistance of a circuit from one point in this circuit from the generator to the motor, and back again to the generator, has to be calculated, except in those cases where there is an earth return. But an earth return is not usually admissible; therefore, if the distance from the generator to the motor is one mile, the length of the circuit is two miles. ECONOMY OF HIGH TENSION 163 II. The loss of voltage on any given length and size of cable is propor- tional to the current as measured in amperes—that is to say, the drop in pressure between the positive and negative mains at the generator, assuming that there is no resistance in the circuit other than that of the cable, will vary with the number of amperes, or current passing through the cable. Although the motor is only at a distance of one mile from the generator, yet the heat generated by this current must be considered. It is not only the re- istance of the mile of cable from the generator to the motor, but also the mile of return cable from the motor to the generator, because the pressure required to force the current back from the motor to the generator is equivalent in effect to the back pressure in the cylinder of a steam-engine. Thus, if 100 amperes pass through a circuit carrying 100 amperes (more exactly 74.6 amperes) at 60 volts, or 60 E.H.P., and the drop in voltage, say, a mile of conductor were 5 per cent. or 30 volts, then if the motor were half a mile from the generator, the circuit therefore being one mile, the voltage at the motor would be 50—30=20. If the amount of current should be doubled, or 150 (149.4) amperes sent be through the same mains, then the drop in voltage would be 60 per cent. and will be 60 volts, and the effective pressure at the motor will be 340 volts. III. The loss of power on any given circuit is proportional to the square of the current—that is to say, with a current of 200 amperes there will be four times as much power lost as with a current of 100 amperes, because of this doubling of power. In discussing the size of a cable for transmitting electricity, the first consideration is how many amperes can be carried without heating the conductor so as to injure the insulating material with which it is covered. The second consideration is how much of the power is lost or wasted in heating up the conductor. The third consideration is purely commercial: the larger the cable, the more it costs; and the smaller the cable, the more power is wasted; and the more power that is wasted, the larger must be the engine and dynamo at the generating station to produce the required power at the motor. In order to see how the matters work out, we will take a concrete example, say, 100 E.H.P. (e.g. 846 amperes), and let us assume that we have a current flowing from the generator, the motor working 10 hours a day for 365 days. Multiplying 100 x 10 x 365 we have 365,000 h.p. hours of power. If we value the horse- power delivered at 1d. per hour, we have 365,000 = £1,250. If we allow a loss of voltage of 10 per cent. of that at the motor in the circuit we shall require a drop in voltage of £125 per mile. For each mile a cable of 91 square inches has an area of .76 square inch. The current density in this cable will be 245 amperes per square inch. We will value this single-conductor cable at £990 per mile, or, say, £3,560 for the circuit, and the cost of fixing it, including the cost of putting it down the shaft and along A diagram showing a simple electrical circuit with two parallel paths: one labeled "Generator" and another labeled "Motor." The diagram includes labels for various components such as "Current," "Voltage," "Resistance," and "Power Loss." There are also annotations indicating calculations like "If 100 amperes...," "If电流 doubled...," and "The loss of power on any given circuit is proportional to...". The bottom part shows calculations: "If电流 doubled...," "The loss of power on any given circuit is proportional to...," "The larger...," "The smaller...," "The more power that is wasted...," "The more power that is wasted...," "The larger must be...," "In order to see how...," "We will take a concrete example...," "If we value...," "If we allow a loss of voltage...," "For each mile...," "The current density in this cable will be...," and "We will value this single-conductor cable at...". The bottom right corner shows calculations: "If电流 doubled...," "The loss of power on any given circuit is proportional to...," "The larger...," "The smaller...," "The more power that is wasted...," "The more power that is wasted...," "The larger must be...," "In order to see how...," "We will take a concrete example...," "If we value...," "If we allow a loss of voltage...," "For each mile...," "The current density in this cable will be...," and "We will value this single-conductor cable at...". The bottom left corner shows calculations: "If电流 doubled...," "The loss of power on any given circuit is proportional to...," "The larger...," "The smaller...," "The more power that is wasted...," "The more power that is wasted...," "The larger must be...," "In order to see how...," "We will take a concrete example...," "If we value...," "If we allow a loss of voltage...," "For each mile...," "The current density in this cable will be...," and "We will value this single-conductor cable at..." 2 164 ELECTRICITY AS APPLIED TO MINING mines, at £150, making a total of £4350. The power of the generator, being 10 per cent. in excess of that of the motor, is 110 E.H.P., and the I.H.P. of the engine being, say, 25 per cent. more than the E.H.P., is 137. Taking the basis of cost given on page 155 for generator, engine, boilers, condensers, &c., at £250 per I.H.P., we have the cost of the generating station, 137 x 250, or £34250. If instead of a loss of 10 per cent. we had a loss of 20 per cent. in the cable, the cost of the cable would be reduced, because it would be less than half its value when its loss was 10 per cent., and £2480 for the four-mile circuit. The cost of erection would be practically the same, say, £400, making a total of £3880; but the power at the generating station would be increased to, say, 120 E.H.P. +25 per cent., making a total power of 156, which at £250 per I.H.P., brings the cost of the generating station up to £3900. The loss in working would be one-tenth of 20 per cent., and the loss in working is one-tenth of the power actually used at the motor. One-tenth of £1250 is £125. Where the 20 per cent. loss was sustained the loss is one-fifth of the utilised power. One-fifth of £3850 is £770. The saving by having only a 10 per cent. drop is £25 a year. If instead of reducing the condenser to half its size we have a total cost of £830 (see Table L, Column C), as compared with £4350 as in the first instance; the loss in transmission would be only 5 per cent. of the power at the motor: the E.H.P. at the generating station would be 105; the I.H.P. would be 131; and the cost of the generating station £6420. The loss would be one-twentieth of £1250, or £62 10s. a year. The above calculations, and some others, showing different percentages of loss in transmission, and distances, are given in Table I. It will be seen in this table that the amount of drop in voltage, or loss of power in transmission, has to be regulated, in the shorter-distance transmissions, not by consideration of the cost of cables, but by the question of heating; that in the shorter transmissions where the losses are larger than would have been necessary if the question of cost of transmission only had been considered. The size of cables of which the cost is given in the table is also slightly modified in order to use standard sizes of cables; these figures have been averaged from makers' lists, and do not necessarily represent the actual cost of cables, which might to some extent be modified by special quotations. In Table I., in Columns A, B, and C, transmissions of half a mile, one mile, and two miles, with a voltage of 400 at the motors, are compared, and with losses of 5 per cent., to per cent., and 20 per cent for the total distance of one mile; and with losses varying from zero to 20 per cent. temperature. We have also, in Column D, F., G., and H., transmissions of half a mile, one mile, two miles, and three miles, with a voltage at the motor of 600, and a total loss in each case of 5 per cent., to per cent., and 20 per cent., except as modified by temperature considerations. The cost of the from 500 Volt to 1000 Volt
in. Two
1 1
5 5
100 100
1000 1000
1000 E 1000 E
1000 L 1000 L
400 400
400 400
632 632
632 G+L+H 632 G+L+H
























































































<
in. No. qixs
18 18
240 240
360 360
5750 5750 E
1299 1299 L
1299 L 1299 L
248 248
Lag (inv h.p.
at No. 6847
or g
no ppr
or--The pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HEATING OF CABLE 165 generating station and cables (including erection) is given in each case; but the insulators and all electrical accessories &c., at the motor station are not included. The value of the power lost is taken at 1¢ per E.H.P. per hour, which no doubt in many cases exceeds the cost of production. Re- ferring to Column B, with 400 volts at the motor, and a transmission of one mile from generator to motor, we can compare the figures where there is a loss of 10 per cent. and 20 per cent., and we find in the latter case an in- creased cost of £283, and that the first cost of £283 + £283 is 40 per cent. greater than the first cost of £283. In other words, if the additional cost of it would be repaid in a little over three years with interest, or, looking at it from another point of view, the extra loss in the 20 per cent. installation is equal to a 10 per cent. interest on the whole cost of the generating station and cables. Considering this matter with a per cent. loss, we have a saving in the latter case of £283 a year working cost ; but the extra capital outlay is £610, and the saving of £283 a year is under 10 per cent. on the increased outlay. It is evident, therefore, that the per cent. installation is not worth the extra expense. Heating of Cable.—The loss in transmitting the current of elec- tricity is represented by heat generated in the wire. In an insulated wire the insulation is liable to damage by heat, and in any case the resistance is largely increased if a high temperature is reached, and therefore it is essential for safety and economy that no excessive current should be con- tinued for long periods. The amount of heat generated in the conductor will show the amount of power wasted. This may be repre- sented by the following formula in horse-power : $$\text{h.p.} = \frac{\text{C}^2R}{746} = \text{C}^2 \times \text{R} \times 0.00134$$ In the above, C represents the current in amperes, and R represents the resistance in ohms. For the sake of example, let us suppose a copper con- ductor, exactly 1 square inch in area and 1,000 feet in length. Then R, the resistance in ohms, will equal "0083". Let us now suppose a current of 1,000 amperes. Then, $$\text{C}^2 = 1,000 \times 1,000 = 1,345$$ Multiplying this figure by "0083" we get 11766, or a loss in round figures of 11 h.p. This represents, perhaps, a very small percentage of the power passing through the main. One thousand amperes at a voltage of 746 equals 1,000 h.p., and 1172 is a little over 1 percent. If, however, the cable, instead of being only 1,000 feet in length, were a mile in length, the resistance would be increased to about four times its former value; so that the loss would be increased more than fivefold. In round figures, the loss would be about 6 h.p., or 6 per cent.; and if the complete circuit were two miles long the loss would be 120 h.p., or 12 per cent. If the conductor, instead of being 166 ELECTRICITY AS APPLIED TO MINING exactly 1 square inch, had been half the area—that is to say, 1 square inch—the resistance would have been exactly double, and the loss would be exactly double; therefore, instead of being 12 per cent, as above, it would be 24 per cent of the power generated, or nearly one-third of the power delivered. But a current density of 1,000 amperes per square inch is as large as it is considered wise to take, and it is often thought advisable to take a loss of only 10 per cent (in the case of copper), but suppose there is a two-mile circuit (the motor being one mile from the generator) with a drop of 10 per cent, the number of amperes in the cable is 1864, the cross-section of the cable is 41 square inch, and the density of the current is 455 amperes per square inch. In the preceding examples of 1 per cent loss, and 2 per cent loss, the size of the conductor is calculated after the following manner. The motor is assumed to be one mile from the generator ; the E.H.P. delivered to the motor is 100; the voltage at the motor is 400, and the drop on the cable is 40, making the voltage at the generator 440. The work done at the motor, if taken in watts, is 100 X 746, or 74,600 watts. Dividing this by 400 we get 1864 amperes. Now, if we assume that we have the amperes—1864. The drop, being taken as 10 per cent of the voltage at the motor, is also 10 per cent of the power at the motor, or one-tenth of 74,600 watts—i.e., 7,460 watts—and the loss of power in watts due to the drop on the line is shown by the formula: $$\text{Loss in watts} = R \times C^2 \text{ (in amperes)}$$ In this case, $7,460 = R \times (1864)^2$, or, $$R = \frac{7,460}{(1864)^2} = \frac{7,460}{34782} = 214$$ ohms. $$R = \frac{7,460}{(1864)^2} = \frac{7,460}{34782} = 214$$ ohms. $$R = \frac{7,460}{(1864)^2} = \frac{7,460}{34782} = 214$$ ohms. $$R = \frac{7,460}{(1864)^2} = \frac{7,460}{34782} = 214$$ ohms. It was stated above that the resistance of a square inch of copper 1,000 feet in length was 0.082 ohm; and do the following sums in proportion we shall get the required sectional area of the conductor. As $$\text{Area} = x \times y$$ $$x = \frac{7,460}{(1864)^2} = \frac{7,460}{34782} = 214$$ In the above manner the sectional area of any conductor can be calculated. An easier way of doing this is to remember that with a current density of 800 amperes per square inch the drops in voltage in a copper conductor is 2 volts per 100 yards, and that the current density is proportional to the drop of voltage, and inversely proportional to the length of cable. Thus with a drop of 40 volts in two miles, the current density MOST ECONOMICAL LOSS 167 equals $800 \times \frac{100}{2 \times 1,760} = 454$ ampères per square inch; therefore, the area of cable required to carry 185'6 ampères is $185.6 = 41$ square inch. Referring to the examples given on pages 103 and 164, of a motor or motors of 100 E.H.P., two miles distant from the generator, and the comparative cost, the results, as seen in the table, are very different if the distance of the motor from the generator, instead of being two miles, is reduced to half a mile, and with the same percentage of loss in each case the size of the conductors is also quartered. In dealing with insulated cables the cost of labour always predominates over that of the conductor, and the cost of fixing in the ship will be about the same in each case. We need not consider here the case of 20 per cent. loss, since the current density in this case comes out to 1,820 ampères, which is much too high. The current density with a 10 per cent. loss—namely, 910 ampères—is also too high, so that instead of 10 per cent. we must take a loss of 8.5 per cent., which gives us a total cost of £3.369 for installation. The cost of laying out of cables and erection for 8.5 per cent. and 5 per cent. loss will be £696 and £870, and adding the cost of the generating station, the total costs are respectively £5,356 and £5,490. Here it appears that the 5 per cent. installation gives the best arrangement ; that is to say, that a drop of 5 per cent. is more economical than one of 8.5 per cent., but if an interest at 4 per cent. annum is effected by an increased capital expenditure of only £94, giving a rate of interest of 45 per cent. The conditions would be again altered if, instead of reducing the distance to half a mile, we made the distance from the generator to the motor one mile, as in the cases shown in Columns B of the table below. By making these alterations we find that in all cases whether the motor is half a mile, one mile, or two miles from the generator but in these last instances, where the motor is two miles from the generator, it is probable that the installation, where the loss of power is 20 per cent., is the most economical arrangement. To reduce this loss from 20 per cent. to 10 per cent. requires an expenditure of £1,120 on additional insulation and £270 on additional cable at a cost of 10 per cent. on the additional outlay. Without making further calculations of this kind, we can see at once that if we increase the length of the circuit we must submit to a greater drop in the voltage and consequent loss of power. Effect of Increasing the Voltage.—In all above instances the somewhat greater drop in voltage when the current taken is increased increases the voltage to 600, we shall again find a still more striking difference in the capital outlay. The effect of increasing the voltage in the proportion of 400 to 600, if the same percentage of power loss is maintained, will be to decrease the necessary sectional area of the cables in similar ratio twice over, because the voltage drop is greater, being 60 instead of 40, the current is 168 ELECTRICITY AS APPLIED TO MINING less, being 12a amperes instead of 185; the loss varies as current squared, and the cable resistance increases as its size diminishes. Thus, for equal percentage power losses the size of cable varies inversely as voltage squared, or as $600^2 : 400^2$—that is, as $3^2 : 2^2$, or as $9 : 4$. And since it will not materially affect the thickness of the insulation of the cables (in fact, the same quality of cable will be used), the cost will be reduced in approximately the ratio of $9 : 4$ to $3 : 2$. Referring to Table I., 600 volts, one-mile transmission, with a 10 per cent. drop of voltage and a 10 per cent. loss of power, Column E., we find the cost of the cable erected is £5.68, the cost of the generating station £7.730, and the total cost £5.818. With an 11% per cent. loss (the maximum allowed) the cost would be £5.93; generating station £7.730; total £5.477; and with a 5 per cent. loss, the cost of the cable would be £5.140; generating station £4.020; total, £4.160. From the table it appears that in this case the most economical drop is from 5 per cent. to 10 per cent. From Column D it appears that a loss of 5 per cent. is the maximum permissible in a half-mile two-volt transmission for no greater loss than that taken up by the pole building. When we come to the two-mile transmission (see Column F.), it is evident that a total drop of 10 per cent. is the most economical arrangement. In all the above instances the cost of the cable is calculated as going down a shaft about 300 yards deep; but if the installation had been entirely on the surface, and if the conductor could be carried on poles, the cost of the cables would be much less. With the above transmission of two miles from generator to motor, it is evident that we approach the verge of the distance to which we can profitably go with low-current tensions, but, in order to make sure, we have worked out the results with a distance three times as long and a tension of 600 volts. Column G. shows us that when we use the two-mile transmission, the length of the cable has to be multiplied by one and a half, and the area has also to be multiplied by one and a half, because it is necessary to increase the sectional area of the conductor in order to maintain the given drop of 10 per cent. (really 9:15 per cent.). Thus, the cost of the cables for this transmission is £6.730; and for a similar transmission at a generating station, £7.730; and we have a total cost of £10.450. With a drop of so per cent. (really 18:1 per cent.) we have the cost of mains £3.344; add the cost of the generating station, £2.945; and we have a total cost of £6.254. It is at once evident that 20 per cent. is the least drop that we can employ in this case, and the next question is, Would it be possible to have a plant built at such a low cost per mile as to make it worth while to \footnote{It will be seen from the table that the cost works out at a ratio of about $9:5$; in most cases the cost of manufacture of the smaller cable is a little greater than that of the larger in proportion to its size.} AVERAGE LENGTH OF MAINS 169 reduce the area of the cable in the proportion of 3 to 2. This gives the area of the cables $8 \times 3$ square inch. A No. $37 / 44$ has this area and costs £334 per mile; making a total cost of cable, including erection, of £5,056. The cost of the generator will be ascertained as follows : 100 E.H.P. + 30 per cent. for loss in transmission = 130 E.H.P. + 25 per cent. = say 162 I.H.P., and this at £2 per I.H.P. = £3,240. Adding this to the cost of the cable, we have a total cost of £5,244. The loss in working is 30 per cent. on £3,240, i.e., £972, and this is compared with 10 per cent. on £5,244, the reduction in capital cost is £4,566, and the loss in working is £261 a year more, or about 6 per cent. Comparing it with a 20 per cent. drop, the latter has an extra capital cost of £510, and an economy in working cost of £149 a year. Therefore, it appears that for a three-mile transmission (from generator to motor), with two voltages of 100 and 80 volts respectively, there would be a drop. **Averaging the Distance to which the Power is taken.—** In distributing power at a works similar to a mine it is generally the case that the power is required at a great variety of places, some much nearer than others to the generator. We might put down low-tension generators and cables for these purposes, but we should then have to use high-tension generators and cables for those which are distant; but this at once leads to complications in working and to additional expense at the generator station and in the cables, due to complications. It is desirable to have spare generators at the station, so that the works may not be stopped by the failure of one machine; but this also requires additional machines and we must also have spare machines. For this reason it is generally advisable to sacrifice some economy in transmission for the sake of avoiding the complication of having a variety of generators which cannot be substituted one for the other, and a variety of cables which cannot be tapped for any machine about the works as required. Therefore, in calculating the required distance between stations where there is no inconvenience to which the power has to be taken. Thus out of 2,000 E.H.P. at a mine, there might be the distribution of E.H.P. shown in the table on page 170. The total of 1,610 E.H.P. will absorb the 2,000 L.H.P. of the generating station. It will be seen that 8½ E.H.P., or more than one-half, does not exist half way between two stations; except half way between two miles not exceed one mile ; 350 E.H.P. is two miles, and 160 E.H.P. is at three miles. Looking at the column of E.H.P. miles, we find that the average distance of transmission is $\frac{7+1}{1+0} = 8$ or a little over one mile. We can, therefore, treat such as a one-mile transmission, and on a basis similar to that given in Table I., Column B., for a transmission of 100 E.H.P. It is, therefore, possible to deal with this economically with 400 volts at the motor and a 10 per cent. drop in voltage—i.e., 440 volts at the generator. 170 ELECTRICITY AS APPLIED TO MINING
Description of Engine E.H.P. Average Distance from Generator to Motor E.H.P. from Motor to Mine
Pumping 200 1 66 miles
Ventilating fan 200 1 50 miles
Screens and washing machines 200 1 38 miles
Workshops and saw-mill 80 1 14 miles
Sunday and holiday work 80 1 14 miles
Lighting works on surface 30 1 25 miles
Lighting village 300 1 100 miles
Pumping engine in pit 30 1 83 miles
"' 400 1 100 miles
"' 600 2 200 miles
Hauling engines"
                                                        <table cellspacing="0" cellpadding="0"> <thead> <tr> <th>Description of Engine</th> <th>E.H.P.</th> <th>Average Distance from Generator to Motor</th> <th>E.H.P.</th> <th>from Motor to Mine</th> </tr> </thead> <tbody> <tr> <th>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>"<br>" <TR/> <TH/>Pumping</TH/> <TH/>200</TH/> <TH/>1</TH/> <TH/>66</TH/> <TH/>miles</TH/> </TR/> <TR/> <TH/>Ventilating fan</TH/> <TH/>200</TH/> <TH/>1</TH/> <TH/>50</TH/> <TH/>miles</TH/> </TR/> <TR/> <TH/>Screens and washing machines</TH/> <TH/>200</TH/> <TH/>1</TH/> <TH/>38</TH/> <TH/>miles</TH/> </TR/> <TR/> <TH/>Workshops and saw-mill</TH/> <TH/>80</TH/> <TH/>1</TH/> <TH/>14</TH/> <TH/>miles</TH/> </TR/> <TR/> <TH/>sunday and holiday work</TH/> <TH/>80</TH/> <TH/>1</TH/> <TH/>14</TH/> <TH/>miles</TH/> </TR/> <TR/> <TH/>lighting works on surface</TH/> <TH/>30</TH/> <TH/>1</TH/> <TH/>25</TH/> <TH/>miles</TH/> </TR/> <TR/> <TH/>lighting village</TH/> <TH/>300</TH/> <TH/>1</TH/> <TH/>100</TH/> <TH/>miles</TH/> </TR/>
THREE-WIRE SYSTEM 171 with the price of copper and the nature of the insulation employed, the number of separate cables the capacity of the shafts, &c. If the current is carried in one wire averaging, say, eight large armoured cables, and down a shaft 500 yards deep, the cost will vary from say £12 to £17,000, including fixing. But if, as is more probable, there is an average of, say, sixteen armoured cables, the cost will vary from, say, £15,000 to £19,000. This represents an outlay of capital upon which it is worth while to attempt to make a saving by increasing the voltage, and in other words if it can be shown that a corresponding increase in other parts of the system and loss in efficiency. **Reduction of Cost by Increasing Size of Cable—** It is probable that in the example given there will be a number of cables, going on different roads and kept well separated from each other both in the shafts and at the surface. The fact that they are so placed does not necessarily lead to the stoppage of the whole work. But sixteen highly insulated, lead-covered, and armoured cables, each carrying 100 E.H.P., will be much more expensive than four cables each carrying 400 E.H.P.; and in the case where all the current is required for a long-distance transmission line, it is probable that a great part of the distance all the current can go in large cables and at a low voltage perhaps even in the average cost per h.p. transmitted. In the case of the transmission two miles, with 400 volts at the motor and a drop of 10 per cent. per mile from generator to motor, the cost of the cables for 100 E.H.P. was put down at £368-80; but if we take 400 E.H.P., we get £468-80. If these figures were multiplied by 16, we should get £1368-80; £468-80 = £468-80, as the cost of the cables and of erection ; but if all the power were taken this distance, it might probably be taken in, say, four large cables, each of 400 h.p., and reducing the cost of the cables by 20 per cent. the cost would be £368-80; and the question at once arises how this figure can be reduced. We have to reconcile two conflicting needs : one is the convenience and safety of low-sentation motors, &c.; the other is the economy of high-tension mains. **Three-wire System.—** If we use continuous, or, as it is otherwise called, direct, current, we may conveniently adopt what is called the three-wire system. By this system we can double the voltage at the power-house by putting two wires through each cable instead of one. All of the mine are used at half the voltage which is generated in the power-house, less the drop in transmission. Thus, in the case of the two-mile transmission, we might have two generators in series in the power-house, each of 400 volts. A 20 per cent. drop over the four miles of cable would give us 400 volts at the motor. The three wires are then connected together so that one middle conductor is the neutral or balancing conductor, and the twoouters are the positive and negative conductors. The motors will be arranged 172 ELECTRICITY AS APPLIED TO MINING alternately—the first motor between the positive conductor and the neutral conductor, and the second motor between the neutral and the negative conductors. It is assumed that the motors will be worked in pairs of approximately equal power. It is not necessary that they should be very near together. They may be hundreds of yards apart, and the pair may consist, say, of one motor of 60 h.p. on the positive side and two motors, say, each of 30 h.p., on the negative side; the endeavour being to arrange the motors so that the current shall be equal at all points on each side of the neutral conductor. If the powers are quite equal, the negative conductor will act as the return cable for the positive conductor, and the practical effect on the voltage is as if each pair of motors were run in series the same as the generators. If the pairs are not of equal power, or do not happen to be working at equal powers, then the neutral conductor takes the place of a return cable, and its size must be suitable for the purpose. Since the size of the cables for a given loss of power varies inversely as voltage squared, the size of the cables on the three-wire system is only one-fourth. But this saving of three-quarters is not really made, because there is the cost of the middle wire. If the power on the positive side is such that it would require a 150-h.p. motor to carry it, we have to be equal to 150. For the purpose of comparison with the two-wire system, we may call the cost of that system 16, the cost of the outer of the three-wire system $x + z = 4$. The cost of the middle wire is half one outer i.e. $r$, and the cost of the two systems will therefore be as $16 : 5$. But it is possible that while one side has a motor running at full load, another motor on the other side; in this case the middle wire would have to carry a current equal to that in the outer, so that it will be better to make the three wires of equal size, especially if the number of motors is not large; in this case the cost of the middle wire will be $z$ instead of $r$, and the relative cost of two- and three-wire systems will be as $16 : 8$ or $8 : 5$. In the case of the three conductors being distributed over a large area by means of wires, it will generally be possible, by means of suitable switches, to connect the motors on to that side, positive or negative, which is best for evenly balancing the current. But if we have a middle wire of the same section as the others, this arrangement will be unnecessary, and if one side is entirely without motors, no current will flow, and the system will become for that time being a simple two-wire system. Applying this reasoning to the example above given—namely, a 400-volt two-mile 1,600 h.p. installation, and allowing the same drop—that is, 10 per cent. per mile—we have the cost of cables on the three-wire system, sixteenths of the previous cost $\frac{1}{16} \times 39,880 \times x = £14,960$. But the current density in this case is only $\frac{1}{2}$ that in our example; hence we can expect that both sides will have motors running at full load. As before stated, if we increase either or both sides by increasing their size and cost of the cable must be increased in ratio of $3 : 8o$, making the cost £17,335 and, taking the cost of erection at £64,400 x £ = £44,800, we HIGH TENSIONS AND TRANSFORMERS have the total cost $33,115. If we raise the voltage to the highest which is generally considered admissible in this kind of transmission—i.e., to 600 volts at the motor-house—and take the greatest drop permissible for a density of 800 amperes as above—that is, 137 volts in the circuit, giving 1,337 volts between the outers at the generators—we can reduce the size and cost of the cables in the ratio of the voltage—that is, as 600 : 400 = £3,735 : £1,156. Adding £800 for erection, total cost £6,355. The total cost of the cable will be about £400 volts, and 113 per cent. with 600 volts at the motor. If it is desired to reduce the cost of the cables still further, there are two possible systems. **High Tension.—The first we will take is the continuous-current system, either three-wire or two-wire, and the voltage may be raised at the generating-house up to, say, 1,200 volts on each generator, or on the threewire system up to 1,500 volts. But this requires special makers who will undertake to make continuous-current generators of this high voltage. Most makers of this class of machinery object to exceed 600 volts, alleging that there is a great liability to failure. The high voltage is also unsuitable for the small motors generally used in mines, and it is necessary to transform the current before it reaches the pump house—a rotary transformer. Sometimes a special machine is used, and sometimes the rotary transformer simply consists of a motor at one end of the shaft driving a generator at the other end of the shaft, the generator being so wound as to give current at the required voltage, which may be, say, 400, or any other suitable value. In this case, or in motor-generator, will absorb from 6 per cent. to 5o per cent. of the power, according to its size, and it will cost from £5 to £6 per E.H.P. **Alternating Current: High Tension.—The other method is by alternating-current machines and static transformers. When we get to this system we are at once at high tension—that is, we shall not exceed 3,000 volts. With this low voltage we shall be able to reduce the cost of the cables very greatly. It is assumed, for the purposes of the following calculations, that the size of cables required for alternating current is not materially different from that required for direct current for approximately the same voltages. Going back to the example, Table I., Column C., we find that if we take a voltage of 1200 volts across the motor and a drop of 10 per cent. from the generator to the motor, or a total drop of 20 per cent., we have a voltage at the generator of 480. If, for the sake of convenience in calculation, we multiply 480 volts by five, we get a voltage at the alternating transformer of 2,400, and allowing a total of 20 per cent. for loss in transmission (that is to 10 per cent. per mile) in the cables from generator to motor-house, we get a voltage at the low end of the cables at the low tension was £248 + £400 for erection, and multiplying these by 16, to make up 1600 h.p., gives totals of £39,680 + £6,400, 173174 ELECTRICITY AS APPLIED TO MINING equal £36,80s, cost of the cables and erection. With the increased voltage we may divide the cost of cable by $^{5}=15$, reducing £39,580 to £1,587; but this cost must be doubled on account of extra insulation and smaller size, so that the cost of the cables (7/17) will be £3,174 + the erection of the sixteen cables at £300 each, or, say, £4,800, making total cost £7,974. But with the above drop of 400 volts, the current density will be reduced to one-third of its former value, and this is too much, therefore the current density should be reduced by enlarging the cable three times, using 19/17, thereby increasing the cost from £3,174 to £8,320, adding erection £4,800, making the total cost £13,120. The total loss in transmission will be reduced from 20 per cent. to 10 per cent. To this we must add the cost of the alternator system at £23 per E.H.P. (including chamber and fixing) would be approximately £25,000, or a total cost of £28,120 for cables and transformers. Compare this with £16,35%, the cost of cables alone on the three-wire system (page 173) with 600 volts at the motor, and no transformers, and a loss of 11% per cent. in transmission : 11% per cent. of 1,600 E.H.P. = 18 E.H.P. of loss, and adding a transformer at 25% = 45 E.H.P., making a total loss of 63 E.H.P. at a cost of £20 per I.H.P. costs £460. The loss on the alternating system is $^7$ of the above, or the capital outlay to meet this loss is £24.80. $^{113}$ The annual loss at 11% per cent. is £444 per 100 E.H.P. delivered, and for 1,600 E.H.P. delivered is £444 x 12 = £5324. If the loss is 7 per cent. it will be £1,400s, so that the annual saving by the alternating system will be £904. The extra cost of generating station for the three-wire system is £1,800. The extra cost of cables and transformers for the alternate-current system is £2,134 ; so that there is a balance of £276 of capital cost in favour of the alternate-current system ; but two systems on the whole are about equal in first cost and working cost in this instance. If we go to a distance of three miles and keep the same drop of potential per mile, or a total drop of $^{3} \times 1$ per cent., the cables on the three-wire system will have 600 volts at the motor and only just over 7 per cent. more than for two miles—that is at £368 instead of £364 adding erection £480. With the cables on the alternating-current system with 2,000 volts at the transformer, and a total drop of 10% per cent., the cost would be £2,480, and erection £6,500; total £8,860; so that which must be added is £5,500 for the transformers, making a total of £14,360. This calculation shows a gain by going to three miles instead of two ; but as it gives us on first cost of generating plant, but after allowing for loss in transformers the working costs will be much the same. For a four-mile transmission and the same drop per mile, making a total of 2 per cent., the cost for the three-wire system is £17,375 x $\frac{3}{4}$ = £12,992; and for electric £7,592; total £96312. A graph showing electricity usage. AERIAL LINES 175 For the alternating-current system with a total drop of 14 per cent, the cables will cost £11,480 x 14 = £16,640, and the erection £3,100, and £5,000 for transformers = £38,840. This shows that the 2,000-volt system is £1,472 cheaper in first cost of cables &c. and 9 per cent. cheaper in first cost of generating plant; after allowing for loss in transformation there may be some saving in working cost. Overhead wires are frequently happen at a mine where there is a long-distance electric transmission the cables can be taken on the surface over private lands, and in this case there is no reason why overhead conductors and bare wires should not be used. The cost of the bare-wire conductor is very much less than that of the insulated cable, so that cables of any large sectional area of wire can be used, and the drop reduced from ten to one cent per mile at 10 per cent., or 1 per cent. at 20 miles. Where this is the case, a long-distance transmission at low voltages presents much fewer difficulties than in the cases previously dealt with. An overhead cable suitable for carrying 100 E.H.P. at 660 volts at the generator a distance of two miles to the motor with a total drop of 10 per cent—that is to say, 100 E.H.P. at 660 volts at the generator, and 100 E.H.P. at 660 volts at the motor—will cost about £25 per mile (see Table I., Column F); the copper would weigh about 8,300 lbs. a mile, therefore two miles each way would weigh 33,200 lbs.; this at 10d. per lb. will cost 332,000d., or £1,138, as compared with the insulated cable £24,80. To take a case of long-distance transmission—say, ten miles from generator to motor—a voltage at the generator of 5,000 and a drop of 10 per cent., for the distance, or 1 per cent. per mile—that is, 500 volts, equal to 50 volts a mile—we should have a current density of 567 amperes per square inch; the size of the cable for 1,600 E.H.P. would be $1,600 \times 54 \times \frac{567}{\pi} = 47$ square inch. This would weigh 9,900 lbs. per mile or, for the twenty-mile circuit, 194,900 lbs.; this at 1d. per lb. costs £8,853. It would therefore be economical to use a larger conductor and a less drop. A per cent. drop would save 8 E.H.P., costing £1,000 per annum—at additional cost of £28,000 giving a return of 14% per cent. on the additional outlay. We shall now consider what is the cost of an electric plant of 2,000 H.P., which works out to about $1/4$ per l.h.p. hour; or if we add the repairs and renewals of plant, to nearly $1/4$ per l.h.p. hour. But in considering the amount of drop in the voltage in a long conductor, or the loss of power in a conductor, at page 183 we find that these losses are negligible; but this is not so; it is, of course, a difference between the E.H.P. delivered and the I.H.P. of the engine driving the generator, the former being say, two-thirds of the latter, and therefore our cost of $225$d. per l.h.p. hour, as given on page 193, must be increased to $337$d. per l.h.p. hour at the motor. If we substitute this
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176 ELECTRICITY AS APPLIED TO MINING smaller cost for the t.d. taken in the above calculations, we might be led to adopt a higher drop in voltage. There is, however, a limit to the drop which is permissible, and that is caused by the heat effect of the current on the cable. The amount of drop permissible is not a percentage in every case, but such a voltage as is sufficient to force the permitted density of current against the resistance. **Heat Conductors and Dangers.**—The doctrine of the con- servation of energy teaches us that whenever power is lost in one form it appears somewhere else in another form, and thus, when the E.M.F. in volts is lost in a circuit, it appears in the form of heat in the wires of the cable. If a bare wire were covered with fine coal-dust—as, for instance, in the vicinity of a seam on the surface, or on a haulage road in a pit—that would not be very likely to happen. But if the tempera- ture in the wire would warm the coal-dust and thus would increase the rate of oxidation of the latter, and so set up what is commonly called 'spontaneous combustion,' and a terrible accident might be the result. This danger is not confined to coal-dust, but is found in many other things, such as coal-dust itself, oil, and grease. It is, however, usual for conductors in or about a mine to be covered with insulating material, such as india-rubber or paper saturated in oily and waxy compounds, and the effect of a small rise of temperature in the conducting wire is to injure the insulating material. A temperature of 120° F., or even 130° F., constitutes an insulation at which the human tem- perature to which india-rubber insulation should be exposed, but it is also believed that any rise of temperature above 60° F. is injurious and shortens the life of an india-rubber cable. It is also stated that a rise of temperature reduces the insulating power, both of the india-rubber and of the paper; so that an insulation which would be perfectly satisfactory at a temperature of 60° F. may become unsatisfactory at 80° F. Cable-makers of great experience say that a current density of 1,000 ampères per square inch may be permitted without injuring the cable, in the case of cables of small size; that is to say, a cable made of nineteen wires, 16 gauge, equal to $\frac{1}{9}$ square inch, or about $\frac{1}{9}$ square inch, might be used safely at 1,000 amperes per square inch; but this is not prac- tice—a current of 60 amperes might be sent along such a cable covered with rubber insulation. But for a larger cable the current density must be reduced to 800 amperes per square inch. The makers of cables covered with bitumen say that their cables will stand a maximum current density of 1,000 amperes per square inch over a sectional area of $4$ square inch; above which they find no current density to be safe. It is, of course, perfectly obvious that for any given current density the larger the conductor, and consequently the larger the current and the larger the amount of heat developed by that current, the greater effect it HEATING OF CONDUCTORS 177 will have in heating the surrounding insulating material ; because, whilst the cooling surface increases in proportion to the diameter of the conductor, the heating current increases as the square of the diameter. If we refer to the example in Table I., showing the size of cable required to transmit 100 E.H.P. with 400 volts at the motor with a drop of 20 per cent. of voltage, or 80 volts in four miles of cable, we find that the sectional area of copper is 36 square inches, and that the current is equal to a density of 455 amperes per square inch. It appears, therefore, that this current is well within the limits given by the makers of cables. But before we take that for granted we must consider the conditions of a cable in a mine. Generally speaking, the conditions of a mine are favourable in this respect that the temperature is uniform. The conductor is not exposed to the hot air of the surface or to dust, but is surrounded by water, or parts of the mine, may have a permanently high temperature, and this is injurious to the insulation of the cables ; and on that account the current density should be kept down so as to avoid overheating. In some few mines the temperature underground is $90^{\circ}$ and upwards, and in many others it is $100^{\circ}$. For this reason it is necessary to keep down the maximum current density in a cylinder to 800 amperes per square inch for small conductors, and to 600 for large conductors. This affects the question of burying conductors or of hanging them on props. In deep mines it will be an advantage to have the cables where the ventilation can keep them cool, instead of hanging them in shafts. When we increase the voltage for a given amount of power, we are able to use a smaller conductor, because a smaller number of amperes is required to give the same power. We are also able to use a still smaller conductor, because we can afford a higher drop of voltage. Ten per cent. of 400 volts is 40, and 10 per cent. of 600 volts is 60. In each case we lose the same percentage of power ; but in one case we lose more than in the other range of voltage ; thus, with a voltage of, say, 440 at the generator, and a 10 per cent. drop and a cable $4^{2}$ square inch area, we have a loss of 10 per cent. of the power at the motor. If we increase the voltage to 660 at the gener- ator and allow a drop of 10 per cent. we can reduce the amperes in similar manner ; but here we lose only 5 per cent. power ; whereas in the second instance—and we therefore only require a cable two-thirds the original size ; but by increasing the drop in voltage from 40 to 60 we are able to increase the current density per square inch in similar ratio, and therefore we can reduce the cable again. The result is that the sectional area of the cable is $\frac{4}{\pi}\times\frac{4}{\pi}\times\frac{4}{\pi}=42\times\frac{4}{\pi}=19$, approximately. The current density is increased because we have $14$ amperes in a sectional area of $19$, then as $19\times124$ here we have a current density of $65$, which is quite high enough. A table showing examples of cable sizes required for different voltages and currents.178 ELECTRICITY AS APPLIED TO MINING CHAPTER IX ELECTRICITY APPLIED TO PUMPING AND HAULING Electric Pumping Plants, Various Types—Electric Sinking-pumps and Centrifugal Pumps—Electric Haulage: Single-rop, Main and Tail Rope, Endless-rop—Electric Locomotives. **Electric Pumping Plants.—One of the most important uses to which electricity can be put at a mine is that of pumping; and the great number of electric pumps in use at present in operation gives ample proof of its reliability and adaptability for the work. The most economical pumping arrangement for a mine, viewed from a fuel-consumption standpoint, is that in which a high-class Cornish engine, or a compound or a triple-expansion condensing engine, is fixed at the shaft top and actuates the pumps in the shaft by means of rods; such plants, however, are best suited for dealing with large volumes of water, in which case the steam-shaft should be opposed to them. The extra losses in pumps worked by rotary electric motors arise from— 1. Conversion of mechanical power of the steam-engine into electrical power of the dynamo. 2. Conversion of electric current from dynamo to motor. 3. Conversion of electric power of motor into mechanical power on motor shaft. 4. Intermediate gearing between motor shaft and pump crank shaft. Although the electric pump may be more costly than the steam-engine on the surface and pumps in the shaft, it has decided advantages over other forms of underground pump, such as steam, compressed-air, and wire-rope driven. **Types of Electric Pump.—The most common type of pump to which the electric motor is applied is the three-throw ram pump, in which there are three working barrels and three rams; the latter being worked by a shaft connected with a pulley on the 1st bar; one on each end. Each ram and barrel thus constitute a separate single-acting pump (the barrels being open at the front end), with separate suction and delivery valves. By this arrangement of three working barrels it will be seen that the work on the crank shaft of the pump is equalised, and a uniform current of water is delivered to the pipes. THREE-THROW PUMPS 179 Double-ram pumps are used, but are not so good as three-throw. An important feature with regard to mining pumps is to have perfect interchangeability, and Messrs. Ernest Scott & Mountain, who have had a Pict. 130.THREE-THROW PUMP DAVIES ELECTRIC MOTOR THROUGH WORM GEARING. very long experience in electric mining work, have designed special pumps with this end in view. The pump barrels are independent and interchangeable, also the valves and connecting pipes, so that, in the event 82 180 ELECTRICITY AS APPLIED TO MINING of a breakdown of any part of the pump, the remaining rams could con- tinue working if necessary. **Speed of Pumps.** The speed at which pumps can work is limited, owing to the fact that water is incompressible and inelastic ; they differ in this respect from air compressors, which rise in efficiency as the speed increases. A usual piston speed for three-throw pumps is 60 to 80 feet per minute, but it may go up to 120 feet a minute for a short period. This low speed necessitates the use of a high ratio of reduction between the motor shaft and the pump crank shaft. Fig. 121. **LARGE THREE-THROW PUMP DRIVEN BY ELECTRIC MOTOR THROUGH A BELT AND SPUR GEARING.** This reduction in speed may be got in several ways : 1. By means of spur-gearing only. 2. By a worm and worm-wheel (fig. 120). 3. By spur gearing and belt, or rope (fig. 121). The method chosen will depend to a certain extent on the circumstances. If the space available were limited the reduction might be obtained by No. 3 method, as this can be done with a single train of gear, the worm on the motor shaft and the worm-wheel on the crank shaft of the pump. Worm-gearing, to be efficient, however, should be run in an oil bath ; 121 RIEDLER PUMP 181 and even then the power absorbed is great owing to the thrust of the worm-wheel on the pump, and therefore it is seldom used. A spur-gearing only, would be adopted in those situations, where owing to the atmospheric conditions, a belt or ropes could not be made to work, or where space was limited. Spur-gearing is also to be recommended for large powers, and in these cases it may be necessary to have the teeth on the wheels shrouded up to the pitch line. Helical teeth are often employed in pumps of large sizes, but the gear-shaft is usually duplicated. The teeth of the wheel must be made in one piece, whereas the teeth of raw hide have been successfully used in some cases. The raw-hide pinion on the motor shaft makes but little noise, whilst the clatter of the high-speed metal pinion is deafening. Riedler Pump.--This well-known pump, made by Messrs. Fraser & Chalmers, is especially suited for electric driving on account of the high relative speed for which it may be designed. Over 1,500 Riedler pumps are now in operation, most of them, however, driven by steam or air, as it is only recently that they have been adapted to electric driving. The pump is the invention of Professor A. Riedler, of Berlin, and its principal feature is a device by means of which the valves are closed mechanically so that the stroke of the plunger is due to the reversal of the plunger. At the beginning of the stroke the valve opens automatically, and remains open practically the entire stroke. When near the end, it is positively closed at the proper moment by the controller. This action increases the effective area of the valve, thus increasing the effective area of the valve; it also permits the pump to be driven at a high piston speed. The speed of the pump varies from 150 revolutions per minute in the smaller sizes to about 80 revolutions in the larger. The Standard Riedler pump is of the differential type, with only one suction and one delivery valve for what is practically a double-acting pump. A pump capacity of 30 gallons per minute at a head of 10 feet would have the following dimensions: Diameter of large plunger, 9 inches; do. of small plunger, 6½ inches; stroke, 36 inches; revolutions, 80 per minute; B.H.P. of motor required, 30 ; suction pipe, 12 inches diameter; delivery pipe, 8 inches diameter. A notable example of the Riedler pump was built by Messrs. A. & J. Powell-Duffryn at Collerith in South Wales. This pump has a capacity of 1,000 gallons per minute against a head of 1,600 feet, and has been running night and day for over three years. The increased rotary speed of these pumps as compared with three-wheel pumps entails course less reduction gear, the motor being either coupled directly or else driving through a belt or gearing with only a single reduction in speed. Fig. 1:22 shows a belt-driven Riedler Pump. 182 ELECTRICITY AS APPLIED TO MINING **Riedler Express Pump.**—The Riedler pumps already referred to are usually not built for heads exceeding 4,000 feet, and in the large sizes the speed of 80 revolutions per minute is somewhat low for direct coupling to the motor. The pump illustrated is a 3,000 H.P. Riedler Express pump, in which the valves are mechanically closed by a buffer attached to the plunger. The speed of these pumps varies from 250 revolutions per minute in the smaller sizes to 150 in the larger. A large number of these pumps are working satisfactorily, especially on the lead and zinc mines, but the best qualities is at the works of the Mansfield Copper Company in Germany. This is the largest electrical installation in Germany, and consists of four Express pumps each with a A black-and-white illustration of a belt-driven Riedler pump. The pump has a large wheel at the front, with a handle on top, and a cylindrical body containing gears and valves. A belt runs around the wheel and connects to a motor at the back. Fin. 122. Belt-driven Riedler Pump. capacity of 1,100 gallons per minute against a head of 1,430 feet when running at 176 revolutions per minute. The pumps are direct coupled to motors, and are of the two-throw type, single acting, with plungers 91 inches long and 13½ inches stroke. The pumps are driven by two engines, each of 1,300 H.P., directly coupled to generators, the total I.H.P. of the plant thus being 2,600 I.H.P. The 'Gutermuth' Pump.—The chief feature of this pump is the 'Gutermuth' Patent Valve.' The valve is of very simple construction, being made simply from a sheet of metal; this may be of the same thickness through which it passes as that for heavy pressures the end of the sheet which forms the valve may be thickened. \footnote{This has recently been introduced by Messrs. Fraser & Chalmers, Edinburgh.}
Capacity 1,100 gallons per minute
Head 1,430 feet
Speed 176 revolutions per minute
Pumps Direct coupled to motors
Type Two-throw type, single acting
Plungers 91 inches long and 13½ inches stroke
Engines Each of 1,300 H.P.
Total I.H.P. 2,600 I.H.P.
GUTERMUTH PUMP 183 The valve is very similar in construction to the main spring of a watch, and is shown in fig. 123. The advantages claimed for the valve are that it is instantaneous in action, offers a minimum obstruction to the moving fluid, and is noiseless. In the case of a two-throth single acting pump, which was built by Messrs. Fraser & Chalmers for a colliery in North Wales, the plungers are 4 inches diameter and the combined stroke 6 inches. When running at 50 revolutions per minute the capacity of the pump is 150 gallons per minute against a head of 600 feet. Fig. 123. Valve Core Length Section Cross Section High-pressure Pump fitted with 'Gutermuth' Pump. This pump is driven through a raw-hide pinion and spin wheel from a 60 H.P. motor. The suction and delivery valves are attached to gunmetal cones, which are held in place by means of screws, and are held in place by means of clamps, and the cones are inserted into the body of the pump from the back, looking towards the crank shaft. In order to get at the valves in case of a break-down it is only necessary to unscrew a flange and withdraw the cone. The simplicity of the valve and the comparative lightness of its construction would make it very suitable for pumps which are coupled to the motor shaft. Belt- and Rope-driven Pumps.—Where the conditions are suitable, this is probably the best method for the reciprocating type of mining 184 ELECTRICITY AS APPLIED TO MINING pumps, the reason being that the belt takes up the shock to a great extent when starting the pumps, and is noiseless. In the larger sizes a double-belt drive may be employed, with belt wheels and gearing at each side of the pump. Ropes are often preferred to belts. The belt pulley is always on the motor shaft, driving a large wheel on the counter-shaft, a pinion on this shaft gearing into a wheel on the crank. **Size of Pumps Required.** In calculating the work to be done, due allowance must be made for the friction in the gearing of the pump, for friction of the water in the pipes, and for slip of water owing to the valves not working instantaneously. **Example.** Required, the size of rams and stroke of a three-throat pump capable of delivering 500 gallons per minute, with a head of 400 feet, and also the E.H.P. of motor to drive the same. Allow 15 per cent. for slip and assume a ram-speed of 100 feet a minute, then each pump-ram moves 50 feet a minute against the load, and Gallons per minute = area (sq. ft.) of one ram x number of rams × effective speed of rams (ft. per minute) × $8\frac{3}{4}$ (slip) = $\frac{7854}{100} \times 3 \times 50 \times 6 \times \frac{8}{4} \times 500$ = 500 gallons per minute $d' = d - f$, say, 11 inches. Three 11-inch rams Stroke, say, twice diameter of ram = 22 inches. Revolutions per minute of crank shaft = $\frac{100}{d'}$ = about 28. $h.p.$ = head x quantity = friction of pumps and motor, say, 50 per cent. = $500$ (gallons per minute) × $400$ (feet of head) × $15$ = say, about 100 E.H.P. motor. A rough but effective rule in h.p. of motor = h.p. in water lifted × z. **Efficiency of Electric Pumping Plant.** The electric generating plant described on page 92 is used to drive pumping plant as follows : The cables from the dynamo terminals to the pumping shaft consist of four overhead conduits each carrying copper wire, each conductor being composed of nineteen strands No. 11 S.W.G. wire. The shaft cables to the pumps are each 430 feet in length, and each cable is composed of sixty-one wires. No. 13 S.W.G. There are two sets of single-shaft pumps, three-throat type, with rams 11 inches diameter and 18 inches stroke, driven by separate motors by means of cotton driving ropes. CENTRIFUGAL PUMPS 185 This plant was tested at the same time as the generating plant, and the results obtained were as follows:
Speed of pump, 24 revolutions per minute.
Average E.H.P. at dynamo terminals, 140.
Average B.H.P. at motor pulleys, 65 = 132.
Average B.H.P. at motor pulleys = 61 + 59 = 120.
Loss in cables, 8 E.H.P. = 57 per cent.
Water delivered, 980 gallons per minute.
Efficiency of transmission:
h.p. in water lifted 102 78 per cent. Efficiency of transmission,
E.H.P. at dynamo terminals 102 74 per cent. Efficiency of motors and pump.
h.p. in water lifted 102 74 per cent. Efficiency of motors and pump.
E.H.P. at motor pulleys 132 91 per cent. Efficiency of pumps alone.
E.H.P. at motor pulleys 132 91 per cent. Efficiency of pumps alone.
h.p. in water lifted 102 85 per cent. Efficiency of pumps alone.
B.H.P. at motor pulley 120
Fig. 133A. Electric Sinking-Pump. HIGH LIFT CENTRIFUGAL PUMPS 187 feet, but two or more pumps may be coupled in series with each other, and so the total head increased. The low efficiency may be counter-balanced in some cases by great simplicity of working parts, and consequent saving upon upkeep. A recent development is shown in Fig. 14, which shows an electric motor coupled to four centrifugal pumps, two being coupled in series on each side of the motor. **High Lift Centrifugal Pumps.**—In order to get over the disability of the rotary pump for high lifts 'Multiple Chamber Centrifugal Pumps' have been recently introduced. In these pumps several sets of vanes run Fig. 124. **Electric Motor Driving Centrifugal Pumps in Series.** upon a common shaft but in separate chambers ; the delivery pressure of the liquid varying directly as the number of chambers used. The water enters the revolving wheel axially, and is discharged tangentially into a stationary guide ring of special construction which conveys it to the annular chamber in the body of the pump, from which it is discharged into the second and subsequent chambers. Centrifugal pumps of this construction are designed to deliver up to 1,000 feet. A pump designed to deliver 1,500 gallons per minute against a head of 450 feet would run at a speed of about 820 revolutions per minute. 188 ELECTRICITY AS APPLIED TO MINING Fig. 124A shows a four stage turbine pump by Mather & Platt of Manchester. **Electric Haulage.**—Electricity is now widely employed as a means of driving the various forms of haulage machinery found in or about the mine. The use of slow- and fast-moving band-ropes worked by an engine on the surface and carried down the shaft to the haulage drums in the mine continues to find very great favour, however, and there are in operation haulage plants driven by steam engines which would be less costly than would be the case if electricity were employed ; the workings, however, have been laid out specially with the idea of using wire-rope transmission. MATHER & PLATT - MANCHESTER No. IV. FOUR STAGE TURBINE PUMP, WITH DIRECT-COUPLED MOTOR, TO DELIVER 400 GALLONS PER MINUTE AGAINST 250 FORT HEAD. The methods of electric haulage may be divided as follows : 1. Single-rope haulage drums worked by electric motor. 2. Main- and tail-rope drums worked by electric motor. 3. Endless-rope drums worked by electric motor. 4. Electric locomotives. Single-rope haulage is applicable when the coal-face is to the dip of the shaft, the empty trains running inbyle under the influence of gravity and the full train being hauled up the incline by means of a drum. STRAIN ON HAULING ROPE 189 Such haulage gears may run at a high speed, ten miles per hour being attained if the roads are well laid out. If a motor of moderate speed were adopted running at, say, 250 revolutions per minute, it could be coupled to the drum by means of spur-gearing with only one reduction. The adoption of a slow-speed motor (though more expensive than high-speed) is sometimes the cause of the frequent stoppages required, as the windings of the armature are not subjected to so much strain, and so there is less liability to damage the insulation. If a high-speed motor is used—say, 500 to 700 revolutions a minute—then two reductions of speed may be required. Fig. 125. BRITISH THOMSON-HOUSTON SINGLE-ROPE HAULING DRUM, Where there is only a single reduction of speed, this cannot always be done effectually by means of a belt or rope-gearing, as in the case of a heavy load a long and heavy belt would be necessary. But where there are two reductions of speed, then a belt is generally preferred for several reasons. One reason is that the belt is less liable to produce shocks, which is beneficial to all parts of the machinery and especially to the electric motor; and the other is that the belt works silently, whereas high-speed gearing, where metallic wheels are used, makes a deafening 190 ELECTRICITY AS APPLIED TO MINING clatter. The noise may be minimised by the use of a raw-hide pinion in cases where it is suitable for the amount of work. In some cases the speed is reduced by means of a worm and worm-wheel. By this means the necessary reduction of speed can be got with one reduction and with a silent gear. The difference between spur-gearing and that from its nature there is always a great deal of friction, the only rubbing between the teeth of the wheel, whereas in properly designed spur-gearing there is no rubbing, only a rolling contact. The difference between the two kinds of gears is illustrated by the consideration that well-designed spur-wheels may be run without any grease on the teeth, and with these teeth quite rough to the touch, while with worm-gears, which have to be driven to work against the teeth of a wheel that was rough to the touch or without lubrication. The difference is similar to that between a carriage on wheels and a sledge. A sledge, however, may run upon a smooth surface such as ice, and similarly worm-gearing may act satisfactorily if sufficiently well lubricated, and if the surface in contact is sufficient to prevent too heavy pressure at the points of contact so as to drive out the oil from between the rubbing surfaces. In no case is worm-gearing as Fig. 126. economical as spur-gearing, but there may be cases where loss of power may be tolerated for the sake of convenience and economy in first cost (though it must be borne in mind that economy in first cost at the motor must not be purchased by a greater outlay at the generator). Worm gearing must work in a bath of oil, and the oil should be constantly led away to a filter, so that all metallic and other grit may be removed before it is returned to the machine. Fig. 125 shows a British Thomson-Houston electric hauling drum for use with a direct-current motor, fitted with a patent friction drum and brake. The drum is loose on the shaft and engages with a double-cone friction clutch by a small end movement along the shaft. The weight of the drum is calculated when the incline is soon calculated. Referring to fig. 126, it will be seen that if a weight, w, is pulled up an incline by a pull, p, acting parallel to the incline, the distance on the incline being called l and the vertical height corresponding to that length being called $a$, then $$\text{Weight} = \frac{\text{Distance}}{\text{Height}}$$ MAIN- AND TAIL-ROPE HAULAGE 191 $$P \times d = w \times A$$ and $$P = \frac{w \times h}{l}$$ Thus, on an incline rising 10 feet vertical in 100 feet measured on the slope, the pull on the rope in pounds for a weight of 1 ton would be $$P = \frac{2,540 \times 10}{100} = 224 \text{ lbs}.$$ To get the total load the friction of the tubs and rope must be added; these vary with the conditions, and can be found experimentally. **TABLE SHOWING STRAIN ON ROPE TO BALANCE DEAD WEIGHT AT VARIOUS INCLINATIONS.**
Elevation (vertical) in 100 feet (horizontal) Corresponding Angle of Inclination Strain on Rope for a Load of 1 ton
Fees — Lbs.
5 2° 125
30 11° 154
40 8° 841
70 35° 1,697
100 45° 1,872
190 60° 1,933
T35 —
The dead weight to be lifted includes the coal or stone, tubes, and rope, and the friction of this may be roughly stated at 5 per cent, for a straight road. The friction of the gearing, consisting of two reductions of speed by spur gearing, or of one reduction by belt and one by gearing, may be roughly put at 20 per cent. In order to get up speed rapidly there must be a considerable margin over power so that the motor to impart necessary motion shall have sufficient power and that it should be not less than 25 per cent of the load ; therefore, the power of the motor should not be less than 50 per cent in excess of the maximum pull required merely to hold the load stationary on the incline. When we come to deal with crooked roads, and with the main and tail rope and endless chains, the friction increases considerably, and the amount of the increase depends to a great extent upon the care with which the wheels and rollers to carry the rope are constructed, arranged, and lubricated; and it sometimes happens that the power of the motor required to do the work with ease is three or four times as much as would be sufficient merely to hold the load stationary on the incline. Where there is a heavy load on a steep incline it is much easier to calculate the size of motor necessary, because here the weight is the chief consideration, and the pull to balance that can be easily calculated; but in the case of a long and crooked road, worked main and tail, with a gradient very 192 ELECTRICITY AS APPLIED TO MINING Fig. 147. Man-and-Tail-sore Hauling Device. slightly against the load, or level, or even in favour of the load, it is often difficult to foresee what amount of power will be absorbed in friction; and cases have been known where a 50 h.p. motor was required to haul a load UNDERGROUND HAULAGE 193 on the main and tail system at the rate of six miles an hour, the road being practically level, though undulating in places, but nowhere steeper than 1 in 20, the total weight of train and rope not exceeding 20 tons. It must be borne in mind that the great secret of satisfactory working of electric haulage is to use the least power possible, and to avoid undue strain and heating. The writers have known cases where, according to a liberal calculation, a 25-h.p. motor would be sufficient, and yet in practice a 35-h.p. motor was barely sufficient, and a 50-h.p. motor was found to give the most satisfactory results, the extra power being required to over- Endless-rope Haulage Plant Driven through Worm Gearing by an Electric Motor. 1966 ENDLESS-ROPE HAULAGE PLANT DRIVEN THROUGH WORM GEARING BY AN ELECTRIC MOTOR. come the friction of the rope in passing round curves ; but there cannot be the least doubt in the mind of any mechanical engineer that if sufficient care had been taken and sufficient expense incurred in the construction, arrangement, and lubrication of the rollers and pulleys, the friction might have been very greatly reduced ; but in the instance above alluded to the result was obtained with such skill and care as an ordinarily good colliery engine-wright and manager are capable of giving. Main- and Tail-rope System.—With a varying gradient the set o 194 ELECTRICITY AS APPLIED TO MINING must be pulled both inbye and outbye, and consequently two drums are required. A plan frequently adopted for large installations is to drive by means of two motors, either of which in case of a breakdown could work the plant with a smaller load on. This method, where space permits, is an admirable one. A main and tail rope haulage plant, with rope wheel and spur gearing, is shown in fig. 127. In calculating the power required on this system the heaviest gradient on the road is taken into account, and, in addition to the friction of tubes and main rope, the friction on the tail rope must be considered. **Endless- Rope System.**—This is essentially a slow-running con- tinuous plant, and it follows that where it is applicable the amount of work to be done is much more equally distributed, and consequently the motors may be of smaller power. The speed of hauling rope is usually about two miles per hour, consequently a high ratio of reduction is necessary. With spur gearing three reductions are usually required, but by driving from the motor with ropes or a belt, one set of spur-wheel and pinion is dispensed with. The strain on the motor, however, is much more uniform, and there is no danger of sparking at the rope-drive as in the case of main and tail. Fig. 128 illustrates a type of endless-rope driving gear in which the speed is reduced by worm gearing. Fig. 129 is reduced from a working drawing, and shows a good arrange- ment of pumps and hauling drums in one engine-house, by the Sandcrylic Foundry Company. **Polyphase Haulage Plants.**—The majority of haulage plants in operation are on the continuous-current system, but polyphase (generally three-phase) current is also employed. At the Park Colliery, Garwood, and the Sandwell Park Colliery, near Birmingham, three-phase plants have been erected by Cofeck Engineering Co., Ltd., Birmingham. One feature of the polyphase system is that the motors can be taken very near the working face without danger from sparking, and used for the secondary haulage between the face and the main haulage planes. At the Park Colliery the plant consists of a three-phase generator. The voltage is 500 volts and the current 80 amperes in each phase. The periodicity is 40 per second, and the speed 400 revolutions per minute. The exciter is a small four-pole continuous-current machine, shunt wound for a voltage of 75. The motors include one of 35 h.p.; working a haulage plant ; one of 10 h.p., working a pump and haulage alternately ; and six of 4 h.p., five of which are driving pumps, and one a small hauling drum. POLYPHASE HAULAGE PLANT 195 The motors are all of the induction type, deriving their motion from the revolution of the magnetism of the field poles (these poles themselves being stationary). The armature shaft is provided with slip-rings for the A technical diagram showing the internal components of a polyphase haulage plant, including various mechanical parts and connections. o 2 Google 196 ELECTRICITY AS APPLIED TO MINING purpose of connecting the starting resistance. These particular polyphase motors will stop with an overload of about 15 per cent. Electric Locomotives.—In some European and in many American mines electric locomotives are used for haulage. In this country, however, the use of electric locomotives has been confined to a few mines. Their chief disadvantage for low roads, or gassy pits, is that the cables required are not insulated. The motors take their current from a bare overhead wire, and the circuit is completed through the rails. This entails a certain amount of sparking, and also involves a risk of electric shock. Fig. 130. NORTON COAL CO. ELECTRIC LOCOMOTIVE. The great weight of the locomotive also entails an expensive road, with heavy rails and sleepers, as the consequences of the train getting off the road might be very serious. For long distances, where the roads are crooked and the gradients suitable, electric locomotives would possibly be more efficient than wire-rope haulage. The main objection to locomotives in a pit is the uncertainty of the gradients and the fact that locomotives are not well adapted for gradients steeper than 1 in 50. In America there are many mines worked from levels driven in the hillside, and here the locomotive used on the surface is much lighter than in Europe, and can be run at a higher speed down the slope. Continuous current is usually employed, the weight of the locomotive being from 2 to 20 tons, the height of the smallest locomotive, excluding the trolley, being 32 inches, and that of the heavy locomotive 48 inches, a usual speed being from eight to ten miles an hour. Fig. 130 shows an Electric Locomotive by the Westinghouse Company. ELECTRIC LOCOMOTIVES 197 A Baldwin Westinghouse locomotive weighing 15,000 lbs. has a height, exclusive of trolley pole, of 36 inches; length, exclusive of bumper blocks, 11 ft. 5 ins.; gauge, 30 ins.; width, 52 ins.; wheel base, 56 ins. It runs at a speed of 18 miles per hour and exerts a draw-bar pull at full running load as follows:
Level 2,100 lbs. 3 per cent. grade 1,650 lbs.
1 per cent. grade 1,950 ** 4 ** 1,500 **
2 ** 1,800 ** 5 ** 1,350 **
198 ELECTRICITY AS APPLIED TO MINING CHAPTER X ELECTRICITY APPLIED TO COAL-CUTTING Pick Machines—Revolving-bar Machines—Disc Machines—Chain Machines. One of the most important applications of electricity to mining work, and one in which there is great room for development, is the driving of machinery for under-cutting coal to supercede the ordinary method of ' holing' by hand. The advantages of machine-holing as compared with hand-holing may be summarised briefly as follows: (a) A considerable saving is made on round coal, owing to the reduction of the width of the holing, and owing to the greater distance the coal is under-cut. With a machine-cut the width of the holing will be uniform from 4 inches upwards, but with hand-holing the width will vary with the depth of the under-cut, because it is necessary for the workman to cut away sufficient coal to make a hole large enough to admit his pick. The width will vary from about 2½ inches at the front down to nothing at the back. (b) The rate of advance of the face can be greatly accelerated. This is an important consideration in seams with a tender roof, and exposes the workman to less risk. (c) The machine-holing produces a straight line of cut, and this induces a straight line of fracture in the roof, and in this way a more systematic arrangement of timbering can be used than when hand-holing is practised. (d) The coal is very easily and safely sprayed. (e) The cost of working is greatly reduced (where conditions are favourable to its use), and owing to the greater output, the quantity of coal raised per man is much greater, and in many cases the wages earned are higher, than when holing by hand. (f) The increased rapidity with which the face is advanced enables a large tonnage to be got from a shorter length of face, and so reduces the length of roads to be kept open. In this chapter Mr. H. Home Secretary was furnished with the inspectors of each district with a return of the proportion of coal-cutting machines in use, and from the inspector's reports the table on page 199 has been compiled. ELECTRIC COAL-CUTTERS 190 NUMBER OF COAL-CUTTERS IN USE IN THE UNITED KINGDOM IN THE YEAR 1900.
District No. of Coal-cutters Two of Coal Wrought
Compressed Air Electric
South-Western 8 1 10,080
East Scotland 32 1 297,290
North and East Lancashire 9 8 86,000
Newcastle 12 150,000
Yorkshire 67 16 1,046,444
South Wales 10 - Not stated
South Yorkshireshire - - 1,367
Midland 43 28 645,865
Liverpool 44 - 3,000,000
West Scotland 12 - 234,300
Durham 10 5 192,524
North Staffordshire - - 3,188,880
From this table it will be seen that on January 1, 1903, there was a total of 316 coal-cutters in operation, which had mined over three and a quarter million tons in the year 1900, or $1$ per cent. of the total output for the country. The number of coal-cutters in use in America in 1855 was, as given in Dr. Foster's report, was $345$. In America the value of the coal-cutter has been more fully recognised, and it is estimated that there were in use, in twenty-two States, in 1900, $3,997$ machines, mining $196$ per cent. of the total output of the country, both anthracite and bituminous. As the cost of electricity increases with the increase of the coal cutter will undoubtedly very largely increase, as it affords an opportunity of working at a low cost, seems impossible to work at a profit by hand-holing. The motive power for coal-cutting machines is limited to electricity and compressed air, as will be seen from the above table, and compressed air has been up till quite recently the chief means of transmitting power from the surface to the workings. As a result of this method there has been great loss with compressed air arises from leakages in the pipes, which prove very expensive. A great deal of time, also, is taken in laying the pipes and in coupling to the machine. With electricity the cables are easily laid along the gates and the coal face, a greater speed of cutting can be attained, and when the coal cutter is used for long periods it is economical. Electric coal-cutters can be divided into the following classes : (1) Pick machines, in which the coal is chipped away in a similar manner to that employed by the miner. (2) Revolving-bar machines, in which a number of teeth are fixed on the circumference of a rapidly revolving bar. (3) Disc machines, in which teeth are fixed on the circumference of a horizontal wheel. (4) Chain machines, in which the cutters are fixed on an endless chain. 200 ELECTRICITY AS APPLIED TO MINING (1) Electric Pick Machines.—Although there are several successful compressed-air pick machines, amongst others the Yoch, Ingersoll-Sergeant, Stiskol, &c., there has so far been very little development in the electrical pick machines, there being only one at present on the market. The Morgan-Gardner Electric Pick Machine.—This machine, shown in fig. 131, has a reciprocating piston actuated by a spring Fig. 131. MORGAN-GARDNER ELECTRIC PICK MACHINE, AS USED FOR UNDER-CUTTING. MORGAN-GARDNER MACHINE, MOUNTED ON HIGH WHEELS FOR SHEARING. and cam, the spring striking the blow and the cam drawing the piston back. The cam is driven by an electric motor. The stroke of the piston is an inch, and it runs from 175 to 225 strokes per minute. The length of the machine is 7 feet; the width over the wheels is 21 inches, the weight being 750 lbs. This machine is used in Australia, America, and is, of course, particularly adapted to pillar and stall working. The method of working is to place the machine on a board about 6 feet long and 3 feet wide, which is raised at the back so as to give the board an inclination of a or 5 inches to the yard towards the face. The machine-man sits behind the coal-cutter, with a hand on each handle, and the recoil is taken up by the inclination of the board and by the weight of the bucket resting at the back end of the cylinder. The speed of under-cutting is estimated at from four to six times that attained by hand. The pick machine makes less slack than ordinary hand-holing. A diagram showing a reciprocating piston mechanism for an electric pick machine. A diagram showing a Morgan-Gardner electric pick machine mounted on high wheels for shearing. HURD COAL-CUTTER 201 Fig. 132 shows a section of the coal-face, with the comparative height of hand-holing, and that of the pick machine, from which it will be seen that the latter makes 30 per cent. less slack than hand-holing. The chief advantage of the machine seems to be that it is light and easily moved. (2) Revolving-bar Machines.—The most successful type of this machine is the "Hurd Coal-cutter," manufactured by Messrs. Mavor & Coulson, of Glasgow, and by Messrs. Cowlishaw, Walker, & Co., Burton, Staffordshire. The Hurd Coal-cutter.—This machine consists essentially of five parts: (1) Cutter-bar, (2) gear-head, (3) motor, (4) switch-box, (5) hauling gear. The cutter-bar is made of mild steel, and is tapered on both ends; at each end of the bar are two holes, through which the cutters are drilled on the thread and also between the threads. By fixing the cutters on the thread a greater width of holing can be got; if a less width of holing is wanted, the cutters are fixed between the threads. Figs. 132. The arrangement of gearing will be seen in figs. 133 and 133a. The motor shaft, which has a bevel wheel, $a$, on the end, projects into the gear-case and drives the upper bevel wheel, $b$, on the vertical shaft, the reduction in speed being $z$ to $y$. The lower bevel wheel, $c$, on the same shaft engages with a pinion, $d$, which drives the bar. A reciprocating motion is given to the bar by means of a worm, $w$ (see fig. 134), on the body of the driving pinion, $e$, which is driven by a cam, $f$, which drives a toggle by means of an eccentric pin. The toggles impart a to-and-fro movement of about 2 inches to the thrust-block, $g$, which is communicated to the cutter-bar, there being a feather on the latter which enables it to pass to and fro over a short distance when engaged with $w$ (of fig. 134), so that the lower part of the gear-case, in conjunction with a pinion not shown in the illustration, enables the lower part of the gear-head with the cutter-bar to be revolved in a horizontal plane round the vertical axis ; by this means the machine can be made to cut its way into the coal at starting, and also the cutter-bar can be brought out for the purpose of renewing the cutters. The complete machine is shown in figs. 135 and it will be seen that 1 "Coal-cutting by Machinery," by Mr. W. Birkenshaw ; *Transactions Inst. Mining Engineers*, xi. 176. 2 Known by the trade name of Pickwick.* A diagram showing a coal cutting machine with various components labeled. Hand Machine 202 ELECTRICITY AS APPLIED TO MINING Figs. 133 and 133A. A diagram showing a motor shaft with a cutter bar attached to it. A diagram showing the gearing in Hurd Electric Coal-Cutter. The motor shaft is connected to the cutter bar via gears. GEARING IN HURD ELECTRIC COAL-CUTTER. FIG. 134. A diagram showing the reciprocating motion of the bar in a Hurd Coal-Cutter. The bar can also be revolved in a vertical plane by means of a worm and worm-wheel. The haulage gear is worked by a worm fixed at the end of the armature shaft, and is of the usual pawl and ratchet-wheel type. the bar can also be revolved in a vertical plane by means of a worm and worm-wheel. The haulage gear is worked by a worm fixed at the end of the armature shaft, and is of the usual pawl and ratchet-wheel type. HURD COAL-CUTTER 203 The electric motor may be either continuous- or alternating-current, but the majority of the machines at work are on the continuous-current system. They were formerly made series-wound, but a compound winding is now preferred. The machines are made in three standard sizes, as follows:
Small Medium Large
Extreme length 6 ft. to 8 in. 9 ft. to 12 in. 9 ft. to 16 in.
Height above rail 1 ft. to 1 in. 1 ft. 4 in. 1 ft. to 10 in.
Weight of machine 19 cwt. 30 cwt. 45 cwt.
Depth of undercut 2 ft. to 6 in. 3 ft. to 4 ft. to 6 in. 4 ft. to 6 ft.
Power of motor 1 B.H.P. 18 B.H.P. 26 B.H.P.
Frequency of voltage 1,100 volts 1,750 volts 2,500 volts
Gauge of rails 2 ft. to 3 in. 2 ft. to 4 in. 2 ft. to 6 in.
Weight of rails
The speed of the bar is from 400 to 500 revolutions per minute, which necessitates only one reduction in gearing, and consequently absorbs less power in driving than a machine requiring a double reduction. One great advantage is that the coal, after being under-cut, can be sprayed close up to the bar and within a few inches of the solid coal ; while with the disc machine this spraying cannot be done without stopping the wheel. The bar, also, is more readily recovered in case of a fall of coal. At the Acton Hall Colliery, Hurd machines are working very successfully in a seam of moderate thickness. On the occasion of a recent visit the machine was cutting 5 feet 8 inches under, the width of cut being 6 inches at the front and 3 inches at the back. A three-phase alternating-current motor was supplied by Messrs. Siemens & Halske, and it was found in starting the machine. The rate of advance, as noted at the time, was 1 foot a minute, but according to the operator the usual speed of cutting was a yard in 24 minutes, while in a run of 6 hours, allowing for stoppages and laying rails, etc., 60 yards had been cut. The time taken to change the teeth (by hand) from thirty to forty minutes. Since this machine was introduced by manufacturers of the Hurd coal-cutters have made several three-phase machines, which are operating with great success. A squirrel-cage rotor is used and an auto-transformer is employed for starting. There is no difficulty in starting a bar-cutter with a three-phase current, since the starting torque required for these machines is very small compared with that required for a motor which will be jammed by the coal being very small. One of these machines at a colliery near Nottingham cuts, on the average, over sixty feet six inches under, in 204 ELECTRICITY AS APPLIED TO MINING 12 hours, including preparing the machine for cutting, &c. With three men in charge, the machine can be run at the rate of a yard in two minutes. At a Staffordshire colliery a machine under-cut to a depth of Pig. 135. LIRD FILD'S CO-CUTTING MACHINE, ADJUSTED FOR UNDERCUTTING. 3 feet 6 inches at a rate varying from $+\frac{1}{2}$ to $-\frac{1}{2}$ feet a minute, and, counting stops, the rate was 24 yards per hour. *Transactions Inst. Mining Engineers*, svi. 449. CLARKE AND STEAVENSON'S COAL-CUTTER 205 **Modifications of the Bar Machine.—There have been many different types of bar coal-cutter, which have met with varying success. A machine recently described, called the 'Lee Coal-cutter,' has one or two novel features. The ordinary form of haulage gear, consisting of a wire rope attached to the machine and carried round a pulley some distance in advance, is dispensed with, and in its place one of the rails on which the machine is noched like a rack, and the machine propel itself by means of two wheels, whose rims are cut into teeth. With this arrangement a straight face is not absolutely necessary. The cutters, also, instead of being held in holes drilled in the bar—which of necessity must weaken it—are carried on a spiral of tool steel, which can be slipped off the bar when the teeth are dull or worn out. This machine is said to have done good work at the Mysore mines, U.S.A., where it was used at work under-cutting a 6 feet inches under a seam 2 feet 6 inches thick. **Heppell and Patterson Coal-cutter.—This machine, which is also of the bar type, differs in important details from those already mentioned. The cutters are fixed in three dovetailed grooves in the cutter-bar; they may be removed from the bar by means of screws of metal, so that they can be arranged in any way desired. The debris from the cutter-bar is also brought out by means of a small endless-chain conveyor which works at the side of the bar, thus getting over one of the difficulties urged against the bar-cutter. **Jeffrey Bar Machine.—In this coal-cutter the bar, instead of working at right angles to the cutting racks parallel to it. It is driven from an electric motor by a chain belt and sprocket-wheel, and is carried on a frame which is gradually advanced into the coal as it is cut away by the bar. The machine is not largely used, having been superseded by the Electrotal Chain Machine (see page 215). (3) DIXIE MACHINE—The most popular type of machine in this country, and there are many varieties successfully working. **Clarke and Steavenson's Electric Coal-cutter.—This machine (fig. 18) consists of a continuous-current series-wound motor, on the armature shaft of which is a bevel pinion with 16 teeth, which gears into a bevel wheel on the driving shaft. On this bevel wheel is mounted an intermediate shaft with 14 teeth gears into another spur wheel on the driving shaft with 39 teeth, and at the end of this shaft a wheel with 18 teeth gears directly into the cutting-wheel, which has 125 teeth. The advance of the machine is obtained automatically in the usual manner. A crank on the first-motion shaft is connected by a rod to a pawl or ratchet wheel on the second-motion shaft. The angle between these can be varied so as to make the ratchet engage more or fewer teeth on the wheel. *An American Longwall Mining Machine,* by H. Foster Bain ; *Transactions Inst. Mining Engineers*, xix. 144. 205 206 ELECTRICITY AS APPLIED TO MINING Photo of a large, industrial machine with large wheels and gears. The text "358" is visible on one of the wheels. Clarke and Stevensons's Electric Coal-Cutter. Fno. 15 CLARKE AND STEAVENSON'S COAL-CUTTER 207 On the same shaft as the ratchet-wheel is a small pinion which drives a winch, and a steel-wire rope is taken from this round a pulley some 50 or 60 yards in advance and back to the machine. The cutting-wheel makes about 30 revolutions per minute, but, the motor being series-wound, the speed of the cutting-wheel is regulated by varying the resistance of the light and v-shaped, arranged alternately, and the width of cut is 4 inches. The motor is enclosed in a gas-tight case, and is of 30 h.p.—the makers having found that the secret of success in coal-cutting lies in having a motor of ample size for the work to be done—the h.p. necessary to drive the machine in average ground being about 20. The motor is provided with a starting switch, so that it can be started at any time during working. The machine is made in three sizes—No. 1, standard type, height from floor, 28 inches; No. 2, medium type, height from floor, 22 inches; No. 3, low type, height from floor, 20 inches. Three sizes of wheel have also been made—a 4-foot wheel, undercutting 3 feet 6 inches; a 5-foot wheel, undercutting 4 feet 6 inches; and a 6-foot wheel, undercutting 5 feet 4 inches. The cutting-wheel diameter varies from 30 inches to a tons. In a recent visit to Lidgett Colliery, near Barnsley, where several of these machines have been at work for some years past, the following notes were made, which will be of interest as showing what is actually being done by coal-cutters. The seams worked of is the following section:
Good house coal 2 feet
Holing dirt 4 to 6 inches
Inferior coal 9 inches
The cut is made in the holing dirt, and the collier gets up the bottom coal, part of which is sent out for completion at the face when the rest is thrown into the pit. Two electric motors are used for work, and there are both cutting at night, but only one during the day. The cutter-wheel is 4 feet diameter, with a 3-foot 6-inch under cut. Two men operate the machine. The machines work on a face of 900 yards; gates are 22 yards apart, with cross-gates every 50 or 60 yards. One collier and a boy work in each gate when cutting begins; each yard gate cuts a load, and the machine is taking along the cross-gate at the start after it has begun cutting of the face. The cables supplying the coal-cutters are brought along the cross-gate and up every other gate to the face. Switches are fixed at each of these gate ends. The cables used are separate cables—not concentric. The actual speed of cutting when the machine was seen at work was 4 yards per hour. It was stated that in a shift of 8 hours no yards had been cut; deducting snap time, i.e., the actual time spent in cutting would be about 64 hours. In 1904 Messrs. Clarke, Steavenson & Co. designed an improved type of coal-cutter, the essential feature of which is the adoption of machine- 208 ELECTRICITY AS APPLIED TO MINING cut worm-gear running in an oil bath to replace the spur gearing originally used. The makers state that this has had the effect of reducing the working current and results in practically silent running. The machine is illustrated in fig. 137. The question of silent running is of course of the highest importance, and Mr. W. E. Garforth, who was consulted before the Electricity in Mines Committee that he had found worm-driven coal cutters a great success in this respect. Fig. 137. **CLARKE'S PATENT 1904 TYPE WORM-GEAR ELECTRICAL COAL-CUTTING MACHINE.** **Diamond Coal-cutting Machine.**—This is another successful disc machine, very similar to that last described—that is to say, on the lines of the original Gillett and Copley machine. The design is largely due to Mr. W. E. Garforth, of the West Riding Collieries, Normanton, who has a very large experience of coal-cutting. He was the first to advocate a deeper under-cut, which has been the means of largely reducing the number of shots required to cut through the coal ; besides which, of course, a smaller number of cuts is required for a given advance of face, so lessening the labour in timber-setting, &c. The machine is illustrated in fig. 138. It will be seen that there are two motors, one on each side of the cutting wheel; this, of course, increases the length of the armature and thus reduces the diameter of the cutting-wheel face. The advantages of having two motors are—first, that the balance of the wheel is improved; and, secondly, that, owing to these motors being smaller than if only one were employed, the diameter of the armature is less, and a lower build of coal-cutter is obtained for very thin seams. The motors are from 10 to 12 h.p., and run at a speed of 750 revolutions per minute. When the voltage is below 200, the motors are connected in A diagram showing a diamond coal-cutting machine with two motors on either side of a cutting wheel. DIAMOND COAL-CUTTER 209 Pic. 138. DIAMOND COAL-CUTTER 310 ELECTRICITY AS APPLIED TO MINING parallel, but above that voltage they are connected in series with each other. The motors are series-wound and started with resistances in series. Another admirable feature of the machine is the patent cutter-box (see fig. 139). The cutter wheel has lugs on its circumference on to which the cutter-steel, in each box holds three teeth, and the number of boxes on the wheel varies with the size of the wheel having ten boxes, and the seven-foot six-inch having fifteen. These cutter-boxes greatly expedite the operation of changing the teeth, and in thin seams, should it be required to cut both ways, this is done by simply reversing the motor and turning the boxes the other way round. Diamond Coal-Cutter : Cutter-Box. The speed of the cutting-wheel is from 10 to 15 revolutions per minute. The standard machine is made to hole to a depth of 5 feet 6 inches, and a width of 6 inches, the gross weight being 40 cwt., the weight without cutter-wheel and bracket being 30 cwt. The length of frame over all is 8 feet 6 inches ; width, 3 feet 4 inches ; and height from floor, 25 inches. The smallest machine built so far has a height from the floor of 18] inches. Fig. 140 shows an arrangement whereby the coal-cutter can be made to cut its way into the face when starting a cut. At the Middleton Colliery, near Leeds, two Diamond machines are employed in the Crow Coal, a seam which has only recently been opened. Particulars supplied by Mr. John Neal, jun. DIAMOND COAL-CUTTER 211 out. The seam lies at a depth of 226 yards from the surface, and the section is as follows :
Roof Blue Blud
Coal 1 foot & inches
Holling dirt 9 inches to 18 inches, very hard
Johnny coal 6 inches
The coal is very tender, which necessitates it being got on the 'end,' the 'bord' faces which are necessary to open out are got by hand-boiling, and the dirt is so hard that the holing has to be done in the coal, the yield from a 'bord' face being consequently practically all small coal. The standard machine is used, with an under-cut of 5 feet 6 inches ; voltage 300, current 100 amperes, the length of face on which each machine works being 100 yards, with gas-coals every 40 pieces. This makes cable Fig. 140. DIAMOND COAL-CUTTER : CUTTING-IN ARRANGEMENT. goes down the gate at the centre of the face, and the feeder to the machine is taken from it to the right or left, as required. The cables employed are armoured but the armouring does not constitute the return. The machine is filtered, not only by water but also by air, so that no dust is produced, and that the haulage-rope has to be carried over both motors and it is found inconvenient. The rate of advance is 50 linear yards in a shift of about 8 hours. The Diamond machine is also made to work with three-phase motors. A wound rotor with slip rings is used, as it has not been found possible to obtain the necessary speed without this form of motor. It can carry any reasonable amount of current with a squirrel-cage rotor. The same firm are now making coal-cutters to go on runners like a sledge, instead of using rails. This saves much labour in rail-laying, and the machine is kept up to its work by a long trailing bar behind it on the side away from the face, while a second bar in front timing cuts off the coal from the wheel from coming out of the cut. This machine has made remarkable performances in the way of fast cutting. In the presence of representatives 2 212 ELECTRICITY AS APPLIED TO MINING from several collieries it cut 200 yards in a foot seam to a depth of 5 feet 6 inches in 34 hours. The holing was done in inferior coal inter- spersed with pyrites. This is probably a record for longwall coal-cutting. The table given below is taken from a valuable paper on coal-cutting machines by Mr. W. E. Garforth, and furnishes details of the performance of Diamond coal-cutters in three different seams.
Section of seam CASE A CASE B CASE C
Boof - Blood 2 ft. 10 in. 4 ft. 3 ft. 3 in.
Seam : Coal 9 in. 15 in. 9 in.
Interior coal and dirt
Thill : Superior coal and dirt
Depth below surface 390 ft. 660 ft. 1,700 ft.
Inclination of seam 1 deg. 4 min. 1 deg. 4 min. 1 deg. 4 min.
Method of working Longwall End-on longwall and gatetool
Depth undercut by machine :
Thickness of cut : 58 in. 5 to 6 in. 5 to 6 in.
Average distance over one month cut by machine : 180 ft. 129 ft. 129 ft.
Number of men employed with machine:
333
333
333
333
333
333
Average number per shift: 170 cwt. 111 cwt. 119 cwt. Weight of each tub : 5 cwt. 84 cwt. 84 cwt. Average number of tubs filled per man : 34 13 16 Average wages over one month earned per shift : stallmen 105. 5c 91. 10d 105. 9d Average wages over one month earned per shift : filters 75. 3d 75. 8d 75. 8d Stallmen do internal filling, machine filling, making height, etc. Special men drive and clean up small coal and dirt after the machine :
Thickness of dirt taken up for packing:
Width between props:
Diameter of chocks:
Width between chocks:
Thickness of ripping:
Width of ripping:
Getting out : hand-holing
Getting out : machine-holing
The Jeffrey Longwall Coal-cutting Machine.--This is an American machine of the disc type. It differs in one or two points from those already described, and will be seen in fig. 141. The parts of the machine are balanced (the motor balancing the wheel), so that it is possible to use only one rail, the side thrust being taken by sleepers held in Transactions Inst. Mining Engineers, xxxii. 312. JEFFREY DISC MACHINE position by screw-jacks fixed against the roof. The cutter-wheel may be tilted up or down by means of a hand-wheel, and by this means inequalities in the floor can be successfully cut over. The gearing is similar to that already described, but is enclosed in a casing so that it can run in oil—a great advantage where it can be applied, as it makes the machine much quitter in action. The machine is driven by one shunt-wound motor of 25 h.p. It is a question whether the shunt-wound motor will start against a heavy load, but the writer thinks not, as it will not start against so heavy a load, and consequently there might be a little trouble in starting the machine if stopped during a cut. The cutter has from 20 to 25 teeth, and makes 40 to 45 revolutions per minute. The haulage-drum is provided with a friction-clutch, which in case of excessive Fig. 141. Jeffrey Electric Longwall Machine. strain, slips and eases the machine. The rate of advance is, of course, variable and can be varied from 8 inches to 24 inches per minute. This machine is in use at a Yorkshire colliery, and was inspected by the writer a short time ago. The seam is 180 yards from the surface, and about 3 feet thick. The coal-cutter is at present about two miles from the pit bottom, the voltage at the generator being 600 volts, and the current going to the machine 550 amperes. The amount of current taken by the machine is 30 to 40 amperes normally, but in cutting through 'brasses' and stone, which are met with frequently, the current goes up to 70 and 80 amperes. The cut is 5 feet 6 inches deep by 6 inches high. Three men are at the cutter, and one man goes in front of the cutter and behind the fillers, timbering and dressing the face down for cutting. Thus, while the cutter is working, the other three men is actually running only about half the shift, the rest of the time being spent in timbering, laying rails, &c. The face is 300 yards long, and the Fig. 143. Jaynes Electric Chain Heading Machine. CHAIN HEADING MACHINES 215 cutter works three shifts, filling being done on two shifts. The gates are 72 yards apart, and there is one filler at each side of each gate. The handle used to tilt the wheel is constantly in use. The cables in the roads are mostly concentric with the return conductor uninsulated ; but the cable in the face, which is dragged after the cutter, is a twin cable wound round with bar band. (a) Chain Machines.--This class of chain-cutter has found great favour in this country, and many of them are comparatively new. It is only fair to say, however, that the first chain machines were designed in this country about thirty-five years ago. Jeffrey Chain Coal-Cutter.--This machine is designed specially for pillar and stall or "room" working, and is used in this country for heading work in coal mines. It consists of a stationary frame, on the outside which slides the cutter-frame, to the rear end of which is attached the electric motor. The motor drives, by means of spur- and bevel-gearing, a sprocket-wheel which works the chain, on to which straight teeth are fastened, the teeth being set like those of a saw. On the under side of the stationary frame will be seen a feed-rack, on which the feed-rack on the other side ; this is the feed-rack, the spur-wheels which enter into the rack are driven by means of the gear-gearing shown in the figure. Screw-jacks are provided at both ends of the stationary frame. In starting a cut, the front and back jacks are screwed firmly against the face and roof ; the motor is then switched on, and this causes the chain-cutter to revolve and advance into the coal. As soon as it enters into contact with the face, cut into the coal; at the same time the feed-gear worked by the worms advances the motor and the frame in which the cutter-chain revolves into the coal, the cut thus getting deeper and deeper. At the end of the cut the gearing is reversed and the sliding frame travels quickly up to its original position in readiness for the next cut. The machine is then moved laterally across the face a distance equal to the width of the cut, and another cut is made, and so on. A trolley is provided on which the machine can be transported from place to place, and, if necessary, the electric motor can be made to work the trolley, thus dispensing with a horse. The electric motor is of 4 h.p., shunt winding type, 3-phase, 200 volts. The cutting mechanism consists of two chains, each 6 feet long. The machines are built to cut 5 feet, 6 feet, and 7 feet, the width of cut being 39 inches or 44 inches, and the depth of cut 4 inches. The length required in which to work the machine is about 6 feet longer than the distance under-cut, which of course makes its use in more longwall faces impracticable. On a recent visit to the Stanton Colliery, Burton-on-Trent, this machine was seen at work driving a heading in the Stocking Seam, which is 6 feet in thickness; only 4 feet 6 inches of coal is taken out in the heading, the rest being left as a roof. The machine took a cut of 39 inches in 216 ELECTRICITY AS APPLIED TO MINING width to a depth of 5 feet 8 inches to 5 feet 10 inches. Owing to the floor lifting a little it was found better to make the cut in two operations---i.e., to cut in half-way, then reverse and start again at the beginning and cut the remaining half. The height of cut was 4 inches. The width of heading was 9 feet, which necessitated three cuts. The first cut was made in about 63 minutes, the second cut was made in about 64 minutes, ready for the second cut, which was made in 66 minutes; 6 minutes were occupied in shifting the machine for the third cut, and 64 minutes were spent in cutting. This makes a total of 357 minutes in under-cutting a place 9 feet wide to a depth of 5 feet 4 inches, or about 51 square feet, and it will be seen that this performance compares very favourably with that of any other cutting machine. Mr. Robert Hay, the certificated manager, informed the writers that he had driven a length of 30 yards of coal-heading 9 feet wide in six days. At the mines of the Youghiogheny Coal Company, Pennsylvania, a large number of Jeffrey machines are in operation. The seam is worked on a model of the Jeffrey longwall system with sides 36 feet wide, separated by ribs of coal 6 feet wide. In this longwall system cannot be adopted, as there is no material available with which to build packs. The following table is given by Mr. Gresley:
PARTICULARS OF RESULTS OF ELECTRIC COAL-CUTTING MACHINES.
Type of Machine and Number in Use, 11,500-200 Chain-cutter Breast Machines,
Builders of machine, including motors Jeffrey Manufacturing Company,
Ohio.
Length of cut made by machine 69 inches.
Width of cut made by machine 4 inches.
Height of cut made by machine 4 inches.
Weight of machine 2,800 lbs.
Horse power used on machine
Area under-cut per run or cut 17 square feet.
Time occupied in making one cut 4½ minutes.
Tower capacity when working 10 b.h.p.
Power consumed when backing out 2½ b.h.p.
Average number of cuts per shift 10.
Number of tons produced per day
Tons (2,000 lbs.) produced per shift per machine 60.
Time occupied cutting About 3 hours.
Time occupied loading machine about, changing bits,
&c;About 6 hours.
Average number of cuts made in each 21-foot room:7.
Number of tons per chain used up:15.
Weight cut per chain used up:35,000 tons (2,000 lbs.).
Production per man by machine per day:6 tons.
' Central Station Electric Coal-mining Plant in Pennsylvania, U.S.A.', by W. S. Gresley; Proc. Inst. C.E., Part I. **CHAIN HEADING MACHINES** | | | |---|---| | Production per man by pick work. | 4 tons. | | Number of 21-foot rooms apportioned to each machine per double shift. | 24. | | Number of men operating machines. | Fewer men employed where machines operate or increased production per miner by use of machines. | 33 per cent. | | Less pit room occupied by machines. | Cost of blasting and hoisting up machine-cut coal. | One-half pick mining rate. | | Output per man per day. | Cost of under cutting by machines. | One-eighth pick mining rate. | | Approximate cost per year, including renewals, interest, and depreciation. | Per 2,000-th ton (=412). | In a recent paper in the *Transactions of the Institute of Mining Engineers,* a comparison is instituted between a disc machine and a chain-breast machine, and the writer of the paper comes to the conclusion that for their particular mine the chain-cutter has decided advantages. The mine is worked on the double-stall system, with stalls 60 feet wide and 30 feet long, and the face is cut at an angle of 50 degrees, which allows of the necessary width in the face for the Jeffrey machine. The advantages claimed for the chain machine are that it requires about 30 per cent. less power, that it only makes half the noise, that it is impossible for it to climb up or down in the coal. There are very few mines, however, where it can be said that they have a space of 12 feet between the face and the timbering, and this could not be accomplished at the mine in question were it not for the ribs of coal left. **Other Chain Machines.** A similar machine to the Jeffrey is the Goodman Electric Chain-breast Machine (see fig 143). It differs from A diagram showing a Goodman Electric Chain Breasting Machine. GOODMAN ELECTRIC CHAIN HEADING MACHINE. the Jeffrey in having rollers fixed at the rear end, which materially add to the speed with which the machine is shifted across the face. The stationary * Mechanical Under-cutting in Cape Colony, by John Colley; Trans. Inst. Mining Engineers, xxi. Part I. 217 218 ELECTRICITY AS APPLIED TO MINING frame also is of different construction, the cutter chain being supported entirely under the frame, thus enabling a cut to be taken directly at floor level. The motor winding is so compounded that it automatically adjusts its speed to the amount of work it has to do. It runs at a moderate speed with light cutting and slows down with heavy cutting or extremely dull bits. **Longwall Chain Machines.** Another modification of the chain machine is to have a narrow chain working at right angles to the face (as shown in fig. 144) similar to the original form of Baird Coal-cutter, and such machines are now being introduced. A diagram showing a Longwall Chain Machine. FIG. 144. **MORGAN-GARDNER ELECTRIC LONGWALL MACHINE.** The Diamond Coal Cutter Company are now making a Longwall Chain Machine. It is similar to their disc machine in having a motor on each side of the cutter chain. The standard depth of undercut is 6 feet by 6 inches, the width of cut being about 4 inches, and the extreme width of gap under the coal is 18 inches. For a machine cutting at floor level the overall height is under 20 inches. In very tender or friable seams the coal is liable after boling to break away, with the possibility of clogging the ordinary type of disc cutter, and in such cases a special cutter head may be more satisfactory. **Stanley's Coal-heading Machine.** This well-known heading machine, usually driven by compressed air, can also be driven electrically. The double form is shown in fig. 145. This is a 4-foot by 4-foot machine, the dimensions over the frame and gearing being 6 feet 3 inches by 5 feet.
The Diamond Coal Cutter Company are now making a Longwall Chain Machine.
It is similar to their disc machine in having a motor on each side of the cutter chain. The standard depth of undercut is 6 feet by 6 inches, the width of cut being about 4 inches, and the extreme width of gap under the coal is 18 inches. For a machine cutting at floor level the overall height is under 20 inches.
In very tender or friable seams the coal is liable after boling to break away, with the possibility of clogging the ordinary type of disc cutter, and in such cases a special cutter head may be more satisfactory.
Stanley's Coal-heading Machine.
This well-known heading machine, usually driven by compressed air, can also be driven electrically. The double form is shown in fig. 145. This is a 4-foot by 4-foot machine, the dimensions over the frame and gearing being 6 feet 3 inches by 5 feet.
TRAILING CABLES AND JUNCTION BOXES 219 to inches by 5 feet. It is fitted with a 30 h.p. motor, and the total weight is about 34 tons. **Trailing Cables and Junction Boxes.—One of the most important details connected with the safe working of an electrically driven installation of coal-cutters is the connection of the machine to the main supply of current. The usual plan in a longwall face is to have a junction and fuse box fixed, say, at every other gate along the face, and a trailing cable to carry the current from this box to the machine. A black and white illustration of a coal cutter with a long trailing cable. F10. 145 **STANLEY'S ELECTRIC HEATING MACHINE.** Callenders, Limited, make a useful form of Gate End Switch Fuse Box, which is shown in fig. 146. It consists of two parts : the main body of the box contains the switch fuses, which are controlled by a removable handle in the possession of some responsible person. In order to insert new fuses it is first necessary to work the handle into its normal position, so that it is impossible to renew fuses with the terminals "alive." The ends of the trailing cable are led into a box which fits on to the main body of the apparatus by means of a socket, and it will be seen that the design is such as to make contact with the plugs some little time after the socket has been fitted in, so that in the improbable event of the trailing cable becoming disconnected while in use, it will still be in contact with an enclosed chamber. The main body of the box is kept full of resin oil. The trailing cable in the case of a bad top should be very well protected either with armouring or a strong leather case. In order to avoid damage 230 ELECTRICITY AS APPLIED TO MINING to the trailing cable Mr. M. H. Habershon has established special rules at his collieries, which make it an offence for a collier to go into his stall. Figs. 146. A diagram showing the mechanism of a switch locking gear, marble base, lid locking gear, marble base, trailing end locking gear, and a mica shield over fuses. CABLE SWITCH LOCKING GEAR MARBLE BASE LID LOCKING GEAR MARBLE BASE TRAILING END LOCKING GEAR MICA SHIELD OVER FUSES CALLENDER'S GATE END SWITCH AND FUSE BOX. for the purpose of filling out &c. until the trailing cable has been taken away. 1 Departmental Committee—Electricity in Mines. . . . Level 1500 Dip 110 221 CHAPTER XI TYPICAL ELECTRIC PLANTS RECENTLY ERECTED. Continuous-current Plant—Three-phase Plant—Continuous-current and Three-phase Side by Side. The first plant described—illustrated diagrammatically in figs. 148, 149—is a continuous-current plant recently erected at a large Yorkshire colliery under the supervision of one of the authors. As will be seen from the detailed particulars given, the plant consists of two separate electric generators, each driven by a turbine (De Laval). The total output of each turbine generator is 100 kilowatts, or about 133 E.H.P., and the total h.p. is therefore 266 E.H.P., equivalent to, say, 300 I.H.P. in the steam-engine. The steam-pressure as delivered to the turbines is about 100 lbs. per square inch. The exhaust steam from each turbine is condensed by means of a jet condenser. The water for this purpose is drawn from a reservoir adjoining by means of a small centrifugal pump driven by a De Laval turbine, which raises the water to a height of about 7 feet above the steam-turbine. There is a separate condenser for each engine. A vacuum of 25 inches is obtained. The plant works with great steadiness and has given satisfactory results. A current of 550 amperes at present always in use, at 500 volts is sufficient. The higher voltage will be used when the power is taken to greater distances from the generator. Cables.—From generating station along overhead line and down shaft to (1) pump, 91/05', insulated with vulcanised india rubber, taped and braided; carried by leather suspenders from 2-inch straining wire, stretched on insulating fibre; then carried by wooden casing on wooden poles similar to those used for guide-ropes; carried down shaft in wooden casing carried on bunts six feet apart, as shown in fig. 148. The cables are laid in the 1-inch grooves, and cemented in with bituminous solution. From (2) pump along level to top of incline, 19/14 ; down incline, 19/14 ; along level to turbine (3), 7/14 ; down shaft to pump (4), 7/16 ; down shaft to turbine (5), 7/16 ; down engine-plane to (6), (7), and (8), 37/13 ; along level to (9) and (10), 7/16. Since this description was written another turbine and electric generator have been added, and the waste gases from the coke ovens have been used to generate steam. A diagram illustrating the layout of an electric plant. *91/05' = 91 wires, each 05' diameter. 222 ELECTRICITY AS APPLIED TO MINING Cables down the pit are kept, where possible, at opposite sides of the road. Where the roof and sides are strong they are fastened to props or bars by wooden cleats screwed on to the timber. Where the roof is not so good they are hung by tar-band from the bars, and a little slack left between adjacent points of support. All cables have vulcanised rubber insulation, taped and braided. Fig. 148. METHOD OF CARRYING CABLES DOWN SHAFT.
CURRENTS TAKEN : Ampères
(1) Eighty lamps on surface about
(2) Three-thro pump 135
(3) Single-rope haulage 25
(4) Three-thro pump 6
(5) Three-thro pump (duplicate) 40
(6) Main and tail haulage 65
(7) Three-thro pump 40
(8) Three-thro pump (duplicate) (40)
(9) Centrifugal pump 6
(10) Centrifugal pump 6
(11) Thirty-six lights underground 4
Total current liable to be on at once (not including duplicate pumps) 335 **TYPICAL ELECTRIC PLANTS** 223 Further plant in course of erection will utilise the whole of the power of the two generating sets. **Switchboard.**—Fig. 149 shows the connection of the switchboard, &c., in the generating station. The thick lines represent the cables along which the main currents pass, and the thin lines the shunt, voltmeter, and equaliser wires. Starting from the positive brush of one generator, the A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the connections of a switchboard for electric power generation. 149 A diagram showing the layout of an electric installation at Lilley Drift, Co. Durham. ELECTRIC INSTALLATION AT LILLEY DRIFT, CO. DURHAM. TYPICAL ELECTRIC PLANTS 225 The shunt-fields are connected from outside the series-winding, through the regulating resistance, to the negative terminal. These regulating resistances must be adjusted until both machines give exactly the same voltage, so that the readings of the two ammeters are identical, showing that each machine is taking its proper share of the load. The feeding mains are connected to the bus-bars through the main feeder switch. Fig. 151. LILLY DRIFT INSTALLATION: ELECTRIC GENERATOR. Other Switches.—One double-pole switch in generating station for surface lighting; two single-pole switches at junction near pump for cutting off and ; four single-pole switches at junction of engine plane and level for cutting off and ; four single-pole switches near pump for cutting off and , and also , , , , and . Three-phase Plant.—The following is a description of a plant recently erected by the Corlett Electrical Engineering Company, Limited, 226 ELECTRICITY AS APPLIED TO MINING at Lilley Drift, Rowland's Gill, belonging to the owners of the Priestman's Collieries, Limited, Newcastle-on-Tyne. A sketch plan of the installation is shown in fig. 150. **Engines.**—Pair of horizontal high-pressure engines having cylinders 14½ inches diameter by 16 inches stroke, and running at about 140 revolutions per minute ; steam pressure, 45 lbs. Engines fitted with rope pulley between the engines 8 feet diameter, grooved for six ropes 18 inch diameter. Engines fitted with high-speed governor and suitable lubricators to all journals. **Generator.**—Of the alternate-current three-phase type (see fig. 151), with stationary armature and revolving fields. The armature windings, carried in suitable tunnels through the laminated cores, insulated with micanite tubes. The field windings consist of twelve poles, all wound and carried on centre casting, securely keyed to main shafts, with suitable collecting rings for conveying exciting current to the fields. Bearings three in number, of the self-oiling ring type ; topologically, 37 inches diameter ; speed of rotation, 300 r.p.m., consuming 550 h.p. at 6 h.p. **Exciter.**—A four-pole continuous-current shunt-wound machine, bolted to main shaft of generator and running at same speed as generator. **Main Switchboard.**—The switchboard shown in fig. 152 is built up of wrought steel framework and suitable number of marble panels. **General Purpose Fuses.**—Fitted with porcelain fuse-cases ; main three-pole switch and fuse ; multiple contact field rheostats. **Feeder Panels,** arranged for two circuits, each circuit being fitted with ammeter, main three pole switch and fuse : No. 1 of 60 h.p. for fan circuit ; No. 2 of 25 h.p. for extensions. Also with three similar two-pole circuits on the primary sides of the transformers. The low-tension distribution cabinet is one panel similar panel arranged for a suitable number of circuits for controlling the various sections of lighting. All the high-pressure switches are arranged at the back of the board, the handles only projecting through the face, so that there is no possibility of short to the attendant. The fuses are of the detachable type, the fuse-carrier consisting of a massive porcelain block, and so arranged that it can be removed and a new fuse fitted without danger. The low-tension distribution panel for the lighting is fitted with switches and fuses mounted on the face of the board. There are at present two astatic transformers fitted behind the main switchboard for reducing the tension from 550 to 210 volts, each to kilowatts capacity. **Transmission Line to the Fan Station.**—This line is about one mile long, and is carried as nearly as possible in a straight line from the power house to the fan station. The poles are extra-stout croosseed poles, TYPICAL ELECTRIC PLANTS 227 fitted with specially strong oak arms and special high-quality porcelain insulators. Six bare wires, each No. 2 S.W.G., carry the current. A diagram showing a fan installation with multiple fans and ducts. LILLEY DRAFT INSTALLATION | SWITCHBOARD Fan.—The fan is of the Capell double-inlet type, 7 feet diameter by 8 feet wide, and is designed for an output of 140,000 cubic feet of air at 6/2 228 **ELECTRICITY AS APPLIED TO MINING** 1-inch water-gauge, and running at 275 revolutions. The fan is fitted with four bearings, also of the self-oiling ring type. **Motor.—The motor is a standard pattern three-phase alternate-current induction motor, rated output 60 B.H.P. at 600 revolutions. It is fitted with three brass slip-rings and with clutch-gear, so that when the motor has been started, the clutch-gear is engaged, and the slip-rings are short-circuited and the brushes raised, so that the moving part of the motor is only in contact with its two bearings. The power from the motor is transmitted to the fan through a double orange tan endless belt. As all bearings of both fan and motor are of the self-oiling ring-type, and as there are no special precautions in use on the motor, the amount of attention required by the plant is very small, and it is quite safe to arrange for periodic visits and not to keep a permanent attendant. **Electric Pump.—There is also in the mine a self-contained electric pump of the three-throth type, having rims a inches diameter by 6 inches stroke and running at 450 revolutions per minute. This is driven by a 3 hp. motor, the whole mounted on one bedplate and driven by means of a wide gear. **Lighting.—The whole of the colliery machinery in the vicinity of the power-house is supplied with incandescent lighting, and also the workshops and offices. Further, the residences of the various officials are lighted, and, as also, the main station building and other works outbuildings. Besides the private lighting above mentioned, the streets of the village of Rowlands Gill, extending for about one mile from the power-house, are also supplied with incandescent lamps, the wires being carried in a similar way to the main transmission line. The main station building, which is situated about half a mile beyond the fan station, at a distance of about a mile and a half from the power-house, is also being fitted with electric lighting in the streets, the power being taken from the main fan line, and, as there is no constant attendant in the fan station, the lighting of this village will be controlled by an automatic electric time switch, which will switch the lights on and off at predetermined times. The whole of the plant, which has now been working for some months without giving the slightest trouble, was erected by the Corlett Electrical Engineering Company, Limited. **Combined Continuous-current and Three-phase Plant.—The following is a description of a plant erected under the supervision of one of the authors at a large Derbyshire colliery. The first plant to be erected was of the continuous-current type, intended for electric lighting. A pair of tandem compound horizontal engines ; two high-pressure cylinders, diameter 11 inches ; two low-pressure TYPICAL ELECTRIC PLANTS 229 cylinders, diameter 23 inches, stroke 1 foot to inches, driving-belt wheel 12 feet diameter, with leather belt a foot wide ; counter-shaft with belt-wheel 3 feet diameter for main belt ; belt-wheels on the counter-shaft 7 feet diameter, driving two Hartnell shunt-wound two-pole generators, on which the belt wheels are 14 inches diameter. The average speed of the lighting engines is 50 revolutions per minute. The driving pulleys on the counter-shaft are loose on the shafts, but can be thrown into work by a brush-and-brake. The dynamo is driven at full speed, and the current can be thrown in and out at full speed. The dynamos are each capable of producing 95 amperes at 230 volts and 970 revolutions a minute, and they have worked very satisfactorily. There is also a Crompton dynamo, 230 volts, 100 amperes, 860 revolutions 15-inch pulley, two-pole, shunt-wound. The electric cars are drawn by two horses, one on each side of the enclosed type are used, and also glow lamps. All the engine rooms, pit-bank screens, sidings, offices, and the pit-bottom, are lighted by this plant. The current is also taken to the village, a distance of about half a mile, and used for lighting dwelling-houses, shops, and hotel. The cables are arranged as shown in the diagram. **Three-phase Hauling Plant.—For the underground haulage, which may eventually be extended to a considerable distance, a new plant has been recently erected, consisting of a pair of horizontal tandem compound engines, two high-pressure cylinders, 10 inches diameter; two low-pressure cylinders, 16 inches diameter, steam-pressure in engine room about 90 lbs., stroke 14 inches ; driving-belt wheel 12 feet diameter ; counter-shaft 3 feet 6 inches diameter ; dynamo driving-wheel on counter-shaft 5 feet diameter, pulley on dynamo 2 feet 1 inch diameter. The average number of revolutions of the engine is 96 per minute. Three-phase Westinghouse ten-pole generator, 136 amperes per phase, 450 volts, 72 revolutions and 60 periods. The power factor is say $8\frac{1}{2}$ x $3 \times 450$ (power factor say $8\frac{1}{2}$ = $90\%$) and the E.H.P. is $90\%$ of the power factor times the current. It is taken in one three-core cable down the pit, which is suspended in the shaft inside an iron pipe to protect it from injury. It is taken to the engine-room near the pit bottom, where it is connected to a Westinghouse motor, $4 \frac{1}{2}$ volts, $60$ amperes per phase. The power factor is say $8\frac{1}{2}$ x $3 \times 450$ (power factor say $8\frac{1}{2}$ = $90\%$) and the E.H.P. On the motor shaft is a belt-wheel, 2 feet diameter, driving on to a pulley-wheel 9 feet 6 inches diameter. A pinion on this shaft, 27 inches diameter, gears into a spur wheel on the counter-shaft 79 inches diameter, the pinion on which shaft, 19 inches diameter, gears into a spur wheel on the drum shaft 83 inches diameter. The drum is 6 feet diameter, and takes four and a half turns of a $\frac{1}{4}$ inch rope. This plant works efficiently and gives satisfaction. Makers: Cowlishaw, Walker & Co., Etnuria, Staffordshire. 230 ELECTRICITY AS APPLIED TO MINING CHAPTER XII ELECTRIC LIGHTING BY ARC AND GLOW LAMPS Arc Lamps—Electric Glow or Incandescent Lamps. We will now consider briefly the relative advantages and disadvantages of electric lighting by means of both arc and incandescent lamps and their suitability for mining work. Arc Lamps consist of cylindrical carbon rods, about ½ inch diameter in the majority of cases, attached to a mechanism controlled by electro-magnets through which the current flows. If the carbon ends touch one another, and the current is switched on, the electro-magnets, then energised, cause the mechanism to separate the carbon rods by a short air-gap, which may be anything up to ½ inch long. In the act of doing this a spark is caused which isolates the ends of the carbon rods, forming a conducting path for the current to flow across in the short air-gap between the ends. The current then intensely heats the carbon vapour and tips, so that they emit a powerful light. At least 40 volts must be maintained at the carbon tips in order to keep the arc burning. A difference of about 50 volts is allowed at the terminals of the lamp, if of the open or "inverted" type. There is, however, another type of arc lamp very widely used at the present day, called the "enclosed" form, in which the carbons, or rather the arc and part of the carbons, are enclosed in a glass globe arranged so that there is practically no air gap between them. In this case, when the oxidation of carbon goes on very slowly, and carbons that would last only about twenty hours in the open types will last about 200 hours in the enclosed type. The length of arc in the former reaches to ½ inch, against about ¼ inch in the former, thus allowing more of the light to get out. The efficiency of an arc lamp is usually reckoned in watt per candle-power, though it should be noted that its illumination is only one-third that of lamps requiring from 300 to 600 watt per candle, and it is usual to allow roughly one l.h.p. at the engine of an electric generating set for each lamp of 1,000 candle-power (c.p.). A diagram showing an electric arc lamp with two carbon rods connected by a short air-gap. ELECTRIC LIGHTING 231 Comparing the open and enclosed types, the advantages lie mostly with the latter, and are as follows: (1) Much smaller consumption of carbon. (2) Will burn for much longer periods. (3) Require re-trimming much less frequently. (4) Require smaller currents (roughly, one-half). (5) Will burn on higher voltage circuits (100 to 200 volts). (6) Immersed in oil, the lamp, enclosed in a tight-fitting chamber, cannot set fire to anything through dropping sparks or fragments of red-hot carbon. The initial cost of open and enclosed arc lamps is about the same, but when we consider the renewing cost after first installation, the difference is very evident; for instance, compare a 5-volt 10-ampere arc of the open type, burning twenty hours, with a 100-volt 5-ampere one enclosed, burning 150 hours without a renewal of carbons. In the latter the cost of carbon and labour of re-trimming are each about only $\frac{1}{5}$th, or $\frac{1}{8}$ that of the open type, but the luminosity is less (from 10 to 30 per cent.). The smaller size of the enclosed lamp makes it more convenient and cheaper in those at the outset. Arc lamps of all types are much more successful with direct than alternating-current, and with this latter take roughly 25 per cent. more electrical energy for the same amount of light than with direct currents, while the distribution of light, so far as open spaces is concerned, is much more uniform. The enclosed lamp does not break satisfactorily at a lower frequency than 40 periods per second. As the direct current arc particles of carbon are carried from the upper or positive carbon to the lower or negative one, and in the upper carbon a crater or hollow is formed at the end, which tends to concentrate the light and throw it down to the ground; with alternating currents both carbons burn away equally, the light passing from one to another alternately. **Electric Glow or Incandescent Lamps.** These consist of a specially manufactured carbon thread, or filament, enclosed in a small glass bulb from which nearly all the air has been removed. When the current passes through this filament it heats it to intense or white heat, causing it to glow. The efficiency, or watts per candle emitted, varies from $z_{1}$ to $z_{2}$, and the life from 600 to 1,400 hours, without the lamp becoming too dull to use. Glow lamps are made nowadays in enormous numbers for any voltage, from up to 420 volts, and of almost any c.p. from 1 up to 2,000, the higher c.p. lamps being more efficient. The higher voltage lamps, such as 200, last a shorter time than the lower voltage ones, such as those for 100 volts. A good average is to allow $z_{3}$ watts per candle for lamps of or about 100 volts, and $z_{4}$ watts for 200-volt 232 ELECTRICITY AS APPLIED TO MINING lamps, each of c.p. up to about 50. It is usual to allow eleven 16-c.p. 60-watt glow lamps per I.H.P. at the engine of a generating set. Comparing the lighting, by arc and glow lamps, of mines or other places, we see that, roughly speaking, 1 I.H.P. will produce about 160 c.p. with arc lamps and about 1,000 c.p. with arc lamps. For large open spaces lighting by direct-current enclosed arc lamps is more economical both in first cost and running cost afterwards, and should therefore be adopted where practicable. One single circuit can then be made to do for both arc and glow lamps— namely, that shown in fig. 76, of either one 100-volt arc lamp in parallel with 100-volt lamps or two 100-volt lamps in series with zero-volt mains with zero-volt glow lamps. It is, however, preferable to have at least two arc lamps in series, as one tends then to steady the other. Such an arrangement or combination might in some cases prove more economical than separate arc and glow lamp circuits, notwithstanding the zero-volt glow lamp being less efficient than the arc lamp. In offices and rooms it is preferable to use glow lamps, but they cannot compete with arc lamps for large open spaces. The height of the posts on which they are fixed and the distance apart must be governed by the brilliance of the illumination required. For underground lighting the incandescent lamp should be enclosed in a gas-tight glass bulb. **Nernst Lamp.—** This is an incandescent lamp, of which the filament is composed of substances which will only conduct the current when warm. It is therefore necessary to provide a heating arrangement to start the lamp, which takes about a minute to light up. A small wire is placed round the bulb and heated by a current flowing through this wire until the lamp is switched on. When the glowers gets warm enough to carry current, and the heater coil is automatically switched out of circuit by an electro- magnetic switch, and the lamp burns in the same way as an incandescent lamp. This lamp has a very good efficiency—about 1 r2 watt per candle power. For a detailed description of the construction of electric glow and arc lamps, see Maycock's "Electric Wiring, Fittings, Switches, and Lamps," or Parr's "Electrical Engineering in Theory and Practice," vol. I. **Separate Lighting System.—** It is often considered desirable to have an entirely separate system for the lighting, because in the event of an overload on one power circuit it might temporarily be broken, and if the lighting were on the same circuit, it would also be cut off just when most required. Variations in load also might affect the lighting system if it were combined with the power circuit. 233 CHAPTER XIII MISCELLANEOUS APPLICATIONS OF ELECTRICITY Telephones—Signal Bells—Electric Blasting—Electric Safety Lamps—Lighting Safety Lamps by Electricity—Electric Dolls—Electric Welding—Electric Winding. **Telephones.—The principle on which the telephone works is so well known as to require little or no description in these pages ; suffice it to say that the vibrations of a very thin soft iron disc, caused by the human voice speaking at it, are electrically transmitted along a wire and** FIG. 153. A diagram showing the operation of a telephone. A thin iron disc vibrates when struck by the sound waves from a human voice. The vibrations are transmitted through a wire to another similar disc, causing it to vibrate. This vibration is then amplified by a series of electromagnets, which cause a needle to move up and down, indicating the sound being transmitted. DAMP, DUST, AND GAS-PROOF MINING BELL. exactly reproduced at the other end on a similar iron disc actuated by an electro-magnet, itself controlled by the intermittent currents generated electro-magnetically by the voice wave service bell. The line connecting the two places may be suspended overhead or laid underground, and may consist of iron, copper, or phosphor-bronze wire, of comparatively small 234 ELECTRICITY AS APPLIED TO MINING gauge, as the current used in the telephone itself is extremely small. Up to comparatively recently, it has been the common practice to have a separate outward wire for each telephone and return by the earth for them all. Now, however, metallic wires are being used and the earth not used so much for a return, as this arrangement entails less interference in the telephone, while at the same time it reduces the resistance of the circuit, which is an advantage. Sometimes, however, at mines, the ordinary signal-bell circuits are used temporarily for telephone circuits, to save having separate ones for the Fig. 154. Fig. 154a. **The 'Peerl' Watertight Bell** **Watertight Mining Tappe.** latter. This cannot be recommended, since it is possible for the bells and telephone to interfere with each other and cause complication. Telephones are of great use in mining work, and should be installed in a systematic manner all over the workings. Should a mishap then occur at any point, assistance could be at once telephoned for, and the nature of the mishap reported. **Signal Bells.** These are electric bells, usually of the 'single-stroke' type, though sometimes of the 'trembling' form, which are employed to call the attention of some one, perhaps in the engine-house, to listen at the telephone, or in the case of winding, hauling, &c., as a signal to start, stop, &c., according to a prearranged code of signal rings. 154 **SIGNAL BELLS** For mining purposes electric bells should be damp-, dust-, and gas-proof, and one form having these qualifications is shown in side elevation, fig. 153. It is made by Messrs. Davis & Sons, Limited, of Derby, and consists of two small electro-magnets side by side, which act on a soft iron plate armature, pivoted on a short horizontal spindle. A rod fixed to the SINGLE-STROKE BELL. COVER REMOVED. upper edge of this armature terminates in a knob for striking the inside of the gong. The play of the armature is limited by two adjustable set screws working in an upright standard. All the working parts are enclosed in a metal case, fig. 154, and the two insulated wires, $i$, are led in through a stuffing-box or gland, $c$, in the case. SINGLE-STROKE BELL. OUTSIDE VIEW. Fig. 154 shows a good form of mining bell by the General Electric Company, while fig. 1544 shows a water- and gas-tight signalling key. The case of the latter is of gun metal, and all springs are of German silver and contacts platinum pointed. Figs. 155 and 156 show a table form of single-stroke bell and push-key combined. 235 236 ELECTRICITY AS APPLIED TO MINING It is not necessary for a single-stroke bell to be gas-light, as the circuit at the bell is never broken. A trembling bell, however, must be, as the contact lever carrying the armature makes and breaks the circuit many times a minute, creating a little spark each time. In such cases the containing box must be strong enough to withstand the explosion of the small amount of gas contained in it without fracture or contact with the outside gas. If the same line be used for both signal bells and telephones, there is the liability for a telephone call to be mistaken at the engine-room for a signal, with possibly the starting or stopping of the engine at a moment when it was not wanted. This might even entail loss of life. Separate circuits are, therefore, to be recommended for both telephones and signal lines, as the cost of installation for each is small, the initial cost may and probably will be much more than outweighed by greater ease and facilities in working afterwards. Fig. 157. A diagram showing two electric signals on a haulage road. **Electric Signals on Haulage Road.** R. Bell. P. Push. 1. Battery of seven Leclanché Cells. Both sets could be carried together either underground or overhead, thus entailing only one expense in laying them down. Tolerably well insulated copper for both kinds of circuits would be the best to use (except haulage signal wires, which are usually bare in the haulage road, to enable signals to be made instantly at any point), the gauge depending on the distance run. For a house-bell circuit a single copper wire $\frac{1}{2}$ to $\frac{1}{4}$ gauge would be used. For outside work and long distance, $\frac{1}{8}$ to $\frac{1}{16}$ gauge, though this is not so severe for a bell two miles distant. Telephone wire need not be any larger. **Electric Signals.—** Fig. 157 shows the arrangement of an electric signalling apparatus for a main and tail haulage road. Both bells R can be rung from either end of the line by depressing the push P, which makes a connection between two fine wires. The bells are Mercier's patents, which have been tried and found that a battery of seven Leclanché cells, connected up in series at each end is powerful enough to ring them through 2,900 feet of line. The line wire is of No. 104 B.W.G., and is supported on earthen ELECTRIC SIGNALS 237 ware insulators, to which it is attached by fine wire ; the insulators, which are $2\frac{1}{2}$ inches in diameter, with a groove $\frac{1}{4}$ inch deep and $\frac{1}{4}$ inch thick, are screwed on to props or bars; and the two line wires kept about 8 inches apart, which is a convenient distance for making contact between them. Fig. 158. A diagram showing a signal arrangement for a haulage rope where the engine is situated on the surface; in this case there is no down. Electric Signal along Road and Up Shaft. B, Bell. P, Push. L, Battery of six Leclanche Cells. with a lamp or metal tool of any kind, when it is necessary to signal from some point on the road. It is found convenient to do the wiring at both ends with insulated wire—say, No. 16 B.W.G. Fig. 158 shows a signalling arrangement for a haulage rope where the engine is situated on the surface; in this case there is no down 238 ELECTRICITY AS APPLIED TO MINING signal. The wires in the shaft, 48 yards deep, are No. 16 B.W.G., and are india-rubber covered, taped and braided, and supported by two insulators fixed to the brickwork at the top of the shaft and attached to two more insulators at the bottom of the shaft. The wires in the underground road are No. 10 B.W.G., and are supplied as in the case previously described. The bell is an Edwards with 5-inch gang, and six Leclanché cells in series are found to be powerful enough to ring it. In both cases clattering bells are used, not single-stroke bells. Fig. 159 shows an arrangement for signalling between an engine-house and turn-outs on a haulage road. The bells are rung by local batteries, i.e., The circuit is completed by the action of an armature magnet in the relay, and caused by current from the battery which by itself would not be strong enough to ring the bell. When a single battery can be used it is undesirable to use relays, as they introduce a new com- ![Diagram of Three-wire Signal Circuit with Relay](image) plication, and in wet places especially may seriously increase the liability to break down ; when kept dry, however, they can be used satisfactorily. Wire No. 1 is an earth or return wire, and need not be insulated. The relays are connected in parallel between wires Nos. 1 and 2, and the pushes, K, connect wires 2 and 3. All the bells are rung on depressing any of these relays. Electric Blasting.--The use of electricity as a means of firing shots, although first practised in this country about the year 1845, is a comparatively recent innovation in the mining of coal and other minerals. In the Coal-mines Regulation Act, 1885, no restriction was placed upon the method of electric lighting while in use. The principle involved is made use of the use of the pricker, which was employed when firing a shot with a squib or 'German.' About this time, however, considerable attention was being paid to the question of coal-dust in mines, and the necessity of having a safer explosive than gunpowder for fiery and dusty mines was recognised ELECTRIC SHOT-FIRING 239 by the passing of an 'Act to amend the Coal-mines Regulation Act' in 1865, which gave the Secretary of State power to prohibit the use of any explosive deemed likely to be dangerous. Following upon this came several orders regulating the use of explosives in coal-mines, in which it is provided that no explosive shall be fired by a powder charge, nor by a charge shall be fired by an efficient electrical apparatus, or by some other means equally secure against the ignition of inflammable gas or coal-dust.' The use of electricity for firing shots, however, had become very common in mines some time before the issue of these orders. **Advantages of Electric Firing—The advantages of this method of firing shots may be summarised briefly as follows :** 1. The person firing the shot can do so from a distance, and thus the risk of a premature explosion is avoided. 2. There is very little danger of a shot hanging fire ; with a tape fuse any damage caused by the explosion might cause it to smoulder and delay the explosion, with the possibility of injuring fatally or otherwise the shot-firer, who had returned to the place under the impression that a 'miss shot' had occurred. 3. A saving of time in the case of a ' miss shot.' It is provided in the special rules that no person shall return to the place until after a certain stated interval has elapsed. This is very frequently there is a provision enabling this to be dispensed with in case the attempt to fire the shot has been made with some electrical appliance. 4. No burning fuse being employed, there is nothing (apart from the detonating explosive) to fire gas into the pit. 5. If necessary, any two shots may be fired simultaneously. With regard to the first named of these advantages, it is of special importance when considering the firing of shots in shaft-sinking. With the ordinary tape fuse, sufficient length is provided to enable the sinkers to be withdrawn before the shots explode; but any mishap in connection with the winding apparatus might involve very serious consequences. With electric firing, however, all at the pit top, and the shots are not fired until everyone is out of the pit. There are three methods of electric shot firing in use : 1. With low-tension fuse and exploder. 2. With high-tension fuse and exploder. 3. With high-tension fuse and the necessary electric current derived from the lighting or power mains. **Low-tension System.—A low-tension fuse is shown in fig. 160, and consists of two tinned copper wires 0.22 inch in diameter, the length depending on the depth of the hole; these wires are wrapped with cotton saturated with insulating compound, and for wet situations (such as shaft-sinking) are covered; in addition, with gutta percha or india-rubber.** 240 ELECTRICITY AS APPLIED TO MINING The fuse wires terminate in the detonator, and are joined together by a bridge of fine platinum wire, which is embedded in a flashing mixture adjoining the detonating compound. The passage of the electric current heats the platinum wire to redness, the flashing mixture is ignited, and the detonator explodes. **High-tension System.** A high-tension fuse is shown in fig. 161, and differs from the low-tension fuse in only one respect--namely, the absence of the platinum wire bridge. A current of much higher voltage is used, Fig. 16a. Low-tension Electric Detonator. and the explosion is caused by the heat generated owing to the resistance of the sensitive chemical compound by which the terminals are surrounded. **Explosives.** 1. Primary batteries. Low-tension fuses, which only require a low voltage, may be fired by a primary battery. This may consist of a Leclanché cell (see figs. 9 and 10); the ordinary open-topped glass jar being replaced by an ebonite cell, the top of which is sealed so that loss by evaporation or spilling is avoided. Fig. 161. High-tension Electric Detonator. Dry batteries (see fig. 14) are more commonly used, however, and are preferred for mining work owing to their greater portability. The E.M.F. of a Leclanché cell or of a dry battery is nominally 155 volt. The cell should have as low an internal resistance as possible. Mr. William Maurice, in a very valuable paper on the subject contributed to the Institution of Mining Engineers, gives the following interesting example on this particular point: *Suppose, for example, treating the question in an off-hand manner, a* Transactions of the Institution of Mining Engineers, vol. 169. MAGNETO-EXPLODERS 241 dry cell, say, 2 inches in diameter and 7 inches in length, with an internal resistance of 0.7 ohm, be selected to fire Nobel low-tension fuses through a go-foot twin wire of No. 20 S.W.G. It can be seen almost at a glance that the apparatus is foredoomed to failure, for the total resistance of the circuit (resistance of cell + resistance of line + resistance of fuse) exceeds 3 ohms, and since the E.M.F. required to fully induce a fuse has been shown by Ohm's Law (E = current x resistance) that the E.M.F. of the cell (1.55) is barely sufficient to overcome the resistance of the circuit. If, on the other hand, a cell of lower internal resistance, say, a C size Obach (with an internal resistance of 0.25 ohm) be selected together with a fusing line containing more copper, say, a 3/22 A diagram showing a magnetoelectrode device. FIG. 16a. Magneto-Exploder. m m, Magnets; a, Armature; t t', Terminals; k, Firing key; c, Handle. S.W.G., this apparently trifling modification makes all the difference between failure and success.' The total resistance in the latter example is as follows :
Rc. (internal resistance of cell) Ohm
R.L. (resistance of 18o feet /22 S.W.G.) 0.75
R.f. (resistance of fuse bridge) 0.75
Total resistance 1.79
On taking the product of 1.79 x 0.6 (fusing current), it is found that the E.M.F. required to force that current through the circuit is less than the 242 ELECTRICITY AS APPLIED TO MINING E.M.F. available at the terminals of the battery; hence this set may be expected to perform its work in an efficient manner. Magneto-exploders are very commonly used for firing both high- and low-tension fuses. They depend upon the principle that when an armature is revolved between the poles of a permanent magnet an electric current is produced. A diagrammatic view of a magneto machine is shown in fig. 162.¹ Fig. 163. **METHOD OF FIRING SHOTS FROM LIGHTING OR POWER MAINS** When a shot is to be fired the fuse wires are attached to the terminals, $r^{\prime} r^{\prime\prime}$, the handle, $a$, connected by gearing to the armature, $a$, is revolved, and when a good speed has been attained the button, $k$, is pressed in and the circuit through the fuse is completed. Fig. 164. Fig. 165. TUMBLER SWITCH. DOUBLE-Pole PLUG SWITCH. The winding of the armature decides whether the fuse shall be high or low tension; in the former the armature is wound with a greater number of turns of finer wire. In course of time the magnets lose some of their strength and require to be re-magnetised. 3. Dynamo-electric exploders have the same principle as the ordinary ¹ Reproduced from a drawing in Mr. William Maurice's paper. FIRING FROM MAINS 243 continuous-current dynamo (see page 38). The electric current generated by revolving the armature is carried round the field magnets, converting them into electro-magnets and producing a much stronger magnetic field. The 'Rackbar' exploder is constructed on this principle, but, instead of the armature being revolved by turning a handle, the teeth on the rack-bar engage with those of a pinion on the armature shaft, and by forcing the rack-bar rapidly to high speed the armature strength is sustained. At the moment when the current reaches its maximum strength (a), then the rackbar is nearing the end of its stroke) the current is directed into the fuse wires by an automatic switch. Fig. 166. Messrs. John Davis & Sons' SHOT-FIRING KEY. Dynamo-electric exploders are the most powerful, and are largely employed in shaft work and in large drifts. They possess the advantage of durability. The dynamo or battery used in sinking must be locked up, and then it cannot be connected to the firing line, except by the foreman after all the sinkers have left the shaft. **Testing Exploders:** A voltmeter or electric lamp may be used to test the power of a dry battery or other exploder. **Firing from Lighting or Power Mains—In the case of shaft-sinking or driving a storm drift, shot-firing can be effected by using the** *For details of this and other exploders the reader is referred to Mr. W. Maurice's paper in Transactions of the Institution of Mining Engineers, xiv., xv., and xvi.* 2 2 244 ELECTRICITY AS APPLIED TO MINING current from the lighting or power circuits about the mine where such exist. Special precautions must be taken to prevent accidental firing of shots—this being more likely to occur when using current derived from this source than when using an exploder. A double-pole switch, which makes contact with both mains, is employed, and this is placed under the sole control of one man. The shot-firer should be in a position to tell whether the mains are alive or not. This can be done by having an incandescent lamp or series of lamps joined across the mains before the double-pole switch is reached. Fig. 167. A diagram showing a double-pole switch with two poles connected to two mains. One pole is connected to a lamp or series of lamps, while the other pole is connected to the main circuit. Fig. 168. A diagram showing a shot-firing arrangement manufactured by Messrs. John Davis & Sons. A double-pole plug switch is inserted in one of the mains, and in addition there is a double-pole plug switch (see fig. 165). High- or low-tension fuses may be employed, and these are generally arranged in series if the mains are of high tension. METHOD OF ARRANGING FUSES IN SERIES. METHOD OF ARRANGING FUSES IN PARALLEL. A sketch showing the necessary connections is shown in fig. 163. In this case a tumbler switch (see fig. 164) is inserted in one of the mains, and in addition there is a double-pole plug switch (see fig. 165). High- or low-tension fuses may be employed, and these are generally arranged in series if the mains are of high tension. Fig. 166 shows a shot-firing arrangement manufactured by Messrs. John Davis & Sons for use in firing from the mains in the case of either lighting or power circuits. It will be seen that there is a switch in each main, one being a tumbler switch and the other a press-down switch. There is also ARRANGEMENT OF FUSES 245 an incandescent lamp, by means of which the apparatus can be tested ; by placing a piece of metal across the fuse terminals and switching on the positive main, and then depressing the key in the negative main, the lamp should light up if the apparatus is in working order. The whole is contained in a strong lock-up case, so that it cannot be tampered with. Arrangement of Wires for Simultaneous Blasting.—Where more than one shot has to be fired electrically there are two possible methods of arranging the wires—namely, 'series' and 'parallel.' In the 'series' method, shown in fig. 167, the current passes from one firing main through all the fuses in turn and back to the other firing main. As the resistance varies directly as the length, it will be seen that with the 'series' arrangement the total resistance is the sum of the resistance of each fuse added to the resistance of the line conducting the current. The number of shots fired is represented in proportion to the number of shots fired simultaneously, but the voltage rises in proportion to the number. Fig. 169. CABLE REEL. In the parallel method each fuse is connected directly to both mains, so that the current divides amongst the various fuses (see fig. 168). In this system the resistance of any fuse is less than that of a single fuse, the section area of the main conductors or cables must be in proportion to the current required—that is to say, in proportion to the number of fuses to be fired simultaneously. Low-tension fuses which have a very low resistance are commonly arranged so that when several fuses are arranged in 'parallel' if the number is too great to enable them to be fired in 'series.' The 'series' arrangement is more convenient in practical work than the 'parallel' arrangement, especially in shaft-sinking, where it permits a ring of side shots to be easily arranged round a shaft without interfering with the work carried on in the sump, whereas with the 'parallel' method there will be more care and more wire required to avoid taking some of the wires across the shot. High or Low Tension.—Both high- and low-tension systems have their advocates, and both are working satisfactorily. The number of 246 ELECTRICITY AS APPLIED TO MINING recorded mine-shots are about the same with each system. For firing one shot or two shots with low-tension fuses a very light and cheap form of dry battery may be used, and this allows the operator to have one hand at liberty, with which he can pull back the firing cable as soon as he hears the report, and so save it from damage by falling rock or coal. Low-tension fuses can also, if desired, be tested by passing through them a weak current (insufficient to fire) and noting if the needle of a galvanometer in the circuit is moved. In this way, however, it is only possible to test each fuse, but the amount of excellence that the testing is perhaps unnecessary. For shaft work, how-ever, where the length of the firing line may be very long, the high-tension system permits of a thinner wire being used, and so it tends to reduce the cost of the firing line. A reel is necessary to carry the firing cables for use in shaft and other work is shown in fig. 169. Mr. F. T. W. Brain, in his evidence before the Departmental Committee on Electricity in Mines, gave the cost of firing 1,000 shots, including detonators and depreciation of cable and exploder, as follows : With high-tension electric system, cost per shot equals 1.73d. With low-tension electric system, cost per shot equals 1.48d. With Bickford's patent fuse 2.17d. Figs. 170. Nobel's Arrangement of Detonator and Explosive. This does not include the time of the shot-firer, which would be more in the case of the electric than the Bickford fuse. Missiles fired by Nobel's Patent fuse is a factor for successful electric blasting is to see that the firing-line from the exploder to the fuse is not short circuited --i.e., that the two wires carrying the current shall be perfectly insulated from each other. In wet ground it is advisable to insulate the connection of the line-firing cable to the fuse wire by wrapping it with rubber tape or other insulating material. With this arrangement all three wires of the fuse wires in the shot-hole Messrs. Nobel recommend the use of a fuse protector, which is simply an iron tube in which the fuse wires are placed whilst stemming. To avoid striking the detonator during stemming Messrs. Nobel A diagram showing a detonator and explosive arrangement. SHOT-FIRING IN SINKING PIT 247 Fig. 171. A diagram showing a shot-firing mechanism in a sinking pit. The top view shows a vertical cylinder with a horizontal rod inside, connected to a trigger mechanism at the top. The side view shows the internal components, including a spring (W), a trigger (T), and a firing pin (F). The diagram also includes labels for various parts such as R.W., W, M, B1, B2, C, A, W-M-B2-C. ELEVATION Side of box removed A sectional plan of the shot-firing mechanism. The diagram shows the internal components, including a spring (W), a trigger (T), and a firing pin (F). The diagram also includes labels for various parts such as C1, C2, W-M. SECTIONAL PLAN. DYNAMO ELECTRIC EXPLODER 248 ELECTRICITY AS APPLIED TO MINING recommend that the detonator be placed at the back of the primer cartridge as shown in fig. 173. Electric Shot-firing in Sinking Pit.—Fig. 171 shows a small series-wound dynamo used for this purpose and capable of giving a Fig. 172. SHOT-FIRING IN SINKING PIT. pressure of 500 volts. Current is generated in the armature, $A$, and passes from the brush, $B$ (working on a two-part commutator), through the windings, $W$, and thence to the contact, $C$, and to the terminal, $T_1$. **SHOT-FIRING IN SINKING PIT** From the brush $h$, the current goes straight to the terminal, $t_1$. The armature is made to revolve by means of the arrangement shown in the figure. When the handle, $h$, is drawn up, the rider, $w$, rotates the spindle, $s$, which rotates the ratchet wheel, $k$, without turning the wheel, $c_1$. On the down stroke, however, the ratchet wheel engages the pawls which are fixed to the gear wheel, $c_1$, which rotates the armature through $c_2$. The ratio of $c_1$ to $c_2$ is about two and a half to one, and the spindle, $s$, rotates about seven revolutions per second. The current then flows through the brushes, $b$, takes to push down the handle the armature makes five revolutions. On reaching the bottom, $x$ pushes down the contact, $c$, and the terminals, $t_1$ and $t_2$ are connected to the brushes ; the armature now continues to rotate as long as its momentum lasts, the ratchet allowing it to do so, and a current is sent round the circuit. The brushes are No. 16 copper wire, india-rubber covered, taped, and braided. A pair of these wires are wound on a wooden reel mounted on a trellis with a handle for winding and unwinding ; they are let down the shaft when a shot is to be fired, and one is attached to each of two detonator wires on adjacent shots ; the remaining detonators are connected up in series ... etc., so that all detonators are fired at once. The battery box is kept locked while the connections are being made, and the charge-man down the pit has the key. The detonators used with this arrangement are No. 7 low-tension fuses and ignite a charge of $\frac{1}{8}$ to 2 lbs. of gelignite. The detonators are buried in a small cartridge of the explosive used for filling holes in sandstone or brickwork. The holes are driven on to sticks; these are put down the holes after the rest of the charge is in, the top of the hole is plugged up with clay, and the wires connected up. In firing 'sumpers,' as shown in fig. 173, there are seven detonators connected in series. After the 'sumpers' have been fired and the loose rock seen out of the pit, eleven side holes (shown dotted in the elevation) are driven into the ground in this way; thus, in one way, eleven detonators are set. These particular are for a 15-foot shaft, inside the brickwork, or 17 feet excavated, in average coal-measure shales and sandstones. The holes are drilled about 6 feet deep, and deepen the shaft about 5 feet; hence, each hopper holding about 4 tons each, the sumpers take six times together about four hours for filling full. The sumpers take four or five hours to drill, and the side holes five feet in ordinary measures, though in strong rock they may take three times as long. The dirt will be cleared out in about eight hours with a dozen men in the pit. Thus, if there are no delays the pit should advance, in soft ground, 5 feet 6 inches in eighteen to twenty hours, allowing one hour for charging, firing, and examining the sides after firing, which comes to about 7 feet per diem. A diagram showing how shot-firing in sinking pit works.
250 ELECTRICITY AS APPLIED TO MINING
Name Description Weight with Weight Differ- Time be- Approxi- Circular
Test Test ent een between uate Cost Area Lt.
First and First Test Mineral Cost Head
Davy - Round stick, burning a mixture of part paraffin, part zinc chloride L36 Lbs. Lbs. Minutes Pence Degree of Arc:
c83 c73 c70 90 921 plo
Unshunted Clammy Round stick, burning a mixture of part paraffin, part zinc chloride, part zinc chloride, part zinc chloride, part paraffin, 3 parts colza c86 c75 c71 90 928 plo
Ashworn - Defector - Thorneberry - Wolf's - Ashworn's Unswick - Ashworn's Twowick - Johnson - Morgue - A. H. G. - Brigid Electri- Headland Electric - Headland head lamp, not safety Flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass; flat stick, clear glass;
NameDescriptionWeight with TestWeight TestDiffer-Times between First and First TestApproximate Cost (pence)Circular Area Lt.
Davy - Unshunted Clammy - Aluminum Defector - Thorneberry - Wolf's - Ashworn's Unswick - Ashworn's Twowick - Johnson - Morgue - A. H. G. - Brigid Electri- Headland Electric - Headland head lamp,Round stick, burning a mixture of part paraffin and zinc chloride (part zinc chloride) (part zinc chloride) (part zinc chloride) (part zinc chloride) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin) (part paraffin); Flat stick burning two parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flat stick burning mix- ture of three parts zinc chloride and one part colza Flatstick burning two parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts zinc chloride and one part colza Flatstick burning mixture of three parts锌chlorideandoneparaf-filcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszincchlorideandone-partcolzatbackFlatstickburningmix-tureofthreepartszn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-partsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-PartsZn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mix-tur-of-three-Pars-Zn-chlorid-and-one-paraf-fil-col-za-at-backFlatstick-burning-mixt-Zn-chlorid-and-one-paraf-fil-col-za-at-backMixture of cola (3 parts) and paraflin (1 pence) Paraffin Benzinine Note: These observations on candle power must be considered in view of the fact that the light given by the above lamps would be greatly varied ; generally the because lamp gives the best light,
TESTS OF SAFETY LAMPS 251
Time Hours of burning Height of Water Level Place of Filament, Edge or Base Glass Base of Bulb, Upper or Lower Size of Reflector Remarks
Candle power Candle power Inches Inches Inches Inches
17½ — — 0-66 — — —
2½ — — 0-66 — — —
17½ — — 0-66 — — —
2½ — — 0-66 — — —
Equation:
e.g. = z2 Reflector parallel with wick
e.g. = z1 Reflector perpendicular with wick
e.g. = z3 Reflector at right angles to wick
e.g. = z4 Reflector at right angles to wick
e.g. = z5 Reflector at right angles to wick
e.g. = z6 Reflector at right angles to wick
e.g. = z7 Reflector at right angles to wick
e.g. = z8 Reflector at right angles to wick
e.g. = z9 Reflector at right angles to wick
e.g. = z10 Reflector at right angles to wick
e.g. = z11 Reflector at right angles to wick
e.g. = z12 Reflector at right angles to wick
e.g. = z13 Reflector at right angles to wick
e.g. = z14 Reflector at right angles to wick
e.g. = z15 Reflector at right angles to wick
e.g. = z16 Reflector at right angles to wick
e.g. = z17 Reflector at right angles to wick
e.g. = z18 Reflector at right angles to wick
e.g. = z19 Reflector at right angles to wick
e.g. = z20 Reflector at right angles to wick
e.g. = z21 Reflector at right angles to wick
e.g. = z22 Reflector at right angles to wick
e.g. = z23 Reflector at right angles to wick
e.g. = z24 Reflector at right angles to wick
e.g. = z25 Reflector at right angles to wick
e.g. = z26 Reflector at right angles to wick
e.g. = z27 Reflector at right angles to wick
e.g. = z28 Reflector at right angles to wick
e.g. = z29 Reflector at right angles to wick
e.g. = z30 Reflector at right angles to wick
e.g. = z31 Reflector at right angles to wick
e.g. = z32 Reflector at right angles to wick
e.g. = z33 Reflector at right angles to wick
e.g. = z34 Reflector at right angles to wick
e.g. = z35 Reflector at right angles to wick
e.g. = z36 Reflector at right angles to wick
e.g. = z37 Reflector at right angles to wick
e.g. = z38 Reflector at right angles to wick
e.g. = z39 Reflector at right angles to wick
e.g. = z40 Reflector at right angles to wick
e.g. = z41 Reflector at right angles to wick
e.g. = z42 Reflector at right angles to wick
e.g. = z43 Reflector at right angles to wick
e.g. = z44 Reflector at right angles to wick
e.g. = z45 Reflector at right angles to wick
e.g. = z46 Reflector at right angles to wick
e.g. = z47 Reflector at right angles to wick
e.g. = z48 Reflector at right angles to wick
e.g. = z49 Reflector at right angles to wckle=reflector parallel with Wick z=reflector perpendicular with Wick z=reflector top, side z=reflector bottom, side z=reflector top, side z=reflector bottom, side z=reflector top, side z=reflector bottom, side z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angle z=reflection angel 252 ELECTRICITY AS APPLIED TO MINING Electric Safety-lamps. The use of the electric lamp in the working places of a mine has, up to the present, not had a very extensive application. The importance of having a good light cannot be over-estimated, and a better light would no doubt decrease the number of accidents by falls* of roof and sides, and would be beneficial to the eyesight of those employed ; but these advantages are far outweighed by the disadvantages of other prin- ciples of safety which are found in the ordinary oil safety-lamp. The disadvantage of an electric lamp for mining work is that it will not without cumbersome and delicate attachments detect the presence of poison ous or inflammable gases, consequently it can never in any case be used where gas detectors are employed. At most collieries blasting operations are in progress, it is essential that the state of the air in the mine should be known to the workmen. The question also arises as to whether the electric filament of the lamp is capable of igniting an explosive mixture of fire-damp and air, while the bulb itself may be considered as perfectly safe, but in the event of it getting broken there is a possibility of such a mixture being fired.* It should be borne in mind, however, that the ordinary oil lamp, with its exposed glass, stands an equal or even greater chance of being broken than one in which case the flame would un- doubtedly fire gas. The great difficulty found in designing an electric hand-lamp was to procure a sufficiently powerful, and at the same time portable, form of battery. The principal forms of hand-lamps are the Headland lamp, Suss- man lamp, and British Patent lamp. **The Headland Lamp.** This lamp, which is shown in fig. 173, uses the Headland battery already de- scribed on page 21. The lamp is made in two forms, one with a reflector behind and the other without, and it is the latter type which is illustrated in fig. 173. The photometric value of the two types is given in the table on pages 250 and 251. **The Sussman Lamp.** Another lamp which has met with success in mining work is the Sussman lamp, which is in use at the Murton Collieries.† † In 1904, 437 per cent. of the total number of deaths in mines were due to falls of roof and sides, and only 11 per cent. to explosions. ‡ Transactions Inst. Mining Engineers, vol. 8R. * The Sussman Electric Miner's Lamp by W. O. Wood ; Trans. Inst. M.E. xxi. 189. A diagram showing an electric safety-lamp. HEADLAND ELECTRIC LAMP. A diagram showing a Headland electric lamp. HEADLAND LAMP. A diagram showing a British Patent electric lamp. BRITISH PATENT LAMP. ELECTRIC LIGHTING OF LAMPS 253 The lamp, as described by Mr. Wood, is made in two patterns—No. 1 with an ordinary or fixed bulb, and No. 2 with a top (patented by himself) containing a removable bulb, so that in the event of a bulb falling in the mine it can be replaced immediately by a new one. The secondary battery is of the Faure or pasted type, consisting of two rectangular ebosite cells, each cell containing three elements (one positive and two negative), connected in series, and having a metallic framework tapering inwards and pasted with oxide of lead, incorporated with a special binding material and made into a paste with dilute sulphuric acid or sulphonate of ammonia : the mass being pressed into the leaden grids and allowed to set and dry for four days. When dry, the plates are placed in a bath containing dilute sulphuric acid, and are then formed or converted, the positive into peroxide of lead, and the negative into spongy metallic lead, by means of an electric current. These plates, connected by leaden strips, are placed in the cell, which is then filled with the electrolyte, a semi-liquid compound of dilute sulphuric acid and some absorbent. The dimensions of the lamp are 2½ by 2½ inches by 8 inches high, and the weight is from 3 to 4 lbs. The batteries maintain the charge for from eight to ten hours, and are then recharged by coupling up to a dynamo. The following table * given by Mr. W. O. Wood shows the cost of upkeep of lamps during the first six months, including all charges except interest on capital and the cost of running the dynamo :
Per cent Week
All labour at basis rates, including superintendence, cleaning, making ready, maintenance, &c., 2½
Material for renewing batteries, repairing, and maintaining the lamps in every part and lamp glasses 100
Incorporated lamps or bulbs 0½
Total 379
With wages at 27½ per cent. above basis rates the cost is increased by -90
Total 439
The life of the battery is said to be about twelve months. Tests of Safety-lamps.—The table on pages 250 and 251 gives the result of tests made on various safety-lamps by the authors, and will be interesting to readers who have not had much experience with Lighting Oil Safety-lamps by Electricity.—An application of electricity which is widely used is an arrangement for lighting the lamp electrically. There are two principal methods in use, a high-tension current being used in one, and with this system the ordinary paraffin or colza-fed wicks can be lighted by means of a spark ; the other system is to use a low- * Originally published in the Electrical Engineer. 254 ELECTRICITY AS APPLIED TO MINING Tension current and cause a platinum wire over the Wick tube to incandescence and so light the vapour which is given off in a lamp burning light oils, such as colatlene. The Protector Company were the first to sell a lamp of this kind lighted by an electric current heating a wire. When this kind of lamp-lighting is used, there are generally several lamp-lighting stations in the mine, where lamps can be taken out of the box and put into the box again. The lamps are contained in a box; there are arrangements for prevention of sparking when putting the lamp on or removing it from the battery. The whole system was perfected under the direction of one of the authors; since then many modifications of these lamps have been introduced. Messrs. John Davis & Sons and others make an excellent type of low-tension lamp; it consists—as shown in fig. 174—of an ordinary safety-lamp. **METHOD OF LIGHTING SAFETY-LAMPS.** with a platinum wire loop over the Wick tube. One end of this wire is connected to a brass plate insulated from the rest of the lamp; the other pole is formed by connecting the other end of the platinum wire to the lamp body. A current of electricity passes through this wire and makes it incandescent, so lighting the lamp. Messrs. Ackroyd & Best make lamps on the high-tension system. A section of the lamp bottom is shown in fig. 175, and it will be seen that there is an insulated conductor going down into the lamp with a pointed end at its lower extremity. The lamp body forms the other conductor, and on connecting the lamp to an induction coil connected to an accumulator sparking takes place between the pointed conductor and the Wick-tube, and the lamp is lighted. A diagram showing the method of lighting safety-lamps. Pot. 778 ELECTRIC LIGHTING OF LAMPS 255 Another form of high-tension lighting has been recently described in the "Transactions Inst. M.E." by Mr. Edward Brown. The apparatus (see fig. 176) consists of an induction coil connected up to a storage battery, or other source of electricity, in the usual way. A copper conductor A diagram showing an induction coil connected to a storage battery. Fig. 175. **ACKROYD & BRYN'S ELECTRICALLY LIGHTED SAFETY-LAMP.** is fastened into the glass of the lamp and extends so as nearly to touch the wick-tube. The lamp to be lighted is placed on an iron plate, and the circuit is completed by connecting one terminal of the induction coil to this plate and the other terminal with a copper rod, and the sparking between the end of this rod and the wick tube lights the wick. A diagram showing a lamp with a copper rod and a spark gap. Fig. 176. APPARATUS FOR LIGHTING ELECTRIC LAMPS IN LAMP CABIN. This apparatus seems to present a very simple means of lighting lamps in the lamp-cabin just prior to being given out ; but in taking it down the pit it must be borne in mind that sparking will be produced outside the lamp, if connection is made, say, to the bottom plate and to one of the Vol. xxii. Part II. page 186. 1 Vol. xxii. Part II. page 186. 256 ELECTRICITY AS APPLIED TO MINING pillars supporting the bonnet, and the apparatus is therefore only suitable for a naked-light station. With the low-tension system the spark on breaking the circuit will only be momentary, and will be much less powerful than with the high- tension system. The use of this system has been employed at least a few flash-point, and this introduces another danger. Electric Drills.—The application of electricity to drilling has been so far comparatively rare in the coal-mines of this country. In those countries where metal-mining is largely carried on, and in coal- mining countries where the large number of faults entails the driving of long tunnels, electric drills have been employed with some frequency, met with. Under the ordinary conditions of many coal-mines, where, perhaps, the only shot-holes are in the ripping down of the roof in a long- wall gate, the use of electric drills would not effect a saving sufficient to justify the capital outlay; but where stone heads have to be driven, or in sinking shafts and driving cross-measure drifts, their use might in many cases effect great economy. There are two classes of drilling machinery—rotary, on the auger principle, and percussive. The rotary drills are worked with most success in rocks of a comparatively soft nature; but in harder or softer rocks the percussive drill is usually employed. Electricity has also been employed successfully for driving the diamond drilling machinery used in prospecting; and quite recently the small diamond drill has been used in a Continental mine for boring in very hard rocks, such as granite, and it is quite possible that there may be a great future for a machine in granite and slate quarries. Electric Drills in Coal-Mines.—In 1870 Mr. A. L. Steavenson, mining engineer to Messrs. Bell, the well-known ironmasters of Middles- brough, is largely responsible for the successful application of electricity to drilling, as employed in Messrs. Bell's Cleveland ironstone-mines. The machines employed were originally driven by compressed air, but electricity was introduced into this connection the following remarks of Mr. Steavenson are interesting : *For twenty years compressed air had been used in mines, about two miles from one end to the other, with numerous machines, so that it was impossible to make a direct test, and say that they would start at a given hour and continue every day. He had managed to obtain a result by taking diameters over several holes with compressed air from one bank, and these showed that with compressed air 111 h.p. was required for eight drills ; with electricity, about 17 h.p. was required for the same work. The great loss with compressed air arose from the leakages of some eight or nine miles of pipes, whether the drills were cutting or not.* Transactions Inst. of Mining Engineers, vol. 1849-9, page 222 ELECTRIC ROCK-DRILLS 257 At the Cleveland ironstone-mines all classes of rotary drills have been used—the hand ratchet drill, the compressed-air rotary drill, the hydraulic drill, the petrol-engine drill, and the electric drill—and the following table gives the result obtained with the several drills: ¹
Description of Drill First Cost of Machine only Holes Drilled per Hour² Ironstone cut per Shift
Hand Jumper
Hand Ratchet Drill
Compression Air Drill
Hydraulic Turbine
Petroleum Engine
Electric Drill -		 L
3
100
800
375
350		 43 feet in 45 minutes
Not yet known
About 8 hours
About 8 holes
About 6 holes
About 6 holes		 Tons
1 to 3
100 to 130
100 to 130
100 to 130
140
Fig. 177 illustrates the construction of the drill. The motor, m, is used to drive the drill-gear, and also acts as a counterbalance to the weight of the drill itself. The main shaft is coupled directly to a shaft about 10 feet long, which passes through the long hollow carrying-bar, c. By means of bevel-gearing the vertical spindle, s, is made to revolve, and a bevel-wheel on this spindle gears into another bevel-wheel on the boring-bar, b, and so rotates the drill, d. As the hole bored the drill is automatically advanced by means of two pairs of gear-wheels, n, the gear of which can be varied to suit the nature of the rock being bored. The nut, n, is thus made to revolve, and the boring-bar, b, advanced. As soon as a hole has been drilled the full length of the screw on the boring-bar, b, the split-nut, x, is opened, the boring-bar brought back, and a longer drill inserted. In ordinary drilling operations when the ground machine is provided with a semicircular wheel, a, which can be moved by means of a worm. The machine will drill a hole 12 feet above ground level. The whole machine can be moved in a horizontal plane by means of the worm-wheel, w. The third worm wheel, t., turns the long hollow bar, c, which carries the drill. The drill can also be moved in a horizontal plane on the plate, j., by unclamping a screw. The drill is mounted on a trolley to run on the ordinary roads of the mine. The motor makes about 1,300 revolutions, the drill about 400 revolutions per minute. The voltage used is 300 at the dynamo, and the amperes used are each 25 ampere for each motor and drill being operated. ¹ Description of the Electric Rock-drilling Machinery at the Carlinville Ironstone-mines in Cleveland," by A. L. Stevenson; Proceedings Inst. of Mechanical Engineers, August 1893. The figures are reproduced by their courteous permission. ² This includes the time lost in moving the machine to different working places. 2 258 ELECTRICITY AS APPLIED TO MINING about 6 h.p. The holes are 1 inch diameter, and they are bored at the rate of about 3 feet a minute. The weight of the machine is 35 cwt. The motors are completely enclosed, and the current is supplied through A diagram showing a drilling machine with various components labeled. The top left shows a drill bit, the top right shows a motor, the bottom left shows a power switch, and the bottom right shows a cable. Fig. 177: Electric Rock-drill is used at Cawthorne Ironstone-Miners, in Cirencester. twin flexible cable, there being junction boxes in each district, so arranged that when the drill is moved from one working place to another a junction box is always to be found within 50 yards. Fig. 178 is from a photograph of the machine. GRANT'S ELECTRIC DRILL 259 Grant's Electric Drilling-machine. This machine is of the rotary type and is shown in section in fig. 179. The electric motor is connected to the drill by means of a telescopic shaft, and gives motion to the bevel-pinion, $a$, which gears into a bevel-wheel, which is fastened to the tube, $b$. The drill-spindle, $c$, is inside this tube, and is threaded along its entire length : it is also provided with two grooves in which the projecting keys, $d$, rest, and so the rotary motion of the tube, $b$, is imparted to the drilling-spindle, $c$. At the front end of the drill-case is the feed-cutter, $e$, which is held in the nut-holder, $f$ ; the latter is connected to a sheave, $h$, which has brake-shoes, $j$, working on its circumference. A wire rope encircles these brake-shoes, A diagram showing a drilling machine with a telescopic shaft connecting an electric motor to a bevel-pinion, which drives a bevel-wheel attached to a tube. The drill-spindle is inside this tube and has two keys resting in grooves. The feed-cutter is held in a nut-holder that is connected to a sheave with brake-shoes. Fig. 178. ELECTRIC ROCK-DRILL (see also fig. 177). the two ends being taken to the two bolts, $k$ ; thus on tightening the wing-nuts, $l$, the brake-shoes press on the sheave, and prevent it and the nut-holder from revolving, and so the drill-spindle is caused to advance. The working parts being entirely enclosed enable the machine to be run in an oil-bath. The standard motor is a continuous-current one of 4 h.p., and slow speed (400 revolutions per full load), which enables it to be coupled direct to the telescopic shaft without intermediate gearing being necessary. The ratio of the gearing in the drilling-machine is 4 to 1. The motor is series-wound, and this causes it to run at a higher speed when on light load, which is found to lift the dirt out of holes bored in a downward direction. 32 260 ELECTRICITY AS APPLIED TO MINING Fig. 179: GIANT'S ELECTRIC DRILLING MACHINE. A detailed diagram of an electric drilling machine, showing its components and parts. The top part shows the motor and gears, while the bottom part shows the drill bit and other mechanical elements. The diagram is labeled "Fig. 179" on the left side. JEFFREY DRILL 261 The drill-case is attached to a standard provided with the usual adjusting screws, and the motor is swung in trunnions attached to a turntable. The weight of the drill is about 10 cwt., and with regard to the speed of drilling it is stated that in a tunnel 12 feet by 6 feet two men can set up Photo of a drilling machine. GRANT'S ELECTRIC DRILLING MACHINE. the plant, drill in fairly hard rock a round of 10 holes, each 5 feet 6 inches to 6 feet deep, 13 inch diameter, and clear away all their tackle for shot-firing, in eight hours. **The Jeffrey Drill.** This is essentially a coal-drill, and is largely used in the anthracite mines of America. The motor is series-wound, and is 262 ELECTRICITY AS APPLIED TO MINING connected to the drill by gearing in the ratio of about 54 to 1. The drill is attached to a long screw with grooves in it, into which projecting feathers on the boss of the larger wheel fit loosely, and the feed is operated by a nut which is held in a friction-clutch which can be so adjusted as to cause the drill to slip upon encountering any hard substance which might be detrimental to it. The weight of the machine is about 170 lbs., and it is Fig. 183. The Jeffrey Drill. Messer, John Davis & Sons, Derby. said to drill a hole 6 feet deep in less than a minute. These drills have also been designed for drilling in rock, and in this duty they are fitted with a double reduction gear, allowing a slower feed. An illustration of this type is shown in fig. 181, and it weighs complete 300 pounds. **Other Rotary Drills.**—There are other forms of rotary drill in use, but their principle is similar to those described above. Messrs Ernest Scott & Mountain have supplied drills for use in the Rosedale mines of the **MARVIN ELECTRIC DRILL** 263 Carleton Iron Company, while on the Continent Messrs. Siemens & Halske, of Berlin, have several rotary drills in operation in iron and salt mines. **Percussive Drills.**—There are two forms of percussive drill. In one class the percussive action is got by electrical means, the Marvin and Dugdale drills being examples of this class, in which the property of the solenoid is employed. There are usually two solenoids, the drill being made of phosphor-bronze, with a short length of iron in the centre. The two solenoids are so arranged that by supplying the current to each alternately the drill is first advanced and then drawn back. The Marvin Electric Drill shown in fig. 185, has achieved success in American mines, and a number of these drills made by the Union Company are at work on the Continent. It consists of a solid steel plunger surrounded by two coils of wire through which electric currents are caused to Fig. 182. **Marvin Electric Drill.** pass. The coils of wire, by pulling the steel plunger backward and forward, produce the stroke of the machine. Referring to the figure, it will be observed that the plunger (t) of the machine is very similar to that of an air-drill ; it has an enlarged portion (a), which is surrounded by the coils of wire (3, 3), and a shank (q), which passes through a bearing in the front head of the machine, and is provided with a chuck for holding the steel drill or bit. The magnetic pull of the coils (3, 3) draws the plunger backward and forward as the current alternately passes through one or the other. The turning of the plunger is obtained by a rilled ratchet-rod (b) and ratchet-wheel. The spring (7) is a very heavy coil-spring, which is intended to check the backward stroke of the plunger and supply energy to the forward stroke. 264 **ELECTRICITY AS APPLIED TO MINING** The electric generator used is a two-phase alternating-current slow-speed machine, giving 135 volts. A 5-inch stroke drill will bore a hole 3 feet deep, $\frac{1}{4}$ inch diameter, with about 1 h.p. An 8-inch drill will bore a hole to a depth of 30 feet with about 10 h.p. The high speed of the motor seems to be owing to the heating of the coils of the solenoids, and also that they have not the power of withdrawing the bit out of the bore-hole in case it should get wedged. The other class of drills employ rotary motors, and have a mechanical contrivance to turn rotary into reciprocating and percussive movements ; and it is to this class we must look for future developments in electric percussion drills. **The Gardner Electric Percussive Rock Drill.—This is an American drill, at present being introduced into this country. It is said to have met with considerable success in the American mining districts.** Fig. 183 shows a sectional elevation and plan of the drill. The motor is enclosed by a box, the connection to the drill being made by a flexible shaft. This shaft imparts motion to the driving-wheel which is at the end of the driving-s axle of the drill. The driving-s axle passes through the crosshead of the drill and is cranked, the crank shaft working in a slot at the rear end of the crosshead, which is so contrived that one-fourth of a revolution of the axe-crank gives the full striking blow. The next step in the revolution withdraws the drill, during which the turning motion is imparted. During the remaining portion of each revolution the drill remains stationary. On the opposite side of the axis is a gear which drives the fly-wheel, the latter being so arranged that it makes $\frac{1}{4}$ revolutions to each revolution of the motor. On this wheel are mounted two ratchet-wheels which act as a buffer between the crank and the drill when striking rock. A portion of the drill has a spiral groove in it, and by means of two ratchet-wheels, one left hand and the other right hand, the drill is rotated after each blow. The machine is carried on a vertical standard, and is provided with a feed-screen and handle, by means of which the drill-cylinder can be raised or lowered according to need. This machine is made in two sizes. One size, for general work, weighs 150 lbs., will cut holes up to 5 feet deep, requires a 1 h.p. motor weighing 175 lbs., and strikes 600 blows per minute with a $\frac{1}{4}$-inch stroke. The other size, for heavy work, weighs 260 lbs., will cut holes to 8 feet deep, requires a 2 h.p. motor weighing 225 lbs., and strikes 1200 blows per minute with a $\frac{1}{4}$-inch stroke. The drill has so far hardly had a trial in this country, but it seems mechanically good, though no doubt some arrangement for automatically clearing the hole would be an advantage. **Portable Drills for Shop Work.—It is, of course, a well-estab- lished practice at the present time to drive all kinds of shop tools** PORTABLE DRILLS FOR SHOP WORK 265 electrically ; the usual practice at collieries being to instal a single motor to drive the shop shafting in place of the ordinary steam engine. This method A diagram showing a sectional view of a portable drill. A plan view of the same portable drill. A close-up view of the chuck and spindle of the portable drill. leads to an undoubted economy, especially where the shops are situated at a distance from the boilers, and the losses of economy due to long ranges of steam pipes and the inefficiency of small steam engines are done away with. CARBONE ELECTRIC PNEUMATIC DRILL 266 ELECTRICITY AS APPLIED TO MINING It is, however, in the direction of portable tools, and especially drills, that electricity is seen to the best advantage, and will certainly be a more convenient agent about the works of a mine than compressed air for this purpose. A useful tool of this description has recently been brought out by Messrs. Charles Wicksteed & Co. of Kettering, and is illustrated in fig. 184. The drill itself is similar to an ordinary hand socket drill, and can be used in any situation in which such a drill could be placed. It is driven through a pair of bevel wheels by a flexible shaft of a new and A diagram showing a portable shop drill with various parts labeled: A, motor; B, sliding pulley with lever and weight adjustment; C, flexible shaft; D, drill; E, ratchet; F, standard bar for fixing. WICKSTEED'S PORTABLE SHOP DRILL. A., motor ; B, sliding pulley with lever and weight adjustment ; C, flexible shaft ; D, drill ; E, ratchet ; F, standard bar for fixing. particularly efficient type. The shaft consists of a number of short spindles united by universal joints, and runs in a flexible steel tube which is filled with oil and is entirely dust-proof. This shaft can be bent to a radius of about eighteen inches, and thus enables the drill to be put in very inaccessible positions. The motor and gearing for driving the flexible shaft are mounted on a small trolley, which may be conveniently mounted on wheels to the gauge of the mine tram roads. The motor drives the shaft through a pair of rope wheels, one of which is mounted on slides in such a Fig. 184. ELECTRIC WELDING 267 way that the tension on the rope can be adjusted by a weighted lever mounted on the trolley. By the proper adjustment of this weight it is arranged that if the tool sticks in the hole the rope wheel will slip, and the motor will not be overloaded nor the drill broken. The current taken by the motor is only two or three amperes; it thus can be used at any part of the mine, on the surface or underground, where electric light or power is laid down, the current being supplied by a generator. Where such apparatus is used, wherever there is machinery or constructional work of any description on which it may be desired to use the drill. A length of cable is attached to the switchboard on the trolley, which can be connected to the mains. The weight of the trolley, motor &c. is about 15 cwt.; that of the drill and holder about 30 cwt., the apparatus being very portable, and can be moved by one man. The utility of such an apparatus would be appreciated in the erection of screening plant, steel headgears, &c., to say nothing of the time and labour saved by its use in the ordinary work of repairs and breakdowns. The apparatus can also be fitted with tapping tools, tube cutters, and other expanders and holders for locomotive boiler work. It may also be driven into a more spacious arrangement in place of the motor trolley, and then takes two men to work it. It is proposed to apply the principle of this drill to coal and rock boring, but the apparatus has not yet been put on the market. Another form of portable electric drill has the tool attached directly to the armature shaft, which has been found to be much less cumbersome than previously a less easily handled arrangement than a drill on the principle above described. Electric Welding.—At a large colliery a system of welding by electricity could no doubt be adopted with advantage. One chief advantage seems to be that awkward breaks, which cannot be welded in the ordinary way, can be welded by electricity, and also repair work can be done without breaking up (which is a danger of igniting gas) as on the surface. The electric arc can be used for cutting metals, such as girders or plates, and this operation can be done in situ when it is not possible to bring the work into the shop. Sections such as are used in making the top and bottom frames for cages, channels, angles etc., can be repaired by filling in cracks with metal shavings or arms of fly-wheels or drums can be repaired by filling the crack with small chippings of metal and subjecting the whole to the electric arc. While it would not, of course, pay to introduce an electric current merely for the sake of occasional electric welding, yet where the dynamic is already installed for haulage, pumping, or lighting, an electric welding plant will be found useful. There are two systems of electric welding—the Benardos or Arc system and the Thomson system. In the Arc system the heat is generated by making the article to be operated upon one pole of an electric 268 ELECTRICITY AS APPLIED TO MINING circuit, while a carbon pencil attached to a portable insulated holder constitutes the other pole, and an electric arc is formed which produces the heat necessary for the weld. In the Thomson system currents of electricity are passed through the abutting ends of the pieces of metal which are to be welded, and so heat is generated at the point of contact, while, at the same time, pressure is applied by means of a hammer or together. The Benardos System.—This plant consists usually of low-tension continuous-current dynamos, which during light load are employed in charging accumulators. The dynamos supply the current direct to the welding machines, and when extra current is required a demand is made on the accumulators. A flexible cable goes from the dynamo to a carbon pencil held in insulated hands, and from this to the heated bar of iron or brass, while the other terminal is connected to the table on which the work lies or to the work itself. Each welder has a regulating resistance, so that the current and pressure can be varied to suit the work. The voltage used in the Benardos system is about 85. It is necessary that the hands of the workman should be covered to protect them from the glare of the arc. The Benardos system is employed at the works of Messrs. Lloyd & Lloyd, Birmingham, and that firm have made many tests which prove the capabilities of the process. Two hundred and ten bars of iron and steel of varying thicknesses were tested both heated and welded by ordinary workmen (called smiths), and, together with some well-wrought bar-steel engine-smiths, were submitted to tensile tests. The average strength of the 150 electrically welded iron bars equalled 85 per cent. of the solid, and of the sixty electrically welded steel bars 80% per cent., which was a better result than was obtained in the case of the hand-welded bars. A modified version of this system is known as the Modified Arc system. The dynamo mains are connected direct to two terminals similar to an ordinary arc lamp ; but on one side of the arc an electro-magnet is arranged, through which a portion of the electric current is sent. This magnet draws out the flame of the arc. It will be seen that in this system the article being welded may be connected at any way to the dynamo, and the arc can be moved nearer to, or further from, the article, and the temperature varied in this way. The Thomson System.—This system is used by the Electric Welding Company. The plant consists of a generator producing low-pressure alternating currents, a rheostat and reactive coil for controlling the current intensity, and a transformer for increasing its value so that the pieces to be welded, and mechanical (sometimes hydraulic) arrangements by which the abutting ends of these pieces are forced together. The transformer converts the current—which is delivered to it at a ELECTRIC WINDING 269 Voltage of about 200—down to about 1 volt, with the result that a current of very great strength passes through the metals to be united, and raises their temperature to that requisite for welding. Electric Winding.—Electricity has not come largely into use for winding, owing to the fact that given days are required for small plants in operation in this country, and on the Continent large plants have been built. Fig. 185 illustrates an electric winding gear which has been made by Messrs. Ernest Scott & Mountain for the Heckmondwike Collieries for winding from a staple pit about 100 yards deep. The gear is driven by a four-pole open-type motor, capable of working up to so effective Fig. 185. Electrically Driven Winding Gear. horse power at a speed of about 600 revolutions per minute. At the end of the motor-shaft an automatic electric brake is fitted, which sustains the load immediately the current is switched off. The coils of the electro-magnet of this brake are in circuit with the armature of the motor, and immediately the current is switched on to the motor the armature is attracted by the electro-magnets and releases the brake-wheel. The drum is driven by a spur gear of 3 feet pitch (the first-motion pinion being forged steel, machine cut) into a machine-cut cast-iron spur wheel, which is supported by a counter-shaft on which a pinion is carried, gearing into a spur wheel on the drum shaft. The drum is 3 feet 6 inches diameter by 2 feet wide, lagged with elm, and has strong cast-iron sides, a brake-strap 270 ELECTRICITY AS APPLIED TO MINING being fitted on one side, which is controlled by the attendant. An indicator is provided which shows the position of the cages in the shaft; this is driven by machine-cut wheels from the shaft. The whole gearing is mounted upon a cast-iron bed-plate made in sections for getting down the pit and into position by the stope. The motor and winding gear is controlled by a liquid reversing and regulating switch. A Scott & Mountain 50 k.w. multipolar dynamo supplies current to the motor. The Gelsenkirchen Bergwerks-Aktiengesellschaft work all the machines in their new Zollern 50 z pit electrically, and there was exhibited at the International Exhibition (1924) a winding engine, which was at the close of the exhibition still in use. In this case the Koepe system of driving pulley is used, the pulley being about 20 feet in diameter. There are two electric motors, one on each side of this pulley or drum, on the same shaft. Direct current is employed, the voltage being 600. Each motor has a maximum power of 1,400 h.p. The motors are used in conjunction with a steam engine, which drives a belt-driven fly-wheel. The load of coal to be lifted is 4100 kilograms, equivalent to about 4 tons, from a depth at first of 300 metres, and ultimately from a depth of 300 metres (546 yards), with a maximum speed of 20 metres a second. There is a compressed-air starting and reversing engine, and a compressed-air brake. This plant, since being installed at the pit, has been made to work on what is known as the Ijger system, which is one of the most promising methods of electric winding yet introduced. In place of storage batteries the energy stored in a heavy fly-wheel is used as a buffer between the motors and the generator, and by taking energy from the fly-wheel at starting and stopping it is in stopping and allowing it to get up speed during the stoppages for changing tone, a fairly uniform load is obtained on the generating plant. The fly-wheel is of steel, with a heavy rim 12 feet 6 inches in diameter by 31 inches wide, and weighs about 40 tons. It is mounted on a shaft between two motors, w (for winding) and a generator, g (for generating). Motor w receives power from the generator g through two volts, and drives the fly-wheel and the generator. The generator supplies current to the two winding motors, w. Regulation is made by introducing resistance into the exciting circuit of the generator, which is supplied by another small generator, g. As the exciting current of g is increased, the voltage supplied to the motors, w, increases until power developed by them becomes so great at which they tend to run increases in the same ratio. When the exciting current is very small no current is supplied to the motors, which may, running on by the inertia of the load, send a reverse current to the generator, making it act as a motor and speeding up the fly-wheel. The increased ELECTRIC WINDING 271 energy demanded by the winding motors during starting is not supplied by an increased current from the central station, but by a slowing down of, and yielding up of, energy by the fly-wheel, which regains its speed, and in so doing provides a load for the motor, $m$, during the stop at the end of the wind. In winding from a depth of 388 yards with a load of 4 tons 9 cwt. of coal the engine takes 45 seconds from start to stop; the current taken by the winding motors varies from — 800 to + 1,700 amperes, that taken by the motor, $m$, from 300 to 450 amperes. The machine is under perfect control, being operated by one lever (except for emergency brakes, &c.). The current which has to be switched about is a small one, the exciting current only, and the plant works efficiently and satisfactorily.* Trolley to Power Motor **Fig. 286.** *For the information as to the present method of working this plant, the authors are indebted to a paper by Mr. W. C. Mountain, of the North of England Institution of Mining and Mechanical Engineers ; a paper by Mr. G. M. Stevenson to the South Wales Institution of Engineers; and to an article in the *Iron and Coal Trade Review* of April 15, 1904. **ELECTRIC WINDING. IONIER SYSTEM.** A plant of this description could be made to give as good, if not better, results than any steam winding-engine so far as fast winding and efficient control are concerned, and much better results in steam economy; whether it would be really more economical in ordinary cases, taking into consideration the depreciation and interest on first cost, than a high-class steam winding-engine driven by a motor-generator set is doubtful. Moreover, where power is obtained from coke ovens, blast furnace gases, or other sources at a distance, it might prove the most economical method of A diagram showing the electric winding system. 272 ELECTRICITY AS APPLIED TO MINING winding. It must be remembered, however, that in the system just described there are no fewer than six electrical machines, a breakdown in any one of which would mean the stopping of the winding plant. There are also on the Continent several examples of large winding engines, but they are not so well known as those in England, and some particulars of two of the largest of these, and is taken from the evidence given by Mr. D. Selby Bigge to the Committee on Electricity in Mines:
Coal lifted, net 2 tons cwt. 2 tons cwt.
Coal turned per hour
Depth of shaft 50 metres 1,000 metres
Speed per min. Cwt. 700 metres 16 metres
Speed per sec. Min. 3 metres 1 metre
Speed per setting Inspection 0.5 metre 0.5 metre
Time in winding 16 sec. 78 sec.
Time normal full speed 22 sec. 78 sec.
Time altering 17 sec. 39 sec.
Time starting and changing 14 sec. 34 sec.
Home-power at starting 1.3Bo 93%
Home-power running
System -phase -phase
Voltage -500v -575v
Pulsation -25 periods -231 periods
Morse revs. mean full load
Method of driving:
Direct coupled
Direct coupled
Uninterrupted power:
Weight of rope:
14.8 lbs. per metre
Tubular blocks:
No. of blocks:
2
Weight of cage:
3 tons cwt.
Total weight of cage:
1 ton cwt.
Total weight of lift:
1 tons cwt.
4 tons cwt.
The method of control at the Preussen II pit is described in 'Engineering', August 1, 1902, and consists of a liquid resistance inserted in the rotor circuit which is provided with slip rings ; the resistance is regulated by varying the height of the liquid by means of a little centrifugal pump and a drain tap. 273 CHAPTER XIV ELECTRICITY AS COMPARED WITH OTHER MODES OF TRANSMITTING POWER Steam—Rods—Wire Ropes—Compressed Air—Hydraulic—Gas and Oil. The sources of power at a mine may be various, and are generally as follows : Steam boilers, gas generators, waterfalls, and windmills ; steam boilers being at the present time more than 99 per cent. of the whole quantity. It is quite reasonable to anticipate that in the future gas generators may to a considerable extent take the place of steam boilers. At most mines the steam boilers are all close together at the base of a tall chimney, but the engines which use the steam are scattered about over considerable areas. The differences between them on high ground or on the top of pillars, others at the bottom of deep shafts, and other machines are situated in places where it is difficult or dangerous to convey steam. The best method of transmitting power from the boilers to the machines is a matter which has to be considered by every mine manager, and the various methods may be grouped under the following headings : 1. Steam transmission. 2. Rods worked by steam-engines. 3. Ropes worked by steam-engines. 4. Compressed air in pipes. 5. High-pressure water in pipes. 6. Gas and oil. 7. Electricity in metallic conductors. No. I. STEAM TRANSMISSION is at once the most obvious, the simplest, and cheapest in first cost, and is free from danger except in the case of unusual roadways or shafts, where the passage of steam might be dangerous. It is, however, largely adopted underground, and accidents from its use are very rare. Steam has been conveyed a distance of one mile from the boiler. There is great waste of power, owing to the loss of heat by radiation and convection from the pipes. The greater the length the greater the loss. This loss may be much reduced by 274 ELECTRICITY AS APPLIED TO MINING covering the pipes with non-conducting material. It is difficult to measure the amount of loss. The amount of water formed in the steam-pipes which may be caught in a cistern and taken out and measured does not represent the total loss, because a considerable amount of water passes through the steam-engine. The percentage of loss varies inversely as the amount of power transmitted, and directly as the length of pipe that is trans- mitted the less will be the percentage of loss. The loss is constant for a given length, size, and temperature of pipe, but the power varies with the speed at which the steam passes through the pipe and the regularity of its passage; thus, if the steam is passing through the pipe at full speed the whole twelve hours, the amount of power consumed might be, say, 20 per cent, but if the steam were only passing through at half speed for twelve hours, and the other twelve hours only little more than sufficient to warm the pipes, there would be as much steam lost as in the first case ; but, as there would only be half the power, the percentage of loss would be 40, and if the total amount of power transmitted only represented eight hours' work, then two weeks' work would be lost, and it would be increased to 60 per cent. The above figures are all hypothetical. Loss from the steam-pipe increases directly with its length and cir- cumference, but the percentage of loss decreases as the diameter increases, if the diameter is properly proportioned for its work. That is to say, a 4-inch pipe, having a surface area of 3 square feet, will lose twice as much surface for cooling, but it will allow five times as much steam to pass with the same percentage of frictional resistance as the 2-inch pipe; therefore, if the 2-inch pipe and the 4-inch pipe were each transmitting the full amount of power for which they were suited, the percentage of loss in the 4-inch pipe would be two-fifths as that in the 2-inch pipes--viz., that in each such pipe would be one-fifth. But when in the 4-inch pipe would be only so per cent, the other circumstances being the same. When, however, a pipe is covered with non-conducting material, the surface exposed to the air does not vary in the same proportion as the internal diameter of the pipe, because the thickness of the non-conducting composition has been made equal to that of a similar pipe. Thus 4-inch pipes were made of cast iron $\frac{1}{2}$ inch and $\frac{3}{4}$ inch thick respectively, and were each covered with $\frac{1}{8}$ inch of non-conducting material, the external diameters of these pipes would be $1$ inches and $\frac{3}{4}$ inches respectively--i.e., the radiating surface of the 4-inch pipe, instead of being twice as great as the 2-inch pipe, was only $\frac{5}{8}$ times as great. The foregoing remarks show that steam transmission may be economical with a large power and wasteful with a small power. On the other hand, it must be borne in mind that a large steam-pipe in a mine might be dangerous if taken a long way into the workings, and that the immunity from serious accidents in the past is very likely due to the fact that it is seldom that a WIRE ROPE TRANSMISSION 275 large steam-pipe is taken far from the pit bottom. One objection to steam- pipes is that an exhaust-pipe is generally necessary for the return of the steam, except in those few cases where all the steam can be condensed, as is the case with pumping-engines where the height to which the water is raised is small. No. 2 Transmission by Rods—In this case a crank driven by a steam-engine gives a backward and forward movement to a connecting- rod carried on vibrating levers. Rods may be joined together end to end, carried on levers, and the power transmitted for great distances on land, or down a pit shaft, by means of a rope or wire rope, or by means of a belt driven by bell-crank wheels. This arrangement is good for certain cases, but is seldom adopted nowadays on account of the ponderous nature of the connecting- rods when they are of considerable length, and the room occupied, and the awkwardness of using the reciprocating motion for any other purpose than that of transmitting power. No. 3 Transmission by Wire Ropes is very largely used both for carrying power on the surface and transmitting it down the shaft of a mine and along the levels and inclines. Wire-rope transmission is of two kinds. The first is where the wire rope which is connected to the drum of the engine at the boilers is the same rope that does the work underground for which it was designed, and in which it is made up. In such cases no surface haulage is done by one rope. If the rope is used for endless- rope haulage, the speed is generally slow; if for single-rope or tail-rope haulage, the speed is generally fast. The second method of using wire- rope transmission is where the rope is merely used like a strap or belt in a factory, for transporting materials from one place to another, or as pulleys to some other machinery in the pit. In this case the strap-rope is generally high-speed; the higher the speed the lighter is the machinery and the smaller the rope necessary. Some engineers prefer a thick rope and slow speed machinery. Next to steam transmission, the wire rope is the cheapest mode of transmitting power, as regards first cost. All that is necessary is the driving engine, which may be either an engine and running-wheel, wheel, or drum, on the machinery in the mine, the various guide- and angle-pulleys on the way, and the rope itself. This method of transmitting power is therefore very largely used, and is particularly applicable where the number of turns in the direction of the pipe is not great. In some cases on earth-workings it may be carried out with only one side- pulleys from the generating engine to the receiving machine. In such a case as this it is probably the most economical mode of transmitting power that is known. But these cases are rare. From the mine manager's point of view the objections to the rope transmission are the space required in the shaft for the ropes and the space required in the underground roads for the supporting pulleys and turn-wheels, and the complications of T 2 276 ELECTRICITY AS APPLIED TO MINING subsidary driving, and receiving drums where the machinery to be worked is scattered in many directions and places. There is also the question of the safety of men in the shafts down which the strap-ropes are working, and the necessity for boxing up the ropes. Where, however, strap-ropes can be conveniently used, it is doubtful if any other means of transmitting power is more economical. It should, however, be noted that in many places they have been superseded by electricity. No. 2. Compressed air has been much used, and is the most obvious means of superseding steam transmission. The same arrangement of pipes will do in each case, but instead of the steam going direct from the boiler, it goes into an air-compressing engine, and compressed air is taken into the pipes which go into the mine. The advantages of compressed air are very great. An escape of compressed air would be very dangerous. An escape of compressed air, though it means a loss of power and of money, is otherwise beneficial in a mine. It is useful, not merely for power, but for ventilation in confined places. On the other hand, it is very costly. The air-compressing engine and the air-using engines or air-motors cost a great deal more than steam-engines when they were supposed to. It may be true that the air-motors are due to the coal of a steam-engine which would have done the work, but on the surface we have the air-compressing engine, and this probably costs five times as much as the steam-engine would have cost to do the work direct had steam transmission been used. Also, owing to the loss of power in air-compression, there will be a great loss in efficiency. This is a point which must be considered, because the loss by steam transmission might have been so great as to equal the loss by air-compression, or even to exceed it, so that, as compared with steam transmission, it does not follow that the boiler cost would be increased, but the boiler cost would be greatly increased as compared with rope transmission. In working cost the loss of power depends on many circumstances, some of which are under the control of the engineer. They are as follows : 1. Leaks in the pipes. This is entirely a question of management. It is not necessary that there should be any leak, and, if existing, it may be easily disposed of. 2. Friction of the air transmitted in the pipes. This can be reduced to a very small percentage by having the pipes of sufficient diameter. The friction varies inversely as the fifth power of the diameter, so that a very small increase in the diameter is sufficient to make a very great reduction in the loss through friction. The fifth power of five is $3125$, and the fifth power of two is four hundredths; so that if only one pipe were substituted for a pressure of $777$ lbs., to overcome it, a $6$-inch pipe would be substituted, a pressure of only $312$ lbs. would be required. 3. Heating of the air during compression—the necessary abstraction COMPRESSED-AIR TRANSMISSION of this heat from the air, which heat is necessary for the free expansion of the air in the motor, and is not there when wanted. The loss due to the generation of heat may be reduced by efficient cooling of the cylinder and by stage compression, but this loss can be expected to amount to 20 per cent. of the engine power. The useful electrical power of the air-motor, as measured by a friction-brake, cannot be expected to be more than 40 per cent. of the L.H.P. of the air-compressing engine. In some cases the useful effect is only to per cent., in many cases only as per cent., so that per cent. is considered a good return. If compressed air is used on the surface, it becomes place in the mine where there is no danger from fire and smoke, then it may be re-heated before entering the air-motor. In that case, the power which was taken out of it on compression can be restored, and, indeed, more than restored ; this involves an expenditure of fuel ; but the efficiency of such an air-engine is very low, and its cost of operation is high. The cost of re-heating is inconsequential, and, where re-heating by means of a fire is admissible, compressed-air transmission is exceedingly efficient, and it is not probable that the efficiency can be exceeded by any other means. But it rarely happens that re-heating by fire is admissible. Compressed air is applicable to almost every species of machine used for work. A compressed-air transmission and electric transmission has distinct advantages for some situations. Power is transmitted by electricity to a point in the workings, and there driven an air compressor from which power is transmitted to the face for coal cutting, drilling, &c. Messrs. Russell and Co. of Ipswich make an electrically driven air compressor which may be coupled direct to the electric motor without the intervention of gearing. In the single stage type of machine for working pressures of about 60 lbs. per square inch or less, four cylinders are arranged radially round a common crankpin. The stroke of the pistons is short, and a high speed can be attained. A combined system of transmitting power is destined to have important developments. No. 5. Hydraulic Transmission of Power.—In this case the generating engine, instead of compressing air, pumps water, which is incompressible, along pipes. The water is pumped against a heavy head maintained by accumulators, and the pressure used generally varies from 500 lbs. to 1000 lbs. per square inch. It is largely used in docks for working hoists and capstans, and small motors for opening and closing dock gates. It is also used for driving motors in warehouses and elsewhere. It has many advantages and conveniences, but for mining purposes it is not generally so convenient as compressed air. It is generally 1 Mining, by Lupton, p. 369. 278 ELECTRICITY AS APPLIED TO MINING necessary to have a return pipe for the exhaust water, and, being non- elastic, it is often used at full pressure when one-half or one-quarter of the full pressure would be sufficient. In cases where a single generating engine on the top forces water through a pipe to drive a single pump at a distance in the mine, the power of the generating engine may be exactly that which is necessary for driving the pump at a distance without any waste power. In such a case, as well as the efficiency of an hydraulic transmission system would be greater than that of any other system (assuming it is admissible), and probably at least equal to that of any other system of transmission. But the mine manager, as a rule, desires a system of trans- mission of power applicable to a great variety of motors working at various speeds and in places far distant from one the other. Hydraulic trans- mission is not applicable to all these conditions. But it can be employed for those cases where a small power constantly generated at the central station can be applied to exerting great efforts for very short periods. **No. 6. Transmission of Power by Gas and Oil.—The last forty years have witnessed a great development of gas-engineering. These engines are made for burning either coal gas or oil gas, but they burn no kind of inflammable gas. The cheapest kind of gas is that known as "producer" gas. This gas, when once produced and purified, may be transmitted to any distance in pipes, and the gas-engine can be erected at the place where the work is required. The main objection, however, to the transmission of "producer" gas to a great variety of places is the poisonous nature of this gas, which contains carbon monoxide, or carbon dioxide gas, called by chemists carbon monoxide, and shown by the chemical symbol CO. For this reason, as well as for its explosive quality, no mine manager would think of taking the gas down the pit; and if he wished to use a gas-engine at all, he could only use it at some engine-house so situated and ventilated that the danger is avoided. A gas-engine might very properly be used at a central power-generating station, for generating power to be transmitted either by wire ropes, compressed air, hydraulic pressure, or electricity, and in this way probably it will be largely used in the future. If we consider the exhaust gases by means of gas, which is to be burnt in the cylinder of an engine at a great distance from the gas generator, it is reasonable to talk of conveying power by oil, not conveyed in pipes but in barrels to the place. The oil, on its way into the cylinder of the engine, is vapourised, and therefore for practical purposes becomes a gas in the cylinder of the engine. The oil-engine is very much less both in the pit and on the surface than can be said with respect to those situations where, the exhaust will do no harm. Although the exhaust from an oil-engine is not poisonous, still it does not conduct to the health and strength of those who breathe it, and is unsuitable for discharge into an air course ELECTRIC TRANSMISSION 279 conveying air to working places. It should, therefore, only be used where the exhaust is discharged into the return. An objection to the oil-engine is the amount of fire required to heat a large oil flame being sufficient to heat the vapour until the engine has not fairly to work. This is enough to prevent it becoming very popular in most collieries, and in a great many other mines where fire is the most dreaded risk; but the economy of oil-engines is such that for surface work it will be difficult to provide anything more cheap in working cost. Oil-engines, however, as well as steam engines, are not suitable for starting at night or on Sundays, when once started, are generally kept running through the day, and, as usually made, are not suitable for reversing, although the reversing can be effected by means of suitable gearing. No. 7. Electric Transmission. Twenty years ago was both novel and rare; it is now common and very common. It cannot be said to have superseded wire-rope transmission, but it has come into favour in comparison with transmission by steam and compressed air; indeed the real contest for superiority lies between compressed air and electricity. The advantages of electrical transmission may be stated as follows: 1. Electrical generators are revolving instead of reciprocating like most air-compressors; they are more easily started; they do not make about a steadily revolving machine than about a reciprocating one; and consequently the engine driving it may go at a much greater speed than is practicable with a reciprocating air-compressor, which tends to reduction in the comparative size of the electric generating machinery. 2. The motor runs more quickly and conveniently taken down pits and along roads and round numerous corners than compressed-air pipes, and occupies less space. 3. The efficiency of transmission by electricity is greater than with compressed air (except where re-heating is permissible). The efficiency of an electric motor is nearly equal to that of a steam engine of the same power put into it, and the efficiency of the motor is about the same. The loss in transmission through the cables is probably about equal to the loss that takes place in compressed-air transmission through pipes, and may be much or little according to the amount of money laid out upon the cables. Electrical motors are also cheaper than those driven by compressed air, similar to the elasticity of compressed air. The cost of an electrical installation is probably rather more than that of compressed air, but very few mining engineers would-to-day put down an air-compressing plant in preference to electricity, unless the particular situation of their mine made electricity peculiarly dangerous. The drawbacks to the use of electricity are gone into in Chapter XV. Means, Possessors of Hennes Works, Newcastle-on-Tyne, make a revolving air-compressor, being one of their turbines reversed. 280 ELECTRICITY AS APPLIED TO MINING CHAPTER XV DANGERS OF ELECTRICITY Dangers of Electricity—Methods of Obviating the Dangers—Testing of Cables—Wheatstone Bridge—Ohmmeter, &c.—Electric Shock. Every new method or appliance involves new dangers. Perhaps these new dangers may be less than the old dangers, but they are none the less real. **Fire.** At a mine, one of the most terrible dangers is that due to the burning of woodwork or coal. The reason of this great danger is because of the slow movement of the passage through the mine, which supplies the oxygen necessary for rapid combustion, and also the fact that there may be no possibility of escape from the mine without passing through a burning fire on the one hand or a suffocating smoke on the other. This danger applies equally to underground mines of all kinds which contain inflammable materials, such as wood, and to surface mines and collieries, because here the fire, once started, may take hold of a great mass of coal. The danger is not limited to the underground works, but also exists on the surface works. At the top of the pit there are frequently a great many buildings and machines, and if in and about these there is inflammable material such as wood or coal, the smoke and flame from these will spread rapidly over the whole district, thus causing the ven- tilating current, and may cause a terrible disaster. For this reason the mining engineer, when deciding to use electricity, must spare no reasonable precaution to avoid danger from fire. **Arrangement of Cables to Avoid Fire.** If well-insulated cables are used, and if they are placed in such a way that they do not come into contact with any inflammable material, past all the buildings and woodwork by being placed in the earth, care being taken to avoid the proximity of any wooden posts, or any ground containing coal, pit shale, or other inflammable material. If this is carefully done, the electric cable, however defective it may be or become, will not set fire to anything, for the simple reason that there is nothing near it which it can feed. Another method is to lay the conductor in pipes in the ground. These pipes may be of iron or earthenware, care being taken that the pipes do FIREPROOF ENGINE HOUSES 281 not go too near to any timber, coal, or pit shale; but even if the pipes do pass some coal or similar inflammable material, it is not likely that they will cause a fire, because the current in passing through the earth will be spread over a large area, and not get into the earth. The pipes may be laid in a bed of sand, clay, gravel, or ash. A third plan is to lay the cable in a brick culvert, which contains no timber crossbars or other inflammable material, the crossbars being made of iron, and the cables carried on porcelain insulators. Care must be taken to exclude coal-dust from this culvert, and therefore it must not have an open end, and must be ventilated. If the air entering the coal screens would carry dust into the culvert. It is well known that coal-dust from screens is easily inflammable, and explosive when mixed with air. The culvert must also be ventilated, so that no accumulation of gas can occur. If the conductor is carried overhead, on poles or otherwise, it must be so carried that in case of defect in the insulation the current will not come in contact with any wood, coal-dust, coal, or inflammable shale. The conductor must also be carried in such a line that in case it should be broken, either by its own weight or the weight of snow, or by some weight accidentally or carelessly thrown upon it, it will not fall within contact with any wood or other inflammable material. There may be some difficulty in fulfilling this condition, but the difficulty must be overcome. Fireproof Generator-house.—The dangers from fire on the surface are not only in the conductors, but are to be found in the generators, terrible accidents having occurred from fires arising from sparks produced near to the pit top. For this reason the generators should always be placed in buildings which are entirely separate from all inflammable materials at or about the pit top, so that a fire originating at the generator or in the generator-house cannot possibly extend to the winding-engine, pit bank etc., and cannot extend to heaps of pit timber, shavings, &c. Accidents from chafing of conductors are very rare. Fires arise in some manner that ordinary human intelligence does not foresee. In new installation there is no difficulty in having a complete fireproof separation between the generators and the rest of the plant at a mine. Fireproof &c. Motor-rooms.—The precautions which are necessary for the motors must be observed as far as practicable with the motors; but the danger from motor motors is greater than from a generator, because if there is any defect in the insulation of the conductors it is likely to lead to the stoppage of the motor and the current, whereas, on the other hand, it would not stop the generator or the current generated. But still every reasonable precaution must be taken in fixing up motors at or about a pit top. They should be placed as far as possible on foundations of stone, brick, or concrete, and walled round with similar 282 ELECTRICITY AS APPLIED TO MINING non-inflammable material, and the roof carried on iron instead of wood; it being always remembered that near the top of a coal-mine there may be under every roof an accumulation of coal-dust, which is easily fired, and in its turn will set fire to woodwork. Where brick, stone, or concrete cannot be used for the support of the motors and for surrounding them, they should be placed upon iron plates and surrounded with iron plates, so that in case of a short-circuit at any point in the conductors, or in the consequent passage of electricity, there will be no inflammable material to catch fire. To place electric motors and conductors anywhere about wooden pit frames, screens &c. near a pit top, is to court terrible disaster. Fuses, &c.—At a generating station there are fuses and automatic circuit-breakers, the intended effect of which is to shut off the current in case of an excessive or dangerous passage of electricity. The motor owing to a short-circuit or any other cause, such as an overload on a motor; but in the case of a circuit where there are, say, half a dozen motors, either in parallel or series, there might be a short-circuit at one of these without the amount of current being necessarily so large as to cause the circuit-breakers or fuses at the generator to operate. In this case the current would pass through also a fuse in an air-iron light case at each branch, so that every provision may be made to avoid any evil consequences from a short-circuit at the motor or in the conductors, just as if it was certain that the generator would continue to send the maximum possible current. The many fires which have originated at mines through the use of electricity have been known from time immemorial by mining engineers and mine managers, and also from the fact that the advocates of the use of electricity and the vendors of electrical machines have not sufficiently dwelt on the dangers against which provision must be made, and also from the fact that the mining engineer is frequently induced to adopt the use of electricity by representations as to its safety. The convenience with which electric cables can be carried about the works, and the convenience with which electric motors may be placed on stages and in corners of buildings. But when it comes to carrying out the work, it appears that if it is executed with every precaution that prudence can suggest, it is much more costly than appeared from the representations. The mining engineer must treat electrical generators, conductors, and motors as if they were fires, because they may become fires. Having once made up his mind so to deal with electric appliances, and having in consequence made the proper arrangements, it is probable that he will never have a fire or any serious difficulty with his electrical plant. Conductors in Vertical Shafts.—The most common wiring from electric cables in a vertical shaft, as a rule, are not so imminant as those on the pit top, because the shaft is usually damp, and frequently wet; and if it is walled with brick or stone there is nothing to burn. But in the case of a shaft UNDERGROUND CABLES 283 lined with timber, and which is not permanently wet, then the utmost pre- cautions must be taken to avoid fire. Sometimes a bare copper cable is stretched from top to bottom of the shaft, and is not in contact with any material at all. In such a case as this, no accident can arise from the cable unless it should break, or be brought in contact with some con- ductor, such as one of the cages, or a metal trunk, swinging against it. In case such accidents should happen, the greatest damage (except, the spark on the breaking of the circuit) would happen to the shaft or below, because the circuit would be entirely cut off. In case one con- ductor should break and fall down, then the part at the pit bottom would be earthed, but if the end at the pit top remained well insulated the circuit would be broken and no current would flow; but if there was a fault, then the current would flow through the shaft and the shaft would very likely get hot. In brick-laid shafts a heavily insulated cable is often carried down in a species of wooden pipe. In case from any defect a short-circuit should happen, and if the shaft was perfectly dry, the wooden box might be fried. In an ordinary brick-lined shaft it is not likely that such a fault will occur, because the shaft is usually lined with dry timber and the consequences might be disastrous. In order to avoid this danger it would be better to have the cables covered with armour of galvanised-iron wire, instead of covering them with wooden boxes; and in case, owing to the depth of the shaft or other circumstances, it is necessary to carry the cable in water, it can be fixed to porcelain insulators. If the shaft is dry, the galvanised-iron armouring will last for a great many years. **Cables along the Passages of a Mine.** The dangers due to these cables are in the first instance arising from the use of bare wire con- ductors, which may be due to that cause or overheating through excess of current, causing coal-dust or other inflammable material in contact with the wires to take fire. The dangers due to bare wires, however, are so obvious that it is seldom, if ever, they are used in a coal-mine so we will consider the dangers due to the use of insulated cables. These are, firstly, due to excess of current; secondly, due to faults in insulation; thirdly, due to main cable, can be met by the fuses or automatic circuit-breakers at the generator-house, which will cut off the current as soon as it becomes excessive. The next danger is that due to some defect in the cable causing an escape of current to the earth. In the case, however, of all ordinary arrangements where the generator is not connected with the earth, a short-circuit has no effect except to cause an escape to the earth. There must be a corresponding defect in the other cable, which also then connects with the earth, and the current goes from one cable through the 1This is now forbidden in colliery shafts. See Appendix, Section 5, Rule 40. 284 ELECTRICITY AS APPLIED TO MINING earth into the other cable, thus causing a short-circuit. Suppose, for the sake of argument, that the motor being supplied with electricity is one mile from the generator-house, and the defect in the cable is half a mile from the generator-house, then the current which can pass at the defective place has only a circuit of half the distance it was intended to have; and not only that, but it avoids the resistance of the motor. Suppose the current at the motor to be 600 amperes, and 660 volts, then in the circuit was represented by 60 volts, then the voltage at the motor would be 600, and if there was a short-circuit the motor would be cut out, and only the resistance would be that of the cable of half the length, over which the drop would be 30 volts for the number of amperes in the ordinary current. But we have 660 volts at the generator, which is equal to forcing a current twenty two times greater than that which will pass through the circuit. This will quickly cause a configuration at the place where the current passes from the cable to the earth, or across from one cable to the other in case the two cables are close together. We thus have a fire lighted in the mine which can only be stopped by the stoppage of the current by some automatic means. The stoppage of this current may be effected by increasing the increase on the ampere-meter or some other sign. If before this happens the timber or coal has been fired, the salvation of the mine depends upon some man being at hand to extinguish the fire. In the case of main cables, the automatic cut-out should always act immediately. In the case of branch cables they are connected to (the generator-house) main cable; but that will depend on the relative size of the branch cable to the main cable, because the automatic cut-out on the generator can only act in consequence of the current exceeding the normal amount. But there might be an excessive current on a branch cable, whilst the total actual current was below the normal. It is therefore necessary that there should be some means of cutting out any such excessive current, so that in case of an excessive current passing along any branch cable it should be immediately cut out. Thus automatic cut-out should be as much part of the necessary apparatus as use of a safety-lamp is in a mine known to contain fire-damp. The cables used in mines are a mine insulated with vulcanised india-rubber, and protected by wrappings of tape and braid. In case there should be a short-circuit, this stuff is easily inflammable, and if it should be in contact with dry wood or coal, a fire might be produced instantly. Every experienced mining engineer is aware that it does not take long to produce a fire. Coal, timber, brattice-cloth, or coal-dust, has been often fired by short circuits; therefore it is possible for a flame of very short duration to set fire to a mine. The liability to firing from a sudden flame in the cable would be reduced to a minimum if the cable was armoured with iron wire. The UNDERGROUND CABLES 285 armouring should be well carried every hundred yards or nearer, because this would tend to prevent the burning of the braiding tape &c. caused by a sudden and quickly stopped short-circuit. Mine managers do not always think it necessary to adopt armoured cable for the sake of the additional safety, but there is no doubt that in a very warm and dry mine it is a mistake to run any unnecessary risk. It may, however, be borne in mind that when the mine is quite dry, and the positive and negative cables are at a distance apart, and where ventilation is excellent, it is almost impossible for a short-circuit to take place between two well-protected taped and braided cables, unless there is some accident or mischief. A short-circuit can only take place by both the conductors being connected with the earth at the same time. The armouring of the cable, by means of which the current is conducted through the earth, facilitates the making of faults and short-circuits, and makes detection and repair of faults less easy. On the other hand, it reduces the chance of injury by mischief or by violent accident. Where the mine is wet, short-circuiting is much more likely to take place, because the water in the air, being wet, become comparatively good conductors, and water making its way into the cable forms a wet track from the copper conductor in the cable to the timber or ground, and the slightest fault in the insulation may be the forerunner of a short-circuit. Happily, these conditions are also those where a general conflagration is not likely to take place. The main cables, where there is a considerable current, are often impregnated with a mixture of clay and oil or paraffin, or with clay and oily compound. This paper, with its compound, is much more durable than india-rubber. The paper covering is drawn into a lead pipe to protect it from water, as contact with water will destroy the insulation. In order to protect the lead, it is armoured, sometimes with a steel strip, like a tape, wrapped round it; sometimes with a galvanised iron band; sometimes galvanised. This armouring will form an effectual protection against a great many accidents, and the galvanising saves the iron from rapid corrosion. When a steel strip is used, it is covered with yarn or tape and protecting composition, to save it from corrosion by water or damp air. The heavy lead-covered cable is laid in trenches below roof level on timber or ground; and in the latter case it may be covered red with clay to keep out water, and also to provide a non-inflammable material in proximity to the cable. A cable laid in a trench is safe from injury even from the heaviest fall of roof, and no cable suspended from props can be regarded as safe from the effect of a fall of roof. The cable may be laid in an iron pipe in order to protect it from falling rocks or mischievous blows, or damage by coal wagons getting off the road, &c. Heat Generated by Filament Lamps.—The ordinary incandescent or filament lamp is not usually regarded as a heat generator, but 286 **ELECTRICITY AS APPLIED TO MINING** as a matter of fact it produces great heat. The greater part of the electric energy (over go per cent.) used in the lamp is converted into heat. When such a lamp is exposed to a current of air, the heat is so quickly removed by the air and by radiation that the glass remains only moderately hot ; but if the lamp is covered up so as to prevent the rapid escape of the heat, then the glass will soon get hot enough to set on fire any inflammable material within reach, such as wood, cotton waste, hemp yarn, brattice cloth, wood coal, and more particularly coal dust. If an incandescent lamp is embedded in coal dust, a fire may be lighted in a few minutes. It must always be borne in mind that the electric lamp contains a fire, like any other lamp, and must be kept cool by constant exposure to a current of air, or to a considerable volume of air, as in a chimney. **Sparking at Switches.—Another source of danger is the sparking at the switches which is liable to occur when using either alternating current or continuous current. This danger may be diminished by putting the switches into strong iron boxes, and working them through stuffing-boxes, the spark being thus prevented from reaching no point except that of explosive gas could be inside, and the openings on the outside being so small that if there were any explosion of gas inside the flame would not be able to pass through the openings. Another plan is to work the switches in oil or water (see page 120). Each of these methods has its advantages. **Short-circuiting with Coal-cutting Machines.—Some of the precautions above named are not practicable in dealing with electric coal-cutting machines. But there is no reason why the switches in the gates should not be protected, either in stout boxes or by oil. The cables used in the face may be armoured or leather covered, and the connections on the machine may be such as to make short-circuiting almost impossible. Short-circuiting with a coal-cutting machine is most likely to be produced by some mistake in connecting the cables to the machine. This might be made impossible if there are only two places on the machine to which cables can be connected, and if these places are separated by a distance sufficient to prevent sparking with the motor, but there is also a danger with the cutting parts of the machine striking hard substances, so that the additional danger from an electric spark is not perhaps very serious. The real danger to be avoided is that of a short-circuit above referred to. No amount of care and attention to all precautions above named will entirely eliminate the danger arising from the use of electricity in mines containing inflammable gas, or liable to blowers of fire-damp. All that is possible is to treat the fire, or possibility of fire, in the electric circuit in the same way as a fire in a lamp is treated in a safety-lamp when it is A diagram showing a coal-cutting machine with cables connected. **SHORT CIRCUITING** 287 covered with wire gauze. The fire is there, but it is rendered harmless. The electric circuit is much more complicated, and for its safe manage- ment requires much greater foresight and knowledge. On the other hand, the electric circuit is only introduced into parts of the mine, and into those parts which are considered quite free from inflammable gas, and is only used when the danger of explosion by the deputies with their safety lamp has shown that it is free from gas. The safety-lamp, though the safety-lamp has shown that it is free from gas, is simpler, is deliberately introduced by the deputies into parts of the mine which are likely to contain gas, for the purpose of exploration and estimation of the percentage of carburetted hydrogen ; but no colliery manager in his senses will allow such an instrument to enter into any part of a mine where he does not know that a fact that there is gas in that place, and the possibility of gas occurring in that place is remote in the extreme. The difference between the danger from gas in contact with an electric current and the danger from coal-dust, dry wood, coal or shale, is that gas may be fired by a single spark of momentary duration, such as might occur at the fracture of a stone or at the impact of a falling rock, while coal-dust may from a great variety of accidents, and before any automatic cut-out could stop the circuit; and indeed the action of the automatic cut-out itself would be liable to cause a spark which might fire gas in the vicinity unless the instrument was put in a case so arranged as to preclude the possibility of firing gas by any accidental spark. The danger from coal-dust and inflammable materials arises only from more prolonged heat or fire. The coal-dust may be fired without any visible signs of heat in the electric circuit, but the heat must be continued for some time, and this should be prevented by a properly arranged system of cut-outs. **Prevention of Accidents due to Short-circuiting—Two classes of permanent causes exist in short-circuiting. One may be called permanent, and the other accidental. In class No. 1, the chief cause is the gradual deterioration of the insulation owing to inherent defects in the insulating material, which tends to decay, the decay being often facilitated by heating of the cable. With the deterioration of the insulating material comes a gradual tendency for water to penetrate through the material, and thus in course of months or years the cable that was perfectly good when laid down becomes quite useless and dangerous. The other permanent causes are those due to the attack of water or damp air, or corrosive gas. The effect of water is to corrode iron arrounding and to soften or destroy rubber; damp air contains carbonic acid material, bitumen, lead, and paper, and in this attack the quality of the water is a matter of utmost importance. The attack of damp air is similar. The attack of corrosive gases can seldom arise except from the smoke due to the combustion of coal or shale, and would happen in a furnace shaft, or in any other atmosphere charged with gas. In an upcast shaft with 288 ELECTRICITY AS APPLIED TO MINING a furnace at the bottom, the fumes of sulphur are likely to mix with water on the shaft side, and thus form a highly corrosive mixture. The accidental causes are storms on the surface, and many other matters of uncertainty which it is not necessary to detail; in the shafts falls of rock, and the destruction of roofs by falling rocks; and in the roadways of the mine damage by coal-wagons getting off the road, falls of roof, upheavals of the floor, blows from pickaxes, shovels, &c. Many people advocate the use of concentric cables, on the ground that if a short-circuit occurs the fire is confined to the cable itself. Others advise the use of separate cables, on the ground that short-circuiting is less likely to occur than in a single circuit. The cause of roof breaking in concentric cable in two or crushing it flat would almost certainly cause a short-circuit at the point of fracture or crushing, but if there were two separate cables it would be necessary that each cable should be simultaneously so fractured as to be connected with the earth before a short-circuit could take place. If three separate cables were in separate rooms it would be impossible to imagine continuous fractures would occur from falls of roof; and even if two separate cables were in the same roadway, but on opposite sides, it is probable that one of the cables would escape fracture. **Detection of Faults.** It must however, be borne in mind that it is quite possible that although in an electric mining circuit there is an undetected fault in one of the cables, because, as previously stated, there can be no short-circuit unless each cable is connected with the earth, so that a fault, however bad, in one cable will probably escape notice altogether until the other cable has some fault made in it; if, therefore, any fault exists in one of these cables, care should be taken to maintain all the cables always in perfect condition, so that no short-circuit can possibly take place except by sudden injury to both the cables at once. If this care is taken, not only are the dangers reduced to a minimum, but the liability to stoppage of work through failure of this kind is also reduced to a minimum, and the cost of maintenance and repairs of the cables is reduced to the slightest degree, because all faults have to be repaired at some time, and everybody will admit that it is much cheaper to repair them before harm has arisen than afterwards. For this reason there should be a fault- or 'ground' detector in each generator-room, so that the man in charge can at once detect an undetected fault. This detector might be placed in a mine or premises connected with his generators. This defect of insulation being only in one conductor of the circuit, which is not earthing, will not cause any damage, and if it is repaired before any defect occurs in the other conductor, no harm will have happened. This detector might be placed in duplicate in the manager's office on the top, and also at some convenient WHEATSTONE BRIDGE 289 place at the pit bottom, so that attention would be immediately called to any defect in any one of the cables, and the underground manager would then be able to telephone up to the engine-room for information as to the position of the fault in the cable, and would be able to give directions for the repair of the cable. This scheme would be out of date. **Position of Fault.**—The men know that there is a fault somewhere in one part of a circuit would be of very little use to the manager of the mine or electrician in charge. He might be entirely unable to ascertain the place of the fault by simply looking at the external casing or feeling for damages, and particularly in the case of an armoured cable, in which case it is impossible to detect any defect in the insulation ; all that he might find would be some sort of distortion. It is therefore necessary to ascertain the position of the fault by means of suitable electrical apparatus. **Measuring Position of Fault.**—The position of a fault in a cable is found by measuring the resistance of the cable from the generating Fig. 187. Station to the fault. For this purpose the apparatus known as Wheatstone's bridge is used. This consists of— (1) A galvanometer, preferably a D'Arsonval mirror galvanometer, very sensitive and dead beat. This instrument has a moving coil which is suspended vertically by a fine wire, and has attached to it a small mirror, and when a current passes through the instrument the needle is deflected, and this deflection is reflected on to a scale or glass window through a ray of light through a lens on to the mirror, which reflects the ray as a spot of light on to any convenient mark or scale, the motion of the spot of light following the motion of the needle. It is necessary to arrange a dark place for the use of this instrument. Instead of this, a 'detector' may be used, which is a sensitive portable galvanometer, in which the deflecting force is supplied by a battery instead of by a current passing through it. (2) A battery consisting of a few galvanic cells, or an accumulator cell. (3) A wire of German silver or other high-resistance metal, at least a yard long and preferably several yards long; the longer it is the more 0 290 ELECTRICITY AS APPLIED TO MINING accurate will be the result, provided that it is of sufficiently uniform cross-section. This wire is stretched in several lengths along a scale which may conveniently be divided into millimetres. Fig. 187 shows a convenient form of this wire, or, in place of the clamp joining the ends of the several sections of wire, a wooden slider may be placed on the scale by means of knife-edges attached to copper bars, short-circuiting that part of the wire which passes round the pulleys; by this means a uniform stretch is given to all parts of the wire. This is known as the bridge wire. (a) A "slider," so arranged as to make contact with the wire at any point, when the resistance between two points is measured. If the wire and scale are mounted on a board this contact may be conveniently mounted on a wooden slider running in a groove on the board. A diagram showing the principle of the Wheatstone bridge is given in fig. 188. Current from the battery, $a$, passes to the points, $b$ and divides Fig. 188. between the two paths, $b$ and $d$, in a ratio depending on the resistances, $a$, $b$, $c$, and $d$. The galvanometer, $e$, is connected between the points $c$ and $e$, and when the resistance, $a$, $b$, $c$, and $d$, are such that $$\frac{a}{b} = \frac{c}{d},$$ then the points $c$ and $e$ must have the same potential, and no current can flow through $e$. Thus, if the ratio $\frac{a}{b}$ and the value of $c$ be known, the resistance, $d$, can be determined. For the purposes of this test* the two leads at the far end of the line are connected together, and the bridge connected up as in fig. 189. The two parts of the bridge wire on either side of the slider form the resistances, $a$ and $d$, and their ratio is that of the lengths of wire as read * The apparatus may be fixed at the generating station, or at the terminals of any cable under test, wherever they may be. 189 WHEATSTONE BRIDGE 291 on the scale. One terminal of the galvanometer is connected to the slider, $c$, the other to earth ---i.e., to some line of piping or to a plate buried in damp earth or a pond; earth corresponds to the point, $x$, in fig. 188, having no appreciable resistance. The battery is connected to the ends of the bridge wire, and the ends of the cables to the points, $a$ and $b$. The two parts of the cable (in fig. 188) which is included in the connection on either side of the fault form the points, $d$ and $e$. The point, $p$, where the fault is situated, is electrically equivalent to the point, $x$, in fig. 188. The slider, $c$, is moved about until on making contact there is no deflection of the galvanometer; this shows that the relationship $d = c \times d$ has been established. The ratio, $\frac{a}{b}$, is read off on the scale, and the position of the fault can now be calculated if the sectional area of all parts of the cable Fig. 189. TESTING BY MEANS OF THE WHEATSTONE BRIDGE. and the lengths are known. Take first the case in which the area is the same throughout. Let the resistance of a yard of cable be $R$ ohms, and let the distance from the generator to the end of the cable be $l$ yards, the total length of cable being $z l$ yards; $c$ a resistance of cable from $m$ to fault; $d$ a resistance of cable from $o$ to fault. Then $$\frac{c}{b} = \frac{a}{b} \text{ and } c = \frac{a}{b} \times d,$$ also $c + d = R \times z l;$ $$\therefore \frac{a}{b} + v) d = z l R,$$ $$d = \frac{2}{R},$$ $$1 + \frac{a}{b}$$ v2 292 **ELECTRICITY AS APPLIED TO MINING** The length of cable having resistance $d$ is $\frac{d}{R}$ :. length of cable to fault = $\frac{z}{1 + \frac{d}{R}}$ yards. If the circuit is made up of cables of various sizes, the lengths of the smaller sizes must all be reduced to the lengths of the main cable which would have the same resistance. Thus, if part of the circuit is composed of cable having half the area of the main cable, its length must be multiplied by two before being added to the length of the main cable to form the length, $z$. The figure obtained as the distance of the fault will not in this case be the true distance, but to obtain this true distance the length of each size cable must be multiplied by one and the remainder divided by two; this will give the length of the small cable from the junction to the fault. It is not sufficient to measure the resistance of one cable only to the fault, as the fault may itself (if it is not a ' dead chord') have a resistance equal to that of the length of cable. By this method, however, it is easily seen that the resistance of the fault is eliminated. It will be seen that it is necessary to have all branches, except the one in which the fault lies, disconnected at the time of the test, so as to form a single continuous circuit. It is easy to find in which branch the fault lies by connecting a telephone receiver to one end of a wire, and listening until the ground detector indicates a fault. For this purpose, as for many others, it would be convenient to have a telephone system all over the mine. Another rather less cumbrous method of finding a fault may be used in certain cases when alternating current is available. An alternating current is passed through one cable in which the fault lies and back through another section of the same cable. The electric sound is heard along the cable with a telephone receiver connected to a coil of wire. The alternating current in the cable will cause an induced current to flow round the coil and produce a buzzing in the telephone receiver. So long as the alternating current remains of the same strength, the sound which it causes will remain constant. When a short-circuit occurs, however, when a fault is passed, however, the current in the cable will diminish or cease, and the sound in the receiver will be similarly affected. Some little practice is needed to understand the indications of this instrument. The position of a fault having been located, it will be the business of the electrical man to see that repairs are made as soon as possible. If this system of testing the cables and repairing them is carried out thoroughly, it will be hardly possible for a short-circuit to occur, except as the result of some serious accident to the mains, or mischief, and the repairs will be effected at the minimum possible cost. OHMMETER 293 Another plan which is frequently adopted for keeping the insulation in order is to make insulation tests of the circuits at certain intervals—say, every week. For this purpose the ohmmeter, an instrument for measuring very large resistances, is used ; it measures the resistance of the insulation, and if there is a serious fault in the circuit it will indicate an insulation resistance of zero, while if the insulation is in perfect order it will register infinity. The reading on the ohmmeter does not necessarily imply that when a fault is found the resistance is always zero, but it may be only a few ohms, which will not be indicated on the ohmmeter, or that when the insulation is good its resistance is really infinite, but it may be millions of ohms, which the ohmmeter cannot distinguish from infinity. One obvious disadvantage of this method over that explained above is that, one FIG. 190. OC Cable To Earth T1 T2 C C P O O O O Ohmmeters. fault being formed subsequently to one week's test, another may occur before the next week and a short-circuit arise; whereas with the ground detector on the switchboard the first fault would immediately declare itself. The ohmmeter also forms a rough way of finding the position of a fault, as it can be taken round to different sections of cable and the insulation resistance measured between each section and earth. This method of efficiency and safety to that above described, is adopted in many cases. With some installations even this is not done, no systematic attempt being made to find the first fault, the secondary ones being repaired as they show themselves. The risks involved in this plan are obvious. 294 ELECTRICITY AS APPLIED TO MINING The ohmmeter is also a useful instrument for testing the insulation of apparatus such as generators, motors, and switches. Its construction is shown in diagram form in fig. 190. c is a little magneto-generator turned by hand, which should give, when turned at a moderate speed, a voltage about the same as that of the supply at the station, because the so-called insulation resistance depends on the voltage (unlike a true resistance). These machines are now made to give voltages up to 1,000. The generator must be ordered with a scale graduated in megohms instead of volts. The generator sends current round two circuits which are connected in parallel; one circuit is formed by the two coils, c, c, which control the position of a soft iron needle pivoted and attached to the pointer, p. The tendency of this current is to point the needle along the axis of the coils, c, and cause p to point to the part of the scale marked Insulation. The axis of the needle is perpendicular to the axis of the coils (the axis of which is at right angles to that of the coils c), the cable under test, its insulation, and the earth. A current passing round this coil tends to deflect the needle along its own axis, and the position taken up by the pointer is determined by the relative strength of the currents in each c'. The current in c is directly proportional to the voltage applied by means of which it flows in c', and therefore in c' is proportional to this and to the conductivity of the insulation of the cable. Thus, the position taken up by the pointer depends solely on the insulation resistance of the cable, and is independent of the rate at which c is turned (which determines its voltage); an increase in voltage, and consequently an increase in current through c', will not affect this position, and therefore the needle is not affected by such increase. The effect of voltage on the insulation resistance pointed out above does not have any serious effect with the small range of voltage which would correspond to turning the handle a little too quickly or too slowly. It is not always necessary that the insulation of the circuit should be considered infinite; it may be said that it "does not register infinity". A less value may be quite good enough. A good rule is that the insulation resistance in megohms should not be less than the figure obtained by dividing 10 megohms by the number of amperes at full load. Thus, in the case given in the table (page 164) for a 460-volt two-mile transmission, the current was 1863 amperes. The minimum insulation resistance of this circuit should therefore be $\frac{10}{1863} = 53.6$ megohm = 53,600 ohms. If cable of 600 megohms per mile be used, the insulation of the cable (four mile circuit) itself will be $\frac{600}{4} = 150$ megohms ; but for fittings, switches, motors &c. may each have an insulation equal to a few megohms, which will soon pull down the total insulation of the circuit.
Page Number 294
Description ELECTRICITY AS APPLIED TO MINING
Diagram fig. 190
Object Description The ohmmeter is also a useful instrument for testing the insulation of apparatus such as generators, motors, and switches.
Construction Details c is a little magneto-generator turned by hand.
Details Description which should give, when turned at a moderate speed, a voltage about the same as that of the supply at the station.
Details Description because the so-called insulation resistance depends on the voltage (unlike a true resistance).
Details Description These machines are now made to give voltages up to 1,000.
Details Description The generator must be ordered with a scale graduated in megohms instead of volts.
Details Description The generator sends current round two circuits which are connected in parallel;
Details Description one circuit is formed by the two coils, c, c,
Details Description which control the position of a soft iron needle pivoted and attached to the pointer, p.
Details Description The tendency of this current is to point the needle along the axis of the coils, c,
Details Description and cause p to point to the part of the scale marked Insulation.
Details Description The axis of the needle is perpendicular to the axis of the coils (the axis of which is at right angles to that of the coils c),
Details Description the cable under test, its insulation, and the earth.
Details Description A current passing round this coil tends to deflect the needle along its own axis,
Details Description and the position taken up by the pointer is determined by the relative strength of the currents in each c'. The current in c is directly proportional to this and to conductivity of insulation of cable.
Details Description This thus position depends solely on insulation resistance cable and is independent rate at which c is turned (which determines its voltage); an increase in voltage,
Details Description and consequently an increase in current through c', will not affect this position,
Details Description &therefore needle is not affected by such increase.
Details Description The effect voltage on insulation resistance pointed out above does not have any serious effect with small range voltage which would correspond to turning handle little too quickly or too slowly.
Isolation Resistance Requirement:Minimum Insulation Resistance Required (in Megohms)Equivalent Insulation Resistance (in Ohms)
Current (in Amperes):1863 Amperes)536 Megohm = 53600 Ohms)
Cable Insulation Resistance (in Megohms per Mile):600 Megohm per Mile)150 Megohm = 15000 Ohms)
Fittings & Switches & Motors &c.:Each has an Insulation Equal To Few Megohms)Which Will Soon Pull Down Total Insulation Of Circuit.)
OHMMETER 295 Thus, if we have insulation of cable = 150 megohms 10 motors, of insulation = 3 each 50 switches and fittings = 5 the total insulation resistance being R (and, as the conductivity is the reciprocal of the resistance is $\frac{1}{R}$), we have for conductivity: $$I = \frac{1}{R} + \frac{1}{R} + \frac{1}{R} + \frac{1}{R}$$ $$= \frac{1}{150} + \frac{1}{3} + \frac{1}{5}$$ $$= \frac{1}{150} + \frac{10}{3} + \frac{10}{5}$$ $$= \frac{1}{150} + 3 + 2,001$$ And $$R = \frac{150}{2,001} = 79.7\text{ megohm}.$$ And this might easily be reduced further by a little bit of slightly defective insulation, or damp. The rule for insulation resistance proposed by the Departmental Committee on "Electricity in Mines" is that the leakage current shall not exceed one ten-thousandth part of the maximum supply current. In the above example the insulation resistance for this standard would work out as follows: Leakage current allowed = 186.5 amperes. Insulation resistance = Voltage $$= 400 \times 10,000$$ ohms. $$= 400 \times 10,000$$ megohms = 0.0214 meg. This regulation is, therefore, less stringent than that given above, and the rule made by the Board of Trade for electric lighting is less stringent still; the leakage current allowed being one thousandth of the supply current, which gives on the above circuit a minimum insulation resistance of over 4 million ohms. The rule adopted by the Board of Trade for coal mines mentioned would give the same result, and the first rule would be less stringent for higher voltages than the second. The rule finally adopted for coal mines is the same as that of the Board of Trade. See Appendix, Section 1., Rule 6. In making a test of the insulation of a circuit, the terminal $T_a$ is connected with the earth—a water-pipe forms a good earth connection—and great care must be taken to be sure that it is a good earth, as upon this depends the efficiency of the test. Care must be taken that all the 296 ELECTRICITY AS APPLIED TO MINING connections of the circuit are made in such a way that the terminal, $r_1$, of the ohmmeter is in electrical connection with every conductor of any description which forms a part of the circuit when it is at work. Thus, if it is desired to include the generators in the test (and this should be done if practicable), all connections must be made as if the entire plant were in operation ; the terminal, $r_1$, is then connected to any convenient point in the armature winding of one generator, and the other end of the ohmmeter is connected to any convenient point on the field winding of another generator—-and the test is made ; an open switch somewhere may cause only a part of the circuit to be included in the test, and the resulting insulation resistance will be higher than is actually the case for the entire installation. It is advisable before using an ohmmeter to test the voltage given by the generator on an ohmmeter, so that the proper spot at which to turn the handle can be known. The ohmmeter may be taken to any part of the circuit and connected to any part of it, or to any separate generator or motor, or part of it. For instance, it may be connected to the windings of the field magnets, or to the armature, and each may be tested separately, and each branch line may be tested separately by opening the switches connecting them. Treatment of Electrical Shock.—Electric shock is the result of simultaneous contact of the body with two conductors at different potentials. Its effect depends mainly on the strength of current passing through the body and the time for which it is maintained; it also depends on the physical condition of the body. A high potential shock is more dangerous than a low potential shock, since it tends to cause a greater flow of current through the body; but other conditions, such as the extent of contact, the moisture of the skin and clothing, may enter into the result and make a low potential shock produce a greater flow of current than a high potential shock under different conditions, and therefore some caution is necessary. On a perfectly insulated circuit, contact with one point of the circuit and the earth would produce no shock, as the point touched would take up earth potential (without appreciable flow of electricity), and there would be no through path for the current to take to the other pole. The greater the leakage between points in a circuit, however, and especially if this leakage is through water or other conducting material, a shock may be obtained by contact with a point in the circuit and earth. If contact be made with both poles, however, or between one pole and earth when there is an earth on the other pole, the current taken will only be limited by the resistance of the body. With alternating current circuits, even if perfectly insulated from earth, very dangerous shocks may be obtained on high-voltage lines containing capacity by contact with one line and earth; in other respects alternating-current shocks are somewhat different in their effects from continuous-current, but it is not universally agreed that they are more dangerous at ELECTRICAL SHOCK 297 equal voltages. A high potential alternating-circuit containing capacity should not be regarded as safe to handle, even when the current has been switched off, until it has been connected to earth and so statically discharged. It is possible to touch live parts of circuits without inconvenience when standing on an indoor rubber-mat or even on a dry wood floor, but contact with metal connected to the circuit by a person who is standing on the ground with the other hand or part of the body at the same time might have serious results. It is a safe rule not to touch any part of an electrical apparatus of any kind without thoroughly understanding what one is doing. The physiological effect of shock is not completely understood, but its practical results are readily as follows: (1) Contraction and stiffening of the muscles. (2) Stopping or weakening of the action of the lungs. (3) Stoppage or weakening of the action of the heart. The effect on the contraction of the muscles is that on grasping a live part it may be impossible to leave go, and a shock which instantaneously taken would not be serious may prove fatal. A good habit to get into is to lightly touch any part of an electrical apparatus before actually beginning grasping it. A habit of this kind might result some day in saving one's life. A person who has received a severe shock may exhibit the following symptoms: (1) Unconsciousness. (2) Cessation of breathing. (3) Cessation of the heart's action. (4) Turning blue and green in the face. These symptoms do not necessarily indicate death, and artificial respiration should never be attempted until a doctor has decided life to be extinct. Mr. F. B. Asmali, in a paper to the Institute of Electrical Engineers (vol. xxxi page 761), gives amongst others the following case of electric shock : "B received a 2,000-volt shock from hand to hand but was not badly burnt. He was insensible; we could not feel his heartbeat, and his breath did not come easily; he was carried up so that we could only see the white of the eyes, his jaw drooped, and thus seeing the man I could have been certain he was dead.... After forty-five minutes' artificial respiration this man recovered." There is no doubt that many people have been given up for dead after an electric shock had occurred, and have been struck by lightning, when proper measures would have restored them. As to remedial measures, artificial respiration should be applied—Dr. Silvester's method, as described in the ambulance books. This has been known to be successful after many hours' continuous application by relays of workers. Dr. L. H. Jones, in a discussion at the Institute of Electrical Engineers (vol. xxxii page 795), in addition to artificial respiration, recommended 'the elevation of the lower limbs and trunk, the rhythmic traction on the tongue which has lately been advocated, and a smart tap over the region of the heart repeated a few times in the course of the first half-minute.' 298 ELECTRICITY AS APPLIED TO MINING The elevation of the body and legs is to send the blood to the brain as a remedy for syncope, which may be a result of the shock ; a blow over the heart may start that organ again if it has stopped beating, and the drawing in and out of the tongue is a form of artificial respiration. Hypodermic injections of ether and alcohol beneath the skin are also advocated as a means of disending the arteries and so helping the heart's action, but this method is dangerous, and all persons who have accidents should be provided with the apparatus and instructed in its use ; in any case, it would be desirable to have the apparatus there for the doctor to use on his arrival. Many of the serious accidents from shock have been due not to the normal usage of the system, but to the very high pressures which are set up on suddenly breaking the circuit, especially in the field coils of a motor. A man took hold of the terminals of a motor at a colliery thinking the current was off ; the field circuit, however, was connected up. He could not get away, and a man who was with him switched off the current, with the result that the man who had jumped through him died, and he was killed. A better plan would have been to seize him by the clothing and try to pull it off the terminal. Dry clothing is a good insulator, and a man in contact with live metals may safely be seized thereby ; but to take him by a bare part or under the armpits, where the clothing is apt to be damp, is attended with risk. When current is being switched off a machine, it is advisable to wait until it has been switched off for some time when an incipient fault may develop and an existing fault becomes most dangerous. APPENDIX SPECIAL RULES FOR THE INSTALLATION AND USE OF ELECTRICITY, ISSUED BY THE HOME OFFICE UNDER THE COAL MINES REGULATION ACT, 1887. (Adopted 1905.) The following Rules shall be observed, as far as is reasonably practicable, to the same. Definitions. The expression 'pressure' means the difference of electrical potential between any two conductors through which a supply of energy is given, or between any part of either conductor and earth as read by a hot wire or electrostatic voltmeter, and (a) Where the conditions of the supply are such that the pressure at the terminals where the electricity is used cannot exceed 250 volts, the supply shall be deemed a low-pressure supply. (b) Where the conditions of supply are such that the pressure at the terminals where the electricity is used, between any two conductors, or between one conductor and earth, may at any time exceed 250 volts, but cannot exceed 650 volts, the supply shall be deemed a medium-pressure supply. (c) Where the conditions of supply are such that the pressure at the terminals where the electricity is used, between any two conductors, or between one conductor and earth, may at any time exceed 3,000 volts, but cannot exceed 5,000 volts, the supply shall be deemed a high-pressure supply. (d) Where the conditions of supply are such that the pressure at the terminals where the electricity is used, between any two conductors, or between one conductor and earth, may at any time exceed 5,000 volts, the supply shall be deemed an extra high-pressure supply. Section 1. General. t. (a) All electrical apparatus and conductors shall be sufficient in size and power for the work they may be called upon to do, and so far as is reasonably practicable, efficiently covered or safeguarded, and so installed, worked and maintained as to reduce the danger through accidental shock or fire 300 ELECTRICITY AS APPLIED TO MINING to the minimum, and shall be of such construction, and so worked that the rise in temperature caused by ordinary working will not injure the insulating materials. (d) In any place or part of a mine where General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies, the covering shall be constructed so that, as far as is reasonably practicable, there is no danger of firing gas by sparking, or flashing which may occur during the normal or abnormal working of the apparatus. (c) All metallic coverings, armouring of cables, other than trailing cables, and the frames and bedplates of generators, transformers, and motors other than portable ones, shall be completely enclosed in strong armouring or metal casing efficiently connected with earth at the terminals where the electricity is supplied, so that the pressure at the terminals does not exceed the limit of low pressure. 2. Where a medium-pressure supply is used for power purposes, or for arc lamps in series, the wires or conductors forming the connections to the motors, transformers, arc lamps, or otherwise in connection with the supply, shall be, as far as is reasonably practicable, completely enclosed in strong armouring or metal casing efficiently connected with earth, or they shall be fixed at such a distance apart, or in such a manner, that danger from fire or shock may be reduced to the minimum. This rule shall not apply to trailing cables. 3. Where a medium-pressure supply is used for incandescent lamps in series the wires or conductors forming connections to the incandescent lamps, or otherwise connected with the supply, shall be of a form as reasonably practicable, completely enclosed in strong armouring or metal casing efficiently connected with earth, or they shall be fixed at such a distance apart, or in such a manner that danger from fire or shock may be reduced to the minimum. 4. Motors and generators used for supplying current at a pressure higher than medium pressure, and no motor using such current, shall be of less normal rating than 3 b.h.p. for use under ground. No higher pressure than a medium pressure shall be used in any place or part of the mine to which General Rule No. 8 of the Coal Mines Regulation Act, 1887 applies. 5. No higher pressure than a medium-pressure supply shall be used other than for transmission or for motors, and the wires or conductors other than overhead lines between points of supply and motors, transformers or otherwise in connection with the supply shall be completely enclosed in strong armouring or metal casing efficiently connected with earth, or they shall be fixed at such a distance apart, or in such a manner that danger from fire or shock may be reduced to the minimum. The machines, apparatus, and lines shall be so marked as to clearly indicate that they are high pressure, either by the use of the word 'Danger' at frequent intervals along their length on poles and overhead lines, 6. The insulation of every complete circuit other than telephone or signal wires used for the supply of energy, including all machinery, apparatus, and devices forming part of or in connection with such circuits, shall be so maintained that the leakage current shall, as far as is reasonably practicable, not APPENDIX 301 exceed 1/20 of the maximum supply current, and suitable means shall be provided for the immediate localisation of leakage. 7. In all complete insulating circuit, switch, or fault detectors shall be kept connected up to every generating and transforming station, to show immediately any defect in the insulation of the system. The readings of these instruments shall be recorded daily in a book kept at the generating or transforming station. 8. Main and distribution switch and fuse boards must be made of incunable insulating material, such as marble or slate free from metallic veins, and must be so arranged that they can be opened without risk of shock. 9. Every sub-circuit must be protected by a fuse on each pole. Every circuit carrying more than 5 amperes up to 125 volts or 3 amperes at any pressure above 125 volts, must be protected by one of the following alternative methods: (a) By an automatic maximum cut-out on each pole. (b) By a detachable fuse on each pole. (c) By a switch and fuse on each pole. 10. Fire buckets, filled with clean, dry sand, shall be kept in electrical machine rooms, and shall be used in case of electrical extinguishing fires. No repair or cleaning of the live parts of electrical apparatus except upon mere wiping or siling shall be done when the current is on. Gloves, mats, or shoes of india-rubber or other non-conducting material shall be worn by persons working on live parts of switches or machines working at a pressure exceeding the limits of low pressure, have to be handled for the purpose of adjustment. 11. A competent person shall be on duty at the mine when the electrical apparatus or machinery is in use ; and at such time as the amount of electricity delivered down the mine exceeds 200 h.p., a competent person shall be on duty at the mine above ground, and another below ground. Every person appointed to this duty shall have been instructed in his duty and he must perform for the work that he is set to do. 12. No person shall wilfully damage, interfere with, or without proper authority remove any part of any electrical apparatus, or part thereof, used in connection with the supply or use of electricity, 13. Instructions shall be posted up in every generating, transforming, and motor house containing directions as to the restoration of persons suffering from electric shock. 14. Direct telephonic or other equivalent means of communication shall be provided between the surface and the pit bottom or main distributing centre in the pit. 15. Within three months after the introduction into any mine of electric motive power, notice in writing must be sent to H.M. Inspector of Mines for the district, who must also be sent of any existing electric motive power installation at any other mine in his district, and which are brought into force of these rules. 16. A plan shall be kept at the mine showing the position of all permanent electrical machinery and cables in the mine, and shall be corrected as often as may be necessary to keep it up to a date not more than three months previously. 302 ELECTRICITY AS APPLIED TO MINING Section II.—Generating Stations and Machine Rooms. 17. Where the generating station under the control of the owner or manager of the mine is not within 400 yards of the working pit mouth, an efficiently enclosed locked switch box or boxes, or a switch-house, shall, where reasonably practicable, be provided near the pit mouth, for cutting off the supply of electricity to the mine. 18. There shall be a passage way in front of the switchboard of not less than 3 ft. in width, and if there are any windows at the back of the switchboard, and if there are any windows on the switchboard, they shall be less than 6 ft. clear. This space shall not be utilised as a storeroom or a lumber room, or obstructed in any manner by resistance frames, meters, or otherwise. If space is required for storage purposes, electrical apparatus behind the board the passage way must be widened accordingly. No cable shall cross the passage way at the back of the board except below the level of the floor. The space at the back of the switchboards shall be properly floored, accessible from each end, and, except in the case of low-pressure switch-boards, must be kept locked up, but the lock must allow of the door being opened from the outside without use of a key. The floor at the back shall be combustible, firm and even. 19. Every generator shall be provided with a switch on each pole between the generators and the main switch. Where continuous-current generators are paralleled, reversed current cut-outs shall also be provided. Suitable instruments shall be provided for measuring the current and pressure of all circuits. Every feeder circuit shall at its origin be provided with an ammeter. 20. If the transmission lines from the generating station to the pit are overhead lines, they shall be protected against damage by lightning circuits. 21. Automatic cut-outs must be arranged so that when the contact lever opens outwards no danger exists of striking the head of the attendant. If unenclosed insulators are used they must be placed within 2 ft. of the floor or be otherwise stably protected. Where the supply is at a pressure exceeding the limits of medium pressure, there shall be no live metal work on the front of the main switchboard within 5 ft. of the floor or on any part of any other switchboard. In this case this section shall be not less than 4 ft. in the clear. Insulating floors or halls shall be provided for medium pressure boards where live metal work is on the front or back. 22. All terminals and live metal on machines over medium pressure above ground, and over low pressure under ground, where practicable shall be pro-tected with insulating covers or with metal covers connected to earth. 23. No one other than an authorized person shall enter a machine or motor room, or interfere with the working of any machine, motor, or apparatus connected therewith. APPENDIX 303 Section 111.—Cables. 24. All conductors (except as hereinafter provided) shall in every case be maintained completely insulated from earth, but it is permissible to use the concentric system with earthing outer conductor, if proper arrangements are made to reduce the danger from fire or shock to the minimum, but the neutral point of polyphase systems must be earthed and the wires of three-wire continuous-current systems may be earthed at one point. 25. Unless fixed as far as is reasonably practicable out of reach of injury, all conduits, open cables, and other exposed parts, must further be protected by a suitable covering. Where lead-covered cables are used, this covering must be applied, and electrically continuous throughout. The exposed ends of cables where they enter the terminals of switches, fuses, and other apparatus, must so that moisture cannot creep along the insulating material within the waterproof sheath, nor can the insulating material, if of an oily nature, come into contact with the apparatus. 26. All joints must be mechanically and electrically efficient, and, where reasonably practicable, must be suitably soldered. In any place or part of a mine where power is supplied, suitable power boxes must be used, and the conductors connected by means of metal screw clamps, connectors or their equivalent, constructed in a safe manner. Provided that in any place or part of a mine where a shot may be fired, pipes or other devices are provided for discharging the gas produced by that shot for the safety of persons employed in that work on behalf by the manager, but the same precautions in regard to examination and removal of workers as are prescribed by paragraphs (f) and (g) of General Rule 12 are required. 27. Overhead bare wires on the surface must be efficiently supported upon insulators, and clear of any traffic, and provided with efficient lightning arrestors. 28. All cables used in shafts must be highly insulated and substantially fixed. Shaft cables, not capable of sustaining their own weight, shall be properly supported at intervals varying according to the weight of the cable. Where the cable is suspended from a metallic sheathing or from insulating material, space shall be left between them and the side of the shaft that may yield, and so lessen a blow given by falling material. 29. Where the cables in main haulage roads cannot be kept at least 1 foot from any wall or timber, they shall be electrically protected. When separate cables are used they shall, if reasonably practicable, be fixed on opposite sides of the road. The fixing of supports and fastenings of cables and wires not provided with metallic covering to walls or timbers is prohibited. Cables underground when suspended shall be suspended by leather or other flexible material in such a manner as to allow of their readily breaking away when struck, before the cables themselves can be seriously damaged. 304 ELECTRICITY AS APPLIED TO MINING Where main or other roads are being repaired, or blasting is being carried out, suitable temporary protection must be used so that the cables are reasonably protected from damage. 30. Trailers for portable machines shall be specially flexible, heavily insulated, and protected with either galvanised steel wire armouring, extra stout braiding, hose pipe, or other effective covering. Trailings cables shall be examined at least once in each shift by the person in charge of the machine, and any defective cable replaced immediately. At points where the flexible conductors are joined to the main cables, a fixed terminal box must be provided, and a switch shall be fixed close to or in the terminal box capable of entirely cutting off the supply from the terminal box and motor. Section IV.---Switches, Fuses, and Cut-outs. 31. Fuses and automatic cut-outs shall be so constructed as effectively to interrupt the current when a short-circuit occurs, or when the current through them exceeds the working current by 200 per cent. Fuses shall be stamped or marked, or shall have a label attached, indicating the current with which they are intended to be used, or when fuse wire is used each coil in use shall be so stamped or labelled. Fuses shall only be adjusted or replaced by an authorised person. 32. All live parts of switches, fuses, and cut-outs in machine rooms, or in connecting tunnels specially arranged for this purpose, must be covered. These covers must be of combustible material, and must be either non-conducting or of rigid metal, and, as far as practicable, clear of all internal mechanical parts. 33. All points at which a circuit, other than those for signals, has to be made or broken shall be fitted with proper switches. The use of hooks or other makeshifts is prohibited, and in any place or part of a mine where General Rule No. 87 of the Coal Mines Regulation Act, 1887 applies, the use of open-type switches, fuses, and cut-out is prohibited; they must either be enclosed in gas-tight boxes, or break under oil. Section V.---Motors. 34. All motors, together with their starting resistances, shall be protected by switches capable of entirely cutting off the current, and fixed in a convenient position near the motor, and except in cases of no-h.p.h., or over in a machine room underground shall be provided with a suitable annometer to indicate the load put upon the machine. 35. Where unarmoured cables or wires pass through metal frames or into boxes or motor casings, the holes must be substantially bushed with insulating bushes, and, where necessary, with gas-tight bushings which cannot readily become loose. 36. Terminal boxes of portable motors must be securely attached to the machine, or be designed to form a part thereof. 37. In any place or part of a mine where General Rule No. 8 of the Coal Mines Regulation Act, 1887 applies, all motors, unless placed in such rooms as APPENDIX 305 are separately ventilated with intake air, shall have all their current-carrying parts, also their starters, terminals, and connections completely enclosed in flame-tight enclosures, made of nonflammable material, and of sufficient strength as not to be liable to be damaged should an explosion of firelamp occur in the interior, and such enclosures shall be opened only when the lamp is lit, and then only when the current is switched off. The pressure shall not be switched on while the enclosures are open. 38. In every mine where General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies, a safety lamp or other suitable apparatus for the detection of firelamp shall be provided for use with each machine when working underground. When a firelamp is detected by any person using the safety lamp or other apparatus used for the detection of firelamp, the person in charge shall immediately stop the machine, cut off the current at the gate end or nearest switch, and report the matter to an official of the mine. 39. If any motor is not being used underground for a period of time exceeding a maximum period which shall be specified in writing by the manager, so that the roof may be carefully examined. 40. All electric motors used underground and the casings of their switches and other appliances shall at least once a week be opened by a competent person appointed by the manager, and the parts so disclosed shall be examined by him. In special cases requiring a motor to run continuously longer than one week, the motor shall be examined at the end of the run. A report of such examination shall be entered in a report book. 41. No coal-cutter or drilling machine shall not leave the machine while it is working, and shall, before leaving the working place, see that the current is cut off from the trailing cables. He must not allow the cables to touch any live wire until he has seen that they are free from any portable machine until the pressure has been cut off from the trailing cables. 42. If any electric sparking or arc be produced outside a coal-cutting or other portable motor or by the cables or rails, the machine shall be stopped, and not be worked again until the defect is repaired, and the occurrence shall be reported to an official of the mine. Section VI.—Electric Locomotives 43. Electric haulage by locomotives by the trolley wire system is not permissible in any place in part of a mine where General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies. On this system no pressure exceeding the limits of medium pressure may be employed. 44. Trolley wires shall be placed on poles or other supports so that they are at least 7 ft. above the level of the road or track, or elsewhere, if sufficiently guarded, or the pressure must be cut off from the wires during such hours as the roads are used for travelling on foot in places where trolley wires are fixed. The board on which these rules are printed shall be displayed prominently and indicated by notices and signals placed in a conspicuous position at the ends of the roads. At other times no one other than a duly authorised person shall be permitted to travel on foot along the road. X 306 ELECTRICITY AS APPLIED TO MINING On this system either insulated returns or uninsulated metallic returns of low resistance may be employed. 44. In order to prevent any other part of the system being earthed (except where the whole system with earthed outer conductor is used); the current supplied for use on the trolley wires with an uninsulated return shall be generated by a separate machine, and shall not be taken from or be in connection with electric motors otherwise completed in such places from each other. 45. If any place, or part of a mine, is in any place or of a mine where General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies, the rules applying to motors in such places shall also be deemed to apply to the bosses containing the cells. Section VII.—Electric Lighting. 46. All arc lamps shall be so guarded as to prevent pieces of ignited carbon falling from them, and shall not be used in situations where there is likely to be danger from the presence of coal-dust. They should be so screened as to prevent risk of contact with persons. 47. Where electric lamps are used they must be either conveyed in pipes or casings, or suspended from porcelain insulators, or tied to them with some non-conducting material which will not cut the covering, and so that they do not touch any part of the body of a person who may come into contact with them. If metallic pipes are used they must be electrically continuous and earthed. If separated uncased wires are used they must be kept at least 2 in. apart, and not brought together except when swivelled by a fitting. 48. In any place or part of a mine where General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies, electrical lamps, if used, must be of the vacuum or enclosed type ; they shall be protected by gastight fittings of strong glass, suitably arranged to allow of ventilation without loss of light, and shall only be accessible to the authorised shot-firer. 49. In all mines and other places underground, where a failure of electricity is likely to cause danger, some safety lamps, or other proper lights, shall be kept for use in the event of such failure. Section VIII.—Shot-Firing. 50. Electricity from lighting or power cable shall not be used for firing shots, except in sinking shafts or stone drifts, and then only when a special firing plug, button, or switch is provided, which plug, button, or switch shall be placed in a fixed locked box, and shall only be accessible to the authorised shot-firer. The firing cables or wires shall not be connected to this box until immediately before the time of firing of shots, and shall be disconnected immediately after the shots are fired. When shot-firing cables or wires are used in the vicinity of power or lighting cables, sufficient precautions shall be taken to prevent the shot-firing cables or wires from coming in contact with the lighting or power cables. APPENDIX 307 The foregoing rules shall not apply to telephone, telegraph, and signal wires, to which the rules of this section only shall apply. Section IX. -- Signalling. 51. All proper precautions must be taken to prevent electric signal and telephone wires from coming into contact with other electric conductors, whether installed or not. 52. Contact makers or push buttons of electric signalling circuits shall be so constructed and placed as to prevent the circuit being accidentally closed. 53. In any place where the provisions of General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies, bare wire shall not be used for signalling circuits except in haulage roads, and the pressure shall not exceed 15 volts in any one circuit. Section X. -- Electric Relighting of Safety Lamps. 54. In mines to any place or part of which General Rule No. 8 of the Coal Mines Regulation Act, 1887, applies, when safety lamps are relighted underground by electricity, the manager shall select a suitable station or stations, which are not in the return airway, and in which there is likely to be any accumulation of inflammable gases. The relighting apparatus specified shall be used in any other place. All electrical relighting apparatus shall be securely locked, so as not to be available for use except by persons authorised by the manager to relight safety lamps, and such persons shall examine all safety lamps brought for relighting before they are re-lit. Section XI. -- Exceptions and Miscellaneous. 55. Notwithstanding anything contained in these rules, any electrical plant or apparatus installed or in use before the coming into force of these rules may be continued in use until such time as it is otherwise direct, or subject to any conditions affecting safety that he may prescribe. In case any difference of opinion shall arise between an inspector and an owner under this Rule, the same shall be settled as provided in section 42 of the Coal Mines Regulation Act, 1887. 56. Any of the foregoing requirements shall not apply in any case in which exemption is obtained from the Secretary of State, on the ground either of emergency or special circumstances, or such conditions as the Secretary of State may prescribe. X 2 308 ELECTRICITY AS APPLIED TO MINING NOTES ON THE HOME OFFICE RULES It will be found that practically all the dangers contemplated by these rules have been very fully dealt with in the body of this work, and the general conclusions arrived at will for the most part be found to be in accordance with the tendency of the rules. It cannot, however, be too strongly insisted on, that mere compliance with the rules is not enough to ensure the safety of an installation and of the persons employed about it. The rules necessarily allow a very large discretion to the management of the mine, to take whatever precautions may be necessary for the particular circumstances of each case; any attempt on the part of the framers of the rules to impose upon them a rigid and unalterable and hard and fast restrictions, which would probably have had the effect of making use of electricity impossible in many cases where it could otherwise have been used with safety and advantage. It is just as necessary—in fact more necessary—to take the additional precaution against accidents which might happen in other ways than others—as common sense and experience may dictate, as it is in the working of a coal mine to take precautions which are not compulsory under the Coal Mines Regulations Acts and General and Special Rules. It remains to point out some points in the rules which may require explanation or discussion. Section 1 (a) (Rule I (a))—with some general advice on the necessity of having everything of the best. There can, of course, be no economy in buying cheap plant, or installing it in a cheap way, if we thereby increase the liability to breakdown or accident. This general maxim is peculiarly true of electrical plants. Rule I (b)—For parts of a mine "in which there is likely to be any such quantity of inflammable gas as to render the use of naked lights dangerous," nothing can be better than a totally self-contained electric motor (see p. 71). Rule I (c) to state the provision of the earth connection for cables and cable armouring. Probably the greatest difficulty which will be experienced by mining engineers in carrying out the rules will be in the provision of efficient earth connections. An earth connection for conveying a small current such as is required for a telephone or signal circuit is not difficult to obtain, but it is not always easy to obtain an earth connection for conveying away the current which comes from a fault on an electric power or lighting circuit. The rails even on a thoroughly wet road must on no account be used as an earth connection. A case occurred a short time ago where a motor frame was connected to one rail, and a man standing on another rail was killed. The yards almost covered with water and mud; a fault occurred on the motor, and a horse standing on the rails was killed, and a shock was experienced by a man in contact with one rail. The rails being connected to an earthed pump main cannot be regarded as wholly satisfactory inasmuch as if the main does not make a thoroughly good earth, the whole of the metal in connection with it becomes alive when a fault occurs. Probably the safest plan is to use an earth plate entirely unconnected with other metal—if this plate is not a good APPENDIX 309 earth the danger is at any rate not extended over the rails or a range of pipes. The earth plate should be of copper with a cable soldered to it connected to the frame or armouring which is to be earthed. The plate should be at least two feet square, and should be buried in coke breeze and kept thoroughly wet. A coil of old cable with the insulation removed having an equivalent surface area may be used instead of the earth plate. In this case, however, cables can be obtained armoured cables are a source of danger, and the alternatives allowed in the rules should be adopted in its place. Rule 8.—All instruments and apparatus are now made by many manufacturers to comply with this rule ; they mostly measure the relative insulation and leakage of the two mains, but are said to be officially recognised as complying with the rules. They are of a form of the fault-detecting voltmeter mentioned on p. 130, and are calibrated in milliamperes of leakage current, in ohms of insulation resist- ance, or in volts of difference of potential between line and earth. The same instrument may be used for both purposes. Rule 7.—It is generally regarded as sufficient if the earth detector is pro- vided with a switch to be connected to the circuit when necessary. If it is kept connected all the time, it will give a false indication that no connection provides the first fault necessary before a second fault can produce a short circuit, and so the rule is made to defeat its own object. (See p. 283.) Rule 10.—The sand mentioned in this rule must, of course, only be used in the case of a fire on the rails, because if it were used on anything that flamed could be effectively dealt with in this way ; the promiscuous throwing of sand on anything that gets hot may result in considerable trouble and damage. Water is, of course, more effective than sand, but it is liable to cause short-circuiting and earthing, and is more likely to increase fire than diminish it. Section II. Rule 19.—A reverse current cut out is an electro-magnetic instrument which automatically opens the circuit when a current goes through it in the opposite direction to that for which it was designed. Direct current generators are run in parallel, if for any reason the voltage of one should fall below that of the others, current will flow through it in a reverse direction and destroy it. A reverse current cut-out is sometimes inserted in each generator circuit to prevent this action from continuing. A feeder circuit may be taken as any distributing current starting from the generating room switchboard. Whether a circuit starting from a distributing centre elsewhere is a feeder circuit, is a point on which there might be differ- ence of opinion. Rule 20.—Lightning arresters (see page 123). These would usually be fixed to the feeders coming into the generating room. Rule 21.—A simple way of protecting an unenclosed fuse for low voltages is by slipping it through a piece of asbestos tube. This should also be done in the case of fuses in iron cases, or better still the case should be lined with a non-conducting material such as felt or cork. The asbestos tube should connect the fuse and the case. Section III. Rule 25.—Electrically continuous throughout. That is to say, wherever a wire is made in the lead covering, for instance, at a junction box, there shall be a metallic connection between the lead on one side of the break and that on the other side, such that a current can pass between them. The 310 ELECTRICITY AS APPLIED TO MINING second part of this rule applies mainly to paper insulated cables; there is no waterproof sheath to an india-rubber or bitumen insulated cable. Rule 29.--It is convenient to have the cables so fixed that they can be lowered on to the floor and covered up while repairs are going on, and replaced by raising them again. It is desirable to avoid the use of nails in any way involving the use of nails except a properly qualified man. Rule 30.--Wire armouring is an undesirable covering for trailing cables, as it can seldom be efficiently earthing, and so any fault in the cable is a source of danger to the men who must constantly handling it. Leather or strong braiding is a better protection. Section V. Rule 37.--It is difficult to avoid regarding it as a matter for regret that the coal-cutter motor is usually provided with a squirrel-cage induction motor with a squirrel-cage rotor. The most dangerous part of a coal-cutting machine as regards the firing of gas is undoubtedly the trailing cable: this cannot be enclosed in a gas-proof case, and is liable to blow at any time, which means that it may be exposed to the air at all times. In such a case it is not safe to introduce a squirrel-cage motor (a piece of apparatus which is most unlikely to produce a spark of any kind) without enclosing it in a flame-tight case, if it is possible to do so. The introduction of such a seam is a most dangerous thing, and should of no account be allowed. It should be remembered, however, that a coal-cutter motor and a trailing cable are not necessarily connected together, and that there may be a gap between them. There is likely to accumulate without warning such a quantity of inflammable gas as to render them dangerous. One would be inclined to say that where it is safe to fire shots, it is certainly safe to use an open motor of the squirrel-cage type. The disadvantage of using an open motor in such cases increases the liability to heating and consequent breakdown of the insulation. INDEX A ACCUUMULATEUR, 18 Ackroyd & Best safety lamp, 254 Aickon Hall Colliery, Hard coal-cutter, 203 installation of turbine dynamos, 100 Advantage of low-vision current, 160 Agglomeration block Lechuchet cell, 74 Alternating-current, 27, 138, 173 electro-motors, 53 elementary principle of, 46 periodicity, 50 transformation, 50 Alternating multiphase motors, 75 Alternations, 27 L.C.C., 34 Aluminium conductors, 113 Ammeter, 127, 129 Amphere, 6 Arc lamps, 430 cathode type, 230 system, electric welding, 267 Armature, 32 core, 39 dross wound, 39 ring wound, 40 Arrangement of cables to avoid fire, 280 of engine-house, engines, dynamos, 49-51 Armature lighting, 123 Asynchronous motors, 53 Atmospheric electricity, 1 Averaging distance to which power is taken, 169 BAIRD coal-cutter, 218 Bar machine, modifications of, 205 Battery, 21, 17 house, 25 Horse-power oven, 78 Bell's Cleveland ironstone-mines, electric drills in, 256 Bellis signal, 234 Bell-driven pumps, 184 Bellows motor, 185 Bells, horse-power transmitted by, 85 Benardos system of welding, 267 Binkenhed Electricity Works, test of Plant, 88 Blacksmiths electricians advantages of, 299 firing from lighting or power mains, 443 high tension system, ago low-tension system, ago Board of Trade unit, 8. Boiler plant, working costs of, 155 Brake horse-power, 87 British Electric tramway, British Thomson-Houston hauling drum, 190 electric locomotive, 196 Bowers Lindsay engine, 44 Edison's high-tension lighting oil safety lamps, 253 Buckling in secondary cells, 21 Burning of inferior fuel, 28 Byng Hawkins dynamo, 44 312 ELECTRICITY AS APPLIED TO MINING
TABLE, arrangement of, to avoid fire, 280
concentric, 136, 137
construction of, 134
erection of, 143
heating of, 146, 156
in shaft, cost of putting, 145
installation of, 134
loss in, 137, 162
method of carrying down shaft, 145, 281
overhead, 175
reduction in cost by increasing size of, 171
sizes of, 133, 166
sizes of, 133, 141
suspender, 143
tensile tests of copper, 147
cutting-out switch in workings, 245-283
Calender's feed switch box, 125
gate end switch box, 219
system, 130
Caliper, 7
Carpus cell, 14
Celsis, carposus, 14
Century, 15
Daniels', 19
Double head, 9
D.P., see D.P.
dry, 9, 17
galvanic, 10
Headlandi', 21
Lecithin', 18
 Clarke & Stevenson's coal-cutter, 205
Cleveland Mines, electric drill-in, 298
Coal-cutter, Bainel, 218
Clarke & Stevenson's, 205
Diamond, 208
Gossel's electricric, 217
Heppell & Paitterson, 205
Hardt, 201
Jeffrey bar machine, 205
chain machine, 215
longwall machine, 218
Lee, 205
Morgan-Gardner electric pick ma-chine, 200
chinee, 200
coal-cutting machines, 215
disc machines, 205
electric pick machines, 200
modifications of revolving bar ma-chines for cutting coal in shafts,
number in use, 199
revolving bar machines, 203
 Cool-cutting by electricity, 198
Coal-cutting machines short - circuiting machine for cutting coal in shafts,,
Coke oven utilization of waste heat, 78
Beechey', 78
Colliery compression,,86
installations,,84
Combined plant at Derbyshire colliery,,228
Commutator,,38
Compound dynamo,,43
Compressed air transmission of power by,,276
Construction details of,,141
in vertical shafts,,145,28s
Continuous current,,27
high-sension,,173
Core,,39
Cost of electrical estimate,,156
high-sension concentrate cable,,161
working $50$ h.p. water-power plant,,76
working $100$ h.p. steam plant,,76
 Cut-off blockage in the coal cutter,,28
Efficiency of single fluid,,9
Voltaic,,30
Centrifugal plants,,153
Centrifugal pumps,,185
Century cell,,5
Chemical effect of electric current,,5
Circuits electrici,,2
 Cut-off blockage in the coal cutter,,28
Efficiency of single fluid,,9
Voltaic,,30
Centrifugal plants,,153
Centrifugal pumps,,185
Century cell,,5
Chemical effect of electric current,,5
Circuits electrici,,2
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,28
} 		Cut-off blockage in the coal cutter,,313 Costs, working, 175 Condensh, 8 Current, alternating, 27, 158 continuous, 27, 157 Foucault, 39 lag, 47 of electricity, 2 properties of an electric, 3 single-phase, 34 three-phase, 36 two-phase, 34 unit, 6 Curit turbine, 105 Cut-outs, 124 magneto, 126 DANGERS from heat of conductors, 280 heat of electricity, 280 Daniell cell, 11 D'Anroval galvanometer, 289 Davis & Schrader low-tension lamp, 254 signal bell, 255 Davis's junction box, 152 Deacon's system of carrying cables, 149 Decomposition of water, 5 De Laval steam engine, 101 steam composition of, 105 weight and dimensions of, 103 Derbyshire Colliery, electric plant, 228 three-phase heating plant, 229 Description of electric power station, 57 Destructor, Meldman's refuse, 79 Details of conductors, dimensions, caps, city, weight, &c., 141 Detection of electricity, 388 Detonators, electric, 239 Diamond coal cutter, 208 chain coal-cutter, 218 Diode tube, 134 thickness of, 134 Direct current, 27 transformers, 60 Disc coal-cutting machines, 205 Distance to which power is taken, averaging, Distribution of current in shafts and work- ings. Distribution of electrical energy; series and parallel circuits. three-phase system, three-wire parallel system, two-phase four-wire system, two-phase three-wire system, three-phase four-wire system, double fluid cell, double leather beltin. h.p. transmitted by. Double pole liquid switch, D.P. secondary cell, Drill. Gardiner electric, Gauss's electric, Helmholtz coil, Marvin electric, Wickstead's shop drill, Drop of voltage, Dynamo-wound armature, Dry dynamo, Dynamic electricity, Dynamo, compound, direct current, shunt, Dynamic-electric exploder, Dynamo-meter, EARTH detector, Easton & Co., Erth; sinking pump by, Economical generation of electric power, Efficiency of electric plant, electric motor, plant, pumping plant, secondary cells, Electric blasting. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. A diagram showing the arrangement of a three-phase electrical system with a transformer and a motor. 314 ELECTRICITY AS APPLIED TO MINING Electric blasting, advantages of, 239 cables, 238 circuits, 2 current, properties of, 3 drills, 236 Gardiner's, 264 Grant's, 259 Jeffrey, 261 permissive, 263 heating, 188 booming, 190 pick machines, 200 Morgan-Gardner, 200 plant, working costs of, 156 power, cost of, 153 efficiency of generation, 86 pumping plants, 178 pumps, types of, 178 direct current, 182 safety lamps, 253 Healand, 252 Summan, 252 shot firing in sinking pit, 248 dynamic description, 248 signals, 256 transmission, 273 welding, 267 Electrical current, description of, 157 measuring instruments, 166 shock, treatment of, 266 units, 5 Electricity, atmospheric, 1 current dangers of, 280 definition of, 1 dynamic, 27 frictional, 1 Electricity, meters, 129 monophase, 34 three-phase alternating, 36 two-phase alternating, 34 Electrolysis, electro-motive force, 6 Electro-motors, 48 direct current, 48 electrically generated, 53-71 Electro-negative, 10 Electro-positive, 10 Enclosed switches, 120 Exhaustive type arc lamp, 230 Endurance life of bulbage, 193 Energy, electrical unit of, 8 Engines, compound, 81 gas, 83 high or slow-speed, 82 oil, 84 single-cylinder, 81 steam, 81 vertical or horizontal., R3 Estimation of cost of electric station of $2.000 h.p., 156 Examination and repair of plant facility for safety. Exciter, 33; 226 Exploders, 240 magneto-, 240 receiver-, 243 testing-, 243 E.C.C. alternator., 31 E.C.C. dry cell., 17 Electrical current description of $k$ A.L.T.S. detection of $k$ A.L.T.S. position of $k$, $k$ A.L.T.S. secondary circuit on $k$, $k$ A.L.T.S. feeder panels., $k$ A.L.T.S. magnetic field., $k$ A.L.T.S. double magnetic circuit., $k$ A.L.T.S. single magnetic circuit., $k$ A.L.T.S. Fire-proof generator-house., $k$ A.L.T.S. motor-room., $k$ A.L.T.S. INDEX 315 Firing from lighting or power mains, 243 Flaming's rule, 28 Force, elastic, 26 Tinnes of, 3 Foucault currents, 39 Fraser & Chalmers, Eichh, Redler pumps 54, 70 Free magnetism, 4 French thermal unit, 7 Frictional electricity, 1 Fuel, burning of, 78 Fall and rise, 188 Fusees, …., 124, 182 G ALVANIC cell, 10 Garforth, W. E., referred to, 208, 212 Gas, transmission of power by, 278 Gas-engines, 83 Gas-light motors, 71 Gaspump, electric wire, 133 Gearing of pumps, 180 spun, 181 worn, 180 Gelbowski's Bergwerks-Akriegssellschaft, electric winding at, 270 Generator-house, fire-proof, 282 Generator-pipe, 226 Generator-plate, multiple, 43 starting and stopping, 113 Gilbert & Cooley coal-cutter, 208 Glass lamps, 230 Goodwin's breast machine, 217 Grant's electric drill, 259 Greenfield Mine, Patterdale, electrical plant, 76 water-power at, 76 Greensley, particulars of results of electric coal-cutting machines, 216 Guttermuth pump, 183 H AULAGE, electric, 188 calculations, 191 endless rope systems, 193 Harness, main and tail-rope system, 193 single rope, 188 Hauling drag by British Thomson-Houston electric, 190 Hay's insulator, 49 Healdon electric safety lamp, 232 Heat of conductors, dangers from, 176 Heating effect of electric current,, 5 of cable,, 165; 176 Hickman-white culverts, electric winding of cables,, 205 Hoppell & Patterson's coal-cutter,, 305 High-pressure steam,, 70 High tension economy of,, 60 High-voltage concentric cable,, cost of,, 84 currents,, 160; 173 system,, electric blasting,, 240 Holding by hand and machine,, 205 Hot-water heater,, indicated,, 87 lost in cable,, 137; 165; 165 transmitted by belts,, 95 Johannsen's cable,, cost of,, 84 ropes,, 84 Highton Main Colliery destructor at,, 79 Hard coal-cutter,, 304 Hydraulic transmission of power,, 377 L IONER'S system of winding,, zyo Incandescent lamps,, zji Increasing the voltage effect of,, of size of cable,, reduction of cost by,, of cost by, Indicated home-power,, by, Induction engine,, by, Induction motors,,, by, Inductors,,, by, Inductor feed,, burning of,, by, Insulation of colliery,, by, Instruments,, electrical measuring,, by Insulation of cables,, by, Insulation of cables,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by, Insulation resistance,,, by. A page from a technical manual or textbook. 316 ELECTRICITY AS APPLIED TO MINING Insulators, 145 Hay's, 149 Holliday's, 145 mushroom oil, 22 oil, 145 po, 149 single-leaf, 145 Low power of transmission, 353 of voltage in transmission, 360 Low tension current, advantage of, 166 system, electric blasting, 239 MAGNET, 2 electro-, 4 Magnetic cut-outs, 126 field, 30 Magneto-electric fuse, 4 Magneto-exploders, 240 Magnets, field, 30 Main and tail rope system of haulage, 193 main and tail shafts, 803 Maurice, William, on electric blasting, 240 Maycock's electric wiring, fittings, switches, and lamps, 237 KAPP, Giobert, referred to, 38 Kilowatt, 8 Lag currents, 47 Lahmeyer motor, 54 Lamps, arc, 230 electric safety, 252 insulators, 231 Laval, Dej. steam turbine, 101 steam consumption of, 105 weight and dimensions, 105 Lochlanche aggregate block, 14 celli, 14 Lee coal-cutter, 205 Legal ohm, 6 Ladgers Colliery, coal-cutters at, 207 Lighting of safety-lamps by electricity, 531 or power mains, firing from, 243 Rowlands-Gill, 226 Lightning arresters, 193 Thomas' lightning arresters, 184 Liquid switches, double pole, 120 Load, full and light, 88 Locomotives, electric, 196 Loss of power in transmission, of voltage in transmission, low tension current, system, electric blasting, Mechanical efficiency of plant, Mechanical efficiency of plant, Method of carrying cable down shaft, Method of supporting cable in shafts, Middleton Colliery, method of supporting cable, Millameter, electric characteristic currents, Morgan-Gardner pick coal-cutter, Motor-room, Motor starting rheostats, Motor starting alternating multiphase- 67, asynchronous, electric, efficiency of, generator, starting and stopping, synchromotor, 53 57 67 75 89 97 107 117 127 137 147 157 167 177 187 197 207 217 227 237 247 257 267 277 287 297 307 317 327 337 347 357 367 377 387 397 407 417 427 437 447 457 467 477 Jeffrey tar-coal cutter; chain machine; electric drill; longwall machine; Joulie; effect; Joulie's law; Junction-box; Davis's; Kilowatt; Lahneyer motor; lamp; arc; electric safety; insulators; Laval; Dej. steam turbine; steam consumption of; weight and dimensions; Lochlanche aggregate block; celli; Lee coal-cutter; legal ohm; Ladgers Colliery; coal-cutters at; lighting of safety-lamps by electricity; or power mains; firing from; Rowlands-Gill; lightning arresters; Thomas' lightning arresters; liquid switches; double pole; load; full and light; locomotives; electric INDEX 317 Multiphase electric currents, 34 Multipolar generators, 43, 44 Multipole chamber centrifugal pump, 197 Mushroom oil insulators, 22 N ETTLEFOLDS, Limited, test of plant, 90 Noels referred to, 246 North pole, 4 O BACH et al., 15 Ohm's law, 6 Ohmmeter, 293 Oil engines, 84 Insulators, 24, 143 transmission of power by, 278 Ovens, coke, benneite type, 78 distilling type, 78 Overhead wires, 175 P ARALLEL, 42 method, arrangement of wires for simultaneous blasting, 245 Park Collingwood, glasswork, enlume rope haulage, 194 Parsons' steam turbine, 96 dynamos, installation at Ackton Hall Colliery, 100 size and weight, 100 steam consumption, 99 Particulars of results of electric coal-cutting machines, 112, 116 Permeable electric drills, 256, 263 Periodicity, 29 Permanent bar magnet, 7 Plant, cost of working: 1,000-h.p. steam, 76 machine room efficiency of, 86 Plated cells, 19 Plants, central electric, 153 tests of, 88 typical, 721 Polariation, 11 Polyphase alternating current, 34 haulage plants, 194, 229 motors, 53 Position of fault, 289 Power cutting, 289 Potential, 6 difference, 6 Power, steam, 78 use of electrical of, 85 waste heat Pressure definition of unit of, 6 Prevention of accidents due to short-circuiting, 286 Petrol-electric Co., Limited; three-phase plant by Colet Electricar Company; 226 Primary cells, 9 Protection of insulation; 134 Protection against; electric lighting of oil safety lamps; 285 Pumping-plant; efficiency of; 184 size of pumps required; 184 central haulage pumps; 185; 187 Pump and motor-super-olives; 183 centrifugal; 185 electric slinging; 185 gearing of; 180 Gordian knot; 181 piston speed of; 180 Kiebler; 181 size of; required; 184 R ACKBAR exploder; 243 Rectifier; 38 Reduction of cost by increasing size of cable; 265 Refuse destructor; Mediumum-, 79 Resistance; 2 definition of unit of; 6 of metals; IYI. Reversal motor-cutter; 201 Eheostats; motor-startering; IYI7 Eicker pump; I84 express pump; I82 318 **ELECTRICITY AS APPLIED TO MINING** Ring-wound armature, 40 Rock-drill, Gardner electric, 264 Rods, transmission of power by, 275 Rope-driven plant with high efficiency, 93 Ropes, horse-power transmitted by, 84 transmission of power by, 275 Rotary converters, 60 electric bells, 236 Rotors, 55, 286 Rules for size of cable, 137, 166 of pumps, 284 Simultaneous blasting, arrangement of wires for, 245 Single-cylinder engines, 81 Single-fluid cell, 9 Single-phase alternating current, 34 motors, 53 Single- Rope haulage, 188 Single-shaft inductor, 245 Sinking pit, electric shoe-holding in, 248 single-pole pumps, electric, 185 Size of pumps regulated, 184 Slips, 58 Solenoïd, 60 Standard Coal Company, test of plant, 92 South pole, 40 Space plant in case of emergency, 95 Spacious tunnels, 120, 286 State of motion, 69 Specific resistance of materials, 193 Speed of pump, 180 Speed of turbine wheels, 193 repulsion motors, 66 Spur gearing, 180 Stanley voltmeter, 177 Stanley self-induction machine, 218 Stanley coal-bending machine. Stanley Colliery. Jeffrey chain machine at, Stanley Colliery. Jeffrey chain machine at, Starting and stopping generators, 113 Motors, 115 Serious-worm motors, 120 Serious-worm motors, 120 torque of motors, 69 Static transformers, 60 cost of fuel oil. Steam consumption. De Laval turbine. Pansen steam turbine. Steam engine indicator. Steam pressure. plant cost of working $1000 h.p., 76 power. transmission. turbine advantages of, 106 Safety lamps, electric, 253 lighting oil by electricity, 253 rest of lamp, John C. Colliery. method of supporting cables. Sandwell Park Colliery. endless-rope haulage. Sandpiper motor starter. Scott & Macdonald. tests of plants. interchangeability of pumps. Secondary cells. D.P., Headland, Self-induction. Series arrangement for simultaneous blasting. dynamo. system of electrical distribution. wooden pulleys starting of. Shafis. cost of putting cable in. Shafts. conductors in vertical. suspension of cable in. Short-circuiting. prevention of accidents due to. with coal-curing machines. Shunt dynamos. long. short. wound motors. starting of. Signal bells. electric. Short-circuiting. Shunt-dynamos. Long short long short long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长期以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来长久以来已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的已久的 INDEX 319 Steam turbine, Curtis, 105 De Laval, 101 Parsons, 96 Storage cell, 17 Strength of electric cable, 147 Suspender, cable, 143 Susman electric safety lamp, 253 Switchboards, 116, 223, 226 Switches, 130 double-liquid liquid, 121 enclosed, 120 single-pole liquid, 121 sparking at, 286 Synchronous motors, 53 Synchronous motors, 53 TELEPHONES. 233 Tenile tests of copper cable, 147 Testing explosives, 243 Tests of plants, 88 Birkenhead Electricity Works, 88 Northfield Limited, 90 South Durham Coal Company, 92 Tests of safety lamps, 253 Therm., 7 Thermal units, 7 Thomson, Silvamus P., referred to, 48 Thomson-Houston lightning arrester, 124 Thomson-Houston electric welding, 268 Three-phase electric motors, 53 Three-phase plant, Corlett Electrical Com- pany, 225 Three-throat pump, 178 Three-wire parallel system of electrical dis- tribution, 109 Three-wire system, 109, 171 Torque, 50, 67 Training ovens, 219 Tramway cars motors on, 157 Transformer, cost of static, 160 Transformers, 59 alternating current, 60 continuous current, 60 Transmission of power by compressed air, 276 by electricity, 279 by gas and oil, 278 by steam and oil, 279 by steam, 273 by wire ropes, 275 hydraulic, 277 Transmitters electric bells, bells, Tumbler switches. 241 Turbine. Curtis. 105 dynamic. instability at Ackton Hall, Parsons steam. 96 consumption. consumption. consumption. whirlwind load. of oil. oil. Two-fall cell. of Two-phase electric currents. currents. four-wire system. system. three-wire system. system. motor. motor. motor. Two-phase parallel system of electrical dis- tribution. distribution. Typical electric plants recently erected, UNIT current. current. of electrical energy. energy. of power. power. of quantity of electricity. electricity. thermal. thermal. Units. electrical. electrical. Utilization of waste heat from coke ovens, kilowatts. VERTICAL engines. engines. Virtual voltage. voltage. Volt. volt. Voltage effect of increasing. effect of increas- ing. for mines. for mines. less of oil. less of oil. A page from an index with page number "INDEX" and page number "319". The text is divided into sections with headings and subheadings. 320 ELECTRICITY AS APPLIED TO MINING Voltaic cell, 10 Voltimeters, 126 WASTE heat, utilization of, 78 Water power, 76 plant, cost of, 76 Watt, 8 Waterer, 199 Welding, electric, 267 * Wicksteed's drill, 266 Winding, electric, 154, 270 Igniter system, 270 Wire gauge, standard, 133 s Wires, arrangement of, for simultaneous blasting, 240 parallel method, 240 series method, 240 connections to electric and steam plants, 156, 175 Workings, carrying cables in, 149 Worm gear coal-cutter, 208 gearing, 190 VOUGHIOGHENY Coal Company, Pennsylvania, Jeffrey machines at, 210 PRINTED BY SPOTTISWOODE AND CO. LTD., NEW STREET SQUARE LONDON 2 [API_EMPTY_RESPONSE]