LIBRARY 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Deceived -SfSl^L . . 
 
 Accessions No. U?//} 74^ - Class No. 
 
ELECTRIC MOTIVE POWER: 
 
 THE TRANSMISSION AND DISTRIBUTION 
 
 OF ELECTRIC POWER BY CONTINUOUS 
 
 AND ALTERNATE CURRENTS. 
 
 WITH A SECTION ON THE APPLICATIONS OF 
 ELECTRICITY TO MINING WORK. 
 
 BY 
 
 ALBION T. SNELL, 
 
 it 
 
 ASSOCIATE MEMBER OF THE INSTITUTION OF CIVIL ENGINEERS ; 
 
 MEMBER OF THE INSTITUTION OF ELECTRICAL ENGINEERS; 
 MEMBER OF THE FEDERATED INSTITUTES OF MINING ENGINEERS. 
 
 LONDON : 
 "THE ELECTRICIAN" PRINTING AND PUBLISHING COMPANY, 
 
 LIMITED, 
 SALISBURY COURT, FLEET STRKKT, E.C. 
 
 1894. 
 [All Rights Reserved.] 
 
Printed and Published by 
 
 THE ELECTRICIAN " PRINTING AND PUBLISHING CO., LIMITKI), 
 
 1, 2, and 3, Salisbury Court, Fleet Street, 
 London, E.C. 
 
PREFACE 
 
 "ELECTRIC MOTIVE POWER" is designed to be a 
 practical treatise for Mechanical and Mining Engineers 
 and other Students of Applied Electricity. Taking for 
 granted an elementary knowledge of the theory of 
 Electricity and Magnetism, I have endeavoured to give 
 a clear exposition of the principles governing Electric 
 Transmission of Power, and to develop them in accor- 
 dance with the best practice of the present time. In 
 doing this I have deemed it advisable to discuss the 
 design of Dynamos, Alternate and Continuous Current 
 Motors, Alternators and Transformers, as well as the 
 various systems in vogue for transmitting and dis- 
 tributing power. 
 
 A large part of the book is devoted to a careful con- 
 sideration of the alternate current systems, both single 
 and polyphase. This is necessitated by their growing 
 importance, as it is now well recognised by the leading 
 electrical engineers of Europe and America that for 
 power transmission they are superior to continuous 
 -current systems. In a short Appendix are given some 
 figures with reference to the prime cost of plants for 
 long-distance transmission. 
 
 The last two Chapters deal exhaustively with the 
 applications of electricity to mining work, particularly 
 with reference to collieries and coal-getting. It is hoped 
 they will prove to be of interest to the large number 
 of Mining Engineers who are now using or are advising 
 the use of electric plants for lighting and power purposes. 
 
iv. PREFACE. 
 
 The contents are arranged in ten chapters, each of 
 which is, as far as possible, a monograph on one portion 
 of the subject. Cross references enable the engineer or 
 student to find required information with the greatest 
 possible facility. The mathematics employed are of 
 the simplest kind, entirely practical in character, and 
 inch-pound-minute units are used throughout. Most 
 of the tables of data and tests are new, and all have 
 been compiled from recent and trustworthy informa- 
 tion. The illustrations, about 250 in number, are 
 chiefly diagrams rather than perspective drawings, as 
 the former are more useful in practice, and the majority 
 have been prepared specially for this work. 
 
 In endeavouring to treat the subject as comprehen- 
 sively as possible, I have necessarily drawn on the ex- 
 perience of my confreres on both sides of the Atlantic, 
 and have to some extent availed myself of the store of 
 information contained in the columns of the Technical 
 Press and in the Proceedings of the Technical Societies. 
 
 I am much indebted to Dr. W. E. SUMPNER for 
 many instructive hints with reference to the alternate 
 current sections ; and my thanks are also due to many 
 firms and companies who have kindly supplied me 
 with information referring to special features in their 
 manufactures. 
 
 Although written primarily for Engineers and Students, 
 I hope the book will be found of value to users of 
 electric plant generally. 
 
 ALBION T. SNELL. 
 
 November, 1894.. 
 
CONTENTS. 
 
 {A Complete Synopsis of the Contents of each Chapter appears 
 under each Chapter Heading.) 
 
 PAGE 
 PEE PACE ... iii. 
 
 INTRODUCTION 
 
 CHAPTER I. 
 
 THE GENERAL PLAN OF ELECTRICAL TRANSMISSION OF 
 POWER 5 
 
 CHAPTER II. 
 THE DYNAMO AND MOTOR 13 
 
 CHAPTER III. 
 THE LINE AND THE DISTRIBUTING MAINS 69 
 
 CHAPTER IV. 
 
 DIRECT-CURRENT SYSTEMS OF TRANSMITTING AND DISTRI- 
 BUTING ELECTRICAL POWER 115 
 
 CHAPTER V. 
 
 SINGLE-PHASE ALTERNATORS AND ALTERNATE-CURRENT 
 MOTORS 149 
 
 CHAPTER VI. 
 TRANSFORMERS 187 
 
 B2 
 
vi. CONTENTS. 
 
 PAGE 
 
 CHAPTER VII. 
 
 SINGLE-PHASE ALTERNATE-CURRENT TRANSMISSION AND 
 DISTRIBUTION 211 
 
 CHAPTER VIII. 
 POLYPHASE ALTERNATE-CURRENT WORKING 251 
 
 CHAPTER IX. 
 
 ELECTRIC TRANSMISSION OF POWER IN MINING OPERATIONS 301 
 
 CHAPTER X. 
 COAL CUTTING AND ROCK DRILLING BY ELECTRIC POWER... 345 
 
 APPENDIX 381 
 
 INDEX TO CONTENTS 385 
 
 INDEX TO ILLUSTRATIONS 399 
 
 LIST OF TABLES OF DATA AND TESTS 405 
 
 LIST OF FORMULA , 407 
 
'BRSITY' 
 
 INTRODUCTION. 
 
 CIVIL ENGINEERING has been defined as " the art of directing 
 the great sources of power in Nature for the use and conveni- 
 ence of man." This definition was apt in the days which saw 
 the incorporation of the Institution of Civil Engineers; it is 
 doubly so now. Then gravitation, the expansion of vapours 
 and gases, and the pressure of wind, formed the sum total 
 of forces at the disposal of man for engineering purposes. 
 The study of Electricity and Magnetism was in its infancy, 
 and neither physicist nor engineer dreamed of the possibilities 
 of the new force. It was not until nearly half a century 
 later that the principles of electro-dynamics were developed, 
 giving a fresh meaning to the old definition of Civil Engineer- 
 ing by the inclusion of another force, all-pervading as 
 gravitation, and powerful and manageable in a measure 
 suggesting boundless possibilities of application. 
 
 As yet we do not know what Electricity is, but its 
 utilisation is by no means limited by our want of insight into 
 its nature. We do not know what Gravity is, but we have 
 a clear conception of weight, and we measure horse-power in 
 gravitation units, estimating the effects of gravity, though 
 its real nature is still a mystery. In like manner the 
 electrician defines electricity in terms of the magnetic effects 
 produced by it, having no exact knowledge of the causes, 
 
2 ELECTRIC MOTIVE POWEK. 
 
 although empirical suggestions, which fit the phenomena 
 more or less completely, may give "educated guesses" as to 
 their nature. Thus the utilisation of electricity demands no 
 closer acquaintance with its nature than does that of gravity, 
 which is made use of without direct reflection in almost every 
 contrivance devised by man. 
 
 At any rate, the laws governing the production and regula- 
 tion of electricity have been very completely investigated, and 
 in most practical cases are capable of treatment by the simplest 
 mathematics. Electric Power is measured in definite units 
 bearing an easily defined ratio to the mechanical expressions 
 for horse-power and work. In fact, electric quantities are 
 gauged with a degree of readiness and accuracy impossible with 
 the corresponding quantities in steam, compressed air, or any 
 system involving the transference of matter as well as of power. 
 It is to this transmission of power without the conveyance 
 of matter that electricity owes its superiority to all other 
 means of transmitting power where distance is a factor in the 
 problem. Since neither gravity nor material friction enters 
 into the question, power is transmitted equally well horizontally 
 or vertically, round the sharpest curves and by the most devious 
 routes. The advantages of electricity from this point of view 
 are at once apparent, and have already been widely recognised, 
 both at home and abroad ; more especially where natural 
 water power is available, and the distance between the source 
 of the prime power and the point of application is considerable. 
 On the Continent, water power is largely used for generating 
 electricity; and at home, although less abundant, is steadily 
 gaining favour. 
 
 As might be expected, Electricity has found ready favour with 
 mining engineers, and in more than one case the application of 
 Electricity to Mining has resulted in profit where previously 
 such a result was unattainable. But the importance of electric 
 
INTRODUCTION. 3 
 
 power in this connection has not yet been fully appreciated. 
 So far back as the year 1889, the author pointed out in the 
 Engineer that in districts where coal pits are in close proximity 
 to each other considerable economy of power might be obtained 
 by the establishing of central power stations on a basis- 
 similar to that used for electric lighting. The idea has not 
 commended itself to mine owners as yet, but it is certain to- 
 do so as the conditions for economic working become better 
 understood. At present it is not customary to keep so close an 
 account of the cost of power at mines as is done at electric 
 supply stations, and therefore it is not generally recognised 
 how much waste may be involved in this item. Experience 
 in lighting stations shows that the cost of power per unit is- 
 largely reduced when the load curve is kept fairly constant 
 during the night and day. Now the conditions of many 
 collieries are such as to permit of a nearly straight line load 
 curve ; for pumping, &c., can often be done at times when 
 coal is not being drawn, and hence the one load can, in great 
 measure, be made to balance the other. 
 
 At the central power station alternators could be kept run- 
 ning day and night. The power could be transmitted by 
 duplicate mains at high pressure to transformers at each 
 colliery, where the current could be transformed to a working 
 pressure of, say, 500 volts. The power consumed by each 
 colliery could be measured by recording wattmeters, or by 
 ordinary current meters if the pressure were kept approxi- 
 mately constant. The mains between the central station and 
 the collieries could generally be carried on poles, but in a few 
 cases it would pay to lay them underground. 
 
 Such an arrangement would have the great advantage of 
 localising all the generating plant under one roof, and in the 
 charge of a special staff, whose only duty would be to supply 
 power at all times at the lowest possible cost. It is evident 
 
4 ELEGTEIG MOTIVE POWEE. 
 
 that the cost of a unit of power would be much less under such 
 conditions than when it is produced by a number of small 
 plants, and that the cost of upkeep would be less because of 
 more efficient supervision, while the reserve plant could be 
 reduced very much below that absolutely necessary with 
 independent plants. 
 
 When a number of adjacent pits belong to the same pro- 
 prietors, the introduction of a central power station would 
 be simple. If the pits belong to different owners it would be 
 easy to arrange an equitable system of charging on the basis 
 of actual power consumed. The station might be financed by 
 a single proprietor, or by an independent company, whose 
 function would be to supply power exclusively. 
 
 In the future developments of coal working some modification 
 of this scheme will, no doubt, find a place. 
 
 An electric power plant is more easily erected and controlled, 
 and is generally more flexible, than any other. It is no more 
 experimental than the steam engine and boiler, and, if built and 
 erected by competent men, is cheap and trustworthy. 
 
CHAPTER L 
 
 THE GENERAL PLAN OF ELECTRICAL TRANSMISSION 
 OF POWER. 
 
 1. Electricity not a Prime Power. Steam and Water Power. The 
 Dynamo. The Line. The Motor. The Machines Driven. 
 
 BEFORE entering on the general subject of the Electrical 
 Transmission of Power it will be well to define what is 
 understood by the phrase, and also to note at this early 
 stage that electricity is not a prime power (see foot-note on 
 page 2). The problem will be at once understood by con- 
 sidering a hypothetical case. On a hillside there is a water- 
 fall, with its power running to waste. A few miles from 
 this is a mine, where power is urgently required to drive 
 the winding engines, pump the dips, and work the fans. A 
 turbine or waterwheel is so arranged as to rotate by means 
 of the falling water, and a dynamo is coupled to it. The 
 electrical power generated by the dynamo is led by two 
 copper conductors to the mine, and there runs the motors 
 which reconvert the electrical into mechanical power and drive 
 the mining machinery. 
 
 Now, it is clear that electricity is simply the medium by 
 which the power of the water is carried to the mine, and, 
 therefore, is not a prime, but a secondary power. Natural 
 water power is not often available, and, therefore, in the 
 majority of cases, a steam engine is employed to drive the 
 dynamo ; but the function of electricity remains the same. 
 
6 ELECTRIC MOTIVE POWER. 
 
 The essential factors of an electrical power plant may be 
 summarised as follows : 
 
 '. f Boilers. 
 
 Steam - 1 Engines. 
 
 (a) Prime Motors -j Water... Turbine or waterwheel. 
 
 f Gas Producers. 
 Gas -{ Engines. 
 
 (b) Dynamos to convert the mechanical power of the prime 
 motors into electrical power. 
 
 (c) Copper conductors to transmit the power from the 
 dynamos to the motors. 
 
 (d) Motors* to reconvert the electrical into mechanical power. 
 
 (e) Machines to absorb the work given out by the motors. 
 
 We will now briefly examine these before developing the 
 main scheme of the book. 
 
 (a) Prime Source of Power. The consideration of this part 
 of the working plant belongs more properly to a treatise on 
 prime motors ; but the requirements of electrical work have 
 introduced quite a revolution into the building of both steam 
 engines and turbines, and probably a few hints as to the class 
 of plant most suitable will not be unacceptable or out of place. 
 
 The dynamo is essentially a high-speed machine, one of the 
 fundamental factors of both output and efficiency being a high 
 peripheral speed of the armature. Practical considerations con- 
 fine this to about 3,000ft. per minute ; and hence, for a given 
 output, economy of material and labour will limit the diameter 
 and length of the armature so as to make the speed a maximum. 
 Now the stationary engine of 15 or 20 years ago was, on the 
 contrary, a slow-speed machine, i.e., the flywheel made rela- 
 tively few revolutions per minute, although the piston velocity 
 may have been in some cases as high as 300 or even 400ft. 
 
 * Probably most installations will be used for lighting as well as power 
 purposes. By suitable arrangements, either the line or the distributing 
 mains can be tapped at any part, and lamps of the proper voltage can be 
 coupled across them. The power absorbed by the lamps, however, will 
 not be available for use at the motors, and so a margin must be made in 
 the output of the dynamos to allow for the lighting work, 
 
PEIME MOTORS. 7 
 
 per minute. The cylinders were of large diameter and long 
 stroke, being designed to work with low boiler pressures. 
 Therefore, when it became necessary to drive dynamos by 
 this type of engine, difficulties arose in belting the dynamo 
 pulley to the flywheel ; and it soon became evident that the 
 engine was in many ways unsuitable. Firstly, it was in- 
 efficient for the class of work ; secondly, it governed badly ; 
 thirdly, it was costly in relation to its output ; and, fourthly, 
 it required foundations of a strength out of all proportion 
 to the load. These weak points in steam-engine design were 
 gradually forced on the notice of mechanical engineers by the 
 introduction of the efficient and high-speed dynamo. But im- 
 provements were made before long. Boilers were built for 
 higher pressures, enabling engines to be designed with smaller 
 cylinders and to rotate at higher speeds. Next, governors 
 were improved, and soon the tell-tale pilot lamp scarcely 
 showed the variations of speed, and now it is by no means un- 
 common to find engines governing to within 2 per cent, or even 
 less. Compound and triple expansion engines have now become 
 general. The vertical type of engine, with inverted cylinders, 
 finds most favour for driving dynamos. It is used either 
 single, compound, or triple, with or without condensers, ac- 
 cording to output and circumstances. It is coupled direct or 
 by belting, requires small foundations, gives a minimum of 
 trouble, and is cheap and efficient. Compared with the beam 
 or horizontal engines of from 20 to 30 years ago, it is as David 
 to Goliath. 
 
 The change in boiler design has been equally remarkable. 
 Fifty pounds to the square inch used to be considered a 
 fair working pressure with boilers of the Lancashire type ; now 
 from SOlbs. to 1201bs. is not uncommon; and with multitubular 
 and watertube designs 1401bs. or even a greater pressure fre- 
 quently occurs ; and water-heaters, economisers, and mechanical 
 stokers are now thoroughly appreciated. All these improve- 
 ments, it is needless to say, tend to raise efficiency and decrease 
 the running charges. 
 
 In choosing steam plant for an electrical transmission instal- 
 lation, therefore, the rules and precautions adopted in laying 
 
 c2 
 
8 ELECTRIC MOTIVE POWER. 
 
 down a central station for lighting should be followed. First 
 choose a convenient unit of power, i.e., select the best size of 
 dynamo to give the minimum work required during the twenty- 
 four hours, and then if this be of convenient size keep to it as 
 the standard, and simply multiply the unit as the work increases. 
 If the minimum output be very small, it will probably be best 
 to have a separate plant to deal with this part of the work, 
 and to select a larger unit for the main installation. Having 
 fixed the output of the dynamo, the size of the engine which 
 has to run it, either direct or by belting, is easily determined, 
 and the boiler power is also known. The boiler unit may 
 be sufficient to run one or two engines ; this will depend on their 
 size, and cannot be definitely fixed without a knowledge of each 
 case. The boilers should each be coupled with a stop-valve to 
 the steam main, so that any boiler can have the fires banked 
 or be shut down separately. One boiler and an engine and 
 dynamo should be held in reserve in all large installations. 
 
 If water-power be used, the turbine or water-wheel can ad- 
 vantageously drive the dynamo direct, or belting may be used. 
 The general considerations for determining the size of dynamo 
 unit will hold just the same as with the steam plant; and, indeed, 
 the whole station arrangements will be similar, except that the 
 turbine will replace the boiler and engine. It is unnecessary 
 to point out the simplicity of such a power station, or to refer 
 to the advantages of the utilisation of waste water power by 
 electricity they are self-evident. It is not surprising that 
 magnificent transmission plants have sprung into existence 
 during the past few years ; the wonder is that the number of 
 such plants is not tenfold greater. 
 
 Surprise may be felt that the author includes gas engines 
 among the prime motors for electric transmission plants, 
 but experience most decidedly points to them as a feature of 
 future installations for small outputs, especially if the load 
 be intermittent, and the load factor consequently low. It has 
 been thoroughly demonstrated that it is more economic as 
 regards the quantity of light obtainable to consume coal gas 
 in an engine, and to utilise the energy for driving a lighting 
 dynamo, than to burn the gas direct in the ordinary way ; 
 
THE DYNAMO. 9 
 
 and the cost of power may be still farther reduced if producer 
 gas be employed. The gas-making apparatus is to the gas 
 engine what the boiler is to the steam engine, and the cost of 
 wages and repairs in each case is about the same. As regards 
 the cost of producing this gas, the Dowson Economic Gas and 
 Power Company state that, on a moderate scale, with coke cost- 
 ing 12s. per ton, the average cost of the volume of Dowson 
 gas required to give the same heat as 1,000 cubic feet of 
 ordinary town gas is only 10d., while in large plants the cost 
 falls as low as 6|d. Allowing for the variations of price of 
 coal gas and fuel, it may be fairly assumed that producer gas 
 will be about one-quarter as costly. Considerable improvements 
 have of late years been introduced in the design of gas 
 engines ; and double-cylinder engines of 100 B.H.P. are now 
 running successfully, and prove more economic for intermittent 
 and varying loads than steam plant of similar capacity. It 
 may be assumed, therefore, that gas engines will find a field for 
 transmission of power work in certain cases. 
 
 At Schwabing, a suburb of Munich, a lighting plant with 
 gas engines and Dowson gas runs ten arc lamps and 270 glow 
 lamps of 16 c.p. each, and 30 of 32 c.p. each, at a cost for fuel 
 of Id. per brake horse-power per hour, the fuel consumption 
 being 2Jlb. of mixed coke and anthracite per brake horse- 
 power hour. This result compares favourably with the per- 
 formance of good steam plant. And in England the Morecambe 
 Electric Light Company use gas engines, with Dowson gas, 
 indicating 110 H P. each. 
 
 (b) The Dynamo. The function of the dynamo is to convert 
 mechanical into electrical power, and the machine, if well 
 made, accomplishes this end in a most satisfactory manner. 
 The ratio of conversion of a good dynamo at full load varies 
 between 90 and 94 per cent., and this high efficiency is main- 
 tained through a considerable range of output ; at half load 
 it is about 85 to 90 per cent., and at one quarter load will not 
 fall below 75 per cent. ; while running on open circuit the 
 power absorbed is only sufficient to overcome friction. If 
 the plant be divided into suitable units, it will always be 
 possible to run at nearly the full output of the dynamo, 
 
10 ELECTRIC MOTIVE POWEll. 
 
 and so secure a high working efficiency. In addition to the 
 high-speed dynamo already referred to, there is a slow-speed 
 type, which is used when it is desired to couple dynamos 
 directly to the engine shaft. 
 
 The output of a dynamo is roughly proportional to the length 
 of the armature into the square of its diameter = say I D 2 . 
 Let the surface velocity be fixed. If D be large then I will be 
 small, and vice versa. In order to design a slow-speed dynamo 
 it is necessary, then, to take a large diameter and a short 
 length for the armature proportions. This type of machine is 
 heavy and costly, in consequence of the large field-magnets, 
 yokes, and extended beds, and the increased cost of machining 
 and labour ; but these disadvantages arc, in the opinion of 
 some designers, more than counterbalanced by the decreased 
 speed. 
 
 American practice favours dynamos running at from 800 to 
 1,000 revolutions per minute, coupled with belts to high- 
 pressure engines making from 200 to 350 revolutions. Con- 
 tinental engineers are leaning to the slow-speed steam dynamos, 
 and recently huge plants, with units of 500 H.P. and upwards, 
 and triple expansion condensing engines, have been laid down 
 for town lighting. English engineers are very much divided in 
 opinion upon the subject ; but there certainly seems to be a 
 tendency to use steam dynamos of about 100 unit capacity 
 running at from 175 to 250 revolutions per minute. 
 
 Viewing the question of the electrical power station from all 
 sides, and laying particular stress on the high pressure that will 
 be required, it appears that the dynamo will generally be belt- 
 driven, and therefore it will be permissible to use high-speed 
 dynamos. A convenient size for a high-pressure transmission 
 dynamo for use in mines is 50 or 100 H.P. ; the speed may be 
 from 450 to 600 revolutions per minute. 
 
 (c) The Line. The success of an installation will very largely 
 depend on the care and forethought shown in the selection of 
 the gauge of copper and the erection of the line. As the 
 general conditions to be observed will be discussed hereafter 
 
THE MOTOR. 11 
 
 seriatim, it need only be noted here that the power wasted 
 in the conductor may be made as small as desired, for the 
 loss varies directly as the length and inversely as the area 
 of cross-section. There are, of course, limits both to gauge 
 and length of conductor ; but these are determined as much 
 by financial as by practical considerations. The essential con- 
 ditions of a successful line are : Firstly, a continuous metallic 
 circuit between the dynamo and the motor ; and, secondly, 
 high insulation resistance, so that the current does not leak to 
 earth ; for defective insulation means not only a direct loss of 
 power but may also entail a risk of fire. 
 
 (d) The Motor. The motor is similar in appearance to the 
 dynamo, though its function is the converse, viz., to convert 
 electrical into mechanical energy. It is, in fact, a reversed 
 dynamo, is governed by precisely the same laws, and its action 
 is accompanied by similar phenomena ; and, moreover, it is 
 quite as efficient. The reversibility of the dynamo is said to 
 have been discovered by an accident, though more probably it 
 was the result of the steady and laborious study of early inven- 
 tors. Be this as it may, the fact has been thoroughly recog- 
 nised, and full advantage has been taken of the opportunities 
 afforded by it. 
 
 The motor is as varied in the details of construction as the 
 dynamo, the design being modified to suit the work expected 
 from it. It is made to give either high or low speeds, and for 
 high or low pressures. 
 
 (c) The Machines Driven. The machines to be driven need 
 no special reference, but care must be taken to couple them to 
 the motors in a suitable manner. High-speed machines may 
 often be connected directly to the motor-shaft ; or they may 
 be coupled by gearing or belts ; and countershafts, friction 
 clutches, and other devices may be used. But whatever 
 arrangement be adopted, care must be taken that the motor 
 speed, pressure, and current be approximately those designed 
 by the makers. If any of these points be neglected, the motor 
 will not be working under the best conditions, and there will 
 be a loss in efficiency. ^^^flR**^****, 
 
 
 
12 ELECTRIC MOTIVE POWER. 
 
 The importance of this will be made clear in the sequel. 
 Attention will be drawn to the different types of electric 
 motors and the kind of work for which they are specially 
 adapted. And an electric plant will be shown to be more 
 flexible and suitable for a greater variety of conditions of 
 working than any other means of transmitting and distributing 
 power now in vogue. 
 
 The general idea involved in the expression " an electrical 
 transmission plant" has now been explained, and the way is 
 cleared for a more minute study of the electrical part of the 
 subject, which is the main theme under consideration. 
 
CHAPTER II. 
 
 THE DYNAMO AND MOTOR. 
 
 2. Introductory. 3. Units and Symbols. 4. First approximation to 
 the Size of Armature for a given Output and Speed. 5. Funda- 
 mental Equation connecting the total E.M.F. with the Armature 
 Constants. 6. The Ratio between Radial Depth of Core and 
 Diameter of Armature. 7. Determination of the Number of 
 Conductors on the Armature. 8. Design of Field - Magnets. 
 9. Calculation of the Magnet Details. 10. Calculations of the 
 Field Excitation. 11. Special Points to be observed in Designing 
 Motors. 12. Practical Example of preceding Methods of Design. 
 13. Tabulated Form for use in Designing Dynamos and Motors. 
 14. Series, Shunt, Compound, and Separate Excitation. 
 
 2. INTRODUCTORY. 
 
 THE author has not thought it advisable to enter into an 
 elementary discussion of the principles underlying the con- 
 struction of dynamo-electric machines. This is the province 
 of text-books on electricity and magnetism. Here the object 
 aimed at is rather to develop the lines on which these princi- 
 ples are applied in practice, and to give useful information to 
 those using electric plant. Although machines suitable for 
 the transmission of power are chiefly considered, yet the method 
 advanced is equally applicable to dynamos intended for lighting 
 and other purposes. 
 
 First, it is necessary to distinguish between the dynamo 
 and the motor. In the introductory remarks it was mentioned 
 that the dynamo was a reversible machine, the motor being the 
 converse of the dynamo. The functions of the two machines 
 must now be compared. The chief conditions of working in 
 the two cases are widely different, and these distinctions must 
 be duly considered in the design. The dynamo will usually 
 be driven at a speed as nearly constant as possible, and will be 
 required to give a variable output at a constant pressure. 
 The motor will usually be run at a fairly constant pressure 
 of supply, and will be required to give a variable torque at a 
 
14 ELEGTEIC MOTIVE POWER. 
 
 constant speed. The dynamo in most cases will be placed in a 
 dry engine-room, and will meet with at least as much attention 
 and care as ordinary steam plant. The motor, on the other 
 hand, must be in close proximity to the machine driven, and 
 will frequently be in a dusty, dirty, and more or less exposed 
 position ; and the attendance will be most likely a few visits 
 daily at the most. The dynamo will probably run with fairly 
 steady loads, and not be subjected to large sudden variations of 
 output. The motor must, in the nature of things, meet with 
 rougher usage. If the work be intermittent, the stoppages 
 frequent, or overloading common, the motor must meet the 
 demand, no matter how great the strain. It follows, therefore, 
 that the motor should be built with a larger factor of safety than 
 is usually necessary with a dynamo ; and that extra care should 
 be given to the insulation and general details. To ensure due 
 prominence being given to these conditions, it is the author's 
 practice to base the design of the dynamo on the total armature 
 E.M.F., and to give special attention in the building of the 
 motor both to the maximum and the average torque. 
 
 3. UNITS AND SYMBOLS. 
 
 To make the equations suitable for the drawing office, inch- 
 pound-minute units will be used. The author has for some time 
 endeavoured to use the C.G.S. system in dynamo design; but he 
 has invariably found it necessary to convert the measurements 
 into English quantities before the drawings could be used 
 in the shops, because the workman uses a foot-rule and thinks 
 in eighths and quarters of an inch. The decimal system has 
 not met with much favour in this country ; and until it is in 
 more general use there is nothing to be gained by making 
 the duplicate measurements. To the pure physicist the 
 C.G.S. system of units offers a real advantage, for it simplifies 
 his work ; but to the construction engineer, at present at any 
 rate, it only means an increased number of figures with a cor- 
 responding chance of error from clerical slips. The symbols 
 will be as nearly as possible those used in Prof. Silvanus Thomp- 
 son's "Dynamo-Electric Machinery," because these are, no 
 doubt, already widely used, and so will be readily followed by 
 the majority of men interested in the subject. 
 
UNITS AND SYMBOLS. 15 
 
 SYMBOLS USED IN THE FOLLOWING CALCULATIONS AND EQUATIONS. 
 A = area in square inches. 
 
 ATs = ampere turns with armature on open circuit. "iBoth refer to field- 
 OTs compensating ampere turns for full load. / magnets. 
 
 B" a = magnetic induction per square inch of armature cross section. 
 B"f = magnetic induction per square inch of field-magnet cores. 
 B" g = magnetic induction per square inch of area of air gap. 
 N a , Nf , N K = total magnetic fluxes severally in armature, field-magnet, 
 
 and air gap. 
 
 C d = total number of conductors in armature, counted all round the 
 external periphery, and therefore applicable to either drum or 
 Gramme winding. 
 
 C m , C 8 = total number of turns of wire on series and shunt coils respec- 
 tively of field-magnets. 
 
 D = outer diameter of armature, > 
 
 d = inner diameter of armature, 
 
 T- radial depth of iron of armature, J in 
 
 1 = length of armature reckoned over iron, ) 
 
 E = total electromotive force generated in armature, either "| 
 
 dynamo or motor, J-in volts. 
 
 e = difference of potential from terminal to terminal, 
 i = current in external circuit, 
 t a = current in armature, 
 
 i 8 = current in shunt coils, 
 
 in amperes. 
 
 ? m = current in series or main coils, 
 
 n number of revolutions per minute. 
 
 p number of pairs of poles. 
 
 K resistance of external circuit, 
 r a = resistance of armature coils, ^ . 
 
 r s -resistance of shunt coils, ( iu olun3> 
 
 r m resistance of series or main coils, ) 
 
 T torque or turning moment. 
 
 v coefficient of magnetic leakage. 
 
 5 = length of air gap on one side only, 
 
 10 = width of polar cavity, 
 
 Lg length of polar cavity, J> in 
 
 , a , 7Hf , mfi = mean length of magnetic paths, respectively, 
 in armature, cores and pole pieces, and yoke, 
 
 FOR ALTERNATE CURRENTS. 
 
 ' nip = total impressed electromotive force of armature, \ 
 'imp = impressed difference of potential from terminal 
 
 to terminal, 
 
 in effective volts. 
 
 = resultant electromotive force, 
 
 =self induction electromotive force, 
 
 impressed current, 
 
 = dynamic or working current, [-in effective amperes. 
 
 = wattless or condenser current, 
 
 = coefficient of self-induction in lieuries. 
 
16 ELECTRIC MOTIVE POWER. 
 
 4. FIRST APPROXIMATION TO THE SIZE OF ARMA- 
 TURE FOR A GIVEN OUTPUT AND SPEED. 
 
 The first point to determine in designing a dynamo is the 
 size of armature that is best suited to meet all the require- 
 ments of the problem. Usually the volts, amperes and speed 
 are fixed, and frequently the selling price too. The latter item 
 cannot be specially discussed, but a few hints gathered from 
 practice will be given when possible. We know, then, e, i and 
 n. The size of machine can generally be selected by reference 
 to a price list, and by making a few rough calculations. But, 
 if the type be new to the shop, no data are available, and it is 
 necessary to resort to a few figures and to solve a few simple 
 equations. 
 
 The output of any dynamo is proportional to the volume of 
 the armature, the number of revolutions per minute, and the 
 magnetic flux. Hence output oc ZD 2 ?iN a &, where k is a dimen- 
 sional constant to reduce the product to watts or horse-power. 
 The author first published empirical equations of this form 
 in 1888, and coupling N a and k, he based the resultant value 
 on an average magnetic density and radial depth of core. Such 
 assumptions are found to be fairly correct for the greater 
 number of bipolar machines with outputs of from 10 to 
 100 H.P. ; but below and above these limits the equations are 
 not so nearly in accord with actual results. Small machines 
 realise smaller outputs than this theory indicates, owing to the 
 disproportionate resistance of the air gap ; while in large 
 machines the output is so largely affected by the design of the 
 field-magnets and the arrangement of the poles that separate 
 calculations are needed for every type. Nevertheless, similar 
 equations can easily be determined for any size and type, but 
 the constant must be adjusted to suit the conditions. For 
 Gramme armature dynamos the author finds the coefficient 
 O'Ol to give fair results between the limits stated ; and for 
 drum machines 0-015, the output in each case being in watts. 
 Thus for dynamos : 
 
 Watts = ID 2 n 0-01 (Gramme) . . . (1) 
 Watts = lD 2 n 0'015 (drum) ... (2) 
 
EQUATIONS FOE OUTPUT. 17 
 
 and for motors, estimating the output in brake horse- 
 power : 
 
 B.H.P. = / D 2 n 0-00001 (Gramme) . , (3) 
 
 B.H.P. = ZD 2 n 0-000015 (drum). . . (4) 
 
 It should be rioted that a drum armature is capable of 
 giving an output which is 50 per cent, in excess of a Gramme 
 ring of the same over-all dimensions. This enormous advan- 
 tage in the weight efficiency of drum armature machines is 
 well recognised, and where the conditions of running admit of 
 the design it should be invariably used. Indeed, if it were 
 not for the difficulties in the winding and insulation of drums, 
 Gramme armatures would soon be obsolete. The superiority 
 of the drum lies chiefly in the following points : (a) it has 
 no inside wires to cause an internal field, as well as to increase 
 the dead resistance of the winding ; (5) there is less in- 
 equality of E.M.F. in the coils, since each turn of wire embraces 
 the whole armature field ; and (c) the iron can be carried 
 right to the shaft, and therefore the magnetic induction per 
 unit of area may be made low, though the total flux be very 
 large. 
 
 The Gramme winding, although causing an internal field 
 and a more irregular distribution of the lines of force, is better 
 adapted for rough work, since coils of very different potentials 
 are not in close contact, as occurs with ordinary drum wind- 
 ings. There are arrangements of drum bar windings with 
 spiral or crank-shaped end connections which obviate this 
 objection to some extent ; but experience has all along pointed 
 to the Gramme type of winding as the safer for hard work. 
 The author has tried both methods, and has no hesitation in 
 advising the Gramme winding for high-tension transmission 
 dynamos and motors. For lighting dynamos of, say, from 
 100 to 200 volts, the drum type, with bar windings, and spiral 
 strip connections, is the better, and is generally adopted now 
 when other considerations do not prevent its use. 
 
 The rough dimensions of the armature for dynamo or motor 
 may now be assumed to be known, and the calculation of the 
 winding may be proceeded with. 
 
18 
 
 ELECTRIC MOTIVE POWER. 
 
 5. FUNDAMENTAL EQUATION CONNECTING THE 
 TOTAL E.M.F. WITH THE ARMATURE CONSTANTS. 
 
 The factor of prime importance in armature design is E, the 
 total E.M.F. The value of E is determined by the relationship 
 
 E = e4r a ....... (5) 
 
 The positive sign must be used for the armature of a generator, 
 and the negative for that of a motor. The armature current 
 i a is known from the conditions of the problem ; but r a has to 
 be determined, as the winding is not yet chosen. An assumed 
 value must be substituted in (5), which can be amended 
 when the gauge and number of turns of wire are definitely 
 fixed. The numerical value of * a r a cannot yet be fixed, but it 
 can be conveniently expressed as a percentage of E. The follow- 
 ing table of approximate values will be of use at this stage : 
 
 Table A. 
 
 Output of dynamo 
 or motor in H.P. 
 2-5 
 
 a ?*a expresse 
 percentage o 
 6 per 
 
 d as a 
 EE. 
 cent. 
 
 I 
 9 
 
 j 
 9 
 
 j 
 
 5 
 
 5-5 
 
 10 
 
 5 
 
 15 
 
 4 
 
 20 
 
 3 
 
 25 
 
 ... . 2-75 
 
 30 
 
 2-5 
 
 50 
 
 2-25 
 
 100 , 
 
 , 2 
 
 The total armature E.M.F. is given by 
 
 (G) 
 
 C a , the number of turns of wire counted all round the 
 periphery, is the most important variable ; n is the number 
 of revolutions per minute, and is given by the conditions of 
 running. N a is the total magnetic flux, and is numerically 
 equal to the product of the number of square inches in the 
 cross section of the armature iron and the magnetic density per 
 square inch, or 
 
 N a =B/2r/0-85 ...... (7) 
 
 The coefficient 0'85 is introduced to allow for the insulation 
 between the plates of the armature. 
 
DEPTH OF ARMATURE CORE. 19 
 
 The value of I was fixed when the over-all dimensions of 
 the armature were chosen ; and so in practice would be that of 
 r ; for the radial depth of core has presumably a fixed ratio to 
 the diameter of armature for a given type of field- magnet. 
 Various considerations determined by experience fix the mag- 
 netic flux per unit area at from 90,000 to 110,000 lines of force. 
 Therefore N a can readily be determined as soon as the size of 
 armature is selected. 
 
 $6. THE RATIO BETWEEN RADIAL DEPTH OF CORE 
 AND DIAMETER OF ARMATURE. 
 
 The following ratios between radial depth of core and diame- 
 ter in Gramme armatures have been taken from practice, and 
 illustrate the difference of opinion on the subject. They all 
 refer to bipolar machines. It will be noted that they vary 
 from i to y. 
 
 Table B. 
 
 Diameter of core = D. Radial depth of core = r. 
 
 7 inch 1-0 inch. 
 
 8 : 1-25 
 
 8-875,, 1-125 
 
 10 1-5 
 
 10 2-0 
 
 12 1-875 
 
 12 2-25 
 
 13 2-0 
 
 M 2-5 
 
 15 2-25 
 
 15 3-0 
 
 16-875,, 3-0 
 
 18 4-0 
 
 24 4-5 
 
 In multipolar design the radial depth will be given approximately by 
 
 -, where r is the depth for a bipolar machine and p is the number of 
 
 P 
 
 pairs of poles. 
 
 7. DETERMINATION OF THE NUMBER OF CON- 
 DUCTORS ON THE ARMATURE. 
 
 The factor neither known nor approximated to as yet is the 
 number of conductors. This is found by putting 
 
 E GO 
 
20 ELECTRIC MOTIVE POWER. 
 
 which determines the trial value of C a . Dividing the necessary 
 number of turns into the space for winding on the periphery 
 gives the width over the insulation of one turn. Deducting 
 the insulation thickness, say, from 7 to 8 mils on each side 
 of the wire, the remainder gives the net width of copper. 
 C a may be a number that will conveniently form one or two 
 complete layers on the armature ; if such be the case, a round 
 wire may be generally chosen ; but if the number will not fit 
 into one or two layers, it will be advisable to use a wire of 
 rectangular section, the width being such as to fill up com- 
 pactly the winding space, and the depth such as to give a 
 suitable air gap. 
 
 The current density has been thus far purposely neglected, 
 as the determination of a suitable number of turns is of the 
 first importance ; and therefore special stress has been laid 
 upon it. The relation of cross-section of copper to the current 
 must now be carefully considered, since the heating of the 
 armature (i & 2 r & ), as well as the fall of potential (* a r a ), and the 
 efficiency of the machine depend on this. No current density 
 can be said to be the best in all cases j it is a question of size 
 of machine, cooling surface of armature, character of output, 
 peripheral velocity, ventilation, and time of duration of 
 maximum load. The practical limits are between 1,500 
 amperes per square inch in large machines, and 3,000 with 
 outputs as small as 5 H.P. The permissible total heat waste 
 for a given rise of temperature in degrees Fahrenheit may be 
 roughly estimated for any ordinary armature by the following 
 empirical equation, which is based on a surface velocity of 
 about 3,000 feet per minute : 
 
 Rise of temp. Fahr. deg. = Watts 64 . . (8 A ) 
 
 Surtace in sq. in. 
 
 The constant is determined for the exterior surfaces only ; 
 hence, for Grammes, the surface is given by the perimeter 
 x the length + the area of the two end winding spaces ; and 
 for drums the length may be measured from the commu- 
 tator along the extreme length of the winding in order to 
 allow for the two ends which are usually covered up if 
 
ARMATURE CURRENT DENSITY. 21 
 
 they are open their areas should be included in the cooling 
 surface. 
 
 The usual rise of temperature arranged for by designers is 
 about 70F., and then the equation allows an area of rather 
 more than one square inch per watt. This is an ample 
 allowance for most small and medium size armatures, but it is 
 not nearly sufficient with very large machines having massive 
 iron cores. Internal ventilation and, perhaps, a " forced 
 draught" have then to be resorted to as is seen in the case of 
 the modern bar wound drum armatures. The watts must be 
 understood to comprise the heat from all sources, hysteresis, 
 eddies, and 4 2 r a loss in the copper. If the number of volts to 
 be lost over the armature resistance be determined from Table 
 A, then r a is known, and the cross-section of copper to give this 
 value with the calculated number of turns can be found. It is 
 advisable also with Gramme armatures to consider the depth of 
 copper in relation to the radial depth of armature core, since 
 the output of the machine will vary as the depth of the copper 
 up to a certain practical limit. This point is reached where 
 the air gap becomes so deep that the resistance is too high for 
 the field excitation to give the necessary magnetic density; 
 
 or else when the circumflux,* -, on the armature causes 
 
 a 
 
 cross and back fields of such magnitude that sparking ensues. 
 It should be remarked, however, that the output of the arma- 
 ture is not greatly affected by even considerable differences in 
 the length of the gap. For, with the standard sizes of fields 
 the excitation is practically fixed, and so a large gap with deep 
 copper implies a weak magnetic density ; and a small gap with 
 less copper means a corresponding gain in the number of lines 
 of force. 
 
 Two empirical equations, due to Mr. W. B. Esson, connecting 
 the circumflux and the armature diameter, are important, for 
 they give at once the permissible value of C a . 
 
 * This expression for the circumflux of an armature is applicable to 
 machines with any number of poles, and must not be confounded with the 
 ampere turns. The numeral occurs simply because the symbol i & is chosen 
 to represent the total armature current and not that in each conductor. 
 
22 ELECTRIC MOTIVE POWER. 
 
 For Gramme armatures 
 
 Circumflux = ^ = D 1000; . . ^ . (9)* 
 for drum armatures 
 
 Circumnux = ^l a = D 1500, . . . (10) 
 
 2i 
 
 D in each case being the diameter measured over the winding 
 in inches. 
 
 Practically, with Gramme ring armatures, it is found that 
 the best all-round results are obtained with a ratio of radial 
 depth of, copper to iron of from J- to -f%. With drums some 50 
 per cent, more copper may be allowed, as is evident from the 
 empirical equations (1), (2), (9), and (10). 
 
 This ratio is important in Gramme armatures, because the 
 narrow section of iron must be magnetised to at least B a " = 
 1 00,000. In drums this ratio has no meaning, since the iron 
 is made as deep as possible, reaching even to the shaft itself, and 
 the depth of copper is determined simply from the heating 
 limits and the possible length of gap, having regard to an 
 economical field-magnet and excitation. 
 
 The thickness of the copper being known, the length of the 
 air gap can be determined approximately. It may have to be 
 adjusted when the field-magnet excitation is calculated from 
 the trial equations. 
 
 8. DESIGN OF FIELD-MAGNETS. 
 
 The function of the field-magnets is simply to supply the 
 necessary magnetic flux, with due regard to an economical dis- 
 tribution of iron and copper. The shape does not directly enter 
 into the question, and is really a matter of convenience, 
 and perhaps fancy. Broadly speaking, field-magnets may 
 be divided into the bipolar and multipolar types, each of 
 which is subdivided into a number of forms, sometimes pos- 
 
 * In (9) and (10) the diameter is expressed in inches instead of centi- 
 metres, and so the coefficients have different values from those given by 
 Mr. Esson. 
 
TYPES OF FIELD-MAGNETS. 
 
 23 
 
 sessing real advantages, but often only expressing individual 
 taste. Some of the more general designs are shown in 
 Figs. 1 to 9. 
 
 (a) The Single Magnetic Circuit Bipolar Field-Magnet. 
 This popular shape of magnet is simply a development of 
 
 FIG. 1. Wrought Cores, Cast Yoke, 
 Upright Type. 
 
 FIG. 2. Cast Iron. 
 
 FIG. 3. Wrought Iron Inverted 
 Type. 
 
 FIG. 3A. Wrought Iron Cores and 
 Yoke, Cast Iron Pole Pieces. 
 
 the primitive horseshoe magnet, and is largely used in both 
 the upright and inverted forms (Figs. 1 to SA). The former 
 arrangement has a lower leakage coefficient than the latter, and 
 
 D2 
 
24 
 
 ELECTRIC MOTIVR POWER. 
 
 is generally adopted for small armatures ; but since the centre 
 of gravity of the rotating part is high there is a tendency to 
 instability, and, consequently, with large diameter armatures 
 the latter is more generally adopted. 
 
 The single magnetic circuit field-magnet is cheap to build, 
 and is excited with a relatively small quantity of wire. Its 
 principal disadvantage lies in the unequal distribution of the 
 magnetic flux in the polar space, and hence inequality of in- 
 duction in the armature wire. For this reason drum arma- 
 tures are most frequently used in these fields. Large Gramme 
 rings are practically impossible with the single magnetic cir- 
 cuit. The largest size, in the author's opinion, that is advisable 
 is a diameter of about 12in.; if this be exceeded there is a 
 tendency to spark with large outputs, so that the armature 
 cannot be loaded to its proper limit. Larger sizes should be 
 run in double magnetic circuit bipolar fields, or, if very large, 
 in multipolar fields. 
 
 (b) The Double Magnetic Circuit Bipolar Field-Magnet. 
 Figs. 4, 5, and 6 illustrate this well-known form of field- 
 magnet, and show the variety of ways in which it is built. 
 
 Fia. 4. " Snell " Type of Double Magnetic Circuit Magnet. Wrought 
 Iron Cores and Cast Iron Pole Pieces. 
 
 The resulting distribution of magnetism is symmetrical, though 
 the "field" as a whole may be rotated considerably (see Fig. 14 A, 
 page 51). Therefore a Gramme armature as large as, say, 15in. 
 in diameter may be profitably excited in this type. The chief 
 objection is that the number of ampere-turns in the exciting 
 
TYPES OF FIELD-MAGNETS. 
 
 25 
 
 coils has to be about twice as great as in the single-circuit 
 type for a given induction in the armature. This type has 
 also a larger coefficient of leakage than the single magnetic 
 circuit. 
 
 If the armature be 18in. or more in diameter, it will be 
 advisable to increase the number of pairs of poles, although by 
 doing so the exciting turns will be somewhat more numerous. 
 
 FIG. 5. Wrought Iron Cores and Cast Iron Pole Pieces. 
 
 FIG. 6. Wrought Iron. 
 
 (c) Multipolar Field-Magnets. The two types most in 
 vogue are shown in Figs. 7 and 8. It is not easy to state 
 exactly at what diameter of armature it is commercially pro- 
 fitable to depart from the bipolar type. But there seems to be 
 some connection between the angular width of the polar cavity 
 and the diameter of the armature. The ratio of interpolar space 
 to the width of the poles, with Gramme rings, is usually 
 about 1:3; and the width of the pole, w, may be expressed 
 in terms of the diameter, as 
 
26 ELECTRIC MOTIVE POWER. 
 
 If the maximum economic diameter with a single magnetic 
 
 circuit field be 12in. the width of pole will be ^_ = 12*5 
 
 o 
 
 inches approximately. With a double magnetic circuit the 
 diameter may be as large as, say, 15in. ; and the polar 
 
 width will be - =15'75. The limiting width of the polar 
 3 
 
 FIG. 7. Wrought Iron. Four Poles. 
 
 cavities therefore varies between 12-5in. and 15'75in. These 
 limits accord well with practice, poles seldom being found 
 shorter than the former or longer than the latter in multi- 
 polar designs. Continental engineers have lately developed 
 large Gramme rings with multipolar field-magnets, and have 
 attained considerable success. Recently this design has been 
 
 FIG. 8. Multipolar Design in Cast Iron. 
 
 appreciated in England and America. Some four years ago 
 the author selected it for large transmission-of-power machines, 
 and also for central-station dynamos ; Mr. Kapp has developed 
 the same idea, using, however, drum armatures ; and the Thom- 
 son-Houston Company have also adopted the device for their 
 traction dynamos. 
 
TYPES OF FIELD-MAGNETS. 
 
 27 
 
 (d) Armour-clad fields, as in Fig. 9, are coming into use for 
 certain purposes, particularly for motors which have to work 
 in very exposed positions. For tramcars, they are often built 
 with only one exciting coil, one pole piece being made very 
 short. 
 
 FIQ. 9. Made in Wrought or Cast Iron or with Wrought Poles and 
 Cast Yokes. Armour-clad Type. 
 
 This type (see Fig. 10) offers no special advantages other 
 than the excellent protection it affords to the armature and 
 field winding. 
 
 (e) General Conclusions with reference to Field-Magnet 
 Design. A point to be specially observed before discussing the 
 proportions of iron and copper is that no one design of field is 
 equally well adapted for every size of armature. If cost of 
 production be the ruling factor the single-magnetic circuit type 
 is preferable for armatures up to, say, 12in. in diameter; from 
 this to 15in. the double circuit may be used ; but from 18in. 
 and upwards one of the multipolar forms should always be 
 employed. 
 
 This feature of dynamo design has received little attention 
 outside of the workshop. The author has been led to investi- 
 gate it when designing large machines ; and he some time ago 
 discussed at length the relation of weight to cost, with varying 
 
23 ELECT HIC MOTIVE POWER. 
 
 types of machines.* He finds that the curves connecting 
 weight and output, and cost per kilowatt with output, follow 
 very different laws with distinct types of machines. The 
 
 FIG. 10. Thomson-Houston Tramcar Motor. 
 
 subject is interesting, but rather outside the scheme of this 
 book, and therefore only two diagrams will be given, which 
 show the genera] shape and order of the curves. It will be 
 seen that the weight - output curve follows practically a 
 
 * Electrical Review, July, 1892. 
 
MAGNETIC INDUCTION IN IRON, ETC. 
 
 29 
 
 straight line law, and that the output of a dynamo or motor 
 is proportional to the cube of the linear dimensions of the 
 machine, practically of the armature diameter (see Figs. 11 
 and HA). 
 
 The question of material has not yet been mentioned, because 
 this is rather a matter of cost and convenience. The necessary 
 sections and corresponding inductions with wrought and cast 
 iron are given in the following Table C, and the student 
 can estimate for himself the relative advantages. Practically, 
 with bipolar machines at the inductions usually employed, 
 viz., B" f = from 77,000 to 100,000 in wrought iron and from 
 45,000 to 50,000 in cast iron, the relative magnet cross- 
 sections should be in the ratio of 1 6 : 3, the effective armature 
 section being taken as unity. Of late, cast steel magnets have 
 been used with some success. The induction curve for cast 
 steel lies nearer to that of wrought than to that of cast iron, 
 and so there seems a probability of its use becoming more 
 frequent. 
 
 Table C. 
 
 TABLE giving Area of Wrought and Cast Iron to correspond to 
 given inductions in air, assuming all the lines to pass through 
 the iron and none to leak. 
 
 Wrought Iron. 
 
 Cast Iron. 
 
 Air. 
 
 Area. 
 
 B" a . 
 
 Area. 
 
 Bn 
 a. 
 
 B. 
 
 1 
 
 70,750 
 
 2-2 
 
 36,500 
 
 100 
 
 1 
 
 90,750 
 
 217 
 
 42,000 
 
 200 
 
 1 
 
 100,250 
 
 2-17 
 
 46,000 
 
 300 
 
 1 
 
 100,575 
 
 2-07 
 
 48,500 
 
 400 
 
 1 
 
 100,650 
 
 1-9 
 
 52,500 
 
 500 
 
 1 
 
 100,770 
 
 1-9 
 
 54,500 
 
 600 
 
 1 
 
 110,050 
 
 1-93 
 
 57,250 
 
 700 
 
 1 
 
 110,150 
 
 1-83 
 
 59,750 
 
 800 
 
 1 
 
 110,250 
 
 1-8 
 
 61,750 
 
 900 
 
 1 
 
 110,265 
 
 1-74 
 
 63,500 
 
 1,000 
 
 1 
 
 110,500 
 
 1-7 
 
 65,250 
 
 1,100 
 
 1 
 
 110,650 
 
 1-7 
 
 66,500 
 
 1,200 
 
 1 
 
 110,750 
 
 1-6 
 
 67,750 
 
 1,300 
 
 1 
 
 110,800 
 
 1-6 
 
 69,250 
 
 1,400 
 
 1 
 
 110,900 
 
 1-57 
 
 70,250 
 
 1,500 
 
 1 
 
 120,000 
 
 1-4 
 
 71,250 
 
 1,600 
 
 From George Halliday's " Notes on Designs of Small Dynamos," p. 33. 
 
30 
 
 ELECTRIC MOTIVE POWER. 
 
 tt 
 
 e 
 
 3 
 ' 
 
 a 
 
 |; 
 
 2 
 1 
 C 
 
 8000 
 7000 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 & 
 
 n 
 
 s 
 
 
 ""*>..,_ 
 
 Watts 
 
 ^ 
 
 
 \ 
 
 ? 
 
 
 bUUU 
 
 z 
 
 i 
 / 
 
 
 
 ^"^ 
 
 '""^ 
 
 ^,- 
 
 
 
 
 
 5000 
 
 I 
 
 
 
 
 
 ^ 
 
 7 
 
 
 
 
 
 ED 
 
 
 
 y 
 
 / 
 
 
 
 
 
 20001 
 
 g 
 
 
 
 / 
 
 
 
 
 
 
 
 ^ 
 
 X 
 
 
 
 
 
 
 
 
 
 s* 
 
 
 
 
 
 
 
 
 
 35, 15 20 25 30 35 40 4 
 KILOWATTS. 
 
 FIG. 11. 
 
 Curve I. connects total weight with output. 
 Curve II. connects watts per pound with output. 
 
 COST PER KILOWATT IN POUNDS. 
 
 "'-'_. K> 10 K> CO C 
 
 O*CJoo>o-^CDK}fl 
 
 320 
 280 
 240 
 
 
 
 
 
 
 
 
 
 
 i 
 
 
 
 
 
 
 
 
 
 / 
 
 1 
 l 
 
 1 
 l 
 
 
 
 
 
 
 
 / 
 
 y i 
 
 i 
 
 
 
 
 
 ^ 
 
 0;^ 
 
 
 
 
 1 
 
 
 
 
 / 
 
 
 
 
 
 120 
 80 
 40 
 
 Q 
 
 r\ 
 
 
 / 
 
 s 
 
 
 
 
 
 
 8\ 
 
 ^-V 
 
 / 
 
 
 
 Com 
 
 
 
 
 _i 
 
 L- 
 
 / 
 
 
 
 ..... 
 
 
 
 KilovH 
 
 'att. 
 
 r 
 
 
 
 
 
 
 
 
 
 6 10 15 20 26 30 3& 40 4 
 KILOWATTS. 
 
 FIG. HA. 
 
 Curve I. connects total cost with output. 
 Curve II. connects cost per kilowatt with output. 
 
CALCULATION OF MAGNET DETAILS. 31 
 
 Annealed mild steel has met with some favour, but is not 
 likely to supersede wrought iron at present. Good samples 
 show a permeability equal to that of Swedish iron, and there- 
 fore better than that of ordinary magnet forgings, which are 
 simply heavy machine scrap, and often of doubtful quality. 
 
 The shape of the magnet cores is of importance, because the 
 length of the mean turn of the winding is affected thereby. 
 The circle has the minimum perimeter for a maximum area en- 
 closed, and therefore possesses the ideal section ; and the square 
 gives, from this point of view, better results than any other 
 rectangular form. Structural considerations, however, carry 
 great weight, and it is found in many designs that a rectangular 
 core, with length equal to twice or three times the depth, most 
 nearly meets all the conditions, in spite of the extra expense in 
 copper. 
 
 9. CALCULATION OF THE MAGNET DETAILS. 
 
 (a) Winding Space. In the preceding section the usual areas 
 of cross-section of armature and field-core were shown to be in 
 the ratio of 1 ; 1 *6 for wrought-iron magnets, and 1 ; 3 for cast- 
 iron. These ratios are not absolutely the best in all circum- 
 stances, but in practice they give economical results. It may 
 be assumed, then, that a type of field is chosen, and that the 
 material has been decided on. The area of the field-magnet 
 cores will also be known from its relation to the area of the 
 armature iron. The open factor at present is the length of core 
 to carry the exciting coils. This cannot be settled without 
 calculation ; but a few trial-and-error determinations will show 
 the minimum length required to carry the necessary turns of 
 wire, allowing at the same time the proper area of surface for 
 the watts spent. For bipolar single magnetic circuit machines 
 0-75 D, for double circuit 0'9 D, and for multipolar fields from 
 0-4 D to 0-5 D will be very nearly correct for all ordinary 
 designs. And when the other quantities are properly balanced, 
 the number of square inches in the surface of the winding, 
 not including the ends, will be equal to about twice the number 
 of watts wasted in heat ; or each watt of energy wasted in the 
 magnet windings will be dissipated through a superficial area 
 
32 ELECTRIC MOTIVE POWER. 
 
 of two square inches. This is found to be sufficient in machines 
 with windings not exceeding 2'5in. in depth ; but for large 
 machines a greater margin is preferable if it can be obtained. 
 A useful equation for determining the probable rise in tem- 
 perature in degrees Fahrenheit is 
 
 Rise of temp. Fahr. deg. = 
 
 Here, the cooling surface does not include the ends of the 
 shapes, but is expressed by the perimeter of the coils x their 
 height. The permissible rise of temperature is generally about 
 60F. ; under these conditions the empirical equation allows a 
 radiating surface of nearly 17 sq. in. per watt. This is suffi- 
 cient for small coils with shallow winding ; but if the depth be 
 more than 2 'Sin., the margin must be increased. 
 
 (6) Poles and Polar Extensions. The next point to determine 
 is the length of the polar cavity. This has already been referred 
 to in 8, division (c), and the practical limits have been given. 
 It may be further noted here that since the magnetic resistance 
 is lowered by increasing the width of the polar cavity, it appears 
 advisable to make the arc embraced as large as possible. But 
 this is limited by the strength of the cross field, which is 
 directly proportional to the extent of the pole pieces. With 
 drum windings the lead of the brushes is comparatively little, 
 as the field disturbance is small, and so the polar angle may 
 be somewhat larger than is permissible if a gramme ring be 
 used. A safe rule is to make the angle 135deg. for drums 
 and 125deg. for Grammes; but both of these figures may 
 require to be slightly modified by the peculiar conditions of 
 the design.* In a well-proportioned dynamo, when the arma- 
 ture is carrying the maximum circumflux for its diameter,! the 
 brushes will require a lead just sufficient to bring them under 
 the tips of the pole piece, so that the lead and the width of the 
 polar cavity roughly depend on each other. The ratio of area of 
 polar cavity to the cross-section of the magnet core is determined 
 mainly by the induction B" g in the gap. It is not found 
 economical to push the flux beyond B% = 33,000 ; and, there- 
 fore, neglecting leakage, the ratio of area of core section to polar 
 
 * See 8, division (e). t Sec equations (9) and (10), p. 22. 
 
LEAKAGE COEFFICIENT. 
 
 33 
 
 cavity will be, approximately, 1;2'75 if wrought iron be used, 
 and 1 ; 1 6 for cast iron. But there is always a certain loss of 
 magnetism from leakage, and it is necessary to allow for this 
 in estimating the total field core flux. Calling the leakage co- 
 efficient v, the relation between the field lines and the armature 
 lines is expressed by 
 
 N f = ^N a or v N g . 
 
 The assumption that the armature induction is equal to the 
 induction in the gap will not introduce a large error for 
 ordinary designs, and it simplifies matters. The stray field is 
 not constant for every type of magnet, but varies with the 
 armature induction, increasing in value as B" a approaches its 
 maximum. The practical values of v in the types of machines 
 most in vogue are given in Table D. 
 
 Table D. 
 
 TABLE giving Approximate Values of v, the leakage coefficient ^ for 
 various types of Field- Magnets. 
 
 Type of Magnet. 
 
 Open Circuit. 
 
 Full Load. 
 
 Single magnetic circuit, ) 
 upright type,Figs. Iand2j 
 
 v=r5 
 
 ^ = 1-32 
 
 Single magnetic circuit, in- } 
 ' ver ted f orm,Figs. 3 and 3 A J 
 
 v=l'31 
 
 ^=1-33 
 
 Double magnetic circuit,) 
 Figs. 4, 5 and 6 J 
 
 0=1-4 
 
 1=142 
 
 Four-pole machines, Fig. 7 
 
 v = from 1*4 to 1'45 
 
 V! = from 1'42 to 1'47 
 
 Fig. 8 
 
 v = from 1'45 to 1'5 
 
 i^from 1'47 to 1'6 
 
 Armour-clad field, Fig. 9 
 
 v=l'2 
 
 ^=1-22 
 
 (c) The Yoke. The yoke is that part of the magnet which 
 connects the cores. It is not usually covered with windings, and 
 is made either in wrought or cast iron so as to suit the design. 
 Generally cast iron is preferred, since the bed and yoke can, in 
 many cases, be combined in the same pattern. If this device 
 be adopted, as in Fig. 1, p. 23, care must be taken that 
 the path through which the magnetic flux will principally 
 nass is at least as wide as the area of cross-section of the 
 
34 ELECTRIC MOTIVE POWER. 
 
 cores, and usually it is found advisable to make it about 1'2 
 times larger, if the cores and yoke are of the same material. 
 If the cores be made in wrought and the yoke in cast iron, the 
 area of the cross-section at right angles to the lines of force 
 should be about 1'6 to 2*0 times as great for the latter as for 
 the former. Attention must be given to the joints between the 
 cores and the yoke, that they be truly machined and be in close 
 contact over the entire face. In machines of the double-magnet 
 circuit type there is no yoke, since the pole pieces are connected 
 directly to the magnet cores ; these joints must be carefully 
 made. 
 
 In some designs the polar cavity can be bored from the 
 solid iron (see Figs. 2, 3, and 6) ; but in others it is necessary to 
 attach extension pieces, so as to increase the polar cavity 
 (see Figs. 1, 4, and 5). But the dimensions of the pole-pieces 
 will in all cases be governed by the rules already given. 
 
 10. CALCULATION OF THE FIELD EXCITATION. 
 
 (a) The following points have now been determined : The size 
 of armature, the gauge of wire, and the number of turns for its 
 winding ; the shape of field-magnets, and length of the winding 
 space ; the bore of polar cavity and the width of the poles ; the 
 magnetic flux, and the densities in armature, gap, field cores 
 and yokes. 
 
 The excitation necessary to give these fluxes and the corre- 
 sponding winding have now to be calculated. The method 
 adopted by the author does not take into direct consideration 
 the permeability of the iron, this being understood in the 
 proposed limiting values of B. It is based on curves (see 
 Fig. 12) connecting induction per square inch of core with 
 ampere-turns per linear inch of distance through which the 
 induction has to be maintained. (The principle of these curves 
 was first suggested by Drs. J.and E. Hopkinson in 188G.) The co- 
 ordinates are the mean values of the ascending and descending 
 curves of the magnetisation. The variation of induction due to 
 hysteresis is most marked with cast iron, and is of little import- 
 
MAGNETIC INDUCTION CURVES. 
 
 35 
 
 ance with well annealed Swedish plates, such as are used for 
 armature cores, especially if the induction be about B" = 100,000. 
 Therefore in practice no trouble is caused by hysteresis other 
 than the loss of energy due to the cyclic change of magnetism, 
 which is rendered evident by a development of heat. The 
 calculation for this is given in Table L and footnote on page 173. 
 
 1 
 
 4* 
 
 H~ 
 
 o rx 
 
 ^ 
 ^ 
 
 ***~ 
 ^ 
 
 1Z=* 
 
 ,* 
 
 , ^ 
 
 ^ 
 ^ 
 
 
 
 ' 
 
 ^^- 
 
 - 
 
 i 
 
 ^*~ 
 
 *m " 
 
 ITT 
 
 ^ 
 
 
 s=: 
 
 - 
 
 ' 
 i.^ 
 
 ^T" 
 
 
 
 
 "TT 
 
 a^2 
 
 ' 
 
 
 MMe= 
 
 ^ 
 
 
 
 
 ^^ 
 
 ntz 
 
 
 / 
 
 x" 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 10 14-A 
 
 f 
 
 
 t 
 
 
 " 
 
 ***** 
 
 .- 
 
 7\T 
 
 ^ 
 
 
 
 
 
 
 
 / / 
 
 
 ^ 
 
 ^^ 
 
 ^****' 
 
 
 
 
 
 
 
 
 
 
 
 ^^ 
 
 4\\r 
 
 ^^ 
 
 
 
 
 
 
 
 
 
 
 
 ^- 
 
 -" 
 
 
 
 M 
 
 
 
 
 
 
 
 
 , 
 
 *^ 
 
 -^ 
 
 V 
 
 
 
 
 
 m HI / 
 
 
 
 
 
 ^-- 
 
 ^^ 
 
 -^ 
 
 
 
 
 
 
 
 
 
 / 
 
 
 ^ ' 
 
 . 
 
 ^** 
 
 
 
 
 
 
 
 
 
 
 
 
 r/U^ 
 
 ^^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 
 
 Magnetising Force in Ampere-Turns per lineal inch. 
 
 FIG. 12. Curves of Magnetic Induction in Iron and Air. 
 Scale of Ampere-Turns lor Iron : One division =50 Ampere-Turns per inch. 
 I. Annealed Swedish Charcoal Plates, as used for Armatures. 
 II. Wrought Iron Forcings, as used for Field-Magnets. 
 
 III. Cast Steel, as used for Field -Magnets. 
 
 IV. Cast Iron, as used for Field-Magnets. 
 
 V. Air To read Ampere-Turns multiply abscissa by 20. 
 
 All the Headings have been determined iron* the Mean ot the Ascending and 
 Descending Curves of Magnetisation. 
 
 In Fig. 12, Curve I. refers to annealed charcoal iron plates, 
 from 22 to 24 B.W.G. The data are taken from experiments 
 made by Mr. George Halliday. 
 
36 ELEGTEIC MOTIVE POWER. 
 
 Curve II. relates to magnet iron, large scrap, hammered 
 and allowed to cool slowly after forging. The values of the 
 co-ordinates were originally taken from Drs. J. and E. Hopkin- 
 sons' Paper on the characteristics of the dynamo ; but they 
 have been altered to suit the samples of iron used by the author, 
 most of which have been supplied from Yorkshire forges. 
 
 Curve III. is taken from the recent experiments of Mr. Stein- 
 metz with annealed cast steel, which is likely to be largely used 
 for magnets. 
 
 Curve IV. is for the class of grey cast-iron generally supplied 
 in London for magnets. The samples which have recently 
 come under the author's notice have perhaps a slightly higher 
 permeability than that implied in this curve. However, it is 
 best to be on the safe side with cast iron, as the cooling so 
 largely affects the results, and with these values there will be 
 no doubt about the output of the machine. 
 
 Curve V. shows the number of ampere-turns required to 
 send the induction through the gap. Its abscissae are cal- 
 culated from the equation 
 
 ATs 1 
 B ~ 0-31328 ' 8 
 
 where <5 = lin., and 0-31328 is a dimensional coefficient to con- 
 vert the absolute measures into practical units. To use this 
 curve find the ampere-turns corresponding to the density, and 
 multiply by the length of the gap expressed in decimals of an 
 inch. 
 
 (b) Excitation for Open Circuit. The data must now fee 
 collected in order to make the calculations for the number of 
 ampere-turns. It is necessary to know for the armature : 
 
 B" a = the induction per unit area. 
 
 A a O'85= the area of cross section, less 15 per cent., which 
 will be occupied by the insulating material 
 between the plates. 
 m & mean length of magnetic path in armature core. 
 
EXCITATION FOE OPEN CIRCUIT. 37 
 
 For the gap : 
 
 B" g = the induction per unit area. 
 
 8 = length of the air gap on one side. 
 
 A g = area of one of the polar cavities. 
 
 For the field-magnets : 
 
 B" f = the induction per unit area in the cores. 
 
 B" fl the induction per unit area in the yoke. 
 
 A f = area of core in square inches. 
 
 A fl = area of yoke in square inches. 
 
 m { = mean length of magnetic path in cores and pole-pieces. 
 
 m n = mean length of magnetic path in yoke. 
 
 Since the design is symmetrical about the perpendicular 
 line a b (see Figs. 1 and 4), it is only necessary to measure the 
 path of the lines of force around one-half of the circuit. The 
 excitation thus determined refers to one limb only, and must 
 be doubled to give the full number of ampere-turns. This 
 method is usually adopted because it both saves time and 
 gives smaller figures. 
 
 The excitation for the several parts of the circuit is : 
 Armature: ^a/(B" a ). 
 
 Field cores : m f f(B" { ). 
 
 Field yoke: n/(B" a ). 
 
 Gap:- S/(B" g ). 
 
 And, summing up- 
 
 . . (12) 
 
 (c) Excitation to be added at Full Load. In the preceding 
 equation the demagnetising effect of the armature is neglected, 
 and no allowance is made for the fall of pressure over the 
 internal resistance of the armature and series coils. 
 
 The demagnetising and cross-magnetising fields due to the 
 armature have been already referred to, and it has been shown 
 that these effects are most pronounced with the Gramme type 
 of armature. The subject has been exhaustively treated 
 during the past two years, and need not be discussed in 
 detail here. Suffice it to say, that the band of conductors 
 
38 ELECTEIC MOTIVE POWER. 
 
 lying between the pole tips will, as a whole, tend to produce a 
 field in the opposite direction to that due to the main excita- 
 tion ; while the coils lying under the pole-pieces, i.e., in the 
 polar cavity, will tend to cause a cross magnetism. The latter 
 is not of serious importance if the pole-pieces be very thin near 
 the centre, as at a and b in Figs. 4 and 6. If, however, the 
 pole-pieces be very massive in the region of the centre line there 
 will be a cross field, which will have a tendency to twist the 
 main field by crowding out some of the forward induction. But 
 this can be almost entirely obviated with the relative propor- 
 tions and shapes of magnets given in 8. 
 
 The demagnetising or back ampere-turns are of more im- 
 portance, and must be allowed for by an equal forward induc- 
 tion. The disturbance is, with Gramme rings, proportional to 
 the armature current multiplied into the number of turns 
 included in the angle of lead ; and this product must be 
 multiplied by the coefficient of leakage between the field and 
 armature at full load, since a certain quantity of the field induo 
 tion is lost in space. Therefore the compensating turns are 
 
 BTs = C a *>! = C a - L v, . (13) 
 360 180 
 
 where 6 = the angle of lead in degrees. 
 
 With drum armatures the disturbance is about one-half as 
 great, and so the compensating turns are 
 
 To find the turns to compensate for the fall of potential, it 
 is necessary to make two calculations for the ATs, see (12), one 
 with no current flowing in the main circuit, and hence no arma- 
 ture reactions, and one at full load. The former will determine 
 the strength of the shunt coils if the machine be compound 
 wound ; and the latter will give the shunt excitation plus extra 
 turns to be added for the fall of potential due to the internal 
 resistance. The total excitation is : 
 
 ATs for open circuit + BTs for demagnetising effects + CTe 
 for fall of potential. 
 
W. B. SAYEBS' COMMUTATOR COILS. 39 
 
 Practically, with large Gramme rings the two latter quan- 
 tities will be about one-third of the first. 
 
 If the machine be shunt wound the winding must be deter- 
 mined for full load, and the BTs must be added before calcu- 
 lating the gauge of wire. The same remark also applies to a 
 series machine. A few trial windings will soon illustrate the 
 point, and facility will come with experience. 
 
 Mr. W. B. Sayers' device* for the " prevention and control 
 of sparking " is likely to introduce important modifications in 
 dynamo and motor design. It is well recognised that the limits 
 of output of a given machine are determined simply by the 
 heating and the sparking effects. The former is controlled by the 
 current density, the eddies in the conductors, and hysteresis ; 
 the latter depends mainly on the circumflux and the relative 
 proportions of the machine, or, briefly, on the armature 
 reactions. It is well known that a dynamo requires a forward 
 lead and a motor a backward lead to prevent sparking, and it 
 is also recognised that the amount of the lead is proportionate 
 to the load. The effect of the angular displacement of the 
 diameter of commutation is to cause the convolutions, included 
 in the angle of lead to be traversed by currents in an opposite 
 direction to the E.M.F. induced in them, and in a direction 
 such as to tend to demagnetise the field-magnets. The demag- 
 netising effect of this belt of back-ampere turns is merely an 
 accidental condition, and is compensated for by extra excitation 
 as already shown; but the reversed E.M.F. is an absolute 
 necessity in order to check the self-induction of the current in 
 the coils short-circuited beneath the brushes at the moment of 
 current reversal. The explanation is simple. If the current in 
 these coils is acted upon by an E.M.F. in the same direction 
 as itself, or even if the coils are in a neutral field, it is clear 
 that the current cannot be stopped and reversed without a self- 
 induction spark ; but if the coils, at the moment of reversal, 
 are in a magnetic field of such direction and intensity as to 
 induce an E.M.F. just sufficient to counter-balance the self- 
 induction of the current, then the reversal may be made 
 without a spark, and the collection of the current will be 
 
 * Proceedings of Institution of Electrical Engineers, Vol. XXII., Part 107. 
 
 E2 
 
40 
 
 ELEGTEIG MOTIVE POWER. 
 
 sparkless. This apparently complicated action is easily accom- 
 plished hi practice by simply giving the brushes sufficient 
 lead ; and it is obvious that since the armature reactions are 
 proportionate to the lead that the best designed machines 
 will have the least lead. The converse of the problem is also 
 suggested for consideration, for if a forward lead tends to 
 demagnetise the field-magnets of a dynamo it may be reason- 
 ably inferred that a backward lead would tend to strengthen 
 them. This is found to be so in practice, and many attempts 
 
 FIG. 12A. P, Trailing Horn of Pole-piece. B, Positive Brush, a, I, c, d, 
 Commutator Coils to furnish the E.M.F. necessary for Sparkless Reversal. 
 S, S, S, S, Segments of Commutator. The current in main coil C has 
 to be reversed in passing under the brush ; this is effected by the E.M.F. 
 in the coil b, which is actively cutting Magnetic Lines in the Dense Field 
 under the Pole Horn, and hence can overcome the Self- Inductive E.M.F. 
 of the current in C. P, the Axis of Poles, and N the Neutral Axis at 
 right angles to P. The large arrow shows the direction of armature 
 robation, and the small ones the direction of the E.M.F. acting in the 
 several coils. The current is flowing round both sides of the armature to 
 the positive brush, B. 
 
 have been made to assist the field excitation by the armature 
 reactions, but without success, until Mr. Sayers invented his 
 Commutator Coils, as he calls them. Their application will be 
 
W. B. SAYE&S' COMMUTATOR COILS. 41 
 
 seen at once by reference to Fig. 12A, and a little study will 
 make clear the theory of their action. In the diagram, which 
 represents a Gramme ring dynamo, the commutator bars, S, 
 are drawn, for convenience, outside the armature, the main 
 windings are seen to be continuous, and each coil to be coupled 
 to a segment of the commutator through a commutator coil, 
 a, I, c, d, the order of connecting up being one main coil back- 
 wards. It is of special importance to mark that these coils 
 only carry current when touching the brushes, and then the 
 whole armature current passes through them momentarily ; 
 also the brush, B, is made sufficiently wide to always bridge 
 over two segments. The direction of motion of the armature is 
 clockwise, as indicated by the large arrow, the brush has, 
 therefore, a backward lead, and the active commutator coil, b, 
 is, practically, just beneath the edge of the trailing pole piece, 
 while the main current in C is commutated at a diameter not 
 far removed from a line at right angles to the axis of the pole 
 pieces. In this position the field due to the armature magne- 
 tism is partly added to that of the field-magnets, and sparking 
 is prevented by the E.M.F. induced in the commutator coils, 
 which are cutting lines of force in the dense magnetic field at 
 the trailing horn of the pole piece. These effects will obviously 
 increase in intensity as the brush is brought nearer to the pole 
 piece. The importance of this device lies not so much in the 
 application, just illustrated, as in the new line of investigation 
 suggested by it. In the first place, it permits a much smaller 
 air gap, and, therefore, less excitation in the field coils, which 
 means smaller magnets ; secondly, since sparking can be 
 obviated, or its magnitude restricted, a greater output may be 
 expected from a given armature, heating being the only 
 limiting condition. A host of applications at once present 
 themselves for immediate consideration, amongst which may 
 be instanced : /Series-wound motors for traction purposes, to 
 which light compact machines are essential; and regulators, 
 for compensating the fall of potential on feeders, which, 
 carrying widely different loads, are always subject to sparking 
 troubles (see 25, page 137). 
 
 The winding details present some difficulties in practice, 
 especially as regards insulation ; and generally the device 
 
42 
 
 ELECTRIC MOTIVE POWER. 
 
 lends itself most readily to slotted cores in which the main 
 winding can be wound underneath and the commutator coils 
 on the top, so as to be all but flush with the surface. In one 
 form Mr. Sayers has used the commutator coils as keys to 
 hold the main winding in position, as shown in Fig. 12B, which 
 refers to a slotted drum. In drum windings the ends of the 
 main windings are brought to the back end of the armature 
 away from the commutator, and the commutator coils are 
 simply lengths of wire sufficiently long to couple up to the 
 bars. In Gramme rings the commutator coils are wound in a 
 
 FIG. 12B. Sayers' Winding as applied to Drums. 
 a, Commutator Coils, c, Main Coils. 
 
 plane at right angles to the main coils, and may therefore lie 
 on the outside of the armature. Various methods of carrying 
 out the device will suggest themselves to different designers, 
 but it is too soon to predict the probable results. 
 
MOTOIi DESIGN. 43 
 
 11. SPECIAL POINTS TO BE OBSERVED IN 
 DESIGNING MOTORS. 
 
 So far the author has considered the design of either dynamo 
 or motor simply from the point of view of the total E.M.F. 
 of the armature, and has given no special attention to the 
 torque. But it was mentioned in 2 that in designing motors 
 it was essential to give due weight to both the maximum and 
 average couples likely to obtain. The importance of this will 
 be best seen from an inspection of the equation for the torque 
 in terms of the armature constants : 
 
 Torque = T = Ga * aNa , .... (15) 
 /c 
 
 or the torque is directly proportional to the product of the 
 number of turns of wire, the magnetic flux, and the arma- 
 ture current; k being simply a dimensional coefficient. The 
 numerical value of fr is 852'3 x 10, if T be in pound feet, so 
 that, adopting the practical unit, 
 
 Now, in all ordinary motors the number of turns of wire in 
 the armature, C a , is constant, and only the current 4, and the 
 magnetic flux N a vary. There are two simple cases for con- 
 sideration in practice. First, when both i & and N a vary. This 
 occurs with a series-wound motor, and it should be noticed 
 that a relatively small increment of current may cause a con- 
 siderable increase in the value of the torque, for the magnetism 
 will be greatly varied at the straight part of the characteristic 
 with low values of B, and therefore N a will at this point 
 increase much more rapidly than i A ; whilst after the knee of 
 the curve is passed the rate of increase of N a for increments of 
 * a will slowly decrease, until a point is reached where the torque 
 varies simply as the current. 
 
 The second case is where N a is constant, and i A alone varies. 
 This condition is given with a shunt-wound motor, if it be as- 
 sumed that the armature reactions are negligible. The torque, 
 therefore, will simply vary as the armature current ; it will be 
 a minimum with small loads, but will finally attain the same 
 
44 ELECTRIC MOTIVE POWER. 
 
 maximum value as it would if the motor were series wound. 
 Both of these windings give special advantages to a motor, and 
 hence they are severally selected for particular work. The 
 compound-wound motor is a combination of the two methods, 
 and has not met with much favour as yet. The uses of the 
 three types of winding are more fully discussed in 14. 
 
 It remains now to investigate the best lines upon which 
 to build armatures for special turning moments. 
 
 If the desired brake horse-power and the speed of the motor 
 be known, the work per revolution and the torque can be easily 
 determined. 
 
 The work per revolution in foot pounds 
 
 B.H.P. 33000 , 
 
 = -footpounds. , . (17) 
 
 n 
 
 and the torque T^= RH R 3300 pound feet, . . . (18) 
 2 TT n 
 
 where n = revolutions per minute. 
 
 It should be noticed that T is independent of the diameter 
 of the armature. 
 
 The peripheral pull exerted by the armature 
 _ RHJ>.SSOOO 1 l pounds 
 
 2 TT n g 
 
 where g = the radius of the armature in feet. 
 
 This force will act tangentially on the winding, practically 
 at two parts diametrically opposite each other, and just under- 
 neath the leading horns of the pole-pieces. If the value repre- 
 sented by B.H.P. be the average load, we can thus determine 
 the average strain on the wire ; and if it equal the maximum 
 output the maximum running torque is known. But suppose 
 the current to fluctuate largely, and at times to exceed that 
 corresponding to the maximum output at starting, for 
 example, the momentary rush of current may easily be twice 
 or even three times as great. Then, from what has been 
 
EXAMPLE OF DYNAMO AND MOTOR DESIGN. 45 
 
 said, it is clear that the torque will be increased also, and 
 it may be so sufficiently to endanger the stability of the 
 winding, or even to bend the shaft. Therefore, in motors it 
 is usual to allow a very large factor of safety by supporting 
 the winding with specially-devised driving horns, and by the 
 use of much stiffer shafts than are necessary for dynamos 
 of the same average output. In designing motors, then, it 
 is advisable first to examine the limits of the turning couple, 
 and then* to take precautions to ensure sufficient mechanical 
 strength. For tramcar and coal-cutting motors it is usual 
 to provide shafts from 2in. to Sin. in diameter for outputs of 
 from 15 to 20 H.P. 
 
 15. PRACTICAL EXAMPLE OF PRECEDING METHOD 
 OF DESIGN. 
 
 The application of the principles of dynamo and motor design, 
 described in the preceding sections, is fairly illustrative of 
 the ordinary methods followed in most drawing offices, but 
 the details are varied to suit the fancy or necessities of each 
 engineer. 
 
 The design has been treated in a general way, so as to include 
 as many cases as possible, and hence appears to be complicated. 
 In practice, however, owing to fixed types of field magnets, 
 definite sizes of armature plates, and data from machines 
 already built, it is very simple. A machine can be designed 
 for a given output at the required speed in a very short time ; 
 indeed, nearly all of the calculations will be in simple pro- 
 portions that can be seen at a glance on a slide rule. 
 
 However, to illustrate the use of the equations, the following 
 calculations for a 25-unit dynamo and a 25'5-B.H.P. motor are 
 appended. 
 
 It is assumed that the problem is to build a pair of machines 
 to transmit 25'5 B.H.P. a distance of one mile from the dynamo. 
 Since there is only one motor, the series winding will be most 
 suitable for both machines,* this arrangement giving fairly good 
 
 *. See Chap. IV., 23. 
 
46 ELECTRIC MOTIVE POWER. 
 
 speed regulation of the motor for variable load if the generator 
 speed be kept constant. 
 
 The efficiency of the motor may be safely taken at 90 per 
 cent., and, therefore, the watts absorbed by it at full load 
 will be 
 
 OFJ.K 
 
 1^1 = 28-32 EH.P. 
 0-9 
 
 If the line loss be fixed at 5 E.H.P., the output of the dynamo 
 will be 33-32 E.H.P. = 24,857 watts. On referring to Fig. 13, 
 which gives the volts and horse-power lost per mile of cable, 
 with different currents and areas, it is seen that a wire of 
 sectional area corresponding to No. 6 S.W.G. will carry 35'75 
 amperes with a drop of 52 volts and a loss of 2'5 E.H.P. per 
 mile. This is a convenient size of wire,* as small as it is prac- 
 tically safe to erect for a permanent line and is not too costly ; 
 it weighs 5891b. per mile, and has a resistance of 1'4694 ohms 
 for the same length. The two miles of line wire will therefore 
 have a resistance of nearly 3 ohms and the fall of potential will 
 be 104 volts, and the watts lost in it 3,720. Dividing the 
 dynamo output by the current (35 -75 amperes), the potential 
 difference is seen to be approximately 695 volts. Since the 
 pressure is high the armature winding will be of the Gramme 
 type, and the double magnetic circuit two-pole field (Fig. 4) 
 will be selected for both machines. The armatures will be 
 made interchangeable in order that one " spare " may cover 
 the two machines ; and the field carcases will also be identical 
 save in respect of the number of ampere-turns, and perhaps the 
 area of the magnet cores. 
 
 Determination of Armature Dimensions : 
 
 From ZD 2 7*0-01 (1) 
 
 15 x!5 2 x 800x0-01 = 27,000, 
 
 where 1=15, D = 15, w = 800. 
 
 This will give a safe margin in output. 
 
 Now, E = c + t ft (r. + rj (5) 
 
 and, also E = 695 + (0-06 x 695) = 736. . From Table A. 
 
 (Take one and a-half times the percentage, since the field resistance must 
 
 be included.) 
 
 * The economic area is purposely neglected in this case, as the subject has 
 not yet been referred to in the present work. 
 
CURVES OF DROP OF PRESSURE. 
 
 47 
 
 6 7 H.P. LOST PER MILE. 
 
 600 
 
 476 
 
 460 
 
 10 CO 30 40 60 60 70 80 90 100 
 
 FIG. 13. Curves showing Volts lost per Mile and per 100 Yards with 
 different Currents and usual Sizes of Cable. Calculated from the 
 formula E = (0'0427/area) C. Ako H.P. lost per Mile. 
 
48 ELECTRIC MOTIVE POWER. 
 
 N a = B" a 2 rJO-85 ........... (7) 
 
 (See Table B.) 
 
 1=15, let r = 2-5, and B' a = 110,000. 
 .'. N a = 110,000 x 5 x 15 x 0-85 = 7-0 x 10 6 . 
 
 Put this trial value of N a in (8) 
 
 6C 736 x 60 
 
 7-0 x!0x 800x10^ 
 
 C = 
 
 a 
 
 The circumference of armature = ird = 47", deduct a space of 2" 
 for the driving horns, leaving 45" for the winding. No, 11 
 S.W.G. will carry 17 875 amperes at a density of 1,800 amperes 
 per square inch : it is O 1 11 6" in diameter when bare, and with 
 16 mils of insulation = 0-1 32". Therefore, with No. 11 wire 
 
 45 
 the turns per layer will be - - = 340. Two layers will 
 
 = 680 turns, i e., say 68 coils of 10 turns each. The resistance 
 when cold will be about 0'37, and when hot about 0'4 ohm. 
 Ampere-turns per coil = 10 x 17-875 = 179 say 
 Permissible circumflux = D 1000^=15,000 ... (9) 
 
 A J - *a C a 3575x680 -.ooAH 
 
 Arranged crrcumflux = - * = -- = 12,200. 
 
 Two layers of this wire give fewer turns than the value of C a 
 determined from (8\ and three layers give too many, besides 
 increasing the air gap unduly and raising the circumflux above 
 the safe limits given by (9). This is a common case in 
 practice ; the remedy is to raise the speed, or to increase the 
 armature induction : or to do both, 
 
 780 
 
 making n = 800 x _ = 920, say ; 
 
 680 
 
 or, making N a = 7'0 x 10 6 x = 8'00 x 10 6 ; 
 
 680 
 
 or, by varying both n and N a . 
 
 To illustrate these points the combined method is chosen. 
 
 Since B" a is already 110,000, in order to increase N a it is 
 necessary to make the cross section of the armature larger, 
 and to alter the radial depth of core, r, from 2 -5" to, say, 2-75". 
 Therefore 2lr will = 82-5 sq. in., and the effective armature 
 section will be 82'5 x 0'85 = 70 sq. in. 
 
EXAMPLES OF DYNAMO AND MOTOR DESLGN. 49 
 
 The new value of N a is therefore 
 
 7-Oxl0 6 x^I5 = 7-7xl0 6 . 
 2-5 
 
 The speed must be raised to give the required conditions. 
 Substitute these new values in (8), and solve for n 
 E 60 736 x 60 
 
 n 
 
 C a N a 10- 680 x 7 -7 x lO" 2 
 
 = 835. 
 
 Fia. 14. Cross Section of Dynamo, showing Method of Design. 
 
 The bore of the poles must next be determined. Allowing 
 for two layers of No. 11 S.W.G., suitable insulation, bands, 
 and clearance of, say, 0-0937 (3/32") of an inch all round, the 
 required bore is 15-75, and .*. 8 = 0-375. And let the angular 
 width of the pole pieces be 120 at the edges, and 125 in the 
 centre ; i.e., let the " horns" be curved in a direction parallel to 
 the winding (see Fig. 14). This tends to destroy the abruptness 
 of the magnetic flux, and to distribute it more gradually on the 
 
50 ELECTRIC MOTIVE POWER. 
 
 leading horns. The average width of the pole pieces, w, is 
 122-5, or, say, 17". 
 
 The length of the poles should be a little less than that of 
 the armature core, say, in this case, 14 -5 ; therefore the area 
 of each cavity is 17 x 14'5 = 247 sq. in., and B" g , the density in 
 the gaps, is found by dividing N a by 247, 
 
 or, 7 ' 7xlQb = 31,000. 
 
 247 
 
 This is a convenient density. 
 
 Next, to determine the field induction : 
 
 N f = ^ N a = l-42x7-7xlO G . (^ = 1-42) 
 
 = 10-95 xlO 6 , 
 
 and half of this flux, 5 -47 5 x 10, must be supplied by each 
 limb of the magnet. 
 
 Let the cores be wrought iron, and B" f = 85,000 say, and 
 
 5*475 x 10 fi 
 the area of each core = _ =65 sq. in. nearly. 
 
 Now the length of the pole pieces has been fixed at 14'5"; let 
 the magnet cores be the same, then the width of each core is 
 
 65 . - 
 
 = 4'5 . 
 
 14-5 
 
 The dimensions of the machine are now sufficiently deve- 
 loped to enable a drawing to be made showing a cross-section 
 of the field and armature cores ; let this be to, say, an inch and 
 a-half scale, as in Fig. 14. The length of the magnetic paths 
 can be estimated, and the field winding calculations made. 
 
 m t = 23"; m tl = 16"; w a = 7"; A a = 2 lrO'S5 = 70; A f = 14-5 x 4-5 
 = 65-0; A fl = 110; B" a = 110,000; B" f = 85,000; B" n = 50,000; 
 and B" g == 31,000. 
 
 The path of the induction in the armature can only be 
 averaged, since the density is not constant (see Fig. 14A, which 
 has been drawn specially to show the probable distribution of 
 the lines of force). It is noticeable that the maximum induction 
 
RULES FOR FIELD-MAGNET EXCITATION. 51 
 
 only takes place in the areas bounded by the lines a a v and b b r 
 Averaging the path to the density chosen, B" a = 110,000, it is 
 seen that the mean distance between the pole tips is a fair 
 approximation to the value of m & * 
 
 FIG. 14A. 
 
 The excitation per limb for full load is from (12) 
 
 ATs for armature = wi a /(B" a ) = 7x 280 = 1,960 
 
 cores = m f /(B" f ) = 23 x 77 = 1,771 
 
 pole pieces = m fl /(B" n ) = 16 x 130 = 2,080 
 
 gap = 2 S/(B" g ) = 0-75x9,750 = 7,300 
 
 13,111 
 
 The various " functions " for the determined values of the 
 inductions are read direct from the proper curves in Fig. 12 
 (p. 35). Let the value of the ampere-turns be taken at 13,100. 
 
 To find the back ampere-turns : Let the maximum angular 
 lead be 27, i.e., nearly to the tip of the leading " horn." 
 Then, by (13), the back turns are 
 
 a1( 
 = 680 x x 35-75x1 -42, 
 
 = 5, 1 50 nearly. 
 (For value of " vf see Table D, p. 33.) 
 
 * It is also interesting to mark how the direction of the lines is altered 
 after leaving iron and entering air. This is to be expected from the very 
 different permeabilities of the two media, a subject which is engaging the 
 attention of experts. 
 
52 ELECTRIC MOTIVE POWER. 
 
 And 2,575 ampere-tarns must be added to the excitation of 
 each limb, since (13) estimates the total demagnetising effect. 
 Therefore the total excitation per limb is 
 
 ATs + BTs = 13,100 + 2,575 = 15,675. 
 
 With a series machine no calculation can be made for " open 
 circuit," since it will not excite unless the main circuit be 
 closed. If the dynamo be compound wound, this calculation is 
 necessary in order to determine the shunt winding ; and for a 
 simple shunt machine it is usually sufficient to determine the 
 excitation at full load only. 
 
 It now remains to find the gauge of wire for the field wind- 
 ings. This can be done in several ways, according to the 
 result aimed at. 
 
 In a series machine* the current is fixed, and thus the 
 gauge of wire is given by the heating effect, see 9 (a) ; 
 and the number of turns must be selected .to give the proper 
 excitation. In the design under consideration the convolu- 
 tions required 
 
 = 15675 = 440 per limb, 
 37-75 
 
 * If the field coils were in shunt, then the potential difference would be 
 fixed, and the current and turns be variable, the efficiency increasing with 
 the number of turns for a given value of the excitation. Now there is 
 one gauge of wire, and one only, which will give a particular excitation, for 
 the given section of core, at a fixed potential difference. This wire being 
 used, the number of convolutions affects the efficiency and not the value 
 of the excitation. Therefore, it is necessary to find the proper area of 
 cross section of wire, and usually as it is not a stock size the next size larger 
 is chosen. This increases the number of ampere-turns ; but an external 
 resistance is easily put in series with the coils, and hence the potential 
 difference can be adjusted for any required excitation. The method 
 followed is to first find the length of a mean turn of the coil, say it = / 1 
 inches ; then, the resistance of a mean turn 
 
 I = l 
 ATs' 
 
 where e x = the potential difference available ; and the gauge of wire can 
 be found by reference to a table of wire resistances. 
 
RULES FOR FIELD-MAGNET EXCITATION. 53 
 
 and No. 5 B.W.G. will carry 38 amperes at a density of 1,000 
 per square inch of section ; and this will give an ample margin. 
 The resistance when hot is 
 
 440 x L x 0-00064 440 x 34 x 0-00064 n 
 
 - 1 - = - Q - = 0-258 ohm, 
 36 36 
 
 where l^ = length of a mean turn in inches, and 0'00064 
 = resistance of one yard of the wire. 
 
 The winding space is 14in. long; allowing 0'75in. for the 
 two flanges of the " shape," and for insulation, there is left a 
 net length of 13'25in. Now 4-25 turns of No. 5 B.W.G. (with 
 16 mil insulation) will occupy lin., and so the convolu- 
 tions per layer will be 56. The turns per coil are 440 ; hence, 
 8 layers are required. The depth of the winding is approxi- 
 mately 2 in. 
 
 The resistance of the two coils is about 0*516 ohm, and 
 the total resistance of the dynamo 0*916 ohm. The pressure lost 
 over this is 
 
 35-75 x 0-916 = 33 volts nearly. 
 
 Now, in fixing the value of E, a fall of 6 per cent., or 41 
 volts, was allowed. The winding is thus within the required 
 conditions. If desirable, the value for the total E.M.F. can 
 now be readjusted and the calculations amended accordingly. 
 Practically, however, it is sufficient to decrease the speed in 
 the ratio of 
 
 _ = _ = 
 
 695 + 40 735 
 
 and speed = 835 x 0-99 = 830 revolutions per minute. 
 
 The design of the dynamo is now complete. That of the 
 motor is substantially the same ; but special care must be 
 given to the driving horns and general details of the armature. 
 The speed of the motor is proportional to the counter E.M.F., 
 i.e., to 
 
54 ELECTRIC MOTIVE POWER. 
 
 and the torque is 
 
 
 
 Now gj, the volts at motor terminals, =e 104 = 591 volts. 
 
 t a =3575. 
 r a =0-4. 
 and let r m =0*3, say (trial value with six layers on field cores, 
 
 C =680 turns in all). 
 
 And n and N a are the two unknown variables. If there be any 
 particular speed, then n is also fixed. Let 720 be the required 
 number of revolutions per minute, then n = 720, and the torque 
 is given by (18) 
 
 25-5 x 33.000 ^gy^ft 
 2?r720 
 
 The peripheral pull on the armature at full working load is 
 given by (19), and 
 
 -B.H.P. 33.000. l_ 9pnb , 
 
 2irn g 
 
 where g = radius of armature in feet, =0'G25. 
 
 The maximum stress at starting may possibly be twice as great, 
 or even more, so it is necessary to allow for a pull of, say, from 
 800 Ibs. to 1000 Ibs. Equating to (16), the value of the torque 
 found from (18), and taking out N a , the required value of the 
 armature induction* is found, thus 
 
 1 87 x 852-3 xlO 6 
 
 680 x 35-75 
 = 6-55xl0 6 , 
 
 * The value of N a cannot be determined frpm equation (6) with sufficient 
 accuracy, as the exact value of the counter E.M.F. is not known until 
 the field- winding is settled and its exact resistance, r m , arrived at. 
 
and, 
 
 DESIGN OF MOTOR. 55 
 
 R ,, 6-55 xlO 6 Q - 7ft 
 D a = - - = yj,570, 
 
 N, =1-4N. =9-17xlO, 
 4-585 x 10. 
 
 B ,, 
 
 
 The excitation is by (12) : 
 
 ATs for armature = w a /(B" a )= 7 x 53 =371 
 
 cores = *f/(B" f ) = 23x35 = 805 
 
 pole pieces = ??i fl /(B" fl ) = 16x66 =1,056 
 
 gap = 25y(B" g ) = 0-75x8,280 = 6,210 
 
 8,442 
 
 The back ampere-turns* are practically the same as with 
 the dynamo, and, therefore, the full excitation per limb is 
 
 ATs + BTs = 8,442 + 2,575 = 11,017. 
 
 Dividing this by the number of amperes, the number of 
 turns per coil is 310, or six layers, as was assumed in the 
 approximation. 
 
 This finishes the calculation for the motor. The figures 
 have been estimated by means of a slide rule, and so are not 
 always quite exact ; but they illustrate the use of the equations, 
 and show the general method of designing dynamos and motors. 
 It is interesting to notice that the motor is rather large for the 
 output, and that the inductions in the field cores and gap are 
 consequently much less than those required in the dynamo. 
 This can be obviated to some extent in practice, by reducing 
 the section of the motor cores, making them, say, 4in., instead 
 of 4-5in., in depth. Such alterations will readily suggest them- 
 selves. 
 
 * Probably the lead at full load will be from 5deg. to lOdeg. less than 
 that required with the dynamo. If this be so, the extra turns will be 
 correspondingly less. 
 
 F2 
 
56 
 
 ELECTRIC MOTIVE POWER. 
 
 13. TABULATED FORM FOR USE IN DESIGNING 
 DYNAMOS AND MOTORS. 
 
 In practice, the difficulties attending the designing of 
 machines are very much lessened if the constants and variables 
 are classified under their proper heads and in the order in 
 which they come under the notice of the designer. The author 
 has found the following tables of great utility; they can, of 
 course, be modified to suit special requirements. One of the 
 great advantages possessed by such a regular system is that 
 the details of different machines can be readily compared, and 
 hence fresh designs can be got out with the minimum of trouble 
 and figuring. To derive the greatest possible benefit from this 
 arrangement the printed sheets should be bound together into 
 a volume of suitable size, with an index. 
 
 Table E. 
 
 DYNAMO OR MOTOE. 
 
 Pules. Output K.W. I Volts 
 
 Magnetic Circuits I Kevs. per min. 
 
 Type 
 
 Amps 
 
 Length in. 
 
 Diameter in. 
 
 Dimensions over iron .... in. x in. 
 
 Effective area (. .a&. 0-85) . . . .sq. in. 
 
 Size of hole in. 
 
 No. of coils , TT d in. 
 
 No. of convolutions per coil. . . C a . . 
 Area of bare wire : 
 
 in.x in.=: sq. in. 
 
 Dimensions over insulation : 
 
 ..in.x in. 
 
 AEMATUEE. 
 
 Weight of copper .Ibs. 
 
 Weight of iron Ibs. 
 
 Current density per eq. in. 
 
 Length of mean turn ft. 
 
 Eesistance of copper (hot) from brush 
 
 to brush ohms 
 
 Bore in. 
 
 Circumflux 
 
 Permissible ditto 
 
 COMMUTATOE. 
 Over- all dimensions I Metal 
 
 No. of segments 
 Size of ditto . 
 
 Insulation, mica in thick 
 
TABLE FOR DYNAMO AND MOTOR DESIGN. 57 
 
 COKES Iron. 
 
 YOKK Iron. 
 
 FIELD MAGNETS. 
 
 Area sq. in. Weight Ibs. 
 
 Area sq. in. Weight Ibs. 
 
 AREA OF POL AR CAVITY sq. in. Arc of poles cleg. 
 
 LENGTHS OF MAGNETIC PATHS, TAKEN AS FOLLOWS : 
 
 
 LEAKAGE 
 
 COEFFICIENT, open cii 
 
 
 
 E a = 
 .-. N a 
 
 OPEN CIRCUIT EXC 
 
 .. ..=C a N tt 10- 
 , B" a , 
 
 ITATION PER COIL. 
 
 cap . 
 
 B" K . 
 
 
 
 N f = V Na 
 
 
 field -{ 
 
 B 
 
 
 
 
 
 
 E a i 
 
 FULL LOAD EXCITATION PER COIL. 
 
 .'.B" a 
 
 N f =r x N a 
 
 d// 
 
 D f 
 
 
 Rap . . 
 
 
 
 B"fi 
 
 ,, field -! wt a 
 
 
 a 
 
 Total . . .... 
 
 
 
 'Turns per coil to compensate for fall of pressure over internal resistance 
 = CTs = minus . . . . = . . 
 
 Total back ampere- turns = BTs 
 
 = C a *a ^10/360 = 
 
 {/a = armature current) (6 = , 
 
 and is the angle of lead in 
 degree ? ; take 2 for Grammes) 
 
 .Total excitation per coil: 
 ATs + CTs + BTs . . 
 
 (11 = number of coils) 
 
 SHUNT COIL WINDING. 
 Mean turn . . feet 
 
 Res. (Hot) mean turn ohms. 
 
 Res. (Cold) mean turn ( hms. 
 
 Gauge of wire S.W.G. 
 
 Shunt current 
 
 Convolutions . 
 
 layers of turns each. 
 
 Depth inches. 
 
 Weight per coil Ibs. 
 
 Total weight Ibs. 
 
 Res. per coil ohms. 
 
 Total resistance . , , ohms. 
 
58 ELECTRIC MOTIVE POWEE. 
 
 MAIN COIL WINDING. 
 How wound 
 
 Area of bare wire in. x in. = sq. in. 
 
 Dimensions over insulation in. x in. 
 
 Convolutions 
 
 Weight per coil Ibs. Total weight Ibs. 
 
 Resistance per coil oams. Total resistance ohms. 
 
 HEAT WASTE IN COPPER, 
 
 Watts in armature , . 
 
 ,, shunt coils 
 
 main coils 
 
 .Total watts 
 
 Pressure lost over internal resistance volts 
 
 Electrical Efficiency % 
 
 PROBABLE EISE OF TEMPERATURE. 
 FIELD : area of surface of each coil sq. in. 
 
 BHaottemp.-"* 100 - V 
 
 Surface 
 
 ARMATURE : area of surface sq. in. 
 
 (3,000ft P er| Riseof Watts 64 = FO 
 
 mia.) / Surface 
 
 These Tables show at a glance the most important details 
 of continuous-current dynamos or motors, and can be easily 
 adapted for alternate-current machines. They should be used 
 in conjunction with cross-sectional diagrams, similar to that 
 shown in Fig. 14, page 49. Special requirements for indi- 
 vidual designers can be incorporated without difficulty, and 
 small modifications will readily suggest themselves. 
 
PROPERTIES OF THE VARIOUS WINDINGS. 59 
 
 14. SERIES, SHUNT, COMPOUND, AND SEPARATE 
 EXCITATION. 
 
 It is of the utmost importance to appreciate fully the differ- 
 ences between the various methods of exciting the field-mag- 
 nets, as all are not equally well adapted for all kinds of work,, 
 some being specially suitable for one class, and worse than 
 useless for others. It is probable that the connections of the- 
 several windings are well known to most readers ; yet it will 
 serve to fix ideas and to give a firm grip of the various pur- 
 poses to which each type of machine may be most profitably 
 applied if the following sets of diagrams are carefully examined* 
 
 In each set the first figure shows simply the diagram of con- 
 nections, irrespective of the arrangement and number of the 
 field coils and poles. 
 
 The second figure refers to a dynamo wound as indicated in 
 the first figure, and shows the order of the curve connecting the 
 terminal, or omnibus bar pressure, with the main current. 
 
 The third figure refers to a motor wound in the same style, 
 and indicates roughly the shape of the curve connecting the 
 number of revolutions per minute with the main or armature 
 current. 
 
 In each case the dynamo speed is assumed to be constant, 
 and so also is the pressure at which current is supplied to the 
 motor except in the case of series winding* These conditions are 
 usually essential, and are conformed to, as nearly as possible, 
 in all central stations. 
 
 The diagrams are so explicit as to need little comment, 
 but a few suggestions may be useful. The pressure in series- 
 wound dynamos varies with the current, and these machines 
 are chiefly suitable for working on a fairly constant resist- 
 ance, such as a definite number of arc lamps or a fixed 
 number of incandescent ones ; they are more especially 
 adapted for the former work. They present one very ap- 
 parent advantage in the fact that, since the main current 
 passes through the field winding, the number of turns of 
 
60 
 
 CURVES OF SEBIES-WOUND MACHINES. 
 
 u 
 oc 
 
 
 
 
 
 / 
 
 MAIN CURRENT 
 
 15c. Mechanical Characteristic Curve of Series 
 Motor. Ab cissrc Proportional to Torque. 
 
 b 
 
 CO 
 (0 
 Ul 
 
 C 
 
 5 
 
 
 
 
 / 
 
 CL 
 
 H 
 
 z 
 
 ^ 
 
 SUPP 
 
 
 
 / 
 
 
 I- 
 (0 
 
 z 
 o 
 
 u. 
 o 
 
 
 / 
 
 / 
 
 
 o 
 
 
 
 __ 
 
 ^^ 
 
 
 
 
 3 O O O c 
 
 1 8 S S 2 
 
 NIIAI 
 
 ^> 
 
 
 
 => I i 
 
 o O 
 <O C S 
 
 S 
 
 S1TOA N 
 
 O 
 
 1VNIWW31 
 
 1 
 
CURVES OF SHUNT-WOUND MACHINE'S. 
 
 61 
 
 
 
 
 / 
 
 Ul 
 
 cc 
 
 
 
 
 MAIN CURRENT ARMATURE CURRENT 
 
 FlG. 16B. External Characteristic Curve of Shunt T?T^. thn Tvr^^v.or : n i nVmi-n/>fn<:^t;n Pnv-tro /-.f Slmnf 
 
 Dynamo. Motor. Abscissa) propot tional to Torque. 
 
 
 
 
 5 
 
 w 
 
 CO 
 Ul 
 
 x 
 
 
 
 
 
 
 Q. 
 
 H 
 Z 
 
 Q. 
 0. 
 
 3 
 (0 
 
 
 
 
 
 
 ONST/ 
 
 u 
 
 o 
 
 
 
 
 
 
 O 
 
 
 
 
 ? 
 
 3 />>* CO OJ 
 
 "NIW 3d '9A3H 
 
 c 
 S 
 
 s 
 3 
 
 3 
 S 
 
 
 CVJ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 /^~ 
 
 > 
 
 x^ 
 
 
 
 
 
 
 
 / 
 
 
 
 
 \ 
 
 
 
 
 I/ 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 \ 
 
 \ 
 
 
 / 
 
 
 
 
 
 
 
 \ 
 
 
 _J 
 
 
 
 
 
 
 
 
 \ 
 
 !< 
 
 " CO CM 
 
 SJ.1OA NI 3nsS3Ud bva sneiNwo 
 
62 ELECTEIC MOTIVE POWER. 
 
 wire on the magnets is comparatively small, and the wire is- 
 large, and therefore strong. High insulation resistance is 
 thus easily obtained without mechanical difficulty or the use 
 of costly material. For power purposes this winding possesses 
 special value. Consider Fig. 15c, which refers to a series motor 
 running at constant pressure. It will be seen that as the 
 current in the motor is increased the speed decreases ; and, since 
 the torque is proportional to a function of the current, it 
 follows that the torque varies inversely with the speed. Now, 
 look at the series dynamo curve, Fig. 15B. The pressure 
 increases with the current, which is exactly the condition of 
 supply to make the series motor run at a uniform rate of 
 speed. Therefore, if a series-wound dynamo be coupled to a 
 series-wound motor, and be driven at a constant speed, it will 
 supply current at a pressure varying directly as the current, 
 and the motor will tend to run at a constant speed. This is 
 such an important problem in the electric transmission of 
 power that it is referred to again,* and the conditions for 
 success are examined more closely. 
 
 Series motors are suitable for a variety of purposes, and are 
 generally used where a large torque is required at starting and 
 a fairly constant load obtains afterwards. When run off 
 constant-pressure circuits, they are regulated by a variable 
 resistance placed in series with them, which may be altered 
 by hand as occasion requires. 
 
 The feature of a series motor to be specially remembered is 
 that the torque is greatest at starting, when the current is a 
 maximum : this is so because the main current passing through 
 both field and armature coils then magnetises them to the 
 greatest possible degree. 
 
 Shunt-wound dynamos give variable current at fairly con- 
 stant pressure. They are, therefore, suitable for running loads, 
 such as banks of lamps, which require varying power ; and also 
 for running series-wound motors with constant loads, or shunt- 
 wound motors with varying loads. This is seen to be the case by 
 
 * See 23, especially pages 121 and 122. 
 
CURVES OF SHUNT-WOUND MACHINES. 63 
 
 comparing the shunt motor mechanical curve (Fig. 16c) with 
 the shunt dynamo curve (Fig. 16B). The shunt motor when 
 supplied at constant pressure maintains a fairly uniform rate 
 of rotation with varying load : the shunt dynamo approximately 
 conforms to the supply conditions. 
 
 The shunt motor is not so well adapted for starting 
 against a heavy torque as the series motor. For the shunt 
 field excitation cannot exceed that determined by the supply 
 pressure ; and a sudden rush of current through the armature 
 at starting tends to weaken the field magnetism unless the 
 brushes have a forward lead, when there will be considerable 
 sparking. When starting a shunt motor the field circuit 
 should be first closed, and time be allowed for the magnets 
 to become fully excited before the armature circuit is 
 closed. A starting resistance is always used in the arma- 
 ture circuit of these motors. After the armature has com- 
 menced to revolve the resistance may be gradually cut out 
 and the speed raised to its normal value. 
 
 The feature of a shunt motor to be specially recollected is 
 that it will run at nearly constant speed, with varying load, 
 when supplied with current at constant pressure. This is the 
 supply condition of nearly all central power stations, and hence 
 shunt motors have a special field in this connection. 
 
 As will be gathered from Fig. 17 A, compound winding i& 
 a combination of the shunt and series methods of excita- 
 tion. The shunt coils may be coupled across the brushes 
 in " short shunt," or across the mains in " long shunt " as in 
 Fig. 17A. The theory is as follows : The shunt coils give 
 an excitation proportional to the terminal pressure, and the 
 series coils an excitation varying with the armature or 
 the main current, according as the long or short shunt is 
 adopted. It is evident that, by suitably proportioning the 
 two windings, the external characteristic, Fig. 17B, can be 
 made to slope at any angle to the current line ; or briefly, 
 the pressure may be made to increase with the current, 
 and by compensating for the fall in the feeders, maintain 
 a constant pressure at the centre of distribution. The 
 chief difficulty with high-pressure compound dynamos is the 
 
64 CURVES OF COMPOUND-WOUND MACHINES. 
 
 
 
 / 
 
 
 UJ 
 
 
 
 ARMATURE CURRENT 
 
 G. 17c. Mechanical Characteristic Curve of Com 
 pound Motor. Abscissa) proportional to Torque. 
 
 (O 
 
 u 
 cc 
 
 UJ 
 
 
 /I 
 
 
 S 
 
 (O 
 t/3 
 U 
 
 cc 
 
 lj 
 
 Q_ 
 
 
 to 
 1 
 
 
 / * 
 
 
 a. 
 
 h- 
 
 z 
 
 Q. 
 CO 
 
 
 i 
 
 (0 
 
 
 /I 
 
 
 ONST/ 
 
 u. 
 
 
 
 
 
 
 / 
 
 
 o 
 
 
 
 3 O O O O O C 
 ^ 
 
 j "5 * co eu 
 
 NIIAI W3d 'SA3d 
 
 
 
 
 
 
 
 
 
 
 
 10 20 30 40 50 60 7 
 MAIN CURRENT 
 
 G. 173. Extrrnal Cliaract eristic Curve of Over 
 Compounded Dynamo. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 300 o o c 
 000 
 CO CM 
 
 SIIOA NI 
 
 ava snaiwwo 
 
 ^ 
 
PROPERTIES OF COMPOUND-WOUND MACHINES. 65 
 
 shunt windings, which consist necessarily of many turns of 
 fine wire, and give trouble from a variety of causes, besides 
 being costly. 
 
 The mechanical characteristic of the compound-wound motor 
 is shown in Fig. 17c. It will be seen that there are two methods- 
 of coupling up the coils, as illustrated in curves 1 and 2. In 
 the top curve the shunt and series excitations are differential,, 
 i.e., the series coils demagnetise the field-magnets ; in the lower 
 curve they are cumulative, and both act in the same direction. 
 The differential coupling is theoretically the better arrange- 
 ment, and gives a more constant speed than even a shunt 
 
 motor, as is apparent from equation (6)*E m = C a N a 10 ~ 8 ,, 
 
 60 
 
 where E m is the motor armature counter-electromotive force, 
 C a the number of armature conductors, N a the lines of force,, 
 and n the revolutions per minute. Since C a is constant, and 
 N a may also be regarded as such when a constant pressure acts 
 on the shunt coils, it is clear that the speed, n t varies directly 
 as the counter electromotive force, E m . And the armature 
 reactions do not affect the problem if we assume the armature- 
 current to have no effect on the value of N a . Now, 
 
 E m = e-*> a , 
 
 where r a is the armature resistance, and ^' a the current in the- 
 armature, and e the pressure of supply which is assumed 
 to be constant. It is clear that E ra varies inversely with i v 
 and, therefore, the shunt motor tends to slow as the current 
 increases (for the speed is proportional to E m ). Referring 
 again to equation (6), it is seen that to prevent this falling 
 off in speed it is necessary to decrease the lines of force,, 
 N a , in the same proportion as the fall of counter-electro- 
 motive force. This is most readily accomplished by decreasing 
 the excitation, either by intercalating resistance in series with 
 the shunt circuit, or by adding compound series coils wound 
 in an inverse sense to the shunt turns. It should be noticed 
 that the armature reactions always weaken the main magnetic 
 field, and therefore tend to make a shunt-motor run at a. 
 constant speed. It is thus possible to obtain nearly a straight 
 line speed regulation with a well-designed shunt motor. It is- 
 
 * >SVe page 18. 
 
36 ELECTRIC MOTIVE POWER. 
 
 also evident that a motor armature should have a very low 
 internal resistance, the lower the better. 
 
 The cumulative method of winding is more frequently used 
 than the differential, for although it obviously does not 
 regulate so well as a simple shunt motor, yet it combines to 
 some extent the starting power of the series winding with 
 the speed regulation of the shunt. Sometimes a few series 
 turns are wound on large shunt motors, and are used only at 
 starting, being short-circuited as soon as the armature is fairly 
 under weigh. 
 
 Broadly speaking, differentially-wound compound motors do 
 not seem to be a success, and, except for the few cases where 
 absolute regularity of speed is required, they certainly have 
 not found favour. One great objection to them is a liability 
 to start in the wrong direction owing to the reversed series 
 winding. This difficulty is met to some extent by short- 
 circuiting the series coils at starting. Cumulative compound 
 motors have also a very limited field of usefulness, well-designed 
 *hunt machines being more efficient, and giving better speed 
 regulation. Yet there are some few applications where this 
 winding will meet the conditions better than any other. 
 
 The feature of compound winding to be specially noted is 
 that, by applying it to the generator, the pressure at a distant 
 point can be maintained (within limits) automatically constant 
 with varying current. 
 
 The idea of separate excitation is so easily grasped that 
 there is no need to do more than refer to Fig. 1 SA, which is 
 a diagram of the connections, the top scrolls representing the 
 field-magnet coils. It is obvious that the number of turns on 
 the field-magnets and the gauge of wire can be varied to suit the 
 exciting current so long as the necessary ampere-turns are pro- 
 Tided. In Fig. 18s the external characteristic of a separately- 
 excited dynamo is shown at curve 3. It will be noticed that the 
 pressure falls as the output is increased, owing to the volts lost 
 over the internal resistance of the armature. To obviate this 
 the excitation must be proportionately augmented, so as to give 
 urve 2. And if it be desired to compensate for the fall of 
 
CURVES OF SEPARATELY-EXCITED MACHINES. 67 
 
 8 
 
 'SA3H 
 
 
 
 *., -2 
 
 a 'a 
 
 .55 
 
 
 
 S110A Nl 3tiHSS3b!d dV9 SHQINWO 
 
 -2 j? 
 
68 ELECTRIC MOTIVE POWER. 
 
 pressure in feeders, then the exciting current must be still 
 further increased until the slope of curve 1 suits the required 
 conditions. The regulation can be accomplished automatically 
 by some mechanical device controlled electrically ; but it is- 
 usual in central stations, where this type of machine is most 
 frequently used, to alter the excitation by hand. 
 
 The separately excited motor is not yet much in use, but it 
 will steadily grow in favour as transmission of power by elec- 
 tricity becomes more general. The mechanical characteristic 
 of this type of motor, supplied at constant pressure, is shown in 
 Fig. ISc. The fall of speed with increase of load can be com- 
 pensated by decreasing the excitation either by hand regulation 
 or reversed series coils. 
 
 These motors are specially adapted for large powers, because, 
 since the main current only passes through the armature, a 
 high pressure can be safely used ; and moreover, the speed 
 regulation is much superior to that obtainable with the 
 series-winding. The objection is the independent source of 
 power for the exciting current, which will probably have to- 
 be supplied in most cases from accumulators. Yet it must be 
 recollected that a small shunt-dynamo, driven by the large 
 motor, will not only charge the cells, but also excite the motor 
 field-magnets after the proper speed is once attained ; and, 
 further, this dynamo is available for lighting during the period 
 the motor is at work. 
 
 The advantages of separate excitation are : firstly, the dynamo 
 fields can be excited at a low-pressure, and hence are free from 
 insulation troubles ; secondly, lightning discharges are less 
 likely to affect the magnet coils, since they are not connected 
 with the line circuit (unless reversed series coils be used) ; and, 
 thirdly, the pressure at any distant point can be easily varied 
 at will. With motors a high-pressure service is permissible, 
 and the speed can be regulated either by the dynamo or motor 
 exciting current. 
 
CHAPTER III. 
 
 THE LINE AND THE DISTRIBUTING MAINS. 
 
 15. Definition of the " Line." 16. The Theoretical Considerations 
 affecting the Loss in the Conductor. 17. The Conductor in Practice: 
 Aerial Lines. 18. Insulated Cables. 19. Insulators and Details 
 of Aerial Lines. 20. Underground Lines ; Conduits ; Built-in and 
 Drawing-in Systems. 21. Lightning Discharges ; Lightning Con- 
 ductors ; and Lightning Arresters. 
 
 15. DEFINITION OF THE " LINE." 
 
 BY the "line" is here understood not only the conductor, but 
 also the posts or culverts, insulators and protective devices. 
 Treated in this way, the subject may be regarded from two 
 distinct points of view : one relating only to the method of 
 erecting and protecting the conductor, and the other having 
 reference to the kind of material, to the area of cross section 
 of the wire, and to the conditions which limit the distance 
 through which a definite quantity of power can be profitably 
 transmitted. 
 
 Practice usually precedes theory, and the first attempts at 
 electrical transmission of power were made without any definite 
 conception of the governing laws. An ordinary galvanised iron 
 telegraph wire was the handiest conductor when Marcel Deprez 
 made his memorable experiments between Munich and Miesbach 
 in 1882. And even to-day considerations of convenience or 
 necessity often invite, or compel, the use of material which is 
 not exactly what would be chosen if the engineer had a free 
 choice, and cost were no object. 
 
 It will be more in keeping with the fitness of things, how- 
 ever, if the theoretical conditions are first discussed. 
 
70 ELECTRIC MOTIVE POWER. 
 
 16. THE THEORETICAL CONSIDERATIONS AFFECT- 
 ING THE LOSS IN THE CONDUCTOR. 
 
 The two conditions absolutely essential in the line circuit 
 are continuity and insulation, i.e., the line must consist of a 
 conducting substance or several conducting materials carefully 
 joined together, and must be so arranged that at least one of 
 the circuits between the dynamo and motor is insulated. The 
 earth, under certain conditions, may be used as part of the 
 circuit if sufficiently good " earth-plates " are provided at each 
 end of the metallic conductor. This is the case with sub- 
 marine cables and land telegraphs, and is oftentimes so with 
 telephone circuits. An "earth return," as it is generally called, 
 presents serious objections with relatively large currents at high 
 pressures ; and hence, with but few exceptions, existing power 
 plants are designed with insulated return circuits. A compromise 
 between the two methods is found in a continuous metallic 
 uninsulated "return," as used with electric street railways, in 
 which the metals are coupled by copper connections, and so 
 form a continuous conductor. In this case, the action of the 
 current is not confined to the iron rails, but considerably affects 
 the adjacent earth ; so that telegraph and telephone lines, using 
 the " earth " in the immediate neighbourhood, may suffer con- 
 siderable disturbance. A purely practical objection to the use 
 of the earth as part of the circuit, or to an uninsulated wire 
 for any portion of the line, is that the leakage may be largely 
 increased and the difficulties of handling the line and plant 
 intensified. Street railway practice has hitherto been limited 
 to a maximum pressure of 500 volts, and it is probable that 
 300 volts will be more generally used in the future. At any 
 rate, in Great Britain the Board of Trade regulations restrict 
 the pressure to this limit. 
 
 In mining work the author has selected 500 volts as the 
 most convenient pressure for general purposes, as it permits 
 the transmission of considerable power without an excessive 
 outlay in copper, and yet is not dangerous in the event of 
 accidental " shocks." The use of an uninsulated separate 
 return is particularly unsafe in mining, owing to the serious 
 risk of fire which is inevitable with such a circuit.* Even 
 
 * This remark does not refer to a concentric cable with the outer con- 
 ductor earthed, as used in the Andrews or Fowler-Waring systems. 
 
LOSS IN THE LINE. 71 
 
 with pressures as low as 100 volts, both mains should be care- 
 fully insulated. 
 
 It may be granted, then, that the practice of the future, 
 excepting in tramways and railways, will demand an insulated 
 lead and return, and that the whole of the circuit, including 
 dynamos, motors and switches, will be insulated in the most 
 careful and thorough manner. 
 
 The simplest case of transmitting power, and one that often 
 occurs, is where there are only two machines, a dynamo and 
 a motor. The circuit in this case is obviously a lead and 
 return of the same diameter throughout, since the current is 
 the same at all points (neglecting the small loss from leakage). 
 The line may be an insulated cable buried in a culvert ; or a 
 bare copper wire carried by insulators on poles a so-called 
 aerial line or it may combine the two methods, being partly 
 buried, or cased-in, and partly in the open air. Yet since the 
 diameter is the same throughout the circuit, the resistance of 
 the copper per unit length will be constant. Now, for this 
 simple case what is the best area of conductor to carry a given 
 current or to transmit a given quantity of power if the pres- 
 sure be not fixed 1 It is oftentimes not easy to give a definite 
 answer to this apparently simple query, as so much depends 
 upon the result desired. Is the prime power at disposal practic- 
 ally unlimited 1 If so, the line loss may be large. What is the 
 cost of power at the generator 1 If it be cheap, then the line 
 loss may be high ; but if it be dear (although, perhaps, plenti- 
 ful), the waste in the line must be restricted. But this limita- 
 tion of loss will mean a corresponding increase in the weight of 
 the copper, and copper is costly. So the answer is practically 
 a compromise, and each case must be settled on its own merits. 
 Lord Kelvin as early as 1881 pointed out that a general solution 
 is obtained by equating the annual cost of the horse-power lost 
 in the cable to the interest on the capital invested in the line 
 and supports, including the cost of erection, plus the annual 
 cost of maintenance. But this condition does not always meet 
 all the practical requirements. Indeed, the size of the con- 
 ductor is often fixed between narrow limits by the simple 
 conditions of permissible fall of pressure and mechanical con- 
 siderations. Theoretically, the higher the supply pressure the 
 
 G2 
 
72 ELECTEIC MOTIVE POWER. 
 
 smaller the conductor ; but prudence, to say nothing of Board of 
 Trade regulations, limits the pressure, and copper circuits are 
 not often used smaller than, say, No. 10 or No. 12 S.W.G., or 
 equivalent cross section in stranded wires, from considerations 
 of strength and durability. And an excessive outlay in copper 
 is prevented not only by the price of the cable, but by the 
 increased expense in the erection of the line. The latter objec- 
 tion has less weight when the conductor is laid in the ground, 
 for it is practically as cheap to lay a large cable as a small 
 one ; and large mains are invariably run underground. 
 
 Prof. G. Forbes, in his Cantor Lectures, 1885,* showed that 
 Lord Kelvin's law is most accurately expressed in the following 
 form : The most economical area of conductor is that for 
 which the annual value of the energy lost in the copper is 
 equal to the annual charges on the total cost of the line (includ- 
 ing erection, &c.), minus that part of the capital outlay which 
 is independent of the area of the conductor. Or, in other words, 
 the annual value of the i^R loss must be made equal to the 
 annual interest on that portion of the total capital which may 
 be considered proportionate to the sectional area or weight of 
 the copper. 
 
 Symbolically, this may be written 
 
 where p = the annual cost of one E.H.P. at the dynamo 
 
 terminals, 
 
 R = the resistance of the line, 
 
 i m = the equivalent or average current in the conductor, 
 Cj = the total cost of line, 
 
 C 2 = part of capital proportionate to area of conductor, 
 C 3 = part not proportionate to area, 
 k = the rate per cent, of annual interest to cover total 
 charges in the several capital sums, C 15 C 2 , C 3 . 
 
 The equivalent current, i m , must be calculated from the 
 estimated load curve, and it is by no means easy to predict 
 
 * The student should carefully peruse these Lectures, especially the 
 tables formulated to facilitate the calculations. They appeared iu The 
 Electrician, Vols. XV. and XVI., and are published iu pamphlet form 
 by the Society of Arts. 
 
LOSS IN THE LINE. 73 
 
 its value. The annual heat waste in the mains may be written 
 i^ KT, where T is the number of hours in a year (8,760 hours, 
 say), and i m the equivalent current. And suppose it is esti- 
 mated that in one year a current of value i l will be required 
 for, say, t l hours, a current of i^ for t 2 hours, &c.; then the 
 total waste is equal to the summation of the quantity of 
 energy spent severally in the periods t lt t v 3 , &c. Whence 
 
 T 
 
 It is thus evident that there is likely to be a difficulty in find- 
 ing the value of the equivalent current, and yet on the accurate 
 determination of this quantity depend the estimate of annual 
 heat waste and the selection of the economic ratio between 
 current and area of conductor. 
 
 The equation for determining the section of the copper for 
 the predetermined current is usually given in some such form 
 as the following : 
 
 . Iwpl 
 
 a=lm j~cnc 
 
 \/ <u 2 /C 
 
 <> 
 
 where a the area of conductor in square inches, t m , k, p t T, 
 and C 2 have the values already given to them here, and w is a 
 constant depending on the energy wasted in the conductors. 
 
 If the current be constant (or nearly so, as in a pumping 
 plant), the economic area is most readily determined from a 
 modification of (21), which takes into direct consideration the 
 load factor. This equation, (21 A), is due to Mr. E. Tremlett 
 Carter. It may be written 
 
 1-168 C / (2lA) 
 
 The new symbols are : 
 
 W = the watts spent in one yard of copper, one square inch 
 in cross section, at a density of one ampere per square 
 inch. This is a constant for commercial samples of 
 copper, and equals 2'56 x lO^ 8 . 
 
74 ELECTRIC MOTIVE POWEE. 
 
 F = the load factor expressed as a rate per cent. 
 c = the cost of additional copper per square inch per yard 
 length in sterling. 
 
 The values of c used by the author are given in Table El. 
 
 Table El. Values of "c" for different costs of laying one yard 
 of additional copper one square inch in section. 
 
 Copper, including additional cost 
 of laying, at 
 
 Corresponding values of c in 
 sterling. 
 
 d. 
 
 
 6 per Ib. = 56'0 per ton. 
 
 0-2900 
 
 7 = 65-3 
 
 0-3383 
 
 8 
 
 , - 74-6 
 
 0-5866 
 
 9 
 
 = 84-0 
 
 0-4349 
 
 10 
 
 = 93-3 
 
 0-4837 
 
 11 
 
 -102-7 
 
 0-5316 
 
 12 
 
 -112-0 
 
 0-5800 
 
 13 
 
 = 121-3 
 
 0-6287 
 
 14 
 
 -130-6 
 
 0-6766 
 
 15 
 
 = 139-9 
 
 0-7249 
 
 16 
 
 = 149-2 
 
 0-7732 
 
 17 
 
 = 158-5 
 
 0-8215 
 
 18 
 
 = 168-0 
 
 0-8700 
 
 19 
 
 = 177-3 
 
 0-9187 
 
 20 
 
 = 196-6 
 
 0-9666 
 
 It should be noticed that (21 A) is applicable to constant 
 pressure circuits with variable currents if F be taken as 100 
 and i m be determined from (20). 
 
 When the pressure and current vary approximately in the 
 same ratio, (21 A) is applicable, if the equivalent current be 
 determined from (20) ; and is specially useful when only two 
 machines, both series wound, are used. 
 
 There are three separate difficulties to be overcome before 
 the above conditions can be fulfilled, even assuming that prac- 
 tical and financial requirements necessitate no modifications. 
 In the first place, what is the annual cost of an electrical horse- 
 power? This is by no means easy to predetermine, judging 
 from the wide limits of figures recently published in the reports 
 of Central Lighting Stations. Secondly, it is not easy to esti- 
 mate with certainty how much of the total capital outlay is 
 
LOSS IN THE LINE. 75 
 
 independent of the area of the conductor. And, thirdly, the 
 determination of the equivalent current presents such difficulties 
 as to make this quantity indeterminate in many cases. Further- 
 more, the question of fall of pressure in the conductor is not 
 taken into consideration in the equation. 
 
 The cogency of the above reasoning will be more easily 
 appreciated after studying some of the theoretical considerations 
 which affect the problem. Transmission of power may usually 
 be classed under one of the following heads : 
 
 (a) Definite loss in line and fixed quantity of power at motor 
 terminals. This usually implies that the power at the gene- 
 rating station is either costly or limited, or sometimes that 
 only a definite horse-power can be profitably spent at the 
 motor. 
 
 (b) Unrestricted loss in the line and fixed quantity of power 
 at motor terminals. Here the prime energy is cheap and un- 
 limited, and only a portion is required for use. 
 
 (c) Fixed quantity of power at the generating station and as 
 much at the motor as possible. In this case the line loss must 
 be made a minimum, and the weight of copper as great as the 
 capital at disposal will permit. 
 
 The limiting conditions of pressure, current, weight, cross 
 section and length of conductor, are determined by relatively 
 simple laws. 
 
 In (a) the loss in the line, i^R, and the energy at the motor 
 terminals, i m e 1? are fixed. Therefore the sum of these two 
 quantities that is, the output of the dynamo is also fixed. 
 Let this be i m e. 
 
 Now, the resistance of the line, R, will vary directly as the 
 length and inversely as the area of cross section of the cable ; 
 and therefore 
 
 where d is the diameter, w the weight of the copper wire, and 
 k a constant. And by the assumed conditions of transmission, 
 
70 ELECTRIC MOTIVE POWER. 
 
 i* Ik 
 
 i m e = i m e l -{- =a constant. It is evident that the value 
 
 d 2 
 
 of this quantity may be kept the same with considerable 
 variations of i, e, I, and d that is, with alterations of current 
 and pressure, and with different lengths and gauges of wire. 
 
 In (b) the line loss is assumed to be unlimited, and therefore 
 the dynamo output may be made as large as convenient. The 
 size of conductor may be determined simply by the maximum 
 pressure deemed advisable and the condition of a definite 
 quantity of power at the motor terminals. 
 
 In (c) the power of the generator is fixed, and that of the 
 motor is to be as large as possible ; therefore, the line loss must 
 be as small as consistent with an economic outlay in copper for 
 the conductor. Practically the pressure selected should be as 
 high as the peculiar circumstances permit, and then as much 
 money should be spent in copper as can be spared for this 
 part of the plant. 
 
 If the generating station be situated in the middle of the 
 line and power be distributed equally along each branch of the 
 circuit, the weight of copper will be one-fourth what it would 
 be if the station were at one end. With several supply stations, 
 the weight of copper will vary inversely as the square of the 
 number of stations, assuming they are properly placed, i.e., at 
 equal intervals with reference to the load distribution. And 
 the pressure required will vary inversely as the number of 
 stations. 
 
 These few general statements of the laws governing power 
 transmission and distribution by electricity show distinctly 
 two facts : firstly, that a high pressure is necessary for econo- 
 mical transmission ; and secondly, that for distribution a 
 number of stations is advisable to minimise weight of 
 copper and line loss. But it must also be recollected that 
 every additional power station involves increased outlay for 
 land and buildings and an additional staff for the control and 
 working ; therefore it will not be expedient to lay down a new 
 station from considerations which simply refer to copper and 
 line loss. But it is evident that the first station should be 
 
LOSS IN THE LINE. 77 
 
 placed as near the centre of the district as convenient, and that 
 the subsequent erection of inter-dependent or independent 
 stations will have to be determined by the load factor* and 
 the centre of gravity of the load with reference to the station 
 and mains. 
 
 It has already been stated that the most simple case of 
 transmission of power is where the circuit simply joins up a 
 pair of machines. But no practical difficulty, as far as the 
 line is concerned, is involved if a number of motors, in close 
 proximity, be supplied from one distant power station ; for the 
 whole of the current will be carried by the main conductors. 
 Yet, it is evident that there must be one particular gauge of 
 wire which will give the most economic results for the average 
 current ; and this will be the case when Lord Kelvin's rule in 
 a modified form is adhered to. 
 
 Various attempts have been made to give a general expression 
 for the current in terms of area of wire, cost of horse-power at 
 generator, &c., and it is noteworthy that the term for the length 
 of the line cancels out of the final equation. This implies that 
 the length of the circuit does not affect the problem a state- 
 ment only true when the economic law is adhered to. The 
 length of the line does not affect the economic area of a 
 conductor for the given current, but it obviously affects the 
 power lost in the line. The length of the line is always 
 understood in the terms expressing the resistance of the 
 circuit and the area of the conductor for, mathematically, 
 the line may be defined, both as regards its length and weight, 
 by the resistance of the circuit and the gauge of the wire. 
 This is done in the usual equations, and hence the difficulty. 
 Profs. Ayrton and Perry, in March, 1886, read an important 
 Paper on "Economy in Electrical Conductors,"! showing that 
 Lord Kelvin's law holds good only when the current is fixed, 
 and is not taken as an independent variable in the equation 
 for determining the area of conductor. 
 
 * The load factor is the ratio of the average output to the maximum 
 output of the plant, and is usually expressed as a percentage. For central 
 lighting stations the load factor is found to vary from about 10 per cent, 
 to 15 per cent. With power plants it will generally be much higher, and 
 in collieries, &c., may reach as high as from 80 to 90 per cent. 
 
 t Jour. Proc. Institution Electrical Engineers, Vol. XV., p. 120. 
 
78 ELECTRIC MOTIVE POWER. 
 
 Mr. Kapp, in his Cantor Lectures,* gives the subject close 
 attention from the transmission of power point of view, and 
 takes as a basis the elementary case of one generator and one 
 motor. (The case of several generators at one power station 
 feeding into a single pair of mains and delivering energy to 
 a group of motors lying near to each other at the end of the 
 line is practically the same.) He shows that a complete 
 solution requires the following conditions to be taken into 
 account : 
 
 Annual value of brake horse-power at generating station. 
 
 Pressure of supply at generator terminals. 
 
 Brake horse-power required at motor. 
 
 Length of line. 
 
 Capital outlay per ton of copper erected. 
 
 Cost per electrical horse-power at dynamo terminals. 
 
 Interest and depreciation of the whole plant. 
 
 It is important to note that Mr. Kapp takes into considera- 
 tion the pressure of supply ; in fact, he fixes the pressure and 
 finds the best current for the given conditions. And he shows 
 that in no case will it be economical to lose more than half the 
 total power in the line, and that for every transmission problem 
 there is one pressure for which the annual cost of a brake 
 horse-power at the motor is a minimum. 
 
 This conclusion was foreshadowed in one of the questions asked 
 a few pages back, " What is the best area of conductor to carry 
 a given current if the pressure be not fixed?" But, as already 
 suggested, the limitations of the Board of Trade or other con- 
 siderations practically fix the pressure of supply in most cases, 
 and the problem consequently narrows itself to the relative 
 weight of copper for the predetermined current, and the real 
 variable quantity is the amount of energy to be wasted in 
 the line. Under these assumptions, by a few trial-and-error 
 calculations, a gauge of conductor may be selected to permit 
 such an average heat waste that the approximate annual cost 
 of the energy lost shall be as nearly as possible equal to the 
 interest on the capital outlay in the copper (including erection) 
 and maintenance of the line. 
 
 * Cantor Lectures delivered before the Society of Arts, February, 1891. 
 
AERIAL LINES. 79 
 
 Exception may be taken to this treatment of the subject; 
 yet it accords with practice. Local circumstances have 
 such weight that no hard-and-fast rule can possibly meet 
 the requirements of every case. And the indeterminate 
 character of some of the quantities of the problem, such 
 as the equivalent current and the cost of an electrical horse- 
 power, on the constancy of which the truth of the equations 
 depends, renders a general mathematical solution inferior to a 
 determination based on a practical knowledge of the require- 
 ments of the particular case, and a simple application of 
 Kelvin's law to select the gauge of wire and heat waste for 
 a given current. This method does not necessarily give a 
 minimum cost per brake horse-power of the motor ; but if the 
 steam (or turbine) plant and the electrical machines be of good 
 make and properly proportioned for the average load, the best 
 results will be very nearly realised. 
 
 In concluding this section on the theoretical conditions 
 affecting the line loss, it is noteworthy that the simple condition 
 of fall of pressure may be of paramount importance, and hence 
 may alone determine the gauge of wire and consequent copper 
 loss. This is specially likely to be the case in the distri- 
 buting network when the current and pressure are fixed. 
 Yet the author wishes to emphasise the importance of the 
 economic law, and to state that whenever circumstances admit 
 of its application its requirements should be carefully satis- 
 fied, A competent engineer will attend to this in a thorough 
 manner. 
 
 17. THE CONDUCTOR IN PRACTICE AERIAL LINES. 
 
 Proceeding to practical requirements, the first consideration 
 is the metal for the conductor. In selecting this, the main 
 consideration is all-round cost i.e., which metal will be the 
 cheapest, taking into account the predetermined heat waste 
 and conditions of erection and insulation. To guide the choice 
 it is necessary, first of all, to know the specific resistance of 
 the metals usually sold in the form of solid or stranded wire. 
 Copper, or its alloys, and iron and steel are practically the only 
 materials at present available for the purpose. 
 
80 
 
 ELECTRIC MOTIVE POWER. 
 
 The resistance to the passage of an electric current in a 
 wire of unit length and of unit area of cross section is denned 
 as the specific resistance of the substance. The quantity thus 
 designated gives a ready means of comparing the relative 
 electrical suitability of the different metals. 
 
 Table F. 
 
 Metal. 
 
 Sp. resistance 
 in microhms. 
 
 Specific 
 gravity. 
 
 Breaking 
 strain in Ibs. 
 per sq. inch. 
 
 Price per 
 Ib. in 
 pence. 
 
 Copper, annealed 
 
 1-615 
 
 8-8 
 
 
 
 8-5 
 
 Copper, hard-drawn .. 
 
 1-642 
 
 8-9 
 
 56,000 
 
 8-5 
 
 Bronze (tin and copper) 
 *Silicium bronze 
 Phosphor bronze 
 Aluminium bronze 
 
 4-42 
 1-7 
 
 } 3-0 to 60 
 
 I 8-4 to 8-7 
 
 84,000 
 ( 65,000 
 
 10-0 
 
 ) 8-5 
 \ to 
 ) 12-0 
 
 Iron, ordinary, soft ... 
 Iron, galvanised, killed. 
 
 9-827 
 10-0 to 11-0 
 
 } 7-6 to 7-8 
 
 / 47,000 to 
 \ 85,000 
 
 } i-o 
 
 Steel, ordinary, soft ... 
 
 14-5 to 15-0 
 
 7-8 to 7-9 
 
 135,000 
 
 1-5 
 
 Steel pianoforte wire... 
 
 16-0 to 16-5 
 
 7-75 
 
 335,000 
 
 2-0 
 
 Manganese steel 
 
 73-0 
 
 7-8 
 
 110,000 
 
 3-0 
 
 The above figures are necessarily only approximate. The specific resist- 
 ance determinations have been made at 32F. 
 
 In Table F. are arranged the copper alloys and various other 
 metals in the order of their specific resistances ; the specific 
 gravity, breaking strain in Ibs. per square inch, and the ap- 
 proximate price per Ib. of the several metals are also given for 
 rough comparison. It will be seen that iron has nearly seven 
 times the specific resistance of copper, and so for a given resist- 
 ance of line, the area of an iron wire will have to be, say, seven 
 times that of a copper one. The specific weights are in the 
 ratio of 8-9 : 7 '8, and the weight of the iron line, relative to 
 that of the copper line, will be expressed by 
 
 7 W ?, or 6-18 W, 
 
 O * / 
 
 where W is the weight of the copper in Ibs. And the costs 
 will be respectively 8 '5 W for the copper, and 6 '18 W x 1 for 
 
 Per sq. in. 
 
 * Class A, 97% conductivity of pure copper, tensile strength ... 65,0001b. 
 Class B, 80% ... 90,0001b. 
 
 Class C, 45% ... HO.OOOlb. 
 
IRON VERSUS COPPER. 81 
 
 the iron. A bare copper line will, therefore, cost about 27 per 
 cent, more than an electrically-equivalent iron wire. From 
 the simple consideration of first cost of conductor, then, it 
 appears that iron is superior to copper. In relation to the 
 total line cost, however, the reverse of this is the case. For 
 the weight and breaking strain have to be taken into considera- 
 tion. The breaking strain of hard-drawn copper of 98 per 
 cent, conductivity is about 56,0001b. per square inch ; whereas 
 that of well-killed galvanised iron is about 85,0001b., so the 
 spans may be made longer with iron wire in the proportion of 
 85 to 56; or, say, 50 per cent. Yet, since the iron wire is 6'18 
 times as heavy as the copper, the supporting devices must be 
 correspondingly stronger. The figure of merit of a metal per se 
 may be expressed as the product of the reciprocals of the specific 
 resistance, the specific weight and price multiplied by the break- 
 ing strain the best metal giving the largest figure of merit. 
 Thus, for copper 
 
 o ^ x o x x breaking strain . . . (22) 
 Sp. R Sp. W Price 
 
 x^xA-x 56,000 = 450. 
 
 1-642 8-9 8-5 
 
 For iron -^ x ^ xj x 85,000 =1,040. 
 
 From these general considerations, galvanised iron appears 
 to be, roughly, about twice as well adapted for overhead lines 
 as hard-drawn c6pper. But this proportion is not maintained 
 when all the conditions of the problem are taken into considera- 
 tion. In the first place, the greater bulk of the iron is objec- 
 tionable on account of the expensive posts and attachments 
 necessary to carry the weight ; and secondly, iron, although 
 galvanised,* rusts, and has a shorter life than copper ; and so the 
 cost of maintenance is far greater than with copper or its alloys. 
 Again, old iron wire has practically no marketable value, whereas 
 copper is always saleable. As regards total prime cost of line 
 it is not easy to say which metal is the cheaper for all cases ; 
 but generally for light work, such as telegraph and telephone 
 
 * Galvanised iron wire is usually served with linseed oil before erection. 
 
82 
 
 ELECTEIC MOTIVE POWER. 
 
 lines, there is very little to choose between the two. With 
 heavy power lines, however, copper will always work out the 
 cheaper (unless the relative costs be very different from those 
 assumed here), for the weight of the iron becomes practically 
 prohibitive. And even with large copper cables of, say, 19 
 strands each of No. 16 S.W.G. (and larger) the supports have to 
 be so frequent as to form a very important item in the cost of 
 the whole line. In such cases it is usual to suspend the copper 
 conductor from a small steel wire carried on insulators (see 
 Fig. 19) ; by this means the spans can be materially lengthened, 
 and the total cost largely decreased. 
 
 If, however, the conductor is insulated, copper is by far the 
 cheaper metal, since the necessary bulk of insulation material is 
 
 FIG. 19. Cable Hanger and Bearer Wire. 
 
 much less. The areas of the two metals are for equal resistance 
 as 1 I 7, say, and the diameters will be as Ji I ^7, or 1 ; 2 '6 3 ; 
 and for an equal thickness, t, of insulation, the volumes of insu- 
 lation material will be as (1 + 2 ) 2 - 1 2 1(2-63 + 2 *) 2 -2'63 2 . 
 But since to maintain the insulation resistance constant 
 per unit length of conductor it is necessary that the depth 
 of the dielectric vary as the diameter of the conductor, the 
 diameter of the iron cable over the insulation will have to be 
 
 2-63 + 2 t ?^? = 2-63 + 5-26 t. To illustrate the meaning of 
 
 these ratios let the copper conductor be No. 10 S.W.G., and let 
 = 0-lin. Then the diameter of copper conductor = 0'128in., 
 and the diameter over the insulation = 0'328in. The diameter 
 of iron conductor = 0'336in., and the diameter over the 
 
SILICIUM BRONZE. 83 
 
 insulation = 0'862in. Now the insulating materials generally 
 used, such as pure rubber, vulcanised rubber, &c., are costly, 
 and, therefore, the iron conductor is commercially impossible. 
 
 These comparisons have been made between iron and copper, 
 because they are the metals most likely to be used for power 
 circuits. The copper alloys, however, present many advantages 
 and combine the high conductivity of copper with the tensile 
 strength of the best iron. Bronze wires are in general use for 
 telephones and telegraphs, and will inevitably come into favour 
 for transmitting power. Silicium bronze has a conductivity 
 about the same as that of hard-drawn copper and a breaking 
 strain of at least 90,0001b. to the square inch. Bearing in 
 mind the relative weights for a given resistance, it is evident 
 that the spans can be made longer with the bronze than with 
 galvanised iron ; in practice, it is found that they may be made 
 about 50 per cent, longer, and less bulky and costly insulators 
 are necessary. One reason why the copper alloys have not 
 been used much as yet is a real or fancied difficulty in pro- 
 curing a wire of a specified resistance and tensile strength, it 
 being found that one of these qualities is frequently gained at 
 the expense of the other. This difficulty is merely a detail of 
 manufacture, and will, no doubt, soon be overcome. The cost, 
 from about 8'5 to 12 pence per lb., is high, but will be reduced 
 as the demand increases. 
 
 The figure of merit of silicium bronze (Class A), as previously 
 estimated for hard-drawn copper and iron, is 
 
 ^x^x 65,000 = 517. 
 
 It is better than copper by about 12 per cent.* It should 
 be noticed that the atmosphere has practically no effect upon 
 copper or its alloys, and that the alloys are not so liable to 
 crystallise as hard-drawn copper. These qualities largely reduce 
 the cost of line maintenance, and in conjunction with the 
 other points already discussed render silicium bronze perhaps 
 the best all-round metal for power lines. 
 
 * For the comparative data of Classes B and C, see foot-note, p. 80. 
 
84 ELECTRIC MOTIVE POWEE. 
 
 18. INSULATED CABLES. 
 
 The materials used for insulating electric wires are not very 
 numerous, being chiefly pure or vulcanised rubber, rubber 
 compounds, resins, and hydrocarbons, such as the paraffins, bitu- 
 mens, &c. ; vulcanised heavy hydrocarbon oils have also been 
 tried with some success. The covering on the cables has 
 usually two functions of entirely different characters. That 
 part nearest the wire is strictly the electric insulator or 
 dielectric ; while the outside portion simply serves as a 
 mechanical protection to the real insulator, and is not neces- 
 sarily a non-conductor of electricity. 
 
 It follows then that the insulating properties depend not on 
 the total thickness of the covering, but only on the nature 
 and depth of the dielectric. The insulation resistance varies 
 with the character and quality of the material used, with 
 the temperature, and even with the pressure at which the test 
 is made. It is usual to define the insulation resistance of 
 cables and wires in megohms per mile of cable at 60F., but 
 the pressure at which the test is made is not often given. 
 This, however, is important, and a pressure bearing a suitable 
 proportion to that at which the cable will be worked should 
 always be used in the test. Thus, for a cable to be used 
 in a colliery at 500 volts the testing pressure should be not 
 less than 1,000 volts, which gives a factor of safety of four, 
 assuming the insulation resistance to be inversely proportional 
 to the squares of the two pressures. It is also usual to immerse 
 the cables in water at 60F. for 48 hours before making the 
 test. These precautions are sufficient for the short lengths of 
 conductors used for most power plants, but if the cable be 
 many miles long it is necessary to make capacity tests, espe- 
 cially if it be armoured by a continuous metal sheathing of 
 lead or steel ribbon. This is most important with alternating 
 currents, since then the condenser effects may cause very 
 unexpected results. But this will not be a serious question 
 in general power work, and may be neglected if continuous 
 currents be used. 
 
 It is difficult to place the various dielectrics in order 01 
 absolute merit, but their specific resistances have been deter- 
 mined with close accuracy, and form a rough guide. 
 
SPECIFIC RESISTANCES OF DIELECTRICS. 85 
 
 The specific resistance of some of the dielectrics in common 
 use are given in the following table, being compared with that 
 of mica : 
 
 Table G. Specific Resistances of various Dielectrics compared 
 with that of Mica. 
 
 Insulator. 
 
 Specific resistance 
 compared with mica. 
 
 Temperature at 
 which the determi- 
 nation was made. 
 
 Mica 
 
 ro 
 
 68 F 
 
 Callender's cable, bitite 
 Callender's cable, bitumen 
 Gutta-percha 
 Vulcanised india-rubber 
 Ozokerited india-rubber 
 
 1-8 
 
 5-33 
 5-33 
 18-0 
 77-0 
 
 59 J F. 
 59 D F. 
 75F. 
 59F. 
 f-9F 
 
 Fowler- Waring cable 
 Shellac 
 
 86-0 
 107-0 
 
 59F. 
 
 82 F 
 
 India-rubber (untreated) 
 Siemens' fibre cables 
 Hooper's submarine cable 
 India-rubber, Siemens' special.. 
 Ebonite 
 
 130-0 
 141-0 
 ]78-0 
 200-0 
 333-0 
 
 75 F. 
 59 3 F. 
 75 C F. 
 59 F. 
 115F 
 
 Paraffin 
 
 405-0 
 
 115F. 
 
 
 
 
 It will be seen that prepared para rubber has the highest 
 resistance (with the exception of ebonite and paraffin), and 
 from its dense, hard texture it is peculiarly adapted for in- 
 sulating cables. But it is very expensive, and becomes hard 
 and cracks when exposed to the atmosphere, if great care has 
 not been taken in the manufacture. Vulcanised rubber, i.e., 
 rubber mixed with a certain proportion of sulphur and subjected 
 to a dry heat of about 360F. for a certain time, has a lower 
 insulation resistance, but is practically acid-proof, and is not so 
 liable to become hard and brittle as pure rubber. It is specially 
 adapted for use in water, and hence is largely used for under- 
 ground mains. In applying it to conductors there are usually 
 three servings : the first of pure rubber, the second a separator 
 containing chloride of zinc (to absorb the sulphur and prevent 
 the centre rubber from vulcanising), and the third a mixture 
 of rubber and sulphur; the whole being vulcanised together. 
 In some cases the rubber is vulcanised right through, but there 
 is then a probability of uncombined sulphur damaging the 
 copper. The sulphur blackens the copper, and appears to 
 permeate its substance and to make it brittle. To obviate 
 
86 ELECTRIC MOTIVE POWER. 
 
 these difficulties the copper should always be tinned before 
 being coated with rubber : this also assists the soldering of 
 joints. 
 
 Paraffin, although having a very high resistance, is not 
 stable and soon shrinks, becoming pierced with small holes. 
 This appears to be the result of the different melting points of 
 the various waxes composing the whole. Paraffin is largely 
 used for electrical instruments, and the melting point is care- 
 fully specified when it is purchased ; a good sample has a 
 melting point of not less than from 180F. to 200F., or even 
 higher. 
 
 Several forms of resins have been used for insulators, but 
 they all require to be carefully protected from excessive heat 
 or cold, or they will melt or crack. The Berthoud-Borel lead- 
 covered cable is the best example of this class. It is not much 
 used, however, now, as various heavy hydrocarbons have an 
 equally high insulation resistance, and are not so liable to crack. 
 
 One of the best examples of yarn fibre saturated with heavy 
 hydrocarbon compounds is found in the Fowler- Waring lead- 
 covered cables. The copper is first covered with a braiding of 
 hemp, or jute, and then thoroughly soaked with the wax at a 
 high temperature, so as to remove all traces of moisture. 
 Lead* is then squirted, under hydraulic pressure, in a fluid 
 state around the core, and so forms a continuous metallic 
 coating. Since lead is a soft metal and is easily damaged by 
 friction, kinking, or blows, it is usual to cover it with hemp 
 braiding, or servings of tape or yarn. With tape there should 
 always be two servings, one right-handed and one left-handed, 
 and the whole should then be coated with a preservative 
 compound, such as ozokerite, Stockholm tar, or pitch. 
 
 Preparations from bitumen are largely used for insulating 
 cables, but have much lower specific resistances than the pre- 
 viously-mentioned dielectrics. If used in a solid form without 
 any fibrous material they are practically impervious to water; 
 but being viscous, the conductor may become decentralised 
 
 * The best Spanish lead, guaranteed to contain less than 3 per cent, of 
 tin or other alloys. 
 
BITUMEN INSULATED CABLES 
 
 87 
 
 in course of time. To obviate this, Calender's Bitumen, Tele- 
 graph and Waterproof Company (Limited), who are the makers 
 of this class of cable, vulcanise the bitumen by adding just 
 enough sulphur to combine with the hydrocarbon. This 
 form of insulation, known as Bitite, is firm, and yet flexible, 
 and appears to be unaffected by water* ; but its insulation re- 
 sistance is very low compared with that of vulcanised rubber, 
 and, hitherto, it has been chiefly used for the distributing and 
 service mains on low-pressure systems. When used for high- 
 pressure mains, the makers recommend laying the cables in 
 solid bitumen in iron or wood troughing (see 20, pp. 91 102). 
 
 J.B. 
 
 c: 
 
 FIG. 20. 
 Armoured Cable. 
 
 FIQ. 21. 
 Concentric Armoured Cable. 
 
 J.B., jute braiding; S.T., steel tape; J., jute; L.S., lead sheathing; 
 I., insulation ; C., conductor. 
 
 The bituminised fibre cables are usually laid in special 
 bitumen culverts, which form a watertight envelope. Both 
 classes of these cables are frequently covered with lead and 
 served with jute ; and when culverts are not used it is cus- 
 tomary to armour the outer serving of jute by one or two 
 spiral windings of galvanised iron wire. 
 
 Steel tape, braided steel, or spirally-wound galvanised iron- 
 wire is now frequently used to armour all the classes of cable 
 referred to (see Fig. 20) ; but it is most generally applied to 
 lead-covered cables, to protect them from mechanical injury, 
 and it is especially useful when they have to be buried in earth 
 the chemical reactions of which are unknown. 
 
 * It is stated by some that the sulphur works out of the bitumen in 
 course of time. 
 
 H2 
 
88 ELECTRIC MOTIVE POWER. 
 
 Armouring is also largely used for concentric feeders (see 
 Fig. 21). The use of a concentric distributing network is open 
 to criticism, but the difficulty of making sound joints is practi- 
 cally overcome by the use of iron service boxes. After the 
 wires have been properly soldered the box is filled with insu- 
 lating material, which can be vulcanised if desired. Fig. 22 
 shows a service box adapted for use with single mains ; and 
 Figs. 23 and 24 two views of one suitable for concentric cables. 
 These illustrations refer to the Callender Company's system of 
 armoured mains. In all cases, however, the iron or steel 
 armour must be well covered with jute yarn, thoroughly soaked 
 
 FIG. 22. Junction Box for Single Cables. Plan. 
 
 S.T., Steel Tape, covered with jute ; L.C., Lead Coating ; B.I., Bitumen 
 Insulation ; C.C., Copper Conductor. 
 
 in a preservative compound, and the cable should be laid in 
 pitched wood troughing, or in fresh-water sand, or in puddled 
 clay, and thoroughly protected from the atmosphere. Neglect 
 of these precautions is sure to end in breakdowns in course of 
 time. 
 
 The chief source of trouble with lead-covered cables seems to be 
 corroding agents, and if they be found in even very small quan- 
 tities the lead will be attacked at one or more places, and holes 
 will soon be eaten through to the dielectric ; the erosion then 
 rapidly increases from the inside as well as the outside, being 
 
JUNCTION BOXES. 
 
 89 
 
 assisted by leakage of electricity, which is sure to take place 
 at about the same time. Again, lead- covered cables should 
 
 FIG. 23. Junction Box for Concentric Cables. Plan. 
 I., Inner Conductor ; 0., Outer Conductor. 
 
 never be laid in contact with iron pipes, as the slight leakage 
 of electricity, which nearly always exists in damp weather, will 
 
 FIG. 24. Junction Box for Concentric Cables. Sectional Elevation. 
 
 rapidly cause the iron to corrode, iron being electro-negative to 
 lead. 
 
 19. INSULATORS AND DETAILS OF AERIAL LINES. 
 The various considerations affecting the choice of metal for 
 the conductor having now been discussed, it remains to 
 investigate the methods of erecting the line. There are two 
 conditions which cannot be neglected, viz., the mechanical 
 
90 
 
 ELECTRIC MOTIVE POWER. 
 
 FIG 29. FIG. 30. 
 
 Iron Stalls, Shackles and Solid Insulators used for Low-Tension 
 Aerial Lines. 
 
DETAILS FOR AERIAL LINES. 91 
 
 strength of supporting devices and high insulation resistance 
 of the conductor. The general plan of carrying wires on insu- 
 lators is familiar to us all, by reason of the large number of 
 telegraph and telephone wires in use everywhere, and there is 
 thus no need for a detailed description. The most frequent 
 forms of iron stalks, shackles and solid insulators, suitable for 
 low-tension bare wire lines, or insulated cable circuits run 
 overhead, are shown in Figs. 25 to 30. The uses of the various 
 types of support will be apparent on inspection. The stalks are 
 usually of galvanised iron, either with coach screw threads, or 
 screwed for tapping into metal ; for very heavy power lines, in 
 which copper cables of about 19/16 S.W.G. are used, the stalks 
 are forged or cast steel, sometimes galvanised. The insulators 
 are made of glazed white or brown ware, the latter being the 
 stronger and more durable. 
 
 The line wire is laid in the groove at the top or on the sides 
 of the insulators, and is lashed securely in position by binding 
 wire. 
 
 If the line pressure be high, and the dielectric covering around 
 the cable of a low quality, it is usual to use an insulator, 
 with a recess moulded in the inside of a bell-shaped bottom, 
 which is filled with a resinous oil of high specific resistance. 
 This plan is illustrated in Figs. 31 to 34, p. 92. Figs. 33 
 and 34 are adapted for straight lines ; Fig. 31 for curves and 
 straining posts, or for attachments at the end of a line; and 
 Fig. 32 shows a form used for carrying a bare steel wire to 
 which the copper cable is suspended by leather loops (see 
 Fig. 35), or by a metal hook as shown in Fig. 36, p. 93. The 
 function of the oil is, primarily, to prevent surface leakage of 
 current, especially during wet weather, when it necessarily 
 occurs with the solid type of insulator. To maintain the oil in 
 an efficient state, it is important to prevent an accumulation of 
 dust or water on its surface. The arrangement shown in Fig. 33 
 is the Indian Government pattern, and is, perhaps, superior to 
 any of the other devices ; but the types of insulators shown in 
 Figs. 31 and 34 have given excellent results in most of the 
 plants installed by the author. In very dusty places, or where 
 the insulators are difficult of access, it is better to use the form 
 illustrated in Fig. 33. 
 
ELEOTEIC MOTIVE POWER 
 
 FIG. 33 FIG. 34. 
 
 Oil Insulators used for High-Pressure Aerial Lines. 
 
DETAILS FOR AERIAL LINES. 93 
 
 With regard to posts, it is clear that these will only be 
 employed where there are no buildings or other convenient 
 objects to which the insulators may be attached. For cross- 
 country lines wood posts are usually found to answer all the 
 requirements; but along roadways, and in other special cases 
 hollow steel posts will be found to be nearly as cheap, and 
 certainly more durable and ornamental. It is probable, however, 
 that in the class of work to be carried out in the near future, 
 wood posts will be more generally preferred. They should be of 
 well-seasoned larch or pitch pine, and be firmly embedded in the 
 ground to a depth of from four to six feet, according to their 
 
 FIG. 35. FIG. 36. 
 
 Hangers for Suspending Cables from a Bearer Wire. 
 
 height and the strain they are designed to carry. It is not easy 
 to give definite rules for sizes of posts, lengths of spans, and 
 strain in conductor; but an examination of a few trunk telegraph 
 lines will exemplify the principles involved. Some idea of the 
 length of spans and dip of the cable with and without a 
 bearer wire may be gathered from Table H, p. 94, which has 
 been compiled by the Silvertown Company. 
 
 These figures agree fairly well with the author's own experi- 
 ence. In connection with this part of the subject, it should be 
 noticed that the Board of Trade Regulations fix the maximum 
 length of span at 200ft. where the direction of the conductor 
 
94 
 
 ELECTEIG MOTIVE POWER. 
 
 is straight, and at 150ft. where it is curved, or when the 
 conductor makes an angle with the support. These regulations 
 also require a factor of safety of at least 6 in the conductors 
 and suspending wires, and of at least 12 in the posts, standards, 
 and fixed attachments, taking the maximum wind pressure to 
 be 501b. to the square foot. The accumulation of snow may 
 be neglected, since the slight heat generated in the conductors 
 is sufficient to melt the snow in this climate. 
 
 Table H. 
 
 
 
 Span. 
 
 Copper conductor 
 strands of 
 
 Dip. 
 
 
 
 Without steel bear- 
 
 With 7/16 steel 
 
 
 
 ing wire. 
 
 bearing wire. 
 
 7/20 
 
 5 feet 
 
 64 feet 
 
 238 feet 
 
 7/18 
 
 5 
 
 80 
 
 ??0 
 
 7/16 
 
 5 
 
 102 
 
 220 
 
 19/18 
 
 5 
 
 118 
 
 210 
 
 20. UNDERGROUND LINES, CONDUITS, BUILT-IN 
 AND DRAWING-IN SYSTEMS. 
 
 It is becoming generally recognised that aerial lines, whether 
 of bare copper or insulated cable, though permissible in open 
 country where they are free from wilful or accidental damage, 
 are a source of possible danger in towns, &c. And it is also 
 apparent that such lines are especially subject to interference 
 from lightning. (The aesthetic view of the question is irrelevant 
 to the purely engineering aspect, and need not be discussed.) 
 It is evident, however, that if a line has to be made permanent,, 
 and be required to work continuously at a minimum cost of 
 maintenance and depreciation, it must be protected from both 
 atmospheric influences and accidental injury. These ends are 
 best attained by laying the cables in the earth. Experience 
 gained with gas and water mains points to two methods : in 
 one, the cables are simply buried in the soil at a safe depth ; in 
 the other, they are run in conduits or pipes. 
 
 If the first system be adopted, it is evident that the insula- 
 tion of the cable must be protected from mechanical injury 
 during laying, and afterwards from a possible erosive action of 
 
UNDERGROUND CIRCUITS. 95 
 
 the soil, and, perhaps, from the effects of water. This suggests 
 the use of a metallic sheathing of lead if the Water Company's 
 practice be followed, or iron if the Gas Company's experience 
 be accepted. Both kinds of armouring have been used with 
 success here and failure there. The causes determining the 
 result are often obscure, but electrolytic decomposition, assisted 
 more or less by the peculiar constituents of the soil and water, 
 can always be shown to have taken place. In fact, the problem 
 is complicated by the inevitable leakage of at least one main, 
 usually the negative, and sometimes by weak points on both. 
 The passage of an electric current through water breaks up the 
 chemical combination into its component gases, oxygen and 
 hydrogen. The liberated oxygen immediately attacks any 
 oxidisable material in contact with it the lead sheathing, for 
 example and soon changes its nature. Other more obscure 
 chemical changes, resulting in the formation of free chlorine, 
 hydrochloric, and sulphuric acids, and other corrosive agents, 
 may also occur if the soil be impregnated with suitable salts. 
 Hence, wherever there is a leakage of electricity, there is a 
 tendency to destroy the metallic sheathing, the insulation, and, 
 finally, the copper conductor itself. It is, therefore, necessary 
 to protect the metallic armouring by servings of jute soaked 
 in tar, ozokerite, or some preservative compound. And in all 
 cases it is wise to lay the cables in clean soil, clay, or fresh- 
 water gravel, and to protect them by boards from mechanical 
 injury such as an inadvertent blow from a pickaxe. The 
 armoured concentric mains laid at Bradford for the lighting ser- 
 vice are a good example of this class of work. They have been in 
 continual use for about four years, and the author is assured by 
 the engineer that the insulation resistance has increased since 
 they were laid in the ground, and that no trouble is expe- 
 rienced with the coupling-up of consumers through the iron 
 junction boxes already referred to.* 
 
 The second method may be considered under two heads: 
 (a) the "built-in" systems, in which the cables, usually strips of 
 bare copper, are laid in conduits, and covered-in permanently, 
 so that they cannot be reached without taking up the top soil ; 
 
 * See pages 88 and 89. 
 
96 
 
 ELECTEIC MOTIVE POWER. 
 
 and (6) the " drawing-in " systems, in which continuous lines of 
 waterproof conduits, or pipes, or troughing are laid, with 
 drawing-in boxes at intervals of from 80 to 100 yards in the 
 straight, and at all sharp bends. 
 
 A complete discussion of these systems of laying cables is not 
 .suitable here, since this properly belongs to a treatise on central 
 
 ^\^ ^^ 
 
 Fia. 37. Callender Solid Bitumen System. Cross Section of Trough. 
 
 stations for distributing light (and perhaps power) over a large 
 area to a number of points in close contiguity ; whereas this 
 book treats, primarily, of the transmission of power between 
 
 FIG. 38. Callender Solid Bitumen Svst< m Sectional Elevation of Trough. 
 
 places relatively far apart, and, secondarily, of its distribution 
 to a comparatively few points in a limited area. And should 
 it be deemed advisable to run the cables underground, some 
 modification of the first method (cables simply buried) is sure 
 
CALLENDEKS SOLID BITUMEN SYSTEM. 97 
 
 to be selected from the mere consideration of first cost. If, 
 however, the cables are likely to be frequently disturbed for 
 coupling-up supply mains to new points, or if they are likely 
 to be increased in number within a reasonable time from the 
 first installation, then one of the second systems may prove to 
 be the cheaper in the long run, but this contingency is improb- 
 
 FIG. 39. Callender Solid Bitumen System. Cross Section of Service Box. 
 
 able on the supposition laid down in the preface and introduc- 
 tion. It will be, therefore, sufficient to illustrate the methods 
 by a few characteristic drawings. 
 
 (a) One of the best "built-in" systems is the Callender Solid 
 Bitumen system (illustrated in Figs. 37 and 38). The troughs 
 are made in cast iron, in lengths of 6ft., with sockets at one 
 
 FIG. 40. Callender Solid Bitumen System. Plan of Service Box. 
 
 end, so that when fitted together there is a free " run " inside. 
 The lengths are coupled together by counter-sunk bolts and 
 nuts. Bends and circular pieces are used to carry the mains 
 round corners and for changing the levels at crossings, but 
 considerable deviation from the straight line is possible with 
 the standard type. The method of laying is as follows : The 
 
98 
 
 ELEGTEIG MOTIVE POWER. 
 
 troughs are first connected in position and then a small 
 quantity of molten bitumen is run in, and before it sets bridges 
 of bituminised wood are placed in it at intervals of 18in. The 
 insulated cables are next laid in position and held in place by 
 .these bridges so as to be clear of the sides and bottom of the 
 
 FIG. 41. Callender-Webber Drawing-in System. Section of Manhole. 
 
 trough and of each other. The interior space is then carefully 
 filled up with bitumen to within half an inch of the top, and 
 the whole is finished off with a covering of Portland cement or 
 cast iron lids. It is obvious that cables laid in this manner 
 
 FIG. 42. Callender-Webber Conduit. 
 
 are completely protected from the action of damp, water, or 
 gases, and yet repairs can be easily and rapidly made. The 
 method of coupling up the service mains is illustrated in 
 Figs. 39 and 40, p. 97. 
 
CALLENDEE-EAWOETH CONDUIT. 99 
 
 (I) The drawing-in systems are illustrated by Fig. 41, 
 which refers to the Callender-Webber system and shows the 
 conduit, a brick pit, and the method of coupling the feeders or 
 service mains without junction boxes. The conduit consists of 
 bitumen concrete formed in blocks 6ft. in length and pierced by 
 "ways" (see Fig. 42). The standard sizes of these cases are for 
 2, 3, 4, and 6 ways of either 1 Jin., 2in., 2f in. or Sin. in diameter. 
 To join the lengths, the cases are brought together and an iron 
 mandrel placed in each of the ways. Melted bituminous 
 concrete is then run between the two cases and rammed home. 
 When the bitumen is cold the mandrels are withdrawn, leaving 
 a perfect joint as strong as the main case itself. The u ways " 
 are quite smooth throughout their entire length. It is said 
 that the cables occasionally sink into the bitumen in course 
 
 FIG. 43. Callender-Ra worth Drawing-in System. Cross Section of Trough. 
 
 of time and cannot, therefore, be withdrawn. The Callender- 
 Raworth system (Fig. 43), however, entirely obviates this 
 difficulty, and combines the mechanical strength of the solid 
 bitumen system with the facilities of a drawing-in system. It 
 consists of cast iron troughs with flanges. The spaced wooden 
 bridges are fixed at short intervals, and on these are laid tubes 
 of specially-made paper impregnated with bitumen. The tubes 
 are joined together by sleeve pieces and are then buried in 
 solid bitumen. The conduit is closed on the top by a cast-iron 
 lid with flanges fitting over the sides of the trough. The high- 
 pressure lighting mains at Huddersfield are thus laid, and are, 
 in the author's opinion, a first-rate example of conduit work. 
 
100 
 
 ELECTRIC MOTIVE POWER. 
 
 Messrs. Doulton and Co. make a variety of designs in 
 vitrified stoneware casing, suitable either for built-in or 
 drawing-in systems. They are made both with solid and 
 removable tops, and in every respect are suited to the various 
 requirements of conduit work. The stoneware is acid proof,, 
 
 .mpervious to water and gas, and is made in convenient 
 lengths, easily jointed in position by cement or bitumen. 
 The lengths of casing are laid, with a small space between 
 them, in cast-iron or stoneware jointing-collars. Mandrels with 
 india-rubber heads are then introduced into the " wavs " and 
 
STONEWARE CONDUITS. 
 
 101 
 
 expanded opposite the joints by a screw. Molten cement is 
 next poured in to fill up the space between the lengths. The 
 cement sets in a few minutes and then the mandrel is removed, 
 leaving a smooth joint in the inside, and insulated cables may 
 be safely drawn into the u ways." Solid casing with two 
 " ways " is shown in Figs. 44 and 44^, in vertical and horizon- 
 tal section at a joint ; while the method of using the mandrel 
 is made clear in Figs. 45 and 45A, (a) showing the mandrel as 
 introduced into the " way," and (b) showing it expanded and 
 closing the duct. This ware can be made in almost any shape, 
 and will probably be largely used in the future instead of 
 cement for conduits with removable tops. In price it competes 
 favourably with most of the methods now in use, and its great 
 strength, combined with its insulating qualities, commend it 
 for a number of purposes incidental to electrical work. 
 
 The same firm make round spigot and socket pipes in stone- 
 ware with Standford's patent watertight joint. The principle 
 will be at once seen on reference to Fig. 46. The black part 
 
 FIG. 46. Standford's Watertight Joint. Section of Joint as Laid. 
 
 in the joint refers to a special preparation of bitumen fitted 
 around the spigot and inside the socket. That in the spigot 
 is curved, while that in the socket tapers gently inwards from 
 the mouth. To joint the pipes it is only necessary to place 
 the ends in line and push them together a little thick grease 
 being first put on the surface of the bitumen. It is obvious 
 that an exact alignment of the pipes is not necessary, and that 
 the joints will remain watertight with even a considerable 
 settlement or displacement of the line of pipes. This method 
 of jointing will, no doubt, find favour for electrical purposes, as 
 it already has for water works. 
 
 i 
 
102 ELECTRIC MOTIVE POWEE. 
 
 Iron pipes have not been referred to because their use is 
 sufficiently obvious. It may be mentioned, however, that cast 
 pipes are more durable than wrought ones. They form the 
 cheapest kind of conduit and are most frequently used for 
 high-pressure feeders, which, of course, are heavily insulated. 
 Generally, iron pipes are not so well adapted for the dis- 
 tributors as the Callender or kindred systems. 
 
 21. LIGHTNING DISCHARGES, LIGHTNING CON- 
 DUCTORS, AND LIGHTNING ARRESTERS. 
 
 It has already been mentioned that aerial lines are specially 
 liable to trouble from lightning. It is, therefore, important to 
 study the conditions under which lightning discharges take 
 place, and also to examine the various methods in vogue for 
 protecting the line and plant from damage. It is well known 
 that trees, tall buildings, and elevated structures in general, 
 are more likely to be damaged by lightning than low-lying 
 parts ; and it has also been observed that a certain area around 
 a tall building, such as a church spire, is comparatively safe, 
 and rarely receives damage from this cause. It is therefore 
 customary to erect lightning conductors with barbed platinum 
 points on the summits of tall or isolated buildings, and to con- 
 nect the rods with the earth by means of suitable earth-plates 
 buried in moist ground. The theory of lightning conductors 
 has been elaborated by Prof. Oliver Lodge, to whose excellent 
 work* the reader is referred for more complete information. 
 It will now serve practical purposes to make a few generalisa- 
 tions with reference to the subject. The function of a lightning 
 conductor is to prevent disruptive discharges by rendering 
 the accumulation of a dangerous charge an impossibility. To 
 attain this end experience shows that the discharging rods 
 should be of stranded wire or ribbon so as to offer a low resist- 
 ance to the high periodic current of the discharge. The metal 
 may be copper or galvanised iron according to the view taken 
 of the relative values of electro-magnetic induction and ohmic 
 resistance. Prof. Lodge inclines to an iron conductor, but 
 usage certainly points to copper. The conductor should form 
 
 * " Lightning Conductors and Lightning Guards," by Prof. Oliver J. 
 Lodge, D.Sc. 
 
LIGHTNING DISCHARGES. 103 
 
 a network, completely surrounding the structure of the 
 building, and discharging points should be placed at every 
 prominent place, or semi-detached part. Insulators should be 
 used to carry the rods, in order to lessen the liability of lateral 
 discharges to metallic things inside the building, and all 
 external metal-work, such as water-pipes and iron ornaments, 
 should be connected to each other and to earth, but not to the 
 lightning conductor. The conductor is usually composed of 
 seven strands each of No. 9 S.W.G., or of a ribbon lin. wide 
 by -Jin. thick. The earth-plate (or plates) should not be of less 
 area than, say, 20 sq. ft., counting both sides, and should be 
 buried about 20 ft. below the surface, so as to ensure contact 
 with moist soil, even during times of drought. Sometimes an 
 iron pipe is used, in which case the area must be reckoned 
 from the outside only. 
 
 Lightning conductors as ordinarily understood, however, are 
 not the only protection required for aerial lines, which, as a 
 rule, lie much nearer to the earth than adjacent trees and 
 buildings. Indeed, the difficulties in connection with atmo- 
 spheric electricity which have to be encountered are of a varied 
 character and demand special precautions. Lightning discharges 
 may be conveniently divided into four classes. 
 
 (a) The most dangerous of all is a stroke falling on the line 
 from clouds directly overhead, which raises the potential to a 
 dangerously high point. 
 
 (6) If discharges of electricity occur between clouds in a direc- 
 tion parallel to or approximately parallel to that of the line, they 
 tend to induce charges in the line and cause a disturbance in 
 the line pressure, which may, or may not be compensated for 
 by the effect being in the same direction in the lead and return. 
 
 (c) If part of the line be earthed at two or more places, 
 disturbances in the surface potential of the earth may cause 
 relatively large currents to pass through the copper line before 
 equilibrium is restored. 
 
 And (d) The electrostatic induction of banks of clouds may 
 cause rapid and violent changes of pressure in the line. These, 
 
 i2 
 
104 ELECTRIC MOTIVE POWEIi. 
 
 in a modified degree, are probably of frequent occurrence, but 
 fortunately, the pressure can be rapidly adjusted through 
 lighting arresters. 
 
 From an examination of the preceding divisions it may be 
 assumed that time is an important factor in the effects of the 
 discharge. Indeed, it seems probable that even a well-designed 
 lightning conductor is insufficient to carry off the enormous- 
 discharges which sometimes fall directly on the line from low- 
 lying clouds. Yet the true function of a lightning conductor, 
 in the author's opinion, is not to form a safe path for the surg- 
 ing current of the discharge, but to prevent the accumulation 
 of a dangerous potential by a continuous silent discharge. 
 Viewed in this light, lightning conductors are useful in power 
 work and may be placed at intervals on the posts, as indicated 
 in Fig. 30, p. 90 ; but to be effective they must be efficiently 
 earthed to plates, preferably of copper, of not less than 6 or 
 8 sq. ft. in surface on each side. The wire connecting the dis- 
 charger and the earth-plate may be of galvanised iron wire,, 
 say, seven strands of No. 12 S.W.G. not solid wire. 
 
 Inductional disturbances produced either by discharges 
 between clouds or by the electrostatic effects of heavily-charged 
 layers of clouds can be generally guarded against by means of 
 devices known as lightning arresters. (These are referred to 
 later.) Or, the line wires may be almost certainly protected 
 from these troubles if a bare grounded wire be run on the tops 
 of the poles, a foot or so higher than the insulated circuit. 
 This is usually of bare stranded iron wires, and is earthed at 
 either end and at intervals along the lines, and is, therefore, at 
 the same pressure as the earth. As soon as there is a tendency 
 to produce a disturbance of pressure in the system, it is 
 annulled by a discharge between the earth and the grounded 
 wire, and there can be no great difference of pressure between 
 the earth and the insulated copper mains from this cause. 
 The safety grounded line is shown in Fig. 47. 
 
 The iron wire might with advantage be connected to dis- 
 charging rods placed on the tops of posts on high ground or 
 exposed positions. Although the use of an earthed wire 
 
GROUNDED DISCHARGING WIRE. 
 
 105 
 
 parallel to the insulated ones is fully appreciated in the 
 telephone and telegraph services, it seems to present several 
 difficulties with high-tension circuits. Since the " live " cir- 
 cuits, i.e., the wires carrying the high-pressure current, are 
 close to the " grounded " wire there is a chance of linesmen 
 receiving shocks, especially if an earth return be used as in street 
 railway work. This risk is obviously increased if the " live " 
 wires be bare, and there is also the chance of short-circuiting 
 the " live " and " ground " wires by accidental contact with 
 telegraph and telephone wires. If both lead and return be 
 covered with insulation, these objections have less weight, but 
 
 FIG. 47. Grounded Discharging Wire. 
 
 ven slight leaks may give serious shocks, when the working 
 pressure is high. 
 
 However, this method is so efficient and so generally avail- 
 able that the author strongly recommends it for power lines 
 running across open country, where the previously mentioned 
 difficulties are not of so much importance. But the cheapest 
 and in many respects the most practical way of securing com- 
 parative immunity from lightning discharges with an overhead 
 line is to use a concentric cable in which the outer metallic 
 sheath forms the return and is grounded. This arrangement 
 only requires one conductor in place of two and the earthed 
 
106 
 
 ELECTRIC MOTIVE POWER. 
 
 wire, and as the positive wire is protected by the insulation 
 between it and the negative wire and also by the grounded 
 wire itself, it is absolutely impossible for a linesman to get a 
 shock. And in the event of a short circuit the worst that can 
 happen is the blowing of one of the cut-outs. A concentric 
 cable used in this manner must not be confounded with a 
 
 To Line 
 
 J 
 
 Spark 
 "X3ap. 
 
 To Earth 1 ? 
 
 FIG. 48. Lightning Arrester. 
 
 simple earth return or an insulated bare wire return, in both of 
 which cases there is always the chance of severe shocks to men 
 working on the line. 
 
 Lightning arresters have already been referred to, and two 
 elementary types are illustrated dia grammatically in Figs. 48 
 and 49. The theory underlying the use of these devices is 
 
 To Lm< 
 
 wwwwwvwvwv 
 
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 To Earth 
 
 FIG. 49. Lightning Arrester. 
 
 found in the power possessed by high pressure and high fre- 
 quency currents to leap across air-gaps of high ohmic resistance 
 rather than traverse the line and dynamo circuits which have 
 large self-induction but small ohmic resistance. 
 
 In Fig. 48 there are two plain-faced discs of metal insulated 
 from each other by an adjustable air-gap. The "live" wire 
 
LIGHTNING ARRESTERS. 107 
 
 is coupled to one disc, and the other is " grounded." The 
 action is as stated above. 
 
 In Fig. 49 another property of an electric charge is utilised 
 to render the device more sensitive and certain in its action. 
 It is found that an electric charge does not distribute itself 
 equally over the surface of a body but tends to accumulate at 
 points, from which it is also most readily discharged. This type 
 of arrester has therefore a dual purpose. In the first place it 
 acts as a discharger, and tends to prevent a slow accumulation 
 of electricity ; and, secondly, the spark-gap forms a path of low 
 resistance, compared with that of the line, dynamos and 
 motors, in the event of a large disturbance of pressure in 
 the system. Referring to Fig. 49, a is a plate connected to 
 the line, b a plate of similar size connected to the earth, the 
 two serrated edges being placed opposite to each other. It is 
 evident that a will be at the pressure of that part of the line 
 it is in contact with, whilst b will be at the same pressure as 
 the earth 
 
 The distance of the spark-gap can be regulated to suit the 
 ivorking pressure of the line. If this pressure be exceeded by 
 a predetermined amount dependent on the distance between the 
 teeth, a discharge will take place from one plate to the other, 
 thus preventing an excessive difference of pressure. Since the 
 inductive disturbances due to lightning are of a periodic 
 character, it follows that there will not be the same tendency 
 for disruptive discharges all over the area of the system at any 
 moment. In fact, there will be a series of nodes continually 
 shifting their position. To ensure efficient protection, then, the 
 arresters should be placed at intervals, and also at the entrance 
 to the power station, and across the terminals of all dynamos 
 and motors coupled to the circuit. The method of applying 
 them will be clear, after a perusal of the preceding statements. 
 Owing to the fact that a much lower pressure is required to 
 maintain an arc than to initiate it, the simple forms of arrester 
 (shown in Figs. 48 and 49), although affording efficient protec- 
 tion to telegraph and telephone circuits, are not satisfactory 
 on lines which are worked at a high pressure. For there is a 
 risk of short circuiting the dynamo through one of the arresters. 
 In fact, it is necessary to provide some means for extinguishing 
 
108 
 
 ELECTRIC MOTIVE POWER. 
 
 the arc as soon as it is formed. This can be accomplished in 
 various ways, the most apparent being to move the plates apart 
 sufficiently to break the circuit after each discharge, and then 
 immediately to return them to the original position. Another 
 plan is to blow out the arc by means of the heated air resulting 
 from the energy of the spark. Or the spark-gap may be sub- 
 divided into a series of very short intervals, which effectually 
 prevent the formation of an arc by the lower pressure of the 
 line, but across which the high pressure discharge readily leaps 
 in a series of small sparks. Or the short-circuiting current 
 
 TO EARTH. 
 
 FIG. 50. Keystone Pattern Lightning Arrester. 
 
 may be passed through a few turns of thick wire on a powerful 
 electro-magnet, and so break the arc by electromagnetic repul- 
 sion. All of these devices have been tried with more or less 
 success, and a number of patents embodying the principles 
 have been taken out at various dates. It is proposed only to 
 illustrate a few of the most recent or best tried forms, since 
 these are sufficiently characteristic of the whole. 
 
 The air-blast arc-breaking and self-adjusting type of light- 
 ning arrester is illustrated in Fig. 50, which shows the so-called 
 
LIGHTNING ARRESTERS. 
 
 109 
 
 "keystone" pattern, made by the Westinghouse Company. 
 It is largely used in the United States on power circuits. The 
 action will be clear on reference to the diagram. The black lines 
 show the normal position of the discharging points, the dotted 
 lines refer to the movement which takes place immediately 
 after a disruptive discharge, when the heated air has driven the 
 dischargers out of the box. This action takes place on either 
 pole, and so the arc is effectually broken. The swinging arms, 
 after striking against the mallet-shaped stop at the top, 
 rebound again into their normal position, and the arrester is 
 again ready for action. In theory this device is admirable, and 
 practically, perhaps, the only objection is the moving arms 
 which are likely at times to stick. The apparatus as actually 
 
 FIG. 51. The Power Circuit Lightning Arrester. 
 
 installed is shown in Fig. 51 ; the box is opened in order to 
 show the interior arrangements. 
 
 The magnetic " blow-out " type is generally not so satisfac- 
 tory as the cc air-blast," it being found that there is a tendency 
 for the spark-gap to fuse across before the magnet has time to 
 act. To obviate this, the strength of the magnet must be 
 carefully adjusted, and in some designs carbon buttons are 
 used on the dischargers. The carbon gradually disintegrates 
 by the heat, and the length of the gap, therefore, requires 
 adjustment from time to time. 
 
 Fig. 51 A illustrates the Thomson-Houston arc line protector. 
 The spark is formed at the base of the curved metallic horn 
 
 
 
110 
 
 ELECTRIC MOTIVE POWER. 
 
 plates, but is instantly repelled by the strong magnetic field 
 produced by the electro -magnet in the air-space, the lines of 
 which are perpendicular to the path of the discharge. The arc 
 is thus pushed along the horns and increasing in length is finally 
 broken. 
 
 One of the most successful of the devices depending on the 
 bringing into action of new points after each discharge is the 
 
 TO EARTH 
 
 FIG. 5lA. Thomson-Houston Magnetic Blow-Out Lightning Arrester. 
 
 Wasson arrester. In this the arc is formed between carbon' 
 buttons, and the current melts a fuse, causing a lever to drop 
 and thereby bringing into position a fresh couple of carbon 
 points. The arrangement contains, say, four or five sets of 
 carbons, and therefore is able to deal with as many discharges, 
 after which fresh fuses are required, and also a readjustment 
 'of the buttons. It has therefore a more limited scope than 
 the Westinghouse air-blast type. 
 
LIGHTNING ARIiESTEES. 
 
 Ill 
 
 The " Wurts" lightning arrester depends on the subdivision 
 of the arc into a number of minute sparks, and practically 
 assumes that an arc is never formed. In its most usual form, 
 and as largely used on tramcars by the Thomson -Houston 
 Company and others, it consists of a number of thin iron 
 washers threaded on an insulated column and separated from 
 each other by thin discs of mica. The top sheet of iron is 
 coupled to the " line," and the bottom to " earth." In order 
 to form a circuit between the two end plates it is necessary for 
 the current to leap from the periphery of one plate to that of 
 the next, and so on through the series. Occasionally, beading 
 takes place, finally resulting in a short circuit ; but generally, 
 
 FIG. 52. The "Wurts 
 
 Lightning Arrester for Continuous Current 
 Circuits. 
 
 after properly experimenting with the particular line pressure, 
 it is possible to select such a size and number of plates that 
 excellent results are obtained. The portability and automatic 
 action of the arrester commend it specially for tramcar work. 
 
 The action seems to be due to the large number of gaps, the 
 length of which is short compared with the breadth, which is 
 equal to the length of the periphery of the plates. The general 
 appearance is given in Fig. 52. 
 
 The preceding remarks refer chiefly to direct- current systems ; 
 with alternate currents a much simpler device is sufficient. It 
 
112 
 
 ELECTRIC MOTIVE POWER. 
 
 has been demonstrated by Mr. Wurts that zinc, antimony, 
 bismuth, cadmium, and mercury have the properties of 
 breaking an alternate-current circuit without sparking, and 
 hence are called by him non-sparking metals. This gentle- 
 man has availed himself of this peculiar fact to design a very 
 interesting and effective form of lightning arrester which will 
 .permit a 1,000-volt alternator to be short-circuited through 
 it as often as desired, without even affecting the lamps on the 
 circuit, so rapidly is the high-pressure arc destroyed. It is 
 
 MAIN. 
 
 MAIN. 
 
 FIG. 53. The " Wurts 
 
 TO EARTH. 
 
 Double-Pole Alternate Current Lightning 
 Arrester. 
 
 shown in Fig. 53, which is of the double-pole type. The seven 
 pillars, each Sin. long by lin. in diameter, are solid and cast 
 from an alloy of zinc and copper, and have knurl td surfaces. 
 The spark-gaps are each ^ths of an inch in length, and as 
 the earth wire is joined to the middle column, there are three 
 spaces on each side equal to a total gap of /^ths of an inch. 
 
 Incidental to non-sparking metals it may be mentioned that 
 the inventor says they only show these remarkable properties 
 
LIGHTNING ARRESTERS. 113 
 
 where the spark-gaps are very small ; and if the gap be 
 increased to half an inch the short circuit will be maintained 
 with a vicious arc. The author has had no actual experience 
 with these interesting arresters, but Mr. Wurts has given the 
 subject much attention, and his investigations in this field are 
 undoubtedly of importance. Some thousands of these non- 
 arcing arresters are already being used in the United States 
 with success.* 
 
 Fortunately, owing to our climate, there is not the same 
 need for lightning arresters in Great Britain as in some other 
 parts of the world. This happy circumstance explains to a 
 large extent why the power circuit and station devices are 
 chiefly of American origin. And due allowance must also be 
 made in the relatively small number of overhead power circuits 
 in the United Kingdom. All telegraph and telephone lines 
 are, of course, protected by arresters, but, as already stated, 
 the overhead grounded line and the devices shown in Figs. 48 
 and 49 prove amply sufficient. 
 
 The author has had, however, in the course of his professional 
 work, to erect a number of overhead circuits for power purposes, 
 and accordingly he designed some years ago the double-pole 
 lightning arrester, shown in Fig. 54, p. 114. The base is of 
 metal, and is grounded ; the faces of the discs are serrated in 
 straight lines, placed at right angles to each other when in the- 
 position for action. The two discs are severally coupled to the 
 positive and negative pole mains outside the main fuses, so that 
 in the event of a disruptive arc forming across the plates and 
 short-circuiting the dynamos, the cut-outs shall at once open 
 the circuit. Although the author has used a number of these 
 lightning arresters in some of the most open and mountainous 
 parts of Great Britain and the Continent, he has never known 
 a case where an arc has been formed. This does not prove that 
 (under suitable conditions) an arc cannot form, but seems to 
 suggest that the device acts as a silent discharger within the 
 limits of the tests to which it has been subjected. It should 
 also be marked that it is the author's invariable practice to use 
 
 * Vide The Electrician, December 2, 3892, and Paper read by Mr. Wurts 
 before the American Institute of Electrical Engineers on March 15, 1892, 
 entitled i! Lightning Arresters and Non-arcing Metals." 
 
114 
 
 ELECTRIC MOTIVE POWER. 
 
 toothed arresters at various parts of the line, mounting them 
 on the posts in water-tight metal boxes, and also to use dis- 
 charging rods or lightning conductors at the most exposed parts 
 
 I 
 
 of the circuit. And he has always used the overhead grounded 
 line as well as these arresters in mountainous districts, which 
 are subject to heavy thunderstorms. 
 
CHAPTER IV. 
 
 DIRECT CURRENT SYSTEMS OF TRANSMITTING 
 AND DISTRIBUTING ELECTRICAL POWER. 
 
 22. Introduction and Definitions. 23. Series Working : Constant Cur- 
 rent System ; Single Dynamo and Motor in Series. 24. Parallel 
 Working : Simple Parallel ; Reverse Parallel ; Feeder System ; Three- 
 Wire System ; Multiple- Wire Feeder System with Regulators. 
 25. Continuous Current Transformers or Dynamotors. 26. 
 Methods of Compensating for Fall of Pressure on Feeders and 
 Mains. 
 
 22. INTRODUCTION AND DEFINITIONS. 
 
 IN the previous chapter the line and its details were discussed 
 in theory and practice, but the electrical systems of transmit- 
 ting power were not differentiated. As it is of the utmost 
 importance to understand the essential differences between 
 them, and to notice the cases to which they are severally 
 applicable, these branches of the subject will be treated in 
 separate chapters. 
 
 The electrical systems of transmitting and distributing power 
 may be classified, according to the direction of the current, as 
 continuous or alternate ; or, according to its division, as series 
 or parallel. The former classification will be adopted here. 
 
 A continuous current is one producing electromagnetic 
 effects which are continuous in direction. An alternate current 
 is one producing electromagnetic effects which regularly and 
 rapidly alternate in direction. One complete alternation is 
 called a period, and the number of periods per second the 
 frequency of the current. This varies in ordinary practice from 
 about 40 to 130. 
 
116 
 
 ELECTRIC MOTIVE POWEE. 
 
 A series system is one in which the current is used in a 
 single circuit ; a parallel system, one in which the current is 
 divided into two or more circuits. Both continuous and alter- 
 nate systems are capable of parallel working, but only the 
 continuous is suitable for series running. 
 
 23. SERIES WORKING. 
 
 The simplest case of transmission is that in which the 
 current is of the same value at all parts of the circuit, and in 
 which therefore the dynamos and motors are all coupled in 
 series with each other (see Fig. 55). The current is usually con- 
 stant, the system being denned as series or constant current, 
 and its application is limited by considerations of pressure of 
 dynamos and speed regulation of motors. It is not likely to 
 
 FIG. 55. Series System. 
 
 be largely used, and is worthy of consideration chiefly on 
 account of the particular case in which there are only two 
 machines one generator and one receiver. (This, although a 
 true series system, is not necessarily a constant current one, 
 and indeed is generally worked with both pressure and current 
 varying with the load ; it is specially discussed at the end of 
 this section.) The constant-current series system, as its 
 name implies, requires the current to be maintained approxi- 
 mately of the same intensity; and alterations of load are 
 brought about by changes of pressure. Since all electrical 
 work can be expressed by the product of the numbers of 
 volts and amperes in the circuit, or symbolically e x i m} it 
 is clear that for a given value of i m , the output of the 
 dynamo (or dynamos) is limited merely by the pressure, e, 
 at which it is found practicable to work. The insulation 
 
CONSTANT CURRENT SYSTEM. 117 
 
 of the circuit and machines presents difficulty, and the 
 collection or commutation of the current is influenced by 
 the pressure. It is found that a pressure of about 3,000 
 volts is as high as can be conveniently dealt with in machines 
 of the open coil type, such as the Thomson-Houston and the 
 Brush arc dynamos ; while with closed-coil drum, disc, or 
 Gramme armatures, one of 2,000 volts is, perhaps, all that can 
 be safely handled. (The difficulty will be increased if the 
 current exceed the 6*8 or 10 amperes required for arc circuits.) 
 
 The pressure difficulty can be met to some extent by coupling 
 two or more dynamos in series with each other, and this plan 
 has been tried with success. It is a common practice to couple 
 mechanically two dynamos in line with the prime motor 
 say, a turbine and to connect the armatures electrically. 
 But in order to maintain the current constant, it is necessary 
 to vary the pressure to suit the work. This may be done in 
 several ways. For instance, the speed of the prime motor, 
 and, consequently, that of the dynamo, may be varied ; or a 
 governor may be used to alter the excitation, or to change 
 the position of the brushes, and so cause the armature to have a 
 greater or less demagnetising effect on the field-magnets. The 
 controlling power for either of these governors is the main 
 current which energises the soft iron core of a solenoid, and 
 connection is made by suitable link motion to variable 
 resistance coils, or to the brush rocking frame. The effect 
 of altering the excitation is sufficiently obvious; but that 
 of shifting the brushes is not so clear. It may be roughly 
 stated thus : When the external resistance of the circuit is 
 lowered, by cutting out one of the motors or arc lamps, or 
 by a reduction of the motor speed, more current tends to 
 flow in the circuit. This increase of current energises the 
 solenoid, causing it to shift the brushes to a fresh position 
 where the pressure is lower, and thereby the current is 
 decreased. When the current falls below the designed value, 
 the governor also shifts the brushes, but in the opposite sense, 
 and increases the pressure so as to tend to raise the current 
 to its normal value. However, in the nature of things, with 
 such a method of control, it is clear that the current cannot be 
 maintained constant, for the governing depends on a departure 
 
118 ELECTEIG MOTIVE POWER. 
 
 from the designed value. In fact, it is found that the current 
 varies between certain limits for full load and no load, the full- 
 load current being less than the no-load current by an appreci- 
 able amount. The result of this variation is, that there is always 
 a tendency to stop any motor which is temporarily loaded 
 beyond the torque corresponding to the particular current. 
 
 The constant-current dynamo has hitherto been chiefly 
 used for arc lighting, and as many as 60 lamps, each of 
 2,000 c.p., and requiring about 50 volts and 10 amperes, have 
 been run in series off a single machine. As a rule, it is not 
 necessary to vary the number of lamps on an arc circuit either 
 all lamps or no lamps being required to be in use and, there- 
 fore, the fluctuation of the current is not of much moment, 
 and, in any case, would only increase or diminish the illuminat 
 ing power. But with motors it is a very different matter. 
 
 The torque of a given motor [see equation (16), T = a l& " a , 
 
 K 
 
 p. 43], depends simply on the current in the armature, if the 
 field excitation be kept constant. In considering the action 
 of motors in series on a constant current circuit, first let it 
 be assumed that there is only one motor in the circuit, and 
 that its field excitation is constant. The motor will run at a 
 -stable speed, determined by the relative values of the torque, 
 excitation, and current. Next suppose that the main current 
 is decreased by the interpolation in the circuit of another 
 motor, also with a constant field excitation. As soon as the 
 current passes through the second machine its armature will 
 start into motion, if the current be large enough to overcome 
 the static resistance, and will steadily increase in speed until 
 the counter pressure is just sufficient to permit the particular 
 value of current to flow under the forward pressure ; or when 
 
 where i m the main current, 
 
 e = the forward pressure, 
 E = counter pressure of the motor, and 
 r = the resistance of the motor. 
 
CONSTANT CURRENT SYSTEM. 119 
 
 But if the torque be too large for the particular current and the 
 excitation, the armature cannot move ; and the introduction 
 of the second motor will increase the resistance of the circuit, 
 and so lower the current, the immediate effect of which will be 
 to stop the first motor, unless the dynamo is governed so as to 
 keep the current constant. 
 
 These effects are intensified by the fact that a constant cur- 
 Tent dynamo or motor is usually series wound, and that con- 
 sequently the field coils are traversed by the main current ; 
 therefore, a variation of current affects the torque not only by 
 virtue of its effect on the armature, but also by altering the 
 -excitation. Thus the constant current motor (as well as the 
 dynamo) requires a governor, controlled by the current or the 
 speed, which will strengthen the excitation as the speed 
 ^diminishes, and vice versd. The problem of running each 
 motor independently of all others in series with it has 
 been attempted by means of centrifugal governors, which, 
 essentially, move a short-circuiting piece so as to cut in or out 
 of circuit more or less of the winding, or else vary the resist- 
 ance of a circuit placed as a shunt to the main exciting coils. 
 The same end might be attained by means of a solenoid 
 actuated by the main current, if only one motor were used ; 
 but there do not appear to be any published accounts of prac- 
 tical devices embodying this idea. The author has built a 
 number of motors with centrifugal governors, which have 
 worked satisfactorily enough on power circuits ; and in the 
 United States similar motors are used on arc lamp circuits, 
 but probably the number is not large. The power plant at 
 Genoa is, at present, the most interesting illustration of con- 
 stant-current working. It is noteworthy that the designers 
 have found it convenient to control the pressure by vary- 
 ing the speed of some of the generators, and by altering the 
 excitation of others. But the motors are all controlled by 
 governors which vary the field excitation. This plant is most 
 successful. 
 
 Evident objections to constant current working are the 
 heavy field-magnets necessary to prevent stoppages with tem- 
 porary overloading, and the difficulties of maintaining the 
 
 K2 
 
120 ELECTRIC MOTIVE POWEH. 
 
 current constant. In cases where the load is constant, sucb 
 as in driving ventilating-fans, for example, no governor is re- 
 quired, and series motors are more convenient than parallel 
 ones, since no starting resistance is necessary. And in 
 cases where the excitation can be controlled by hand, as 
 on tramcars, there is an apparent field for series motors on. 
 constant current circuits. In practice, however, it is found 
 that if more than three or four cars are worked in series 
 the one with the heaviest load is likely to stop, and cannot be 
 re-started until the load on the others is decreased. In fact,, 
 the possible output of the dynamo is fixed by its speed 
 and the arrangement of the brush-rocking governors, and 
 there is only a limited number of volts free at any particular 
 moment to deal with sudden temporary demands. As is well 
 known, the load diagrams of tramcars show large variations 
 occurring far too suddenly for the steam governors to control ;. 
 and therefore, just when the dynamo should increase rather 
 than decrease its speed, the engine is checked and an increase 
 of pressure is impossible for the moment. 
 
 The series traction system was tried between Gravesend and 
 Northfleet from 1888 to 1891. There were two trams with, 
 series motors regulated by hand, and a self-regulating constant 
 current dynamo. No special difficulty was met with from the 
 reasons just discussed ; but if there had been five or six trams 
 the experience would have been similar to that of the Short 
 Company and others in America. At the present moment 
 there seems to be no probability of series traction competing 
 with parallel. 
 
 Some stress has been laid on constant current trans- 
 mission because the system presents such favourable 
 conditions for the conductor. Since the current is only 
 fixed by the limits of pressure deemed advisable for the 
 given work, it follows that in cases where the distances are 
 considerable a current of few amperes can be chosen, and 
 the copper wire be as small as mechanical considerations will' 
 permit. And further, assuming the governors to work properly, 
 two or more motors far apart from each other may be run in 
 series without variations of load affecting the speed of either 
 
SERIES DYNAMO AND MOTOR. 
 
 121 
 
 through changes in the fall of pressure in the line. The con- 
 stancy of loss in the line, i m 2 R, whilst advantageous as regards 
 speed regulation, is objectionable from economic considera- 
 tions, for the same quantity of energy is lost under all 
 conditions of load. With parallel working, the line loss is 
 roughly proportional to the load ; but the speed of the 
 motors falls off as the load increases, owing to the loss or 
 drop of pressure. Some means of compensating for this drop 
 
 L.UU 
 
 450 
 400 
 350 
 
 .300 
 til 
 
 o: 
 
 3 250 
 
 2 
 
 cc 
 a- 200 
 
 
 
 
 
 
 
 ^ 
 
 
 
 
 
 / 
 
 ^ 
 
 
 
 
 
 / 
 
 ^ 
 
 ^ 
 
 *~ **' 
 
 
 
 / 
 
 X 
 
 
 
 
 
 / 
 
 77 
 
 / 
 
 
 
 
 
 
 2 
 
 
 
 
 
 
 , 
 
 // 
 
 
 
 
 
 
 100 
 5C 
 
 
 
 1 
 
 / 
 
 
 
 
 
 
 I 
 
 
 
 
 
 .*-- 
 
 -" 
 
 L 
 
 ^.e** 
 
 *"~* 
 
 
 
 
 
 IO 20 30 40 50 60 7 
 
 MAIN CURRENT 
 
 FIG. 56. 
 
 Curve 1 is the internal characteristic of a series dynamo running at 
 a constant speed. Curve 2 is the internal characteristic of a series 
 motor designed to be driven by the dynamo. Curve 3 represents 
 geometrically the summation of the resistances of dynamo, motor, and 
 cables. Note that with series machines the field and main currents are 
 the same. 
 
 is necessary. The usual methods are referred to in 26, 
 p. 142. A compromise between these two systems is found in 
 a particular case of the series system in which high pressure, 
 Small current, light conductor, line loss proportionate to load, 
 
122 ELECTRIC MOTIVE POWER. 
 
 and good speed regulation, can be obtained ; but it is only 
 applicable to one dynamo and one motor. The necessary 
 conditions are : 
 
 Both machines must be series wound and designed for the 
 same maximum current ; the dynamo must be run at a con- 
 stant speed ; and the characteristics of the two machines must 
 be such that the difference in pressure between the total 
 E.M.F., E d> of the dynamo and the counter E.M.F. of the 
 motor, E m (at least on the working part of the curve), is always 
 equal to the pressure lost over the entire circuit, viz., line and 
 internal resistance of the two machines. The idea will be 
 readily grasped on reference to Fig. 56, in which the top 
 curve connects the total dynamo pressure with the current, 
 and the dotted curve shows the motor counter-pressure for cor- 
 responding values of the same current. The difference between 
 the heights of the ordinates is suggested by the dotted line 
 at an angle to the abscissae, the slope of which is proportional 
 to the total resistance of the circuit. 
 
 It is not easy to fulfil exactly the conditions necessary to 
 give a perfectly constant speed through a long range of the 
 curve ; but there is no difficulty in a close approximation 
 through, say, a current variation of about 60 per cent., and 
 this is really all that is required in most cases, and no attempt 
 is made to get a better regulation. When the load is taken off 
 the motor the speed will increase by perhaps 15 per cent., but 
 this is of little importance in general work. 
 
 Fortunately, many power problems are capable of solution 
 by this method, and no doubt its simplicity has done much 
 to commend the use of electrical transmission of power. 
 Further reference is made to it in the chapter describing 
 actual plants, in which some figures taken from practice illus- 
 trate the usual limits of speed regulation. 
 
 24. PARALLEL WORKING. 
 
 The term " parallel " is applicable to a number of systems 
 of distribution, in all of which the current is divided into- 
 branch circuits, and the power is supplied to the distributing. 
 
PAEALLEL SYSTEMS. 123 
 
 mains at approximately constant pressure. The conducting 
 circuit may be complex, and as it will be necessary to make 
 frequent reference to its various parts, it will simplify matters 
 to define the terms used. 
 
 The feeding mains, or simply the feeders, are the large con- 
 ductors which carry (high or low pressure) current to a central 
 point of distribution, or to the nearest motor, in a simple 
 parallel case. They may be long or short, according to circum- 
 stances, but since their function is that of carriers only, they 
 relate to the transmission part of the problem, and are distinct 
 from the distributing network of mains. Feeders, therefore, 
 are never tapped by branch connections. 
 
 The distributing mains, or briefly the distributors, are 
 designed to carry the current from a centre of distribution, i.e., 
 the end of a feeder, to the service mains which supply each 
 consumer. They vary in section and length according to the 
 area to be served, and may form simple radial lines from the 
 distributing station, or in some cases a complicated network 
 looped at the extremities by ring mains. Distributors may 
 be tapped at any point to supply current to consumers. 
 
 The Service mains are the pairs of conductors connecting 
 a motor or bank of motors (or lamps) to a distributor. 
 
 The parallel systems suitable for power work are : 
 
 (a) Simple parallel, reverse parallel. 
 
 (b) Feeder system. 
 
 (c) Three-wire system. 
 
 (d) Multiple-wire feeder system with regulators. 
 
 Inspecting Figs. 57 to 64,* which illustrate diagrammatically 
 the several arrangements, it will be seen that the condition of 
 constant pressure between the distributors is common to all, 
 and that since the energy at any point is equal to the product 
 ei m , it follows that variations of load have to be made by 
 changing the current of the particular motor (or lamps). 
 
 * Pages 124 to 136. 
 
124 ELECTRIC MOTIVE POWER. 
 
 This is exactly the converse of the series system of supply. 
 The immediate effect of the variable current is to cause the loss 
 in the mains (feeders and distributors) to vary with the load. 
 This is an advantage, since the line loss is then roughly 
 proportionate to the useful effect ; but it also causes the pres- 
 sure at different parts of the line to vary inversely as the load, 
 which is exactly the opposite of what is required. 
 
 The line loss is expressed in watts by : 
 
 I + i 2 . . . +4 2 R n , 
 
 where i, i^ i 2 . . . i n are the several currents in different 
 parts of the circuit, of which the respective resistances are 
 R, RJ, R 2 . . . R n . And the fall of pressure in volts on the 
 several sections is given by i R, i R I} i 2 R 2 . . . i n R n . 
 
 FIG. 57. 
 Incandescent Lamps in Series on High- Pressure Motor Circuit. 
 
 The aim, then, in a parallel system is to compensate for 
 the drop in the feeders, and to keep the pressure constant at 
 the centre of distribution. There will remain a slight fall of 
 pressure uncompensated for between this centre and the points 
 of supply, i.e., the motor terminals. But this is practically 
 unavoidable, and in most cases can be made so small as to give 
 no trouble if sufficient copper be put into the service mains ; 
 for then the product iR becomes so small, even at full load, 
 as to be insignificant. The weak point in simple parallel 
 working is that the pressure is limited by the voltage of the 
 smallest motor, or that of the lamps run in parallel with 
 the motors. In lighting circuits, where incandescent lamps 
 requiring about 100 volts are mainly used, the pressure at the 
 centre of distribution is necessarily about 100 volts, although 
 at the station it must be higher by an amount determined by 
 
WEIGHT OF COPPER IN LINE. 125 
 
 the resistance of the conductors. In power circuits where 
 lighting is of secondary importance the pressure on the distri- 
 butors may be, say, from 300 to 500 volts ; and lamps may be 
 coupled in series to make up the required voltage. Thus three 
 incandescent lamps of 100 volts each may be coupled in series 
 across a pair of mains having a pressure between them of 300 
 volts ; and, similarly, five 100- volt lamps may be placed across 
 a 500-volt circuit (see Fig. 57). 
 
 Raising the pressure of distribution permits a reduction in 
 the weight of the copper, but at the same time introduces 
 complications which require careful attention. 
 
 In the first place, consider a constant length of conductor, 
 with constant power at supply station, and let the pressure of 
 supply be varied. As before : 
 
 a = area of conductor. 
 
 R = resistance of conductor = - k, where I = the length of 
 
 a 
 
 conductor and k is a constant. 
 e = pressure of distribution. 
 
 Then we have 
 
 i 2 R = line loss ; - = the current density. 
 a 
 
 i R = drop of pressure in volts. 
 e i = the power transmitted = a constant. 
 
 Now, assume that the pressure is doubled, so that it may be 
 presented by 2 e, then the 
 is to be the same as before. 
 
 represented by 2 e, then the current will be -, since the power 
 
 The line loss will now be (-) R= , i.e., only one-fourth 
 \2/ 4 i 
 
 as great ; and the drop of pressure will be - R, or one-half. 
 The section of the conductor may, therefore, be safely reduced. 
 
 There are two cases of importance in practice. 
 
126 ELECTRIC MOTIVE POWER. 
 
 Firstly, when it is essential to keep the line loss, i 2 R, constant, 
 In this case the new conductor need have only one-fourth of 
 the area of the old one, but will have four times the resistance;. 
 
 (" \ 2 
 - J 4 R = i 2 R, the same as before ; 
 
 and the line drop will be ~ 4 R = 2 i R, or the same percentage 
 
 drop as before, which is equivalent to saying that the line loss 
 remains constant when the percentage of the line drop is kept the 
 same for variations of supply pressure. It must be noticed that 
 the current density has been doubled, for it is now equal to 
 
 1 _._ a _ i 
 
 2 ' 4 a 
 
 Secondly, when it is necessary to keep the line drop,, 
 i R, constant. This can be accomplished by using a 
 conductor of half the area and twice the resistance, for 
 
 then the drop = ^ 2R = ^R. And the line loss will be 
 
 f~\ 2 R = z -^, or only one-half as great as before. The 
 
 economic current ratio, -, is constant, for both current and 
 a 
 
 area of conductors are halved. Or, generally, when the line 
 drop is kept constant for variations of supply pressure, the 
 current density is also constant. 
 
 The latter of these two cases is, perhaps, the more important 
 with respect to the feeders, for in relation to them the 
 economic current density is the chief consideration. And the 
 
 ratio - should have the same value in both long and short 
 
 a 
 
 feeders, the drop of pressure in each being compensated for at 
 the supply station. Strictly speaking, on the score of economy, 
 the same considerations should determine the gauges of the 
 distributors, but frequently the fall of pressure will be the 
 only criterion, and then the conditions suggested in the first 
 case will obtain, viz., a constant percentage of drop. 
 
 If the current density, -, be settled, as it should be, in< 
 a 
 
 accordance with the economic law, the current varies as the 
 
DEOP OF PRESSURE. 127 
 
 area of the conductor, and the drop, i R, is simply proportional 
 to the length of the conductor. For each pressure of supply 
 and current density, then, there will be a definite distance 
 through which the power can be transmitted for a given drop. 
 This distance includes the feeders, the distributors, and the 
 service mains, if the drop be measured from the dynamo to 
 the. motor terminals ; but in most cases it will be convenient 
 to consider the three drops separately. 
 
 The fall of pressure for any section may be determined in 
 
 the following manner if the current density, -, be fixed : 
 
 a 
 
 (23) 
 
 And if / be expressed in yards and a in square inches (the most 
 convenient and practical units for these calculations), then K 
 will be the numerical value of the resistance of one yard of 
 copper of one square inch in cross section, and will be approxi- 
 
 95-6 
 mately equal to - for ordinarv commercial samples. There- 
 
 fore 
 
 (24) 
 
 From this equation, if we know the drop, we can find the 
 possible length of conductor ; and if I be given, then the 
 corresponding drop can be estimated. 
 
 It will sometimes be more convenient to estimate the drop- 
 in terms of the length of the section (measured along the 
 conductors) rather than in terms of the length of conductor. 
 To do this it is only necessary to put I equal to the length of 
 the section, and multiply the right hand member by 2 ; thus 
 
 9 9Pvft 
 
 iR, = 2L-. _ ...... (25) 
 
 l a 10 6 
 
 If i'R = l, then J x will give the number of yards per volt 
 
 drop for the particular value of the economic ratio -. 
 
 a 
 
128 
 
 ELECTRIC MOTIVE POWER. 
 
 The following table shows the relation between drop and 
 distance for various pressures and current densities : 
 
 Table I. Tivo Wire Mains. Drop of 1 per cent. 
 
 Current density 
 in amperes 
 
 Di.-tance in yards from the power station for a drop 
 of 1 per cent, at the following pressures of supply. 
 
 per square inch. 
 
 100 v. 
 
 200 v. 
 
 500 v. 
 
 1,000 v. 
 
 1,000 
 
 19-5 
 
 39-0 
 
 97-5 
 
 195-0 
 
 900 
 
 21-75 
 
 43-5 
 
 108-75 
 
 218-0 
 
 800 
 
 24-5 
 
 49-0 
 
 122-5 
 
 245-0 
 
 700 
 
 27-75 
 
 55-5 
 
 138-75 
 
 278-0 
 
 600 
 
 32-5 
 
 65-0 
 
 162-5 
 
 325-0 
 
 500 
 
 39-0 
 
 78-0 
 
 175-0 
 
 390-0 
 
 4 
 
 49-0 
 
 98-0 
 
 245-0 
 
 490-0 
 
 300 
 
 65-25 
 
 130-5 
 
 316-0 
 
 653-0 
 
 To find the distance for a drop of n per cent., multiply the distance in 
 the columns against the given pressure and current density by n. 
 
 In the above table the percentage of drop in the line and 
 the current density are kept constant for pressure variations, 
 both current and area of the copper varying inversely as the 
 pressure of supply. The line loss, i m 2 R, will remain constant 
 under these conditions. 
 
 It is thus sufficiently clear that a parallel system requires 
 conductors of larger section than is necessary for a series 
 one ; and a compromise involving a series transmission and a 
 parallel distribution is at once suggested. 
 
 Let us examine the various methods a little more in detail. 
 
 The simple parallel system is shown in Fig. 58. In this the 
 whole of the current necessary for motors 6, c, and d has to 
 traverse the two conductors included in the section of the line 
 a b, and that required for motors c and d has to pass through 
 section b c, while c d only carries the current for motor d. Now, 
 the areas of the conductors in the several sections of the line 
 will be suited to the maximum, or more probably the average, 
 current required to pass through them. There will, therefore, be 
 
SIMPLE PARALLEL. 
 
 a different pressure of supply at each of the points 6, c, and d. 
 This in itself would not matter if the actual values remained- 
 constant, but since the pressure of the line varies inversely 
 as the current, it follows that the pressure will be lowest 
 when the three motors are each absorbing their greatest 
 current, and that it will vary at the several points of 
 supply according to the current at each. Now, the speed 
 of a motor is proportional to the pressure of supply for 
 given currents, and therefore the speed will decrease as the 
 load increases, from this cause, as well as from the drop over 
 the internal resistance of the motor, which is necessarily 
 greater with large than with small loads. 
 
 This difficulty is enhanced if the distance from the dynamo 
 to the first motor, from thence to the second motor, and from 
 thence to the third motor, &c., be considerable. Indeed, in this 
 
 FIG. 58. Simple Parallel System^ 
 
 case a limit is very soon reached where the system hopelessly 
 breaks down from the difficulties of regulation already re- 
 ferred to, and from the cost of the copper required to keep 
 the line drop and loss within practicable bounds. To fix ideas, 
 suppose the maximum current to be carried in the feeders 
 a b to be 100 amperes, and the current density in them to be 
 selected such that one square inch of area of cross section 
 of copper is allowed to every 500 amperes. Then, since the 
 maximum current is 100 amperes, the area of the conducting 
 
 wire will be x 1 = 0'2 sq. in. The resistance of this wire is 
 
 500 
 
 '2 2 of an ohm per mile, and if we assume t&e feeders a b to be 
 one mile a very fair assumption for a transmission of power 
 problem the fall in pressure from the dynamo to motor b will 
 be 100x0-44 = 44 volts. Now, if the pressure at b be 100 
 volts, it is clear that that at the dynamo- mus-t be 144, and the 
 
130 ELECTEIC MOTIVE POWER. 
 
 line efficiency, assuming all the power to be absorbed at point 6, 
 is only =-jT"*- or ^ P er cent> * fc * s nee ^ ess to sa J tnat a l ss i n 
 
 the line of 30 per cent, is prohibitive in most cases. If the points 
 c and d bo severally far removed from b and from each other the 
 line loss h still further increased, and ultimately may become 
 so large as to absorb the greater part of the total power. 
 
 The apparent remedy is to raise the pressure. Let it be 
 doubled (say, 200 volts), and the current consequently halved 
 (50 amperes), and let the same section of conductor be used. 
 The current being halved, the line loss will be one quarter as 
 great, the fall of pressure will be halved, and the line 
 
 efficiency becomes - or 91 per cent, at the point 6, and is 
 correspondingly raised at points c and d. But the current 
 
 FIG. 59. Reverse Parallel System. 
 
 density in the conductors will be at the rate of only 250 amperes 
 to the square inch, for the current is now only 50 amperes. 
 This implies an extremely costly conductor for the current. 
 Practically, a current density of from 500 to 800 amperes 
 will usually be found to be about correct. 
 
 If it be required to cause the pressure at different parts of the 
 circuit to vary approximately equally, the arrangement shown 
 in Fig. 59, which is sometimes useful in lighting circuits, may 
 be adopted ; but the weight of copper in one of the feeders is 
 evidently increased, and if the distance a b be at all consider- 
 able compared with be and cd, the variation of pressure in the 
 feeders may still be so great as to practically outweigh the 
 better regulation gained over the distributors by the looped 
 wire. The scheme at best is but an approximation, and only 
 likely to be used in cases where the motors are not grouped 
 
FEEDER SYSTEMS. 131 
 
 but lie conveniently in a line. In practice, they are much 
 more likely to lie in a group, and some point fairly equidistant 
 from the majority of them will be selected as the centre of 
 distribution. The pressure there will be maintained as nearly 
 constant as possible ; and each motor or group of motors will 
 be run on separate distributors, the areas of which will be so 
 calculated with reference to their length and to the maximum 
 or average current, that the maximum or average fall of 
 pressure will be fixed between reasonable limits. 
 
 This brings us to the feeder system (b) shown in Fig. 60' 
 As before, D represents the dynamo, and a the power station 
 if there be a number of dynamos in parallel; a b represents the 
 
 153 
 
 FIG. 60. Feeder System. 
 
 transmission mains or feeders, b the centre of distribution and 
 the point at which the pressure is to be maintained constant ; 
 and c, d, e, f and g severally represent motors at the ends of 
 distributors be, bd, be, bf, and bg. Let the pressure of dis- 
 tribution be fixed at 300 volts at sub-station b, and let the 
 feeder efficiency at full load be 90 per cent., a drop of, say, 30 
 volts being permitted along ab. The pressure at the power 
 station will be 330 volts at full load, and will fall as the load 
 is decreased until the smallest output is reached, when the 
 pressures at the station a and the distributing centre b will be 
 approximately the same. The fall of pressure in each of the 
 distributors may be regulated according to the class of work 
 and regularity of speed required an average drop of about 
 
132 ELECTRIC MOTIVE POWER. 
 
 five per cent, will usually be permissible for power work. 
 The efficiency of the entire line will be, say, 85 per cent, 
 under these assumptions ; but it must be remembered that the 
 conditions selected are entirely arbitrary, and the economic 
 limitations discussed in Chapter III. have not been taken into 
 consideration in any way whatever. 
 
 The next practical point bears on the limiting distances 
 through which power can be efficiently transmitted. The two- 
 wire feeder and distributor system (Fig. 60) has already been 
 examined, and it is clear that the distance of transmission is 
 practically fixed by the station pressure. Now, the pressure 
 of the distributors cannot be greater than that permitted by 
 the conditions of supply to the consumers 500 volts will 
 
 Middle Wire. 
 
 FIG. 61. Three-wire System. 
 
 probably be the maximum. But there is no restriction to 
 the pressure between the feeders, other than that of con- 
 venience. The three-wire feeder system, shown in its simplest 
 form in Fig. 61, meets this difficulty to some extent, for by its 
 use the feeder-pressure may be doubled without raising that of 
 the distributors. In this, essentially, two dynamos are coupled 
 in series across the feeders, each giving, say, 500 volts and 50 
 amperes, and thus making a line pressure of 1,000 volts. 
 
 At the end of the feeders two suitable motors are also coupled 
 in series, and a third wire is coupled to the junction of the 
 dynamos and also of the motors as is indicated in the diagram. 
 The path of the current, when the motors are equally loaded, 
 is shown by the arrow heads, and it is seen to be entirely con- 
 fined to the machines and the two outer conductors, the middle 
 
THREE-WIRE SYSTEMS. 133 
 
 or third wire carrying no current. But if one motor absorbs 
 more current than the other, the difference between the two 
 currents will pass through the middle wire. Let i be the cur- 
 rent in the circuit D 1 M x ; i 2 that in the circuit D 2 M 2 ; and i z 
 that in the middle wire. Now when ^ = i^ then i.^ = ; and also 
 when 1 >i' 2 , then i l -i^ = i y When i 2 is zero, i.e., when M 2 
 is not running, then ^ 1 = ^' 3 , and the middle wire has to 
 carry the whole current in the circuit Dj M r In this case 
 it is not advisable to make the middle wire of less section 
 than the outer ones ; but if M : and M 2 each represent a 
 group of motors, it is reasonable to assume that full load 
 on one circuit and no load on the other will be unlikely to 
 occur at the same time, and, therefore, the middle wire will 
 have to carry only the difference between the two currents. 
 In practice, with a load of lamps, it is found to be sufficient 
 if the area of the middle wire be half that of one of the outer 
 wires. And it should be recollected that if the distributing 
 centre be a large one it will be provided with suitable switching 
 arrangements for transferring part of the load on the distribu- 
 tors from one circuit to the other, as may be necessary to 
 secure a proper balance, and to keep the current in the middle 
 wire as small as possible. 
 
 Assuming the load to be so arranged that it will be 
 generally divided nearly equally between the two circuits, it 
 will be safe to follow the practice usual in lighting and to 
 make the middle wire of one-half the area of the outer 
 ones. This will permit a sufficiently wide difference of load 
 in the two circuits to meet the requirements of practice. 
 Now it is seen that for the same output from the power 
 station the three-wire system requires twice the pressure 
 and half the current necessary in the simple parallel systems 
 shown in Figs. 58 to 60. And, if the same gauge of wire 
 be used in the outer mains, the weight of copper in the 
 three feeders will be 25 per cent, greater than that in the 
 two, being in the ratio of 5:4. But since the middle wire 
 is nominally idle, and the outer feeders carry a current 
 which is equal to only half of that in the two-wire mains, 
 it is clear that the power lost with the three wires will be 
 only one- quarter of that with the two wires, and that the fall 
 
134 ELECTRIC MOTIVE POWER. 
 
 of pressure will be only half as great. For equal conductors 
 and the same drop of pressure, the three-wire may be there- 
 fore twice as long as the two-wire system ; and for equal line 
 loss or feeder efficiency and the same percentage of drop, four 
 times as long. And if the relative weight of copper for equal 
 lengths and line drop be considered, since the area of each 
 outside conductor is one-half, and of the middle conductor one- 
 fourth, of the area of either conductor in the two- wire system, 
 the ratio of total weights will be as 5 : 8, showing a net 
 saving of 37 J per cent, in favour of the three-wire system. 
 
 FIG. 62. Three-wire System in Feeders and Distributors. 
 
 This system is evidently capable of extension. For if four 
 wires be used and coupled to three dynamos and three motors, 
 the pressure may be 1,500 volts over the outer feeders and the 
 current only 33-3 amperes ; and if the outer mains have the 
 same section as before, the distance through which the power 
 may be transmitted with the same drop is again increased by 
 50 per cent., or is three times as great as that with the parallel 
 system. And the weights of copper for equal lengths and line 
 drop are as 5 : 1 2, when compared with the two-wire system. At 
 Manchester a five-wire system has been erected for lighting, to 
 the specifications of Dr. J. Hopkinson. The outer mains are 
 at 420 volts, approximately, and between each pair there is & 
 pressure of 105 volts. 
 
THEEE-WIRE SYSTEMS. 
 
 135 
 
 The three-wire system is applicable to the distributors as 
 well as to the feeders, and is an important means of limiting 
 the fall of pressure in the distributing network (see Fig. 62, 
 which represents the disposition of feeders, distributors, and 
 motors). It is apparent, however, that the three-wire feeder 
 system, as well as the two-wire, ultimately reaches a limit 
 beyond which it does not pay to transmit power ; for though 
 the pressure may be raised almost indefinitely as regards the 
 feeders, it is not so with the distributors 500 volts being 
 probably a maximum for the supply service even in power 
 work. 
 
 rO 
 
 M 
 
 R) 500 
 
 t ) 1 
 
 o v Q M OM 
 
 i Y Y 
 
 500 V 
 
 5 
 
 
 500 V 
 
 -4 
 
 500 V 
 
 M_ ONI 
 
 M AM 
 
 M 
 
 o- 
 
 FIG. 63. Feeder System with Regulator. 
 
 A convenient combination of the feeder and multiple- wire 
 systems consists of a pair of high-pressure feeders coupled to 
 a regulator, which may be a motor having as many armatures 
 as there are sub-circuits, and feeding each pair of distributors 
 at the desired pressure. The general idea is represented in 
 Fig. 63. In this diagram the armatures are sketched as if 
 they were independent of each other. But in practice they 
 are rigidly coupled or built on one shaft, and essentially 
 run in magnetic fields of the same strength and direction 
 or in a common field. The armatures are of identical con- 
 struction, and their internal resistance is made as low as 
 possible. The field excitation is preferably supplied from an 
 independent source, such as accumulators, which may be 
 
 L2 
 
136 ELECTEIC MOTIVE POWER. 
 
 charged in parallel across one of the sub-circuits at suitable 
 times. Normally, when the load is equally divided between 
 the sub-circuits, the four armatures absorb just sufficient 
 current to overcome friction. But when one of the circuits 
 takes more current than the others the armature in shunt 
 to it will tend to lag. And the armatures in parallel with 
 the other circuits will resist this alteration, and being coupled 
 to the lagging armature will spend power in keeping the 
 speed constant. Thus the lagging armature will be driven as 
 a generator and supply the increase of current to its circuit, 
 and the necessary power for the increase of torque will be 
 supplied from the generating station. The degree of regu- 
 lation attainable with this device depends on the internal 
 resistance of the armatures ; and it can only be efficient when 
 
 
 3 1=1 ioo volts (M) (M) 
 
 > -=r- i V V 
 
 Volts.(M) 
 
 100 Volts 
 
 FIG. 64. Feeder System with Accumulators. 
 
 the pressure lost over the armatures is negligible. The regu- 
 lator principle is capable of considerable modification, and is 
 Jikely to be used, in various forms, for lighting in cases where 
 the feeders are of great length. 
 
 Secondary batteries have already been used with undoubted 
 success in connection with feeder systems with sub-stations, 
 by Mr. Crompton and others. The method is illustrated in 
 Fig. 64. It will be apparent that the secondary battery acts 
 as an accumulator as well as a regulator, and can be arranged 
 to store up, during periods of light load, energy which will be 
 available for temporary outputs exceeding the rated engine 
 and dynamo power. In many cases also the accumulators 
 can deal with the small loads during parts of the early morn- 
 ing, and the steam plant may be shut down for that period. 
 
DYNAMOTOES. 137 
 
 It is probable that secondary batteries will be found profit- 
 able for simple power work in some cases ; and for combined 
 light and power they certainly are always worth serious con- 
 sideration. The author believes strongly in the future of the 
 storage battery, and insists upon its importance in all direct 
 current installations when the output is intermittent, or the 
 load factor small. 
 
 25. CONTINUOUS CURRENT TRANSFORMERS 
 OR DYNAMOTORS. 
 
 The continuous current transformer, motor generator, or 
 dynamotor (Fig. 65, p. 138) presents some points of interest 
 ior long-distance transmissions; for, when used in parallel, 
 ^ach dynamotor forms an independent sub-station. 
 
 The principle of the dynamotor has already been mentioned 
 when considering regulators. The essential difference between 
 the two machines is, that in the dynamotor the primary and 
 secondary circuits are separate, whereas in the regulator they 
 are connected. In its simplest form the dynamotor consists of 
 a motor coupled to a dynamo, the motor being connected to the 
 primary circuit, usually a high-pressure one, and the dynamo 
 feeding the secondary circuit with a low-pressure current. Of 
 various feasible arrangements the most common consists of 
 two armatures mounted on one shaft and run in common field 
 magnets excited in shunt from the low-pressure secondary 
 mains. The magnets have a few series turns which are tem- 
 porarily coupled to the primary circuit at starting. Or, the 
 motor and dynamo windings may be placed on the same 
 armature, and the commutators arranged one at each end, 
 the field magnet details being as before (see Fig. 65, p. 138). 
 These machines are adapted for parallel working, and have been 
 used by the Electric Construction Company at the Crystal 
 Palace and Oxford Central Stations and elsewhere. They are 
 placed at the distributor end of high-pressure feeders, and 
 feed the distributing network with current at a pressure of 
 about 100 volts. Each dynamotor forms a sub-station. The 
 general arrangement is illustrated in Fig. 66. It will be seen 
 that the transformers feed into different parts of a network of 
 
138 
 
 ELECTRIC MOTIVE POWER. 
 
DYNAMOTOE SUB-STATIONS. 
 
 139 
 
 distributing mains. The proportion of the total current given 
 out by any one transformer depends simply upon its pressure, 
 and since voltmeter wires are taken from each point to the 
 central station it is easy to alter the pressure of the feeders, 
 and consequently that of the distributors. By this simple means 
 the load can be divided as desired between the dynamotors, the 
 number of which in use at any time can also be readily controlled. 
 Thus supposing it is necessary to start a fresh dynamotor at, say, 
 
 
 Volts 
 
 
 
 
 Volts. 
 
 
 A A 
 
 (M) (M) 
 X X 
 
 
 A gfl/j 
 
 , V ?> Sr 
 
 ) p (M) 
 
 (M) 
 
 ; $ 
 
 
 
 
 
 -vl O-,>J- 
 
 FIG. 66. Dynamotor Parallel Sub-Station System. 
 
 sub-station b v the field of the machine is first excited, then the 
 armature is started and the pressure adjusted approximately 
 to that between the distributors to which it is to be coupled. 
 The circuit between the dynamotor and the distributors is 
 next closed, and finally the pressure is raised at the station 
 end of the feeder until the dynamotor has its proper share of 
 the load as gauged by the feeder current. A dynamotor is 
 removed from the circuit in a similar way, the feeder pressure 
 
140 ELECTRIC MOTIVE POWER. 
 
 being lowered until the feeder ammeter shows only the exciting 
 and friction current; the distributor circuit can then be opened 
 without sparking or disturbing the pressure of the distributing 
 network. The whole of the necessary switching and regulating 
 can be done from the power station. 
 
 It may, however, fairly be questioned whether parallel 
 dynamotors present any real advantages for power work, or 
 for installations in which power work is of more importance 
 than lighting. For it is evident that the feeders can be 
 brought close to the motors in most cases by means of one of 
 the multiple-wire and feeder systems already described. Yet 
 the author has found dynamotors to be of use as a detail in 
 certain cases, especially where the pressure of the main distri- 
 butors (500 volts) is too high for the small motors, or where 
 from any other cause it is deemed to be unsafe to carry the 
 high-pressure mains into certain parts of the area of supply. 
 A dynamotor may then be driven off a pair of distributors 
 to deliver power at a suitable pressure, as at the Greenside 
 Lead Mines. The author drew attention to this field for 
 continuous-current transformers as far back as 1889, and 
 he has already installed several for running rock drills, and 
 also for working electric locomotives inside mines with bare 
 overhead conductors. Dynamotors with the primary circuits 
 in series and fed with a constant current are not, in the 
 author's opinion, likely to prove of much use in practice ; for 
 the arrangement requires the secondary circuits also to be in 
 series, and governors would probably be required to control the 
 field excitation. But there are a few cases, perhaps, where 
 series dynamotors might prove commercially profitable. The 
 general scheme for series coupling is illustrated in Fig. 67, 
 which is sufficiently explanatory to show the chief features. 
 It will be seen that the system has all the drawbacks of a 
 simple series transmission, and although some economy may 
 be obtained in the weight of the conductors, yet, since the 
 loss in both primary and secondary conductors is necessarily 
 constant and independent of the useful load, the system can- 
 not be economical unless the load factor is very high. The 
 difficulties attending its use are further increased by the 
 fact that the dynamotors are necessarily designed for the 
 
DYNAMOTORS. 
 
 141 
 
 maximum power of the motors, while the average output is 
 not likely, except in special cases, to exceed, say, 50 per cent. 
 of this. 
 
 If it were possible to run the primary circuits in series at 
 constant current, and to feed the various secondary circuits at 
 constant pressure, dynamotors would combine the advantages 
 of the series system as regards the transmission mains and that 
 
 FIG. 67. Dynamotor Series Sub- Station System. 
 
 of the parallel as regards proportionality of line loss to load in 
 the distributing circuits. But the solution is too complicated 
 for practical consideration. And, moreover, the author is 
 strongly of opinion that, assuming such a scheme to be 
 practicable, it would not be so economical, all things being 
 considered, as a two-wire feeder and multiple-wire distributor 
 system, with or without regulators, unless the load factor 
 were very high a rare thing indeed, even in power work. 
 
142 
 
 ELECTRIC MOTIVE POWER. 
 
 38. COMPENSATING FOR FALL OF PRESSURE 
 ON FEEDERS AND MAINS. 
 
 In the preceding description of continuous current systems 
 of transmitting power, it has been assumed that the pressure 
 at the distributing centre the sub-station or sub-stations 
 can be maintained practically constant. This is evidently a 
 necessary condition of satisfactory distribution either of power 
 or light. In order to secure it the pressure of supply at the 
 
 500 
 
 
 
 
 1 
 
 . a . -i 
 
 ... 
 
 .. 
 
 " 
 
 b - j - ' 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 400 
 
 
 
 
 
 
 
 
 1 
 
 2 350 
 P 
 
 
 
 
 
 
 
 
 
 ^ 300 
 CO 
 CO 
 
 
 
 
 
 
 
 
 e 2 
 
 D 260 
 CO 
 
 h 
 
 
 
 
 
 
 
 
 e, 
 
 < 200 
 
 UJ 
 
 cc 
 
 
 
 
 
 
 
 
 
 CO 
 Ul 
 
 
 
 
 
 
 
 
 
 Q. 100 
 
 
 
 
 
 
 
 
 
 50 
 
 
 
 
 
 
 
 
 | i 
 
 
 
 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 O 6 
 
 7 
 
 j 
 
 CURRENT IN FEEDER. 
 
 FIG. 68. 
 liising Characteristic for Compensating for Fall of Pressure in Feeders. 
 
 power station must vary in some definite ratio to the output. 
 (It has already been shown that the rise of station pressure is 
 simply proportional to the resistance of the transmission feeders 
 and to the current in them.) The easiest way of accomplishing 
 this end in a small installation with one dynamo is to over- 
 compound the dynamo so that it gives a rising characteristic. 
 See Fig. 68, where a b shows the characteristic for constant 
 
DYNAMOS IN PARALLEL. 
 
 143 
 
 pressure, and ac that required to compensate for the loss in 
 the feeders. The number of volts lost over the feeder resistance 
 is given by the heights of the ordinates between ac and ab; e l 
 being the pressure at the sub-station, and e 2 that at the power 
 station. 
 
 Shunt excitation is only practicable with moderate pressures, 
 and, speaking broadly, is not advisable with terminal pressures 
 of more than about 350 volts, although the author has found 
 it convenient to run compound dynamos at even 600 volts. In 
 such cases it is necessary to subdivide the shunt winding as 
 much as possible, and to use extra precautions in insulating 
 
 FIG. 69. Compound Dynamos Coupled in Parallel. 
 
 the coils ; and further, to guard against lightning discharges, 
 which are a frequent source of injury to shunt wound dynamos 
 coupled to power circuits (see 21, page 102). 
 
 If two or more compound dynamos be run in parallel, it will 
 be necessary to connect the machines at three points, i e., at the 
 positive and negative brushes, and also at the extremities of the 
 series coils, as shown in Fig. 69. This coupling will tend to divide 
 the load between the dynamos according to their designed out- 
 put, and help to ensure that all the machines give the same 
 pressure at any load. Yet, since compounded dynamos, even 
 
144 
 
 ELECTRIC MOTIVE POWEE. 
 
 if of similar design, fail to give characteristics of exactly the 
 same slope, it follows that the output may not be properly 
 divided between them. 
 
 To obviate these difficulties central station dynamos are 
 generally separately excited by small dynamos specially 
 arranged for the work. The plan which is the most flexible 
 and allows most scope for regulation of load and pressure is to 
 excite all the dynamos in parallel through separate variable 
 resistances for each set of field coils, as represented in Fig. 70. 
 By these means the pressure of any dynamo can be varied, and 
 its output altered, at will. 
 
 Fuse. 
 
 Feeder. 
 
 FIG. 70. 
 
 FIG. 70. Diagram showing three separately excited dynamos coupled in 
 parallel and supplying current to one pair of feeders. Dj, Do, D 3 , dynamos. 
 F.M.J, F.M. 2 , F.M.3, Field Magnets. E, Exciting dynamo. E^, R.,, R 3 , Regu- 
 lating resistances in series with field magnets. A], A 2 , A 3 , Ammeters for 
 reading current given out by each dynamo. Vi, A^oltmeter for reading the 
 separate pressure of the dynamos. V 2 , Voltmeter for reading pressure at 
 sub-station. 
 
 If the load varies slowly the regulation for fall of pressure 
 on the feeders can be adjusted by hand, as occasion requires, 
 by simply altering the resistances. But if the work necessitates 
 rapid fluctuations of load, and consequent changes of pressure, 
 then a few series turns may be wound round each of the 
 
MOTOR REGULATORS. 145 
 
 field magnets and the main current of the particular dynamo 
 caused to pass through it. By this simple means the pressure 
 can be regulated sufficiently well for most purposes. 
 
 Another method rapidly coming into favour involves the use 
 of a series dynamo, both the field and armature of which are 
 placed in series with the feeder whose pressure is to be regulated, 
 the dynamo being driven at a constant speed. Since a series 
 dynamo running at uniform speed gives a pressure proportionate 
 to the current, it follows that when it is to be used as a regu- 
 lator it must be designed so that with the minimum current in 
 the feeder (and consequently through its windings) it will 
 generate pressure just sufficient to drive the current. Then as 
 the feeder current increases it must give a terminal pressure 
 equal to the drop due to the feeder resistance. These conditions 
 require a straight line characteristic, and therefore the weight- 
 output efficiency is small. Machines used for regulating may 
 be placed either at the dynamo station or at the distributing 
 end of the feeder ; and they are frequently driven by shunt- 
 wound motors. 
 
 Mr. W. B. Sayers' self-exciting dynamos are specially suited 
 for this purpose, as in them the field excitation is provided by 
 the armature reactions; and the difficulty of controlling sparking 
 with the weak field is entirely obviated by the use of commu- 
 tator coils. One of these regulators is in use at the Bradford 
 Station Hotel, and is found to compensate for the drop in the 
 mains with great exactitude. The relation between the current 
 and the terminal pressure of the regulator, in this case, is 
 shown in Fig. 71, p. 146, which is taken from a Paper by 
 Mr. W. B. Sayers, entitled " The Prevention and Control of 
 Sparking and Self Exciting Dynamos."* 
 
 It will be observed that at 10 amperes the pressure is about 
 0'4 volt, which is just sufficient to drive this current through 
 the armature coils. And at the maximum load of 350 amperes 
 the regulator adds 13 volts to the pressure of the mains. 
 
 A modification of this idea has been successfully applied to 
 a three-wire system. In the case in point two armatures are 
 
 * Proceedings of Institution of Electrical Engineers, Vol. XXII., Part 107. 
 
146 
 
 ELECTEIC MOTIVE PoWEE. 
 
 coupled on the same shaft and fitted with separate keepers or 
 fields, the current in one of each of the outer mains passing 
 through one of the armatures, and that in the middle conductor 
 through a few turns of wire wound around the keepers. By 
 this means the pressure between the middle conductor and each 
 of the outer mains is kept practically constant. An objection 
 to the dynamotor regulator is the Initial cost, which will be 
 sufficient in many cases to prohibit its use. 
 
 Automatic regulators, depending on variations of pres- 
 sure or current, have been tried from time to time, but 
 
 12 
 
 10 
 
 
 Y 
 
 40 80 120 160 200 240 280 320 360 
 
 Amperes. 
 
 PIG. 71. Curve showing Relation between Volts and Amperes with a 
 Savers' Regulator. 
 
 their use has not been attended with much success, and in 
 most of the present central lighting stations hand regulation 
 is found to be preferable. The objection to mechanical devices 
 which depend on minute changes of pressure or current to 
 work them is the delicate construction of the mechanism. 
 Since they work by reason of the very error which they are 
 designed to correct it is obvious that makers will arrange things 
 so that the smallest possible variation will cause a maximum 
 
MO TOE EEGULATORS. 14? 
 
 effect on the moving parts, which are necessarily light and 
 delicate. From very slight causes, such as dust, thickening of 
 oil in bearings, &c., a large increase of power may be required to 
 move the governor, and the pressure or current may vary by a 
 dangerous quantity before it comes into action. The author 
 is in favour of separately excited dynamos with hand regulation 
 for most transmission of power cases ; and of compensating 
 series excitation by the main current when the load variation 
 is too irregular for hand adjustment. Motor regulators will 
 generally be found to be unnecessary and too costly for trans- 
 mission of power plants. 
 
CHAPTER V. 
 
 SINGLE-PHASE ALTERNATORS AND ALTERNATE 
 CURRENT MOTORS. 
 
 27. General Features ; Classification of Types of Machines. 28. Maxi- 
 mum, Instantaneous, Average, and Effective Values of Harmonic 
 Functions ; Ratio between Effective and Average Values ; Equa- 
 tions for Electromotive Force ; Pitch of Poles and Coils ; Diagrams 
 of Typical Alternators ; Self-induction of Armature Coils ; Equation 
 for Self -Induction. 29. Impressed, Dynamic or Working, and 
 Condenser or Wattless Currents ; Relation between Phases and 
 Magnitudes of Pressures and Currents; Power Measurements; 
 Effects of Current Lag. 30. Effect of Iron Cores ; Power Wasted 
 by Hysteresis ; Effect of Frequency. 31. Design of Alterna- 
 tors ; Magnetic Density ; Effect of different Shapes of Pressure and 
 Current Curves ; Peripheral Speed. 32. Excitation ; Parallel 
 Working ; Critical Field Current of Alternators and Alternate-Cur- 
 rent Motors. 33. Self-Starting Single-Phase Current Asynchronous 
 Motors. 
 
 27. GENERAL FEATURES ; CLASSIFICATION OF TYPES 
 OF MACHINES. 
 
 IN Chapter II. the general principles of dynamo design were 
 discussed with special reference to continuous currents, because 
 these are at present in more general use for power transmission 
 than alternate currents. The reasons for this preference are 
 made clear in the sequel, and practical deductions drawn from 
 experience are used to suggest the probable systems of the 
 future. Now, it is necessary to examine the alternator, both 
 as a dynamo and as a motor. This chapter is confined to the 
 consideration of single current machines, because these have 
 already established a justly wide reputation for lighting, 
 and also for certain cases of power transmission, while the 
 multiple current systems are not so well known, although 
 
150 ELECTRIC MOTIVE POWEE. 
 
 they possess many features that render them especially suitable 
 for the distribution of power. 
 
 An alternator may be defined as a machine giving periodic 
 electric currents which are reversed in direction many times 
 per second, the number of complete changes per second being 
 called the frequency of the current. Most types of the 
 modern alternator have a series of salient pole pieces arranged 
 symmetrically around the axis of the shaft and excited by 
 a continuous current derived from an independent source, or 
 else by a redressed current. The armatures have also a series 
 of coils, with or without iron cores, the number of which is 
 either equal to, half as great, or twice as great, as that of the 
 pairs of field poles. The current induced in them is of course 
 alternating in direction, with a period depending simply on the 
 number of pairs of poles (i.e., pairs of N and S poles) in the 
 field coils and the number of revolutions per second. Thus 
 the frequency, usually expressed by the symbol ^u , is equal 
 
 to p , where p is the number of pairs of field poles and n is 
 60 
 
 the number of revolutions per minute. This cyclic change in 
 the armature currents is produced by causing the magnetic in- 
 duction through the coils either to alternate rapidly in direction, 
 or else alternately to increase and decrease in density. These 
 distinctions divide alternators into two classes 
 
 (a) Alternators in which the magnetism changes in direction, 
 and consequently passes through all values of density between 
 a positive and a negative maximum ; 
 
 (b) Alternators in which the magnetism is of constant direc- 
 tion, but alternately increases and decreases in density, vary- 
 ing between the maximum and a lower limit depending on the 
 design, and which is always nearly zero. 
 
 The first class is the older, having been devised by Wilde in 
 1867. 
 
 Frequently the field-magnets consist of two opposing crowns 
 of poles of alternate sign, between which the armature coils 
 revolve, as in the old Siemens alternator and the modern 
 Ferranti machines; or the cores are spaced radially around the 
 
FIELD MAGNETS OF ALTERNATORS. 
 
 151 
 
 inside of a cast-iron ring, forming the yoke of the field-magnets, 
 with the faces of the poles pointing to the axis of the shaft ; a nd 
 the armature coils are laid flat on the periphery of a drum- 
 shaped core, and so face the field-poles, as in the Westinghouse 
 and Giilcher machines. 
 
 In the two examples just referred to the field-magnets are 
 stationary, but in some designs they revolve inside the arma- 
 ture coils, which are then fixed radially so as to form an outer 
 ring, with the faces of the coils opposite to the field-poles. A 
 
 FIG. 72. Field Magnet of Mordey Alternator. 
 
 recent development of this type of field-magnet is C. E. L. 
 Brown's multiphase alternator, in which the armature is 
 stationary and the field-magnets are excited by a single coil. 
 The Lowrie-Hall alternator also has revolving field-magnets, 
 but each of the poles has a separate exciting coil. 
 
 The second class (6) is of comparatively modern development, 
 and is most familiar to us in the Brush alternator, the field- 
 magnet of which is shown in Fig. 72 and the armature in 
 
 M2 
 
152 ELECTRIC MOTIVE POWER. 
 
 Fig. 73. In this case the armature is stationary. This arrange- 
 ment, although not typical of the particular class, is very 
 common with the alternators of many makers, as it offers 
 facilities for coupling up the armature coila in different groups 
 
 FIG. 73. Armature of Mordey Alternator. 
 
 without the complication of brushes. The field-magnets con- 
 sist of two crowns of pole-pieces, but all the poles on one side 
 are N poles, and on the other all are S poles, not alternately 
 N and S, as in type (a). 
 
VALUES OF HAEMONIC FUNCTIONS. 153 
 
 28. MAXIMUM, INSTANTANEOUS, AVERAGE, AND 
 EFFECTIVE VALUES OF HARMONIC FUNC- 
 TIONS ; RATIO BETWEEN EFFECTIVE AND 
 AVERAGE VALUES ; EQUATIONS FOR ELEC- 
 TROMOTIVE FORCE ; PITCH OF POLES AND 
 COILS; DIAGRAMS OF TYPICAL ALTERNATORS; 
 SELF-INDUCTION OF ARMATURE COILS; EQUA- 
 TION FOR SELF-INDUCTION. 
 
 It will be apparent that the laws which govern the deter- 
 mination of the excitation for a given magnetic flux in 
 continuous-current machines are equally applicable to the 
 alternate-current designs, it being necessary, however, to modify 
 the equation for the total electromotive force of the armature, 
 and to arrange suitable current densities in the copper and 
 magnetic densities in the iron. These modifications are of 
 importance, and require elucidation. The total armature im- 
 pressed electromotive force, V imp , may be expressed in terms of 
 the number of turns, the number of lines of force per pair of 
 poles, and the number of revolutions per minute, as in the con- 
 tinuous-current machine in equation (6), p. 18. But since 
 the pressure is periodic, and therefore changes in value through 
 each cycle, there will be a maximum pressure considerably 
 greater than either the effective or average value. It is evident 
 that both the pressure and current have all values between 
 zero and a maximum (one positive and one negative) twice in 
 every period, and it is, therefore, necessary to define what is 
 understood by the terms alternating pressure and current. 
 
 In practice we require to know the effective* values, and not 
 the maximum or instantaneous values. The readiest means 
 of comparing the effective values of an alternating and direct 
 current is afforded by the several heating effects produced in a 
 wire of known resistance having no self-induction. When the 
 heat caused by the current is the same in either case, then the 
 effective pressure of the alternate current is equivalent to the 
 continuous pressure. 
 
 * Prof. Silvanus P. Thompson prefers to use the older term, " virtual " 
 value. 
 
154 ELECTRIC MOTIVE POWEE. 
 
 Instruments which depend on the heat waste, i 2 R, are cali- 
 brated by continuous currents, the effective values of which 
 are proportional to the squares of the uniform currents, and 
 therefore the readings of the instruments are proportional to 
 the square root of the mean square of the currents (usually 
 written \/niean square). For example, the heat produced 
 by a continuous current of 10 amperes is proportional to 10 2 , 
 and the readings of an ammeter of the dynamometer type 
 placed in the circuit are proportional to \/i0 2 . If the same 
 instrument were placed in an alternate-current circuit and gave 
 the same deflection, the ^mean square value of the current 
 would be as before, 10 amperes, and this is called the effective 
 strength of the alternate current. This /s/mean square value 
 is independent of the shape of the curve, which, however, 
 determines its ratio to the maximum value. 
 
 FIG. 74 Sine Curve of Pressure or Current. 
 
 If it be assumed that the pressure and current are harmonic 
 functions of the periodic time, the curves of either will be 
 sinusoidal, and may be expressed by an equation of the order 
 
 y = a sin t, 
 
 where y is the instantaneous value of the pressure or current, 
 t is the time of one period (it is most conveniently reckoned in 
 angular measure), and a is a constant numerically equal to the 
 maximum value of y, which occurs when 
 
 7T , 7T -. 
 
 * = -, for sin -=1. 
 
 Fig. 74 shows a sine curve of wave length t. The ordinates 
 y l and y. 2 may represent either values of pressure or current, 
 at the corresponding instants ^ and t 2 . 
 
 The arithmetical mean value of the ordinates y v y^ y, &c., 
 
 n 
 
 for the first half of the curve in Fig. 74 is times that of the 
 
 7T 
 
VALUES OF HARMONIC FUNCTIONS. 155 
 
 maximum ordinate ; or the average value = 0'637 times the 
 maximum value. 
 
 But the total heat generated per second in the wire is a 
 numerical measure of half the square of the maximum ordinate 
 of a simple periodic current ; for the mean of the squares of 
 the sine, either for one quadrant or a whole circle, is one-half 
 
 of the square of the maximum value, or ~ a - . And therefore 
 
 2 
 
 instruments depending on the estimation of the heat waste 
 will measure- = - 707 of the maximum value of a 
 
 N/2 
 
 periodic current or pressure.* And the continuous current which 
 will give the same deflection on the instrument, that is, the 
 
 same heating effect, is equal to j= times the maximum value 
 
 v2 
 of the alternate current. 
 
 Therefore, an alternating current, if it follow the sine law, 
 will cause a heating effect greater than that of a continuous 
 current of the same average strength by the ratio of 0'707 to 
 0'637, that is, about 1*1 times greater. In these pages when 
 an alternate current or pressure is referred to, unless otherwise 
 stated, it is assumed that effective values are understood, and 
 that pressure measurements are made by hot wire or electro- 
 static voltmeters, and currents are read by dynamometers. The 
 effective value of the total armature electromotive force may be 
 written 
 
 V e = &N a C a ^10-s . . , . (26) 
 if all the coils are in two parallels, or 
 
 V e = 2&N a C aP ^10-3 .... (27) 
 if they are in one series. 
 
 * For a more complete explanation of the interesting phenomena of 
 alternate currents reference should be made to " The Alternate Cur- 
 rent Transformer, in Theory and Practice," by Dr. J. A. Fleming, F.R.S. 
 
156 ELECTRIC MOTIVE POWEE. 
 
 The symbol V e = the effective pressure in volts, 
 
 p = the number of pairs of poles, 
 n = the number of revolutions per minute, 
 N a = the magnetic flux from one pair of poles, 
 C a = 2pw, where w = ihe number of active conductors on 
 one side of one coil; or, briefly, C a = the number of turns 
 counted all round. 
 
 The symbol k requires special explanation. 
 
 The quantity expressed by Jc is a constant for the particular 
 arrangement of pole-pieces and armature coils ; it is the ratio 
 between the effective and the average pressures. If the pressure 
 curve obeys a sine law (it does very nearly in many types of 
 alternators), k = I'l, and this value may be taken in most 
 rough approximations. The equations for the electromotive 
 forces of alternators, assuming a sine laiv to be followed, may 
 be written thus : 
 
 Two parallels : average total volts = N a C a p 10~ 8 . . (28) 
 
 60 
 
 One series: ,. = 2N a C a 10~s . (29) 
 
 Two parallels: maximum = 7 L N a C a /? 10~ 8 . . (30) 
 
 One series: =TT N tt C a /> ^ IQ-s. . (31) 
 
 Two parallels : effective =_^ N a C a ^ - 10~ s (32) 
 
 2 x/2 
 
 One series: = ^ N a C a/? ^ 10~s (33) 
 
 The numerical value of the constant in equation (32) is 
 approximately 1*1. In equation (33) for series coupling the 
 coefficient must be twice as great as that in equation (32), if 
 the same value of C a be retained, and it is therefore equal to 
 2x1-1 = 2-2. In practice it is found to vary between 2*2 and 
 2-3 when the coils are all in series. 
 
SHAPES OF PRESSURE CURVES. 
 
 157 
 
 The pitch of the pole-pieces and the width of the armature 
 oores are of great importance in designing alternators, for they 
 affect the shape of the pressure curve, and consequently the 
 effective pressure and the output. Four of the more frequent 
 
 Aperture. 
 
 immiHmm 
 
 fs/ /// 
 
 isi 
 
 m% 
 
 - Coil -- 
 
 FIG. 75. Diagram of Typical Alternator giving a Triangular Pressure 
 
 Curve. 
 
 arrangements are illustrated in Figs. 75, 76, 77 and 78, which it 
 will be necessary to examine carefully before proceeding with the 
 calculation of the output. In these diagrams the field poles are 
 
 Pitch 
 hi of Coil. *\ 
 
 Core. 
 
 FIG. 76. Diagram of Typical Alternator giving a Flat-Topped Triangular 
 Pressure Curve. 
 
 developed horizontally, and the polarity is indicated by capital 
 letters. The winding and bare spaces on the armatures and 
 the apertures of the coils are clearly marked. The pitch of the 
 field poles is equal to half a wave length, and, therefore, in 
 
158 
 
 ELECTRIC MOTIVE POWER. 
 
 Figs. 75, 76 and 78, it is equal to the angular width of one pole 
 and one space ; whilst in Fig. 77 (which refers to the Brush 
 alternator) it is equal to that of one pole only, for there the 
 spaces, strictly speaking, act as if they were poles of opposite 
 sign to those hatched in the diagram. It will be noticed in this 
 case that the armature coils are twice as numerous as the pairs 
 of field poles. 
 
 _ Pitch ^ ^poie-.* 
 
 
 Aperture. 
 
 FIG. 77. Diagram of Typical Alternator giving a nearly Kectangular 
 Pressure Curve. 
 
 Below each figure is the theoretical shape of the pressure 
 curve in full lines, determined on the assumption that there 
 
 Armature Core. 
 
 M Ooil. - 
 
 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 
 
 Agerture 
 
 FIG. 78. Diagram of Typical Alternator giving a Triangular Pressure 
 
 Curve. 
 
 is no magnetic leakage between the poles, and, therefore, 
 no magnetic fringe at the corners of the pole-pieces. This is 
 not the case in practice, and the actual curve will therefore 
 be less angular, and approximate more or less closely to sine 
 curves, as shown in dotted lines. This is easily seen to be 
 
SHAPES OF PRESSURE CURVES, 159 
 
 the case with Figs. 75 and 76, and practically it is found 
 that the design in Fig. 77 gives an almost perfect sine 
 surve. 
 
 In tracing the pressure curve for any arrangement of pitch 
 and angular width of poles and coils the zero points may 
 be first determined and marked off on the line a b, which 
 shows the angular position of the coils. Having found these 
 points the wave-length is known, and the positions of maximum 
 pressure should lie midway between the zero points. . If the 
 winding spaces are exactly equal to the width of the poles, then, 
 assuming no magnetic fringe, the intermediate values lie in 
 straight lines, joining the maximum and zero points as in 
 Figs. 75 and 78. The height of the maximum ordinate depends 
 on the scale selected, but its numerical value can be found 
 from the known magnetic flux and the linear speed of the coils. 
 In these diagrams an arbitrary height has been selected. If 
 the coils be in one series, and a simple zigzag curve be assumed, 
 then 
 
 V max = 4 C a N a , ID-* volts, . . . (34) 
 
 where the maximum pressure is assumed to be twice the aver- 
 age pressure. If the winding space be shorter than the polar 
 width, then the maximum pressure will be maintained for that 
 period of time during which the coil as a whole is passing 
 under the pole piece. Thus, if the polar arc be lOin. in 
 width and the winding space only Sin., then for the fraction of 
 time required for the coil to move through the angular dis- 
 tance of 10-8 = 2in., the pressure will have the same 
 value, and hence the top of the crest will be flat, as in Fig. 76, 
 which refers approximately to the design of the Kapp 
 alternator. 
 
 In some designs the coils are made to exceed the poles in 
 width by a small percentage, the arrangement being found to 
 give a very close approximation to a sine curve. Prof. Elihu 
 Thomson is in favour of such proportions. 
 
 The Giilcher alternator, designed by Mr. W. B. Esson, also 
 embodies this idea, but the armature coils are only half as 
 
160 
 
 ELECTRIC MOTIVE POWEE. 
 
 numerous as the field poles, and are equal to the width of 
 three and a-half poles. There is a distance piece between the 
 
 FIG. 79. Dia^ian of Typical Alternator showing a Stepped Pressure 
 
 Curve. 
 
 V s 
 
 j? ma> 
 
 FIG. 80. Diagram showing Effective Pressure Sine Curve. 
 
 coils equal to half a pole, the pole being of the same width 
 as the space (see Fig. 79). The theoretical curve of pressure is 
 
SHAPES OF PRESSURE CURVES. 
 
 161 
 
 here shown in full lines, and a sine curve is indicated by dotted 
 lines. It is evident that this arrangement approximates very 
 closely indeed to the necessary conditions for obtaining a sine 
 function. 
 
 The curve of pressure for any arrangement of pole pieces and 
 armature coils can be traced approximately, and then it is easy 
 
 V 7 max 
 
 O X 
 
 FIG. 81. Diagram show n? Effective Pressure Triangular Curve. 
 
 to determine the effective pressure with sufficient accuracy for 
 practical purposes. Consider the sine curve in Fig. 80. The half 
 wave length is equal to, say, OX, and V max equals the maximum 
 ordinate. Now, bearing in mind that the instruments used 
 for measuring alternate currents give readings proportional 
 to the ^/mean square, it is necessary to square the ordinates 
 of OVX and plot a new curve on the same base, and then 
 to find the mean ordinate, say V e 2 , of the squares. The 
 
162 ELECTEIC MOTIVE POWER. 
 
 square root V e will be the effective pressure. This can be 
 readily found for most symmetrical figures by integration ; but 
 many engineers prefer the graphical method, which is as 
 follows : Plot the curve to any convenient scale on squared 
 paper and divide the base line X into any number of equal 
 parts the more the better. Measure the heights of the 
 ordinates at the middle of each and divide the sum of their 
 values by the number of parts : the result is the numerical 
 Talue of the mean square, V e 2 , in the selected units. Next 
 
 V ? max 
 
 FIG. 82. Diagram showing Effective Pressure Semicircular Curve. 
 
 extract the square root, and then the effective pressure, V e , 
 js known. It will be seen that for a sine curve 
 
 V, = 0707 V max . 
 
 The same operation has been performed in Fig. 81, for the 
 triangular pressure curve OV max X, and the value of V e is 
 found to be 0'58 of the maximum pressure ; or 
 
 Thus equation (34), which gives the value of the maximum 
 pressure with a simple zigzag curve, must be multiplied by 
 
EATIO OF EFFECTIVE TO AVERAGE PRESSURE. 163 
 
 0'5S to equal the effective pressure. Therefore, if the coils 
 be in one series, since 
 
 2-320.^^10-8 
 
 (35) 
 
 And if the coils be coupled in two parallels, then the effective 
 pressure is half as great as the value given by (35). 
 
 In Fig. 82 the same operation has been performed for a 
 semi-circular curve, and it is found that 
 
 V e - 0-835 V 
 
 The relation between the effective and average values, for 
 the more common curves, and the theoretical values of k are 
 shown in Table J : 
 
 Table J. 
 
 Order of 
 curve. 
 
 Average value 
 as a decimal 
 of the 
 max. ordinate. 
 
 Effective value 
 as a decimal 
 of the 
 max. ordinate. 
 
 & = llatio of effective 
 to average 
 value. 
 
 Sine 
 
 0-637 
 
 0-707 
 
 1-1 
 
 Semicircle 
 
 0-7854 
 
 0-855 
 
 1-063 
 
 Triangle 
 
 0'5 
 
 0-58 
 
 1-16 
 
 Rectangle 
 
 ro 
 
 ro 
 
 ro 
 
 
 
 
 
 The above values will not all be realised in practice, since the 
 magnetic fringe will cause the angles to be rounded. The 
 slope of the curves will also be largely affected by the pitch 
 ratio of the poles and coils. The shape of the coils is also 
 important in determining the pressure curve. For example, 
 a rectangular coil will tend to give a very rapid increase and 
 decrease of pressure, because it will become instantly active 
 or inactive; while a coil of rounded form will cut the lines 
 of force in steadily increasing or diminishing numbers. 
 
164 
 
 ELECTRIC MOTIVE POWER. 
 
 There is some difficulty in finding the actual value of fe in 
 practice, since the exact magnetic density B" a is not easily 
 found without special experimenting. The designer, of course, 
 knows the calculated value of N a , and consequently B" a , and 
 then from the measured pressure may calculate k with a fair 
 degree of approximation to the truth. But if a small test eoil 
 be arranged so as to explore the gap at various places, then the 
 average value of B" a may be readily found ; this, however, is 
 not an easy thing to do in an ordinary shop with workshop 
 instruments. 
 
 Mr. Kapp has made a special study of the coefficient Jc, and 
 the following table is based on his researches. The windings 
 are assumed to be in one series, and the width of the poles and 
 coils is expressed in decimals of the pitch. 
 
 Table K. 
 
 Proportions of Field-Poles and Armature Coils in Terms of 
 the Pitch, and Consequent Values of k. 
 
 Kef. No. 
 
 Pitch. 
 
 Pole. 
 
 Total breadth of copper in each 
 coil. 
 
 2Jc. 
 
 1 
 
 1 
 
 1 
 
 1 (covering whole surface) 
 
 116 
 
 2 
 
 1 
 
 1 
 
 0'5 (covering half surface) 
 
 1-635 
 
 3 
 
 1 
 
 0-5 
 
 1 (covering whole surface) 
 
 1-635 
 
 4 
 
 1 
 
 0-62 
 
 0'5 (covering half surface) 
 
 2-06 
 
 5 
 
 1 
 
 0-5 
 
 0"5 (covering half surface) 
 
 2-30 
 
 6 
 
 1 
 
 033 
 
 0'33 (covering third of surface) 
 
 2-832 
 
 7 
 
 
 Sine Function 
 
 2-22 
 
 Nos. 4 and 5 approach most nearly to designs found in practical 
 machines. 
 
 Having thus determined the total effective pressure of the 
 alternator, it is easy to find the terminal effective pressure. 
 
 This will be given as for continuous currents by equation (5), 
 page 18. Thus, if <y e be the effective terminal pressure, then 
 
 v e =V e ir, 
 
 where i is the current in the armature and r the resistance of 
 the coils. 
 
SELF-INDUCTION PRESSURE. 165 
 
 It remains to examine more closely the armature pressure 
 and to consider the relation of its components. Essentially 
 there are three active electromotive forces in every alternate- 
 current device, the capacity of which is either zero or negligible, 
 whether it receives or generates power ; they are the impressed 
 pressure V imp , which represents the total electromotive force 
 of the circuit, v a the electromotive force of self-induction, 
 and v x the resultant or current- driving electromotive force. 
 
 The self-induction pressure is always in advance of the 
 resultant pressure by 90deg. The pressures may be repre- 
 sented by the diagram in Fig. 83, and numerically their 
 maximum values are related by the equation 
 
 imp 
 
 (36) 
 
 FIG. 83. Diagram of Phases of Pressure. 
 
 These three pressures have different amplitudes, but the 
 same wave length, and it is necessary to distinguish between 
 their maximum and effective values ; but equation (36) is 
 equally true for both. In practice, however, the effective 
 pressure is of the greater importance, and is generally under- 
 stood when the word pressure is used without any qualification. 
 
 The self-induction electromotive force requires further expla- 
 nation. Whenever an electric current is started in a conductor 
 there is a tendency to stop the passage of the current, and 
 when the circuit is opened so as to break the path of the current 
 there is a tendency to continue the current, which is evidenced 
 by a spark at the gap. These tendencies are clearly of a 
 different nature to the ohmic resistance, which absorbs pressure 
 (^R), and simply regulates the quantity of current. Since it 
 
166 ELEGTEIG MOTIVE POWER. 
 
 requires an electromotive force to start a current, an electro- 
 motive force will also check a current; and hence in alter- 
 nate-current working, as the current is rapidly alternated, i.e., 
 first started in one direction, stopped, and then restarted in the 
 other direction many times in each second, it is clear that this 
 apparent inertia of the electrical current may at times assume 
 considerable proportions. 
 
 The phenomenon is said to be due to self-induction, which is 
 expressed as a quantity whose magnitude is measured in units 
 called henries. The self-induction of a circuit is proportional to 
 the square of the number of turns of wire. The number of lines 
 of force embraced by the circuit is proportional to the product 
 of the self-induction and the current. The electromotive force 
 generated by the variation of the lines of force is proportional 
 to the frequency of the current, and also to the total flux of 
 lines, and may therefore be expressed in volts by an equation 
 of the form 
 
 V s max = 2;r oo ^ C 2 N 1-41, .... (37) 
 
 where <*r is the frequency, or number of complete cycles per 
 second, C is the number of turns of wire, N is the number of 
 lines of force due to one turn of the winding, and i is the 
 current in one coil in amperes. 
 
 The quantity expressed by the product N C 2 is called the 
 coefficient of self-induction of the circuit, and is usually sym- 
 bolised by the letter L. 
 
 .'. V s max = 27r ^ i L 1-41, . . (38) 
 and the effective pressure 
 
 Vv s max n .1 /orv\ 
 
 se = =2?r f\, t L. . . (39) 
 
 In considering the self-induction of an alternate-current 
 armature, N is the number of lines of force due to one turn of 
 the winding where one ampere flows through the coil, and C 
 is the number of turns in. series. The self-induction can 
 therefore be made small by keeping the quantity C 2 N small. 
 But C is obviously proportional to the required pressure, and 
 
CAPACITY PEESSUEE. 167 
 
 therefore practically fixed between narrow limits for a given 
 design; and the magnitude of N will depend on the perme- 
 ability of the field. If the magnetic density be sufficiently 
 great to practically saturate the iron of the cores (field and 
 armature) the self-induced-field will be small compared with the 
 main field, and will have little effect. In designing alternators, 
 then, it is advisable to make the field induction strong in 
 order that this induction per turn of armature may be as 
 large as possible. In armatures with air cores it is possible 
 to work at a high magnetic density, and therefore the self- 
 induction can be made low; but with iron-cored armatures at 
 the density usually permissible the self-induction is considerable. 
 It must also be noted that the electromotive force of self- 
 induction of an armature depends on the frequency (i.e., the 
 rate of change of induction), and also on the armature current 
 if the permeability of the magnetic path be constant. 
 
 FIG. 84. Diagram of Phases of Current. 
 
 It is found in a practical alternator that L has a smaller 
 value at full load than on open circuit. This is the case, 
 because the permeability of the field is then less. The electro- 
 motive force of self-induction in most cases will be greatest 
 at light loads, since, though i is then small, L, on the con- 
 trary, attains its greatest value. 
 
 If a circuit have capacity, such as is caused by a condenser, 
 an armoured cable, or a concentric cable of considerable length, 
 then there will be an electromotive force lagging 90deg. behind 
 the resultant pressure, and therefore in opposition to the self- 
 induction pressure if the circuit be an inductive one. The 
 relative position of the phases and magnitudes of the pressures 
 are shown in Fig. 84, where V imp is the impressed pressure, v t 
 the resultant pressure, and v c the capacity pressure. 
 
 N2 
 
168 
 
 ELECTRIC MOTIVE POWER. 
 
 29. IMPRESSED, DYNAMIC OR WORKING, AND 
 CONDENSER OR WATTLESS CURRENTS ; RE- 
 LATION BETWEEN PHASES AND MAGNITUDES 
 OF PRESSURES AND CURRENTS ; POWER 
 MEASUREMENTS; EFFECTS OF CURRENT LAG. 
 
 It has been shown that the resultant pressure can be divided 
 into two components, the impressed pressure in phase with the 
 dynamic current, and the self-induction pressure at right 
 
 FIG. 85. Clock Diagram. 
 
 angles to it. A similar relationship is true of the impressed 
 current, i.e., the current measured by a dynamometer, which 
 can be resolved into a dynamic or work-producing current in 
 phase with the impressed electromotive force, and a wattless or 
 condenser current lagging 90deg. behind it. Let the three be 
 distinguished by the several symbols ^ imp , i &1 and i c . Then 
 
 imp 
 
 (40) 
 
CLOCK DIAGRAMS. 
 
 169 
 
 The relation between the pressures and currents is illustrated 
 in Fig. 85, which is known as a clock diagram. The radius A 
 is supposed to revolve clockwise, and to represent the impressed 
 electromotive force, one complete revolution being equal to one 
 period. It is evident that the projections of A on any straight 
 line, say, Y Y 15 passing through the origin 0, will represent 
 the fluctuations of pressure from instant to instant, assuming 
 the curve to follow a sine law. The instantaneous values of 
 the several pressures may therefore be read on the axis Y Y r 
 
 d& 
 
 Yl 
 
 Y 
 
 FIG. 86. Work Diagram Current aucl I'rossui e in Phase. 
 
 On A describe the semi-circle ABO. Let any convenient 
 unit length be chosen to represent a volt and an ampere. 
 Through draw. B, touching A B at B, proportional to the 
 resultant pressure and lagging behind A by an angle <, 
 and join A and B. Then the line A B will be equal to the 
 self-induction pressure expressed in the same units. Draw 
 E parallel to B A, and complete the parallelogram B A E 0. 
 Then E shows te relative phase of the self-induction 
 electromotive force, V s , which is seen to lead V imp by an angle 
 
170 
 
 ELECTRIC MOTIVE POWER. 
 
 a . Now the impressed current is due to the resultant pressure 
 and therefore is in phase with it. Hence on B (prolonged if 
 necessary) mark off a part C equal to the impressed current. 
 From C draw C F perpendicular to A, draw D parallel to 
 C F, and join C and D. Then the resolved parts of C, viz.,0 F 
 and D, will severally be proportional to the dynamic and the 
 wattless currents, for they are the components of C resolved 
 at right angles. It is seen that the impressed current lags 
 behind the impressed pressure by an angle </> arbitrarily chosen 
 in this case and the dynamic current leads the impressed 
 
 FIG. 87. "Work Diagram Current lagging 45deg. in Phase behind 
 Pressure. 
 
 current by the same angle <, while the wattless current lags 
 behind it by the angle 6. It is also apparent that the greater 
 the difference in phase between the impressed pressure and 
 impressed current the greater the magnitude of the wattless 
 current. The import of this will be more readily appreciated by 
 reference to Figs 86 and 87, which show respectively curves 
 of instantaneous values of pressure and current coinciding in 
 phase in Fig. 86, and the same curves differing by 45deg. in 
 Fig. 87. The work performed in each cycle, corresponding to 
 products of V&', is shown in the shaded areas bounded by dotted 
 lines, the ordinates of which are simply proportional to the pro- 
 
MEAN POWER. 171 
 
 duct of the corresponding ordinates of V and i, with due regard 
 to the algebraic signs. The curves beneath the time line are 
 considered as of negative value ; and, since the product of two 
 negative quantities is algebraically positive, the work areas are 
 chiefly positive or useful. In Fig. 87 the small negative areas 
 denoted by negative signs must be subtracted from the positive 
 areas when estimating the total or mean power, 
 
 We are now in a position to deduce the equation for the 
 mean power. It has just been shown that the mean power 
 is simply the product of the impressed effective pressure and 
 current, as measured by a hot wire voltmeter and a dynamometer, 
 if there be no difference of phase. If there be a phase difference, 
 then the expression must be multiplied by some function of 
 the angle of phase. Referring to Fig. 85, p. 168, it is clear 
 that the mean power is proportional to AO. FO, or to BO. CO ; 
 or to the product of V imp i& or ^ imp V d) all being effective values. 
 
 Now AO.FO = AO.COcosAOB. 
 
 = V imp i imp cos <, 
 and B . C = A . C cos A B. 
 
 = V imp 4np COS <. 
 
 And if we consider the maximum values, since 
 V - /*> V 
 
 max v v e ) 
 
 mean power = Vm " >""> l^jw C os <f>. 
 
 = Y Vmax imp Vax imp COS <j>. 
 
 The effective values are practically the only directly measur- 
 able ones, and therefore it may be stated finally that 
 
 Mean power = V imp i imp cos <. . . . (41) 
 
 It is evident that if the angle < be large the power may be 
 relatively small, although the product of impressed pressure 
 and current be considerable. And it is also seen that a large 
 angle of lag is detrimental in two ways : 
 
172 ELECTEIG MOTIVE POWER. 
 
 Firstly, it decreases the output for a given plant, i.e., lowers 
 the plant efficiency, and 
 
 Secondly, it increases the heat waste in the copper of mains 
 and machines. 
 
 30. EFFECT OF IRON CORES ; POWER WASTED BY 
 HYSTERESIS ; EFFECT OF FREQUENCY. 
 
 There is considerable difference of opinion as to whether an 
 alternate-current armature should have iron cores or not. In 
 the thin disc type there is no doubt that iron is inadmissible, as 
 the armature would inevitably be drawn out of flat and jammed 
 against the pole-pieces ; but in drum and Gramme-wound 
 machines there is no mechanical reason why the magnetic re- 
 sistance should not be decreased by a liberal use of thin iron 
 plates. But the magnetic density must be kept low or the 
 losses from hysteresis will be serious : B" a must not exceed 
 from 40,000 to 65,000. And iron cores are said to have the effect 
 of reducing the loss from eddy currents in the copper. Re- 
 garded from a purely mechanical standing, the author is of 
 opinion that the iron-cored drum-wound type of alternators, 
 such as those built by the Westinghouse and the Giilcher com- 
 panies, are the better adapted for power purposes. The Brush 
 and Ferranti armatures, compared with these, are fragile in 
 make, and appear to be much more likely to give trouble. The 
 experience of Continental and American engineers seems to 
 confirm this, and the gradual development of multiphase alter- 
 nators with iron cores undoubtedly points in the same direction. 
 
 From a purely electrical point of view the Mordey alternator 
 is almost perfect, and certainly possesses the important feature 
 of a high efficiency of conversion at light loads a most impor- 
 tant consideration with some kinds of work. On the other 
 hand, it is an undoubted fact that the light-load efficiency 
 of many of the iron core alternators is extremely low. In 
 designing alternators with iron in the armatures it is always 
 important to estimate the power which will be wasted in simply 
 magnetising and demagnetising the cores, as this will be suffi- 
 
HYSTEEETIG LOSS. 
 
 173 
 
 ciontly large to be comparable with the copper loss. Table L, 
 which is calculated for 50, 83, 100, and 130 periods per second, 
 gives the data necessary for most practical cases. 
 
 Table L. Power Wasted by Hysteresis. 
 
 
 B" 
 
 Watts 
 
 Watts 
 
 Watts 
 
 Watts 
 
 B 
 
 C.G.S. 
 
 per square 
 
 wasted per 
 cubic inch. 
 
 wasted per 
 cubic inch. 
 
 wasted per 
 cubic inch. 
 
 wasted per 
 cubic inch. 
 
 
 men. 
 
 ^=50. 
 
 r^=83. 
 
 r\,=100. 
 
 <\> = 130. 
 
 4,000 
 
 25,800 
 
 0-116 
 
 0-192 
 
 0-232 
 
 0310 
 
 5,000 
 
 3;,250 
 
 01665 
 
 0-277 
 
 0-333 
 
 0434 
 
 6.000 
 
 38.700 
 
 0-2165 
 
 0-360 
 
 0-433 
 
 0-564 
 
 7,COO 
 
 45,lfiO 
 
 0-2575 
 
 0-444 
 
 0-555 
 
 0-695 
 
 8,000 
 
 51,600 
 
 0-3 '05 
 
 0-532 
 
 0-641 
 
 0'832 
 
 10,000 
 
 64,500 
 
 C-450 
 
 0-743 
 
 0-900 
 
 1-170 
 
 12,000 
 
 77.400 
 
 0-597 
 
 0-995 
 
 1-194 
 
 1-550 
 
 14,0-0 
 
 90,300 
 
 0-7tO 
 
 1-260 
 
 1-520 
 
 1-970 
 
 16.000 
 
 103.200 
 
 0-910 
 
 1-510 
 
 1-820 
 
 2-360 
 
 17,000 
 
 109,550 
 
 1-1375 
 
 1-890 
 
 2-275 
 
 2-950 
 
 18,000 
 
 116,100 
 
 1-4075 
 
 2-340 
 
 2-815 
 
 3-660 
 
 It will be noticed that for a fixed induction the waste of 
 power increases in direct proportion to the frequency ; and that 
 for a given frequency the loss varies as the induction raised to 
 the 1'6 power. 
 
 A further loss may also arise from eddy currents unless the 
 iron plates are very thin, but with the usual gauges the waste 
 of power is unimportant.* (See also 37, p. 207.) 
 
 The effect of the frequency on the size of an alternator for a 
 given output demands careful attention. It was shown in 27 
 
 * Mr. Steinmetz's law gives the expression for the loss in core thus : 
 Watts a f ~^ l B ltf3 + b <"\_, 2 B 2 , the first term referring to hysteresis and 
 the second to eddy currents. The constants a and b have to be determined 
 for each sample. The second term decreases in value as the temperature 
 of the core is raised. Generally, eddy current loss will not exceed about 
 five per cent, of that due to hysteresis. See The Electrician, February 12, 
 19, and 26, 1892 ; also Journal of the American Institute of Electrical 
 Engineers, January 19, 1892. 
 
174 
 
 ELECTEIC MOTIVE POWEll. 
 
 that the frequency is proportional to p^-' } that is, it varies 
 
 directly as the number of pairs of poles in the field magnets and 
 the number of revolutions. If the speed be fixed, the frequency 
 determines the number of poles, and, consequently, to a large 
 extent, the size and cost of the machine. The bearing of this 
 point in the design of alternators will be readily appreciated on 
 inspecting Table M, which shows the necessary revolutions per 
 minute at the frequencies customary in this country, with 
 various numbers of pairs of field poles. 
 
 It is seen that for a given number of pairs of poles the revo- 
 lutions vary directly as the frequency, which on this account 
 should be low. But the output of a given alternator, other 
 things being equal, is proportional to the frequency, and there- 
 fore a low frequency means a heavy machine for the output. 
 This disadvantage is, however, counterbalanced to a large 
 extent by the higher induction which is permissible as the 
 frequency is diminished. 
 
 Table M.* 
 
 lumber of pairs 
 of field poles. t 
 
 revs. 
 per niin.= 
 
 "V =80 
 
 revs. 
 per min. = 
 
 no =100 
 
 revs, 
 per min. = 
 
 revs, 
 per min.= 
 
 2 
 
 1,500 
 
 2,400 
 
 3,000 
 
 3,9CO 
 
 4 
 
 750 
 
 1,200 
 
 1,500 
 
 1,950 
 
 6 
 
 500 
 
 800 
 
 1,000 
 
 1,300 
 
 8 
 
 375 
 
 600 
 
 750 
 
 975 
 
 10 
 
 300 
 
 480 
 
 600 
 
 780 
 
 12 
 
 250 
 
 400 
 
 500 
 
 650 
 
 14 
 
 215 
 
 343 
 
 432 
 
 557 
 
 16 
 
 187 
 
 300 
 
 375 
 
 487 
 
 18 
 
 166 
 
 266 
 
 332 
 
 433 
 
 20 
 
 150 
 
 240 
 
 300 
 
 390 
 
 The ordinary values of B" a are given in Table X, and are 
 practically the same as those found advisable in the design of 
 transformers. 
 
 * See also Table T, p. 184. 
 
 t The number of pole pieces in a machine is equal to 2 p. 
 
DESIGN OF ALTERNATORS. 17& 
 
 Table N. 
 
 Frequencies C\j = 
 
 Induction B" a = from 
 
 50 
 
 52,000 to 40,000 
 
 60 
 
 30,000 to 32,000 
 
 70 
 
 28,000 to 30,000 
 
 80 
 
 26,000 to 28,000 
 
 90 
 
 24,000 to 26,000 
 
 100 
 
 22,500 to 24,000 
 
 110 
 
 21,000 to 22,500 
 
 120 
 
 19,500 to 21,000 
 
 130 
 
 18,000 to 19,500 
 
 If the magnetic field vary in strength the induction must be 
 the ^/mean square value. 
 
 31. DESIGN OF ALTEENATOES ; MAGNETIC DEN- 
 SITY; EFFECT OF SHAPE OF PEESSUEE AND 
 CURRENT CUEYES ; PEEIPHEEAL SPEED ; 
 EXCITATION. 
 
 The preceding investigation of alternate-current working 
 will be sufficient to show the modifications necessary to apply 
 the method of designing dynamos and motors described in 
 Chapter II. But special attention must be given to the mag- 
 netic induction, and the current density must be chosen accord- 
 ing to the ventilation provided. For example, in an iron-cored 
 armature having a large heat waste from hysteresis, the loss in 
 the copper must be kept relatively small, whereas in a coreless 
 armature of the Brush or Ferranti type, the current density 
 may be as high as 4,000 amperes per square inch, or even more. 
 The effective pressure and current will in all cases be of chief 
 importance, since the output, heat waste, and average torque 
 are proportional to these values. But the maximum pressure 
 will determine the limiting stress on the insulation, and must 
 be considered accordingly. From this point of view the Kapp 
 and Giilcher alternators appear to be excellent, for they give a 
 flat top to the theoretical curve of pressure (see Figs. 76 and 79, 
 pp. 157 and 159). Also the maximum torque will be proportional 
 
170 ELECTRIC MOTIVE POWER. 
 
 to the maximum value of the current, and the stresses must be 
 calculated with reference to this value. In cases likely to be 
 met with in practice, both pressure and currents closely approxi- 
 mate to a sine law in the outer circuit, and it is probably safe 
 to assume that the armature pressure curve also does so ; at 
 any rate, judging from the illustrations given in Figs. 75, 76, 
 77, 78 and 79, the probable error is not sufficiently large to 
 seriously affect the results. 
 
 The peripheral velocity of alternators is generally greater 
 than that of continuous-current machines, and is a question of 
 mechanical construction. Generally, with non-magnetic core 
 armatures of the Ferranti type, the moving parts are so light 
 that the centrifugal stresses are easily dealt with at even from 
 5,000 to 8,000ft. per minute. And in revolving field machines 
 of the Brush and Brown type it is clear that the speed is only 
 restricted by the tensile strength of soft cast iron. In revolv- 
 ing fields, of the Electric Construction Company's type, since the 
 magnets are mainly built up of wrought iron, the chief factor 
 is the ultimate strength of the bolts and screws holding the 
 parts together, &c. 
 
 32. EXCITATION; PARALLEL WORKING; CRITICAL 
 FIELD CURRENT OF ALTERNATORS AND 
 ALTERNATE-CURRENT MOTORS. 
 
 The estimate of the excitation presents some difficulty, but 
 since the current is almost invariably provided by a separate 
 direct-current dvnamo, often coupled to the shaft of the alter- 
 nator, a limited adjustment can be made by a variable resistance 
 placed in series with the coils. However, it is important to notice 
 that for every driving power there is one value of exciting current 
 which will make the output, and, consequently, the efficiency, a 
 maximum (see Fig. 88). In fact, the output does not vary directly 
 as the field strength, as is the case with continuous-current 
 machines ; and it follows that a proper regulation cannot be 
 made by the exciting current alone, but requires as well an 
 adjustment of the driving power for each load. This is of special 
 importance when alternators are coupled in parallel. If the 
 exciting current be either above or below this critical value, the 
 
BEST EXCITATION OF ALTERNATORS. 
 
 177 
 
 efficiency will be lowered and the output reduced (for the given 
 driving power). In the first case, the armature current will be 
 increased, and the terminal pressure also; but the efficiency will 
 be diminished, owing to the increased loss over the armature 
 resistance. In the second, the current will also be increased, 
 and the terminal pressure will be lowered. 
 
 This effect is not very marked with many alternators, and in 
 most cases the best excitation is nearly the same through the 
 working range of output. Perhaps it is of more importance in 
 the case of alternate-current motors, for then the increase of 
 
 eo 
 
 'JO 
 
 20 
 
 O 
 
 O 
 
 600 1,000 
 
 1,600 2.000 2,600 3,000 3,600 4.0OO 
 Volts. 
 
 FIG. 88. 
 
 * Curves connecting Terminal Volts and Amperes in Alternators sup- 
 plied with Constant Power, the excitation being increased continuously 
 from zero at O to a maximum at E. Curve 1 refers to a 10 k.w. alter- 
 nator ; curve 2 refers to a 60 k.w. alternator ; and curve 3 refers to the 
 larger machine when the armature reactions are taken into account. The 
 Volts and Amperes are in phase at the apex of each curve. 
 
 main current may cause a serious loss in the line, as well as in 
 the armature circuits. With motors it is found that for every 
 load there is one excitation which will give a minimum 
 armature (main) current. 
 
 This important fact was apparently first discovered in 1890, 
 at a lighting plant in Cassel, where the generating station, 
 comprising turbines and alternators, lies several miles from 
 the town. The electric power is transmitted through 
 a concentric cable to two sub-stations, each of which coh- 
 tains a large alternate-current motor driving dynamos. The 
 
 *See " Dynamos, Alternators and Transformers," by G. Kapp, page 409. 
 
178 
 
 ELECTRIC MOTIVE POWER. 
 
 men in charge of these machines found that the main 
 current could be varied for the same load by altering the 
 motor excitation. A similar effect is illustrated in Fig. 88A, 
 which refers to experiments made by Mr. Mordey with two 
 Brush alternate motors running light. The terminal pressure 
 was maintained constant at 2,000 volts and 100 r\^, the speed 
 also being constant throughout the range of observation. The 
 
 20 
 18 
 16 
 
 |14 
 
 5 
 
 < 12 
 
 i_ 
 o 
 
 |io 
 
 c 
 'I 8 
 
 & 
 E 
 <.6 
 
 4 
 2 
 
 
 y 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 "A10" 
 
 
 
 
 Z; 
 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 
 \ 
 
 V 
 
 \ 
 
 
 
 < / 
 
 1 
 
 
 
 
 \ 
 
 \ 
 
 \ 
 
 / 
 
 / 
 
 - 
 
 
 
 
 \ 
 
 
 \J 
 
 ;- 
 
 I 
 
 / 
 
 
 
 
 
 
 X 
 
 Z 
 
 J 
 
 
 
 
 
 
 
 "V 
 
 
 
 
 
 Amperes in Motor Field. 
 FIG. 88A. Curves Connecting Exciting and Armature Currents. 
 
 tests were continued with various loads, and it was found that 
 for every output there was one value of exciting current 
 corresponding to minimum armature current, and that the 
 critical excitation was approximately the same for all loads. 
 Professor S. P. Thompson has shown that minimum current in 
 a motor armature occurs when the pressure and current are in 
 phase, that under-excitation causes the current to lag, and over- 
 excitation causes it to lead. 
 
SINGLE-PHASE ASYNCHRONOUS MOTORS. 179 
 
 Dr. Sumpner, Mr. G. Kapp, Mr. Swinburne, Mr. Mordey, 
 and others have investigated the theory of exciting alternate- 
 current machines. And it appears that the output of an alter- 
 nator depends on the steam admission, or simply the driving 
 power ; while the lag, and, consequently, the wattless current, 
 is controlled by the excitation. And for a given brake 
 horse-power of an alternate-current motor, the excitation deter- 
 mines the lag and wattless current. 
 
 The problem is complicated by armature reactions, self-induc- 
 tion, and capacity effects in the circuit.* Armature self-induction 
 tends to weaken the field, and therefore to necessitate more 
 than the calculated excitation, while capacity in the armature 
 tends to strengthen the field, and hence to reduce the value of 
 the necessary excitation. The actual number of ampere-turns 
 for any load will, therefore, depend on the balancing of these 
 two effects, and no general rule can be stated. When alterna- 
 tors are running in parallel, with separate engines, the steam 
 admission should be controlled by a common single governor, 
 and not by separate governors to each engine. And the real 
 output of each dynamo, after the throttle valve is properly set 
 for the full output of the machine, should be measured by a 
 wattmeter, and controlled (in accordance with its readings) 
 through the field excitation. An ammeter is of no use for this 
 purpose, as it gives no indication of the lag between current 
 and pressure ; and the omnibus pressure, being the same for all 
 the machines, is no criterion of the actual total electromotive 
 force of any of the alternators. It may therefore happen that 
 two alternators in parallel may be giving pressures differing as 
 widely as 2,500 and 1,500 volts, and yet the omnibus pressure 
 will show 2,000 volts, the proper station pressure, say. 
 
 33. SINGLE-PHASE ASYNCHRONOUS MOTORS SELF- 
 EXCITING AND SELF-STARTING. 
 
 This type of motor is not yet in general use in England, but 
 it is steadily gaining favour on the Continent, and is worth 
 some attention in connection with the present subject. 
 
 * For further information and experiments with synchronous motors 
 on long-distance lines, see Proceedings of the Institution of Electrical 
 Engineers, Vol. XXIII. 
 
180 ELECTEIG MOTIVE POWER. 
 
 Messrs. Hutin and Leblanc, Mr. C. E. L. Brown and others, 
 have devised various forms which run with single-phase currents, 
 asynchronously and with good efficiency. By the combination 
 of an additional rotary magnetic field, they can be made to 
 start against a considerable torque, and the auxiliary field can 
 be stopped after the proper speed is attained. Various devices 
 to secure such a field are suggested by Mr. C. E. L. Brown, 
 the most feasible being two parallel windings on the field 
 magnets one with small self-induction (or even capacity), and 
 the other with larger self-induction. These two circuits when 
 supplied with a single-phase current will produce a rotary field, 
 and so cause the armature to revolve.* 
 
 The construction and general appearance of these motors is 
 similar to that of multiphase machines (see 58, page 284). 
 It may be noted now, however, that the armatures have no 
 collectors or brushes, and that the windings consist simply of 
 straight conductors laid in grooves or tunnels, and connected at 
 either end to solid metal rings (see Figs. 169 to 171, and page 
 292) ; or are laid on the periphery of the core so as to look not 
 unlike a squirrel cage. 
 
 The general principle will be readily grasped from diagram 
 Fig. 89, which refers to the ordinary method of coupling the 
 Brown motors. 
 
 The winding of the motor is in two parallels, each of as many 
 coils as there are poles in the field magnets, the coils of each 
 series being wound in the spaces between the coils of the other. 
 The starting set 'is in series with a voltameter or condenser 
 and serves to lead the phase of the current passing through it, 
 while the main coil, having considerable self-induction, retards 
 the phase of its current. 
 
 To start the motor, the paths a 1) and c d are closed by their 
 respective switches. Current from the transformer T then 
 passes from the terminal m to the field magnet coils of the 
 motor M. On dividing between the two coils, one portion of 
 
 * Other forms of this ingenious motor have been suggested by Mr. 
 Brown, for which rference should be made to the technical journals of 
 January and March, 1893. 
 
SINGLE-PHASE ASYNCHRONOUS MOTORS. 181 
 
 the current passes through a b and through the regulating 
 handle J and the inductive resistance R, and returns to the other 
 terminal n of the transformer through the ammeter A. The 
 second current, in parallel with the first, goes through the 
 voltameter V, and passes direct to the other terminal of the 
 transformer. Thus it will be seen that the phase of the 
 current passing through the main coils and the resistance R 
 will lag, while the other will be advanced by means of the volt- 
 ameter V. By a suitable adjustment of the condensers to the 
 
 wwwwvw 
 
 MMAAA 
 Jf n 
 
 FIG. 89. Diagram of Connections of C. E. L. Brown's Single-Phase 
 Asynchronous Motor. 
 
 periodicity, a phase difference of a quarter of a period is 
 obtained between the two currents. A steady deflection of the 
 ammeter A shows the motor has arrived at synchronism. Then 
 the connection a b is broken by the commutator switch, and 
 that through a f is made, the motor then running by means of 
 the main coils only. By a suitable coupling of the switches, 
 the making of the contact a f and the breaking of c d can be 
 accomplished at one operation. 
 
 The resistance R is always in circuit across the transformer- 
 terminals, absorbing a current which, however, is very small 
 
 o 
 
182 
 
 ELECTRIC MOTIVE POWER. 
 
 owing to its large self-induction. This arrangement is used to 
 eliminate the induced current on opening and closing the 
 circuit, and is the subject of a special patent by Mr. Brown. 
 The condenser is built up of layers of V-shaped iron plates, in- 
 sulated from one another, and immersed in a solution of caustic 
 soda contained in an iron jar. It is to be noted that the 
 auimeter, the resistance and the condenser, may be made to 
 serve all the motors of a given installation. 
 
 For very small motors of from -^ to J H.P. a few spirals of 
 German silver or iron wire put in the starting circuit are 
 sufficient to give the requisite lag. This resistance is con- 
 veniently arranged on a hand frame, and can be used con- 
 secutively for a number of machines. 
 
 By the courtesy of the Directors of the Giilcher Company, 
 the author was enabled, in November, 1893, to make, in 
 conjunction with Mr. W. B. Esson, a series of tests with Brown 
 motors of various sizes. The results obtained are given in 
 Tables 0, P, Q, and R, and may be of interest as showing the 
 commercial efficiency and general performance of this type of 
 motor. 
 
 Table 0.100 volts, SO r^, 4 poles, J H.P. 
 
 Absorbed. 
 
 Eevolu- 
 tions. 
 
 Watts 
 given out. 
 
 Actual 
 efficiency 
 % 
 
 Apparent 
 efficiency 
 % 
 
 Amperes. 
 
 Watts 
 actual. 
 
 Watts 
 apparent. 
 
 Power 
 factor. 
 
 3 
 
 .128 
 
 300 
 
 0-43 
 
 2,330 
 
 Empty. 
 
 
 
 
 
 5-5 
 
 343 
 
 550 
 
 0-62 
 
 2,280 
 
 245 
 
 71-4 
 
 44-6 
 
 36-5 
 
 2,188 
 
 3,285 
 
 0-666 
 
 
 
 
 
 At starting. 
 
 Table P. 100 volts, 80 ^, 6 poles, 1 H.P. 
 
 Absorbed. 
 
 Revolu- 
 tions. 
 
 Watts 
 given out. 
 
 Actual 
 efficiency 
 
 7 ' 
 
 Apparent 
 efficiency 
 
 % " 
 
 Amperes. 
 
 Watts 
 actual. 
 
 Watts 
 apparent. 
 
 Power 
 factor. 
 
 8-48 
 
 176-4 
 
 848 
 
 0-21 
 
 1,560 
 
 Empty. 
 
 
 
 
 
 10-0 
 
 364-0 
 
 1.000 ! 0-364 
 
 1,540 
 
 213 
 
 58-5 
 
 21-3 
 
 12-45 
 
 706 
 
 1,245 0-567 
 
 1,520 
 
 500 
 
 70-8 
 
 40-18 
 
 16-7 
 
 1,109 
 
 1,670 (0-663 
 
 1,490 
 
 800 
 
 72-1 
 
 48-0 
 
 22-0 
 
 1,125 
 
 2,200 
 
 0-511 
 
 
 
 
 
 At starting. 
 
SINGLE-PHASE ASYNCHRONOUS MOTORS. 183 
 
 Table Q.1CO volts, 80^,6 poles, 2 H.P. 
 
 
 
 
 
 
 
 Revolu- 
 tions. 
 
 Watts 
 given out. 
 
 Actual 
 efficiency 
 
 Apparent 
 efficiency 
 
 /o 
 
 Amperes. 
 
 Watts 
 actual. 
 
 Watts ' Power 
 apparent. |f actor. 
 
 12-7 
 
 252 
 
 1,270 
 
 0-2 
 
 1,570 
 
 Empty. 
 
 
 
 
 
 14-5 
 
 520 
 
 1,450 
 
 0-38 
 
 1,550 
 
 290 
 
 55-8 
 
 20-0 
 
 18-0 
 
 1,C40 
 
 1,800 
 
 0-557 
 
 1,520 
 
 637 
 
 66-0 
 
 38-1 
 
 24-5 
 
 1,630 
 
 2,450 
 
 0-665 
 
 1,500 
 
 1,100 
 
 67-5 
 
 44-9 
 
 30-5 
 
 2,050 
 
 3,050 
 
 0-672 
 
 1,490 
 
 1,400 
 
 68-3 
 
 45-9 
 
 31-2 
 
 2,150 
 
 3,120 
 
 0-69 
 
 1,480 
 
 1,520 
 
 70-7 
 
 48-6 
 
 30-0 
 
 1,848 
 
 3,000 
 
 0-616 
 
 
 
 
 
 At starting. 
 
 Table E.100 volts, 60^,6 poles, 2 H.P. 
 
 ( 
 
 Absorbed 
 
 
 
 
 
 Revolu- 
 tions. 
 
 Watts 
 given out. 
 
 Actual 
 efficiency 
 
 % 
 
 Apparent 
 efficiency 
 
 % 
 
 Amperes. 
 
 Watts 
 actual. 
 
 Watts 
 apparent. 
 
 Power 
 factor. 
 
 16-5 
 
 320 
 
 1,650 
 
 0-194 
 
 1,185 
 
 Empty. 
 
 
 
 
 
 17-5 
 
 500 
 
 1750 
 
 0-286 
 
 1,170 
 
 210 
 
 42-0 
 
 12-0 
 
 19-6 
 
 1,040 
 
 1,960 
 
 0-53 
 
 1,160 
 
 690 
 
 66-3 
 
 35-7 
 
 23-4 
 
 1,320 
 
 2,340 
 
 0-564 
 
 1,150 
 
 980 
 
 74-2 
 
 42-0 
 
 27-0 
 
 1,710 
 
 2,700 
 
 0-631 
 
 1,140 
 
 1,270 
 
 74-3 
 
 47-0 
 
 44-5 
 
 2,850 
 
 4,450 
 
 0-64 
 
 
 
 
 
 At starting. 
 
 Mr. Banti has made some tests with larger Brown machines 
 of this class, the figures of which are given in Table S. It will 
 be seen that the efficiency is high at from half to full load. 
 
 Table S. 106-3 volts, 45 <^, 6 poles, 5 H.P. 
 
 Absorbed. 
 
 Revolu- 
 tions. 
 
 Watts 
 given out. 
 
 Actual 
 efficiency 
 
 Apparent 
 efficiency 
 
 Amperes. 
 
 Watts 
 actual. 
 
 Watts 
 apparent. 
 
 Power 
 factor. 
 
 15-0 
 
 343-0 
 
 1594-5 
 
 0-215 
 
 902 
 
 Empty. 
 
 
 
 
 
 17-5 
 
 967-7 
 
 1860-25 
 
 0-521 
 
 890 
 
 588-8 
 
 61-0 
 
 31-6 
 
 21-0 
 
 1421-0 
 
 2232-0 
 
 0-638 
 
 870 
 
 1015-0 
 
 72-0 
 
 46-2 
 
 24-0 
 
 1911-0 
 
 2551-0 
 
 0-751 
 
 884 
 
 1479-0 
 
 77-4 
 
 57-1 
 
 27-5 
 
 2401-0 
 
 2923-0 
 
 0-820 
 
 872 
 
 1869-0 
 
 77-9 
 
 64-0 
 
 32-3 
 
 2976-7 
 
 3433-0 
 
 0-859 
 
 872 
 
 2325-0 
 
 78-9 
 
 67-6 
 
 37-0 ! 3503-5 
 
 3933-0 
 
 0-894 
 
 860 
 
 2699-0 
 
 77-0 
 
 68-2 
 
 43-0 
 
 4189-5 
 
 4570-0 
 
 0-917 
 
 868 
 
 3083-0 
 
 73-6 
 
 67-4 
 
 48-5 
 
 4806-9 
 
 5155-0 
 
 0-934 
 
 848 
 
 3383-0 
 
 70-3 
 
 65-6 
 
 55-0 
 
 5439-9 
 
 5846-0 
 
 0-934 
 
 860 
 
 3731-0 
 
 68-0 
 
 67-0 
 
 o2 
 
184 
 
 ELECTRIC MOTIVE POWER. 
 
 ws 
 
 I 
 
 g -I 
 
 " 
 
 
 o 
 
 O3 Ji} 
 
 ijslj 
 
 CO T-l rH CO rH CO 
 
 O 
 
 LO O 
 LO O CO 
 
 i-Ti-f~" io" 
 
 o 10 o 
 
 CO CO CO 
 
 o / \ 
 ^- z:. 
 
 r4W- of rH 1- Of 
 
 s ? 
 
 rHr4' 
 
 o 
 
 o^ 
 
 of r-T 
 
 03 03 ^r 
 
 of of 
 
 gg 
 
SINGLE-PHASE ASYNCHRONOUS MOTORS. 185 
 
 Mr. Banti conducted his tests in the following manner : 
 The motor was started by sending the single-phase current 
 from the transformer through the auxiliary and main windings. 
 As soon as synchronism was attained, the ammeter inserted 
 in the principal circuit assumed a definite deflection. The 
 condenser circuit was then opened and the belt applied to 
 the motor. Readings were then taken of the watts and 
 amperes supplied, the speed, and the torque on the motor 
 shaft ; the volts being kept at constant pressure. The watts 
 were measured by a Ganz wattmeter, a Cardew voltmeter was 
 used for observing the pressure, whilst a previously standardised 
 tachometer indicated the speed. The output was measured 
 by an absorption brake. 
 
 The effect of frequency on the outputs, weight, and speed is 
 of great importance, and has been made the subject of in- 
 vestigation by some of the leading firms. It is illustrated in 
 Table T, which is compiled from a series of tests made by 
 the Oerlikon Company with their stock machine?, designed 
 for frequencies varying from 42 to 100 *\> per second, at a 
 uniform pressure of 110 volts. The figures are only approxi- 
 mate. 
 
 The weights bear no particular ratio, te the outputs, because 
 the same patterns have to be used for machines working under 
 different conditions. It will be clearly seen, however, that the 
 weight for a given speed and output and the same number of 
 poles varies, roughly, inversely as the frequency. The actual 
 watts and the efficiency are practically the same at each 
 frequency within the limits of the Tables. In connection with 
 this, reference should be made to Table M, page 174, which 
 shows the alteration of speed caused by varying the number of 
 poles for various frequencies. 
 
CHAPTER VI. 
 
 TRANSFORMERS. 
 
 34. Introduction; Types of Transformers. 35. Transformers in 
 Parallel ; Primary and Secondary Drop of Pressure ; Magnetic 
 Leakage ; Constant Current Transformers. 36. Curves of Closed 
 Magnetic Circuit Transformers. 37. Design of Transformers. 38. 
 Step-up and Step-down Transformers. 
 
 34. INTRODUCTION ; TYPES OF TRANSFORMERS. 
 
 REFERENCE has already been made to the function of the con- 
 tinuous-current transformer or dynamotor ( 25, p. 138). That 
 of the alternate-current transformer or converter, generally 
 denoted simply by transformer, is essentially the same ; it 
 converts power, ^ V lf at one pressure into a corresponding 
 power at another pressure and current, i. 2 V 2 (say). Assuming 
 no loss in the transforming, 
 
 The left-hand product represents the power in the primary 
 circuit at the high-pressure terminals of the transformer ; 
 ij and Vj severally expressing the primary current and 
 pressure. The right-hand product gives the power in the 
 secondary circuit at the terminals of the consumer's circuit, 
 V 2 being the pressure of supply. It is clear that the primary 
 pressure will be selected as high as possible in order to keep 
 the current small, and thereby to economise in copper. A 
 usual value for V l is 1,000 volts. Let the horse-power be 100 
 (i.e. =74,600 watts) and the secondary pressure, V 2 , be 100 
 volts, then 
 
 Primary circuit, ^ Vj = 74'6 x 1,000, 
 Secondary circuit, ^' 2 V 2 = 746 x 100, 
 or the currents are in inverse ratio to the pressures. 
 
188 ELECTRIC MOTIVE POWER. 
 
 In practice, owing to the loss in conversion, the secondary 
 output will be less than the primary by an appreciable quan. 
 tity, varying with the design, size and load of the transformer 
 It will fix ideas to examine the following results obtained by 
 Dr. J. A. Fleming from some of the best known transformers. 
 Table U shows that with an output of one tenth of the full 
 load a well-designed transformer has an efficiency of rather 
 more than 86 per cent., and with one-twentieth about 76 
 per cent. 
 
 It will be seen also that the efficiency is highest at from three- 
 quarters to full load, and that the efficiency decreases as the 
 load is diminished. It is thus of the highest importance to 
 secure a high-load diagram for each transformer, and to ensure, 
 if possible, that the all-day load shall not be less than 50 per 
 cent, of the maximum. Then the efficiency will average about 
 95 per cent. This is a very important consideration in dis- 
 tributing power, and will be referred to again when discussing 
 power problems. Another point is the poiver-factor, or ratio of 
 true watts (effective volts x effective amperes x cos </>) to appa- 
 rent watts (effective volts x amperes). This is a variable 
 quantity, being from 0'77 to 0'80 at no load, and attain- 
 ing unity at about one-tenth of the maximum output in 
 the best modern designs, such as Mordey, Ferranti, Kapp, 
 Westinghouse, and Thomson-Houston transformers. With the 
 " Hedgehog " type it does not exceed 0'07 at no load, and 
 never exceeds about 0'8 even at full load. This ratio does not 
 give the efficiency of the transformer, but, since it is dependent 
 on the conductivity of the magnetic circuit, it forms a rough 
 guide to the suitability of transformers for given work, and 
 also is a useful figure of merit for determining the probable 
 line and plant efficiency at light loads, especially when the 
 secondary circuits are open.* 
 
 * It is now well recognised that a recording wattmeter is the most satis- 
 factory instrument for measuring the true watts given out from a central 
 station. The integration of the resulting diagram will give the total 
 energy supplied in any given time, and the meter-readings will give a fair 
 idea of the quantity of energy received by the various consumers during 
 the same period (assuming the line to be non-inductive) ; hence it is 
 possible to estimate the plant efficiency with considerable accuracy. 
 
EFFICIENCIES OF TRANSFORMERS. 
 
 189 
 
 O5O3O3 
 
 CT5 
 
 o^rbooro 
 
 O5 SDC5O3O5O5 
 
 O3io 
 
 00 O5 
 
 iHOI^05^00C<3 
 O5O5O5O5COCJ)O> 
 
 - 
 
 cNiihihoaoibto 
 
 C35OG3OO500O5 
 
 OrHOOr-lpCDCpv 
 td-iHlhlbi^COTH^HC- 
 
 ooooooooo 
 
 ptioii 
 
 11 
 
 - 
 
 W 
 
 - f 
 
 - 
 
 y-t n3 
 
 O 01 
 
 ^ W 
 d s 
 
 11 
 
 ^2 ^^ ^^ ^^ ^5 
 
 I 
 
 a 
 
 w 
 
190 
 
 ELECTRIC MOTIVE POWER. 
 
 The types of transformers in general use are not very nume- 
 erous ; indeed, there is not room for much variation, although, of 
 course, the details differ widely. Broadly speaking, transformers 
 may be divided into two classes, which differ fundamentally, viz., 
 the closed magnetic circuit and the open magnetic circuit. The 
 first includes the majority of designs, and has been demon- 
 strated to be generally superior to the second, which is prac- 
 tically only represented by the " Hedgehog " transformer. The 
 specific claim for the open-circuit converter is an assumed higher 
 average efficiency. But Dr. J. A. Fleming, after an exhaustive 
 series of trials,* finds that the closed type is the better, even 
 
 FIG. 90. " Hedgehog " Transformer. The vertical lines in the centre with 
 spiked ends represent the soft iron wire core. 
 
 on this count. With respect to light-load efficiency there is 
 no doubt that the closed circuit is preferable. For the power- 
 factor of the open-circuit type is so low that the wattless current 
 is always large, and the " magnetising current," or primary 
 current with secondary circuit open, appears to be practically 
 prohibitive. 
 
 The "Hedgehog" transformer (Fig. 90) is, however, of com- 
 mercial interest. 
 
 * See Journal of the Institution of Electrical Engineers, Vol. XXL, 
 Parts 101 and 102. 
 
TYPES OF TRANSFORMERS. 
 
 191 
 
 It will be seen that the iron core is surrounded by the copper 
 winding, and the soft wires of which it is built are turned over at 
 the ends into hemispheres. It is a very cheap design, and finds 
 favour from considerations of first cost. 
 
 It is fair to state that Mr. Swinburne explains the indifferent 
 results obtained with the particular "Hedgehog" transformer 
 
 FIG. 91. Westinghouse Transformer. 
 
 tested by Dr. Fleming by the fact that its winding compressed 
 the soft iron wire core so tightly that the films of insulating 
 oxide were partially broken, and the whole core acted as a more 
 or less solid mass. This no doubt militated largely against the 
 efficiency, and it may be that the open-type transformer in 
 
 FIG. 92. Ferrauti Transformer. 
 
 Mr. Swinburne's hands will ultimately attain to as high a power, 
 factor and all-round efficiency as the closed type ; but this is 
 not the case at present. 
 
 The better-known closed-circuit transformers are illustrated 
 diagrammatically in Figs. 91 to 100. The primary windings 
 
192 
 
 ELECTRIC MOTIVE POWER. 
 
 are suggested by fine cross lines, and the secondary by coarser 
 ones. Various methods of disposing of the windings some 
 separate and some interplaced are also indicated, but they 
 are not peculiar to any of the types, being practised as 
 found convenient. With the exception of the Ferranti 
 
 FIG. 93. Mordey Transformer. 
 
 transformer, Fig. 92, which is built up of thin iron strips, 
 all the types illustrated are made from thin stampings of 
 soft iron, the gauges most in use being from No. 24 to No. 30 
 B.W.G. Generally the design aims at reducing the external 
 magnetic leakage to a minimum, thereby causing the whole of 
 
 FIG. 94.- Kapp Transformer. 
 
 the lines of force induced by the primary current to pass 
 through the secondary circuit, and vice versa. The methods of 
 putting the windings in position will be easily understood. The 
 primary and secondary coils are first wound on " formers," and 
 then thoroughly insulated both from each other and from the iron 
 
TYPES OF TRANSFORMERS. 
 
 193 
 
 core. In Fig. 91 the pieces marked p p are bent back, and the 
 coils are passed over the core; when as many stampings as are 
 required are threaded, the plates are distanced with mica and 
 firmly secured in position by the external casing. In Figs. 94, 
 
 FIG. 95. Glibber (Esson) Transformer. 
 
 95, 96, 97 and 98, the coils are simply laid around the centre core 
 pieces before the ends are placed in position. In Fig. 93 the 
 coils are first placed in the centre of the larger plates, and the 
 small punchings are put between consecutive plates, and thus, 
 
 FIG. 96. Snell and Kapp Transformer. 
 
 passing through the coils, form the core. In the Ferranti type 
 the coils are slipped over the iron strips, which are then bent into 
 shape and secured by the outer casing. In Fig. 99 the coils are 
 dropped over the core, and then the plug pieces, k &, are put in 
 
194 
 
 ELEGTEIG MOTIVE POWER. 
 
 position. In Fig. 100, which is the latest Thomson-Houston 
 design, the coils and the core are placed together in the centre 
 of the rectangular stampings. 
 
 The insulation spaces permit a free circulation of air 
 or of oil if the whole be immersed in a resinous oil, as is 
 frequently done in modern designs for very high pressures. 
 
 FIG. 97. Rankin Kennedy Transformer. 
 
 By means of oil insulation converters have been built with 
 a transformation ratio as high as 1 \ 140, viz., from 50 to 
 20,000 volts ; and even this is not necessarily the limit if 
 
 FIG. 98. Brown Transformer. 
 
 special precautions are taken as regards the disposition of 
 the high pressure coils, and especially of the leading-in and 
 out wires, which are usually encased in glass tubes widely 
 separated in order to prevent arcing. It must be recollected 
 that the normal maximum pressure, which in this case should 
 
TYPES OF TRANSFORMERS. 
 
 195 
 
 be about 20,000 x ^j or, say, 28,200 volts, determines the 
 limit of disruptive discharge, and that if there be a resonant 
 effect in the circuit, it is quite possible for this value to 
 be considerably exceeded. No doubt the cumulation of the 
 
 FIG. 99. Thomson-Houston Transformer. 
 
 several harmonic pressures in a circuit explains the otherwise 
 apparently inexplicable puncturing of insulation. Oil insula- 
 tion is especially valuable in such cases, for the puncture closes 
 as soon as the spark has passed, and the insulation is as strong 
 as before. Extra high pressure transformers are at present 
 
 
 1 
 
 
 
 
 FIG. 100. Latest Form of Thomson-Houston Transformer. 
 
 only in use for long-distance transmission, and are used as step- 
 up converters at the power station, and as step-down converters 
 at the distributing end of the line. By their use the main 
 current is made as small as desired, and the extra high 
 
196 
 
 ELECTRIC MOTIVE POWER. 
 
 pressure is reduced to a safer limit before serving the dis- 
 tributing system (see 38, p. 208). 
 
 The performance of a transformer is affected by size, relative 
 quantities of iron and copper, magnetic density, copper loss, 
 stray magnetic field, and resistance of the joints at right angles 
 to the path of the lines of force. Speaking generally, high 
 efficiency at low loads may be attained if the total copper loss 
 be kept small and the magnetic induction be made relatively 
 low i.e., if there be a large quantity of both copper and iron; 
 but this design is necessarily expensive owing to the large mass 
 of metal. If a high efficiency be required only at nearly full 
 load, then the copper loss may be larger and the cross- section 
 
 100 
 
 02 Oo -04 05 0-6 07 
 
 Fractions cf Full Secondary Load. 
 
 
 
 09 
 
 10 
 
 FIG. 101. Efficiency Curves of Various Transformers. 
 
 Top Curve, 6, 500- watt Westinghouse Transformer. 
 
 Middle Curve, 6,000-watt Mordey Transformer. 
 
 Bottom Curve. 4,500-watt Thomson-Houston Transformer. 
 
 NOTE. If the Transformers were of the same power, the curves would 
 probably be nearly identical. 
 
 of the iron less, the design being lighter and cheaper. If 
 high average efficiency with cheap design be aimed at, a com- 
 promise must be made between these two extreme proportions. 
 These remarks are applicable to any of the designs mentioned, 
 and the results actually obtained in practice may be gathered 
 from Table U, p. 189. The efficiency curves of some of the 
 standard transformers are shown in Fig. 101. 
 
TRANSFORMERS IN PARALLEL. 197 
 
 35. TRANSFORMERS IN PARALLEL; PRIMARY AND 
 SECONDARY DROP OF PRESSURE ; MAGNETIC 
 LEAKAGE. 
 
 If a transformer be designed for parallel working and this 
 is the more frequent practice then the aim is to make the 
 mutual induction of the primary and secondary circuits a 
 maximum, and to restrict the drop of volts to as small limits 
 as possible good practice being within 2 per cent, of the 
 open-circuit secondary pressure. The " total secondary drop " 
 at full load is affected not only by the secondary resistance, 
 but also by the primary resistance and the stray magnetic 
 field. The leakage effect increases with the output, for the dis- 
 turbing effect due to the secondary ampere-turns is, of course, 
 proportional to the secondary current, i y The total drop is 
 also affected by the increase of value of the primary drop, 
 *! Rjj for, if the pressure at the primary terminals be kept con- 
 stant, the primary counter pressure decreases as the primary 
 current increases. And as the secondary pressure is propor- 
 tional to the primary counter pressure (not to the primary 
 terminal pressure) the secondary drop Is increased in pro. 
 portion as this diminishes. The number of volts in the 
 secondary drop due to the primary drop is given by the 
 expression 
 
 i x primary drop, or ^R! ..... (42) 
 1 i *i 
 
 In the transformer equations the suffixes o and / are 
 severally used here to distinguish current or pressure at no 
 load and full load ; T 1? T 2 = the respective number of turns in 
 the primary and secondary coils; and R p R 2 = the primary 
 and secondary resistances. 
 
 The relative importance of these three causes of drop in the 
 pressure varies with different designs. In the Westinghouse 
 type the magnetic leakage accounts for about one-half of the 
 drop ; whilst in the Kapp transformer, which has interplaced 
 windings, the leakage is very small (see Fig. 102). Dr. J. A. Flem- 
 ing gives the following rule for estimating the number of volts 
 
198 
 
 ELECTRIC MOTIVE POWEE. 
 
 lost by magnetic leakage with a closed-circuit transformer. 
 Add together the product of secondary resistance and maximum 
 
 T 
 
 secondary current and 2 times the product of primary resist- 
 
 T i 
 
 ance and current, after deducting the " magnetising current " 
 from the primary current at full load; subtract this sum 
 from the total observed drop, and the remainder is the number 
 of secondary volts lost by magnetic leakage. Thus : 
 
 Secondary volts lost by magnetic leakage 
 
 = ( V 2 fV 2 ) - {R$> f*2 + ^ -^i(f*i ~~ 
 T i 
 
 (43) 
 
 102 
 
 650 
 
 1300 1950 2600 3250 3900 4550 5200 5850 6500 
 Output in Secondary Watts. 
 
 FIG. 102. 6,500-watt Westinghouse Transformer. 
 
 a b, Line of Maximum Pressure, b c, Drop due to Secondary Resist- 
 ance, c d, Drop due to Primary Resistance, d e, Drop due to Leakage. 
 
 The order of the "drop" curves is illustrated in Fig. 102, 
 which is taken from observations made with a 6,500-watt 
 Westinghouse transformer. 
 
 These curves indicate the lowering of the secondary pressure 
 for each of the three causes. The top line, a b, shows the 
 open circuit secondary pressure of 101 volts ; line a c marks 
 the loss due to secondary resistance ; a d that caused by 
 primary resistance ; and a e that by magnetic leakage. The 
 maximum drop is about 2*4 volts, or a mean variation of 1'2 
 per cent. 
 
TRANSFORMERS IN SERIES. 199 
 
 If the primary and secondary currents be nearly in opposition 
 to each other, i.e., separated by a phase difference of ISOdeg. 
 approximately, the resultant ampere-turns at any load are 
 simply ij_ T x - ^' 2 T 2 , where T x and T 2 are severally the number 
 of primary and secondary turns. And with the secondary 
 circuit open the excitation is simply due to \ T r This par- 
 ticular value of the primary current is called the magnetising 
 current. It has been determined by Profs. Ryan and Fleming 
 and others that the resultant excitation is constant for all 
 loads ; hence the primary current at no load is a criterion of 
 the merit of a given transformer if the above-mentioned phase 
 difference be preserved. The following figures from a 3,000- 
 watt Giilcher transformer will give some idea of the value of 
 the effective or magnetising ampere-turns. The primary turns 
 are 1,000 in number, and ^^ = 1,570 at full load; the 
 secondary turns are 50, and ^ 2 T 2 = 1,500; therefore the 
 magnetising ampere-turns ^ 1 T 1 -^ 2 T 2 = 70 (approx.), and the 
 magnetising current = 0'07 ampere. 
 
 This law, however, is not true for open circuit transformers, 
 or for those closed circuit types which have defective joints, 
 for then there is a varying lag between the two currents, 
 and the value of the expression ^ T T - ^' 2 T 2 continually 
 diminishes as the load increases. It will be remembered 
 that the "Hedgehog" transformer was specially mentioned as 
 having a low power-factor, the value being a minimum at 
 no load and increasing towards the full output, but never 
 exceeding about 80 per cent., whereas the best closed-circuit 
 types have a power-factor of 100 per cent, at about one-tenth 
 of their output. 
 
 It is theoretically possible to run transformers in series upon 
 a constant current circuit, and to supply the secondaries with 
 constant currents ; but there are so few cases in which such a 
 system of distribution would be economical that few trans- 
 formers have been built for this purpose. It is sometimes 
 desirable, however, to feed a secondary circuit with constant 
 current while the primary is supplied at constant pressure, 
 as occurs with arc lamps in series. Both the Westinghouse 
 and Thomson-Houston Companies have devised such trans- 
 formers; that of the last-named Company being shown in 
 
 r2 
 
200 
 
 ELECTRIC MOTIVE POWER. 
 
 Fig. 102 A. It will be seen that the magnetic flux is carried 
 partly through the side cores and partly across the air-space 
 in the centre marked A. The proportions are so arranged that 
 a decrease in the number of arc lamps, lowering thereby the 
 secondary resistance, increases the reaction of the secondary 
 circuit on the primary, and causes a greater leakage of mag- 
 netism across the gap A. This cuts down the induction in the 
 secondary and increases the self-induction of the primary, thus- 
 proportionately decreasing its current. 
 
 FIG. 102A. Thomson -Houston Parallel Transformer with Constant 
 Secondary Current. 
 
 36. CURVES OF CLOSED MAGNETIC CIRCUIT 
 TRANSFORMERS. 
 
 In examining the performance of a given closed-circuit trans- 
 former, the essential points to be determined are : 
 
 (a) Pressure at primary terminals. 
 
 (6) Magnetising current. 
 
 (c) Primary and secondary resistances. 
 
 (d) Terminal secondary pressure at various loads. 
 
 '(e) Secondary currents at loads corresponding to readings 
 taken under (d). 
 
 The product of (a) and (6) gives the apparent watts con- 
 sumed at no load. Since the power factor or ratio of true 
 to apparent watts is then about 0'8, if this product be 
 
CURVES OF TRANSFORMERS. 201 
 
 multiplied by O'S the true watts are known, and the iron core 
 loss for all loads can be found by subtracting the primary coil 
 watts with open secondary coil. 
 
 Thus, 0-8 V, oh -o*i 2 &i = watts lost in iron. . . (44) 
 
 Then from the ratio of primary to secondary pressure at no 
 load, and from the same ratio with various secondary currents, 
 the corresponding primary currents can be calculated. And 
 hence, knowing the resistances, the primary and secondary i- R 
 losses can be estimated. Then, drawing a total i 2 R loss curve 
 and a total loss curve (i.e., adding hysteretic and eddy current 
 losses) parallel to it, there are sufficient data to determine the 
 efficiency at all stages of the load. 
 
 To illustrate this the following calculations are given with 
 reference to a Ferranti 1892-patteru transformer of 15 H.P., 
 having the following data : 
 
 V 1 = 2,400 volts (kept constant). 
 V 9 = 100 volts nominally. 
 f\> = &2'5 periods per second. 
 R x = 377 ohms. 
 R 2 = 0-0092 ohm. 
 
 T T :T 2 ::24:i. 
 
 o ^ 1 = 0'103 amperes. 
 t i { = 4-837 amperes. 
 f ?' 2 = 113'85 amperes. 
 
 On open circuit : V l = 2,400; V 2 =100; ^ = 0-103; 
 The apparent watts = Vj x *\ = 2,400 x 0-103 = 247'2 ; 
 And loss in core = 08 x 247'2 - 0-103 2 x 3'77 = 197-5 watts. 
 
 At full load .' f V l = 2,400 ; f Vo = 97'8 ; ^ = 4'837 ; 
 /, = 113-85. 
 
 Loss in primary, ^2R 1= 4'837 2 x 3'77 = 885 
 secondary, z 2 2 R 2 = 113-85 2 x 0-0092 = 119-0 
 core =197-5 
 
 Total loss at full load = 405-0 
 
 Efficiency at full load= 11>134 = 96 per cent. 
 J 11,134 + 405 
 
 OF TBOB 
 
202 
 
 ELECTRIC MOTIVE POWER. 
 
 These calculations give the open circuit and full load points 
 of the several curves. The intermediate points can be deter- 
 mined in a similar manner, and the whole series be plotted in 
 succession.* 
 
 * Full particulars of this transformer will be found in Dr. Fleming's 
 Paper CD page 636 of Part 101 of Journal of Institution of Electrical 
 Engineers, to which reference should be made for further information. 
 
CURVES OF TRANSFORMERS. 203 
 
 These results are illustrated in Fig. 103, in which 
 
 Curve a shows the total secondary drop, 
 
 b secondary drop due to the primary 
 
 and secondary circuits, 
 c secondary drop due to the secondary 
 
 circuit, 
 d primary copper loss in watts, 
 
 e secondary 
 / total 
 
 g loss (including hysteresis and eddy 
 
 currents), 
 
 ,, h ,, primary output in watts, 
 i secondary 
 
 k efficiency. 
 
 All the curves are referred to the currents as abscissae, the 
 corresponding primary and secondary currents being plotted to 
 suitable scales. The ordinates are made to different scales in 
 order to condense the whole. 
 
 Some idea of the order and relation of the phases of pressure, 
 current and resulting magnetism in a transformer may be 
 gathered from the following : 
 
 In a transformer there are 
 
 (a) A wave of impressed E.M.F., 1 V imp , in the primary 
 circuit. 
 
 (b) A primary current, i lt lagging behind the primary E.M.F. 
 by an angle <, which may have any value between Odeg. and 
 90deg. 
 
 (c) A wave of magnetism lagging behind the primary current 
 by considerably less than 90deg. 
 
 (d) A counter E.M.F., jVs, due to the inductive action of the 
 magnetism and lagging 90deg. behind the magnetic wave 
 
 (e) An impressed secondary E.M.F., V 2 , due to and measured 
 by the rate of change of the magnetism, and hence lagging 
 90deg. behind it. 
 
204 ELECTRIC MOTIVE POWER. 
 
 (/) A secondary current, * 2 , in phase with the secondary 
 E.M.F. if there be no self-induction in the circuit ; lagging by 
 an angle of any value between Odeg. and 90deg. if there be 
 inductive action in the circuit or leading if there be capacity 
 in the circuit. 
 
 The relative positions of the several curves are roughly 
 indicated in Fig. 104. 
 
 On an open circuit the counter E.M.F. of the primary is 
 nearly in opposition to the primary impressed E.M F. The 
 
 FIG. 104. Curves of Transformer. 
 
 iV imp, Primary Impressed Pressure. jVr, Primary Resultant Pressure. 
 ,V S , Primary Self-induction Pressure. i lt Primary Current. 2 Vim P , 
 Secondary Impressed Pressure, io, Secondary Current, m, Curve of 
 Magnetic Induction. 
 
 Angle of lag between primary pressure and current assumed to be 45 ; 
 and secondary pressure and current taken in phase. 
 
 primary and secondary currents are very nearly in opposition 
 at all loads, but most nearly so at heavy loads. 
 
 It is useful to remember that when the secondary coils are 
 short-circuited the impedance of the primary circuit becomes 
 sensibly equal to about twice its ohmic resistance, and there is 
 a danger of burning out the winding. 
 
TEANSFOEMEE EQUATIONS. 205 
 
 37 DESIGN OF TRANSFORMERS. 
 
 From the preceding practical and theoretical remarks it will 
 be evident that the design of a transformer is from the com- 
 mercial point of view a compromise between conflicting 
 conditions of efficiency and cost. It is generally recognised 
 that efficiency can always be secured if price be of no moment ; 
 but sound commercial engineering requires the annual cost of 
 power wasted in a transformer to be equated to the cost of 
 maintenance plus interest ou capital outlay. This demands a 
 special application of the economic law. There are, however, 
 certain well-established limits for the permissible losses in cop- 
 per and iron cores for different sizes of transformers at each of 
 the more usual frequencies, and if these are observed the results 
 arrived at will not be far from the best attainable. Some of 
 these practical data are to be found in Dr. J. A. Fleming's Paper 
 on "Experimental Researches on Alternate-Current Trans- 
 formers," and as Dr. Fleming tested transformers by some of 
 the best makers in England and America, the figures are, beyond 
 dispute, characteristic of modern practice. 
 
 From the mean of a number of efficiency tests it appears that 
 the copper losses for small transformers do not exceed 1*75 
 per cent, of the full load secondary watts, and for larger ones 
 the loss decreases proportionately to about 1'25 per cent. The 
 losses in the iron are also found to vary between 1*0 and 0'25 
 per cent, of the full load output. So that the total losses may 
 be said to average from 3'0 to 1-5 per cent. 
 
 . Bearing these figures in mind it is comparatively easy to 
 find the permissible resistance for primary and secondary 
 circuits, and also the number of cubic inches of iron fora given 
 output (see Table L, p. 173). In the first instance, the following 
 data will be known : 
 
 Vj = Primary Pressure (assumed constant). 
 V 2 = Secondary no load. 
 i 2 = Secondary current, full load, and 
 TjlTjliV^Vr 
 
 The probable load factor should be calculated first of all in 
 order to determine the equivalent current. Then Rj and R 2 , 
 
206 ELECTRIC MOTIVE POWER. 
 
 the resistances of the primary and secondary circuits, may 
 be selected. Next the current densities may be chosen, the 
 usual values lying between 250 and 1,500 amperes per square 
 inch. The hysteretic and eddy current loss should next be 
 estimated, and then the copper losses and the drop can be 
 approximately determined, the values being chosen between 
 the limits given here. This will enable curves (a), (6), (c), (d) t 
 and (e) (see Fig. 103, p. 202) to be plotted in the following 
 order : 
 
 First plot curve (a) from the assumed value of the total drop r 
 and curve (c) from the calculated value of Rj and the known 
 value of t i v by equation (42), p. 197. Then plot curve (b) 
 from the assumed values of R 2 and t i 2t and the remaining 
 space is the leakage drop. Then curves (d) and (<?), the 
 copper loss in primary and secondary, can be calculated ;. 
 and the iron core loss at all loads can be determined from 
 the selected total loss. Then curves (d) + (e) + core loss give 
 curve (g). The secondary output curve (i) is easily calcu- 
 lated, and then the primary (h) is found by adding the total 
 watts from curve (g) and plotting the result to a suitable 
 scale. The efficiency curve (k) is simply curve (i) divided by 
 curve (A). 
 
 The primary current for any load can be read from the curves 
 now plotted. The ratio of the turns in primary and secon- 
 
 V 2 000 20 
 dary circuits depends simply on the ratio . 1, say? = 
 
 The exact number of turns in each can be found thus : 
 
 First, let the number of primary turns Tj be determined, 
 then the effective value of the primary pressure 
 
 
 which is the fundamental equation for transformers, and corres 
 ponds with equation (6), p. 18, for dynamos. 
 
DESIGN OF TRANSFORMERS. 207 
 
 The only difficulty is to fix the ratio of N to T x when only 
 the product of the two is known. For example let f\> = 100 
 and V x = 2,000; 
 
 then NT 2,000x1 QS = 4 . 5xlQ8t 
 
 4-45 x 100 
 
 Unless there be data available it will require a series of trial 
 calculations to fix the best ratio between these quantities. 
 
 The usual plan adopted in practice is to take a stock size 
 core, and this fixes N, for N = B"A, where A is the area of cross- 
 section and B" the induction. The value of B" is from 
 60,000 to 65,000. Hence T T is known. 
 
 If there be no fixed core to guide the design in respect of N r 
 an approximation may be made from the permissible hysteretic 
 loss (see Table L, p. 173) and the corresponding volume of iron 
 for the given frequency. This is a tedious process, as it leaves 
 the relation of the length of the magnetic path to its cross 
 section an open question, and requires several trial calculations 
 before satisfactory proportions are determined. 
 
 A few calculations, however, for different shapes of carcases 
 will demonstrate the principles. This part of the design neces- 
 sarily involves the consideration of the winding space, which 
 is not a difficult problem when the gauges of wires are approxi- 
 mately known. 
 
 As soon as the number of primary turns Tj is settled, that 
 of the secondary is found by multiplying Tj by the ratio of 
 transformation ; or the secondary turns 
 
 This description of transformer design is incomplete, but the 
 Author feels that this class of plant is not likely to be taken in 
 hand by any except specialists, owing to the many difficulties 
 which arise in connection with the selection of various designs 
 for specific work, such as parallel or series lighting, transmission 
 
208 ELECTRIC MOTIVE POWER. 
 
 or distribution of power ; or, what is still more intricate, a com- 
 bination of power and lighting work. It has been assumed in the 
 calculations, as is customary, that the secondary turns are few 
 in number, and therefore have no appreciable self-induction, and 
 also that the secondary external circuit is a non-inductive one, 
 such as a bank of incandescent lamps. Thus the drop of pres- 
 sure follows simple laws and is easily pre-determined. But if 
 an inductive load, as electric motors or arc lamps, is put in the 
 circuit, then there is a counter-pressure of self-induction ; and, 
 the secondary current lagging behind the secondary pressure, 
 the drop is increased in proportion. Again, if there be resonance 
 in the circuit, such as that caused by a condenser in parallel with 
 the primary mains, or if the power be transmitted through long 
 concentric mains armoured with continuous metallic sheathing, 
 the current may lead the pressure, in which case the "drop" 
 will become negative, and the supply pressure will be raised. 
 These two sources of trouble, being diametrically opposite in 
 their effects, counteract each other, and hence it has been sug- 
 gested that condensers bhould be used at central stations 
 supplying power to inductive circuits. But the treatment of 
 such problems demands a close acquaintance with the under- 
 lying theory, and it would serve no useful end to discuss 
 at length this part of the subject. The engineer desirous of 
 studying self-inductive and resonant effects is referred to Dr. 
 Fleming's "Alternate-Current Transformer,"* in which the 
 subject is treated both practically and mathematically. There 
 are, however, some special problems which occur in alternate- 
 current transmission, and these are referred to in Chapter 
 VII. (see especially 42 and 43). 
 
 38. STEP-UP AND STEP-DOWN TRANSFORMERS. 
 
 In cases where the pressure in the transmission mains is 
 from 4,000 volts upwards it is sometimes found convenient 
 to employ step-down transformers to lower the line pressure 
 at the sub-stations. And in polyphase current systems 
 step-up transformers are sometimes used, because this class of 
 alternator can be most economically designed for pressures 
 of from 50 to 100 volts. It is, however, generally cheaper 
 
 * Published in "The Electrician" Series, in 2 vols., "Theory "and "Practice." 
 
STEP-UP TRANSFORMERS. 209 
 
 to increase the insulation resistance of the windings than 
 to use a step-up transformer, at any rate up to pressures 
 about 5,000 volts. The best authorities appear to differ as 
 regards the pressure at which step-up transformers become 
 advisable or necessary. For example, in the Rome-Tivoli power 
 plant the single-phase alternators are banked in parallel at 
 an omnibus pressure of about 5,000 volts, and at the Ponta Pia 
 sub-stations step-down transformers lower the pressure to 2,000 
 volts, at which the primary circuits of the house transformers 
 are fed. On the other hand at Lauffen-Heilbronn the poly- 
 phase-current alternators give 50 volts and 4,000 amperes 
 each, this pressure being raised by three-phase transformers to 
 5,000 volts. It is reduced at Heilbronn to 1,500 volts to feed 
 the distributing network. 
 
 Intermediate transformers are used primarily to economise 
 copper in the transmission mains by means of extra high 
 pressure, and they secure this important end with freedom 
 from risk to life and with immunity from breakdowns. For 
 it is evident that the distributing network can be easily 
 protected from the inroad of the extra high pressure by use 
 of the safety devices already mentioned ; and that an accident 
 to the step-down transformer only affects the primary circuits 
 of the supply transformers, which in turn are protected by 
 similar devices. The consumer is thus doubly secured from 
 an invasion of the high pressure. From the mechanical aspect 
 there can be no doubt that a transformer properly built and 
 buried in a sealed oil-tank is a far more lasting device than the 
 most carefully constructed high or low pressure alternator. 
 
 The use of intermediate transformers becomes of greater im- 
 portance as the pressure of the transmission service is raised. The 
 experiments with three-phase currents between Frankfort and 
 Lauffen with pressures of from 20,000 to 30,000 volts establish 
 extra high-pressure working on a practical basis, and the difficul- 
 ties arising from capacity and self-induction are not found to 
 present serious obstacles. 
 
 At Cassel an interesting variation in plant design comprises 
 single-phase alternators, step-up transformers, and alternate- 
 current motors at two sub-stations, each motor running two 
 
210 ELECTEIG MOTIVE POWER. 
 
 continuous-current dynamos, which each feed a low pressure 
 three-wire distributing network. And a similar scheme in- 
 volving two-phase generators has been carried out by Messrs. 
 .Schuckert and Co. at Buda-Pesth. 
 
 For power purposes, since the load-factor will generally be at 
 'least 50 per cent., it will be possible to work the large extra 
 high pressure transformers at more than 50 per cent, of their 
 tfull capacity during the greater part of the time, and probably 
 by carefully selecting the units the load-factor of each may be 
 kept as high as 85 per cent. If this be the case the large 
 transformers will generally be less costly to build per unit than 
 ordinary transformers, which require a high average efficiency 
 over a large range of output, and therefore must contain a 
 liberal allowance of copper. 
 
 This is a weighty point in considering the use of step-up 
 transformers, and must largely determine the advisability of 
 their use for any plant. 
 
 The relative merits of low-pressure alternators and step-up 
 transformers versus high-pressure alternators, are not easily 
 estimated. Time is necessary to permit the accumulation of 
 experience before this question can be satisfactorily answered. 
 
CHAPTER VII. 
 
 ALTERNATE-CURRENT TRANSMISSION AND 
 DISTRIBUTION. SINGLE-PHASE. 
 
 39. Advantages and Disadvantages of Alternate Currents. 40. Series 
 Working not Possible ; Single Alternator and Motor. 41. Parallel 
 Working with Alternate Currents. 42. Increase of Resistance in 
 Conductors carrying Alternate Currents. 43. Variation of Pressure 
 in Conductors having Self-induction and Capacity. 44. Effect of 
 Capacity upon the Change Ratio of Transformers. 45. Compen- 
 sating for Drop of Pressure in Conductors. 46. Measurement of 
 Pressure at the End of Feeders. 47. Compensators or Regulators. 
 48. Coupling Separately-Driven Alternators in Parallel. 49. 
 Alternate Current Measuring Instruments. 50. Precautions to be 
 observed in Alternate Current Working. 51. Earthing the Primary 
 and Secondary Circuits of Transformers ; Safety Devices. 
 
 39. ADVANTAGES AND DISADVANTAGES OF 
 ALTERNATE CURRENTS. 
 
 THE special points in favour of the alternate-current systems 
 may be summarised as follows : 
 
 (a) High-pressure machines are permissible, since the 
 currents are not commutated, and, therefore, sparking diffi- 
 culties are entirely obviated. 
 
 (b) Power is readily divided into units of any size and at 
 any pressure by suitable transformers either step-up or step- 
 down. 
 
 (c) The regulation of pressure over a large area of supply is 
 facilitated by parallel high-pressure primary circuits. 
 
 (d) The power in any of the branch circuits can be regu- 
 lated by choking coils without serious loss thus the pressure 
 
212 ELECTRIC MOTIVE POWER. 
 
 of a feeder, or the light of a single lamp, can be graduated 
 without using a wasteful resistance. 
 
 (e) Alternate currents are generally more easily handled than 
 continuous currents, and the sparks at breaking circuit are less 
 destructive. 
 
 (/) Synchronous motors of large sizes have a high all-round 
 efficiency, and necessarily run at constant speed. 
 
 (g) Asynchronous motors can be made without collecting 
 rings and brushes, and are of very simple construction. 
 
 (h) Separately-driven alternators will run perfectly in parallel 
 if the driving power and excitation be properly apportioned. 
 
 (i) Two alternators of unequal pressure may be coupled in 
 parallel, and will give an intermediate pressure if the self- 
 induction of the armatures be of suitable amount too much or 
 too little self-induction lowers the plant efficiency, renders the 
 condition unstable, and prevents the load from being pro- 
 portionately divided between the several machines. 
 
 The chief drawbacks to alternate currents are : 
 
 (1) Transformers, especially small ones, are not efficient with 
 low load factors, and therefore transformer sub-stations are 
 required for economy. 
 
 (2) Synchronous motors are not self-starting, and will stop 
 if temporarily overloaded. 
 
 (3) Asynchronous motors will not start against the designed 
 running torque, unless special precautions are taken in the 
 designing, and are only efficient at high loads. 
 
 (4) The lag between current and pressure in an inductive 
 circuit, such as one loaded with motors, is productive of 
 wasteful current in the copper circuits. This is specially the 
 case when the total useful power is small. 
 
 (5) Synchronous alternators are more costly to build than 
 dynamos of equal output. 
 
 (6) Alternate-current transformers fed with the primaries in 
 series at constant current require the secondary circuits to be 
 also fed with constant current. 
 

 SERIES WORKING NOT POSSIBLE. 213 
 
 (7) Free-running synchronous alternate-current motors will 
 not work in series. 
 
 (8) Alternate currents are not suitable for electrolytic pur- 
 poses. 
 
 (9) There is some difficulty in regulating the pressure of 
 supply in circuits containing induction and capacity. 
 
 (10) Accumulators cannot be used either for storing power 
 or steadying pressure. 
 
 These points will be severally developed in 40 to 51. 
 
 In this chapter only single-phase currents are considered, 
 and hence the term alternate current will be limited here to 
 this meaning. 
 
 HO. SERIES WORKING NOT POSSIBLE; SINGLE 
 ALTERNATOR AND MOTOR. 
 
 In Chapter IV. it has been shown that the simplest case of 
 transmission of power with continuous current is that in which 
 one dynamo and one motor are run in series with each other. 
 The same is the case with alternate currents ; but series 
 working is strictly limited to one alternator and one motor 
 if synchronous machines are used. Properly speaking, series 
 working is impossible with synchronous machines. A reversed 
 alternator runs perfectly as a motor when driven off a similar 
 generator if running in synchronism with it. The term 
 synchronism implies that the periodic time of the alternations 
 of the two machines is the same. Theoretically, it is sufficient 
 that the frequency of one machine either coincides with or is 
 equal to a small multiple of that of the other. 
 
 It has been already shown that the frequency is equal to 
 the number of pairs of poles in the field-magnets multiplied by 
 the number of revolutions of the armature (or field-magnets) 
 per second. Therefore, if the machines be similar, the speeds 
 must be the same when synchronism obtains ; and if the 
 numbers of poles in the two machines differ, the revolutions will 
 vary inversely as the relative numbers of poles. From these con- 
 siderations it is seen that a synchronous alternate-current motor 
 
 Q 
 
214 
 
 ELECTRIC MOTIVE POWER. 
 
 is not self-starting ; and that the moving part must be started 
 and revolved until it falls into step with the generator. This 
 is most readily accomplished by running the generator slowly 
 until the motor has got into step with it, when the speed of the 
 dynamo may be gradually increased and the motor speed will 
 follow it with considerable persistence. This will be compre- 
 hended by reference to the following diagram. 
 
 In Fig. 105 let V be the curve of pressure of the alternator 
 running as a generator ; v the curve of counter pressure of a 
 similar machine running as a motor ; and assume that, by means 
 of suitable adjustment of both field-magnet excitations, V is 
 greater than v by an amount sufficient to drive the current i 
 through the combined circuit. Then the work given out by 
 
 FIG. 105. 
 
 the generator at any instant is equal to the product of the cor- 
 responding ordinates of V and i, as N L x M L ; and the work 
 absorbed by the motor at the same instant is - L NJ x M L. 
 The sign of each curve is taken as positive when above the 
 time line and negative when below it. The total work done by 
 either machine in one complete cycle can be obtained graphically 
 by dividing the time line into any number of equal parts, when, 
 if the time of one-half period be equal to T, then the several 
 
 rp m 
 
 values of V i and v i can be readily found and plotted in 
 
 n n 
 
 new curves whose areas will be proportional to the respective 
 outputs. Care must be taken to use the right signs, and to 
 recollect that the algebraic product of two negative quantities 
 Is reckoned as positive ; hence the values of V i in the second 
 
ALTERNATORS IN PARALLEL. 215 
 
 half of the generator curves are positive, although each of the 
 factors is negative. The negative sign before the product 
 V i implies that the machine is receiving power from the circuit, 
 whilst its absence shows that the machine is giving power 
 into the circuit. The displacement of the current curve i, with 
 reference to the resultant pressure V - v, is called the lag. The 
 amount of the lag is usually measured in degrees, and is deter- 
 mined by the self-induction of the circuit. 
 
 41. PARALLEL WORKING WITH ALTERNATE 
 CURRENTS. 
 
 Alternate-current working is seen at its best when the alter- 
 nators as well as the transformers are coupled in parallel. The 
 term "parallel working," indeed, is usually held to imply this 
 particular arrangement of the generators, and does not refer 
 to the transformers, although these are generally coupled in 
 parallel as well. The ordinary practice is to drive each alter- 
 nator separately. In Continental practice two alternators are 
 frequently coupled, one at each end of a turbine shaft, the 
 whole forming one unit. Machines of unequal size work per- 
 fectly in parallel, dividing the load in proportion to their 
 capacities, if the excitations be properly adjusted and the 
 driving power be of suitable amount. And alternators of 
 even widely different pressures may be coupled in parallel 
 without risk of damaging either, the resultant omnibus 
 pressure being intermediate between the two. But the load 
 will not necessarily be divided according to the capacities of 
 the two; and the machine with the higher terminal pressure 
 will do work on that with the lower pressure during a por- 
 tion of each period. Therefore, the power absorbed will be 
 greater than is necessary, and may be excessive. In fact, 
 the difficulty with parallel working is to proportion the load 
 according to the several capacities of the generators. It is 
 necessary to measure, by means of a wattmeter, the power given 
 out by each machine, and to adjust the excitation and the steam 
 admission until the load is properly distributed. An ammeter 
 is of no use, since it gives no indication of the lag between 
 the current and pressure, and, of course, the omnibus pressure 
 is necessarily the same for all the machines. As already 
 mentioned in 32, p. 179, the engines (or turbines) should 
 
 Q2 
 
21G ELECTRIC MOTIVE POWER. 
 
 not have separate governors, but the throttle valve of each 
 should be first properly adjusted and then fixed. The main 
 supply of steam (or water) should be controlled by a single 
 governor, affecting all the prime motors. Further adjustment 
 of output must be made by the field excitation, the safest 
 condition of working being given, not by the minimum current 
 (see Fig. 88, p. 177), but by pushing the excitation beyond 
 the lowest point of the volt-ampere curve, so that a slight rise 
 of speed will increase the armature current, and the converse 
 will bring it to its normal value. 
 
 Alternate currents lend themselves readily to all of the 
 systems of transmission and distribution discussed in 24 
 (p. 122) with reference to parallel continuous-current working. 
 That is, they are adapted for 
 
 (a) Simple parallel and reverse parallel systems, either direct 
 or with transformers. 
 
 (6) Two-wire feeder systems, direct or with transformers ; and 
 
 (c) Multiple-wire feeder systems, direct or with regulators or 
 transformers. 
 
 It is in the transformer systems that they compare most 
 favourably with direct currents, for dynamotors cannot seriously 
 compete with transformers, either in prime cost, efficiency, or 
 durability. It is important to examine carefully the use of 
 transformers, since they are likely to play a leading part in all 
 large power plants in the future. 
 
 It is sufficiently clear that high pressure is essential in the 
 transmission mains from economical considerations ; and it is 
 also evident that the distributing pressure must not exceed 
 about 500 volts. It has already been shown how these two 
 conflicting conditions are met in continuous-current working 
 by means of regulators, dynamotors, and other devices; all, 
 however, having moving parts, and, therefore, requiring atten- 
 tion more or less continuously, being always liable to break- 
 downs from mechanical defects, and subject, also, to wear 
 and tear from mechanical friction. It would be unfair to 
 the continuous-current systems to lay too much stress on these 
 points, but they undoubtedly must weigh heavily in the 
 
ALTERNATORS IN PARALLEL. 217 
 
 judgment of engineers when comparing these two important 
 classes of electric machinery. 
 
 In alternate-current working the connecting link between 
 the feeders and the distributors is the transformer, which has 
 been fully described in Chapter V. Reference is there made 
 'to the structural details and to the fundamental features of 
 design. It remains to investigate the functions of the trans- 
 former, and to see how admirably it meets the many and varied 
 requirements of power and lighting work. It has been shown 
 that the construction is of the simplest character, involving no 
 moving parts, and comprising merely a laminated iron core and 
 two separate windings of copper wire, the cable being protected 
 by an iron case commonly filled with resinous oil. From the 
 -engineering point of view, it is as near perfection as possible, a 
 machine of the highest efficiency without moving parts, and 
 requiring no attention. If properly designed for the work ex- 
 .pected of it, there is no reason why it should not last for an 
 indefinite period without repairs of any kind. 
 
 In parallel working the transformers may be banked at 
 sub-stations, and any number may be coupled up to the dis- 
 tributors according as the load varies, the necessary connections 
 being made by an attendant on the spot ; or the switching may 
 be controlled by magnetic devices worked automatically or 
 from the central station. The load factor of each transformer 
 must be kept as high as possible, though the load factor of the 
 station may vary largely. To attain this end it is necessary to 
 select the transformer unit so that the minimum load is about 
 75 per cent, of the maximum output of one transformer. As 
 soon as the full load of one transformer is reached, a second is 
 switched in. With this arrangement no transformer will run 
 with a load factor of less than 50 per cent., at which the effi- 
 ciency is about 95 per cent. If the maximum load obtains for 
 only short periods it may be advisable to have some larger 
 transformers in addition to the small ones, but this is a matter 
 of detail which must be settled on the merits of each case. 
 
 Another method, which was largely practised in the early 
 days of alternate-current working (and is still used in com- 
 .mencing a new plant), is to place a transformer in the premises 
 
218 ELECTRIC MOTIVE POWER. 
 
 of each consumer. It has several disadvantages. In the first 
 place, the prime cost of small transformers is necessarily more 
 per unit of output than that of large ones ; and, secondly, the 
 size of the transformer must be chosen with reference to the 
 maximum, and not to the average, load ; hence the all-day 
 load-factor will of necessity be small in most cases, often in 
 lighting plants not averaging more than 10 per cent. When 
 this system is carried out on a large scale for lighting, the 
 result is disastrous as regards coal consumption, which may 
 reach as high as from 201b. to 251b. of coal per kilowatt-hour 
 at the terminals of the lamps (one Board of Trade unit 
 delivered) ; whereas the best continuous-current three-wire 
 systems consume from Gib. to 81b., and sub- station alternate- 
 current plants not more than from lOlb. to 151b. This 
 disparity, is due to the efficiency of transformers being com- 
 paratively small at low loads, and also because the exciting 
 currents must be supplied whether the secondary circuits 
 are closed or open. Hence the primary circuits carry large 
 idle currents, which are wasted in heating them and the 
 transformers. The power thus lost has been variously esti- 
 mated at from 10 to 15 per cent, of the total station power. 
 This drawback, however, is not likely to be so serious in the case 
 of power work, for with large motors the load-factor will gene- 
 rally be high, and the transformers can always be switched out 
 of circuit when the motors are standing, lamps (if used) being 
 supplied from a separate transformer. And in a case requiring 
 a number of small motors, such as that of a manufacturing 
 town or a large factory, a transformer sub-station can be easily 
 arranged to deal with both light and power work. 
 
 42. INCREASE OF RESISTANCE IN CONDUCTORS 
 CARRYING ALTERNATE CURRENTS. 
 
 With periodic currents it is found that the distribution of 
 the current is not uniform over the cross-section of a solid 
 conductor, but is limited to the outer layers of the metal. This 
 skin effect is the more marked as the frequency is raised. 
 A solid or stranded conductor, therefore, offers a higher resist- 
 ance to an alternate than to a continuous current, and the 
 apparent resistance exceeds the measured ohmic resistance by 
 an appreciable amount. 
 
VIRTUAL RESISTANCE. 
 
 219 
 
 This virtual increase of the resistance of conductors was 
 first discovered by Prof. Hughes during his experimental re- 
 searches, and was brought to the notice of practical men by 
 Lord Kelvin in his Presidential Address to the Institution of 
 Electrical Engineers in 1889. It is now customary to speak 
 of the combined ohmic and skin effects in an alternate-current 
 circuit as the virtual resistance of the circuit. Mr. Mordey 
 has had special opportunities for investigating this important 
 feature, and he has formulated some useful tables which give 
 the practical limits of the effect. Lord Kelvin's figures are 
 used in Table V, and for convenience a current density of 
 450 amperes per square inch of cross-section has been selected. 
 This gives a loss of 1 per cent, per mile with 2,000 volts, and 
 1*15 per cent, per 100 yards with 100 volts, these being useful 
 data for primary and secondary mains respectively. The Table 
 also contains the limiting powers for the vanous sizes of con- 
 ductors with this current density at 2,000 and 100 volts 
 respectively. The skin effects are calculated for 80, 100, 
 and 133 <\> per second, which are usual in this country. It 
 is seen that the frequency varies inversely as the square of the 
 diameter of the conductor for the same percentage increase of 
 virtual resistance over ohmic resistance. 
 
 Table V. Increase of Resistance of Conductors in Alternate- 
 Cur ren i Wo 
 
 Diameter in 
 inches. 
 
 Area in 
 sq. in. 
 
 Increase over 
 ordinary 
 resistance. 
 
 Current 
 at 4JO 
 amperes 
 p r sq. in. 
 
 Watts at 
 2,000 volts. 
 
 Watts at 
 i 100 volts. 
 
 ^ per 
 tecoml. 
 
 0-3937 
 
 0-122 : Less than T (ir %! 55 
 
 110.00.1 : 5,500 
 
 
 0-5905 
 
 0-274 
 
 22% 
 
 133 
 
 266,000 13,300 
 
 
 0-7874 
 
 0-487 
 
 8 % 
 
 2^0 
 
 440,000 | '^2,000 
 
 Of) 
 
 0-9842 
 
 0-760 
 
 17 o% 
 
 
 
 
 ao 
 
 1-575 
 
 1-95 
 
 63 % 
 
 
 
 
 
 3-957 
 
 1217 
 
 3'3 times 
 
 
 
 ... 
 
 
 0-3543 
 
 0-093 
 
 Less than i^% 
 
 45 
 
 90,000 
 
 4,500 
 
 
 0-5280 
 0-7G86 
 
 0-218 
 0394 
 
 8^% 
 
 93-5 
 178 
 
 ] 97,00i 9.850 
 356,000 ' 17,800 
 
 100 
 
 0-8826 
 
 0-611 
 
 171% 
 
 ... 
 
 ... 
 
 
 0-3013 
 
 0-071 
 
 Less than T Jvy% 
 
 32 
 
 64,000 3,200 
 
 } 
 
 0-4570 
 0-61C2 
 
 0-164 
 0-292 
 
 8 % 
 
 74 
 131-4 
 
 148,000 
 263,COO 
 
 7,400 
 13,140 
 
 (itt 
 
 0-7622 
 
 0-456 
 
 17 % 
 
 ... 
 
 
 ... 
 
 ' 
 
220 ELECTRIC MOTIVE POWER. 
 
 These figures show that the increase of resistance is prac- 
 tically insignificant with conductors of less than O35in. in 
 diameter with frequencies up to 133 'V per second; that with 
 cables of 0'5in. in diameter it is only 2*5 per cent, of the cal- 
 culated ohmic resistance, while with diameters as large as 
 0'75in. it is as much as 17 '5 per cent. This latter perhaps 
 gives the practical limit of size for stranded conductors for 
 alternate currents. Larger conductors may be built up from 
 insulated strands or tubes, or strips may be used. Probably a 
 fair increase of resistance for ordinary practice will be 10 per 
 cent. ; when, if the calculated ohmic resistance absorbs say 10 
 E.H.P., the virtual resistance will absorb 11 E.H.P. 
 
 Examining the output columns, it is seen that the skin effect 
 is not of much importance with the primary circuits ; for, with 
 a pressure of 2,000 volts and 80 ^ per second, 440,000 watts 
 can be transmitted with an increase of resistance of only 8 per 
 cent, over the ohmic resistance, and with 100 <\> 356,000 watts, 
 the former corresponding to 220, and the latter to 175 amperes. 
 These powers (590 and 465 H.P. respectively) are probably 
 sufficient for single mains or feeders, and for exceptionally long- 
 distance transmission the power can be increased by simply 
 raising the pressure. And if higher pressure be prohibited, the 
 number of feeding points can oftentimes be increased. Thus 
 there is nothing in the virtual resistance effect to hinder the 
 transmission of enormous powers with solid or stranded naked 
 conductors, so far as the feeders are concerned. 
 
 In the low-pressure distributing circuits the virtual resist- 
 ance effect, however, may be of importance, because the watts 
 that can be transmitted with a given percentage increase of 
 resistance vary in inverse proportion to the pressure. Thus, in 
 a 100-volt circuit the load (under the same conditions as 
 referred to in considering the primary mains, i.e., 10 per cent, 
 increase) is only 13,000 for 133 r\>, and 18,000 for 100 ^. If 
 the percentage of increase of resistance be raised, then it may 
 affect the pressure of supply. But it is evident from Table V 
 that the drop from this cause will not be serious in ordinary 
 cases. 
 
DEOP IN CONDUCTORS. 221 
 
 43. VARIATION OF PRESSURE IN CONDUCTORS 
 HAVING SELF-INDUCTION AND CAPACITY. 
 
 The next point to consider is the effect of variations 
 of load on the drop. A bank of incandescent lamps is 
 practically a non-inductive load, and the drop can be esti- 
 mated without difficulty from the known ohmic resistance 
 and the small increase of resistance in the mains due to 
 the frequency. But if the load be partly or entirely com- 
 posed of arc lamps or motors, or other devices containing self- 
 induction, then there will be a further drop in the pressure 
 due to a counter electromotive force, which will vary in mag- 
 nitude with the current. The difficulty is met to some 
 extent in lighting stations by running separate circuits for arc 
 lamps. Motors are not yet used to any appreciable extent 
 in alternate-current circuits, and when used are run chiefly by 
 day, when the lighting work is comparatively unimportant. 
 And in the few cases where power work is sufficiently large to 
 warrant the expense, separate circuits are set apart for the 
 motors. The author believes that most of the Continental 
 stations which have tried the dual load have ultimately found 
 it advisable to use separate circuits (at any rate for the larger 
 motors). For, although a varying pressure may be unimpor- 
 tant, perhaps, as regards motors, ifc is a serious matter with arc 
 lamps, and still more so with incandescent lamps. The effect 
 of capacity in a circuit is precisely opposite to that of self- 
 induction, the condenser electromotive force being phasially 
 180deg. distant from the electromotive force of self-induction. 
 And since both of these effects are functions of the circuit, they 
 may under certain conditions be made to counterbalance each 
 other to some extent. The capacity effect is found in con- 
 centric conductors and in single or concentric mains armoured 
 by continuous metallic sheathing, and it is worthy of considera- 
 tion whether the latter are not of special use for circuits sup- 
 plying alternate currents to inductive loads. At any rate 
 they should tend to cause a smaller drop of pressure than 
 ordinary copper conductors in an inductive circuit. 
 
 Some idea of the order of these effects in the mains may be 
 gathered from the following. 
 
222 
 
 ELECTRIC MOTIVE POWER. 
 
 First, consider a concentric conductor carrying an alternate 
 current, and assume that the alternator and the motors have 
 neither self-induction nor capacity. Let the terminal pressure 
 of the alternator be V. Now the capacity 
 
 Kl 1 
 
 9x 1Q5 microfarads, . . (43) 
 
 I being the length of the two conductors, Rj the radius of the 
 inside of the outer tube, R 2 the radius of the inner conductor, 
 
 FIG. 106. Showing the Effect of Capacity in an Alternate-current Cir- 
 cuit. V is the impressed E.M.F. ; O i A , dynamic current ; O i'i mp , im- 
 pressed current ; and i c , condenser current ; 6, angle by which resultant 
 current leads impressed pressure. 
 
 and K the coefficient of specific inductive capacity of the 
 dielectric in C.G.S. units. And the condenser current in 
 practical units is 
 
 ClQ- < \ .... (49) 
 
 i c and V being effective values. 
 
 The condenser current is, as already stated, a quarter period* 
 in advance of the pressure. In Fig. 106, let V represent V. 
 
SELF-INDUCTION DIAGRAM. 
 
 223 
 
 Then i c represents i c , i d is the dynamic current, and, of 
 course, is in phase with V. Through d , parallel to (H c , draw 
 i d 2 imp . At i e in K erect the perpendicular i c ? imp , meeting 
 i d imp at ^ imp , and join i imp . Then 4n P is t^ e current that 
 must be supplied by the alternator, and it is in advance of the 
 impressed terminal pressure by an angle 0. 
 
 Fig. 107 is constructed in a similar way, and shows the lag 
 <f> due to self-induction. 
 
 FIG. 107. Showing the Effect of Self-induction in an Alternate-current 
 Circuit. V is the impressed E.M.F. ; i c \, dynamic current ; ?'i mp , 
 impressed current ; and 4, current due to E.M.F. of self-induction ; 
 <p, angle by which impressed current lags behind impressed pressure. 
 
 The result of capacity, then, is to cause the impressed cur- 
 rent to lead the line pressure, while self-induction causes it to 
 lag. This phase-difference introduces several important results. 
 If it be positive, the terminal pressure of the generator will 
 be raised, because the armature reactions will then tend to 
 strengthen the field. This is seen to be the case by considering 
 that t.he maximum pressure obtains when the armature coila 
 
-224 
 
 ELECTRIC MOTIVE POWER. 
 
 are midway between the field poles ; and therefore, if the maxi- 
 mum value of the current obtain before this position is reached, 
 the coils will help to magnetise the field magnets ; and, on the 
 contrary, if by reason of self-induction the current lags in phase, 
 it will help to decrease the magnetism. 
 
 The effects of self-induction and capacity with concentric 
 cables may be shown graphically, when the constants of the 
 
 FIG. 108. Showing the Effect of Capacity and Self-induction in a Circuit. 
 
 circuit are known, by combining the diagrams in the preceding 
 figures. And by suitable approximations the effects for a com- 
 plete power plant may be estimated in the same manner. The 
 procedure is indicated in Fig. 108. 
 
 C is the maximum value of the difference of pressure 
 between the condenser plates = ^ ^ , OL is the maximum 
 
COUNTER E.M.F. IN CONDUCTORS. 22& 
 
 value of the counter pressure = L^ 2?r r\j. R is the resultant 
 pressure which sends the current i through the inductive 
 circuit of impedance \/K 2 + L 2 47r 2 ^ 2 . The pressure at the 
 terminals of the condenser is HV by the construction (for 
 KH = OL, and RV = OC-OL). Then 0V represents in 
 phase and magnitude the impressed pressure required to main- 
 tain the current through the condenser. 
 
 The condenser effect is a maximum when the angle H V ia 
 a right angle ; or H V, the condenser pressure, has a maximum 
 ratio to V, the impressed pressure, when this is the case.* 
 
 In practice, with the insulations commonly used and the 
 sizes of conductors most in vogue, the capacity of concentric 
 mains between the inner and outer conductors appears to be 
 about one-third of a microfarad per mile run. But the effect 
 is far more serious between the outer conductor and the earth, 
 being nearly ten times greater. It is, therefore, important, 
 with high frequencies and high pressures, to take into con- 
 sideration the capacity effects as well as the self-induction. 
 
 The impedance of a circuit carrying an alternate current is 
 dependent upon the resistance, capacity, and self-induction of 
 the line, alternators and load, and may be also affected by the 
 mutual induction of a neighbouring circuit. 
 
 Self-induction and capacity in alternators and motors have 
 already been referred to. In a long line the self-induction of 
 the conductors may have an important effect upon the im- 
 pedance. It gives rise to a counter E.M.F. which varies with 
 the diameter of the conductors, the distance between their 
 centres, and the intensity and the frequency of the current. 
 The value in volts per 1,000ft. of double circuit per ampere is 
 shown graphically in Fig. 109, which is adopted from the 
 calculations of Mr. Chas. F. Scott. 
 
 The constants in the abscissae have been calculated for a 
 frequency of one <\j per second. The value for any frequency 
 is thus found by simple multiplication. For example : Given 
 a 100-ampere circuit at a frequency of 60 rvj, supplying power 
 
 * Vide " The Alternate-Current Transformer," by Dr. J. A. Fleming, 
 Vol. II., p. 386. 
 
226 
 
 ELECTEIC MOTIVE POWER. 
 
 0-1662 
 
 0-0103 
 
 Distance between Centres. Inches. 
 
 1 2346 12 21 48 
 
 0-0006 0-0018 
 O'OOOO 0-0012 0-0024 
 
 0-0042 0-0054 
 0-0036 0-0048 
 
 FIG. 
 
 109. Inductive E.M.F. in Volts per Ampere per 1,000ft. of double 
 circuit at 1 r*j per second. 
 
 100 
 
 90 
 
 80 70 
 
 Power Factor. Per cent. 
 
 FIG. 110. Inductive Drop (as a percentage of Station Pressure) for 
 different E.M F.s and different Power Factors of Load. 
 
INDUCTIVE DROP IN CONDUCTORS. 
 
 227 
 
 to a distance of 500ft. with conductors of 0'1662 sq. in. in 
 section, and Gin. between centres. The constant is 0-0027, and 
 
 500 
 
 1 "17)66 
 
 It is seen that the counter E.M.F. is least with small con- 
 ductors lying as close together as possible. This condition is 
 
 the inductive E.M.F. = 0'0027 x _ x 
 
 x 60 = 8-1 volts. 
 
 100 
 
 99 
 
 15 
 
 10 
 
 95 
 
 98 97 96 
 
 Power Factor. Per cent. 
 FIG. 110A. Enlarged view of the commencement of Curves in Fig. 110. 
 
 be st secured by using a concentric cable, when the effect will 
 be reduced to a minimum. 
 
 From these curves it is possible to estimate the counter 
 E.M.F. in most conductors with a fair degree of accuracy. To 
 
228 ELECTEIC MOTIVE POWER. 
 
 predetermine the inductive drop of pressure between the gene- 
 rator and receiver terminals, it is necessary to know the power 
 factor of the circuit. Then the inductive drop for any load 
 can be found by reference to Figs. 110 and 110A (also due to 
 Mr. Chas. F. Scott), after the counter E.M.F. is determined 
 from Fig. 109. 
 
 In these two figures the counter E.M.F. and the drop due to 
 the self-induction of the line, alternators and load, are given as 
 percentages of the supply pressure. The total drop on the 
 line is found by adding to this figure the fall of pressure due 
 to the virtual resistance of the conductors, as given in 
 Table Y. p. 219. 
 
 If two or more circuits are lying in close proximity to each 
 other, there will be mutual induction between them, i.e.,. 
 they will act with reference to each other as the primary and 
 secondary circuits of a transformer. This effect is noticed in 
 overhead lines when two or more circuits are coupled to 
 separate alternators, and is evidenced by a surging of pressure 
 sometimes sufficient to cause an easily discernible fluctuation 
 in the light of incandescent lamps at the end of long circuits 
 parallel to each other. This is also the case with three-wire 
 circuits (either two- or three-phase). The E.M.F. induced by 
 two circuits upon one another is not affected by the distance 
 between the conductors in each j but simply depends upon the 
 relative positions of the two circuits, the intensity and direction 
 of the two currents, and the distance for which they are parallel 
 to each other. 
 
 Some of the usual arrangements of a pair of two-wire 
 circuits, with the mutual induction effects measured in volts 
 per 1,000ft. of double circuit per ampere at frequencies of 
 133 and 60 <\> per second, are given in Fig. 111. 
 
 These data have been determined by Mr. Chas. F. Scott. The 
 wires of the one circuit are marked by black dots, and those 
 of the other by rings. The volts given refer to the E.M.F. 
 induced by each circuit in the other, and will be positive or 
 negative according as the two currents are in opposite or 
 
CHANGE RATIO OF TRANSFORMERS. 229 
 
 the same directions. The diagram is directly applicable to a 
 single-phase system or to a two-phase system with four wires, 
 and can be readily modified to suit a three-wire system. 
 
 Mr. A. E. Kennelly has given much attention to the im- 
 pedance of mutually inductive circuits, and has developed an 
 elegant geometrical treatment, which is applicable to most 
 practical problems, even when they involve resistance, self- 
 and mutual-induction, and capacity. The subject is not suit- 
 able for discussion here. The student is referred to the technical 
 Press for further information.* 
 
 ~ fc / 133 no 0-006 volts. 
 
 1 60 r^ 0-0027 
 
 155 ~ ' 015 
 60 r^ 0-0065 
 
 (133 r^ Q-035 
 I 60 rv 0-016 
 
 n n * -f 133 ~ ' 070 
 O O \ 60 rxj 0-032 
 
 n n * -f 133 ~ ' 112 " 
 1 60 nu 0-050 
 
 FIG. 111. Showing the number of volts per ampere per 1,000ft, of 
 double circuit due to mutual-induction between two circuits in various 
 arrangements. The wires of the two circuits are severally distinguished 
 by rings and black dots. 
 
 H4. EFFECT OF CAPACITY UPON THE CHANGE 
 RATIO OF TRANSFORMERS. 
 
 When transformers are used for testing alternators for insula- 
 tion resistance, it is sometimes noticed that the pressure at the 
 terminals of the transformers is far in excess of that due to 
 the ratio of conversion. This is due to the capacity of the 
 alternator. The effect is of some importance in practice. The 
 theoretical principles on which it depends are difficult to 
 
 * Especially The Electrician, October 27, 1893 ; and the Transactions of 
 American Institute of Electrical Engineers for April, 1893. 
 
230 
 
 ELECTRIC MOTIVE POWER. 
 
 express without the use of coefficients and complicated equa- 
 tions. But some idea of the order and magnitude of the 
 
 1UUUU 
 
 8000 
 
 5 
 
 > 
 
 4000 
 2000 
 
 
 / 
 
 \ 
 
 
 
 
 
 
 / 
 
 V 
 
 
 
 
 
 
 
 \ 
 
 \ 
 
 
 
 
 7 
 
 
 
 \ 
 
 s^ 
 
 
 
 
 
 
 
 
 ^^-^ 
 
 ** 
 
 Microfarads. 
 
 FIG. 112. Curves showing the Variation in Pressure in the Secondary 
 Circuit of a Step-up Transformer, by varying the Capacity in the Secondary 
 Circuit. The experiments made with a No. 2 Siemens Transformer, and 
 Current obtained from a Wl Siemens Machine, frequency 100 c\j . The 
 Exciting Current kept constant, and Alternator kept running at 750 
 revolutions during all the experiments. 
 
 zoo 
 
 160 
 120 
 
 (A 
 
 |so 
 
 40 
 
 
 
 / 
 
 \ 
 
 
 
 
 
 
 / 
 
 \ 
 
 
 
 
 
 / 
 
 ' 
 
 
 \ 
 
 
 
 
 ? 
 
 
 
 \ 
 
 X 
 
 
 
 
 
 
 
 
 ^^ 
 
 * . 
 
 \ '2 -a 
 
 Micr 
 
 4 -5 -6 -7 
 
 afarads. 
 
 FIG. 113. Curves showing the corresponding Variation in Pressure in 
 the Primary (Low Pressure) Circuit in the same Transformer, by varying 
 the Capacity in the Secondary Circuit. 
 
 increase of ratio of conversion will be gathered from the 
 curves in Figs, 112, 113, and 114, taken from "The Alternate- 
 
COMPENSATING FOR DEOP. 
 
 231 
 
 Current Transformer in Theory and Practice," by Dr. J. A. 
 Fleming, Vol. II., pp. 396-398. 
 
 The point to be specially remarked in connection with power 
 work is that long concentric conductors have considerable 
 capacity (about O33 microfarad per mile run), and hence may 
 readily cause effects of the order shown in the experiments. 
 They will be most marked with step-up transformers of large 
 size. It is also important to notice (see Fig. 114) that there 
 is a critical value of the capacity for the given self-induction 
 of the circuit which will produce a maximum rise of pressure, 
 and that the ratio of conversion is steadily increased as the 
 capacity increases (to in this case 0'65 of a microfarad). 
 
 3 -4 
 
 Microfarads.' 
 
 FIG. 114. Curves showing the Dependence of the Ratio of Transforma- 
 tion of Pressure between Primary and Secondary Circuit on Capacity of 
 , -Secondary. Deduced from the experiments illustrated in Figs. 112 and 113. 
 
 It is, therefore, theoretically possible to compensate for the 
 drop of pressure at the far end of a feeder by putting a suitable 
 capacity in the circuit. 
 
 45. COMPENSATING FOR DROP OF PRESSURE IN 
 
 CONDUCTORS. 
 
 There are several methods in use at present for compensating 
 the variations of pressure in the feeders or in the transmission 
 mains. The most common consists in simply adjusting the 
 excitation by hand or automatically, the generators being 
 separately excited ; another consists m compounding the 
 alternators in a manner similar to that adopted with dynamos, 
 the alternators being either self-excited or else partly self- 
 excited and partly separately excited. 
 
 R2 
 
2M ELECTRIC MOTIVE POWER. 
 
 Hand regulation by a resistance in series with the field-coiJ 
 circuit is easily understood, and there is no need to make any 
 special reference to automatic devices whose function is simply 
 to vary the resistance of the exciting circuit. 
 
 The Brush Company, Messrs. Ganz and Co., Messrs. Easton, 
 Anderson and Goolden, and others, make automatic regulators 
 of this description, and the author has spent much time in 
 designing similar devices, but in his opinion they cannot be 
 regarded as complete successes, and certainly are not adapted 
 for use in power plants where there is no skilled super- 
 vision. Compound winding, however, is a practical device. By 
 
 Field Coil 
 
 Field Coil. 
 
 FIG. 115. Thomson-Houston Method of Compounding Alternators. 
 b b, Collector rings. 
 
 means of it alternators can be made to compensate for any pre- 
 determined drop, and therefore to meet most of the regulation 
 difficulties in the feeders. The arrangement adopted by the 
 Thomson- Houston Compivny is to partly excite the field from 
 a dynamo with a steady current and to provide a variable ex- 
 citation by redressing a portion of the main current and passing 
 it through a pair of coils opposite to each other, or through 
 coils on each of the field poles. It is clear that, since the arma- 
 ture current is redressed, the excitation due to it will be 
 proportionate to the load. 
 
 The arrangement is shown in Fig. 115. The two main col- 
 lecting rings are shown at b b. The " redresser " consists of 
 
COMPOUNDING ALTERNATORS. 
 
 233 
 
 "two circular gun metal castings, having each half as many 
 projections as there are pole-pieces in the field; both being 
 insulated. One of these is permanetly connected to one of 
 the collecting rings and the other to the free end of the 
 armature coils as shown. Two brushes, resting on projections 
 belonging to opposite rings of the redresser, form the terminals 
 of the exciting circuit. To reduce sparking at the instant of 
 short-circuiting the field coils when the brushes rest on the 
 same rings, a suitable resistance is placed in shunt to the field, 
 -and this serves in addition to control the proportion of current 
 in the exciting coils, and therefore to vary the compounding 
 within the limits of the design. This is a very complete and 
 convenient device. It is characteristic of the methods generally 
 
 Fig. 116. Kapp's Method of Regulating the Drop of Pressure at the end 
 
 of a Feeder. 
 
 adopted by Messrs. Ganz and others. Alternators thus com- 
 pounded will work in parallel as readily as separately-excited 
 machines.* 
 
 Another very simple plan is to place the primary of a 
 suitable transformer in series with the feeders, and to excite 
 the compound coils from its secondary current, redressed. 
 Since this current is proportionate to the armature current, 
 it is clear that the compounding can be arranged to any slope 
 of characteristic. 
 
 A device patented by Mr. Kapp is interesting in this 
 connection. It is shown in Fig. 116. 
 
 * See letter by Mr. E. M. Mix, The Electrician, March 23, 1894. 
 
 
 
 
234 ELECTRIC MOTIVE POWER. 
 
 One coil, P, of a small transformer is placed in series with 
 the feeder, and the other coil, S, is coupled across the station, 
 mains. By inserting more or less of the coil S in the circuit 
 by a shifting contact, a few auxiliary volts are put into the 
 feeder by the inductive action of the shunt-coil. This regula- 
 tor may be designed to add a sufficient number of volts to com- 
 pensate for any desired drop. It may be applied to each feeder 
 independently, although all the circuits are working off one 
 alternator. This is a distinct advantage, and makes it a most 
 convenient device for power plants. The Westinghouse Com- 
 pany use a similar arrangement, which they call a " booster." 
 
 46. MEASUKEMENT OF PRESSURE AT THE END 
 OF FEEDERS. 
 
 The pressure at the termination of the alternate-current 
 feeders may be read by means of two transformers, without 
 the aid of pressure wires run back to the station. The general 
 
 FIG. 117. Diagram of Station Voltmeter Connections for Reading Pressure 
 at the far end of Feeder. 
 
 arrangement is as follows : The voltmeter is in series with 
 the secondary circuits of two transformers, one of which has 
 its primary in series with the feeder, and the other its 
 primary as a shunt across the feeder. It is illustrated in 
 Fig. 117. 
 
 The pressures of the two transformers are opposed to each 
 other. The secondary pressure of the transformer in shunt is 
 proportional to the station omnibus pressure, V ; and that of 
 the transformer in series to the primary current strength i my 
 and therefore to i m R, if the resistance be constant. The volt- 
 meter will consequently read proportionately to V-^ m R, 
 which is the expression for the pressure at the far end of 
 the feeder. This principle, with important modifications, is 
 employed by Messrs. Ganz and Co. Its application is shown 
 
MEASUREMENT OF PEESSUEE IN FEEDERS. 235 
 
 in diagram in Fig. 118, the adjustable resistance in the 
 exciter circuit being also shown. The feeders are shown 
 at L! L 2 , the shunt transformer at T I} and the series 
 transformer at T 2 . The secondary circuit of T x is closed 
 through the solenoid of the regulator S, the resistances R 3 , R 1 
 and R 2 . The secondary of T 2 is closed through the resistance 
 R 2 . The transformers are joined up so that their electromotive 
 forces are in opposition as regards producing a current in the 
 
 Shunt 
 Transformer 
 
 AltarnAtor. 
 
 /WWVWA MMAM 2 
 
 Feeder. Transformers 
 
 FIG. 118. Ganz and Co.'s Method of Compensating for Drop in Feeders. 
 
 solenoid S. The terminal pressure of Tj is evidently pro- 
 portionate to the station pressure. The difference of pressure 
 between the points a and b, the terminals of R 2 , will be pro- 
 portionate to the feeder current ; and so the pressure between 
 the points e and / will be equal to V i m R, the pressure at the 
 termination of the feeders. The intensity of the current in the 
 solenoid circuit will determine the excitation, and hence the 
 pressure at the sub-station can be kept constant for variation 
 
236 ELECT EIC MOTIVE POWEE. 
 
 of drop. It will be seen that a Cardew voltmeter is used to 
 read the primary pressure. This is made possible by a suitable 
 ratio of conversion in the transformers. It will be evident that 
 these methods of reading the sub-station pressure are only 
 applicable when the virtual resistance of the feeders is 
 practically constant, or varies according to some easily deter- 
 mined law, in which case the voltmeter can be specially 
 calibrated to include these effects. 
 
 47. COMPENSATORS, OR REGULATORS. 
 
 The Thomson-Houston Company use, in particular cases, a 
 compensator, whose functions are similar to those of the " re- 
 gulator " referred to on page 135, with two-wire continuous- 
 current feeders and multiple-wire distributors. This may be of 
 use for power work in cases where the regulation of the supply 
 
 < 
 
 ig r 9 .9 
 
 BOOv. Compensato 
 
 FIG. 119. Prof. Elihu Thomson's " Compensator " System applied to 
 Power Circuit. 
 
 pressure presents difficulties. The plan of connection is shown in 
 Fig. 119, the " compensator" being simply a choking coil, with 
 its circuit divided into as many parts as there are sub-circuits. 
 Each of the distributing circuits is assumed to be at 125 volts. 
 
 When the load is evenly divided between the circuits the 
 compensator will carry practically no current, owing to 
 the impedance of its coils, which are wound round soft 
 iron wire. But if the load be removed from one of the four 
 circuits, the compensator coil, say coil A, in parallel to it will 
 act as the primary of a transformer, and induce in the remain- 
 ing coils of the compensator, B C D, a secondary current which 
 will act in the outer circuit in the same direction as the 
 
MULTIPLE WINE DISTRIBUTION. 237 
 
 main current. The coils B, C, D will supply - amperes, 
 i being the current for two motors (assuming the circuits to 
 
 o 
 
 be equally loaded), and the alternator will supply f i amperes. 
 
 4 
 If the two circuits A and B are opened, then the coils C and 
 
 D will supply *- amperes, and the alternator an equal number. 
 
 The regulating capacity of the compensator is, therefore, seen 
 to be in this case equal to four times the current it is designed 
 to carry. It is evident that small differences of load between 
 the various circuits are readily compensated by this ingenious 
 device, which may be defined as a combination of a transformer 
 .and a direct system of distribution. 
 
 t 
 
 200 v. 
 
 2000 v. < <^ 200 vr 
 
 1 H 
 
 200 v, 
 
 <C * 
 
 FIG. 120. 
 
 As far as the author is aware, this method of multiple- 
 wire distribution with alternate currents has not been applied 
 to power work, although it has been used to some extent for 
 lighting in America. The evident objection te it is that it 
 brings the high pressure of the primary circuit into the 
 consumers' premises, and, therefore, constitutes a source of 
 danger. A better arrangement for power work, when it is 
 desired to use a multiple-wire distributing system, is to divide 
 the secondary circuit of a large transformer at the sub-station 
 into as many circuits as may be required, and at suitable pres- 
 sures for the work (they need not be of the same value) see 
 Fig. 120. This separates the high-pressure mains from the 
 distributing circuit, and the total secondary pressure can be 
 made of any desired value. A transformer thus loaded will 
 work as well as if the secondary circuit were not divided, 
 and it will maintain the same ratio of pressure between the 
 distributors, neglecting the drop due to the resistance, if the 
 
238 ELECTRIC MOTIVE POWER. 
 
 primary circuit, as a whole, adjusts the power according to the 
 output. Difficulty, however, may be found if more than one 
 transformer be used, unless both the primaries are coupled in 
 parallel, and also both the secondaries ; or, better still, if the 
 primaries are coupled in parallel and the secondaries in series. 
 This latter arrangement requires, for four-wire distributors, 
 three transformers, the middle wires being coupled to the 
 junctions of the secondaries. 
 
 It must be understood that these proposals offer no advan- 
 tages as regards the transformers: they simply permit the use 
 of a multiple-wire distribution, which is a point of some import- 
 ance in power work. 
 
 48. COUPLING SEPARATELY-DRIVEN ALTERNATORS 
 IN PARALLEL. 
 
 Before an alternator is coupled in parallel it should be ascer- 
 tained to be as nearly as possible in step with those 
 already in circuit. The practical methods of judging this- 
 condition vary with the types of machines and their relative 
 sizes and speeds, but in most cases there is no difficulty. If 
 the alternators are of similar make, all that is necessary is to 
 raise the pressure to the omnibus voltage, and to notice that 
 the frequencies are approximately the same before closing the 
 parallel switch. At the instant of making circuit there may 
 be a comparatively large rush of current, and then the bank of 
 alternators will drop into step, and the load may be adjusted 
 between them, if necessary, by their excitation. The author 
 has seen Mordey and Giilcher alternators put parallel without 
 further precautions ; but, of course, the lamps pulsated until 
 the machines were in step. 
 
 It is customary, however, in most lighting stations, to use a 
 phase indicator. This usually consists of two transformers, 
 one of the primaries being coupled to the alternator running 
 the load, and the other to the incoming machine. The 
 secondary circuits are coupled in series with two incandescent 
 lamps of suitable pressure. The arrangement of details is as 
 shown in Fig. 121. 
 
SYNCHRONISERS. 
 
 239 
 
 The procedure is usually as follows : The incoming alter- 
 nator, A 2 say, is run up to proper speed, and its field excited 
 so that it gives the terminal pressure of the omnibus bars. 
 Sometimes it is coupled to the station lamps or an artificial 
 resistance, but this is not essential in most cases. The synchro- 
 niser switch W is then closed, and the lamps ^ and Z 2 , being fed 
 with the joint currents from the transformers T x and T 2 , will be 
 black or luminous according as the alternators A 1 and A 2 are 
 in similar or opposite phase. The instant the two are in step 
 the lamps will become steadily black or luminous (according to 
 the coupling up of the secondaries), and then the main switch 
 m 2 may be closed without disturbing the lamps on the outer 
 
 
 P 2 
 
 
 'VVVVVVVVVW 
 
 Pt 
 
 
 
 -^- 
 
 
 _0s- 
 
 
 / 
 
 T! 2 
 
 
 m, 
 
 
 Omnibus Bars. 
 
 f 
 
 FIG. 121. Diagram of Phase Indicator, or Synchroniser. 
 
 circuit. During the short interval of time required for the- 
 machines to get into step, the lamps pulsate with luminous 
 "beats" as the lagging machine overtakes the leading one. 
 The synchroniser can be coupled successively to any pair of 
 the alternators. 
 
 It is found convenient with the Ganz and with most of 
 the Continental alternators to run the incoming machine 
 on an artificial load to approximately the same current as it is 
 required to give on the main circuit before using the synchro- 
 niser and closing the main switch. This precaution seems to 
 
240 
 
 ELECTRIC MOTIVE POWER. 
 
 be only necessary with armatures of large self-induction, in 
 which the terminal pressure varies considerably with changes 
 of current. In England it is not customary to use an artificial 
 load. 
 
 The Thomson-Houston Company insert a variable impedance 
 coil in the armature circuit of the incoming alternator in order 
 to check any in-rush or out-rush of current. The method of 
 using these impedance coils with the synchroniser is shown in 
 Fig. 122. 
 
 SYNCHRONIZING 
 
 rkT 
 
 TRANSFORMER. 
 
 MAIN SWITCH. 
 
 LAMP. 
 
 > <$> i 
 
 ^ SYNCHRONIZING 
 
 ~Si 
 
 FIG. 122. Arrangement of Impedance Coils used in putting Thomson 
 Alternators in Parallel. 
 
 The impedance coil is shown separately in Fig. 123. It consists 
 of a laminated iron ring, with a few turns of well-insulated wire 
 wound round about one-eighth of the circumference. A 
 copper sheath is arranged so as to be capable of rotation about 
 the centre. The sheath is of such width as just to surround 
 the coil. When the winding is covered, the device corresponds 
 to a transformer with the secondary circuit short-circuited, and 
 the impedance is then very slight ; as the sheath is gradually 
 removed the impedance increases; when the stationary coil 
 is entirely uncovered the device has its maximum effect. In 
 
REACTIVE COILS. 241 
 
 some forms the impedance coil is short-circuited when the 
 sheath completely encloses it. These impedance coils are 
 useful for balancing loads between machines and for varying 
 the pressure of feeders ; they generally act the same as 
 rheostats with continuous currents, but have the advantage of 
 consuming less power. 
 
 Parallel working is without doubt assisted by using a 
 moderate rate of frequency, say not more than about lOO'X/ ; 
 but there is no gain in reducing it below about 50 to 60 ^. 
 It is also necessary to synchronise the engines as well as the 
 alternators if the running is to be perfect. This points to the use 
 
 FIG. 123. Impedance Coil, or Dimmer. 
 
 of high-speed engines with flywheels and moving parts of small 
 inertia, so as to readily lead or lag as the conditions of 
 working may require. For the same reason the engine should 
 not be governed directly, as the better the governor the 
 greater the difficulty of varying the rate of speed. The station 
 governor should be designed to control the steam admission to 
 the bank of engines, and the load should be distributed pro- 
 portionately by varying the excitation. A "trip" governor 
 should be used on all engines of this class, to check racing 
 in the event of a belt breaking or slipping off, a fuse blowing. 
 or other similar accident 
 
.242 
 
 ELECTEIC MOTIVE POWEE. 
 
 49. ALTERNATE-CURRENT MEASURING 
 
 INSTRUMENTS. 
 
 It has already been stated that the energy of an alternate- 
 current circuit is proportional to the ^/mean square value of 
 the instantaneous values of the current, and it has also been 
 stated that this x /mean square is equivalent to that of a con- 
 tinuous current that will produce the same heating effects. 
 Therefore, alternate-current measuring instruments are cali- 
 brated by direct currents, and the readings on the scales are 
 accordingly proportional to the ^/mean square of either con- 
 tinuous or alternate currents. 
 
 A few hints of a practical nature will assist engineers in 
 selecting suitable instruments. 
 
 Some of the most practical forms of instruments in general 
 use are tabulated in Table W, the names of typical designs 
 being given. 
 
 Table W. Examples of Practical Instruments suitable for 
 Power Stations. 
 
 
 
 Voltmeters. 
 
 Ammeters. 
 
 Wattmeters. 
 
 Electro-dynamic 
 Electro-magnetic 
 
 fElihu Thomson, 
 -j Evershed,Dolivo, 
 1 Ayrton & Perry 
 
 /Siemens' Dyna- 
 \ mometers 
 (Evershed, Elihu 
 1 Thomson, Kelvin, 
 1 Ayrton & Perry, 
 vDolivo 
 
 /Kelvin, Swin- 
 \ burne 
 
 Electro - thermal 
 Electro-static ... 
 
 Cardew, Holden 
 
 f Kelvin's Multi- 
 cellular, Swin- 
 \burne, Ayrton 
 & Mather ... 
 
 /Holden, Ayrton 
 \ and Perry ... 
 
 ... 
 
 It is desirable, if possible, to have direct-reading pressure 
 instruments, which are directly applicable to the circuits, 
 without non-inductive resistances in series with them, both 
 because the details are simpler and also because the energy 
 wasted in the resistance is often considerable. For this latter 
 reason electro-thermal, or hot-wire, instruments, although very 
 
MEASURING INSTRUMENTS. 243 
 
 convenient for reading low pressures, say up to 150 volts, are 
 objectionable if kept in circuit. For high-pressure work the 
 most practical instruments are of the electro-static type, for 
 they absorb scarcely any power. 
 
 If hot-wire instruments are used to read high pressures, it is 
 advisable to have a voltmeter transformer with a suitable ratio 
 between the primary and secondary. Thus, with a 2,000-volt 
 primary and a 100-volt secondary, a convenient ratio of con- 
 version will be ^j-. This transformer method is more expensive 
 in first cost than direct-reading with a multicellular voltmeter, 
 and it also wastes more power, probably absorbing about 30 
 watts in the hot-wire and 30 in the transformer. 
 
 Current-measuring instruments are generally kept in circuit, 
 and hence it is important that they should waste little power. 
 For reading large currents electro-magnetic instruments of low 
 resistance are perhaps as good as any of the other types. The 
 electro-dynamic method has the disadvantage of requiring 
 mercury contacts. The hot-wire principle is used with success 
 for measuring current, but the instruments are necessarily 
 delicate, and the author does not advise them for power work 
 as a rule. The electro-chemical principle is not generally 
 applicable for ordinary commercial purposes. 
 
 Ohmmeters are useful when erecting station plant, but they 
 they are not necessary for every-day work. 
 
 "Wattmeters, or power meters, are of the utmost importance 
 in all stations where economy of prime power is an object. One 
 should be placed in the circuit of each alternator, to indicate the 
 power given out by the machine. This enables the attendant 
 to divide the load between the several alternators according to 
 their capacity, and thus to work with the smallest excita 
 tion. One of the best forms of this instrument for station use 
 is Lord Kelvin's engine-room wattmeter. The appearance of 
 the interior of the instrument is shown in Fig. 124. It 
 has a main circuit of a double rectangle of copper rod. The 
 pressure coils are made of fine wire wound in the shape ol 
 
244 ELECTRIC MOTIVE POWER. 
 
 a pair of spectacles, and are in series with a non- inductive- 
 external resistance. 
 
 The general arrangement of the movable shunt coils, the 
 suspension springs, and the gravity adjustment for calibrating 
 are shown in Fig. 125. Each of these shunt coils has about 
 1,000 turns of insulated wire of approximately 1,000 ohms 
 
 FIG. 124. Lord Kelvin's Engine-room Wattmeter. General view of instru- 
 ment, with case removed. 
 
 resistance. The scale has nearly uniform divisions, and is 
 graduated to read directly in watts or kilowatts, as required. 
 
 Siemens' dynamometer-wattmeter is also a practical instru- 
 ment, but as usually designed absorbs more power than 
 Lord Kelvin's. 
 
 If a wattmeter be made recording it answers the purpose of 
 an ergmeter, for the load curve is easily integrated by a 
 
PRECAUTIONS WITH ALTERNATE CURRENTS. 245 
 
 planimeter, and thus a continuous record of the station output 
 may be kept without much expense or trouble. 
 
 It is not necessary to put the full primary pressure on the 
 terminals of the shunt-coil of the wattmeter ; a suitable trans- 
 former may be used to reduce the pressure to a convenient 
 fraction, and hence a small movable coil, carrying a mere ciace 
 of current, may be used, and will add considerably to the 
 accuracy of the instrument. 
 
 Ergmeters, or energy meters, are not yet in common use, 
 and are not likely to be much used for power purposes. 
 
 FIG. 125. Lord Kelvin's Wattmeter. Fine-wire Shunt Coils, showing 
 details of suspension. Controlling springs removed. 
 
 50. PRECAUTIONS TO BE OBSERVED IN ALTERNATE- 
 CURRENT WORKING. 
 
 Owing to induction and capacity effects, it is usually neces- 
 sary in alternate- current circuits to observe certain precautions 
 in starting and stopping alternators, in opening and closing 
 circuits, and in adding or withdrawing feeders from omnibus 
 bars. It must be recollected, however, that these effects are 
 largely the result of the self-induction of the alternators, and 
 that generally there will be no difficulty when the armatures 
 have comparatively small reactions, as is the case with the 
 Mordey alternator, for example. 
 
246 ELECTRIC MOTIVE POWER. 
 
 The chief danger to be apprehended is a sudden variation of 
 pressure, which may be sufficiently large to break down the 
 insulation of the circuit at one or more points or to seriously 
 affect the load. This is due both to the change of conversion 
 ratio in the transformers and to the variations of impressed 
 pressure in the alternators. 
 
 The precautions to be observed are : 
 
 Never open a high-pressure circuit containing self-induction 
 or capacity, without first slowing the alternator or weakening 
 its excitation, or reducing the load by an impedance coil. 
 
 Never switch an alternator into parallel with another 
 machine without first adjusting its excitation so that the 
 coupled alternators will give the proper omnibus pressure. 
 
 Never switch an alternator out of parallel without adjusting 
 its excitation (and that of the remaining machines if necessary) 
 so as to leave the omnibus pressure unaffected. 
 
 An alternator of large self-induction requires to be excited to 
 a higher pressure than that of the omnibus bars before coupling 
 in parallel, as the terminal pressure will fall considerably when 
 the circuit is closed. In taking such a machine out of parallel 
 the excitation may be reduced until the current is nearly zero ; 
 or the speed may be lessened until the same end is attained. 
 The switch under these conditions may be opened without a 
 dangerous spark. 
 
 If, however, the conversion ratio of the transformers be 
 affected by the capacity of the circuit (as occurs with step- 
 up transformers and concentric mains), then the incoming 
 alternator with large self-induction must be under-excited, in 
 order to bring it into such a condition that it can be coupled to 
 the active machines without affecting the pressure at the far 
 nd of the feeders. And, conversely, in taking it out of parallel 
 the excitation must be raised a little, in order to avoid serious 
 sparking on opening the circuit. These effects have been noticed 
 in practice, and are explained by an alteration of the capacity 
 and self-induction of the circuit affecting the conversion ratio 
 of the transformer. 
 
SAFETY DEVICES. 247 
 
 ,51. EARTHING THE PRIMARY AND SECONDARY 
 CIRCUITS OF TRANSFORMERS; SAFETY 
 DEVICES. 
 
 In transformer work there is a chance of the primary 
 pressure invading the secondary circuit. If this occurs there 
 may be danger to life. And this is the more serious, since, 
 in most cases, the leakage is entirely unsuspected by the con- 
 sumer. To obviate this danger various arrangements have been 
 suggested, and some have met with the approval of the Fire 
 Insurance Offices and the Board of Trade. The safest plan, 
 perhaps, is to ground one of the primary cables close up to the 
 station (as is done by the London Electric Supply Corpora- 
 tion), and to place between the secondary circuit and earth a 
 device which will ground the secondary circuit the instant 
 the high-pressure current enters the consumer's circuit. This 
 will cause a large primary current to now, and hence the 
 primary fuses will blow and cut off the premises. The 
 danger to life is entirely obviated, unless the transformer or 
 other device has a large electro-static capacity, but this is 
 never the case with ordinary transformers. 
 
 There are three protective devices in general uso : Major 
 Cardew's " Mouse Trap," Mr. Kent's metallic sheath, and the 
 Thomson-Houston film cut-out. 
 
 The Cardew safety device is shown in Fig. 126. It consists 
 of two brass plates placed near together, but insulated from 
 each other. Between them is a strip of aluminium foil 
 attached by ebonite pins at one of its ends to the bottom 
 plate. One plate is connected to earth, and therefore to one 
 of the primary mains, and the other to the secondary circuit. 
 As soon as a leak occurs between the primary and secondary 
 circuits of the transformer there is an electro-static pressure 
 between the plates, and when this equals a definite number of 
 volts the free end of the foil is attracted and makes contact 
 with the top plate. This grounds the secondary circuit, and 
 the primary current blows the primary fuses. 
 
 Mr. Kent's arrangement consists of an earthed metal ring, 
 properly slit to avoid eddies, placed between the primary and 
 
 s2 
 
213 ELECTRIC MOTIVE PuWER. 
 
 secondary windings. Any fault of insulation in either circuit 
 can be made to ground the coils and blow the main fuses. 
 
 The film cut-out of Prof. Thomson is merely a piece of prepared 
 paper between two metallic contacts. This paper is pierced 
 when the pressure between the contacts exceeds a predeter- 
 mined amount. Thus the paper insulation will easily stand 
 the secondary pressure of, say, 100 volts, but will break down 
 instantly it is subjected to the high pressure of the primary. 
 In practice one of these film contacts is connected to each of 
 the secondary mains. 
 
 FIG. 126. Major Cardew's Earthing Device, for protecting Transformer 
 
 Circuits. 
 
 Mr. Ferranti has devised a very ingenious method for pro- 
 tecting transformer circuits from the risk of partial earth 
 (always a source of danger, since a second ground on the same 
 circuit will cause a dead short-circuit on the secondary, and 
 may cause a fire). This method requires the middle of the 
 secondary circuit to be permanently earthed. It is perhaps 
 more useful at present as a testing device for partial grounds, 
 as the Fire Offices' regulations do not generally allow any part 
 of the consumers' circuit to be earthed. But for power 
 plants, where these restrictions do not obtain, the device is 
 most useful. The arrangement is shown in Fig. 127. 
 
EARTHING DEVICES. 
 
 249 
 
 It will be seen that there are two small transformers across 
 the secondary mains, with a ground wire from their point of 
 junction. Their pressures are arranged in opposition, and 
 normally no current flows through them. If, however, another 
 earth, full or partial, is made in one of the secondary mains, 
 the corresponding transformer will cease to produce current, 
 and, the balance of pressure being upset, a current will flow in 
 
 Station. 
 
 To Station J 
 
 Fuce. 
 
 House Transformer. 
 
 VWWWVWVW^ 
 
 AAAAAAA/SAAAAAA 
 
 j-AAAAAAA VWWXA-j 
 
 Safety Device Transformers. 
 
 FusKbleAVire. 
 
 To Lamps. To Earth. 
 
 FIG. 127. Ferranti " Earthing Device," for Transformer Protection. 
 
 their secondary circuit, and will melt the fuse, thereby allowing 
 a conical plug to drop into the split cup shown in the diagram. 
 This short-circuits the secondary circuit and blows the primary 
 fuses. 
 
 It is urged strongly by most of the leading electrical 
 engineers that the secondary circuits should always be earthed 
 
250 ELECTRIC MOTIVE POWEE. 
 
 as well as the primary. If this were generally practised it 
 would be impossible for a plant with defective insulation to 
 remain in connection with the high-pressure circuit, for the 
 main fuses would blow out as often as the main switches were 
 closed. With the existing compulsory regulations, it is possible 
 for a bad earth to exist on an installation until a second earth 
 reveals its existence, possibly by a fire. 
 
CHAPTEE VIIL 
 
 POLYPHASE ALTERNATE-CURRENT WORKING. 
 
 52. Definitions ; Two-phase Currents. 53. Three-phase Currents. 
 54. Relative Merits of Two-phase and Three-phase Currents. 55. 
 Polyphase-Current Transformers ; Diagrams of Connection?. 56. 
 Combination of Two- and Three-phase Circuits. 57. Synchronous 
 Polyphase Motors. 58. Present Practice in Polyphase Alternators 
 and Motors. 59. Winding ; Starting Torque ; Power Factor. 
 60. Combination of Polyphase and Continuous Currents ; Rectified 
 Currents. 
 
 52. DEFINITIONS; TWO-PHASE CURRENTS. 
 
 THE term polyphase is applied to circuits in which two or 
 more alternate currents, of the same wave length and direc- 
 tion, succeed each other at regular intervals. In practice, the 
 number of impulses is usually limited to two or three, and the 
 systems are then severally distinguished as two-phase and 
 three-phase. An alternate current (single or multiple), when 
 used to excite field magnets, produces a rotating magnetic field; 
 that is, the axis of the field rotates with reference to space, 
 though the windings are stationary. This is seen to be the case 
 by considering an ordinary two-pole series-wound motor which 
 is supplied with a single-phase alternate current. The poles 
 will change from north to south polarity at every half period, 
 and the axis of the magnetic field may be regarded as rotating, 
 making one complete revolution in each period. Such a motor 
 is, therefore, said to have a rotary magnetic field, and on the 
 Continent is called a Drehstrom motor. A continuous-current 
 motor, however, is not adapted for working with alternate 
 
 
252 ELECTEIC MOTIVE POWER. 
 
 currents of the usual frequencies, since the self-induction of the 
 field-magnet circuit is too large.* 
 
 If two separate currents, with phase intervals of 90deg., be 
 supplied to a four-pole motor, in which the armature consists 
 of an iron core with windings short-circuited, either through 
 brushes or else by rings of metal at each end, a rotary 
 magnetic field in the stationary part will result. And 
 this will induce currents in the closed coils of the arma- 
 ture whose direction will be such as to cause a magnetic field 
 
 FIG. 128. Four-pole Two-phase Motor. 
 
 tending to stop the rotation of the main field. This will be 
 understood by examining Fig. 128, which represents a four-pole 
 two-phase motor, having neither commutator nor brushes. 
 
 There are two alternating exciting circuits, AA X and B B p 
 in quadrature. 
 
 * Attempts have been made, but with little success, to overcome this 
 difficulty by laminating the magnet?, lowering the frequency, and modify- 
 ing the windings, so as to reduce the self-induction as much as possible, 
 and to limit the hysteretic and eddy-current losses. Vide " The Distribu- 
 tion of Power by Alternate-Current Motors," by Albion T. Snell ; Proc. 
 Institution of Electrical Engineers, Vol. XXII., part 106. 
 
TWO-PHASE CURRENTS. 253 
 
 Suppose that A and A 1 be magnetised so as to cause lines of 
 force to pass from A to A l through the armature C. Then B 
 and B x will be neutral, for there will be no current in them at 
 this moment. 
 
 Next, the current in A Aj will gradually die away ; and that 
 in B Bj will steadily rise in an equal ratio but opposite 
 direction until the magnetic flux is from B to Bj, and A and 
 Aj are neutral. 
 
 This cycle will be repeated, but with this alteration the 
 current in A A x will rise to a negative maximum (assuming it 
 to have been positive before), and the flux will be in the direc- 
 tion of A 1 to A, and in the next wave from B x to B. Carefully 
 noting these changes, it is seen that the magnetic field has 
 rotated once for a complete oo in each of the circuits ; and 
 
 therefore the speed of the field rotation is , where N is the 
 
 P 
 number of ou per second and p is the number of pairs of poles 
 
 in one circuit. In the case referred to, since p = 1, the revolu- 
 tions of the magnetic field will be the same as the number 
 
 of <~v . 
 
 In a two-phase dynamo the closed-circuit armature is re- 
 placed by a suitable electro-magnet, excited by a continuous 
 current. Two-phase periodic currents are induced in the two 
 circuits A A : and B B 15 the phase difference between them 
 being 90deg. 
 
 Mathematically the currents in the two circuits may be 
 expressed thus : 
 
 That in A A! = K sin a, 
 
 B B! = K cos a = K sin ( a - \ 
 
 which shows the phase difference between the two currents. 
 Now K is a constant for the particular machine, and a is a 
 measure of the angular motion of the field. It will be seen 
 that the current in A A! is a maximum when a is equal to 
 
 Q 
 
 - and - TT, and is zero when a is equal to and IT. The converse 
 of this is the case with the current in B Bj. 
 
254 
 
 ELECTRIC MOTIVE POWER. 
 
 Two-phase current circuits are usually worked with four 
 wires two for each circuit but three wires, one having a sec- 
 tion V41 times as great as that of each of the others, may be 
 used. The latter arrangement is possible because the large 
 wire will act as a common return for the two currents, the sum 
 of whose instantaneous values is 1*41 times that of either. 
 This will be seen on reference to Fig. 129, in which the full line 
 curves relate to the impulses acting in the directions A A x and 
 B B x ; and the dotted line curves to those in the directions 
 A! A and BjB. 
 
 Curves A and B show the current-fluctuations in the two 
 circuits, and D that in the common return. The magnetic field 
 
 FIG. 129. Diagram of Two Periodic Currents with a Common Eeturu. 
 Phase Difference of 90deg. 
 
 will be caused by A and B alone, and, therefore, will be pro- 
 portional to their sum at any instant. When A is a maximum 
 B is zero, and the induction at this moment is proportional to 
 
 the current in A alone, and is equal to, say, i sin - = i. When 
 
 A and B are equal, as at 135deg. of field rotation, the induc- 
 tion is proportional to 2 i sin 135deg. = i 1'4. Thus the current 
 varies in intensity between 1 and 1*4. 
 
 To illustrate the fluctuation of the field excitation the sign 
 of the negative curves in Fig. 129 may be changed, and they 
 may be plotted above the time line as if they were positive in 
 value (see Fig. 130). 
 
EXCITATION WITH TWO-PHASE CURRENTS. 255 
 
 This diagram is justified by the consideration that each 
 circuit in the motor contains at least two coils wound in opposite 
 directions, and thus produces poles of different signs. 
 
 In Fig. 130 the excitation fluctuation is shown by the top 
 curves, which may be regarded as having reference to one of 
 the poles of the rotary field, say the north pole. 
 
 Since the theoretical mean variation of the exciting current 
 is as much as 15 per cent., it may be supposed that the 
 magnetic induction also varies by nearly as large a percentage, 
 but this is not the case. These curves are deduced upon the 
 supposition that the exciting circuits have no self-induction, 
 whereas, on the contrary, they have much. And, further, 
 
 SO 160 270 300 6O 
 
 FIG. 130. Diagram of Excitation Variation with Two-phase Current. 
 
 Phase Difference of QOdeg. 
 
 the resultant field cannot vary in intensity between such 
 wide limits as those indicated in the diagram. Indeed, it 
 is safe to assume that the field will be nearly steady as 
 long as the effective values of the two currents are equal and 
 their sum is constant. Armature reactions introduce serious 
 complications, for their effect depends partly upon whether 
 the current leads or lags the impressed electromotive force, 
 and partly on the amount of the phase difference between 
 the pressure and current. As already pointed out, when 
 considering single-phase alternate-current machines, armature 
 reactions tend to magnetise the field when the current leads 
 and to demagnetise the field when it lags. The effect will 
 be a maximum at starting, will decrease as the armature 
 speed increases; and will be nil if the speeds of armature 
 and field coincide, i.e., if the motor works synchronously. 
 
256 
 
 ELECTRIC MOTIVE POWER. 
 
 53. THREE-PHASE CURRENTS. 
 
 Three-phase current working presents several advantages, 
 but entails complications in the design of the machine, and 
 also in the regulation of the pressure, because the three cir- 
 cuits are connected. This is demonstrated in the following 
 abstract from a paper by the author entitled " The Dis- 
 tribution of Power by Alternate-Current Motors," which was 
 read before the Institution of Electrical Engineers in the spring 
 of 1893. 
 
 Fig. 131 represents diagrammatically the phases of three 
 equal periodic currents separated by intervals of 120deg. ; the 
 
 FIG. 131. Three Equal Periodic Currents separated by Phase Intervals 
 
 of 120deg. 
 
 full and dotted lines referring to the direction of the impulses, 
 as already explained for Fig. 129. If A, B, C represent the three 
 currents, then their instantaneous values are severally given 
 
 by:- 
 
 A = K sin a, 
 
 2 
 
 B = K sin (a - - TT), 
 3 
 
 C = K sin (a - 1 TT). 
 o 
 
 o 4 
 
 Now, K { sin a 4- sin (a - " TT) + sin (a - - TT) } = 0, as is easily 
 3 o 
 
 proved ; and, therefore, 
 
THREE-PHASE CURRENTS. 
 
 257 
 
 It may be seen that the algebraical sum of the instantaneous 
 values of the three currents is equal to zero under all condi- 
 tions ; for, even when one of the quantities is equal to nought, 
 the remaining two are equal, and, being of opposite sign, cancel 
 each other. 
 
 Three-phase current working is accomplished by parallel or 
 series coupling of the circuits, the two methods being severally 
 
 FIG. 132. Parallel, or Closed Circuit, or Triangle Three-phase 
 Coupling. 
 
 known as the triangle and star sj stems. In the following 
 diagrams large capitals are used to distinguish the mains, and 
 small letters to denote the windings. Suffixes are used to mark 
 the phase order of the circuits. 
 
 The parallel or closed circuit, or triangle coupling, is shown 
 in Fig. 132. Let I 1? I 2> L be the effective values of the several 
 
 FIG 133. Three-phase Dynamo coupled to a Motor. 
 
 currents flowing in the mains, and i lt i^ i 3 the corresponding 
 effective currents in the coils. Also, let V 15 V 2 , V 3 be the 
 effective pressures at the terminals of i v i z , i 3 . And assume 
 there is no self-induction or capacity. Then, if the load be 
 equally distributed between the mains, we may assume 
 
258 
 
 ELECTEIC MOTIVE POWER. 
 
 Also, since i is in phase with v, and I lies SOdeg. removed 
 from i, it follows, if the mains be fed with a combined 
 three-phase current (as in Fig. 133), that the current in 
 one main will differ in phase by 30 deg. from the pressure 
 between it and the two other mains. This follows from the 
 
 FIG. 134. Diagram of a Three-phase Circuit. 
 
 geometrical relationship of the coils and the mains. And the 
 effective value of the current in each of the mains is equal 
 to 1*732 times the effective current in each of the coils, when 
 the circuits are equally loaded. In Fig. 134 let the currents in 
 the mains and in the coils be considered positive when flowing 
 
 FIG. 135. Diagram of a Three-phase Circuit. 
 
 in the direction indicated by the arrows. The phase and mag- 
 nitude relations of these currents will then be represented by 
 Fig. 135 ; and if this diagram be supposed to revolve uniformly 
 around its centre, the length of the projections of the sides 
 on any fixed straight line will represent the instantaneous 
 
THREE-PHASE CURRENTS. 259 
 
 values of the corresponding currents ; for it is plain that 
 these projections satisfy all the necessary conditions, viz. : 
 
 ^ = ^-^^1 
 
 -*n-h 
 :_.: -H 
 
 All instantaneous values i.e., the 
 lengths of the projections of the 
 x + 1 2 + IB = I corresponding sides of Fig. 135. 
 
 Again, the sides of the diagram are proportional to the effec- 
 tive values of the corresponding currents; and, if the load 
 be equally distributed between the three mains, then for 
 effective values, 
 
 I x = I 2 = I 3> ^ = i 2 = i. 3 ; and Ij = 2 i z sin GOdeg. ; 
 or, generally, 1 = 1*732 i. 
 
 This proof is due to Dr. W. E. Sumpner, Professor of 
 Electrical Engineering at the Battersea Polytechnic. 
 
 v s 
 
 FIG. 136. Diagram of Relative Position of Curves of Currents in Mains 
 and Coils of a Closed-type Combined Three-phase Circuit, with no Self- 
 induction or Capacity. 
 
 By a similar diagram the relation between the effective 
 pressures on the coils and mains in the open or star arrange- 
 ment can be shown to be V = 1'732 v. (See Fig. 137.) 
 
 The relative positions of the phases of currents and pressures 
 in the mains and coils of a combined three-phase circuit of the 
 closed type with no self-induction are shown in Fig. 136, V and e 
 having the same value and coinciding in phase. It will be seen 
 that the maxima of the currents in the mains, I, are always 
 
260 
 
 ELECT 'RIG MOTIVE POWER. 
 
 midway between those of the coils, i, and that the phase 
 difference between them is SOdeg. 
 
 The series, open circuit, or star coupling is shown in Fig. 137- 
 
 As before, let there be no self-induction or capacity, and let 
 I 1= =I 2 = I g - i 1 = i 2 = i 3 ' and V 1 = V 2 = V 3 . Now, since the 
 mains, I, are in series with the coils, i, and there is by hypothesis 
 no self-induction or capacity, I^^, I 2 = *2> anc ^ 1$ = iy But 
 the pressures between the mains are not the same as those at 
 the coil terminals, and v will be in advance of V by SOdeg., and 
 
 will be numerically equal to , or v = , therefore, 
 
 2 sin 60 1-732 
 
 V = 1-732 v. 
 
 FIG. 137. Series, or Open Circuit, or Star Three-phase Coupling. 
 
 The phases of current and pressure in a combined three-phase 
 circuit of the open type are shown in Fig. 138, 1 and i having 
 the same value and coinciding in phase. 
 
 The conclusions thus arrived at are true only on the assump- 
 tions (a) that the coils themselves have no self-induction, and 
 (b) that both the coils and the mains are equally loaded. The 
 first condition is never found in practice, and the second is 
 only likely to obtain with small motors, and then only in an 
 
THREE-PHASE CURRENTS. 261 
 
 approximate degree. The effect of self-induction is to cause 
 the current in the coils to lag behind the pressure at the 
 terminals. 
 
 To measure exactly the power in a three-phase system is 
 difficult ; but it can always be done by measuring the work 
 performed in each of the separate circuits, and adding the 
 quantities. 
 
 In Fig. 136, the power = 3 iv = 3 x V -i-_ = V x I x 1-732, if 
 
 the three circuits be equally loaded and there be no self- 
 induction or capacity. 
 
 FIG. 138. Diagram of Relative Position of Curves of Pressures and 
 Currents in the Coils and Mains of the Open-type Combined Three-phase 
 Circuit, with no Self-induction or Capacity. 
 
 In Fig. 137, also, the power = 3^ = 3x1 ^ = V x I x 1-722, 
 on the same assumptions. 
 
 So it appears, if there be no self-induction and an equal load 
 in each circuit, that the number of amperes in one of the mains 
 multiplied into the pressure between two mains into 1-732 
 gives the power in watts. If there be self-induction, the above 
 quantity must be multiplied by the cosine of the angle of lag 
 between the current and the pressure. The energy absorbed 
 by a motor, therefore, will be expressed by 
 
 Vxlx 1-732 cos <, 
 where c/> is the angle of lag. 
 
262 ELECTEIG MOTIVE POWER. 
 
 Dr. W. E. Sutnpner suggests that when the three circuits are 
 equally loaded the power can be measured by one wattmeter 
 (see Fig. 139). Put the current coil in one of the mains, say I 2 , 
 and take two readings, one with the pressure coil coupled 
 between I x and I 2 , and one with it coupled between I 2 and I 3 . 
 The two readings will be found to be of the same value, and 
 the power will be equal to their sum. 
 
 FIG. 139. Method of Measuring Power in Three-phase Circuit, with 
 Mains equally Loaded. 
 
 When the load is unequally distributed, two wattmeters are 
 required (see Fig. 140). Place the current coils in two of the 
 mains, say I : and I 3 , and couple one pressure coil between I t 
 and I 2 , and the other between I 2 and I 3 . The power is then 
 the sum of the two wattmeter readings. 
 
 o 
 
 o 
 
 FIG. 140. Method of Measuring Power in Three-phase Circuit, with 
 Mains unequally Loaded. 
 
 This method of measuring power is applicable whatever the 
 law of variation of the current, and however unequally loaded 
 the mains may be. 
 
 It has been shown on page 258 that with combined three-phase 
 currents, even when neglecting the self-induction of the coils, 
 there is a constant phase difference between the current and 
 
ROTAEY MAGNETIC FIELDS. 263 
 
 the line pressure. This, although not in itself a direct loss, 
 causes difficulty in measuring, regulating, and controlling the 
 currents. Hence it has been found expedient to work the 
 dynamo circuits unconnected and to use six or more separate 
 coils. The currents from these are combined by a suitable 
 transformer so as to convert the secondary currents into a com- 
 b'nel high-pressure rotary current, with phase differences of 
 120deg. This high-pressure current is reduced at the motor 
 end of the line by another transformer and subdivided as re- 
 quired. 
 
 It is not a simple matter to represent the intensity of the 
 magnetic field by a diagram, for the magnetism is not merely 
 the result of the exciting current in the field coils, but is also 
 largely affected by the magnitude of the induced currents in 
 the closed coils, which vary from instant to instant with the 
 fluctations of load. There are two periodic currents acting 
 in the closed coils one of high frequency, equal to the 
 number of the pairs of field poles multiplied into the number 
 of revolutions per second ; and one of lower frequency, which 
 depends simply on the difference of speed rotation between 
 the revolving field and the rotating coils. 
 
 The low-frequency current produces the torque, and is 
 highest at starting, when the slip of the armature is greatest. 
 
 It is expressed numerically by n , where N = the number 
 
 of revolutions of the magnetic field per minute and n = that of 
 the rotating coils. For example, if N = 2,400 and n = 1,920, 
 then the frequency will be 8 per second. In determining the 
 excitation for polyphase motors and dynamos it is, therefore, 
 necessary to make two calculations, one for full and one for 
 light load, just as with direct-current machines. 
 
 It is clear that it is not possible to represent the changes of 
 the rotary field magnetism by a general diagram. Fig. 141, 
 however, shows diagrammatically the relative position of the 
 current and pressure curves in a combined three-phase circuit, 
 with the resultant excitation, assuming the armature reactions 
 to have no effect i.e., the field and coils are supposed to 
 rotate at nearly the same speed. An arbitrary angle of lag of 
 
 T2 
 
264 
 
 ELECTRIC MOTIVE POWER. 
 
 30deg. between the pressure and the current in the exciting 
 coils has been assumed, and the curves have been drawn to 
 suit the sine law. The pressure is shown in V V, and the 
 current in ii. The resultant excitation is indicated in curves 
 R R, the limits of which are 2i and 1-732*, a mean difference 
 of about 8 per cent. 
 
 This is true only when self-induction and armature reactions 
 are neglected ; if these be taken into account, it is sufficiently 
 clear that the field is approximately constant, and that is ail- 
 that is necessary in practice. 
 
 The torque will vary with the ampere-turns and the number 
 
 of lines of force in the circuit. The torque = ^ , where i & 
 
 K i 
 is the current in the short-circuited coils, C is the number of 
 
 120 180 240 300 360 CO 
 
 FIG. 141. Diagram of the Excitation Fluctuations in a Combined 
 Closed-type Three-phase Circuit. 
 
 turns, K! is a constant, and N is the magnetic flux in the 
 resultant field. 
 
 The magnetism is determined by the excitation, and the 
 number of turns of wire is, of course, fixed for a given design; 
 hence, to increase the torque, it is necessary that the moving 
 part rotate more slowly that is, the slip must be increased. 
 This raises the frequency, and consequently the magnitude, 
 of the current in the closed coils, thereby weakening the 
 resultant field and lowering the counter electromotive force of 
 the exciting coils, and allowing more current to flow in them. 
 But, since the resultant magnetism decreases as the current in 
 
STAETING POLYPHASE CIRCUITS. 265 
 
 the closed coils increases, there is a point at which the torque is 
 a maximum. This corresponds to a definite line current, which 
 is not necessarily the starting current, and hence these motors 
 do not necessarily exert their greatest effort at starting. 
 
 To obtain maximum torque at starting, it is necessary to 
 insert resistances in series with the closed-circuit coils. The 
 function of the resistances is to determine the critical current, 
 thus limiting the armature reactions ; and also to regulate the 
 lag in phase between the currents in the closed and exciting 
 coils. 
 
 In order to avoid large rashes of current when closing the 
 line circuit, it is advisable to insert variable resistances in 
 
 FIG. 142. Combined Series and Parallel Three-phase Couplings. 
 
 series with the closed windings. This is specially necessary 
 with large machines (see 59, p. 293). 
 
 In order to obtain a more constant exciting current than that 
 given by the arrangement shown in Figs. 132 and 137, Mr. 
 Dobrowolski has devised a very pretty combination of the open 
 and closed type windings, which reduces the mean variation of 
 the excitation to 3 '5 per cent. The winding is shown diagram- 
 matically in Fig. 142, and is known as the double-linked 
 winding. The coils indicated by the symbols i' v i' v i' ?j , are 
 severally wound in two parallels, 15deg. removed from the 
 closed coils i v i 9 , i y The complications involved are consider- 
 able, and the gain perhaps not commensurate with them ; yet 
 for large machines the device may prove useful. 
 
266 
 
 ELECTRIC MOTIVE POWER. 
 
 There are some important differences to be noticed between 
 the series and parallel connections. It has been shown that with 
 
 the series coupling v = - (see Fig. 137), and that the cur- 
 1 i o2i 
 
 rents in the coils and mains are the same ; and in the parallel 
 device (see Fig. 132), that each coil carries a current equal to 
 
 , and that the pressure at the coil terminals corresponds 
 T732 
 
 to the pressure between the mains. These differences are 
 suggested by the terms parallel and series, which are in this 
 respect more apt than those of triangle and star. 
 
 FIG. 143. 
 
 Diagram showing Ampere- 
 turns with Parallel Coupling. 
 
 FIG. 144. 
 
 Diagram showing Ampere-turns with 
 Series Coupling. 
 
 Now, the magnetising effect exerted by a given number of 
 turns of wire, and a definite effective current in the mains, will 
 be different with the two kinds of windings. 
 
 Consider Figs. 143 and 144, which severally represent the 
 parallel and series couplings, and the direction of current at 
 the instant when it is entering by one main and returning 
 equally by the two others. Let n be the number of turns of 
 wire in one coil. Then the total ampere-turns in the parallel 
 
 o 
 
 device is n I, and in the series arrangement - n I. Therefore, 
 
 2i 
 
 the magnetising effect, and also the self-induction, of the series 
 
TWO-PHASE AND THREE-PHASE CURRENTS. 267 
 
 device is greater than that of the parallel for a given current 
 and a fixed number of turns. The total power absorbed by the 
 two circuits may be made the same by a proper adjustment of 
 pressure. 
 
 54. RELATIVE MERITS OF TWO-PHASE AND THREE- 
 PHASE CURRENTS. 
 
 Experts differ as to the relative merits of two- and three- 
 phase currents. As regards the alternators and motors there 
 is, perhaps, not much to choose between them. Machines 
 designed for similar outputs, whether synchronous or non- 
 synchronous, are found to have about the same efficiencies, 
 starting torque, weight, and apparent watt consumption. The 
 three-phase machines, however, are the better, according to 
 
 FIG. 145. Single-phase System. 
 
 xx' = Effective Pressure of 3,550 volts (see Table X, p. 268). 
 yy' = Maximum Pressure of 5,000 volts. 
 
 experiments made by Mr. Kolbein, the chief engineer of the 
 Oerlikon Works. As regards the winding details, perhaps the 
 two-phase type is the simpler. The main difference between 
 the two systems lies in the relative weights of copper required 
 in the conductors for given conditions, and the ease with which 
 the currents may be handled, regulated and controlled. 
 
 The first experiments with three-phase working brought to 
 light difficulties in the regulation of pressure between the 
 three mains at the points of supply, when the currents in the 
 three circuits were not approximately the same. These, though 
 since to S)me extent overcome, are greater than those with two- 
 phase currents. The difficulties lie chiefly with the design of 
 
pa.iapisuoo si 
 
 A\.V\ OHUOU009 
 
 ( 263 ) 
 
TWO-PHASE AND THREE-PHASE CURRENTS. 269 
 
 alternators and transformers. Two-phase currents, if worked 
 with two separate circuits, are as easily dealt with as single- 
 phase currents, and serve readily most purposes to which 
 single or poly-phase currents are usually applied. They are 
 specially convenient for distribution. It may be safely inferred 
 that two-phase currents are better adapted for a combined 
 service of light and power work than three-phase currents, 
 which are better adapted for running with equally loaded 
 ircuits, and therefore are not generally so suitable for lighting 
 work. 
 
 For simple transmission of power between two distant points, 
 however, the requirements are different, and the three-phase 
 current system is generally cheaper than either a continuous- 
 current or a one-phase or two-phase alternate-current system. 
 
 FIG. 146. Two-phase System. Three Wires. 
 
 x x' = Effective Working Pressure of 3,550 volts. 
 yy' = Maximum Working Pressure of 5,000 volts. 
 z z' = Maximum Component Pressure of 7,100 volts. 
 
 This interesting and important problem may be considered 
 in three ways, each of practical value : 
 
 (1) On the basis of maximum difference of pressure between 
 conductors ; 
 
 (2) On the basis of effective pressure at the distributing 
 station (or motor terminals, perhaps) ; 
 
 (3) On the basis of the economic law. 
 
270 
 
 ELECTRIC MOTIVE POWER. 
 
 The ratios of the weights of copper under these conditions 
 are stated in tabulated form in Table X, p. 268, in which, to give 
 a practical character to the figures, a definite power has been 
 selected for transmission to the receiving station. Constant 
 line loss and constant fall of pressure have also been considered. 
 
 In the final column the economic law has been considered 
 to the exclusion of both a definite line loss and line drop, the 
 equivalent current being taken as in column 3. In practice, 
 the weights will be modified by local considerations ; but the 
 main ratios will hold good, and the figures given form, 
 therefore, a reliable guide between the relative merits of the 
 systems as affected by the weight of copper. 
 
 FIG. 147. Two-phase System. Three Wires. 
 
 xx' = Effective Working Pressure of 2,500 volts. 
 yy' = Effective Component Pressure of 3,550 volts. 
 z z' = Maximum Component Pressure of 5,000 volts. 
 
 The significance of the difference between effective and 
 maximum pressure is at once apparent if Figs. 145, 146, 147 r 
 and 148 are examined. 
 
 In these figures the electrical centre of gravity is shown 
 graphically. In any electrical circuit of good or bad insulation 
 there is a datum pressure with reference to the variations of 
 pressure in the circuit. For example, in a continuous-current 
 circuit the absolute pressure of the negative pole of the 
 dynamo will have a constant ratio to that of the earth's 
 surface at the particular site, and the difference of pressure 
 between it and the positive pole will simply affect the absolute 
 pressure of the positive pole. And, if the negative pole be 
 
EARTHING THE MIDDLE WIRE. 
 
 271 
 
 grounded, the datum pressure will be that of the earth's 
 surface. 
 
 Again, with an alternate-current circuit, with both poles 
 insulated, the cyclical changes will alternately raise and lower 
 the absolute pressure above and below zero pressure, which 
 corresponds to that of the earth's surface. Thus, in a circuit 
 carrying a current of 5,000 effective volts pressure, the maxi- 
 mum pressure between any two points of the circuit will be 
 5,000 ^2 = 7,050 volts, while the absolute pressure will be 
 only 3,525, positive or negative. The stress through the 
 insulation to earth will, therefore, be only 3,525 volts. 
 
 But if the circuit be grounded, say, at one of the main 
 terminals, the absolute pressure between the other terminal 
 
 FIG. 148. Three-phase System. 
 
 x x = Effective Pressure of 3,550 volts. 
 y y' = Maximum Pressure of 5,000 volts. 
 
 and earth will be 7,050 volts, and therefore the insulation all 
 over the high-pressure circuit will be subjected to this stress, 
 although the effective working pressure will remain as before. 
 
 This possible change in the stress upon the insulation of a 
 circuit carrying an alternate current has led to the general 
 practice of earthing one of the primary mains, usually at both 
 the generating and distributing circuits. The maximum stress 
 is thus accepted, and suitable precautions adopted to safeguard 
 against accidents; and, as already pointed out in 51, p. 247, 
 additional security is gained because a fault in the other main, 
 at any point, at once blows the fuses and disconnects the faulty 
 section. 
 
272 ELECTRIC MOTIVE POWER. 
 
 With two-phase three-wire circuits it is permissible to earth 
 the middle wire at the generating station, which then is at 
 zero pressure. The same precaution is observed with the 
 "star" coupled three-phase system (see Fig. 155, p. 277). With 
 the " triangle " three-phase system it is not permissible to 
 earth either of the mains, since the earth would then form 
 part of the circuit. 
 
 It appears, then, that, from considerations affecting prime 
 cost of line, the three-phase system is cheaper for long-distance 
 transmission than either single-phase or double-phase ones ; 
 and that continuous-current systems are the cheapest of all, 
 but are not admissible for extra high pressures. 
 
 The selection of a system for long-distance transmission 
 of power will mainly depend upon the maximum permissible 
 pressure. If it be possible to work at such pressures as are 
 feasible, in the engineer's judgment, for continuous-current 
 dynamos (coupled in series or otherwise), then there can be 
 no doubt, from considerations affecting the line, that this 
 will be the cheapest and best system. But if extra high pres- 
 sure be deemed advisable, then the three-phase system will 
 generally prove the cheapest.* 
 
 In connection with this question it is important to notice 
 that the engineers responsible for the electric power plant 
 being laid down at Niagara have finally, after mature con- 
 sideration, decided upon a four-wire two-phase system. The 
 reasons for their choice have been given by Prof. George 
 Forbes, in a Paper entitled " The Electrical Transmission 
 of Power from Niagara Falls," read before the Institution of 
 Electrical Engineers on November 9, 1893. The details of 
 the scheme have been strongly criticised by English experts, 
 and the choice of two-phase currents is open to doubt. 
 When it is considered that Buffalo, the nearest large city, is 
 
 * The generating plant may perhaps be a two-phase one, with suitable 
 step-up two-to-three-phase transformers, as proposed by Mr. Scott, of the 
 Westinghouse Company. And it may also prove economical to transform 
 the phase again at the receiving station. The ingenious proposals of 
 Mr. Scott, however, have not yet been verified in practice ; but there seems 
 to be no doubt of their feasibility and usefulness (sec 56, p. 278). 
 
POLYPHASE TRANSFORMERS. 
 
 273 
 
 fifteen miles distant, it is clear that the weight of copper will 
 be a very important item in the cost of the whole plant, and, 
 in the author's judgment, a three-phase system would prove 
 more economical in first cost and give as good pressure regula- 
 tion as the two-phase. For it must be recollected that there 
 is a line difference of pressure of 20,000 volts, and hence step- 
 down transformers will have to be used at every point from 
 which power is taken. The main line drop will, therefore, be 
 as difficult to regulate with two-phase as with three-phase 
 currents ; and variation of pressure of submains of the distri- 
 buting systems can be as readily controlled by using indepen- 
 dent secondary circuits from three-phase transformers as from 
 independent secondary circuits of single or two-phase trans- 
 formers. Prof. Forbes's argument that two-phase currents are 
 
 I 
 
 I ( 
 
 ( f L 
 
 
 
 
 
 
 FIG. 149. Four- wire Two-phase Circuit with two Ordinary Transformers,. 
 Tj, To, one being Coupled to each of the Circuits, A, B. 
 
 more easily redressed than three-phase currents is probably 
 sound, but the fact that for large powers no form of alternate 
 current has yet been made unidirectional in a practical manner 
 cannot be ignored.* The use of a frequency of 25 c\> per 
 second is a departure from the best practice of to-day 
 (see 58, p. 284). 
 
 55. POLYPHASE-CURRENT TRANSFORMERS ; 
 DIAGRAM OF CONNECTIONS. 
 
 The principle of polyphase-current transformers is the same 
 as that of single-phase transformers. With two-phase currents 
 
 * See 60, p. 297. 
 
274 
 
 ELEGTEIG MOTIVE POWER. 
 
 it is usual to couple single-phase transformers across each of 
 the circuits, as shown in Fig. 149, in which there are four con- 
 ductors of equal section. If three were used, the middle one 
 having 1 f 4 times the area of each of the outer ones, the trans- 
 formers could still be coupled as above; but the pressure 
 
 FIG. 150. Dobrowolski Three-phase. Transformer. 
 
 variation would be greater than with the separate arrange- 
 ment, especially if they, or the conductors, or the machines, 
 possessed much capacity or self-induction. 
 
 'With three-phase currents it is usual to employ combined 
 transformers, as it is important to balance the load on the 
 
CONNECTIONS OF POLYPHASE TEANSFOEMEES. 275 
 
 three circuits, but three separate single-phase transformers 
 may be used. Various forms are in use, such as that shown 
 in Fig. 150, which shows the Dobrowolski type. One of the 
 methods of coupling the connections is given in Fig. 151. 
 
 This figure also illustrates one method of winding the primary 
 and secondary circuits. Owing to the phase differences between 
 the currents in the coils of the inter-connected circuit and the 
 pressure between the mains, there is a loss of pressure of r*:x>ut 
 13 per cent, in the ratio of conversion as compared with a 
 single-phase transformer. Hence the number of turns in the 
 
 FIG. 151. General Diagram showing one of the Methods of Winding the 
 Coils of a Three-phase Transformer. A, B, C : High-pressure mains and 
 coils. 1, 2, 3, 1', 2', 3': Low-pressure circuits, which may be combined or 
 separate. The phases are the same in opposite coils. The iron core 
 is indicated by the dotted lines. 
 
 primary circuit must be increased in this ratio. The efficiency 
 is, perhaps, not quite so high as that obtainable with the best 
 designed single-phase transformers, but it certainly exceeds 96 
 per cent, at full load, and the curve can be made to any slope 
 by suitably proportioning the weights of iron and copper as 
 already explained in 37, p. 205, when considering ordinary 
 transformers. 
 
 A three-phase transformer weighs less than three separate 
 single-phase transformers of an equal aggregate power, the 
 
276 
 
 ELECTRIC MOTIVE POWER. 
 
POLYPHASE CUREENT CIRCUITS. 
 
 277 
 
 ratio of weight being about 3 : 4. This increase of weight 
 efficiency is partly due to the magnetic flux dividing in a 
 similar manner to the current. The flux in one leg is in the 
 opposite direction to that in the other two. But the flow in 
 one of the two increases, while that in the other decreases. 
 The cycle is thus continually changing, each leg in succession 
 becoming the return for the magnetic flux in the other two. 
 
 FIG. 154. Diagram of Connections of a Three-phase Circuit. 
 
 A, Alternator. M, Motor. T l5 Step-Up Transformer. T 2 , Step-Down 
 
 Transformer. 
 
 The weight is also reduced to some extent, because the design 
 permits of the framing and yokes being made lighter in pro- 
 portion to the cores than is possible in the single-phase type. 
 
 In high-pressure transformers the high-pressure coils are 
 generally inter-connected, and the ends are usually protected 
 by glass tubes to a considerable height above the level of the 
 coils. The low-pressure coils have separate terminals to permit 
 of coupling up to separate circuits. 
 
 FIG. 155. Diagram of Connections of a Three-phase Circuit. 
 
 A, Alternator. M, Motor. T 1} Step-Up Transformer. T 2 , Step-Down 
 
 Transformer, e, Earth Plates. 
 
 Table Y, p. 285, gives some interesting data of Oerlikon 
 three-phase transformers, designed to work at line-pressures 
 up to about 5,000 volts. 
 
 Three-phase transformers work in parallel just as effectively 
 as single-phase ones. The general schemes for a transmission 
 plant with polyphase currents, with and without step-up 
 
278 ELUGTRIC MOTIVE POWER. 
 
 transformers, is shown in Figs. 152 and 153. The details 
 are clearly illustrated. In the high-pressure system only one 
 transformer is shown, but a bank of transformers might be 
 arranged if necessary. 
 
 Two methods of arranging the connections of three-phase 
 circuits with step-up and step-down transformers are given in 
 Figs, 154 and 155. 
 
 56. COMBINATION OF TWO- AND THREE-PHASE 
 CIRCUITS. 
 
 It has already been shown that three-phase currents present 
 advantages for transmission as distinguished from distribution, 
 and that two-phase currents are specially adapted for serving 
 sub-divided circuits. A combination of the two systems is 
 thus suggested and necessitates a phase conversion at the 
 
 A 
 
 B o c 
 
 FIG. 156. Two- to Three-phase Diagram. 
 
 distributing centre. Various inventors have worked at this 
 problem, and some have attempted to change the frequency as 
 well, for this is also of great importance in long-distance trans- 
 mission of power. A solution of the latter has not yet been 
 published in a practical form, although it is in the air. But 
 the former has been apparently solved by Mr. Chas. F. Scott,* 
 the chief electrician to the Westinghouse Company. The 
 method is simple and practical, and is an ingenious application 
 of the fact that two alternate pressures of different phase in 
 series with each other do not give a combined pressure equal 
 to the sum of the components. 
 
 Thus, if in Fig. 156, AO and OB severally represent in 
 phase and magnitude two pressures in quadrature to each 
 
 * Prof. S. P. Thompson has independently developed the same idea. 
 See British Association Paper entitled " Some Advantages of Alternate 
 Currents" (The Electrician, August 24, 1894, p. 481). 
 
PHASE CONVERSION. 279 
 
 other, and also in series, the resultant pressure will be repre- 
 sented in direction and magnitude by the hypothenuse A B of 
 the right-angled triangle A B. If A and B be so pro- 
 portioned that the angle ABO is GOdeg., the relationship 
 is evidently unaffected. The triangle A B, Fig. 156, is con- 
 structed to suit these conditions, and forms the half of the 
 equilateral triangle ABC, whose sides severally represent in 
 magnitude the t-hree pressures acting in a three-phase circuit. 
 Now, from the construction, and from what has already been 
 demonstrated in 52 and 53, it is evident that the pressures 
 represented by the lines A and C B may be combined to 
 give the two pressures represented by the lines AC and AB 
 or vice versa. 
 
 The three-phase diagram ACB may also be drawn as in 
 Fig. 157, in which A is the resultant of A C and A B. If the 
 
 B 
 FIG. 157. Two- to Three-phase Diagram. 
 
 ooils producing the pressures A and B C be supposed to be 
 each wound on separate ordinary type single-phase transformers, 
 and if there be two separate coils wound outside each of them, 
 ao and 6c, the arrangement may be diagrammatically 
 represented by Fig. 158. 
 
 Then a three-phase system may be supplied from the 
 terminals A, B and C, or a two-phase from those marked a, o, 
 and b, c. (In the diagram, the transformers may be imagined 
 to be feeding three-phase transmission mains at high pressure.) 
 The arrangement is simple and effective, involving no mechanical 
 details. It simply requires suitably-wound transformers at the 
 
 u2 
 
280 
 
 ELEGTEIC MOTIVE POWER. 
 
 phase conversion station, where the pressure may be raised or 
 lowered as required. 
 
 FIG. 158. Diagram of Two Single-phase Transformers combined to give 
 Two- and Three-phase Currents. 
 
 The number of turns in the coils can be adjusted to give 
 any pressure between the several mains ; but if the pressure 
 
 Two Phase- 
 
 < 100 V---> 
 
 wwv 
 
 b o 
 
 .-,00V-, 
 
 1000 V 
 Three Phase. 
 
 FIG. 159. Diagram of Pressure Distribution in Two- to Three-Phase 
 Transformers. 
 
 between the three mains of the three-phase part be of the same 
 value, as is usual, then the number of turns in the two-phase 
 
PHASE CONVERSION. 
 
 281 
 
 coils must be in the ratio of 2 : J3 ; the coil c b having, say, 
 200 turns, and the coil a o 173. This ratio is determined by 
 the phase difference of 30deg. between the line and coil pres- 
 sure (see 53, Fig. 137, p. 260). 
 
 The distribution of pressure will be seen from Fig. 159. 
 
 This system enables a three-phase line to feed three-phase 
 motors direct at the line pressure, two-phase motors through 
 transformers, and lighting circuits in two instead of three units. 
 These various operations are indicated in Fig. 160. If the 
 transformer which supplies direct from the terminals (C B, c b, 
 Fig. 159) be loaded and the other be on open circuit, a 
 single-phase current can be supplied at normal pressure, 
 independently of the other circuit. 
 
 3 Phase Line. 
 
 2 Phase 3 Phase 
 Transformers 
 
 AM AMA 
 
 olio 
 
 21 r* 
 
 2 Phase 
 Motor. 
 
 O] 
 
 3 Phase 
 Motor 
 
 FIG. 160. Diagram of Two-phase Alternators, Three-phase Line, Two- and 
 Three-phase Motors, and Lamps in Single-phase Circuits. 
 
 But if the transformer (A 0, a o) which supplies through 
 the three mains be loaded and the other be on open-circuit, 
 then a single-phase current can be supplied at a pressure equal 
 to only 87 per cent, of the normal. Two mains on one side 
 will be in parallel. The self-induction of the idle transformer 
 will not affect the circuit, for the current entering at the 
 middle of the winding divides equally in opposite directions 
 through the two halves of the coil, and thus completely 
 neutralises the self-induction. The ohmic resistance of the 
 winding has to be overcome, but this loss is^ more than com- 
 pensated by two of the mains being in parallel. 
 
282 ELECTRIC MOTIVE POWER. 
 
 These conditions of running are not likely to be used under 
 ordinary circumstances. Yet one of them may prove of great 
 advantage in the case of damage to part of the system for 
 instance, if one side of the alternator be damaged by lightning 
 or other causes. This feature may be made the means of 
 minimising accidents, especially if the alternators are of the 
 single-phase type coupled rigidly in quadrature. 
 
 The efficiency of two transformers arranged for converting 
 from two-phase to three-phase, or rice versa, is said to be about 
 Y^th per cent, less than when the same transformers are used 
 with single-phase currents at corresponding loads. 
 
 57. SYNCHRONOUS POLYPHASE MOTORS. 
 
 Polyphase alternators make excellent synchronous motors, 
 and are said to keep step better than single-phase machines, 
 the link between the rotating magnetic field and the revolving 
 part being more flexible. The line current is fed to the 
 revolving part of the motor, and the stationary part is con- 
 structed of solid cast iron, with or without windings. 
 
 If there be no windings on the stationary magnets the 
 torque is due to induced currents in the non-laminated 
 pole pieces. This design is not adapted for a high efficiency 
 at starting, and therefore it is usual in most cases to provide 
 secondary windings to assist the initial torque. This need not 
 involve any complicated device. 
 
 Synchronous polyphase motors are specially suitable for very 
 large units, since the bulk of the machine consists of cast iron. 
 They are therefore cheaper than induction polyphase motors of 
 the same capacity. And, what is still more important, they 
 cause no lagging current in the line except at starting, and 
 therefore have a power factor of 100 per cent. This type of 
 motor is likely to be used largely in the near future. Its 
 general appearance is indicated in Fig. 161. 
 
 A good example of large synchronous polyphase motors is 
 afforded by a plant erected early in 1894 at Taftville Cotton 
 Mills, Conn., U.S.A., by which power is transmitted for a dis- 
 tance of about 4J miles. 
 
SYNCHRONOUS POLYPHASE MOTORS. 
 
 283 
 
 There are two 250-kilowatt alternators, delivering power to 
 the line at a pressure of 2,500 volts. They run at 600 revolu- 
 tions per minute, and work perfectly in parallel. The motors 
 are of similar construction, and are separately excited by 
 3-kilowatt exciters driven by belting. They are coupled to 
 the main shafting by clutches, and are allowed to acquire the 
 
 FlG. 161. Diagram of Synchronous Polyphase Alternator, with 
 Windings on the Field Magnets. 
 
 speed of synchronism before the load is applied. In every 
 respect the plant is a great success. 
 
 58. PRESENT PRACTICE IN POLYPHASE 
 ALTERNATORS AND MOTORS. 
 
 Some of the most recent designs of polyphase alternators 
 and motors are shown in this section. They are typical of 
 present practice. 
 
 Polyphase generators for low pressure are frequently designed 
 to work at a pressure of 190 volts between the mains, and 
 
284 
 
 ELECTRIC MOTIVE POWER. 
 
 are used with step-up transformers. High-pressure generators 
 and motors which will work at line pressures of from 5,000 
 to 7,000 volts are generally used for power transmission when 
 the line pressure need not exceed, say, 7,000 volts. The fre- 
 quency adopted is from 50 to 65. The lower value is being 
 
 FIG. 162. Oerlikon Low-Pressure Polyphase Alternator. 
 
 gradually adopted on the Continent, and will soon become the 
 standard. This frequency is too low for the most economical 
 conditions, and 65 or 70 ^ would be better, especially for 
 the transformers, but the practical consideration of slow speed 
 with few poles in generators and motors determines the lower 
 value (see Table M, p. 174). 
 
POLYPHASE 
 
 285 
 
 FIG. 163. Oerlikon High-Pressure Polyphase Alternator, with Exciter 
 attached to the Shaft. 
 
 FIG. 165. Oerlikon Asynchronous Polyphase Motor. 
 

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 < 
 
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 a 
 
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 <z> 
 
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 k 
 
 Efficiency 
 Weight in Ibs 
 Primary n,732 v. between mains 
 amperes inl 3,464 ,, 
 each circuit (5,196 ,, 
 
 ^2 
 
 Is 
 
 O 
 | 
 
 i 
 gj 
 
 H^ 
 
 s 
 
 N 
 
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 Kilowatts 
 Amperes per circuit 
 Horse-power absorbed 
 Efficiency 
 Revolutions per minu te 
 ,.,,. f With exciter 
 Wei S htmtons (without exciter.. 
 
 1 
 
 1 
 
 Kilowatts 
 (1,732 v. betwe 
 Amperes per circuit { 3,464 ,, 
 (5,196 
 Horse-power absorbed 
 Efficiency 
 Revolutions per minute 
 Weight, with exciter, in tons 
 
 ^ 
 
POLYPHASE MACHINES. 
 
 287 
 
 In cases where power work alone has to be considered, and 
 the generators have to be coupled direct to slow-speed engines, 
 it is found convenient to decrease the frequency to even 25 or 
 20 Co per second ; but these are exceptional conditions. 
 
 The Oerlikon Company build excellent polyphase machines. 
 Their low-pressure alternator is shown in Fig. 162, and their 
 high-pressure alternator in Fig. 163. The performances of list 
 
 1,200 
 
 1,000 
 
 800 
 
 "5600 
 
 400 
 
 2CO 
 
 15 
 
 20 
 
 25 
 
 Oi 5 10 
 
 Amperes. 
 
 FIG. 164. Characteristic Curve of a 100-H.P. Three-Phase Oerlikon 
 Alternator. 
 
 machines of these types are given in Table Z and Table A A, 
 and a characteristic curve of a 66-kilowatt alternator, 1,040 
 volts per coil, is given in Fig. 164. The drop practically 
 follows Ohm's law, and capacity and self-induction effects, if 
 present, apparently cancel each other. 
 
 Types of the Oerlikon Company's normal three-phase motors 
 are shown in Figs. 165 and 166; and one of the Allgemeine 
 
288 
 
 ELECTRIC MOTIVE POWER. 
 
 Electricitats Gesellschaft in Fig. 167. Some tests of the small 
 motors are given in Table B B, p. 289. It is seen that the 
 efficiency, although not equal to that of first-rate continuous- 
 current motors of similar output, is sufficiently high to make 
 the machines of great commercial value, especially when the 
 
 FIG. 166. Oerlikoti Asynchronous Polyphase Motor. 
 
 absence of commutator and brushes and all the troubles and 
 risks incidental to them is borne in mind. The Oerlikon 
 Company adopt as standards a pressure of 110 volts per coil, 
 equal to 190 volts between the mains (series coupling, see 
 Fig. 137, p. 260), and a frequency of 50 <~u per second. 
 
TESTS OF POLYPHASE MOTORS. 
 
 289 
 
 Table B B. Tests of small Oerlikon Three-phase Motors, 50 ru. 
 
 
 
 
 Size of 
 
 motor. 
 
 
 
 
 H.P. 
 
 4H.P. 
 
 li H.P. 
 
 3 H.P. 
 
 6 H.P. 
 
 9 H.P. 
 
 Revs, per min., empty 
 Revs, per min., full load 
 Starting torque, in Ib. 
 feet 
 
 1,450 
 1,320 
 
 57 
 
 1,450 
 1,335 
 
 65 
 
 1,450 
 1,350 
 
 137 
 
 1,475 
 1,380 
 
 123 
 
 975 
 910 
 
 570 
 
 970 
 900 
 
 690 
 
 Torque at full load, in 
 Ib. feet 
 
 264 
 
 440 
 
 1 105 
 
 1 200 
 
 2700 
 
 4350 
 
 Pressure in volts 
 Amperes in each cir- 
 cuit, empty 
 
 60-5 
 1'3 
 
 60 
 
 4 
 
 57 
 7 
 
 63 
 
 7'5 
 
 61 
 20 
 
 67 
 25 
 
 Amperes in each cir- 
 cuit, full load 
 
 2'5 
 
 8 
 
 13'8 
 
 IS 
 
 59 
 
 86 
 
 Power absorbed at no- 
 load in watts 
 Power absorbed at full- 
 load in watts 
 
 105 
 325 
 
 670 
 
 346 
 1 900 
 
 404 
 2940 
 
 6620 
 
 10400 
 
 Output in brake H.P. 
 Output in watts 
 Efficiency 
 
 0-23 
 170 
 
 527 
 
 0-49 
 368 
 
 55 / 
 
 'l-5 
 1,140 
 60 7 
 
 2,208 
 
 757 
 
 7-1 
 5,300 
 807 
 
 11-5 
 8,600 
 83% 
 
 Weight of motor com- 
 plete in Ibs. 
 
 92 
 
 132 
 
 220 
 
 330 
 
 608 
 
 925 
 
 
 
 
 
 
 
 
 Some idea of the data of American three-phase motors may 
 be gathered from Table C C, which is taken from the Columbian 
 Exposition Supplement to the Engineering Review. 
 
 Table C C. Data of American Three-phase Motors. 
 
 
 4 H.P. 
 
 i H.P. 
 
 1 H.P. 
 
 5 H.P. 
 
 50H.P. 
 
 Apparent consumption in watts 
 Amperes ivr circuit loaded . . , 
 
 230 
 1-4 
 
 518 
 4-0 
 
 985 
 8-0 
 
 4,380 
 36 
 
 40,200 
 280 
 
 
 60 
 
 60 
 
 60 
 
 60 
 
 60 
 
 Frequency .. .. 
 
 50 
 
 50 
 
 50 
 
 50 
 
 50 
 
 Speed (unloaded) 
 
 2,380 
 
 1,490 
 
 1,490 
 
 1,490 
 
 745 
 
 Speed (loaded) 
 
 2300 
 
 1,400 
 
 1,375 
 
 1,395 
 
 725 
 
 Efficiency 
 
 
 71% 
 
 75% 
 
 84% 
 
 91% 
 
 dumber of poles . 
 
 2 
 
 4 
 
 4 
 
 4 
 
 8 
 
 "Weight in pounds 
 
 396 
 
 1,386 
 
 2,068 
 
 5,390 
 
 26,400 
 
 
 
 
 4-5 
 
 15 
 
 150 
 
 Amperes er circuit at starting ... 
 
 
 
 
 50 
 
 400 
 
 
 
 
 
 5,650 
 
 50,000 
 
 
 
 
 
 
 
 Size of motor. 
 
 
 
 v ;/ 
 
290 ELECTRIC MOTIVE POWER. 
 
 Messrs. Johnson and Phillips have built for use at the Sheba 
 Gold Mines some two-phase alternators which embody an in- 
 vention of Mr. Gisbert Kapps. The construction (see Fig. 168) 
 is similar in general appearance to that of Mr. C. E. L. Brown's 
 machines, the field magnets being of similar design. The 
 armature coils, however, which are, of course, arranged around 
 the outer and stationary part of the machine, are not symmetri- 
 cally placed with reference to each other, but the top half of 
 
 FlG. 167. Allgemeine Electricitiibs Gesellschafo Asynchronous Polyphase 
 
 Motor. 
 
 the coils is set with an angular lead of 90 degrees with respect 
 to the lower half. The two form separate circuits with cur- 
 rents in quadature, and can be used as distinct machines if 
 required. 
 
 One of the advantages claimed by the inventor for this 
 arrangement is that an accident to one of the circuits will 
 probably leave the other uninjured, and so prevent a complete 
 
KAPPS TWO PHASE ALTERNATOR. 
 
 291 
 
 breakdown of the machine. In fact, an alternator thus con- 
 structed is equivalent to two single-phase machines of half its 
 
 output. The design tends to decrease the cost of construction 
 and to economise space. 
 
292 
 
 ELECTRIC MOTIVE POWER. 
 
 59. WINDING; STARTING TORQUE ; POWER FACTOR. 
 
 One of the chief reasons for using polyphase currents is that 
 they admit of motors which are both self- exciting and self- 
 starting ; and in many cases the costly and troublesome brushes 
 and collecting rings may be done away with. 
 
 To obtain the best results it is necessary to reduce tht 
 exciting current to a minimum. This is accomplished by 
 
 FIG. 169. Grooves for Windings of FIG. 170. Grooves for Windings of 
 Polyphase Machines. Polyphase Machines. 
 
 T 
 
 FIG. 171. Diagram of Construction of Polyphase Motors, showing Holes 
 for the Windings. 
 
 decreasing the air gap to a mere mechanical clearance by 
 bedding in iron the windings on the stationary and rotating 
 parts. The coils are sometimes laid in grooves as in Figs. 169 
 and 170, and sometimes wound in tunnels as in Fig. 171. 
 
 Grooves are, perhaps, the best, since they cause less magnetic- 
 leakage. When the motor is designed without brushes, the 
 line current is supplied to the fixed part. This design is chiefly 
 
TORQUE OF POLYPHASE MOTORS. 293 
 
 applicable to small motors. For large designs it is usual to 
 arrange the exciting coils in the revolving part of the machine. 
 This necessitates brushes and rings, it is true, and consequently 
 simplicity of design is departed from. The chief point gained 
 is that the losses due to hysteresis, eddy-currents, and self- 
 induction are practically limited to the relatively small moving 
 part of the machine ; whilst the massive stationary portions 
 are magnetised by currents of frequency equal only to the 
 difference between the speed of the revolving coils and that of 
 the rotary magnetic field. 
 
 When large polyphase motors are designed to work with the 
 line current in the stationary part, it is usual to connect the 
 ends of the revolving coils to collector rings on the shaft, and 
 to close them through separate resistance coils. In this way 
 the speed and torque of the motor can be regulated with com- 
 paratively small waste of power. 
 
 In the most recent designs the resistance coils are built into 
 the rotating part and are thrown in and out of circuit by a 
 lever actuating a device similar to a friction clutch. It is 
 important to notice that a mere impedance coil will not 
 answer the required purpose, since its self-induction causes a 
 lag in the current, and therefore may increase rather than 
 diminish the starting current. A non-inductive resistance 
 coil must be used, the function of which is merely to regulate 
 the current induced in the closed coils. It is found that there 
 is one value of this current which corresponds to a minimum 
 line current for each torque, and therefore gives the motor a 
 maximum power-factor a point of immense importance when 
 a number of motors are running off a common power station. 
 
 By suitably proportioning the motor, the initial torque can 
 be made as large as required, but, if high efficiency and good 
 speed regulation are required it should not usually exceed 
 about three times the running torque. 
 
 The relation between torque and current is well denned in 
 Figs. 172 and 173, which are taken from a paper on Polyphase 
 Transmission* by Dr. Louis Bell. 
 
 * Electrical World (New York), March 17, 1894. 
 
294 
 
 ELECTRIC MOTIVE POWER. 
 
 In Fig. 172 is shown the relation between the static torque- 
 and current for a 10-H.P. (A) and for a 5-H.P. (B) three-phase 
 motor. 
 
 Curve Aj shows the effect of varying the resistance in tha 
 short circuited coils when the pressure is kept constant. 
 
 90 
 
 60 
 
 I s0 
 
 <u 40 
 
 6 
 
 20 
 
 Full Lo 
 
 / 
 
 1O 20 30 4O 50 6O 70 8O 
 
 Weight on Break Arm in Ibs. 
 
 FIG. 172. Curves showing Relation between Torque and Current in 
 Polyphase Motors. 
 
 Curve A 2 shows the same with variable pressure for a 
 constant resistance adapted to give a large torque. 
 
 The full load torque for the particular motor is 35 pounds on 
 the given brake. 
 
 Curves Bj and B 2 are similar curves for the smaller motor 
 whose full-load torque is 17'51b. 
 
TORQUE OF POLYPHASE MOTOR. 
 
 295 
 
 It will be noticed that each motor gives the running torque 
 at less than the full-load current, and that at maximum 
 current each gives roughly 50 per cent, more than the 
 designed full-load torque. There is no rush of current at 
 starting. 
 
 200 
 180 
 160 
 140 
 I2O 
 
 
 
 
 
 / 
 
 
 
 
 
 / 
 
 /B 
 
 
 
 
 / 
 
 / 
 
 
 
 
 / 
 
 / 
 
 
 
 
 
 / 
 
 
 
 
 / 
 
 5 IZO 
 
 c 
 
 a 
 
 |ioo 
 
 c 
 g 80 
 
 L. 
 3 
 
 o 
 
 0) 
 
 .E 60 
 _i 
 
 / 
 
 
 
 
 /k 
 
 / 
 
 
 
 
 / 
 
 
 
 
 
 / 
 
 
 
 
 > 
 
 / 
 
 
 
 
 
 20 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 20 40 60 80 IOO 12 
 Weight on Break Arm in Ibs 
 
 FIG. 173. Curve A: Relation between Current and Torque with a 
 Properly Adjusted Resistance in Series with the Short- Circuited Coils. 
 Curve B : The same, with no Resistance. 
 
 Fig. 173 shows two curves taken from a 10-H.P. motor. 
 They connect amperes in the line with torque. In B no 
 resistance is placed in the closed-coil circuits. In A a care- 
 fully adjusted resistance is added, with the result that the 
 current for a given torque is reduced to about one-half of that 
 
 x2 
 
293 ELECTRIC MOTIVE POWER. 
 
 required in B. The importance of a starting resistance is 
 obvious. 
 
 The initial torque of a well-designed continuous-current series- 
 wound motor is about six times the running torque ; and, 
 therefore, a polyphase motor requires to be, relatively, of about 
 twice the capacity for similar starting power; yet, since the 
 polyphase design is the more compact, there is practically no 
 
 90 
 
 C 
 
 80 
 
 70 
 
 50 
 
 10 
 
 FIG. 174. Curves showing Relation between Power Factor and Output 
 in Polyphase Motors. 
 
 difference in the floor space occupied. Experience undoubtedly 
 shows that the polyphase machine is the cheaper to build 
 and the less costly to maintain in good running order. The 
 power factor of polyphase motors varies with the load and 
 within wide limits ; but at from half to full load it appears 
 to average from 75 per cent, to 94 per cent. There is thus 
 
POLYPHASE AND CONTINUOUS CURRENTS. 297 
 
 no difficulty in running a number of motors upon the same 
 circuit. 
 
 The variation of the power factor of a four-pole 15-H.P. 
 motor at 50 cycles is shown in Fig. 174 by curve A; that of a 
 5 H.P. motor of similar design by B ; and that of a 5-H.P. 
 motor, specially designed to give a high power factor, by C. 
 
 It is sufficiently evident that the requirements of com- 
 mercial working are met in these machines. 
 
 60. COMBINATION OF POLYPHASE AND 
 CONTINUOUS CURRENTS; RECTIFIED CURRENTS. 
 
 It is sometimes urged that alternate currents are not 
 suitable for charging accumulators. In a sense this is true, 
 
 FIG. 175. Diagram of Gramme Armature, designed to work as a 
 
 Polyphase Continuous- Current Converter. 
 
 A, B, C, Polyphase Mains, a, 6, Continuous-Current Mains, coupled for 
 charging accumulators. The field magnet coils may be excited separately, 
 or in shunt to the accumulators. 
 
 but there are two ways in which they can be utilised for this 
 purpose. They possess some interest in a transmission of 
 power plant. Most batteries are useful for dealing with 
 lighting during the light load periods. 
 
 If an ordinary two-pole continuous-current dynamo have three 
 connections made to the armature coils at intervals of 1 20deg. 
 
298 
 
 ELECTRIC MOTIVE POWER. 
 
 each, and these be joined to rings in connection with the 
 mains of a three-phase system, the armature will run as a 
 motor (assuming the field to be separately excited), and a 
 continuous current may be collected at the commutator, which 
 can be utilised for charging accumulators. 
 
 FIG. 176. Rectified Alternate Current. 
 
 This machine may be described as a polyphase continuous- 
 current converter. It is a practical device. Most dynamos can 
 be readily adapted for this purpose, but if a high efficiency 
 of conversion be required it will be necessary to make a special 
 design. A general idea of the arrangement may be gathered 
 from the diagrammatic sketch in Fig. 175. 
 
 FIG. 177. Ferranti Rectifier, as used at Portsmouth. 
 
 Another method of using an alternate current for charging 
 batteries is to " rectify " the negative waves. The resultant 
 current may be diagrammatically represented as in Fig. 176, 
 in which the dotted curves below the time line have been 
 
EECTIFIEES. 299 
 
 rectified. The curves show the instantaneous values of the 
 current (or pressure), and the effective value is J- of that 
 of the maximum ordinate. 
 
 The reversal of sign is accomplished by a two-part commu- 
 tator running synchronously with the alternator, and driven 
 by a small synchronous alternate-current motor. (See the 
 'Thomson-Houston self-exciting alternator. Fig. 115, p. 232.) 
 
 Fig. 177 shows one of the Ferranti rectifiers used at Ports- 
 mouth for running arc lamps in series on a 10-ampere circuit 
 having a frequency of 50 r ^> per second. 
 
 A rectified current appears to be admirably adapted for 
 serving arc lamps, because the rhythmical waves of which it is 
 composed assist the feeding of the carbons. 
 
 It is possible that rectifiers may prove to be useful details of 
 a power plant. 
 
CHAPTER IX. 
 
 ELECTEIC TRANSMISSION OF TOWER IN MINING 
 OPERATIONS. 
 
 61. Introduction. 62. Engine House and Equipments. 63. Shaft 
 Cables. 61. Underground Cables ; Junction Boxes ; Switches 
 and Cut-outs. 65. Uninsulated llcturns ; Safety Cables. 66. 
 Motors and Driven Machines ; Selection of Type of Motor ; Con- 
 tinuous and Polyphase Currents. 67. Safety Motors ; The Mining 
 Motor of the Future. 
 
 61. INTRODUCTION. 
 
 PERHAPS the most obvious field for the utilisation of electricity 
 in power work is to be found in mining. At all events, the 
 first practical plants were applied for mining purposes, and by 
 far the greater number of large electric motors running in Great 
 Britain to-day are used in this connection. The superiority of 
 electricity as compared with compressed air or hydraulic power 
 for driving dip pumps and other underground machinery was 
 recognised as soon as the electric motor was discovered. But 
 it was not until about 1885 that the new plant was well enough 
 engineered to be cheap and reliable. The first application of 
 sufficient magnitude to demonstrate indisputably the possi- 
 bilities of the electric motor for mining work was made at Messrs. 
 Locke and Go's, St. John's Colliery, Normanton, by the General 
 Electric Power and Traction Company (Immisch and Co.), to 
 the designs of the author. The motor was of about 60 brake 
 horse power, and worked a set of ram pumps raising 120 
 gallons of water per minute through a vertical head of nearly 
 900 feet. It was an assured success from the start, and 
 although modifications and improvements have since been 
 introduced in many of the details, the general method still 
 
302 ELECTRIC MOTIVE POWER. 
 
 obtains for continuous-current mining work. The applica- 
 tion of polyphase currents has opened up fresh possibilities, 
 and largely extended the scope of electric power work, by 
 obviating the need for commutators and brushes, and there- 
 fore removing the chief objection to the use of motors in 
 collieries. The author confidently regards polyphase motors 
 as essentially the machines of the future for mining work. 
 
 In the previous chapters this subject has been necessarily 
 discussed in a general way, so as to include all varieties of 
 plants. This chapter is confined to the consideration of mining 
 work and its special requirements. 
 
 There is a wide difference between collieries and metalli- 
 ferous mines in the character of the work ; and, indeed, the 
 local conditions of different mines vary so largely as to require 
 special treatment in almost every case. But the main 
 difficulties peculiar to mining work, i.e., which do not usually 
 obtain with surface plants, may be classified under three 
 heads : 
 
 (a) "Falls" from roofs and sides, and "creeping" of the 
 floors. 
 
 (6) Water, either continuous or intermittent, 
 (c) Explosive gases. 
 
 These may occur singly or conjointly, and evidently require 
 properly selected plant and careful disposition of mains, junc- 
 tion boxes, switches, cut-outs and machines. What is suc- 
 cessful at one pit, or part of a pit, is not necessarily so 
 at other places ; and hence various methods are employed 
 some good and some indifferent. Only an engineer experi- 
 enced in this class of work is able to specify with fair 
 probability of success the best kind of material and most 
 suitable method of erecting plant for each case. Much 
 defective work has been done from want of practical 
 acquaintance with collieries and mines, although the plant 
 supplied has been usually good of its kind. That bugbear of 
 competition, the lowest tender, has also much to answer for 
 here as well as in other departments of electrical engineering; 
 
ELECTRIC WORK IN MINES. 303 
 
 and the practice of asking contractors to tender to their own 
 specification instead of to that of an independent consultant 
 has largely contributed to the breakdowns, which are far more 
 frequent than need be. The absurdity of such a course is 
 rendered the more apparent when it is considered that however 
 up-to-date the mining engineer may be, he cannot possibly 
 have such a close acquaintance with the quality and classes of 
 electric cables, different points in dynamos and motors, and the 
 various details of an electric plant, as the expert who gives his 
 entire time to such work. And, moreover, the expert is not 
 hampered by patents or special types of machines, etc., as a 
 contractor is very likely to be, especially if he is also a 
 manufacturer. 
 
 It is proposed to consider here some of the chief difficulties 
 peculiar to mining work, and to examine the methods most in 
 vogue to meet them, and also to suggest, as far as possible, the 
 best plant and its most effective disposition for working under 
 various conditions. 
 
 It is convenient to treat this part of the subject under the 
 following divisions : 
 
 (1.) Engine House and Equipment. 
 
 (2.) Conductors, Shaft, and Underground Cables, Junction 
 Boxes, Switches and Fuses. 
 
 (3.) Motors and Driven Machines. 
 
 These will be considered in separate sections. 
 
 62. ENGINE HOUSE AND EQUIPMENT. 
 
 The engine-house will generally be upon the "surface," 
 although in some cases, where a steam-engine is already at work 
 near the pit bottom, it may be necessary to erect a dynamo 
 below ground. Electric plant frequently plays so small a part 
 in the main machinery of a colliery that it is not permissible to 
 provide a separate house for the dynamo. In such cases the 
 winding-engine house or the fan-engine house will probably be 
 selected. If the fan-engine has a margin of power, and is kept 
 
304 ELECTRIC MOTIVE POWER. 
 
 running at a constant speed, it may be possible to couple the 
 dynamo to it through a fast and loose pulley. But this is not 
 a common occurrence, nor is the arrangement always advisable. 
 The fan forms a steady continuous load, and its engine should 
 be carefully proportioned, both as regards size and speed, to give 
 the most efficient running. And further, since the safe work- 
 ing of a colliery depends to a very large extent upon the ventila- 
 tion, many mine managers very properly refuse to allow fan- 
 engines to be put to any other work than that of driving the fan. 
 
 It may be assumed, then, that the dynamo (or dynamos) will 
 be driven by an independent engine. In the early days of 
 electric work in mines it was frequently necessary to use any 
 odd engine that happened to be available at the time; and 
 hence some very curious combinations. The author has seen 
 a modern efficient dynamo coupled by belting to a large slow- 
 speed engine of antiquated type, rendered still more inefficient 
 by being supplied with wet steam at less than 401b. pressure, 
 so that the high efficiency of the dynamo was practically 
 annulled. 
 
 Managers, however, are beginning to recognise the saving in 
 coal and steam made by using high-pressure boilers and quick- 
 running engines, and the electrical engineer has now little 
 difficulty in arranging for suitable steam plant. Having regard 
 to the fact that the fuel burnt at collieries is usually the most 
 unsaleable there, and frequently almost dust, mechanical 
 stokers are of special use. Vicars, Proctors, or any of the 
 well-known types, answer the purpose admirably, and will be 
 found to effect economy in labour. The type of boilers will 
 depend upon the class of water and upon local circumstances. 
 Generally, however, Lancashire boilers, with cross tubes, give 
 the best all-round results, and have the advantage of being 
 easily set and repaired by ordinary labour. The working 
 pressure may vary from SOlb. to 1201b. per square inch; 
 whereas multitubular boilers, although admirable for raising 
 steam at short notice, are comparatively difficult to clean out, 
 and require much more careful attention and skilled labour to 
 effect repairs. The same remark applies to water tube boilers 
 in connection with this class of work. 
 
STEAM DYNAMOS. 
 
 305 
 
 In most cases where the engineer has a free hand, 
 and the pressure does not exceed, say, 500 volts, he will 
 advise steam dynamos i.e., engine and dynamo directly 
 coupled (see Fig. 178). This arrangement gives a positive drive, 
 reduces the space occupied to the smallest dimensions, effects 
 considerable saving in first cost, and gives increased facility for 
 repairs ; while a stand-by set can be compactly placed ready 
 for instant use at a moderate increase in the cost of the 
 running plant. Granting the use of a high-speed engine, the 
 question of open or closed type arises. There are various argu- 
 ments in favour of each. The open type (see Fig. 178) has the 
 advantage that all the rods and bearings are in full view, and 
 
 FIG. 178. Steam Dynamo and Open-Type Engine. 
 
 can be inspected while the engine is running, and also that it is 
 readily got at to make repairs. But it is likely to be affected 
 by dust, and is to some extent liable to accidental damage from 
 external causes. 
 
 The closed type (see Fig. 179), on the contrary, is completely 
 protected from dust and chance external damage, and the crank 
 shaft splashes into a lubricant in the crank chamber, thus 
 ensuring lubrication so long as the oil is maintained at the 
 proper level. Messrs. Willans and Robinson supply with their 
 well-known closed-type engines a sight gauge, which shows 
 continuously the height of the lubricant in the chamber. The 
 speed of the engine will be selected with reference to its 
 
306 ELECTRIC MOTIVE POWER. 
 
 dimensions and that of the dynamo to which it is to be 
 coupled. The number of revolutions will be large compared 
 with those to which mining managers are accustomed, being 
 from about 300 to 500 per minute. The question of single'or 
 double-acting engines is an open one, and both sides are admir- 
 ably championed. Perhaps both types, if properly made, are 
 equally good. 
 
 The type of dynamo will depend very much upon the class 
 of work. The discussion of the various windings compound, 
 series, shunt and separate excitation in 14, page 59, gives 
 the special features of each. 
 
 FIG. 179. Holmes-Willans Steam Dynamo and Closed-Type Engine. 
 
 The pressure, as already suggested, will not be much higher 
 than about 500 volts. A number of manufacturers build 
 excellent machines, and it is easy to select a suitable dynamo 
 or motor if a fair price be paid. 
 
 The conditions governing the choice of steam and electric 
 plant are similar to those detailed in 1, page 7. In plants where 
 the work comprises pumping, hauling, coal-cutting and lighting, 
 the generators should be capable of parallel working, feeding 
 omnibus mains. But if there are only large motors, each 
 coupled to, say, pumps or other independent machines, then 
 it may be advisable to run separate pairs of mains to each 
 
SHAFT CABLES. 307 
 
 motor, and to have a separate generator to each circuit. If 
 the loads permit of one type of motor and one type of gene* 
 rator, a couple of spare machines will be sufficient to cover 
 all. For driving pumps series winding is usually preferable 
 for both dynamos and motors if a continuous-current system 
 be adopted. 
 
 If polyphase currents be used, the same suggestions are 
 applicable, with suitable reservations. The pressure at the 
 generating station may be restricted to about 500 volts, and then 
 transformers will not be necessary for stationary motors of from 
 10 B.H.P. and upwards. But in coal-cutters and rock-drillers 
 this pressure is too high for the men to handle the machines- 
 with comfort, and it will be advisable to use step-down trans- 
 formers to reduce it to about 50 volts. At this pressure a 
 " shock " will be impossible, and the machines can be safely 
 worked even in the dampest places. 
 
 In some cases of long-distance transmission it may be advis- 
 able to use a much higher pressure in the " line " between the 
 generating station and the step-down transformers. The ques- 
 tion then arises whether the line pressure should be reduced 
 to, say, 500 volts at the pit bank, be supplied at this pressure to 
 large stationary motors, and be further reduced for small or port- 
 able machines. These are questions, however, which must be 
 decided for each case upon its merits, and a mine manager will 
 do well to take the opinion of an independent expert before 
 signing a contract for plant. 
 
 63. SHAFT CABLES. 
 
 The classes of conductors suitable for the severe conditions- 
 of mining work are not very numerous, and differ chiefly in the 
 kind of insulation, and the means employed to protect them 
 from mechanical injury and from the effects of water, 
 
 Broadly, there are two kinds of insulation which have stood 
 the test of time, viz., vulcanised rubber and bituminous com- 
 pounds.* The former is costly and is thus better adapted for 
 the lighter classes of cables, say up to strands of 19/15 S.W.G., 
 
 These are fully discussed in 18, p. 84. 
 
308 ELECTRIC MOTIVE POWER. 
 
 which, owing to their position, do not require to be covered 
 with a coating of lead or armoured with steel strip or galvanised 
 iron wire. The latter class is cheaper and hence is more suitable 
 for large trunk mains ; but, since it rapidly loses its insulating 
 properties if in contact with water, it is necessary to encase 
 the compound with lead, which should be armoured to protect 
 it from injury. The best form of bituminous cable is made 
 concentric (see Fig. 180). The centre conductor is doubly 
 guarded, so that if the metal sheathing be cut through, only 
 the outer conductor is earthed, and the cable as a whole is 
 workable until an opportunity occurs for repairing the injury. 
 
 FIG. 180. Armoured Concentric Cable, with Lead-Covered Bituminous 
 
 Insulation 
 
 The author finds it expedient, even in large plants, to limit 
 the sizeof each conductor to, say,a cable of 19/14 S.W.G. strands; 
 and, if this be of insufficient cross-section for the current, to 
 duplicate the conductors and connect them to omnibus bars. 
 With this provision, an accident to one conductor does not 
 cause a complete stoppage, for the damaged cable can be 
 disconnected until the fault is localised and repaired. (It is 
 oftentimes a matter of impossibility to make repairs during 
 the drawing of coal, and hence the importance of providing 
 spare conductors.) Another advantage is that small mains 
 are relatively light, and are, therefore, easily handled and 
 
SHAFT CABLES. 309 
 
 fixed in position. And, further, the cost of repairs to the 
 smaller sizes of cables is generally much less than to the larger 
 ones, owing to the greater ease with which the joints can be 
 made. 
 
 The methods of running cables down shafts and fixing them 
 in position differ in almost every case. Pit shafts are usually 
 of circular form, sometimes driven through the live rock, some- 
 times built round at intervals with courses of brick or stone laid 
 on cast-iron cribs, sometimes faced with heavy timber walls, 
 and occasionally lined with cast-iron sections through quick- 
 sands or strata heavily charged with water. If wooden 
 " conductors " are used to guide the cages, heavy cross bearers 
 of timber are built into the sides of the shaft at intervals to 
 carry them. When iron rope guides are used the shafts are 
 practically void of timbering. 
 
 These different conditions evidently demand different 
 methods of supporting the cables. 
 
 In the first place, it is absolutely essential to guard against 
 falling coal and stones, either from the cages or from the sides 
 of the pit. The best position for placing the cables will, 
 therefore, be largely determined by the shape of the cage, and 
 whether its sides are covered up so as to restrict the falling of 
 pieces of coal to the open ends. Again, stands of iron pipes 
 already placed may limit space still further. In general it is not 
 advisable to run electric cables near to pipes, because work- 
 men repairing the latter may damage the cables. Another con- 
 sideration of importance is whether the up-cast or down-cast 
 pit is the better adapted for the purpose. If the " winding " 
 be done from the down-cast, as is usual, it may be convenient 
 to use the up-cast. But care must be taken that the up-cast 
 air is not laden with any erosive vapour, such as is sometimes 
 the case when the exhaust from an underground engine and 
 the gases from a furnace burning coal containing a large per- 
 centage of sulphur, are turned into the up-cast. The author 
 knows of one case in which a pair of lead-covered cables 
 run in such a pit were corroded through in dozens of places in 
 the course of a few months. In some soils there are found 
 erosive agents that attack iron and rapidly rust it away, and 
 
310 ELECTRIC MOTIVE POWER. 
 
 in others a similar action takes place with lead. Generally 
 it has been the author's practice to use the down-cast shaft ; 
 but there is no rule, and in all cases it is possible to ensure 
 success by making suitable arrangements. 
 
 The plans adopted for supporting cables in shafts differ 
 widely. In all cases, however, where it is desired to give the 
 work a permanent character, the cables must be entirely 
 encased in wood, run in iron pipes, or heavily armoured. These 
 three methods are not always equally applicable, for local con- 
 ditions may make one of them the cheapest or most desirable. 
 If there is a set of, say, 4in. pipes already erected in a shaft, 
 and it is desired to run a pair of, say, 19/16 S.W.G. cables 
 down them, and the depth is not great, a pair of vulcanised 
 rubber mains may be suspended in them from earthenware 
 insulators. The weight on the supports will not usually be 
 great, for the cable will generally bend from side to side, and 
 thus may sometimes nearly carry its own weight if sufficient 
 slack be allowed. In running cables down pipes it is advisable 
 to insert a small flexible iron rope, and use this to haul in the 
 insulated conductor, which should be payed out from the drum, 
 perpendicularly into the centre of the pipes, over a wheel of 
 sufficient radius. This method prevents damage to the covering 
 of the cable. The interior of the pipes should be scoured to 
 clean off sand from the cores, or the insulation will inevitably 
 be abraded during the hauling-in. 
 
 If the pipes be of large diameter, and the depth too great 
 for the mechanical strength of the cable, it must be sup- 
 ported at intervals. This may be done in several ways, but 
 it will be necessary to break the continuity of the iron pipes 
 in order to insert wedges, clamps, or similar devices. 
 
 It may be better, then, to adopt a wooden casing which will 
 support the cable along its whole length. In using wood, how- 
 ever, care must be taken that the section is of sufficient strength, 
 and that the " lengths " are spiked firmly to the pit sides. The 
 grooves may be cut in the face, as shown in Fig. 181; or in 
 the sides of the casing, as in Fig. 182, which illustrates a method 
 used most successfully by the author in a number of mines. 
 
SHAFT CABLES. 
 
 311 
 
 The covering boards may be held in position by coach screws. 
 This enables an examination of the cables to be made at any 
 spot which is oftentimes a convenience. The grooves should 
 be cut an easy fit for the cable, which will then be perfectly 
 
 K 10 
 
 FIG. 181. Wood Casing for Pit Work. 
 
 supported, and only a light attachment at the top will be 
 necessary. It is always advisable, however, to loop the top 
 ends securely round earthenware insulators firmly carried at 
 or near to the surface. 
 
 FIG. 182. Wood Casing for Pit Work. 
 
 Concentric cables lead-sheathed and armoured are so stiff 
 that they may be clamped directly to the pit sides, and then 
 form strong compact work. As they are very heavy, there is 
 need for strong brake gear on the surface to pay them out by, 
 
 Y2 
 
312 
 
 ELECTRIC MOTIVE POWEE. 
 
 as they cannot be fastened in position until the whole length 
 is down the pit. It is advisable with very heavy cables to 
 lash ropes at intervals, and pay them out at the same rate, 
 thus relieving the top end of the cable of part of the weight. 
 This class of cable is steadily finding favour for pit work. 
 
 The Electrician, 
 Sectional Elevation. One-third full size. 
 
 Plan (part section). One-third full size. 
 FIG. 183. J. Davis and Son's Junction Box, for use in Mines. 
 
 On no account should joints be permitted in new shaft 
 cables, although it may be necessary to make repairs in old 
 ones. A cable with continuous insulation is much more likely 
 to stand the wet and exposure of a pit than one with joints 
 made with a different dielectric from the rest of the insulation; 
 
UNDERGROUND CABLES. 313 
 
 the more so as the joints will probably be defective from the 
 beginning. And it should be recollected that jointing is an 
 extremely difficult operation in a shaft, owing to falling water, 
 the cramped position, and the strong draught of air which 
 together render soldering an impossibility in many cases, and 
 often make the insulation uncertain owing to the presence of 
 moisture. Of course it is possible, assuming time to be allowed, 
 to provide special apparatus which, in the hands of skilled men, 
 will enable excellent work to be done ; but breakdowns usually 
 occur unexpectedly, and frequently have to be made good by 
 the resident staff with such simple materials and facilities as 
 may be at hand. 
 
 In most cases the author prefers to arrange junction boxes at 
 the top and the bottom of shafts, so that the shaft cables can 
 be severally tested independently of the surface and underground 
 work. These boxes must be housed in dry places, secure from 
 mechanical injury. For this purpose there should be sufficient 
 free length of cable at top and bottom to allow the boxes to be 
 placed well away from the drawing decks and pit bottom. An 
 effective junction box manufactured by Messrs. J. Davis and Son 
 is shown in Fig. 183. 
 
 64. UNDERGROUND CABLES; JUNCTION BOXES; 
 SWITCHES AND CUT-OUTS. 
 
 Each " district " is usually supplied from an independent pair 
 of conductors coupled at the pit bottom to an omnibus bar or to 
 a separate pair of shaft cables. 
 
 These conductors are, therefore, of small size, not often ex- 
 ceeding 19 strands of No. 16 S.W.G. They are carried on the 
 "sides" or "roofs," being supported by wooden cleats or 
 earthenware insulators, or buried in the " floor." The choice 
 of the three methods will be determined by the peculiarities 
 of the "level" in which they are run. The class of insu- 
 lation will depend both upon the method of erection and the 
 difficulties to be dealt with. 
 
 In dry roads, well supplied with timber props and cross 
 bearers, a light vulcanised rubber cable, carried in wooden cleats 
 
314 ELECTRIC MOTIVE POWER. 
 
 or on insulators, will prove to be cheap and efficient. An occa- 
 sional fall of roof or sides will not as a rule cut it if a little 
 slack be allowed between the supports ; and " creeping " of the 
 floor will not affect it. Indeed, in the majority of cases where 
 long lengths of small conductors, about 7/16 S.W.G., have to 
 be carried to the " faces," this is the only practicable method, 
 from the simple consideration of first cost. 
 
 Wood casing is out of the question for " road-work," except 
 for the lighting circuit near the pit bottom, not only because 
 of its cost, but also because of the instability of most of the 
 " roads." 
 
 In main engine planes which have no " weight " upon them, 
 permanent work may be made of the erection ; and it will 
 generally pay to bury the cables beside the metals and to 
 cover them loosely. But a rough wooden trench with a cover- 
 ing board should be laid to receive the cables, unless they are 
 armoured, for otherwise an unlucky blow from a pick may cause 
 a breakdown. If heavily armoured concentric cables, as shown 
 in Fig. 179, p. 308, are used, it is sufficient to bury them from 
 about Gin. to 12in. below the surface of the road, well to one 
 side, where they will not be disturbed during repairs to the 
 metals and roads. If conductors are carried along "travelling" 
 roads, it is specially necessary to secure them from damage 
 at the hands of the miners, who are sometimes inclined to get 
 " lightning " from them by means of their picks. If the road be 
 dry and in a fairly settled condition cables may be laid in fine coal 
 dust, covered by rough boards lin. thick, and then protected by 
 a layer of coal dust of about 6in. deep. This forms a very safe 
 and cheap bed for hemp-braided vulcanised rubber cables, or 
 for lead-covered cables served or braided with hemp. It is 
 specially serviceable in travelling roads, or where the con- 
 ductors are laid for a temporary purpose. 
 
 Whatever the class of cable or the method of laying it, how- 
 ever, the conductor should not be in continuous lengths of more 
 than about 500 yards, in order that a fault may be readily local- 
 ised. At the junctions of sections cast-iron boxes (see Fig. 183, 
 p. 312) should be placed, with screw terminals on porcelain bases, 
 through which the necessary connections can be quickly made or 
 
UNDEEGEOUND CABLES. 315 
 
 unmade. On no account should cut-outs be placed at these 
 points unless special provision be made to enclose them in flame- 
 tight boxes ; they should be confined as far as possible to the 
 generating station and pit bottom. 
 
 At the far ends of the levels where the distributing mains 
 are joined to the feeders, it is good practice to place cast-iron 
 junction boxes in the "gates," so that any or all of the 
 distributors may be coupled up according to requirements. It 
 is imperative, if cut-outs be used here, as is sometimes 
 necessary, that they be placed in flame-tight boxes, for this 
 position is close to the coal faces, from which gas may be given 
 off at any time. 
 
 The "distributors " are necessarily of a temporary character 
 and will be shifted as the work progresses. They must therefore 
 be light and flexible. If convenient they may be carried upon the 
 props, and in the absence of timber may be laid beside the roads, 
 unless they are hung on rough iron spikes driven into the rock. 
 In any case they will often be subjected to rough usage. For 
 attachment to movable machines, such as coal cutters and 
 rock drills, it is usual to provide either concentric or twin cables, 
 insulated with vulcanised rubber and braided with steel wires. 
 Either of these can be made sufficiently flexible, and they are also 
 strong enough to stand being hauled across rough shale and 
 coal. 
 
 If a distributor feed a dip pump it should be erected in a 
 semi-permanent manner ; since such pumps are liable to be 
 drowned-out, and the plant may have to be withdrawn several 
 hundred feet or yards at short notice. In such cases the con- 
 ductors should be brought back with the motor, and not cut. 
 They should be coiled and hung up at the pumping station, 
 being uncoiled again as the water recedes. The best insulation 
 for this work is vulcanised rubber well braided with hemp and 
 ozokerited over all. 
 
 For a polyphase current system it is necessary to run three 
 or four conductors in place of the two required for a continuous- 
 current or single-phase alternate system. For permanent work 
 three conductors may be built into a concentric cable, which 
 
316 
 
 ELECTRIC MOTIVE POWER. 
 
 may be lead-covered and armoured. With a four-wire system 
 two concentric cables of two conductors each are preferable. 
 
 The method of erection is in no respect different from that 
 adopted with continuous-current working. The three wires are 
 carried practically as readily as two. 
 
FLAME-TIGHT SWITCHES, ETC. 
 
 317 
 
 The use of switches and fuses below ground in places likely to 
 contain explosive gas requires careful regulation, and is likely to 
 become the subject of special legislation in the future. The 
 
 author states emphatically that, in his opinion, no switch, cut- 
 out, junction box, or resistance frame that is not absolutely flame 
 and dust-tight should be allowed in a coal mine where naked 
 
318 
 
 ELECTRIC MOTIVE POWER. 
 
 lights are prohibited. It is madness to forbid the use of a naked 
 light and yet to allow the types of switches and'cut-outs some- 
 times found in pits. 
 
 To ensure absolute freedom from firing gas with a continuous- 
 current motor is perhaps impossible, but nojsuch difficulty 
 should be found with small stationary devices, such as switches, 
 cut-outs, and junction boxes. It is merely a matter of expense, 
 and proper precautions should be insisted upon in all permanent 
 plants. 
 
 (D 
 
 
 CD 
 
 [CD 
 
 
 CD 
 
 CD 
 
 
 CD 
 
 
 SLATE 
 
 
 
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 CD 
 
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 CD 
 
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 CD 
 
 FIG. 184B. Showing Plan of Slate Base and Attachments to the Resistance 
 Coils in Figs. 184 and 184A. 
 
 In Figs. 184, 184A and 184s are shown various views of Davis's 
 Mining Switch with resistance coils attached. It is used for 
 starting and regulating motors. The whole is enclosed in an 
 iron case, and the switch contact pieces break circuit in a gas- 
 tight compartment. 
 
 65. UNINSULATED RETURNS; SAFETY CABLES. 
 
 It has been the custom at some pits to use old iron ropes 
 as returns. These are simply laid along the roads or hung up 
 by hooks to the side props. The practice, however, is not to be 
 recommended in pits where there is any chance of gas, and in 
 a large plant, with a pressure of 500 volts, it would in any case 
 be impossible. The author coupled a return in this manner at 
 Andrew's House Pit, Durham, in 1887, the arrangement being 
 
UNINSULATED RETURNS. 319 
 
 adopted simply from considerations of first cost. The pressure 
 at the dynamo terminals averaged 225 volts, and the po\\ er in 
 this part of the circuit never exceeded 8 E.H.P. The rope lay 
 for a considerable part of its total length of 1*25 miles in 
 water. The leakage of current, inevitable in a pit, decomposed 
 water at various places, and the iron corroded rapidly at the 
 junction to the copper return near the motor, and small 
 sparks could often be obtained between the rope and the tram 
 metals near the pit bottom, thus showing that the iron rope, 
 although lying on the wet ground, did not make good " earth." 
 The danger was recognised and caused no difficulty ; but if the 
 pit had been gaseous, it is probable that an explosion would 
 have resulted. 
 
 A bare separate return, if justifiable at all, can only be so on 
 account of cheapness ; but the apparent saving is in most cases 
 more than counterbalanced by after troubles. 
 
 If, however, a system of concentric mains be used with the 
 outer conductor in metallic contact with iron or steel wire 
 armouring,* the conditions are altogether different ; and prob- 
 ably as great an immunity from the danger due to a broken 
 conductor can be obtained by this arrangement as by any 
 of the patented safety cables. 
 
 There are two classes of bare return concentric cables in use. 
 They are shown in Figs. 185 and 186. The former has a lead 
 covering, which is squirted around a copper conductor, the two 
 together forming the outer or return circuit. Armouring can 
 be applied outside, and this adds to the conductivity while pro- 
 tecting the soft lead, whose chief function is to keep water 
 from the insulation between the centre and outer conductors. 
 
 The second class is cheaper, and the return circuit is formed 
 simply by the armouring, which is placed directly upon the 
 insulation. It is only suitable for use in dry places, where it 
 will give good results. 
 
 The chief advantages offered by these cables, in con- 
 nection with mining work, is the security they give against 
 
 * For example, the concentric cable system of Messrs. Mavor and Coulson. 
 
320 ELECTRIC MOTIVE POWER. 
 
 external sparking, and their mechanical strength, owing to 
 which they may be stapled to rough walls, like gas piping. 
 
 There are also conductors which may be collectively classed 
 as " safety cables." In all of them the object is entirely to 
 
 FIG. 185. 
 
 Concentric Cable with uninsulated outer conductor consisting of a spiral of 
 sheet copper under the lead, the lead, and the steel armouring. 
 
 obviate sparking at the point of rupture when an active con- 
 ductor is cut in two. The general method is to provide a 
 small auxiliary cable in parallel to the main conductor. This, 
 J n the event of a fall cutting the cables, is arranged to break 
 
 FIG. 186. 
 
 Concentric Cable with uninsulated outer conductor, consisting of a spiral 
 of sheet copper under the steel armouring no lead. 
 
 either before or after the main conductor is broken; and in each 
 case actuates a magnetic cut-out which opens the main circuit. 
 The idea will be best understood by reference to some of the 
 better known solutions of the safety-cable problem. 
 
SAFETY CABLES. 321 
 
 Mr. LI. Atkinson, of Messrs. Easton, Anderson and Goolden, 
 was the first to patent a cable of this class. It was not 
 designed for concentric wiring, and hence two wires are 
 required for each circuit. The cable (see Fig. 187) consists of 
 two concentric conductors in parallel, insulated from each other, 
 excepting at the main terminals of the circuit to be protected. 
 The current therefore is in the same direction in each conductor. 
 The outer, which is designed to carry the larger part of the 
 current, is of any ordinary make. The inner is constructed 
 of fine wire in the form of a continuous cylindrical coil, the 
 outer diameter of which is covered by insulation, and forms 
 the core around which the main conductor is laid. It is 
 assumed that when a stone falls on the cable the outer con- 
 
 FIG. 187. Atkinson Safety Mining Cable, showing the outer conductor 
 cut through and the inner one drawn out. 
 
 ductor, offering much resistance, may be cut through; but the 
 loosely-coiled centre conductor will be simply drawn out of 
 the insulation. The entire current will then pass through the 
 fine wire, and blow a magnetic cut-out in the engine-room or 
 other convenient place; the arc being thus transferred from 
 a possibly dangerous spot to a safe one. The efficient action 
 of this cable depends upon two conditions : 
 
 (1.) Perfect insulation between the inner and outer con- 
 ductors. 
 
 (2.) The continuity of the fine wire circuit being preserved 
 during the fall. 
 
322 ELECTRIC MOTIVE POWER. 
 
 In order to prevent an accidental contact between the two 
 conductors interfering with the working of the cable, the in- 
 ventor provides a resistance in series with the fine wire at 
 its negative end, so that the fall of pressure is less along the 
 inside than along the outside. The result is that there is 
 always a difference of pressure between the inner and outer 
 conductors at each part of the cable. And a short circuit 
 between the two at any point causes an increase of current 
 in the fine wire which operates the magnetic cut-out. 
 
 Generally it is found that cables hung lightly, with plenty 
 of slack, are dragged down and buried without suffering any 
 damage other than slight surface abrasion ; but if they are 
 firmly fixed they may be cut through as if with a knife. 
 
 It, may, therefore be reasonably questioned whether a fall which 
 is sufficiently heavy to cut a stout cable in two will not sever 
 the fine conductor before the spirals have time to draw out. 
 
 In recent installations a small separated insulated wire has 
 been hung (beside the main conductor) with a few spirals 
 twisted at intervals so as to allow a ready increase of length in 
 the event of a fall or of any undue strain. This is found to 
 be cheaper than the compound cable, and to give equally good 
 results. It should be mentioned that Mr. Atkinson regards 
 the cable as of more importance as a preventive against firing 
 timber from arcs than as an absolute means of preventing 
 sparks. The author is in accord with this view. For the time 
 taken to move the detaching mechanism on the surface will 
 chiefly determine the spark at the break in the cable, though 
 its effect is limited by the resistance of the unbroken fine wire 
 in parallel with the main wire. 
 
 The Charleton cable, patented by Messrs. R. J. Charleton, of 
 Newcastle-on-Tyne, is of similar make, but the inner conductor 
 is merely a small wire, about No. 18 S.W.G. The currents in 
 the two conductors are in the same direction. The action 
 depends upon the fine wire breaking before the large one, when 
 a magnetic cut-out is designed to open the main circuit. Suc- 
 cessful working depends upon the maintenance of the insula- 
 tion between the two circuits, and also upon the time required 
 
SAFETY CABLES. 323 
 
 to move the magnetic cut-out levers. If this is not consider- 
 ably less than the period of time elapsing between the cut- 
 ting of the two cables it is doubtful whether sparks will not 
 occur almost simultaneously at the break and the cut-out. The 
 problem is complicated by self-induction and capacity, which 
 under suitable conditions may seriously affect the time required 
 to open the cut-outs after the breakage of the main conductor. 
 It will be easy, however, to experiment for any particular case. 
 
 FIG. 188. Sectional View of Met Safety Cable. 
 A, Armouring ; C, Main Cable ; C 1 , Auxiliary Conductor. 
 
 Another interesting safety cable is the invention of M. Nolet, 
 engineer of the Cockerill Company, Seraing, in conjunction 
 with M. Jasper, of Liege, and has been applied to several coal 
 mines in Belgium. It provides for a small wire insulated 
 and built into the cable, as in the Charleton cable, but the 
 currents in the two circuits are in opposite directions. 
 
 FIG. 189. Diagram of Connections of Nolet Safety Cable. 
 C, Main Cable ; Ci, Auxiliary Conductor ; E and E 1} Magnetic Cut-outs ; 
 M, Motors. Large arrows show direction of main current ; small arrows 
 that of the shunt current. 
 
 A sectional view showing the method of construction is given in 
 Fig. 188. 
 
 The fine wire is coupled up as shown in Fig. 189. The 
 function of the small wire circuit is to close the main circuit 
 through the two magnetic cut-outs (one to each of the mains). 
 
324 ELECTRIC MOTIVE POWER. 
 
 Therefore, in the event of a breaking of the fine wire, the cut- 
 out will operate and open the principal circuit. 
 
 M. Nolet has recognised that repairs to cables, as generally 
 constructed, cannot be properly carried out without the use of 
 soldering irons, which are inadmissible in a mine. To pro- 
 vide for this contingency his cable is made in short lengths, 
 each section being provided with metal couplings for both 
 of the conductors, as shown in Fig. 190. The rings for the 
 subsidiary wire are shown at C', and those for the main con- 
 ductor at C. The couplings are made to fit together with 
 easy friction, and are so arranged that a sliding movement of 
 about one quarter of an inch is sufficient to open the auxiliary 
 circuit, while a considerable displacement is required to un- 
 couple the outer conductor. The joints are protected by 
 india-rubber tubes. 
 
 The construction appears to admit of a certain time interval 
 between the opening of the main circuit and the lifting of the 
 
 FIG. 190. Sectional View of Nolet Safety Cable, showing the Method of 
 Coupling-up. 
 
 magnetic cut-out, and also provides a ready means of repairing 
 a damaged section. But the cable must be costly, and a slow 
 movement of the measures, throwing weight on the cable, 
 might interrupt the auxiliary circuit unnecessarily. It may 
 be questioned whether the device would have time to act in 
 the event of a fall shearing the cable instantaneously, as some- 
 times occurs. 
 
 In the author's judgment, none of these safety conductors 
 gives more security than that obtainable with an armoured con- 
 centric one buried in the floor, or suspended loosely with plenty 
 of slack between the supports. If a fall occurs, it will be 
 brought down and buried. If the armour and outer conductor 
 are cut through, the incision must first take plaee on one side, 
 and then most probably the severed strands of the outer 
 conductor will be pressed against the inner conductor, and so 
 
SAFETY CABLES. 325 
 
 blow the magnetic cut-out without opening the circuit; 
 because the bottom half of the outer conductor cannot be 
 cut through until the top half is brought into contact with the 
 inner one. The truth of this can be easily demonstrated by 
 cutting the cable through with a cold chisel and hammer. 
 
 Success lies in hanging the cable so loosely that it cannot be 
 cut through instantly. If this occur, it is doubtful whether the 
 two conductors would always be jammed into good contact by 
 a sudden blow, and so a spark might occur coincidently with 
 the blowing of the magnetic cut-out. 
 
 Another solution of the safety cable problem has been pro- 
 posed by Mr. Frederick Hurd. The conductors are run in iron 
 pipes, through which a current of fresh air is forced by a blower, 
 the motors and regulating devices being protected in a similar 
 manner. The plan is good, and, if properly carried out, would 
 seem to afford safety against a large variety of risks. But there 
 is no provision for opening the electric circuit when the pipes 
 are broken. 
 
 Messrs. R. B. Pownall & Son have carried the method a step 
 farther. In a patent dated December 2, 1893, they provide 
 means by which the maintenance of the electric circuit is made 
 to depend directly upon the air pressure in the pipes ; so that, 
 in the event of damage to the pipes, resulting in a leak or 
 in total breakage, the escape of air lowers the plenum and 
 opens the magnetic cut-out controlling the particular circuit. 
 The details of the system have not yet been made public ; but 
 the author is in a position to assert confidently that the scheme 
 is a very practical one, and meets the varied requirements of 
 colliery work in a most complete manner. 
 
 The danger of firing gas through a broken cable is, how- 
 ever, very remote. There is, so far as the author knows, no 
 authentic instance of this having occurred, although the possi- 
 bility is sufficiently evident. 
 
 The fact is that, for obvious reasons, cables are not usually 
 run in roads with "weight" on them, and, where this is un- 
 avoidable, sufficient slack is allowed to prevent the nuisance 
 
326 
 
SELECTION OF TYPE OF MOTOR. 327 
 
 of frequent repairs, quite irrespective of the risk of an explo- 
 sion, and, therefore, broken cables are rare. 
 
 With the gradual extension of electric work in coal mines for 
 driving portable as well as stationary machines, the risks from 
 this cause will be increased, and it is wise to recognise the danger 
 and to adopt the simple and efficacious remedies suggested by 
 experience. This is specially desirable with new plants. 
 
 66. MOTORS AND DRIVEN MACHINES; SELECTION 
 OF TYPE OF MOTOR; CONTINUOUS AND POLY- 
 PHASE CURRENTS. 
 
 The selection of the type of motor for any special work is 
 generally not difficult. An approximate estimate of the power 
 is easily arrived at, and a small error in its determination is 
 of little importance; the crux lies in estimating the average 
 and maximum torque. The cogency of this will be apparent 
 on reference to the equations for torque of motor (11, p. 43), 
 and to the mechanical characteristics of motors ( 14, p. 59). 
 
 For example : Suppose it is required to drive a main pump, 
 as shown in Fig. 191, on a continuous-current circuit at 500 volts 
 pressure at the motor terminals. The theoretical power in the 
 water is known, and the losses in rising main, pumps, gearing 
 and motor, are easily determined. The maximum torque will 
 occur at starting, when the motor has to overcome the statical 
 friction of the pumps and set the column of water in motion. 
 The average torque will be simply that due to the constant 
 load with full delivery of water. To meet these conditions, it 
 is necessary to have a motor with a magnetic field sufficiently 
 strong to give the necessary starting torque without undue 
 sparking or an excessive quantity of current ; and to give the 
 working load at the designed speed with the highest possible 
 efficiency consistent with a fair margin of power. 
 
 The motor winding should, preferably, be in series (see 14, 
 p. 60 ; also 23, pp. 1 16 and 121). 
 
 The chief constructional point is sufficient strength in the 
 armature shaft and bearings to start the pumps safely. If this 
 
( 328 ) 
 
ELECTRIC HAULAGE. 329 
 
 be attended to the motor will be mechanically strong enough 
 for the running load. 
 
 Again, assume it is required to erect an endless rope hauling 
 plant as illustrated in Fig. 192 on the same circuit. The 
 various losses must first be determined, the weight of tubs, 
 loaded and unloaded, debited and credited according to the 
 grades, and a suitable allowance made for tub-wheel and 
 rope friction. From these data the starting and the average 
 torque can be determined. If the running load is fairly 
 constant as is generally the case with an endless rope the 
 series-wound motor is the most suitable, and the suggestions 
 made with reference to the main pumping motor are equally 
 applicable. The margin of power, however, requires to be 
 greater than with pumps, as the load may be largely 
 augmented at times by tubs leaving the rails. There is not 
 much fear of overloading during busy periods, because an 
 increase of torque will slow the motor and so decrease the 
 actual power ; but, since the current will be temporarily in- 
 creased, a margin for the heating effect in the windings must 
 be provided. 
 
 If, however, the system of haulage be a main-and-tail one, 
 as indicated in Fig. 193, where the motor has to be frequently 
 stopped, and to deal with a continually varying load, good 
 speed regulation and perfect control of the motor become of 
 more importance than efficiency. In this case a specially 
 designed cumulative compound motor (see 14, p. 65) may be 
 used, with a switch and starting resistance designed to keep 
 the shunt coils always in circuit during work time, and to 
 introduce a variable resistance in series with the armature 
 and series windings. Reversals of motion should be made by 
 means of friction clutches and gearing, and not by the motor, as 
 this would introduce complications which are better avoided in 
 pit work. The maximum torque in this case is generally that 
 at starting, but not necessarily, as the load may start on the 
 level and afterwards have to climb a steep grade. 
 
 The best arrangement for working main-and-tail rope systems 
 is to run the motor continuously, and to give forward or back- 
 ward motion to the rope drums through friction clutches 
 
( 330 
 
0> 
 
 I 
 
 a 
 H 
 
 ( 331 ) 
 
332 ELECTRIC MOTIVE POWER. 
 
 mounted on an intermediate shaft. If the motor be designed 
 in accordance with the above suggestions, no trouble will be 
 experienced. 
 
 The preceding remarks are also generally applicable to 
 single-rope haulage, although in some cases, where the load is 
 small and the pressure low, a series motor with a large starting 
 resistance may prove satisfactory. A suitable arrangement for 
 single rope haulage or winding is shown in Fig. 194. 
 
 For main winding gear, which is worked practically con- 
 tinuously during a shift, the author prefers a cumulative 
 compound motor having a large regulating resistance, the 
 motor running in one direction only, and the reversals being 
 made through friction clutches as already described for main- 
 and-tail hauling. But if the winding be used only occa- 
 sionally, as frequently occurs with staple-pits, then a series 
 motor with a large regulating resistance is preferable. The 
 time occupied in raising the load is so short that the question 
 of speed regulation does not enter into the problem. What is 
 required is a large power at starting, gradually yet quickly 
 applied, and perfect control over the cage at all parts of the 
 run, but especially when stopping. It is also necessary to be 
 able to raise or lower the cage a few inches. For such work 
 the author has found series motors give excellent results. 
 He finds it convenient to couple the brake lever to the friction 
 clutch, so that when the brake is applied the circuit is opened, 
 and the motor is then ready for raising or lowering as required. 
 
 Dip pumps should invariably be driven by series-wound 
 motors* if continuous currents be used. 
 
 Several typical designs are shown in Figs. 195 and 195A. 
 The first is a view of the Goolden-Atkinson pump, which has 
 been largely used for draining dip areas. The second design 
 is that of the pump made by the Jeffrey Company in America. 
 It is permanently mounted on a trolley with removable wheels, 
 and is a light and efficient pump. 
 
 * See remarks on main pumps, p. 326. 
 
( 333 ) 
 
334 
 
 ELECTRIC MOTIVE POWER. 
 
 Centrifugal pumps have generally a limited use in mines on 
 account of their requiring to be " primed " before they will suck 
 water, and also because there is always a risk of the pum}< 
 ceasing to act if air gets into the suction pipe ; in fact, they 
 require frequent attention. These pumps are not suitable for 
 
 working against higher heads than 28ft. or 30ft. of water unless 
 special precautions are provided for starting. They are suit- 
 able for a low head and a large volume, and will be found useful 
 in cases where the water is too " dirty " for a force pump. A 
 usual arrangement is shown in Fig. 196. 
 
CENTRIFUGAL AND ROTARY PUMPS. 
 
 335 
 
 Fig. 196A refers to an American double rotary force pump, 
 having a capacity of from 50 gallons to 75 gallons per minute. 
 The design forms a very compact combination of pump and 
 motor, which can be easily wheeled about a pit by one man. The 
 
 I 
 
 cu 
 
 C c 
 .~ bo 
 
 II 
 
 H 
 
 a 
 
 ^ 
 
 rotary type of pump doea not find much favour with English 
 mining engineers. It has the disadvantage of requiring to be 
 primed like a centrifugal pump, and is not adapted for working 
 with dirty water. It has, however, practically no slip, and can 
 be worked at varying speeds without seriously affecting the 
 
336 
 
 ELECTRIC MOTIVE POWER. 
 
 efficiency. Both centrifugal and rotary pumps give the best 
 results for dip working when the level of the suction pipe is so 
 arranged as to charge the pump. 
 
 Fans, either main or auxiliary, should also be driven by 
 series motors, if run off a continuous-current circuit. The 
 electric motor is peculiarly adapted for running fans, and it is 
 somewhat surprising that this has not been recognised more 
 fully. The author designed a 25-H P. main fan plant for a tin 
 
 FIG. 196A. Jeffrey Double Rotary Dip Pump, mounted on a Trolley. 
 
 mine at Eger, in Bohemia, in 1889, the dynamo being driven by 
 water power at a distance of about half a mile from the motor. 
 
 Stamp batteries and ore crushers should be driven by shunt 
 or compound-wound motors, if continuous-currents are used. 
 
 Coal cutters and rock drills have hitherto, with few exceptions, 
 been run on continuous-current circuits. In order to avoid 
 gracing " when running light, both differential compound and 
 
POLYPHASE CUREENTS IN MINING. 337 
 
 shunt windings have been tried with indifferent success com- 
 parative regularity of speed has been gained at the expense of 
 torque. After numerous experiments, the author has found 
 series-wound motors to give the best results for driving coal 
 cutters and drills. He prefers to control the speed by an 
 external resistance. Other workers in this field have arrived at 
 the same conclusions. But the success attained has not been 
 sufficient to warrant the hope that coal will be cut largely in the 
 future by machinery driven by continuous-current motors. 
 
 The future appears to lie with the polyphase systems. The 
 polyphase motor, having neither collector nor brushes, obviates 
 the danger from sparking at the brushes a defect always 
 present with continuous-current motors. And, since the speed 
 can never exceed that of synchronism with the generator, racing 
 is impossible. The regulation of polyphase currents can 
 generally be accomplished in a small space by means of 
 impedance coils without any possibility of sparking, for no part 
 of the circuit need be opened or shunted (see Impedance Coil, 
 p. '241), as is necessary with a continuous-current resistance 
 regulator. (See 59, p. 293, for method of obtaining a large 
 starting torque.) 
 
 The sole point on which continuous-current motors are 
 superior to polyphase ones, and then only with series winding, 
 is as regards the starting torque. From this point of view 
 they are undoubtedly the best and most efficient motors that 
 can be built. The three-phase motor at starting is roughly 
 comparable to a shunt motor ; it requires a starting resistance. 
 (See Tables BB and CC, p. 289 ; also curves in Figs. 172 and 
 173, p. 294.) 
 
 It is always possible, however, to start a motor before 
 coupling it to the load if the initial torque be too great for the 
 maximum motive effort. With drilling machines the drill car 
 be slackened if necessary, and with coal-cutters the initial 
 torque of the motor can be made sufficiently great to start the 
 cutter against the friction of the debris, which is all that is 
 necessary. A " fall " should be cleared away by hand. 
 
 In driving large pumps and hauling gear, it will generally 
 be advisable to use friction clutches, through which the load 
 
338 ELECTRIC MOTIVE POWER. 
 
 can be gradually applied after the motor has acquired it 
 proper speed. The clutch does not entail extra expense in 
 most cases, since its use is generally advisable, and often abso- 
 lutely necessary even with continuous-current motors. 
 
 .67. SAFETY MOTORS ; THE MINING MOTOR OF THE 
 
 FUTURE. 
 
 Attempts have been made to design a continuous-current 
 motor perfectly free from the risk of firing gas from 
 sparks at the commutator or from short circuits in the 
 coils. It is admitted on all sides that so long as brushes 
 and commutators (sliding contacts) are used it is impossible to 
 work without sparks, which, under certain conditions, may fire 
 explosive gas. It is, indeed, found experimentally that the 
 small blue sparks occasionally seen on even the best continuous- 
 -current motors are able to fire gas if the metal of the com- 
 mutator becomes heated, as it will at times. It is, therefore, 
 necessary to confine the explosion area to as small a space as 
 convenient, in order to limit the effects and prevent communi- 
 cation to the outer air. 
 
 The first attempts were made in the direction of enclosing 
 the entire motor in a metal case. It was soon found, however, 
 that the quantity of air enclosed was too great to be safely 
 tired, and this method has been abandoned by the majority of 
 motor builders. 
 
 Messrs. Mavor and Coulson have, however, devised a com- 
 pletely enclosed motor (see Fig. 197) of admirable mechanical 
 design, which is by far the best illustration of this class. 
 Whether it may be classed among the " Safety " motors is 
 perhaps a matter for experiment ; but there can be no doubt 
 about the security it affords from accidental damage from any 
 outside cause, and it is thoroughly dust-proof. 
 
 The next obvious course was to enclose only the armature, 
 commutator, and brushes. By suitable arrangements this can 
 be done in a very effective manner, and the space so enclosed 
 can be made small enough to limit an explosion to the protected 
 area. But other difficulties are introduced. The design of the 
 
STEEL-CLAD MOTOR. 
 
 339 
 
340 
 
 ELECTRIC MOTIVE POWER. 
 
 brush gear is complicated, and the adjustment of the brushes 
 is rendered exceedingly difficult, especially to unskilled hands. 
 The author has built motors for coal cutting in this manner, 
 and Messrs. Easton, Anderson and Goolden have patented 
 various devices with a similar object. 
 
 In Fig. 198 is shown a perspective view of the excellent 
 safety mining motor made by Messrs. Easton, Anderson and 
 
SAFETY MOTORS. 
 
 341 
 
 Goolden. The general arrangement is easily understood. The 
 armature, commutator, and brushes are encased in a metal 
 compartment, the free space of which is small. The makers 
 say that an explosion of gas within it cannot be communicated 
 to the outer atmosphere. The author has had no opportunity 
 of testing this important statement, but from his knowledge of 
 the motor it appears to be correct, always assuming that the 
 
 FIG. 199. Sectional view of Davis and Stokes' Inverted Commutator as 
 used on their Safety Mining Motor. 
 
 casing is in good condition and the inspection doors properly 
 fastened. (See remarks on this subject on page 342.) 
 
 Messrs. Davis and Stokes, of Derby, have also invented 
 a " safety motor," a sectional view of part of which is 
 shown in Fig. 199. It will be seen that the commutator is 
 inverted, and the brushes placed inside of it, instead of on 
 the outer perimeter, as is usual. The brush arms are attached 
 
342 ELECTRIC MOTIVE POWER. 
 
 to a movable ring, sliding upon the bearing parallel to the 
 shaft, and arranged to lock in the proper running position 
 through a screw collar. The brushes are thus placed in a 
 small circular space inside the commutator, the cubic con- 
 tents of which in small motors is about the same as that of 
 a safety lamp ; and which, even in large machines, can always 
 be made sufficiently small to render an explosion within it 
 quite harmless. It should be noticed that the brushes can only 
 be got at by the attendant when the motor is standing, and } 
 therefore, when there is no chance of danger. Motors of this 
 make have been tested in an explosive mixture of air and gas 
 by Mr. John Rhodes, at Aid war k Main Colliery, who found 
 that, although the gas continually flashed inside the com- 
 mutator, it did not communicate flame to the outside. This 
 is, perhaps, the safest form of continuous-current motor yet 
 invented, and has been successfully used for the past four years 
 in. connection with a variety of underground work. 
 
 It is questionable, however, whether all these designs are 
 not chiefly important as dust protectors. (It is absolutely 
 necessary that a motor worked on a coal face should be dust- 
 proof.) Though the cases may fit well when new, they probably 
 soon get loose and defective from the rough usage inseparable 
 from the class of work, and thus become useless as safety 
 devices ; indeed, they may then be a positive source of danger 
 from the apparent security attached to them. 
 
 The only real safeguard against explosions from sparking at 
 brushes and commutators (sliding contacts) is to do away with 
 them altogether, and the only way in which this is practicable 
 at present is by the use of polyphase motors. The mechanical 
 simplicity and excellent speed regulation of these machines 
 have already been urged. These advantages, in combination 
 with their safety, must gradually tell. They will necessarily 
 take the place of all other types of motors for driving coal 
 cutters, rock drills, and other machines which have to be 
 driven in a dusty atmosphere liable at any moment to be 
 charged with gas. 
 
 In the author's judgment the mining motor of the future 
 will probably present the appearance of a circular iron box. 
 
MINING MOTOR OF THE FUTURE. 343 
 
 The revolving part will have no electrical connection with the 
 supply mains, and will resemble an iron cylinder with closed 
 ends and steel shaft. A breakdown of the insulation inside 
 the cylinder will simply affect the efficiency ; it will not cause 
 sparking. The windings in connection with the mains will be 
 few in number, laid in recesses, and be entirely protected from 
 external damage by means of the outer framework. The 
 conductors will be of the concentric type ; and in permanent 
 plants it is probable that the Pownall Safety Mining System, 
 or some development of it, will be largely adopted. The 
 junction boxes, switch, cut-outs, and regulating devices will be 
 all flame-tight, and be protected by oil or by air under pressure. 
 An explosion from an inadvertent spark will then be a more 
 remote contingency than one from a safety lamp. 
 
 AA2 
 
CHAPTER X. 
 
 COAL-CUTTING AND ROCK-DRILLING BY ELECTRIC 
 
 POWER. 
 
 68. Introductory. 69. Rotary Bar Coal-Cutters : Goolden- Atkinson, 
 Jeffrey, &c. 70. Rotary Side Wheel Coal-Cutters : Gillott and 
 Copley, Yorkshire Engine Co., Snell-Waterhouse Coal-Cutters, &c. 
 71. Electric Power Drills : Rotary and Percussive Types ; Jeffrey, 
 Marvin, and Van Depoele Drill. 
 
 68. INTRODUCTORY. 
 
 THERE has always been a certain fascination about the appli- 
 cation of electric power to coal-cutting and rock-drilling. It 
 is easy to appreciate this feeling when one has had some 
 experience of coal-getting. Indeed, it has been felt for a long 
 time that a mechanical means of hewing coal is desirable 
 from all points of view. Yet, strange to say, the problem 
 has presented so many difficulties that its successful solution 
 on a large scale still belongs to the future, although coal has 
 been got by mechanical means in isolated cases for a long time 
 past. 
 
 The difficulties are well known, and may be regarded under 
 two heads, each of considerable importance. The first refers 
 to the mechanical side of the question, and hence properly 
 belongs to the present discussion. The second relates to the 
 system of working and the general management of mines, and 
 cannot be treated here at length. But reference must be made 
 to some of the obstacles thrown in the way of mechanical coal- 
 getting by the men and the masters. The use of a coal-cutting 
 machine may seem to be simply a question of the price of coal 
 
346 ELECTRIC MOTIVE POWER. 
 
 hewn by it in comparison with the cost of hand-hewn coal; 
 and this is undoubtedly the case. But the cost of machine- 
 won coal is at present necessarily based on figures resulting 
 from the use of one or two machines, instead of on figures from 
 a dozen worked on " faces " specially laid out for the purpose. 
 It should be noted that the cost of the prime power for two 
 or three machines is practically as great as that required for a 
 dozen, and that the system of mains need not be proportionately 
 increased for the larger number, because the power does not 
 'ncrease in direct proportion to the number of machines, as it 
 is not possible to keep them all running at the same time. It 
 is important, too, that machinery should be run as nearly con- 
 tinuously as possible, which suggests the employment of several 
 shifts of men. And this is only practicable, even if the men's 
 union permit it, when the conditions are such that the coal 
 can be carried away sufficiently quickly. In the majority of 
 faces, as arranged to-day, this is impossible, and a machine 
 cannot be kept at work continuously for even a shift of eight 
 hours ; so that, from causes which are quite independent of the 
 working capabilities of the machine, its theoretical output is 
 never reached. This fact points to the necessity for a coal- 
 cutter of smaller capacity than those now in use say a machine 
 which would hole from 5 to 10 yards an hour, working under 
 normal conditions and allowing the necessary time for clearing 
 away the coal. Hitherto the aim has been rather to produce 
 machines which, under favourable circumstances, will cut from 
 20 to 40 yards of face per hour. Bat a very casual inspection 
 of a coal mine will demonstrate that such a rate cannot be 
 maintained for any length of time, simply from the difficulties 
 of clearing away through gates two chains apart, even neglect- 
 ing for the moment the difficulty of keeping the face clear 
 from falls of roof during the time necessary for removing the 
 coal. 
 
 If mechanical coal-getting is to be carried out on a large 
 scale with the long-wall system, it will be necessary to modify 
 the number and position of the gates in order to increase the 
 facilities for clearing away. The question belongs to mining 
 managers, and not to electrical engineers ; and this is, perhaps, 
 one of the chief reasons why mechanically-driven coal-cutters 
 
, ELECTRIC COAL-CUTTING. 347 
 
 have not made as much progress as their undoubted utility 
 appears to warrant. A colliery is being opened out (October, 
 1894) near Eckington, in which the whole of the work will be 
 done electrically, even to the driving of the fan. The coal will 
 be worked on the pillar and stall system by Jeffrey coal-cutters. 
 This undertaking is due to American enterprise, and will be 
 watched with the highest interest. 
 
 It does not lie in the province of the present work to discuss 
 the question of machines versus hand coal-getting. But it may 
 be remarked that one of the best electric coal-cutters, with two 
 men to work it, will easily hole as much as from 20 to 30 
 hewers; thus largely reducing the number of men required 
 below ground. And since the number of accidents necessarily 
 bears some direct proportion to the number of men employed, 
 the introduction of machinery should have the effect of largely 
 reducing the loss of life. 
 
 The number of mechanical principles applied to coal-getting 
 machines is astonishing, and the ingenuity displayed by many 
 inventors, who evidently have not been in close touch with the 
 principles of mining, is equally surprising. All conceivable 
 methods of cutting, drilling, boring, shearing, and picking coal 
 have been designed, on paper at least, and many of them have 
 been applied to more or less practicable machines. The earliest 
 attempts were made in the direction of a mechanical pick. As 
 might be expected, the pick with the man at the back of it 
 directing each blow did much the better work, and this method 
 was soon dropped. The drill and the circular saw have given 
 the most successful models upon which to work out a cutter, 
 hence a variety of machines embodying these principles. 
 
 The machines which have survived the initial stages and 
 give some hope for future success are divisible into three main 
 
 classes : 
 
 (a) Bar cutters. 
 
 (6) Rotary wheel or disc cutters. 
 
 (c) Drills. 
 
348 ELECTEIG MOTIVE POWER. 
 
 69. ROTARY BAR COAL-CUTTERS: GOOLDEN- 
 ATKINSON, JEFFREY, ETC. 
 
 Under the first head may be included all those machines 
 which have a cutter-bar revolving either at right angles or 
 parallel to the face of the coal. This type has so far proved 
 the most successful in this country and in America, the best 
 known being the Goolden-Atkinson bar cutter of English inven- 
 tion, and the Jeffrey machine of American origin. 
 
 The essential feature of the Goolden-Atkiuson machine is a 
 tapered rotary bar, which is fitted with a series of cutters 
 driven into tapered holes. They are arranged in a spiral form, 
 the object being to clear out the cuttings and prevent the 
 jamming of the bar. The shape, size, and general arrange- 
 ment are the result of long and careful experiment by Messrs. 
 Llewellyn and Claude Atkinson, the staff of Messrs. Easton, 
 Anderson and Goolden, and the Electrical Coal Cutting Cor- 
 poration. The main idea is shown in Figs. 200 and 200A. 
 
 The cutter-bar (see Fig. 201) is usually geared to the armature 
 shaft by a pair of cast steel double helical wheels running in 
 oil, and makes from 300 to 500 revolutions per minute, 
 according to the material being cut. The motors are always 
 wound in series, and hence the speed varies inversely as the 
 resistance ; which is exactly the best condition for work 
 of this kind, since it prevents undue shocks to the motor, 
 and tends to keep the power absorbed within safe limits. It 
 will be seen that the motor is thoroughly protected from 
 mechanical injury either from falls of roof or sides, and that 
 the armature and commutator are fitted into a dust-tight 
 and flame-tight compartment of dimensions as small as are 
 consistent with the safe and effective handling of the brushes. 
 The cutting mechanism as a whole is made to swing through 
 an arc of rather more than 90deg. on the turntable which 
 carries it (see Fig. 20lA). This gives facilities for shifting 
 the machine from point to point in the mine, and also enables 
 the bar to cut its way into the coal until it is at right angles 
 to the face and ready for holing. Fig. 201 A gives a view of the 
 machine as arranged for work. In the early machines of this 
 
GOOLDEN-ATKINSON ELECTEIC COAL-CUTTER. 349 
 
350 
 
 ELECTRIC MOTIVE POWER. 
 
 class the coal was always picked out to give the bar a start. 
 Since the bar has to be carried by bearings it may be thought 
 
 that coal cannot be cut level with the floor by the bar type of 
 machine; but this difficulty is successfully met. By arranging 
 
ELECTRICAL COAL-CUTTING CORPORATION. 351 
 
 the seats of the turntable at a proper angle, and causing it to 
 revolve in an inclined plane, the makers contrive to keep the 
 bar nearly horizontal, and below the level of the rails when at 
 right angles to the face in the cutting position, and yet to 
 lift over the rails when turned parallel to the face. In this 
 way a flooring of regular slope is made at every cut, and 
 forms the floor for the rails during the next cut. In the 
 majority of cases it is not found necessary to level the floor, 
 although when cutting in coal it may pay to do so. The feed 
 is made by a hand winch as in Fig. 20lA, or is worked auto- 
 matically from the motor shaft. The author believes that 
 hand feeding is found to be the more satisfactory method 
 with this machine. 
 
 The Electrical Coal Cutting Corporation have for some years 
 been working coal at a contract price per ton hewn or per linear 
 yard undercut to a specified depth. They supply the whole 
 of the plant and the men to work the machine, while the col- 
 liery proprietors find the labour for clearing away the coal 
 from the faces, leaving them ready for the machines. This 
 system, on the whole, has worked well, and, were it not for 
 the difficulties already referred to, the machines would turn 
 out a, much larger tonnage and, of course, return a larger 
 interest on the capital outlay. 
 
 The experience of the Corporation, whilst it has demon- 
 strated that nearly every mine, working long-wall, can be suc- 
 cessfully operated by electric coal-cutting machines, has also 
 brought into further prominence the essential difference 
 between hand and machine working, and the great difficulties 
 of getting the owners and managers to appreciate the differ- 
 ence, and to lay themselves out to apply to mining the discip- 
 line and regularity obtaining in manufacturing industries. 
 
 A modification of the Goolden-Atkinson machine has lately 
 been introduced by Mr, Frederick Hurd, who for some years was 
 on the staff of Messrs. Goolden and Co. No particulars of its 
 performance have yet been published. A general idea of the 
 machine may be gained from Fig. 202. There is a scoop built 
 at the back of the bar to assist the clearing of the cuttings 
 
352 
 
 ELECTRIC MOTIVE POWER. 
 
 from the hole. This may or may not be an improvement; 
 experience alone can determine; but the author believes its 
 use has been abandoned after a few trials. It should be 
 noted that the Goolden bar is claimed to clear itself by 
 
 virtue of the spiral arrangement of the cutters. There are 
 also some interesting devices for ventilating the motor and safe- 
 guarding it against the risk of exploding gas. These consist of 
 an elaborate system for circulating fresh air under pressure 
 
EIRTLEY ELECTRIC COAL-CUTTER. 
 
 353 
 
 along the conductors, which are encased in pipes for the 
 purpose, and through the interior of the motor, the starting 
 resistance and switch-board boxes, &c. The object in view is,. 
 
 'S - 
 
 J> -4-3 
 O 2 
 
 1-g 
 
 3 
 3 o 
 
 O o 
 
 of course, most desirable ; but the expense necessary to execute- 
 the system described in Mr. Hurd's patent specification is 
 probably too great for ordinary requirements. 
 
354 
 
 ELECTRIC MOTIVE POWER. 
 
 Another bar cutter has been patented quite recently by 
 Messrs. T. Heppell and Patterson, which embodies some novel 
 features. One of these machines, built by Messrs. Ernest Scott 
 
 6 S 
 
 I 
 
 and Mountain, is shown in Figs. 203 and 204. The case is re- 
 ,moved to show the motor and gearing. One of the special points 
 is the endless chain used to clear away the cuttings. This is 
 
BIETLEY CUTTER-BAR. 355 
 
 shielded by a strong guard. The gearing consists of cast steel 
 bevelled wheels and pinions. The machine is propelled by 
 means of a steel rope which passes round a drum geared to the 
 armature shaft by worm gearing, an eccentric, rod, pawl and 
 ratchet. The design is very compact, and appears to be 
 capable of being worked into a less height than the type of bar- 
 cutter adopted by other makers ; but this remains to be proved, 
 for many alterations may be required before the machine gets 
 into regular work. The author believes that at present one of 
 these machines is at work in the Cannock Chase district. Mr. 
 Heppell says that one of these cutters at the Pilner Main 
 Collieries is doing good work, cutting at the rate of about 20 
 yards per hour, 3ft. under. A sectional view of the cutter-bar 
 is shown in Fig. 205. It is arranged to swing about the centre 
 of supports, which permits of the cutters being readily examined, 
 
 FIG. 205. Section of Birtley Cutter-Bar. 
 
 and also facilitates the moving of the machine about a coal 
 pit. It has three dovetailed grooves cut along its length. 
 The cutters have corresponding dovetails, and are distanced 
 by suitable pieces of steel. The cutters can obviously be 
 arranged spirally or otherwise, as may be required. 
 
 The Jeffrey coal-cutter is of different construction from any 
 machine of English origin, and was, no doubt, first designed to 
 meet the conditions of the American coal seams. Special 
 machines are built to suit the conditions obtaining in British 
 mines. The method of working will be readily grasped on an 
 inspection of Fig. 206. The details of construction are shown 
 in Fig. 207. 
 
 It will be seen that the bar lies parallel to the coal face when 
 in the cutting position. It is designed to hole as deep as 7ft. 
 
 
356 
 
 ELECTRIC MOTIVE POWEE. 
 
 Deep undercutting is an especial advantage in pillar and stall 
 working with an average roof. This machine consists of a bed- 
 frame, occupying a space 3ft. wide by 7ft. 2in. long, built up of 
 
 O 
 
 _o 
 
 I 
 I 
 
 two steel channel bars firmly braced, the top plates on each 
 carrying racks with the teeth downwards. Mounted upon and 
 engaging with this bed-frame is the sliding frame, consisting 
 
JEFFREY ELECTRIC COAL-CUTTER. 
 
 357 
 
 BB 
 
358 ELEOTEIC MOTIVE POWER. 
 
 mainly of two steel bars, well braced, upon which is mounted 
 at the rear end an electric motor. Upon the front end of this 
 sliding frame is mounted the cutter-bar, held firmly by two 
 solid steel shoes with brass bearings. The cutter-bar receives 
 motion through an endless steel or bronze chain from the 
 driving shaft, and as it revolves is fed forward by means of 
 straight-cut gear and a worm and wheel. The bar is usually 
 36in. wide. In America the seams are generally much thicker 
 than those in this country, and the roofs are sufficiently good 
 to enable the face to be kept open for the distance necessary 
 with a 6ft. or 7ft. holing, and pillar and stall working is chiefly 
 practised. But in England the conditions of long-wall working 
 rarely permit of such deep undercutting, and from 4ft. to 5ft. 
 is, as a rule, the deepest cut advisable. At the Cannock and 
 Rugeley Collieries, where two of the Jeffrey machines are at 
 work, one holes 4ft. and the other 7ft. 
 
 The electric motor occupies a space of about 20 inches square. 
 It is designed to absorb a maximum of 15 E.H.P., and in some 
 veins of coal does not require more than 7 '5 H.P. The American 
 standard pressure is 220 volts, but this can be altered to suit 
 circumstances. The armature is designed to run at 1,000 
 revolutions per minute, and the cutter-bar at 200. The 
 momentum of the moving parts is such that the machine over- 
 comes ordinary obstacles without difficulty, and, since the 
 motor is not coupled rigidly to the bar, the vibrations are 
 not transmitted to it, and the machine runs steadily and 
 comparatively quietly. Two men are required to work it 
 one at the switch at the back and the other as helper to clear 
 the debris and to assist in shifting. The machine is taken 
 from stall to stall on a special truck, shown in Fig. 208. When 
 at work it is slid on to two boards or bars of iron in front of 
 the coal, and fastened firmly by means of the front and rear 
 jacks against the face and roof of the coal, which is thus sup- 
 ported to some extent during the under-cutting. When the 
 full depth of the cut is reached the feed is thrown off, and the 
 cutter bar is returned to the starting position by means of 
 reversing gear. The machine is then moved sideways for a 
 distance equal to the length of the cutter bar, and another cut 
 is made in the same manner. Each cut is made in ordinary 
 
JEFFREY ELECTRIC COAL-CUTTER. 359 
 
 BB2 
 
360 ELECTRIC MOTIVE POWER. 
 
 working in from three to six minutes. The output depends to- 
 a great extent upon the skill of the men in moving and jacking 
 the machine, and also upon the hardness of the coal, and 
 whether the cut is made in coal or " dirt." In some seams 
 these machines are said to cut from 40 to 66 linear yards of 
 face to a depth of 6ft. in a shift of 10 hours. Since the depth 
 of the undercut is roughly twice as great as that usually made 
 with a bar machine of the Goolden-Atkinson type, this is 
 equal to a length of face of from 80 to 130 yards, in com- 
 parison with the performance of the latter. Assuming these 
 figures to be correct, it would appear that although the Jeffrey 
 machine has to be shifted after every cut, yet it may cut a 
 greater area of coal than the Goolden-Atkinson machine, which, 
 the author understands, does not average more than from 80 to 
 100 yards of face in this time. 
 
 In comparing the two, stress must be laid on their relative 
 suitability for different kinds of work. The Goolden-Atkinson 
 machine requires a space between the face and the props at 
 the goaf of not more than four feet at the outside, whereas 
 the Jeffrey machine cannot be worked in a smaller space than 
 six feet; but with the latter the depth of undercut may be 
 nearly double that with the former, though, of course, the 
 total length of roof unsupported will be then nearly twice as- 
 great. It is, therefore, obvious that the side cutter-bar can 
 be used in places where the front cutter-bar cannot possibly be. 
 It must be remembered also that the Jeffrey coal-cutter makes 
 a capital heading machine, while the side cutter cannot be used 
 for this purpose. The two machines are not, therefore, rivals 
 for the same work in all cases. They cost in the first instance 
 about the same, the motors are of the same rated capacity, 
 and the actual power required in practice appears to be about 
 the same. Hence there does not seem to be much to choose 
 between them as regards the question of cost, but it is rather 
 a matter of suitability for any particular case. 
 
 The English agents for the Jeffrey mining machines are 
 Messrs. John Davis and Son, who are placing these useful coal- 
 cutters in various mines in England, Wales, and Scotland. 
 
TESTS OF ELECTEIC COAL-CUTTERS. 361 
 
 Mr. Williamson, of Cannock and Rugeley Collieries, who is 
 using one of the Jeffrey machines with a 36in. bar, holing 4ft. 
 in a 5ft. seam, says that the output of round coal is increased 
 15 per cent. that is to say, hand cutting makes 75 per cent, of 
 round coal and the Jeffrey cutter 90. And that one machine, 
 working single shifts, has increased the output by at least 60 
 tons per day. 
 
 In Table DD are given a few results from a series of exhaus- 
 tive trials of electric coal-cutters in mines in Ohio, U.S.A., 
 made in 1890 by Mr. R. M. Haseltine, Chief Inspector of Mines. 
 This gentleman found that the only electric coal-cutter in 
 general use was the Jeffrey machine, and, therefore, most of 
 his tests refer to this type. The Leichner and Thomson- Van 
 Depoele machines (see the end of the series) are of com- 
 paratively small output, and do not seem to have made much 
 headway since the date of the report, so far as the author is 
 aware ; while, on the other hand, the use of Jeffrey machines 
 has steadily increased in American mines, and is now gaining 
 a hold on mining engineers in this country. 
 
 The figures are not directly comparable with those now 
 obtaining in England, for the conditions are very different, and 
 the machines have been since improved in many respects; 
 but the results, taken collectively, may prove instructive to 
 future users of electric coal-cutting plant, as they show, on the 
 authority of an independent official and expert, what was 
 being done in this direction as far back as 1890. 
 
 The General Electric Company of America have built a 
 modification of the front-cutter type machine, which is of 
 interest. In it the cutter-bar is made to revolve in such a 
 direction that the coal is cut upwards. It is claimed for this 
 method that it assists the holding down, and hence there is 
 less need for careful fastening by jacks and braces. In this 
 machine there are four chains instead of the single one used 
 with the Jeffrey machine. The Stine-Smith coal-cutter is also 
 similar in design, and appears to be a soundly built machine. 
 
 The author has no information about performances of either 
 of the two last-mentioned machines. 
 
Table DD. Showing Results of Experiments made with Electrical Coal-C utters in Ohio, 1890, ly R. M. Haseltine^ 
 Chief Inspector of Mines. 
 
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364 ELECTRIC MOTIVE POWER. 
 
 70. ROTARY SIDE-WHEEL COAL-CUTTERS. GILLOTT 
 AND COPLEY, YORKSHIRE ENGINE CO., AND 
 SNELL-WATERHOUSE COAL-CUTTERS. 
 
 A number of patents have been taken out for machines with 
 side wheels. In a few the wheels have been designed to cut 
 vertically, or even at any angle, but in the majority the aim 
 has been simply to make a horizontal cut as near the bottom 
 as possible. The diameter of the wheel determines the depth 
 of the undercut, which is limited in practice to from 39 to 
 55 inches, the latter being found to be as deep a cut as is 
 desirable. If this depth be exceeded there is a strong proba- 
 bility of the coal shearing at the back and fouling the wheel 
 before it is clear of the cut, and if the cut be shallower the coal 
 will not break readily by its own weight, but requires wedging 
 down. 
 
 Passing by the early inventors of these machines and coming 
 to modern history, the firm of Messrs. Gillott and Copley 
 stands in the front rank ; but from some cause, which is not 
 very apparent, they have not shown a disposition to apply 
 electric motors to their machines, preferring to continue the 
 use of compressed air. 
 
 The Yorkshire Engine Company have also built some 
 excellent machines of this type, embodying several improve- 
 ments on those of the former firm. They also have confined 
 themselves to compressed air, although they have built to 
 order a few electrically-driven machines. An illustration of 
 one of their machines is given in Fig. 209, in order to show 
 the general requirements of this type of plant, and also because 
 the author has found from actual experience that the machines 
 do excellent work. 
 
 Fig. 210 is an enlarged view of the cutter- wheel, showing the 
 method of supporting it in bearings. The design is arranged 
 to hole at a few inches above the rail tops. 
 
 Owing to the success which this class of coal-cutter has 
 obtained when driven by compressed air, no radical change has 
 
SIDE- WR EEL COAL-CUTTER, 
 
 365 
 
366 
 
 ELECTRIC MOTIVE POWER. 
 
 been attempted by inventors when applying electricity. They 
 have accepted the main principle, and have simply changed the 
 motive power. 
 
 On referring to Fig. 209 it will be seen that the side wheel is 
 fitted with two kinds of cutters. Every alternate one has 
 Y-shaped teeth, while the remainder are straight. The function 
 of the latter is to tear out the centre of the cut, while the 
 former clears it out to the desired width, usually about Sin. 
 In air-driven machines the speed of the wheel varies from 15 to 
 30 revolutions per minute, according to the kind of dirt and 
 its resistance. This speed is found to be too slow for the 
 electric motor, since the reducing gear is costly and cumber- 
 some ; therefore all who have tried to apply electricity have 
 
 FIG. 210. Cutter-Wheel : Yorkshire Engine Company. 
 
 endeavoured to raise the cutter-wheel speed as much as possible. 
 This has necessitated strengthening the machine as a whole, 
 and has increased the dead weight from about 25cwt. to about 
 35cwt. The increased speed, however, has caused no trouble 
 in working. In fact, the momentum of the moving parts is so 
 great that obstacles which caused serious shocks at the slower 
 speed are scarcely felt at the higher speed of from 70 to 100 
 revolutions per minute. And the increased weight tends to 
 prevent derailment. 
 
 While upon the subject of weight, it may be remarked 
 that if it is desirable to limit the weight of a coal-cutter 
 to, say, lOcwt., which is about as much as any one man 
 can shift about a pit with the aid of crowbars, it may be well 
 worth while to pay special regard to the making of a light 
 machine, But if it be necessary, from any cause, such as the 
 need for a machine of greater capacity, to increase the weight 
 
SNELL-WATERHOUSE ELECTEIC COAL-GUTTER. 367 
 
 much beyond the limit just mentioned, then it appears to the 
 author to be a matter of little importance whether the machine 
 weighs 20cwt., SOcwt., or even 40cwt. In each case it is toe- 
 heavy to be handled without special tools, and a screw jack is 
 as much a part of its equipment as it is of a locomotive's. In 
 fact, with the larger type of electric machines the increase of 
 weight is by no means an unmixed evil, for it gives increased 
 stability, and, no matter how rough the road, or uneven the 
 dirt, the machine rides steadily through it all. 
 
 The advantage of the wheel machine compared with the 
 rotary bar lies in the low head room required, and perhaps in 
 the enormous capacity of the wheel on long walls where the 
 conditions are favourable. 
 
 The author has, however, never seen a machine at work which 
 had anything like a chance of working continuously, and so 
 reaching the theoretical output. The majority of pits are so 
 laid out that continuous working is practically impossible, and 
 the most useful machine is that which, under the given con- 
 ditions of normal working, will give the largest average output,, 
 and not a maximum output for a short time under the best 
 conditions. 
 
 A number of patents have been taken out for side-wheel 
 coal-cutters ; but perhaps the first practical machine was 
 designed by the author and Mr. Major Waterhouse, of Glass 
 Houghton Collieries, about four years ago. The general con- 
 struction of this machine will be readily seen on an inspection 
 of Figs. 211, 212, and 213, which are plan, longitudinal, and 
 end views. Much detail is left out in order to show up the 
 main features. It will be noticed that the design roughly 
 resembles that of the Yorkshire Engine Company's compressed 
 air machine. But, in fact, the whole of the details have been 
 modified to suit the increased rate of speed and to accommodate 
 the gearing. The machine was built to hole 3ft. Gin. in a seam 
 24in. in height, on a level with the floor, this being the first 
 machine so designed. The maximum height above the rails 
 was 22in. The cutter made from 80 to 100 revolutions per 
 minute in various kinds of shale and coal, no difficulty being 
 
368 
 
 ELECTRIC MOTIVE POWER. 
 
 experienced either from their hardness or from iron pyrites. 
 The weight was about 35cwt. At least two men were neces- 
 sary to work the machine, and in soft coal with a rapid feed 
 a third was required to keep the wheel from being fouled by 
 
 the coal. The wheel was designed to run in either direction, 
 but the inventors always arranged the cutters to feed out the 
 cuttings at the back of the machine, instead of at the front, as 
 is frequently done in air machines. It was found, however, that 
 
SNELL-WATERHOUSE ELECTRIC COAL-CUTTER. 369 
 
 the wheel did not sufficiently clear itself, and that a man was 
 required to shovel out the loose coal at the front from time to 
 time. The power absorbed did not vary so much as was 
 
 expected. The automatic feed, actuated by a worm and wheel 
 from the first cross shaft, nicely adjusted the speed of the 
 machine to the resistance of the cut. The motor was wound 
 
370 
 
 ELECTRIC MOTIVE POWER. 
 
 in simple series, and thus the current varied in proportion to 
 the torque. And, since the pressure decreased slightly as the 
 load increased, the tendency was to equalise the power absorbed 
 and to vary the rate of feed with the degree of resistance to 
 the cutters. 
 
 The chief difficulties met with at Glass Houghton were due 
 to the peculiar formation of the seam, which is 22in. in height 
 with a thin parting near the middle. The holing was made on 
 the level of the floor in the fireclay, and presented no serious diffi- 
 culty. But the lower coal broke away from the parting, leav- 
 ing the tops in position, and frequently fouled the wheel before 
 a cut could be made sufficiently long for the introduction of 
 Under these conditions the time wasted in clearing 
 
 FIG. 213. Snell-Waterhouse Coal-Cutter. End View. 
 
 the wheel more than counterbalanced any gain from the high 
 rate of speed at which the machine holed when the coal 
 remained firmly in position. A bar-cutter might have suc- 
 ceeded here if more head room had been permissible, for it 
 is obvious that the fall of coal behind the bar, or even on the 
 top of it, would not be so serious as with a side wheel. But in 
 this case the large number of iron balls in the clay were held 
 to be an obstacle in the way of the bar-cutter, and, in addition, 
 the bar could not be made to cut perfectly level with the floor- 
 ing, as was desirable. The power absorbed varied from 10 to 
 17 E.H.P. The motor was designed to give 15 B.H.P. at about 
 800 revolutions, and was of the armoured type adopted by 
 Messrs. Lahmeyer and other Continental firms, but not much 
 known in England. The design is well adapted for use on a 
 
ELECTRIC COAL HEADER. 
 
 371 
 
 coal-cutting machine, as the massive field-magnets protect the 
 field windings and the armature from damage by falls of roof or 
 accidental injury during working. 
 
 At present there are running in Yorkshire collieries a few 
 electric side-wheel machines which have been developed from 
 the Glass Houghton one, but no particulars of their working 
 have been published. Under suitable conditions, however, they 
 must give economic results. 
 
 FIG. 214.- 
 
 - Wan tling- Johnson Electric Coal- Cutter and Heading 
 Machine. 
 
 FIG. 215. Plan of Cutter-Wheels of Wantling-Johnson Electric 
 Coal- Cutter. 
 
 A smaller type of wheel machine of American origin is 
 shown in Fig. 214. It is the invention of Messrs. Wantling 
 and Johnson, and was shown at the World's Fair, Chicago, 
 1893. The author has no particulars about its performance, 
 but it seems to be a practical machine. It is adapted 
 for making vertical or horizontal cuts, but cannot be set to 
 cut level with the floor. It appears to be especially suitable 
 for heading work. The cutter-wheels are shown in plan in 
 Fig. 215. There are two wheels revolving in opposite directions. 
 
 
372 
 
 ELECTRIC MOTIVE POWER. 
 
ELECTRIC POWER DRILLS. 373 
 
 Their diameter can be made to suit requirements. The sketch 
 in Fig. 214 shows the machine arranged for heading, in 
 which case it is always parallel with the rails. For long-wall 
 or pillar and stall working the motor and cutter would be at 
 right angles to the rails and the coal. The method of steadying 
 the cutter is clearly indicated in the figure. 
 
 The Thomson-Houston " Mowing Machine " coal-cutter, 
 shown in Fig. 216, presents several features entirely different 
 from those of the preceding machines. It may be regarded 
 as a modification of the Baird coal-cutter. The cutters are 
 carried on a chain which passes round an overhanging arm 
 capable of rotation through an arc of nearly ISOdeg. The 
 illustration shows most of the working details. It appears to 
 be suitable for long-wall working. There does not seem to 
 be any definite information published with respect to its 
 performances. 
 
 Messrs. Mavor and Coulson have adopted Baird's design, 
 but with several modifications ; the principal one being the 
 use of worm gear for driving the cutter-chain. Their machine 
 is designed to cut at the rate of one foot per minute through 
 ordinary soft Scotch coal, the chain-wheel revolving at 60 
 revolutions per minute. 
 
 The Jeffrey Company are also trying a cutter-chain on one 
 of their special English machines, and expect it will effect a 
 saving in power, since the width of the cut is reduced to 2'5in. 
 
 71. ELECTRIC POWER DRILLS: ROTARY AND PER- 
 CUSSIVE TYPES; JEFFREY, MARVIN, AND VAN 
 DEPOELE DRILL. 
 
 In spite of the obvious advantages to be derived from the 
 application of electric power to rock and coal drills, much pro- 
 gress does not seem to have been made in this direction. The 
 reasons for this are probably partly the same as those, already 
 referred to, which have hindered the introduction of mechanical 
 coal-getting, and, in addition, the cost of the conductors, which, 
 no doubt, has been in many cases prohibitive. But there are 
 also some difficulties peculiar to the work. 
 
 c c 
 
374 ELECTRIC MOTIVE POWER. 
 
 Driving stone drifts is about as rough work as can be 
 imagined, and the men employed are as rough as the work. 
 But even more important than the rough treatment is the 
 question of ventilation. Obviously, when driving a heading, 
 there is a tendency for the air to stagnate, and there is diffi- 
 culty in removing the fumes of powder after shots are fired 
 unless fresh air can be brought up to the face. Now the com- 
 pressed air drill exactly meets this requirement. The exhaust 
 from the drill causes a steady draught, which cools and purifies 
 the air, and powder fumes can always be cleared away by turn- 
 ing on a little air. The electric drill, on the contrary, tends, if 
 anything, to increase the difficulty, for the power wasted in the 
 drill raises the temperature of the air at the face of the head- 
 ing. This disadvantage is probably sufficient, in many cases, 
 to render the use of electric drills impracticable, the mere 
 question of cost of boring (gauged, usually, by work done in 
 well-ventilated spots) being altogether subordinated to other 
 considerations. Of course, for a variety of purposes, such as 
 quarrying, working in well-ventilated spots, &c., these objec- 
 tions have no weight. But even under the most favourable 
 circumstances the electric drill does not seem to have made 
 much progress. 
 
 There are two main principles upon which rock drills are 
 designed : the rotary and the percussive. 
 
 The rotary type is usually driven through gearing from a 
 small motor. The percussive type is sometimes worked by a 
 motor winding up a coiled spring, which at the proper moment 
 uncoils and spends its energy upon the drill shaft. In other 
 types the principle of magnetic attraction is used, the drill 
 shaft being attached directly to the iron core of a solenoid. 
 
 Hitherto the motor-driven drills have proved to be the more 
 successful in general work. Several firms build these machines, 
 each having a different way of gearing the motor to the drill shaft, 
 but the general principle is the same. The best known in this 
 country, as well as in America, is the Jeffrey drill. It is chiefly 
 used for working in hard coal, and is found to give satisfactory 
 results. In general the makers design the motors to work at a 
 
ELECTRIC COAL-DRILL. 375 
 
 pressure of 220 volts ; but this can be varied to suit special 
 requirements. The rated output of the drills for 2in. holes is 
 2 H.P., but a factor of safety of two is allowed. They are stated 
 to drill, in the hard "boney " found in anthracite mines, a 2in. 
 hole, 6ft. deep, in from 2min. to 2Jmin., while in the coal they 
 will cut a similar hole in less than a minute. One of these 
 
 FIG. 217. Jeffrey Rotary Electric Rock and Coal Drill. 
 
 drills is in use at Newbattle Colliery, Dalkeith. The manager, 
 Mr. John Morrison, reports that it makes, in the hard splint 
 coal, a 2in. hole, 4ft. deep, in the space of 5min., including the 
 time taken to set the drill. (This, he adds, is equal to the 
 work of four men, while the Jeffrey drill requires only one man. 
 
 cc-2 
 
376 ELEGTEIG MOTIVE POWER. 
 
 A good idea of the general shape of the drill, and of the 
 method of fixing it in position, will be gathered from Fig. 217. 
 It will be seen that the motor is protected from mechanical 
 injury by an iron shield. Any type of motor can be fitted to 
 the drill, and probably a polyphase machine will ultimately be 
 adopted. 
 
 Various English firms have built electric drills for mining 
 work, but, judging from the number in use, none of these has 
 been so successful as the Jeffrey drill. The designs are usually 
 heavier than that shown in Fig. 217. 
 
 The reciprocating action of a solenoid is apparently well 
 suited for actuating percussive drills. Yet the solenoid, energised 
 by a continuous current, is the least effective means of using 
 magnetic power through comparatively long distances. Its 
 proper function is to attract an armature through a very short 
 range, or to hold it firmly in position.* 
 
 However, in spite of this manifest disadvantage, solenoidal 
 action offers so many advantages in the design of percussive 
 drills that it appears highly probable that this type of drill 
 will ultimately come into more general use than the motor 
 type. Therefore, a brief reference to the subject may be 
 useful. 
 
 The earliest attempts to utilise the principle were made in 
 accordance with the diagram shown in Fig. 218. The current 
 was supplied to the centre of two coils forming the solenoid, 
 and was passed through them successively; the changing being 
 controlled by means of a two-way switch, indicated at a and c. 
 The iron shaft of the drill formed the core, and was attracted 
 first by one coil and then by the other. This is a very waste- 
 ful design, for the power absorbed during the instroke is the 
 
 * Prof. S. P. Thompson has recently demonstrated that an alternate- 
 current magnet possesses very different properties from those of a con- 
 tinuous-current one. He shows that by means of an alternate current 
 the pull of an electro -magnet on its armature may be extended through 
 comparatively long distances, and may be even greater at, say, three inches 
 than when closer. This important fact probably opens up a new field, 
 and may, therefore, assist in the development of percussive drills. 
 
ELECTRIC PERGUS&IVE DRILLS. 
 
 377 
 
 same as that during the outstroke. An additional objection la- 
 the trouble given by the contacts at the switch points. 
 
 An improvement is possible by the use of a single solenoid 
 drawing in a plunger against a strong coiled spring. The blow 
 given by the recoil of the spring is very rapid, and resembles- 
 the impact of a hammer if the bit be at the right striking dis- 
 tance ; and, further, the instroke is made comparatively slowly, 
 and does not shake the drill carriage. In case the stroke is 
 too short to strike the work a spring cushion is provided to 
 receive the blow. A two-way switch is required, however, 
 and this practically renders the design useless. 
 
 FIG. 218. Diagram of Electric Percussive Drill, with two-way 
 switch at a c. 
 
 A distinct advance is made by the use of pulsating or 
 alternate currents, or a combination of them. There are 
 many methods, but they all embody the following conditions : 
 No switch is necessary, three or more wires are required, and 
 also, in some cases, a special dynamo. The first is an absolute 
 gain, for there is no trouble from sparking, and the last two 
 are not serious difficulties when it is considered that these 
 drills are only likely to be used in mines and places where it 
 will pay to provide a suitable dynamo if the use of electric 
 drills is deemed advisable. The simplest drill of this class is 
 one coupled to a two-phase circuit, as shown in Fig. 219; but 
 this has the disadvantage that the power used on the instroke 
 is nearly as great as that on the outstroke. The number of 
 blows given by drills of these types corresponds with the fre- 
 quency of the alternator, and they are therefore adapted for 
 
378 
 
 ELECTRIC MOTIVE POWEE. 
 
 delivering many quick light blows per minute rather than a few 
 heavy blows. A frequency of from four to ten per second is 
 found to give the best results. 
 
 A still better device is that applied in the Marvin drill. 
 This is of the double solenoid type, and is one of the most 
 
 FIG. 219. Diagram of Electric Percussive Drill, for use on 
 circuit. No Switch required. 
 
 two-phase 
 
 successful in America. It is run off a special dynamo giving 
 pulsating currents in two circuits. The connections are shown 
 in Fig. 220. The dynamo armature, in its simplest form, has 
 only one coil. One end of this is fixed to a continuous metal 
 ring d lt on which rubs the brush Bjj the other is joined to a 
 metal segment, d z , extending for 180 deg. and concentric with 
 
 FIG. 220. Diagram of Marvin Electric Percussive Drill. 
 r/ l5 (/o, rings on dynamo ; B 1? Bo, B 3 , fixed brushes. 
 
 d r Two brushes, B 2 and B 3 , placed diametrically opposite to 
 each other, rub against it. Three conductors are required ; 
 one from the middle point of the solenoid is coupled to B I} 
 and those from the ends of the coils are respectively joined 
 to B 2 and B 3 . The two coils successively carry a current, and 
 so give reciprocating motion to the drill. 
 
ELECTRIC PERCUSSIVE DRILLS. 
 
 379 
 
 The best results with reciprocating drills are likely to 
 be obtained by the use of a combination of pulsating and 
 alternating currents ; for the instroke can then be made 
 without much waste of power, and yet a very effective blow 
 can be delivered at each outstroke. This principle has been 
 applied in the Van Depoele electric percussive drill. One form 
 of the arrangement is shown diagrammatically in Fig. 221. 
 The dynamo may be of any continuous current make, with an 
 ordinary commutator, and the two usual fixed brushes at F x 
 and F 2 ; but it requires the addition of two revolving brushes, 
 R x and R 2 , separated by 180 deg. The coils in the solenoid 
 are three in number, 1 and 3, coupled in opposition to each 
 
 FIG. 221. Diagram of Van Depoele Electric Percussive Drill. 
 R 1? R,, revolving brushes ; F 1} F 2 , fixed brushes ; 1, 2, 3, coils of drill. 
 
 other, having few turns relatively to the main coil 2. The con- 
 nections are made as follows : Coils 1 and 3 are in series 
 with each other, and are coupled to the revolving brushes, R x 
 and R 2 ; these coils are, therefore, traversed by an alternate 
 current. Coil 2 is coupled to the fixed brush F l at one end, 
 and at the other to the junction of coil 3 with the rotat- 
 ing brush R x ; it is, therefore, energised by a pulsating current, 
 and exerts the chief effect on the plunger with a polarity 
 always in the same direction. The blow is made mainly by the 
 action of the centre coil, the end coils practically neutralising 
 each other. The return stroke is chiefly due to the resultant 
 pull of the two end coils, as the centre one varies between zero 
 and a maximum. At a first glance there seems to be in this 
 design a departure from simplicity without any compensating 
 
380 ELECTRIC MOTIVE POWER. 
 
 gain, but on a closer examination it will be seen that it marks a 
 considerable advance, and that the chief difficulties referred 
 to in the early part of this section are to a great extent over- 
 come. 
 
 The instroke can be made as light and the outstroke as 
 heavy as required, both of these desirable ends being attained 
 without the use of any switch or complicated gear in the drill. 
 Also only three small conductors are required, and the dynamo 
 gives a continuous current, and can be used for lighting or 
 motor work. 
 
 The chief difficulty likely to be experienced with the Van 
 Depoele drill appears to lie in the heating of the iron cores from 
 hysteresis and eddy currents. This, however, will be gradually 
 overcome as experience suggests better methods of building the 
 core and framing. 
 
APPENDIX. 
 
 LONG-DISTANCE TRANSMISSION OF POWER. 
 
 The distance through which power may be economically 
 transmitted by electricity depends upon so many varying con- 
 ditions that it is not possible to give a definite limit, although 
 one can be found for any particular case.* The theoretical 
 considerations affecting the losses in the line are discussed 
 fully in 16, pp. 70-79, and the practical difficulties in 17 to 
 20. The data given there are sufficient to enable a complete 
 investigation to be made for any case likely to occur in 
 practice. It may be useful, however, to have a table of the 
 relative costs of continuous-current power plants for the usual 
 pressures, and for such distances as are likely to commend 
 themselves for ordinary practice. It will be obvious, however, 
 that the figures must necessarily be but approximations, and 
 in some cases may be even misleading, unless the require- 
 ments of the problem are determined by one who has a prac- 
 tical knowledge of the work. 
 
 The author has prepared Table E E, in which the costs are 
 all expressed in terms of per H.P. delivered by the motor. 
 The line is supposed to be of bare sicilium bronze, erected on 
 wooden posts, with oil insulators. The dynamo cost includes 
 a proportional share of station instrument?, and that of the 
 motor includes switches and starting frames, whilst the line 
 cost is inclusive of posts, insulators, lightning arresters, and 
 other details. 
 
 The prices, which allow a slight margin for market fluctua- 
 tions, include packing. The erection cost is a variable 
 quantity, and must be determined for each case. 
 * See page 132. 
 
382 
 
 ELECTRIC MOTIVE POWER. 
 
 Table E E. Cost of Continuous- Current Plants for Trans- 
 mission of Power. 
 
 -p 2 
 
 s 
 
 OD 
 t 0) 
 
 5 
 
 3^ 
 
 Per H.P. delivered by Motor. 
 
 a II 
 
 l| 
 
 p^ <u 
 9 ;rj O 
 
 g ^ tj 
 
 Cost in . 
 
 ""0,5 
 
 
 "*^ r "O 
 
 | '3 a,9> 5 
 
 __ 
 
 ||| 
 
 ^1 
 
 f 
 
 ll 
 
 6^ 
 
 CL *'* 
 
 I i 
 D ^ na -!Wire. Motor. 
 
 Total 
 Electrical 
 
 O 8 
 
 H 
 
 h ti 
 
 
 M ^** ~ 
 
 mo. 
 
 equip- 
 
 t 1 ) _Q 
 
 
 ^ 
 
 
 11 
 
 i 
 
 ment. 
 
 5,000 
 
 500 
 
 1-62 
 
 75 
 
 1-33 
 
 14-595 4-331 9'975 
 
 28-901 
 
 10,000 
 
 500 
 
 1-62 
 
 65 
 
 1-54 
 
 16-891 ! 7-218 9-975 I 34 084 
 
 15,000 
 5,000 
 
 500 
 800 
 
 1-62 
 1-013 
 
 50 
 80 
 
 2-00 21-945 7-166 9'975 
 1-25 |l5-763 3-715 10-972 
 
 39-086 
 30-450 
 
 10,000 
 
 800 
 
 1-013 
 
 75 
 
 1-33 
 
 16-773 6-825 10'972 
 
 34-570 
 
 15,000 
 
 800 
 
 1-013 
 
 70 
 
 ]-43 
 
 18-033 9-187 10-972 
 
 38-192 
 
 20,000 
 
 800 
 
 1-013 
 
 65 
 
 1-54 19-411 10-828 10'973 
 
 41-211 
 
 25,000 
 
 800 
 
 1-013 
 
 60 
 
 1-67 21-052 i 13-072j 10-972 45 '096 
 
 30,000 
 
 800 
 
 1-013 
 
 55 
 
 1-82 :22-955 14-437 10-972 48'364 
 
 35,000 
 
 800 
 
 1-013 
 
 50 
 
 200 25-22615-2510-972 51423 
 
 10,000 
 
 1,000 
 
 0-81 
 
 80 
 
 1-25 16-405 9-515 11-471 
 
 37-392 
 
 15,000 
 
 1,000 
 
 0-81 
 
 75 
 
 1-33 17-456 9-89611-471! 38'823 
 
 20,000 
 
 1,000 
 
 0-81 
 
 70 
 
 143 118-768 '10-500 11-471 40'739 
 
 25,000 
 
 1,000 
 
 0-81 
 
 65 
 
 1-54 20-212 11-356 11-47 L: 43'049 
 
 30,000 
 
 1,003 
 
 0-81 
 
 60 
 
 1-67 21-918 12-022 11-471! 45-411 
 
 35,000 
 
 1,030 
 
 0-81 
 
 55 
 
 1-82 23-88712-52111-471 47879 
 
 40,000 1.000 
 
 0-81 
 
 50 
 
 2-00 2V250 12-678 11-471 50399 
 
 45,000 1,000 
 
 081 
 
 45 
 
 2-25 29-531 11-812 11-471 52'814 
 
 50,000 
 
 1,000 
 
 0-81 45 2-25 29-531 14-765 ll'47i 
 
 55767 
 
 In many of the problems brought before the electrical 
 engineer the distances are far greater than those assumed in 
 Table E E, and much higher pressures are necessary. The 
 author is of opinion that for extra long distances the best 
 way to deal with power transmission is by polyphase currents 
 {see Chapter VIII., especially 54, p. 267). If the pressure at 
 the alternators can be kept below 7,000 volts, it will not be 
 necessary to use step-up transformers ; but if the pressure 
 exceeds this it will generally be advisable to use them. 
 
 The cost of the alternators and transformers rapidly de- 
 creases as the output is increased. The cost of alternators 
 of, say, 100 kilowatts capacity is about <7 per kilowatt, in- 
 cluding station apparatus; while it has been estimated in 
 connection with the Niagara plant that, with outputs of 4,000 
 
APPENDIX. 
 
 383 
 
 kilowatts, the cost will not exceed '2. Transformers of 100 
 kilowatt capacity average 5 per kilowatt, and in very large 
 sizes are offered by manufacturers at 1 per kilowatt. These 
 figures are so far apart that it is not possible to give any 
 general data for the generating plant, or even for the motors ; 
 but with the line a fair approximation is possible, since the 
 cost of copper may be taken as fairly constant within the 
 limits of the quantities required. Table FF has been pre- 
 
 Table FF. Cost of Line and of H.P. delivered, with Three-phase 
 Circuits, for various Distances and Pressures. 
 
 Distance 
 
 Area of 
 each 
 
 Effective 
 pressure 
 
 Current 
 
 Efficiency 
 of con- 
 
 Total 
 cost of 
 
 Cost per 
 H.P. 
 
 in 
 
 conductor 
 
 between 
 
 
 ductors 
 
 line at Is. 
 
 delivered 
 
 Feet. 
 
 in square 
 
 mains in 
 
 main in 
 
 in 
 
 per Ib. of 
 
 by 
 
 
 inches. 
 
 volts. 
 
 amperes. 
 
 per cent. 
 
 copper. 
 
 motor. 
 
 For 100 brake horse-power. 
 
 
 
 
 
 
 
 s. 
 
 25,000 0-0067 
 
 5,000 8-75 97-0 
 
 200 
 
 2 
 
 50,000 0-0067 7,000 6'25 \ 97'0 
 
 400 
 
 4 
 
 100,000 \ 0-0067 10,000 
 
 4-375 
 
 97-0 
 
 800 
 
 8 
 
 150,000 l 0-0067 
 
 10,000 
 
 4-375 
 
 95-5 
 
 1,200 
 
 12 
 
 250,000 ! 0-0067 10,000 
 
 4-375 
 
 92-5 
 
 2,OCO 
 
 20 
 
 375,000 0-0067 10,000 
 
 4-375 
 
 89-0 
 
 3,000 
 
 30 
 
 500,000 0-0067 10,000 
 
 4-375 
 
 86-0 
 
 4,000 
 
 40 
 
 For 200 brake horse-power. 
 
 1 
 
 
 
 
 
 
 
 25,000 i 
 
 0-0067 
 
 5,000 
 
 17-50 
 
 94-0 
 
 200 
 
 50,000 ' 
 
 0-0067 
 
 7,000 
 
 13-5 
 
 92-5 
 
 400 
 
 100,000 
 
 0-0067 i 10,000 
 
 8-75 
 
 94-0 
 
 800 
 
 150,OCO 
 
 0-0067 ; ] 0,000 
 
 8-75 
 
 91-0 
 
 1,200 
 
 250,000 
 
 0-0067 ! 12,500 
 
 7-0 
 
 90-0 
 
 2,000 
 
 375,000 1 
 
 0-0067 15,000 
 
 5-83 
 
 90-0 
 
 3,000 
 
 500,000 
 
 0-0067 20,000 4-375 
 
 92-5 
 
 4,000 
 
 s. 
 
 1 
 
 2 
 
 10 
 15 
 20 
 
 For 500 brake horse -power. 
 
 
 
 
 
 
 
 
 25.000 
 
 0-0134 
 
 10,000 
 
 26-25 ; 97-5 
 
 400 
 
 50.000 
 
 0-0134 10,000 26-25 94-5 
 
 800 1 
 
 100,000 
 
 0-0134 [ 10,000 
 
 26-25 
 
 92-0 
 
 1,600 3 
 
 150.000 
 
 0-0134 10.000 
 
 26-25 
 
 84-5 
 
 2.400 4 
 
 250,000 
 
 0-0134 I 1^500 
 
 21-00 ! 84-0 
 
 4,000 
 
 8 
 
 375,000 
 
 0-0134 i 15.000 
 
 17-50 I 83-5 
 
 6,000 
 
 12 
 
 500,000 
 
 0-0134 
 
 20,000 
 
 13-15 ' 87-0 
 
 8,000 
 
 16 
 
 16 
 
 1 12 
 
 4 
 
 4 16 
 
 
 
 
384 ELECTRIC MOTIVE POWER. 
 
 pared to give a rough idea of the cost of copper for overhead 
 lines for powers of 100, 200, and 500 H.P., delivered by motors 
 with three-phase currents at various efficiencies and pressures 
 up to 20,000 volts, and transmitted through various distances. 
 
 In these calculations an allowance of 5 per cent, has been 
 made for the sagging of the wires, so that a distance of 
 5,000ft. requires 1 mile, or 5,280ft., of conductor. For first 
 approximations each 5,000ft. in the first column may be taken 
 as a mile. The size of conductor chosen in the first two series 
 is No. 13 S.B.W., which is, perhaps, the smallest consistent with 
 the requirements of mechanical stability. The line losses are 
 chosen arbitrarily. The most instructive thing to notice is 
 the cost per horse-power delivered by the motor. The necessity 
 for high pressures is very apparent. 
 
 Tables EE and FF should be compared with Table X, 
 p. 268. 
 

 It 
 
 I 
 
GENERAL ALPHABETICAL INDEX. 
 
 PAGE 
 
 Accumulators, Use of 136,297 
 
 Aerial Lines - 79,81,93,102 
 
 Air-Blast Lightning Discharger 108 
 
 Air-Gap ... 21,29,36,292 
 
 Alternate Currents, Advantages of 211,216 
 
 Principles of 150,218,221 
 
 Transmission of Power by ... 211, 216, 245, 251, 267, 283, 297 
 
 Alternate- Current Motor, Critical Exciting Current of 176 
 
 Motors, Polyphase 251,283 
 
 Motors, Single-Phase 149,177 
 
 Transformers 187, 197, 273 
 
 Alternator, Allgemeine Electricitiits Gesellschaft .., 290 
 
 - Armatures of 156,172 
 
 Asynchronous ... ... ... ... ... ... 179, 181 
 
 Brown, C. E. L 151,176 
 
 - Compound-Wound 232 
 
 Curves of Pressure of ... ... ... ... ... ... 157 
 
 Design of 156,175,292 
 
 - Effect of Iron Cores in 172 
 
 Efficiency of ... ... ... ... ... ... ... 172 
 
 Excitation of 176,179,231 
 
 Ferranti 172 
 
 - Fields of 156 
 
 Guicher 159,172 
 
 Heating of 172 
 
 -High-Pressure 175,208,269,284,286 
 
 . Kapp 175,291 
 
 Mordey 151,172,176,178 
 
 Oerlikon Polyphase 284 
 
 - Parallel Running of 215,238 
 
 - Polyphase or Multiphase ... 251,283 
 
 Series Working not Possible with Synchronous ... ... 213 
 
 - Speed of 176 
 
 Thomson-Houston ... ... ... ... ... ... 232 
 
 - Types of 
 
 Westinghouse 
 
 Ang.eofLead 32,38 
 
 Arc Lighting Dynamos ... ... ... ... ... ... 117, 232 
 
 Armature, Bar- Wound ... ... ... ... ... ... 17, 21 
 
 Current Density of 20,175 
 
386 INDEX. 
 
 PAGE 
 
 Armature, Design of 18,49 
 
 -Diameter of 16,26,46 
 
 Driving Horns for ... ... ... ... ... ... 45 
 
 -Drum 17,172 
 
 - Equations for 16,18,46 
 
 -Gramme 17,46,172 
 
 -Heating of 20,175 
 
 - Hole in 19 
 
 - Load of (see also Circumflux) ... ... ... ... . . 21 
 
 - Magnetic Flux in 18,29,175 
 
 - Number of Turns in ... ... ... ... ... . . 19 
 
 - Plates for 35 
 
 - Radial Depth of 19 
 
 -Reactions 37,65,179,255 
 
 Section of, for Designing Purposes ... ... ... ... 49 
 
 -Speed of 20,176 
 
 - Total E.M.F 18 
 
 Arresters (see also Lightning Arresters and Dischargers) ... ... 102 
 
 Asynchronous Motor, Single-Phase ... ... .... ... ... 181 
 
 Atkinson's, LI. B., Safety Cable 321 
 
 Average Pressure ... ... ... ... ... ... ... ... 153 
 
 Back Electromotive Force of Motor ... ... ... ... ... 18 
 
 Turns 38,55 
 
 Baird Type of Coal-Cutter 372 
 
 Banti, Mons., Test of Asynchronous Single-Phase Motor ... ... 183 
 
 Bar Armatures ... ... ... ... ... ... ... 17,21 
 
 Bearer Wires for Overhead Lines ... ... ... ... 82, 93 
 
 Birtley Coal-Cutter 354 
 
 Board of Trade Unit 218 
 
 Boilers 7,304 
 
 Bronze for Line Wire ... 80,83 
 
 Brown, C. E. L., Alternator 151,290 
 
 Motors 180 
 
 Transformers 194 
 
 Built-in Systems of Conductors ... ... ..; ... .. ... 96 
 
 Cables, Armoured ,. ...87,221,308 
 
 Concentric 70,221,308,319 
 
 in Mines 70,307,313,318 
 
 in Pit Shafts 307 
 
 Insulated 84,87,307 
 
 Lead-Covered 87,307 
 
 Safety Mining 319,321 
 
 Callender System of Laying Conductors ... ... ... 88, 96 
 
 Callender-Ra worth System of Laying Conductors ... ... ... 99 
 
 Callender- Webber System of Laying Conductors ... ... ... 98 
 
 Capacity, Electrostatic, of Circuit 167,221,225,229 
 
 - of Alternator 229 
 
 -of the Line 221,225,229 
 
 of Transformer 208 
 
 Cardew Voltmeter 242 
 
 Carter, E. Tremlett, on Economic Law... ... ... ... ... 73 
 
 Caasell, Plant at 177,209 
 
 Central Station Dynamos, Regulation of ... ... ... ... 144 
 
 Centre of Gravity, Electrical, of Circuit 270 
 
 Centrifugal Force on Armatures ... ... ... ... ... 176 
 
INDEX. 387 
 
 PAGE 
 
 Centrifugal Governors for Dynamos and Motors ... ... ... 119 
 
 Characteristic Curves, Dynamos, Motors, and Alternators ... 59, 142, 287 
 
 - Mechanical, of Motors ... ... ... ... ... 59 
 
 Choice of Plant for Transmission of Power ... ... ... 7,306 
 
 Circumflux, Permissible, for Armatures ... ... ... ... 21 
 
 Clock Diagram 168 
 
 Coal Consumption, Economised by Use of Sub-Stations ... ... 218 
 
 Coal-Cutter, Atkinson -Goolden 345 
 
 - Baird Type 372 
 
 Birtley 354 
 
 - Kurd 351 
 
 -Jeffrey 347,355,372 
 
 - Snell-Waterhouse 367 
 
 - Thomson-Houston (The General Electric Co., U.S.A.) 361, 372 
 
 - Wan tling and Johnson ... ... ... ... ... 371 
 
 Coal-Cutters, Rotary Bar Types 348 
 
 - Side Wheel Types 364 
 
 Coal-Cutting by Electricity 336,345,360,364 
 
 at Glass Houghton Colliery .... ... ... ... 367,370 
 
 Coal-Drilling 372 
 
 Heading 360,371 
 
 Combined Polyphase and Continuous Current Circuits ... ... 297 
 
 - Two- and Three-Phase Circuits 278 
 
 Commutation of Current in Armature, Act of... ... ... ... 39 
 
 Comparison of Electrical Systems of Transmitting Power, Weight 
 
 of Copper, &c. ... ... ... ... ... ... ... 268 
 
 Compensating Turns 38,231 
 
 Compensators, Electrical ... ... ... ... ... ... 233,236 
 
 Compound Winding 63,142,232 
 
 - Wound Alternator 232 
 
 Condenser Effects (see also Capacity) ... ... ... ... ... 167 
 
 Conductor, The 10, 69, 79, 84, 94, 102, 125, 218, 225 
 
 Conductors, Number of, in Armature ... ... ... ... ... 19 
 
 Conduits for Electric Cables 94 
 
 Connections of Polyphase Circuits ... ... ... ... ... 276 
 
 Constant Current, Distribution at ... ... ... ... ... 117 
 
 - Dynamos and Motors ... ... ... ... ... ... 117 
 
 - Regulators or Governors for ... ... ... ... ... 117 
 
 - Transformers 200 
 
 Constant Pressure, Distribution at ... ... .. ... ... 122 
 
 Consulting Engineer, Necessity for, in Power Work ... ... 303, 307 
 
 Conversion. Efficiency of Dynamo ... ... ... ... ... 9 
 
 - Efficiency of Transformers 189,196 
 
 Converters (see also Transformers) ... ... ... ... ... 187 
 
 Cooling Surface of Electric Machines .. ... .. ... 20. 31 
 
 Copper for Aerial or Overhead Lines 79,81,268,382 
 
 Quality of, for Overhead Wires 81 
 
 Specific Gravity of 80 
 
 Specific Resistance of 80 
 
 Tensile Strength of 80 
 
 Weight of, Required in Different Systems ... ... ... 268 
 
 Cores, Lamination of ... ... ... ... ... ... ... 173 
 
 - Watts lost by Hysteresis per cubic inch of ... ... ... 173 
 
 Cost of Electric Plant for Long Distance Transmission 381 
 
 Counter Electromotive Force in Motor ... ... ... ... 18 
 
 - of Self-induction 203,226 
 
 of the Line - 226 
 
 
 
388 INDEX. 
 
 PAGE 
 
 Counter Electromotive Force in Transformer ... ... ... ... 203 
 
 Coupling of Compound- Wound Dynamos m Parallel ... ... ... 143 
 
 to Engines ... ... ... ... ... ... .. 6, 10 
 
 Critical Excitation of Alternator ... ... ... ... ... 176 
 
 Cross Induction (see also Armature Reactions) ... ... ... 37 
 
 Current, Constant, Distribution with ... ... ... ... ... 116 
 
 Constant, Economy of ... ... ... ... .. 116 
 
 Effective 153 
 
 Current Density in Armature Windings ... ... ... 20,175 
 
 - in Field Windings 31,52 
 
 - in Line 125,219 
 
 Currents in Quadrature ... ... ... ... ... ... .. 252 
 
 Curves, Characteristic 59.142,287 
 
 Curves of Horse-Power in Line ... ... .. ... ... ... 47 
 
 Magnetisation ... ... ... ... ... ... 35 
 
 - Pressure 154, 157, 177 
 
 Pressure, Drop of 47,226,287 
 
 - Torque 60, 6*, 67 
 
 Cycles of Magnetisation ... ... ... ... ... ... ... 150 
 
 - Periodic Current 150,154 
 
 Demagnetising Action of Armature ... ... ... ... ... 37 
 
 Action of Eddy Currents in Armature ... ... ... 39 
 
 Ampere Turns ... ... ... ... . . 38, 55 
 
 Density, Current, Permissible in Armature Coils ... ... ... 20 
 
 - Current, Permissible in Transformer .. ... ... ... 206 
 
 - Magnetic, in Armature Cores ... ... ... ... 29, 172 
 
 Magnetic, in Air Gap ... .. ... ... ... ... 29 
 
 Magnetic, in Field Cores ... ... ... ... ... 29 
 
 Magnetic, in Yokes ... ... ... ... ... 29, 33 
 
 Depoele. Van, Electric Percussive Rock Drill 379 
 
 Depth, Maximum Permissible, of Winding ... ... ... ... 21 
 
 - Radial, of Core 19 
 
 Design of Dynamo, Practical Example of ... ... ... 45,49 
 
 - Motor 43,49 
 
 - Poly phase Alternators 283,292 
 
 Diameter of Commutation ... ... ... ... ... ... 39 
 
 Dielectrics (see also Insulating Materials) ... ... .. ... 85 
 
 Difference of Pressure at Terminals ... .. ... ... ... 18 
 
 Dip Pumping by Electric Power ... .. ... ... ... 332 
 
 Dischargers (see also Lightning Dischargers) ... ... ... ... 102 
 
 Distortion of Magnetic Field ... ... ... .. ... 37.51 
 
 Distributors 123,134 
 
 Dobrowol- ky, von Doli vo, Polyphase Winding ... ... . . ... 265 
 
 Double Magnetic Circuit Field Magnet, Advantage of ... ... 24 
 
 Calculation for ... ... ... ... ... ... 37 
 
 Doulton's System of Stone Casing for Conductors ... ... ... 100 
 
 Dowson Gas Producer ... ... ... ... ... ... ... 9 
 
 Drag 011 Wires of Armature ... ... ... ... ... ... 44 
 
 Dra wing-in Systems of Conductors ... ... ... ... 98, 100 
 
 Drills, Electric Rock 372 
 
 Driving Horns ... ... ... ... ... ... ... ... 45 
 
 Drop Curves of Transformers ... ... ... ... ... ... 198 
 
 Drop of Pressure 47, 124, 142, 226, 231, 234 
 
 Drum Armatures ... ... ... ... ... ... ... ... 17 
 
 Dynamo and Motor in Series ... .. ... ... ... ... 121 
 
 Dynamo, Bipolar ... ... ... ... ... ... ... ... 22 
 
INDEX. 389 
 
 PAGE 
 
 Dynamo, Compound- Wound ... ... ... 65, 143 
 
 Constant- Current 116 
 
 Design of 16,45,49 
 
 Efficiency of ... 9 
 
 Equalising ... ... ... ... ... ... ... 135 
 
 Functions of ... ... ... ... ... ... ... 13 
 
 Governor for Series ... ... ... ... ... ... 117 
 
 Magnetic Leakage, Coefficient of 33 
 
 Multipolar 25 
 
 Output of 10,16,28 
 
 Separately -Excited 66 
 
 Series-Wound 60 116 
 
 Shunt-Wound 61 
 
 Steam 10 304 
 
 Transmission of Power, for 10,26,144 
 
 Dynamometer ... ... ... ... ... ... ... ... 244 
 
 Dynamotor 137,140,216 
 
 Earthing the Middle Wire 272 
 
 Economic Law of Lord Kelvin ... ... ... ... ... ... 72 
 
 E. Tremlett Carter on 73 
 
 Economy of Coal Consumption by use of Sub-Stations ... ... 218 
 
 Eddy Currents 173, 201, 206 
 
 Efficiency of Alternators 172 
 
 Dynamos ... ... ... ... ..- ... ...9, 46 
 
 Dynamotors ... ... ... ... ... ... ... 216 
 
 Motors ... 11,46 
 
 the Line 72 
 
 Transformers ... ... 188, 196 
 
 Electric Coal-Cutting 336 
 
 Coal-Cutting Corporation ... ... ... ... 348, 351 
 
 Power Plant, An ... ... ... ... ... ... ... 5 
 
 Rock Drilling 336,337,373 
 
 Electrical Centre of Gravity of Circuit 270 
 
 Electricity in Mining ... ... ... ... ... ... ...2,301 
 
 Electromotive Force, Average ... ... ... ... ... 153, 166 
 
 Condenser 167, 226 
 
 Effective 153, 166 
 
 Resultant 165, 168 
 
 Self-Induction 165,226 
 
 Square Root of Mean Square... ... ... ... ... 154 
 
 Total of Alternator 153,156 
 
 - Total of Dynamo 18 
 
 Total of Motor 18 
 
 Engine- House Equipment ... ... ... ... ... ...6,303 
 
 Equalising Dynamo ... ... ... ... ... ... ... 135 
 
 Equations of Alternator ... ... ... ... ... ... 156,163 
 
 Current in Line 124,127 
 
 Dynamo 16,20,36,38,43,52 
 
 Line, Conductor, &c. ... ... ... ... ... ... 72 
 
 Motor, Special (see also Dynamo) ... ... ... ... 43 
 
 -Transformer 188,197,198,201,206 
 
 Transmission Problems 72, 75 
 
 Ergmeters 245 
 
 Eeson, W. B., Dynamo Equations ... ... ... ... ... 21 
 
 On Asynchronous Single-Phase Motors 182 
 
 Excitation, Critical 176, 178 
 
 DD 
 
390 INDEX. 
 
 PAGE 
 
 Excitation, Equation for Field-Magnets ... ... . . . . 52 
 
 for Field-Magnets 37, 52, 55, 59, 176, 255 
 
 for Gap Space ... ... ... ... ... ... 36 
 
 Methods of 59 
 
 Exciter, Separate, Use of ... ... ... ... .. 68 
 
 Exploding Gas in Mines 319,322,325,337,338,341 
 
 External Characteristic 60,61,64,67 
 
 Fall of Pressure on Feeders 47, 124, 142, 226, 231, 234 
 
 Fans, Electric, in Mines ... ... ... ... .. ... ... 336 
 
 Feeder System of Distribution ... ... ... ... ... ... 131 
 
 Feeders ... 123,134,136,144 
 
 Ferranti Transformer ... ... ... ... ... ... ... 191 
 
 Field, Magnetic Distortion of ... ... ... ... ... ...24,51 
 
 Field, Rotary Magnetic 251,256,263,266 
 
 Field-Magnets, Bipolar 23 
 
 Coils, Size of, for 31 
 
 Design of 22, 27, 31, 37, 41, 176 
 
 Formula) for ... ... ... ... ... ... ... 31 
 
 Heating of ... ... ... ... ... ... ... 31 
 
 Induction in ... ... ... ... ... ... ... 29 
 
 Joints in ... ... ... ... ... ... ... 34 
 
 Multipolar 25 
 
 Number of Poles 25,174 
 
 Types of 23 
 
 Figure of Merit for Conductors 81 
 
 Fleming. Prof. J. A., on Alternate- Current Transformers ... 155, 197, 
 
 199, 202, 205, 208, 231 
 
 Flux, Magnetic 18,29" 
 
 Forbes, Prof. George, on Niagara Power Plant ... ... ... 272 
 
 Foucault Currents ' 173,201,206 
 
 Frankfort Experiment ... ... ... ... ... .. ... 209 
 
 Frequency of Periodic Currents 150,167,174.185,225,284 
 
 Effect of, on size of Alternators ... ... ... 167, 173 
 
 Fringe of Magnetic Field ... ... ... ... .-. ... 159 
 
 Ganz & Co., Method of Compensating for Drop in Feeders 235 
 
 Gap-space 21,29,36,292 
 
 Gas Engines .. ... ... ... ... ... ... ... 8 
 
 Gas, Producer 9 
 
 Gauge of Wire for Line, Choice of ... ... ... ... ..46,71 
 
 Goolden- Atkinson Electric Coal-Cutter 348 
 
 Pump ... ... ... ... ... ... ... 333 
 
 Safety Mining Motor 328,341 
 
 Governing Series Dynamos ... ... ... ... ... ... 117 
 
 Series Motors ... ... ._. ... ... ... ... 117 
 
 by Steam Pressure 179 
 
 Governors, Automatic 117,145,231,236 
 
 - Steam 179 
 
 Gramme, Dynamos, Advantages of 17 
 
 Armatures of ... ... ... ... ... ... ... 17 
 
 for Transmission of Power ... ... ... ... ... 46 
 
 Grounded Wire (see also Lightning Discharges) 105 
 
 Giilcher Alternator 159 
 
 Harmonic Function, An Alternate Current as an ... ... ... 154 
 
 Haseltine, R. M., Tests of Coal Cutters by 362- 
 
INDEX. 391 
 
 PAGE 
 
 Haulage, Electric 329,330,332 
 
 Heating of Armatures ... ... ... ... ... ... ... 20 
 
 Field-Magnet Coils 31 
 
 Hedgehog Transformer ... ... ... ... ... 190 
 
 Henry, the Unit of Self-induction ... ... ... ... ... 166 
 
 High-Pressure Alternators 175,208,269,284,286 
 
 Dynamos ... 117, 270 
 
 Insulation 194,209 
 
 - Motors .. 1'7, '-470 
 
 -Transformers 195,209 
 
 Transmission of Power at ... 76, 116, 132, 209, 263, 283, 381 
 
 Hole Armatures 292 
 
 Hopkinson's, Drs. J. and E., Curves of Magnetic Induction... ... 34 
 
 Horse-Power of Dynamos and Motors ... ... ... ... 10, 28, 30 
 
 Curves of ... ... ... ... ... ... ... 47 
 
 Hurd, F., Coal-Cutter 351 
 
 Safety Cable 325 
 
 Hysteresis . ... ... ... ... ... ... ... ... 172 
 
 Waste of Power by 173,175,201,205 
 
 Impedance 225 
 
 Coil ... 241,337 
 
 Induction, Magnetic ... ... ... ... ... ... ...29,35 
 
 -Mutual 228 
 
 - Self 165, 223 
 
 Inductive Drop of Pressure ... ... ... ... ... ... 226 
 
 Insulated Cables 84 
 
 Insulating Substances, Table of ... ... ... ... ... ... 85 
 
 Insulators, High-Pressure ... ... ... ... ... ... 92 
 
 Low-Pressure ... ... ... ... ... ... ... 90 
 
 - Oil 92 
 
 Solid 90 
 
 Internal Characteristics of Series Dynamo and Motor ... ... 121 
 
 Iron, Induction Curves of... ... .., ... ... ... ... 35 
 
 for Magnets 22,29 
 
 Proportions of, in Dynamos and Motors ... ... ... 31, 34 
 
 in Transformers ... ... ... ... ... ... 196, 207 
 
 Iron-clad Field-Magnets 27,28,339 
 
 Jeffrey Electric Coal-Cutter 355 
 
 Electric Dip Pump 334,336 
 
 Electric Rock Drill 374 
 
 Johnson and Phillips Two-Phase Alternator ... ... ... ... 290 
 
 Joints in Magnetic Circuit ... ... ... ... ... ... 34 
 
 Junction Box ... ... ... ... ... ... ... 88, 97, 312 
 
 Kapp's Regulator for Drop of Pressure at End of Feeder ... ... 233 
 
 Two-Phase Alternator 291 
 
 Kelvin, Lord, Economic Law of ... ... ... ... ... ... 72 
 
 on Increase of Resistance with Alternate Currents ... 219 
 
 Kennelly, A. E 229 
 
 Kolbein, Mr., on Polyphase Circuits 267 
 
 Lag of Current behind Pressure ..., 168, 214, 223 
 
 Lamination of Armature Conductors ... ... ... ... ... 20 
 
 Lauff en, Transmission Plant at ... ... ... ... ... ... 209 
 
 Law of Alternate Currents 153 
 
 DD2 
 
392 INDEX. 
 
 PAGE 
 
 Law of Magnetic Induction ... ... ... ... 34,37 
 
 Sine 154 
 
 Lead of Brushes, Dynamo and Motor 32,39 
 
 Leakage, Magnetic... ... ... ... ... ... 33 
 
 Magnetic, Coefficient of ... ... ... ... ... 33 
 
 Length of Mean Turn of Wire on Field-Magnets 52 
 
 Lightning Arresters, Functions of 102 
 
 for Power Plants 108,113 
 
 Telegraph and Telephone Types 106 
 
 Lightning Conductors ... ... ... 103 
 
 Lightning Discharger, Air- Blast Type 108 
 
 Alternate-Current 112 
 
 - Blow-out Type ~ 110 
 
 Wurts Ill 
 
 Lightning Discharges ... ... ... 102 
 
 Discharging Wire ... ... ... ... 105 
 
 Limits of Magnetisation ... ... ... ... ... 29, 35 
 
 Non-Sparking Load ... .. ... ... ... 21 
 
 Output of Dynamo and Motor 21,22 
 
 Limiting Distance of Transmitting Power ... ... ... 132, 268 
 
 Line, The 10, 69, 79, 84, 94, 102, 125, 218, 225, 381 
 
 Lines of Magnetic Force ... ... ... ... ... 18,29 
 
 Load Factor 205,210,217 
 
 Locke and Co.'s, Power Plant at Messrs. 301 
 
 Lodge, Prof. Oliver J., on Lightning Discharges 102 
 
 Long-Distance Transmission of Power 132,268,307,381 
 
 Long Shunt 64 
 
 Loss in Line (see also The Line) 125,219 
 
 Machines, Driven , 11 
 
 Magnetic Circuit, Single and Multiple ... 24 
 
 Flux 18,29 
 
 Leakage 33,158 
 
 Pull on Armature ... .. ... ... ... ... 54 
 
 Magnetisation, Curves of ... ... ... ... ... ... ... 35 
 
 - Resultant, with Polyphase Current Excitation 263 
 
 Marvin's Electric Percussive Rock Drill ... ... ... ... 378 
 
 Mean Power of an Alternate Current 171 
 
 Measurement of Alternate Currents ... ... ... ... 155,242 
 
 - of Drop in Feeders 234 
 
 Measuring Instruments ... ... ... .. ... ... ... 242 
 
 Mechanical Characteristics of Motors 60,64,67 
 
 Mines, Coal- Cutting in 345 
 
 - Exploding Gas by Electric Spark in 319, 322, 325, 337, 338, 341 
 
 Fans in ... ... ... ... ... ... 336 
 
 Haulage in 329,330.332 
 
 Pumping in 327,332 
 
 Rock-Drilling in 372 
 
 Winding in ... ... ... ... 331 
 
 Mining, Central Power Station for .... ... ... ... ... 3 
 
 Electricity in 2, 70, 301, 313, 318, 327, 337 
 
 Junction Boxes ... ... ... ... ... ... 312 
 
 Motors 45,327,338,341 
 
 Pownall's Safety System 325 
 
 Safety Cables 319, 321, 325 
 
 Shaft Cables 307 
 
 Switches and Resistances ... ... ... ... 315,316 
 
INDEX. 393 
 
 PAGE 
 
 Mining, Underground Cables ... ... ... ... 313 
 
 - Uninsulated Keturn in 318 
 
 - Use of Cut-outs (Fuses) in 315,317 
 
 Mordey Alternator ... ... ... ... ... ... ... 15L 
 
 - Transformer ... 192, 196 
 
 Motor, Alternate-Current, Asynchronous, Single-Phase 179 
 
 Alternate-Current, Polyphase, 252, 256, 263, 266, 282, 286, 289, 292 
 
 - Alternate-Current, Synchronous 149 
 
 Armature of ... ... ... ... ... ... ... 43 
 
 - Compound- Wound ... ... ... ... ... ... 6 
 
 Counter Pressure of ... ... ... ... 18 
 
 Davis and Stokes Safety 341,343 
 
 Design of Direct- Current ... ... ... ... ... 43 
 
 Function of ... ... ... .. ... ... ... 13 
 
 Goolden Safety 328,340 
 
 - Government of Constant- Current ... ... ... ... 119 
 
 Lead of Brushes 39 
 
 Mavor and Coulson Steel- Clad Mining... ... ... ... 338 
 
 Prime ... ... ... ... ... ... .. ... 6 
 
 Safety 338 
 
 Separately-Excited 68 
 
 Series-Wound 60, 116 
 
 Shaft of 45 
 
 Shunt- Wound 61 
 
 -Torque of 43,54,327,337 
 
 Transmission of Power ... ... ... ... ... ...11,68 
 
 Multiphase Currents (see Polyphase) 
 
 Multiple- Wire Systems (see also Three- Wire Systems) 237 
 
 Multipolar Dynamos ... ... ... ... ... ... ... 25 
 
 - Design of 26 
 
 Mutual Induction of Circuits 228,229 
 
 Niagara, Power Plant at 272 
 
 Nolet Safety Cable 323 
 
 Notation, Algebraic Symbols used ... ... ... ... ... 15 
 
 Oerlikon Company Alternators 284.285 
 
 Asynchronous Single-Phase Motor ... ... ... ... 184 
 
 Oil as an Insulator 194 
 
 Insulators ... ... ... ... ... ... ... ... 92. 
 
 Output of Dynamos 10,16,28,30 
 
 -Motors 10,16,28,30 
 
 Transformers 205, 210 
 
 Over-Compounding Dynamos ... ... ... ... ... ...63,65 
 
 Parallel, Coupling of Compound- Wound Dynamos in... ... ... 143 
 
 Working 122, 176, 179, 215, 246 
 
 Percussive Eock Drills 374,376 
 
 Peripheral Pull on Armature ... ... ... ... ... ...44,54 
 
 - Speed 20, 176 
 
 Phase of Alternator 178,213 
 
 Conversion... ... ... ... ... ... ... ... 278 
 
 Indicator 239 
 
 Pitch of Poles of Alternators 157 
 
 Polar Axis, Shifting of 32,39 
 
 Cavity, Arc of 21,34 
 
394 INDEX. 
 
 PAGE 
 
 Pole-pieces 32,174 
 
 Curving of ... ... ... ... .. ... ... 49 
 
 - Number of, Effect of 174 
 
 Pole Tips, Reversing Fields at 39 
 
 Polyphase Alternators (see also Alternators) ... ... ... ... 283 
 
 - Alternators, Design of 292 
 
 Currents 251, 256, 267, 278, 292, 337, 383 
 
 Motors 252, 256, 263, 266, 282, 286, 289, 292, 337 
 
 - Motors, Starting 265,293,337 
 
 Posts for Overhead Lines ... ... .. ... ... ... 93 
 
 Power, Electric Transmission of, 3, 5, 76, 132, 210, 218, 236, 263, 272, 283, 
 
 301, 383 
 
 Factor 188,226 
 
 Mean ... ... ... ... ... ... ... ... 171 
 
 Plant, Electric 3,6,383 
 
 Product of two Factors ... ... ... ... ... ... 116 
 
 Pownall, R. B. and Son, Safety Mining System 325 
 
 Predetermination of Curves of Dynamos ... ... ... ... 34 
 
 Pressure, Constant, Distribution ... ... ... ... ... 122 
 
 - Difference of, at Terminals 18 
 
 Drop of 47, 124, 142, 226, 231, 234 
 
 - High, Transmission .. 208, iil6 
 
 Producer Gas ... ... ... ... ... ... ... ... 9 
 
 Pumps, Electric, Centrifugal ... ... ... ... ... ... 334 
 
 Dip 332,334 
 
 Main 327 
 
 Rotary 335 
 
 Racing of Motors 337 
 
 Radial Depth of Armature ... ... ... ... ... .. 19 
 
 Ra worth -Callender Conduit ... ... ... ... ... ... 99 
 
 Reaction of Armature on Field 39,179,255 
 
 Rectifiers 297 
 
 Regulating Properties of Compound Winding 65 
 
 Properties of Series Winding 59,62,121 
 
 Properties of Shunt Winding ... . ... ... 62 
 
 Properties of Separate Excitation .. .. ... ... 66 
 
 Regulation, Automatic 117, 145, 231, 236, 337 
 
 by Exciting Circuit 144 
 
 by Shifting Brushes 117 
 
 Regulators, Dynamotors as ... ... ... ... 135,137,146 
 
 Series Dynamos as ... ... ... ... ... ... 146 
 
 Relation between Effective and Average Pressure in Alternators . . . 163 
 
 Relative Merits of Two-and Three-Phase Currents 267 
 
 Resistance, Increase of, with Alternate Currents ... ... ... 218 
 
 for Regulating Pressure 231,337 
 
 - Specific Table of 80,85 
 
 Retardation of Phase of Alternate Currents 168,178,203,214 
 
 ' in Transformers ... ... ... ... ... ... 203 
 
 Return, Uninsulated ... ... ... ... ... ... 70, 318 
 
 Reversal of Current at Commutator 39 
 
 Reverse Parallel System of Distribution 130,216 
 
 Reversibility of Dynamo and Motor ... ... ... ... ... 11 
 
 Reversing of Motors in Mining Operations 329,332 
 
 Rock Drills, Electric ... 372 
 
 Rotarv Currents (see Polyphase) 
 
 Magnetic Field 251,256,263,266 
 
INDEX. 395 
 
 PAGE 
 
 Safety Cable, Atkinson- Goolden 321 
 
 Charleton 322 
 
 Devices 247 
 
 Kurd 325 
 
 for Mining 318 
 
 Nolet 323 
 
 Safety Mining System, Pownall's 325 
 
 Motors 338 
 
 Say ers, W. B., Commutator Coils ... 40 
 
 Improvement in Dynamo Design ... ... ... 39,145 
 
 Winding for Drums ... ... ... ... ... ... 42 
 
 Schwabing, Dowson Gas Plant at ... ... ... ... ... 9 
 
 Scott, Charles F 225,278 
 
 Selection of Plant 7,8,327 
 
 Self-Induction 165,221 
 
 Causing Lag ... 165,171,223 
 
 Coefficient of 166 
 
 - Decrease of Output by 166,171 
 
 Decrease of Plant Efficiency by 171 
 
 Separate Excitation for Dynamos ... ... >.. ... ... 65 
 
 Series, Coupling Dynamos in ... ... ... ... ... 116,117 
 
 Coupling Motors in ... ... ... ... ... 116,117 
 
 Dynamo 59,121 
 
 Dynamo and Motor in 62,116,120 
 
 Motor 59,121 
 
 Power Transmission ... ... ... ... .. ... 116 
 
 Winding, Advantages of ... .. ... ... ... ... 62 
 
 Working 116,213 
 
 Shafts, Proportions of, for Motors 45 
 
 Shunt Dynamo ... ... ... ... ... ... ... ... 61 
 
 Shunt Motor ... ... ... ... ... ... ... ... 63 
 
 Silicium Bronze ... ... ... ... ... ... ... ... 83 
 
 Sine Curve 164 
 
 - Function 154 
 
 Law of Alternate Currents ... 154,163,176 
 
 Single-Phase Alternator (see also Alternators) 149 
 
 Alternate-Current Motor 149 
 
 Alternate Currents, Transmission of Power by ... ... 211 
 
 Size, Relation of, to Output of Dynamo ... ... ... ...28, 30 
 
 Slow-Speed Machine ... ... ... ... ... ... ... 10 
 
 Snell-Waterhouse Electric Coal-Cutter 367 
 
 Spacing Armature Winding ... ... ... ... ... ... 20 
 
 Spark Gap 107 
 
 Sparking, Cause of ... ... ... ... ... ... ... 21 
 
 - Load Limited by 21,39 
 
 Prevention of 21,39 
 
 Specific Resistance of Conductors ... ... ... ... ... 80 
 
 Speed, Dependence of Volts on 18,65 
 
 Effect of Low, in Alternators 174 
 
 in Relation to Cooling ... ... ... ... ... ... 20 
 
 in Relation to Power ... ... ... ... ... ... 18 
 
 Peripheral 20,176 
 
 Square-Section Wire for Armature ... ... ... ... ... 20 
 
 Stamp Batteries, Driving... ... ... ... ... ... ... 336 
 
 Standford's Water-Tight Joint in Piping 101 
 
 Starting Mining Motors 327,337 
 
 Station, Central Power, for Mining ... ... .,. ^ 
 
 OF THX 
 
 0IU7IRSITY 
 
396 INDEX. 
 
 PAGE 
 
 Steam Dynamos 10,304 
 
 Engines 6,303 
 
 Steel, Cast 29 
 
 Induction Curves of ... ... ... ... ... ... 35 
 
 Mild, Use of, in Dynamo Design ... ... ... ... 31 
 
 Steinmetz's Law 173 
 
 Step-up Transformers 208 
 
 Stokers, Mechanical 304 
 
 Stoneware Conduits ... ... ... ... ... ... ... 100 
 
 Sub-stations for Power Distribution ... ... ... ... 139,217 
 
 Sumpner, Dr. W. E 179.259 
 
 Symbols, List of 15,197 
 
 Synchroniser ... ... ... ... ... ... .. ... 239 
 
 Synchronism ... .. ... ... ... ... ... ... 213 
 
 Synchronous Motors 149,213,282 
 
 - Polyphase Motors .. 282 
 
 Systems of Transmitting Power, Weight of Copper required, etc. 267, 381 
 
 Tabulated Form for Designing Dynamos and Motors ... ... ... 56 
 
 Temperature, Rise of, in Armature .. .. ... ... ... 20 
 
 in Field Coils 31,32 
 
 Tests of American Electric Coal-Cutters 362 
 
 American Polyphase Motors ... ... ... ... ... 289 
 
 Oerlikon Polyphase Motors 286,289 
 
 Oerlikon Single -Phase Asynchronous Motors... ... ... 183 
 
 Thompson, Prof. S. P., on Alternate-Current Electro-Magnets ... 376 
 
 Alternate-Current Motors ... ... ... ... ... 178 
 
 - Phase Conversion 278 
 
 Thomson-Houston Air-Blast Lightning Discharger ... ... ... 108 
 
 : Alternator 240 
 
 Arc Dynamo ... ... ... ... ... ... ... 232 
 
 Coal-Cutter 372 
 
 Compensator ... ... ... ... ... ... ... 236 
 
 Transformers 195,196,200 
 
 Winding Engine, Electric ... ... ... ... ... 331 
 
 Three-Phase Circuits, Power in 261 
 
 Connections of ... ... ... ... ... ... ... 276 
 
 - Transmission of Power by ... ... ... ... .. 276 
 
 Three-Phase Alternators 256,273 
 
 Three-Wire Systems of Distribution ... 123, 132, 228, 237, 254, 272, 276 
 
 Size of Middle Wire 133 
 
 Toothed Armatures 42,292 
 
 Torque 43, 54, 118, 175, 263, 293, 337 
 
 Total E.M.F. of Armature 18 
 
 Traction, Motor 28,41 
 
 Series 120 
 
 Tr ansf ormer with Sub-divided Secondary Coil ... ... ... 237 
 
 Transformers, Brown, C. E. L 194 
 
 Brush 192, 196 
 
 Change Ratio of 229,246 
 
 Constant Current ... ... ... ... ... ... 199 
 
 Continuous Current (see Dynamotor) 
 
 Copper Loss in ... 201,205 
 
 Curves of ... 198,202,204 
 
 Design of Alternate Current 190, 197, 200, 205 
 
 Dobrowolski Three-Phase 274 
 
 Drop of Pressure ... ... ... ... ... ... 197 
 
INDEX. 397 
 
 PAGE 
 
 Transformers, Efficiency of 188,196,205 
 
 - Ferrariti 191,201 
 
 Giilcher (W. B. Esson) 193 
 
 Heatingof 201,205 
 
 Hedgehog 190,199 
 
 Hysteretic Loss in ...201,205,207 
 
 - Iron in 207 
 
 Kapp ... ... ... ... ... ... ... ... 192 
 
 Kapp and Snell 193 
 
 - Kennedy, Rankin 194 
 
 Oerlikou Three-Phase, Tests of 286 
 
 Parallel Type 197 
 
 -Polyphase 273,281,286 
 
 - Power Factor of ... ... ... ... ... ... 188 
 
 Series Type 199 
 
 - Step-up 208 
 
 Thomson -Houston 195,196,200 
 
 - Three-Phase 274 
 
 Two-Phase 273 
 
 -Types of 187,190,199,237 
 
 - Use of, in Distribution of Power ... ... ... 187,209 
 
 Westinghouse . ...191,196,198 
 
 Transmission of Power 5, 76, 132, 210, 218, 236, 263, 272, 283, 301, 381 
 
 in Mines ...301,313,318 
 
 Systems of 115,211,267,276,381 
 
 Turbines 8 
 
 Two- and Three-Phase Circuits, Combination of ... ... ... 278 
 
 Relative Merits of 267 
 
 Two-Phase Currents (see also Polyphase Currents) ... ... 251,267 
 
 - Dynamo ... ... ... ... ... ... 253 
 
 - Motor 252 
 
 Two- Wire System of Distribution 124,128,381 
 
 Uninsulated Returns 70,318 
 
 Units and Symbols ... ... ... ... ... ... ... 15 
 
 Van Depoele Electric Percussive Rock Drill ... ... ... ... 379 
 
 Ventilation of Armature ... ... ... ... ... ... ... 21 
 
 - of Transformers 194 
 
 Wattless Currents 168 
 
 Wattmeter 179,188,243 
 
 Weight of Copper in Line 125,268 
 
 W'estinghouse Alternator... ... ... ... ... ... ... 172 
 
 Transformer 191,196 
 
 Willans and Robinson High-Speed Engine ... ... ... ... 305 
 
 Winding Engines, Electric, in Mines .. ... ... ... ... 331 
 
 Wood Casing in Mines 311,314 
 
 Work Diagram 169 
 
 Work per Revolution of Armature ... ... ... ... ... 44 
 
 Wurts Lightning Dischargers ... ... .. ... ,. 111,112 
 
 Yoke, Magnet ... ... ... ... ... ... ... ... 33 
 
 Proportions of ... ... ... ... ... ... ... 34 
 
 Yorkshire Engine Company Coal-Cutter ... ... .. ... 364 
 
I EDISON & SWAN UNITED 
 
 ELECTRIC LIGHT COMPANY, LIMITED. 
 
 Head Offices, Warehouses, and Showrooms : 
 
 EDISWAN BUILDINGS, 36 and 37, QUEEN ST., CHEAPSIPE, B.C. 
 
 Telephone No. 1805. 
 
 The Largest Manufacturers in the World of Incandescence Electric 
 Lamps and Electric Light Fittings and Instruments. 
 
 Fitted with any Desired Tei 
 
 Contractors to the War Office, all Government Departments, many 
 
 Colonial Governments, Largest Steamship Companies, 
 
 Mining Companies, &c., &c. 
 
 SOLE MANUFACTURERS of the SUNLIGHT LAMP, 
 
 Single or Multiple Filament, from 100 to 2,000 candle-power. 
 
 MANUFACTUREBS of the WATERHOUSE ARC LAMP, 
 
 From 1,000 to 10.000 candle-power. 
 
 West End Depot, Warehouses, and Showrooms : 50, PARLIAMENT STREET, S.W. 
 
 Fully Illustrated Catalogue Free to any part of the World. 
 
INDEX TO ILLUSTRATIONS. 
 
 I'IG. PAGE 
 
 1 Field-Magnets, Wrought Cores, Cast Yoke, Upright Type ... 23 
 
 2 Field-Magnets, Cast Iron Cores and Yoke, Upright Type 23 
 
 3 Field-Magnets, Wrought Iron, Inverted Upright Type 23 
 
 3A Field -Magnets, VN rought Iron, with Cast Iron Pole Pieces ... 23 
 
 4 \ Field-Magnets, Double Magnetic Circuit, Wrought Iron Cores, \ 24 
 5/ Cast Iron Pole Pieces ... J 25 
 
 6 Field- Magnets, Double Magnetic Circuit, Wrought Iron 25 
 
 7 Field-Magnets, Four Poles, Wrought Iron 26 
 
 8 Field-Magnets, Multipolar, Cast Iron 26 
 
 9 Field-Magnets, Armour-Clad 27 
 
 10 Thomson-Houston Tramcar Motor 28 
 
 11 Curves connecting Weight and Output of Dynamos and Motors 30 
 HA Curves connecting Cost and Output of Dynamos and Motors.. 30 
 
 12 Curves of Magnetic Induction in Iron and Air 35 
 
 12A Diagram of W. B. Savers' Commutator Coils 40 
 
 12s Sayers' Winding for Drum Armature 42 
 
 13 Curves of Drop of Pressure and Power Wasted per Mile of 
 
 Conductor 47 
 
 14 Diagram showing Method of Designing Dynamo or Motor ... 49 
 
 14A Diagram of Path of Lines of Force in Armature 51 
 
 15A Diagram of Series Winding 60 
 
 IBB External Characteristic Curve of Series Dynamo 60 
 
 15c Mechanical Characteristic Curve of Series Motor 60 
 
 16A Diagram of Shunt Winding 61 
 
 I6u External Characteristic Curve of Shunt Dynamo 61 
 
 ]6c Mechanical Characteristic Curve of Shunt Motor 61 
 
 I?A Diagram of Compound Winding 64 
 
 17fi External Characteristic Curve of Over-Compounded Dynamo. 64 
 
 17c Mechanical Characteristic Curve of Compound Motor 64 
 
 18A Diagram of Separate-Excitation W hiding 67 
 
 18s External Characteristic Curve of Separately -Excited Dynamo. 67 
 
 18c Mechanical Characteristic Curve of Separately-Excited Motor. 67 
 
 19 Cable Hanger 82 
 
 20 Section of Armoured Cable 87 
 
 '21 Section of Concentric Armoured Cable 87 
 
 22 Junction Box for Single Cable 88 
 
 23 Junction Box for Concentric Cable, Plan 89 
 
 24 Junction Box for Concentric Cable, Elevation 89 
 
 25 
 
 Iron Stalks, Shackles, and Solid Insulators used for Low- 
 Pressure Aerial Lines 90 
 
 31 1 
 
 to[ 
 
 .34 J 
 
 Oil Insulators used for High-Pressure Aerial Lines 92 
 
 .34 
 
 35 Leather Line-Suspender 93 
 
 36 Metal Line-Suspender 93 
 
 31 Callender Solid Bitu men System , Cross Section 96 
 
400 ELECTEIG MOTIVE POWER. 
 
 FIG. PAGE, 
 
 38 Callender Solid Bitumen System, Longitudinal Section 96 
 
 39 Callender Solid Bitumen System, Cross Section of Service Box 97 
 
 40 Callender Solid Bitumen System, Plan of Service Box 97 
 
 41 Callender- Webber Dra wing-in System, Manhole 98 
 
 42 Callender- Webber Dra wing-in System, Casing 98 
 
 43 Callender-Raworth Dra wing-in System, Cross Section of Casing 99 
 
 44 Doul ton's Stoneware Casing, Cross Section 100 
 
 44A Doulton's Stoneware Casing, showing Joint 100 
 
 45 \ Doulton's Stoneware Casing, showing Expanding Mandril for 
 45A/ Jointing 100 
 
 46 Standf orth's Watertight Joint for Stoneware Pipes 101 
 
 47 Grounded Wire on Aerial Circuit 105 
 
 48 Lightning Arrester with Plain Gap 106 
 
 49 Lightning Arrester with Toothed Gap 106 
 
 50 Keystone Pattern Air-Blast Arc Breaker 108 
 
 51 The Power Circuit Lightning Arrester ~ 109 
 
 5lA Thomson-Houston Arc Line Protector 110 
 
 52 The Wurts Lightning Arrester Ill 
 
 53 The Wurts Alternate- Current Lightning Arrester 112 
 
 54 Double-Pole Lightning Arrester for Continuous-Current Circuit 114 
 
 55 Diagram of Motors in Series 116 
 
 56 Curves of Series- W ound Dynamo and Motor in Series 121 
 
 57 Diagram of Lamps in Series of Five on 500-volt Circuit 124 
 
 58 Diagram of Motors in Parallel 129' 
 
 59 Diagram of Reverse Parallel Circuit 130 
 
 60 Diagram of Feeder System 131 
 
 61 Diagram of Motors on Three-Wire System 132 
 
 62 Diagram of Three-Wire Feeder System with Three-Wire 
 
 Distributors 134 
 
 63 Diagram of High-Pressure Two-Wire Feeder System with 
 
 Regulator 135 
 
 64 Diagram of High-Pressure Two-Wire Feeder System with 
 
 Secondary Battery Regulator 136 
 
 65 Electric Construction Company's Dynamotor 138 
 
 66 Diagram of Feeder System with Dynamotors at Sub-Stations. 139 
 
 67 Diagram of Dynamotors in Series 141 
 
 68 Diagram of Rising Characteristic Curve of Compound Dynamo 142 
 
 69 Diagram of Connections of Two Compound- Wound Dynamos 
 
 in Parallel 143 
 
 70 Diagram of Separately-Excited Dynamos in Parallel 144 
 
 71 Curve showing Relation between Volts and Amperes with 
 
 Sayers' Regulator 146 
 
 72 Field-Magnet of Mordey Alternator . 151 
 
 73 Armature of Mordey Alternator 152 
 
 74 Sine Curve of Pressure or Current ... 154 
 
 75 Diagram of Typical Alternator Triangular Pressure Curve... 157 
 
 76 Diagram of Typical Alternator Flat-Top Triangular Pressure 
 
 Curve 157 
 
 77 Diagram of Typical Alternator nearly Rectangular Pressure 
 
 Curve 158 
 
 78 Diagram of Typical Alternator Triangular Pressure Curve... 158 
 
 79 Diagram of Typical Alternator Stepped Pressure Curve 160 
 
 80 Diagram showing Relation between Effective and Average 
 
 Pressure Sine Curve 160- 
 
 81 Diagram showing Relation between Effective and Average 
 
 Pressure Triangular Curve 161 
 
 82 Diagram showing Relation between Effective and Average 
 
 Pressure Semi-Circular Curve 162. 
 
INDEX TO ILLUSTRATIONS. 401 
 
 FIG. PAGE 
 
 83 Diagram of Phases of Pressure 165 
 
 84 Diagram of Phases of Current 167 
 
 85 Clock Diagram 168 
 
 86 Work Diagram Current and Pressure in Phase 169 
 
 87 Work Diagram Current Lagging in Phase behind Pressure... 170 
 
 88 Volt-Ampere Curves of Alternator 177 
 
 88A Curves connecting Exciting and Armature Currents of 
 
 Alternator 178 
 
 89 Diagram of Connections of Brown Single-Phase Asynchronous 
 
 Motors 181 
 
 90 Hedgehog Transformer.. 190 
 
 91 W T estinghouse Transformer 191 
 
 92 Ferranti Transformer 191 
 
 93 Mordey Transformer 192 
 
 94 Kapp Transformer 192 
 
 95 Giilcher Transformer 193 
 
 96 Snell and Kapp Transformer 193 
 
 97 Eankin Kennedy Transformer 194 
 
 98 C. E. L. Brown Transformer 194 
 
 99 Thomson-Houston Transformer 195 
 
 100 Latest Form of Thomson-Houston Transformer 195 
 
 101 Efficiency Curves of Transformers 196 
 
 102 Pressure Curves of Westinghouse Transformer 198 
 
 102A Thomson-Houston Constant-Current Transformer 200 
 
 103 Curves of Ferranti Transformer 202 
 
 104 Typical Curves of Transformer 204 
 
 105 Curves of Pressure and Current in Alternator and Motor 214 
 
 106 Capacity Diagram 222 
 
 107 Self-induction Diagram 223 
 
 108 Capacity and Self-induction Diagram 224 
 
 109 Curves of Inductive E.M.F. in Line 226 
 
 110 Curves of Inductive Drop in Line 226 
 
 110A Curves of Inductive Drop in Line (part of Fig. 110 enlarged). 227 
 
 111 Mutual-induction E.M.F. in Line 229 
 
 112 Curve showing Change of Secondary Pressure in Transformer 
 
 due to Capacity 230 
 
 113 Curve showing Change of Primary Pressure in Transformer 
 
 due to Capacity 230 
 
 114 Curve showing Change of Transformation Ratio in Trans- 
 
 former due to Capacity 231 
 
 115 Thomson-Houston Method of Compounding Alternator 232 
 
 116 Kapp' s Method of Regulating Drop of Pressure in Feeder ... 233 
 
 117 Diagram of Connections of Station Voltmeter for Reading 
 
 Pressure at Far End of Feeder 234 
 
 118 Ganz's Method of Compensating for Drop of Pressure in 
 
 Feeder 235 
 
 119 Prof. Elihu Thomson's Compensator System as applied to 
 
 Power Circuit 236 
 
 120 Diagram of Transformer with Secondary Coils divided into 
 
 Separate Circuits 237 
 
 121 Diagram of Synchroniser Connections 239 
 
 122 Impedance Coil as arranged for putting Alternators in Parallel 240 
 
 123 Impedance Coil 241 
 
 124 Lord Kelvin's Engine Room Wattmeter, General View 244 
 
 125 Lord Kelvin's Wattmeter Shunt Coils and Suspension Details 245 
 
 126 Major Cardew's Earthing Device for Transformers 248 
 
 127 Ferranti's Earthing Device 249 
 
402 ELECTRIC MOTIVE POWER. 
 
 FIG. PAGE. 
 
 128 Diagram of Four-Pole Two-Phase Motor ........................... 252. 
 
 129 Diagram of Two Periodic Currents with Common Ke turn ...... 254 
 
 130 Diagram of Excitation Variation with Two Currents in Quad- 
 
 rature .................................................................. 255 
 
 131 Diagram of Three Periodic Currents with Phase Intervals 
 
 of 120deg ................................................................ 256 
 
 132 Diagram of Three-Phase Parallel Coupling ........................ 257 
 
 133 Diagram of Three-Phase Dynamo Coupled to Motor .. 257 
 - 
 
 Diagrams of Three-Phase Circuits ....................... ........ 258 
 
 136 Diagram of Phases of Current in Closed Three-Phase Circuit 259 
 
 137 Diagram of Three- Phase Series Coupling ....................... 260 
 
 138 Diagram of Phases of Current in Open-Type Three-Phase 
 
 Circuit ................................................................. 261 
 
 139 Method of Measuring Power in Three-Phase Circuit with 
 
 Mains Equally Loaded ............................................... 262 
 
 140 Method of Measuring Power in Three-Phase Circuit with 
 
 Mains Unequally Loaded ............................. .............. 262 
 
 141 Diagram of Excitation Variation with Three-Phase Currents.. 264 
 145 Dobrowolski's Combined Series and Parallel Three-Phase 
 
 Coupling ................................................................. 265 
 
 143 Diagram showing Ampere-Turns with Parallel Three-Phase 
 
 Coupling ............................ ........... .. ........................ 266 
 
 144 Diagram showing Ampere-Turns with Series Three-Phase 
 
 Coupling ................................................................ 266 
 
 145 Diagram of Effective and Maximum Pressures in Single-Phase 
 
 Circuit .............................................. .................. 267 
 
 146 } Diagrams of Effective and Maximum Pressures in Two-Phase / 269 
 
 147 / Circuit with Three Wires ......................................... \270 
 
 148 Diagram of Effective and Maximum Pressures with Three- 
 
 Phase Circuit ........................................................... 271 
 
 149 Diagram of Four-Wire Two-Phase Circuit with Two Single- 
 
 Phase Transf ormers .......... ...................................... 273 
 
 150 Dobrowolski Three-Phase Transformer .............. ............. 274 
 
 151 General Diagram of Polyphase Transformer Connections ...... 275 
 
 1 ^9 1 
 
 1531 Dia S rams of Three-Phase Power Plants ......... .................... 276 
 
 Diagrams of Three-Phase Circuit Connections .................... 277 
 
 .jrr-. j- Diagrams showing Conversion of Two- to Three-Phase ........ 1 97Q 
 
 158 Diagram of Two- to Three-Phase Transformer ..................... 280 
 
 159 Diagram of Pressure Distribution with Phase Conversion ...... 280 
 
 160 Diagram of Combined Two- and Three-Phase Circuit ......... 281 
 
 161 Diagram of Synchronous Polyphase Alternator ................. 283 
 
 162 Oerlikon Low-Pressure Polyphase Alternator ..................... 284 
 
 163 Oerlikon High-Pressure Polyphase Alternator ..................... 285 
 
 164 Characteristic Curve of Three-Phase Oerlikon Alternator ...... 287 
 
 166 I ^ er likon Asynchronous Polyphase Motor ................... ..... j noo 
 
 167 Allgemeine Electricitats-Gesellschaft Asynchronous Polyphase 
 
 Motor ..................................................................... 290 
 
 168 Johnson and Phillips Two-Phase Alternator ........................ 291 
 
 170 I Grooves for Windings of Polyphase Machine ............ ....... 292 
 
 171 Diagram showing Construction of Polyphase Motor ............ 292 
 
 17/M fSQ 
 
 173 > Curves of Torque of Polyphase Motors .............................. \2Q5 
 
 j 
 
INDEX TO ILLUSTRATIONS. 403 
 
 FIG. PAGE 
 
 174 Curves connecting Power Factor and Output of Polyphase 
 
 Motors 296- 
 
 175 Diagram of Polyphase Continuous-Current Converter 297 
 
 176 Diagram of Rectified Alternate Current 298 
 
 177 Ferranti Rectifier 298 
 
 178 Steam Dynamo, Open Type Engine 305 
 
 179 Steam Dynamo, Closed Type Engine 306' 
 
 180 Armoured Concentric Cable 308 
 
 182J Wood Casings for Pit Work 311 
 
 183 Junction Box for Mines . 312 
 
 184 ) 
 
 184A \ Gas-Tight Starting Switch and Resistance for Mines 
 
 184s I 
 
 316 
 317 
 318 
 
 1 ft^l ^ 
 
 186 f Concentric Cables with Uninsulated Return . ... 320 
 
 187 Atkinson Safety Cable 321 
 
 188) (323 
 
 189 \ Nolet Safety Cable \ 323 
 
 190 J 1 324 
 
 191 Three-Throw Electric Main Pump 326- 
 
 192 Endless Rope Electric Hauling Engine 328 
 
 193 Main and Tail Rope Electric Hauling Engine 330 
 
 194 Electric Winding Engine 331 
 
 195 Electric Three-Throw Dip Pump 333 
 
 195A Electric Three-Throw Dip Pump on Trolley 334 
 
 196 Electric Centrifugal Dip Pump 335 
 
 196A Jeffrey Double Rotary Dip Pump 336 
 
 197 Mavor and Coulson Steel-clad Mining Motor 339 
 
 198 Goolden-Atkinson Safety Mining Motor 340 
 
 199 Davis and Stokes Safety Mining Motor 342 
 
 200 Goolden-Atkinson Electric Coal-Cutter, Elevation 349 
 
 200A Goolden-Atkinson Electric Coal-Cutter, Cross Section 349 
 
 201 Goolden-Atkinson Electric Cutter-Bar 350 
 
 20lA Goolden-Atkinson Electric Coal-Cutter, Method of Working... 350 
 
 202 Kurd's Electric Coal-Cutter 352 
 
 gQ^j Birtley Rotary Bar Electric Coal-Cutter 353 
 
 205 Section of Birtley Cutter-Bar 355 
 
 206 Jeffrey Electric Coal-Cutter, Method of Working 356 
 
 207 Jeffrey Electric Coal-Cut' er, Perspective View 357 
 
 208 Jeffrey Electric Coal-Cutter on Trolley 359 
 
 208A Rotary Bar Coal-Cutter of General Electric Co 384 
 
 209 Yorkshire Engine Company's Side-Wheel Compressed Air 
 
 Coal-Cutter 365 
 
 210 Cutter-Wheel, Yorkshire Engine Company 366 
 
 211 Snell-Waterhouse Electric Coal-Cutter, Plan 368 
 
 212 Snell-Waterhouse Coal-Cutter, Longitudinal View 369 
 
 213 Snell-Waterhouse Coal-Cutter, End View 370 
 
 214 Wantling-Johnson Electric Coal-Cutter and Heading Machine 371 
 
 215 Plan of Cutter Wheels of Wantling-Johnson Coal-Cutter 371 
 
 216 Thomson-Houston Electric Coal-Cutter 372 
 
 217 Jeffrey Rotary Electric Rock and Coal Drill 375 
 
 218 Diagram of Electric Percussive Drill, with Two-Way Switch.. 377 
 
 219 Diagram of Electric Percussive Drill for Use on Two-Phase Circuit 378 
 
 220 Diagram of Marvin Electric Percussive Drill 378 
 
 221 Diagram of Van Depoele Electric Percussive Drill 379 
 
CABLESi^WIRES 
 
 FOE 
 
 IptNC, WORKSHOP, TRAMWAY, 
 
 AXD OTHER 
 
 ELECTRIC LIGHT AND POWER 
 CIRCUITS, &c. 
 
 CONCENTRBG CABLES. 
 
 SIGNAL CIRCUIT CABLES AND WIRES. 
 
 BOTH 
 LOW AND HIGH 
 
 TENSION, 
 
 To fire 15, 8, & 2 
 
 Shots. 
 
 EXPLODER 
 TWIN CABLES. 
 
 Fuses, and 
 
 all Accessories for 
 
 Export. 
 
 INSULATORS, 
 
 IRONWORK, 
 LINE STORES &TOOLS, 
 
 FOB 
 
 Aerial and Mining, Telephone, Telegraph, 
 Electric Light and Power Circuits. 
 
 ALSO 
 
 TELEPHONES AND BATTERIES. 
 
 THE TELEGRAPH MFG. COMPANY, LTD., 
 
 HELSBY (near Warrington), and 
 
 11, QUEEN VICTORIA STREET, LONDON, E.G. 
 
LIST OF TABLES OF DATA AND TESTS. 
 
 PAGE 
 
 A Permissible Fall of Pressure in Armature Expressed as a Per- 
 centage of the Total E.M.F 18 
 
 B Radial Depth of Core of Gramme Armature 19 
 
 C Comparative Magnetic Inductions in Wrought Iron, Cast Iron, 
 
 and Air 29 
 
 D Magnetic Leakage Coefficients for various types of Field-Magnets 33 
 
 E Tabulated Form for Use in Designing Dynamos and Motors ... 56 
 F Specific Resistance, Specific Gravity, and Tensile Strength of 
 
 various Conductors 80 
 
 G Specific Resistance of Dielectrics compared with that of Mica ... 85 
 
 H Usual Spans with Hard -drawn Copper Conductors 94 
 
 I Percentage Drop of Pressure in Two- Wire Mains for various 
 
 Current Densities and Distances 128 
 
 J Ratio between Effective and Average Pressures in Alternators 
 
 having Pressure Curves of various orders 163 
 
 K Proportions of Alternator Field-Poles and Armature Coils in 
 
 terms of the Pitch, and Consequent Values of Tc 164 
 
 L Power Wasted by Hysteresis in Watts per cubic inch 173 
 
 M Speed of Alternators with various Numbers of Field-Poles for 
 
 the usual Frequencies 174 
 
 N Permissible Induction in Alternators for the usual Frequencies... 175 
 
 P 
 
 Q ]- Tests of Oerlikon Single-phase Asynchronous Motors 182 
 
 It 
 S 
 
 T Data of Oerlikon Asynchronous Single-phase Motors 184 
 
 U Efficiencies of various Transformers 189 
 
 V Increase of Resistance of Conductors in Alternate-Current 
 
 Working 219 
 
 W Examples of Practical Measuring Instruments suitable for Power 
 
 Stations 242 
 
 X Weights of Line Copper required with different Systems of 
 
 Transmitting Power 268 
 
 Y Data of Oerlikon Three-phase Transformers 286 
 
 Z Data of Oerlikon Low-pressure Three-phase Alternators 286 
 
 A A Data of Oerlikon High-pressure Three-phase Alternators 286 
 
 B B Tests of Oerlikon Small Three-phase Motors 289 
 
 CO Data of American Three-phase Motors 289 
 
 D D Tests of American Electric Coal-Cutters {^2 
 
 t 363 
 E E Cost of Continuous- Current Power Plants in terms of per H.P. 
 
 delivered by Motor ... 382 
 
 F F Cost of Line and of H.P. delivered, with Three-phase Circuits, for 
 
 various Distances and Pressures 383 
 
 EE 
 
SIEMENS BROS. & CO., 
 
 LIDMIITIEID., 
 
 (Ekctncal & telegraph (Enptters. 
 
 MANUFACTURERS OF 
 
 DYNAMOS AND . . . 
 . . . ALTERNATORS, 
 
 MOTORS FOR ALL PURPOSES. 
 
 CABLES and WIRES, 
 
 ELECTRICAL and TELEGRAPH INSTRUMENTS. 
 
 CONTRACTORS FOR 
 
 TRANSMISSION* POWER PLANT. 
 
 ELECTRIC TRAMWAYS and RAILWAYS, 
 
 ELECTRIC LIGHTING. 
 
 Head Office : 
 
 12, QUEEN ANNE'S GATE, LONDON, S.W. 
 
 Branch Offices : 
 
 21, GRAINGER ST. WEST, NEWCASTLE ON-TYNE ; 
 
 261, WEST GEORGE STREET, GLASGOW; 
 
 5$, MARKET STREET, MELBOURNE. 
 
 Works: WOOLWICH, KE1VT. 
 
LIST OF FORMULA 
 
 NO. PACK 
 
 (1) Approximate Output of Gramme-type Dynamo 16 
 
 (2) Approximate Output of Drum-type Dynamo 16 
 
 (3) Approximate Output of Gramme-type Motor 17 
 
 (4) Approximate Output of Drum-type Motor 17 
 
 (5) (6) Total E.M.F. of Armature 18 
 
 (7) Magnetic Flux in Armature 18 
 
 (8) Number of Turns of Wire on Armature 19 
 
 (8A) Eise of Temperature of Armature 20 
 
 (9) Permissible Circumflux of Gramme Armature 22 
 
 ^10) Permissible Circumflux of Drum Armature 22 
 
 (10A) Rise of Temperature of Field-Magnets 32 
 
 (11) Excitation for Air Gap 36 
 
 (12) Total Excitation for Dynamo or Motor 37 
 
 (13) Compensating Turns for Gramme Armature 38 
 
 (14) Compensating Turns for Drum Armature 38 
 
 (15) Torque in terms of Armature Constants in C.G.S. Units 43 
 
 (16) Torque in terms of Armature Constants in Pound-Feet Units 43 
 
 (17) Work per Revolution of Armature 44 
 
 (18) Torque in Pound-Feet 44 
 
 (19) Peripheral Pull exerted by Armature 44 
 
 (20) Equivalent or Average Current 73 
 
 (21) Lord Kelvin's Law of Economy 73 
 
 (2lA) Mr. E. Tremlett Carter's Modification of Law of Economy ... 73 
 
 (22) Figure of Merit of a Conducting Metal 81 
 
 <23) (24) (25) Fall of Pressure in Conductor 127 
 
 (26) (27) Effective value of total E.M.F. in Alternator 155 
 
 (28) Average total E.M.F. of Alternator : parallel connection 156 
 
 (29) Average total E.M.F. of Alternator : series connection 156 
 
 (30) Maximum total E.M.F. of Alternator : parallel connection ... 156 
 
 (31) Maximum total E.M.F. of Alternator : series connection 156 
 
 (32) Effective total E.M.F. of Alternator : parallel connection ... 156 
 
 (33) Effective total E.M.F. of Alternator : series connection 156 
 
 (34) Maximum E.M.F. of Alternator with Triangular Curve of 
 
 Pressure 159 
 
 (35) Effective E.M.F. of Alternator with Triangular Curve of 
 
 Pressure 163 
 
 (36) Relation between E.M.F.'s in Alternate-Current Circuit 165 
 
 <37) \Maximum E.M.F. of Self-induction in Alternate- Current 
 
 (38) / Circuit 166 
 
 <39) Effective E.M.F. of Self-induction in Alternate- Current 
 
 Circuit 166 
 
 (40) Relation between Currents in Alternate- Current Circuit 168 
 
 (41) Mean Power in Alternate- Current Circuit 171 
 
 (42) Drop of Pressure in Transformer 197 
 
 (43) Drop of Pressure in Transformer due to Magnetic Leakage... 198 
 
 (44) Number of Watts lost in Iron Cores 201 
 
 (45) Effective Primary Pressure in Transformer 206 
 
 (46) Number of Secondary Turns in Transformer 207 
 
 <48) Capacity of Conductor in Microfarads 222 
 
 (49) Condenser Current in Amperes 222 
 
 EE2 
 
JOHN DAVIS & SON, 
 
 All Saints Works, Derby, and 118, Newgate St., London. 
 
 Electric Xigbt anfc power Engineers. 
 
 ELECTRIC LIGHTING PLANTS FOR COLLIERIES, MILLS, 
 
 WORKS, RESIDENCES, &c. 
 Electric Haulage Plants. Electric Pumping Plants. 
 
 JEFFREY COAL-CUTTING & DRILLING MACHINES. 
 
 The Jeffrey Coal-Cutting Machines are constructed for Long- Wall or Pillar and 
 Stall Work, and are operated by electricity or compressed-air power. Over 600 
 Jeffrey machines are in daily use, and the number is rapidly increasing. 
 
 Estimates furnished and Mines examined to ascertain the suitability 
 of Jeffrey machines. 
 
 THE 
 
 MOTOR 
 
 Is employed in gassy mines for dip pumps, &c. Has withstood the test of time 
 and all attempts to explode. 
 
 FOWLER-WARING GABLES GO. (L-), 
 
 32, Victoria Street, 
 
 Westminster, S.W. 
 
 Works: 
 North Woolwich, B. 
 
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 @CectrtcctC JIaBoraforj? ^lofes & 
 
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 February, 1895. 
 
 THE Publisher of "THE ELECTRICIAN" has compiled the following List of Booka 
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 "THE ELECTRICIAN" PRINTING AND PUBLISHING COMPANY, LIMITED, Salisbury Court, 
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 CONTENTS. 
 
 Electrician " Series* 
 
 PAGE. 
 10 
 10 
 
 11 
 11 
 
 12 
 13 
 
 PAGE. 
 
 "The Electrician" Electrical Trades' 
 
 Directory and Handbook 3 
 
 Electromagnetic Theory 4 
 
 Magnetic Induction in Iron and other 
 
 Metals 4 
 
 Electric Lamps and Electric Lighting 5 
 
 The Alternate - Current Transformer in 
 
 Theory and Practice ( Vols. I. and II.).,. 6,7 
 
 "The Electrician" Primers (Vols. I. and 
 
 II.) 8 
 
 A Digest of the Law of Electric Lighting, 
 
 &c., &c 8 
 
 Electrical Engineering Formulae 9 
 
 The Work of Hertz 9 
 
 The Incandescent Lamp and its Manufacture 
 
 Drum Armatures and Commutators 
 
 Electro-Chemistry 
 
 The Art of Electrolytic Separation of Metals 
 Electrical Laboratory Notes and Forms 
 Practical Notes for Electrical Students ... 
 
 The Steam Engine Indicator, and Indicator 
 Diagrams 13 
 
 The Manufacture of Electric Light Carbons 13 
 
 Electric Motive Power... 14 
 
 Submarine Cable Laying and Repairing ... 14 
 
 Motive Power and Gearing 14 
 
 Fine Art Reproductions, &c 15 
 
 Electrical anb General Xiet 
 
 " The Electrician " l7t 
 
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 Electricity and Magnetism 
 
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 Reference Books, &c. 
 
 Miscellaneous... 
 
 16 
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 27 
 
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 THE 
 
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 BRITISH DIVISION. ^ 
 
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 Summary of Electrical Events for the preceding year. Obituary Notices for the preceding year. 
 PATENTS, DESIGNS, AND TRADE M&~RKS.-( Specially written.) 
 
 DIGEST OF THE LAW OF ELECTRIC LIGHTING and of ELECTRIC TRACTION. (Special) 
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 London County Council Regulations as to Theatre Lighting. [(Special.) 
 
 Rules and Regulations for the Prevention of Fire Risks arising from Electric Lighting : British and Foreign- 
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 Installation Regulations of the London and Provincial Electric Supply Companies. (Special.) 
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 Provisional Orders and Licenses granted by the Board of Trade in the preceding year. 
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 CENTRAL LIGHTING STATIONS OF THE UNITED KINGDOM. -(Specially-.Compiled Folding Sheet.) 
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 ELECTROMAGNETIC THEORY. 
 
 VOL. I. 
 
 BY OLIVER HEAVISIDE. 
 
 IFIROIM: THIS 
 
 This work was originally meant to be a continuation of the series " Electromagnetic Induction 
 an its Propagation," published in The Electrician in 1885-6-7, but left unfinished. Owing, how- 
 ever, to the necessity of much introductory repetition, this plan was at once found to be impractic- 
 able, and was, by request, greatly modified. The result is something approaching a connected 
 treatise on electrical theory, though without the strict formality usually associated with a treatise. 
 The following are some of the leading points in this volume. The first chapter is introductory. The 
 second consists of an outline scheme of the fundamentals of electromagnetic theory from the Faraday- 
 Maxwell point of view, with some small modifications and extensions upon Maxwell's equations. 
 The third chapter is devoted to vector algebra and analysis, in the form used by me in my former 
 papers. The fourth chapter is devoted to the theory of plane electromagnetic waves, and, being mainly 
 descriptive, may perhaps be read with profit by many who are unable to tackle the mathematical 
 theory comprehensively. It may be also useful to have results of mathematical reasoning expanded 
 into ordinary language for the benefit of mathematicians themselves, who are sometimes too apt to 
 work out results without a sufficient statement of then: meaning and effect. But it is only intro- 
 ductory to plane waves. I have included in the present volume the application of the theory (in 
 duplex form) to straight wires, and also an account of the effects of self-induction and leakage, 
 which are of some significance in present practice as well as in possible future developments. There 
 have been some very queer views promulgated officially in this country concerning the speed of the 
 current, the impotence of self-induction, and other material points concerned. 
 
 Vol. II. is in preparation. __^_ 
 
 SECOND ISSUE. 370 pages, 150 illustrations. Price 10s. 6d., post free. 
 
 MAGNETIC INDUCTION IN IRON AND OTHER METALS. 
 
 BY J. A. EWING, M.A., B.Sc., 
 
 Professor of Mechanism and Applied Mechanics in the University of Cambridge. 
 
 SYNOPSIS OF CONTENTS, 
 
 After an introductory chapter, which attempts to explain the fundamental ideas and the 
 terminology, an account is given of the methods which are usually employed to measure the 
 magnetic quality of metals. Examples are then quoted, showing the results of such measurements 
 for various specimens of iron, steel, nickel and cobalt. A chapter on Magnetic Hysteresis follows, 
 and then the distinctive features of induction by very weak and by very strong magnetic forces 
 are separately described, with further description of experimental methods, and with additional 
 numerical results. The influence of Temperature and the influence of Stress are next discussed. 
 The conception of the Magnetic Circuit is then explained, and some account is given of experi- 
 ments which are best elucidated by making use of this essentially modern method of treatment. 
 The book concludes with a chapter on the Molecular Theory of Magnetic Induction, and the 
 opportunity is taken to refer to a number of miscellaneous experimental facts on which the 
 molecular theory has an evident bearing. 
 
 "Prof. Ewing is such a great and well-known authority on Magnetic Induction that it is only necessary to 
 call attention to this second issue of his valuable work to insure its being in the hands of all who require 
 information upon this important subject." Philosophical Magazine. 
 
 "Besides being practically the only book dealing with the subject of magnetic induction, this work considers 
 
 the whole subject very exhaustively, both from theoretical and practical points of view The large number 
 
 of graphic representations illustrating the various phenomena of magnetic induction are of great value. . . 
 We can recommend the book as being of considerable importance." Electrical Review. 
 
 " In this book Prof. Ewing has placed before his readers an accurate record of the more recent and important 
 work done in magnetism. The book as a whole presents the best resume" of this subject extant." 
 
 Electrical Engineer (New York). 
 
 " This is one of the most important books that has appeared, and coming irom an author of such an 
 established reputation it will be looked upon as an undoubted authority upon the important subject of which 
 it treats. No man is better fitted for the task of discussing these questions than Prof. Ewing." 
 
 Electrical World (New York). 
 
 ^_ A German Edition is also published, price 8s. 6d. 
 
 I, 2, and 3, Salisbury Court, Fleet Street, London, E.G. 
 
 
"The Electrician" Printing and Publishing Co., Ltd., 5 
 "THE ELECTRICIAN" SERIES continued. 
 
 Now Ready. Very fully illustrated, handsomely bound, on good paper, price'7s. 6d. 
 
 ELECTRIC LAMPS AND ELECTRIC LIGHTING. 
 
 Being a Course of Four Lectures delivered at the Royal Institution, April-May., 1894, 
 
 By PROF. J. A. FLEMING, M.A., D.Sc., F.R.S., M.K.I., 
 
 Professor of Electrical Engineering in University College, London. 
 
 SYNOPSIS OF CONTENTS. 
 
 I. A Retrospect of Twelve Years Factors in the Development of Electric Illumination The Historical 
 Starting Point Davy's Researches on the Electric Arc The Evolution of Incandescent Electric Lighting- 
 Definition of Fundamental Terms Units of Measurement of Current, Pressure, Work, and Power Board of 
 Trade Units Conditions of Public Supply under Acts of Parliament The Heating Effect of an Electric Current 
 Joule's Law Experimental Proofs Radiation from Incandescent Bodies Temperature of Radient Bodies- 
 Surface Efficiency and Specific Radient Qualities of Various Materials Peculiar Properties of Carbon Bright- 
 ness of Various Lights Methods of Photometric Comparison Standards of Light Sun, Moon, Electric Arc, 
 Incandescent Lamp as 111 uminants Quality and Intensity of Light Luminosity and Candle-power The 
 Physiological Question of Vision. 
 
 II. The Physics of an Incandescent Lamp Characteristic Curves Relation of Candle-power to Current 
 and Pressure Effects of Position on Candle-power Age of Incandescent Lamps Lamp Mortality Causes of 
 it Judicious and Injudicious Arrangement of Lamps Sockets Switches Fuses Decorative Employment of 
 Incandescent Lamps House Wiring Fire Office Rules Good and Bad Work Causes of Destruction of the 
 Carbon Filament Molecular Physics of the Glow Lamp Edison Effect Large Incandescent Lamps Electric 
 Meters Methods of Testing and Comparing Glow Lamps Advantageous Utilisation of Current. 
 
 III. The Electric Arc Lamp Method of Production of the Arc Study of the Arc by Projection Laws of 
 the Electric Arc The Convection of Carbon in the Arc The Crater The Distribution of Electric Pressure in 
 the Arc Arc Lamp Mechanism Recent Improvements Distribution of Light from the Arc Luminous 
 Efficiency of the Arc Comparison with Incandescent Lamps Street and Interior Arc Lighting Proper Distri- 
 bution of Light Arc Light Photometry The Alternating and Continuous Arc The Inverted Arc The Use of 
 the Arc in Metallurgy Electrical Reduction of Metals by the Arc Temperature of Arc Light Crater The 
 Solar Temperature. 
 
 IV. The Production of Current for Electric Lighting Generating Stations Systems of Supply Low 
 Pressure Continuous and High Pressure Alternating Structure of a Dynamo and Transformer Views of 
 Generating Stations at Home and Abroad Underground Conducting Mains Networks of Conductors House 
 Services Long Distance Transmission Electric Lighting of Rome Tivoli- Rome Transmission Utilisation of 
 Water-power Load Diagrams of Stations Supply of Currents for Purposes other than Light. 
 
 PRE2SS NOTICES. 
 
 "Treats the whole subject with the lucidity for which Prof. Fleming's expositions are remarkable, and in 
 language which has been, as far as possible, divested of technicalities. . . . Those who may contemplate 
 the electric lighting of their houses will find in the book many valuable hints and practical suggestions." Times. 
 
 " Prof. Fleming possesses the rare gift of being able to interest almost any audience in his subject. . . . 
 The work under notice consists of a transcript of four lectures delivered at the Royal Institution, and though 
 much of the charm which the lecturer was able to throw round the subject is now absent, still enough remains 
 to render a perusal of these pages a recreation rather than a study. . . . Besides supplying the non-technical 
 student with such information as he is sadly in need of, he has given to the serious student the best account 
 which we have yet seen of the physical properties of arc and glow lamps. . . . The volume contains some 
 220 pages of large type, and is well illustrated throughout. It is the only one we have ever seen that we can 
 thoroughly recommend to the non-technical reader. On the other hand, for the electrical student we cannot too 
 forcibly recommend a careful study of the second and third lectures." Daily Chronicle. 
 
 1 We have no hesitation in recommending all who may be interested in the subject to buy a copy of Prof. 
 Fleming's book, which is well worth the price asked for it ; being fresh as regards matter, and abundantly 
 provided with illustrations which are more or less original. ... As we commenced this article by stating, 
 the book is worth buying for a guide to a fair general knowledge of the principles which underlie the industrial 
 uses of electricity. We can also compliment the publishers upon the way in which the volume is turned out." 
 Journal of Gas Lighting. 
 
 "When one comes across a reprint of a series of afternoon Royal Institution lectures he expects an elemen- 
 tary book, with its contents strung together to connect a lot of experiments that the author was anxious to show 
 to an audience already jaded with a variety of subjects no one human brain could take in. Dr. Fleming has 
 given us nothing of this sort, but a concise account of incandescent and arc lamps, followed by a short sketch of 
 electric lighting generally. It is by no means a mere popular book, however, and few electricians, no matter 
 how well up in their subject, will find they can afford to pass it by. ... The binding, printing, and general 
 get up is admirable." J.S. in The Electrician. 
 
 " Dr. Fleming is known for his high abilities in setting forth scientific facts in a popular lecture in clear and 
 most attractive language, and anyone familiar with the elements of physical science will read the book with 
 great pleasure and derive much sound instruction. . . . The book is very handsomely got up, and would 
 make an exceedingly suitable presentation volume to anyone interested in, and who already knows at least a 
 little of, the subject." Glasgow Herald. 
 
 DIGEST POST FREE ON APPLICATION. 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London, E.G. 
 
6 "The Electrician" Printing and Publishing Co., Ltd., 
 
 "THE ELECTRICIAN " SERIES continued. 
 
 FOURTH ISSUE. 500 pages, 157 illustrations. Price 7s. 6d., post free 8s. 
 
 THE ALTERNATE CURRENT TRANSFORMER 
 
 IN THEORY AND PRACTICE. 
 
 By J. A. FLEMING, M.A., D.Sc., F.R.S., M.R.I., &c. ; 
 
 Professor of Electrical Engineering in University College, London. 
 
 YOL. L THE INDUCTION OF ELECTRIC CURRENTS. 
 
 SYNOPSIS OF CONTENTS, 
 CHAPTER I. Introductory. 
 
 Faraday's Electrical Eesearches Early Experiments on Current Induction Electro-Dynamic 
 Induction First Induction Coil Electro-Magnetic Induction Lines of Force Methods of 
 Ampere, Arago, and Faraday Physical Nature of Lines of Magnetic Force. 
 
 CHAPTER II. Electro-Magnetic Induction. 
 
 Magnetic Force and Magnetic Induction Tubes of Magnetic Induction Kate of Change of 
 Magnetic Induction through a Circuit Inductance Electromotive Force of Induction Electro- 
 Magnetic Momentum Electro-Magnetic Energy Dimensions of the Co-efficient of Self-induction 
 or Inductance Unit of Self-Induction or Inductance Constant and Variable Inductance Curves 
 of Magnetisation Graphical Eepresentation of Variation of Co-efficient of Induction Magnetic 
 Hysteresis. 
 
 CHAPTER III. The Theory of Simple Periodic Currents. 
 
 Variable and Steady Flow Current Curves Simple and Complex Harmonic Motion- 
 Fourier's Theorem Simple Periodic and Sine Curves Current Growth in Inductive Circuits 
 Logarithmic Curves Geometrical Illustrations Graphic Eepresentation of Periodic Currents 
 Mean Value of the Power of a Periodic Current Power Curves Experimental Measurement of 
 Periodic Current and Electromotive Force Wattmeter Method of Periodic Power Mutual 
 Induction of Two Circuits of Constant Inductance. 
 
 CHAPTER IV. Mutual and Self-induction. 
 
 Prof. Joseph Henry's Kesearches in Electro-MagnetismMutual Induction Elementary 
 Theory of the Induction Coil Comparison of Theory and Experiment Magnetic Screening and 
 the Action of Metallic Masses in Induction Coils Transmission of Kapidly Intermittent, 
 Alternate, or very Brief Currents through Conductors Effects of Saturation and Magnetic 
 Hysteresis Characteristic Curves of the Series and Parallel Transformer Efficiency of Trans- 
 . formers Ferrari's Experiments. 
 
 CHAPTER V. Dynamical Theory of Current Induction. 
 
 Electric Displacement Maxwell's Theory of Molecular Vortices Velocity of Propagation of 
 an Electro-Magnetic Disturbance Electrical Oscillations Function of the Condenser in an 
 Induction Coil Impulsive Discharges Alternative Paths Impulsive Impedance Eelation of 
 Impedance to Periodicity Dr. Hertz's Kesearches on Electrical Oscillatory Induction Kesonance 
 Phenomena Interference Phenomena at Various Distances Eecent Experiments Poynting's 
 Views on the Propagation of Electro-Magnetic Energy Possible Direction of Future Kesearch. 
 
 PRESS NOTICES. 
 
 " It would be very difficult to pick out from amongst the electrical literature of the past ten years any work 
 which marks, as emphatically as does Dr. Fleming's book, the manner in which the practical problems of the day 
 
 ave compelled electrical engineers to advance in their knowledge of theoretical science It is a book 
 
 which the electrical engineer of the present and of the future alike will read he of the present, if he can ; 
 he of the future, because he must." Prof, Silvanus P. Thompson in " The Electrician." 
 
 " The practical importance and interest of the subject treated is so great that there should be little need to 
 urge students and electrical engineers to make themselves acquainted with this book, but I do urge them 
 nevertheless ; and they may think it fortunate that Dr. Fleming has managed to find time to issue so instructive 
 and readable and well-timed a volume,," Dr. Oliver J. Lodge in "Nature." 
 
 " Dr. Fleming's book contains an enormous amount of valuable matter . . . . which cannot be got anywhere 
 else in the plain and concise way it is given by Dr. Fleming. It is one of those books every electrician should 
 have." Electrical Review. 
 
 " A most important, timely, and valuable book The author has earned the thanks of everyone 
 
 interested in this great branch of electrical investig'atfon'and practice." Electrical World (New York). 
 
 "If anyone wants this difficult subject treated in the clearest way, he cannot do better than read this book." 
 
 ^^ ' Industries. 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London, E.G. 
 
"The Electrician" Printing and Publishing Co., Ltd., 7 
 "THE ELECTRICIAN" SERIES continued. 
 
 SECOND ISSUE. More than 600 pages and over 300 illustrations. Price 12s. 6d., post free. 
 
 THE ALTERNATE CURRENT TRANSFORMER 
 
 IN THEORY AND PRACTICE. 
 
 By J. A. FLEMING, M.A., D.Sc., F.R.S., M.E.I., &c., 
 
 Professor of Electrical Engineering in University College, London. 
 
 YOL. II. THE UTILISATION OF INDUCED CUEBENTS. 
 
 SYNOPSIS OF CONTENTS. 
 CHAP. I. Historical Development of Induction Coil and Transformer. 
 
 The Evolution of the Induction Coil Page's Eesearches Callan's Induction Apparatus 
 Sturgeon's Induction Coil Bachhoffner's ^Researches Callan's Further Researches Callati's 
 Great Induction Coil- Page's Induction Coil Abbot's Coil Automatic Contact Breakers 
 Ruhmkorff's Coils Poggendorff's Experiments Stohrer's, Hearder's, Ritchie's Induction 
 Apparatus Grove's Experiments Apps' Large Induction Coils Jablochkoff's Patent Fulle' s 
 Transformer Early Pioneers Gaulard and Gibbs Zipernowsky's Transformers Improvements 
 of Rankin Kennedy, Hopkinson, Ferranti, and others The Modern Transformer since 1885. 
 
 CHAP. II. Distribution of Electrical Energy by Transformers. 
 
 Detailed Descriptions of Large Alternate- Current Electric Stations using Transformers in 
 Italy, England, and United States Descriptions of the Systems of Zipernowsky-Deri-Blathy, 
 Westinghouse, Thomson-Houston, Mordey, Lowrie-Hall, Ferranti, and others Plans, Sections, 
 and Details of Central Stations using Transformers Illustrations of Alternators and Transformers 
 in Practical Use in all the chief British, Continental, and American Transformer Stations. 
 
 CHAP. III. Alternate-Current Electric Stations. 
 
 General Design of Alternating-Current Stations, Engines, Dynamos, Boilers Proper Choice 
 of Units Water Power Parallel Working of Alternators Underground Conductors Various 
 Systems Concentric Cables Capacity Effects dependent on Use of Concentric Cables Phenomena 
 of Ferranti Tubular Mains Safety Devices Regulation of Pressure Choice of Frequency- 
 Methods of Transformer Distribution Sub-Stations Automatic Switches. 
 
 CHAP. IV. The Construction and Action of Transformers. 
 
 Transformer Indicator Diagrams Ryan's Curves Curves of Current Electromotive Force 
 and Induction-Analysis of Transformer Diagrams Predetermination of Eddy Current and 
 Hysteresis Loss in Iron Cores Calculation and Design of Transformers Practical Predetermina- 
 tion of Constants Practical Construction of Transformers Experimental Tests of Transformers 
 Measurement of Efficiency of Transformers Calometric Dynamometer and Wattmeter Methods 
 Reduction of Results. 
 
 CHAP. V. Further Practical Application of Transformers. 
 
 Electrical Welding and Heating Transformers for producing Large Currents of Low Electro- 
 motive Force Theory of Electric Welding Other Practical Applications Conclusion. 
 
 PRE2SS NOTICES. 
 
 " In reviewing the first volume of this work we found much to admire and praise, much to raise high 
 expectations for the volume which was to follow. These expectations have by no means been disappointed. 
 The new volume is in many ways of even greater interest than its predecessor." 
 
 Professor Silvanus P. Thompson in " The Electrician." 
 
 "The book is really a valuable addition to technical literature." Industries. 
 
 11 A valuable addition to the somewhat meagre literature on a subject which is sure to grow in importance, 
 and we congratulate Dr. Fleming on his work." The Engineer. 
 
 " Le sujet traite par le Dr. Fleming est un de ceux qui, pour le moment, attirent 1'attention ge"n6rale ; son 
 ouvrage est certainement un des plus importants de la litte"rature electrique. Tous les problemes relatifs h 
 1'application des courants alternatifs y sont trait&s avec une tres grande competence et de plus avec une clarte" 
 et avec une precision sans 6gales. Nous ne ppuvons done que recommander vivement cet ouvrage a 1'attention 
 de tous les 61ectriciens." La Lumibre Electrique. 
 
 " L'ouvrage de M. Fleming est une ceuvre vraiment pratique qui doit rendre a rindustrie de grands services 
 par 1'amas de renseignements qu'elle contient." L' Industrie Electrique. 
 
 " Das Fleming'sche Werk fiillt entschieden eine Liicke in der Literatur aus und kann durchaus empfohlen 
 verden." Elektrotechnische Zeitschrift. 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London, E.C. 
 
8 ; "The Electrician" Printing and Publishing Co., Ltd., 
 
 "THE ELECTRICIAN" SERIES continued. 
 
 In Two Volumes. Price : stout paper, 2s,, post free 2s. 2d. each; strong cloth covers, 2s. 6d., 
 post free 2s. 9d. each. Single Primers, 3d., post free 3%d. 
 
 "THE ELECTRICIAN" PRIMERS. 
 
 (FULLY ILLUSTRATED.) 
 
 A Series of Helpful Primers on Electrical Subjects for the use of Colleges, Schools, 
 and other Educational and Training; Institutions, and -for Young; Men desirous of 
 entering; the Electrical professions. 
 
 OF CONTENTS. 
 
 Yolumell. PRACTICE. 
 
 Volume I. THEORY. 
 
 IWmer 
 
 No. 
 
 1. The Effects of an Electric 
 
 Current. 
 
 2. Conductors and Insulators. 
 
 3. Ohm's Law. 
 
 4. Primary Batteries. 
 
 5. Arrangement of Batteries. 
 
 6. Electrolysis. 
 
 7. Secondary Batteries. 
 
 8. Lines of Force. 
 
 9. Magnets. 
 
 10. Electrical Units. 
 
 11. The Galvanometer. 
 
 12. Electrical Measuring In- 
 
 struments. 
 
 13. The Wheatstone Bridge. 
 
 14. The Electrometer. 
 
 15. The Induction Coil. 
 
 16. Alternating Currents. 
 
 17. The Leyden Jar. 
 
 18. Influence Machines. 
 
 19. Lightning Protectors. 
 
 20. Thermopiles. 
 
 The object of "The Electrician" 
 Primers is to briefly describe in sim- 
 ple and correct language the present 
 state of electrical knowledge. Each 
 Primer is short and complete in itself, 
 and is devoted to the elucidation of 
 some special point or the description 
 of some special application. Theo- 
 retical discussion is as far as possible 
 avoided, the principal facts being 
 stated and made clear by reference 
 to the uses to which they have been 
 put. Both volumes are suited to 
 readers having little previous ac- 
 quaintance with the subject. The 
 mattei is brought up to date, and 
 the illustrations refer to instruments 
 and machinery in actual use at the 
 present time. It is hoped that the 
 Primers will be found of use in 
 Schools, Colleges, and other Educa- 
 tional and Training Establishments, 
 where the want of a somewhat 
 popularly written work on electricity 
 and its industrial applications, pub- 
 lished at a popular price, has long 
 been felt ; while artisans will find the 
 Primers of great service in enabling 
 them to obtain clear notions of the 
 essential principles underlying the 
 apparatus of which they may 
 
 called upon to take charge. 
 
 be 
 
 Primer 
 
 No. 
 
 21. The Electric Telegraph. 
 
 22. Automatic and Duplex Tele- 
 
 graphy. 
 
 23. The Laying and Repair of 
 
 Submarine Cables. 
 
 24. Testing Submarine Cables, 
 
 25. The Telephone. 
 
 26. Dynamos. 
 
 27. Motors. 
 
 28. Transformers. 
 
 29. The Arc Lamp. 
 
 30. The Incandescent Lamp. 
 
 31. Underground Mains. 
 
 32. Electric Meters. 
 
 33. Electric Light Safety De- 
 
 vices. 
 
 34. Systems of Electric Distri- 
 
 bution. 
 
 35. Electric Transmission of 
 
 Energy. 
 
 36. Electric Traction. 
 
 37. Electro-Deposition. 
 i, 38. Electric Welding. 
 
 "The articles are generally so well written, and the subject matter so judiciously condensed, that there is 
 but very little to criticise, though much to praise." Electrical Review. 
 
 " The books are well printed, and we can heartily commend them as stepping stones to more advanced 
 works." Electrical Plant. 
 
 " Clearly written, and all that can be desired in the form of enunciation and explanation." Work. 
 
 " The contents of each one of these volumes is of that quality and description which at once constitute a book 
 a welcome addition to the library of the student or of the artisan." Amateur Work 
 
 Issued annually, price 3s., post free. 
 
 A DIGEST OF THE LAW OF ELECTEIC LIGHTING, 
 
 By 
 
 OTECEII4 STTBJESCTS. 
 
 (Revised to January in each year.) 
 
 A. C. CUBTIS-HAYWARD, B.A., M.I.E.E. 
 
 An abstract of the Electric Lighting Acts, 1882 and 1889, and of the various documents emanating from the 
 Board of Trade dealing with electric lighting, The digest treats first of the manner in which persons desirous of 
 supplying electricity must set to work, and then of their rights and obligations after obtaining Parliamentary 
 powers ; and gives in a succinct from information of great value to Local Authorities, Electric Light Contractors, 
 <fec., up to date. The Board of Trade Eegulations, the London County Council Begulations as to Theatre 
 Lighting, British and Foreign Rules and Eegulations for the Prevention of Fire .Risks arising from Electric 
 Lighting, and the Installation Begulations of the Electric Supply Companies of the Metropolis are also given. 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London, E.G. 
 
"The Electrician" Printing and Publishing Co., Ltd., 9 
 "THE ELECTRICIAN" SERIES continued. 
 
 800 pages, specially bound bookwise to lie open, ?S. 6d. nett, post free ?S. 9d. (abroad, %8., post free) ; 
 large paper edition, with wide margins, 12s. 6d. nett, post free 13s. (abroad, 13s. 6d. post free}. 
 
 A. DOCKET - BOOK: OF 
 
 ELECTRICAL ENGINEERING FORMULA &c. 
 
 BY W. GEIPEL AND H. KILGOUR. 
 
 
 With the extension of all branches of Electrical Engineering (and particularly the heavier 
 branches), the need of a publication of the Pocket-Book style dealing practically therewith 
 increases ; for while there are many such books referring to Mechanical Engineering, and several 
 dealing almost exclusively with the lighter branches of electrical work, none of these suffice for the 
 purposes of the numerous body of Electrical Engineers engaged in the application of electricity to 
 Lighting, Transmission of Power, Metallurgy, and Chemical Manufacturing. It is to supply this 
 real want that this most comprehensive book has been prepared. 
 
 Compiled to some extent on the lines of other pocket-books, the rules and formulae in general 
 use among Electricians and Electrical Engineers all over the world have been supplemented by 
 brief and, it is hoped, clear descriptions of the various subjects treated, as well as by concise 
 articles and hints on the construction and management of various plant and machinery, 
 
 A list of the subjects treated will be found below, from which it will at once be seen how 
 Indispensable the book will be to those engaged in electrical work of all kinds. 
 
 No pains have been spared in compiling the various sections to bring the book thoroughly up 
 to date ; and while much original matter is given, that which is not original has been carefully 
 selected, and, where necessary, corrected. 
 
 Where authorities differ, as far as practicable a mean has been taken, the differing formulae oeing 
 quoted for guidance. 
 
 Units. 
 
 Electrical, Mechanical 
 
 and Thermal Properties 
 
 of Materials. 
 Light and Sound. 
 Gravity. 
 Beams. 
 
 Moments of Inertiae. 
 Approximate Formulae. 
 The Slide Rule. 
 Mathematical Tables. 
 Miscellaneous Tables. 
 
 Ventilation and Heating. 
 
 Buildings & Foundations. 
 
 Water Power. 
 
 Fuel and Combustion. 
 
 Chimneys and Flues. 
 
 Boilers. 
 
 Feed-Water Heaters and 
 
 Economisers. 
 Boiler and Steam-Pipe 
 
 Covering. 
 Boiler Testing. 
 Steam, Gas, and other 
 
 Engines. 
 
 Condensers and Pumps. 
 Injectors. 
 Steam Pipes. 
 Mechanical Transmitters. 
 Dynamos and Motors. 
 Regulators. 
 Transformers. 
 Systems of Distribution. 
 Conductors. 
 Lightning Protectors . 
 Arc Lamps. 
 Incandescence Lamps. 
 
 Primary Batteries. 
 Secondary Batteries. 
 Electro-Chemistry. 
 Electro-Metallurgy. 
 Electric Welding. 
 Electric Traction. 
 Mining. 
 
 Testing and Measuring In- 
 struments. 
 Fire Office Rules. 
 Patent Office Information. 
 Wiring Tables. 
 
 A Prospectus, with Specimen Pages, &c., sent post free on application. 
 
 HOW READY, Fully Illustrated, cloth gilt. Price 2s. 6d. nett; post free 2s. 3d. 
 
 THE WORK OF HERTZ 
 
 AND SOME OF HIS SUCCESSORS, 
 
 BEING THE SUBSTANCE OF A LECTURE DELIVERED AT THE ROYAL INSTITUTION 
 
 By Dr. OLIYEE J. LODGE, F.E.S. 
 
 Reprinted, with Corrections to Text and Illustrations, and Additions, with STEEL-PLATE PORTRAIT Frontispiece 
 
 ! THE untimely end of a young and brilliant career cannot fail to strike a note of sadness and awaken a chord of 
 sympathy in the hearts of his friends and fellow-workers. Of men thus cut down in the early prime of their 
 powers there will occur to us here the names of Fresnel, of Carnot, of Clifford, and now of Hertz. His was a 
 strenuous and favoured youth ; he was surrounded from his birth with all the influences that go to make an 
 accomplished man of science accomplished both on the experimental and on the mathematical side. The 
 front rank of scientific workers is weaker by his death, which occurred on January 1st, 1894, the thirty-sixth 
 year of his life. Yet did he not go till he had effected an achievement which will hand his name down to 
 posterity as the founder of an epoch in experimental physics." Extract from Dr. Lodge's "Introductory" 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London. E.G. 
 
The Electrician" Printing and Publishing Co., Ltd., 
 "THE ELECTRICIAN" SERIES continued. 
 
 Fully Illustrated. Price 7s. 6d., post free 
 THE 
 
 INCANDESCENT LAMP AND ITS MANUFACTURE, 
 
 By GILBERT S. EAM. 
 
 EXTRACT FROM THE PREFACE. 
 
 With the expiration of Edison's master-patent for the carbon incandescent electri< 
 lamp, the attention of electric light engineers, as well as of all those who use the light, is 
 once more directed to the consideration of the lamp itself, to the possibility of obtaining 
 better lamps, and to the probable reduction in price which will naturally follow. Owing 
 to the long prevailing monopoly in the sale and manufacture, there has been littlt 
 inducement for those interested to experiment and to study the problems connected 
 with the incandescent lamp. As a result of this, the literature of the lamp is verj 
 scanty, and is entirely confined to the pages of the leading technical journals. While 
 dynamos, alternators, transformers, arc lamps, and almost every piece of apparatus 
 connected with electrical engineering and lighting, have been written on at length anc 
 discussed at meetings of scientific societies, the incandescent electric lamp, which has 
 been the chief cause of the very existence of these machines and apparatus, has beer 
 comparatively neglected. With the exception of the valuable series of articles bj 
 Mr. Swinburne which appeared in TJie Electrician some years ago, no comprehensive O] 
 detailed account of lamp manufacture has appeared. The manufacture of the incan 
 descent lamp and the principles underlying it are, consequently, but little known, excep 
 to those actually engaged in the work. 
 
 The Author has attempted to impart such information as he has acquired in tht 
 course of a considerable experience in lamp-making, and to give this information witl 
 as little mathematical embellishment as, under the circumstances, is possible. 
 
 Fully Illustrated. Price ?S. 6(1., post free. 
 
 DRUM ARMATURES AND COMMUTATORS 
 
 By F. MAETEN WEYMOUTH. 
 
 A complete treatise of the theory and the different modes of construction of Drum Winding 
 and also of close-coiled continuous-current Commutators, together with a full resume of some o 
 the principal points of consideration that are involved in their design. 
 
 The first chapter relates entirely to the theory of the Drum Winding. An explanation, it i 
 hoped with sufficient fulness, is there given of the generation of electromotive force and curren 
 within that form of winding ; and questions of magnetism connected therewith are also considered 
 Chapters II. to IX. are devoted to the description of various methods in which Drum Winding ha 
 been carried out in practice, with special reference to what is termed the "end-winding.' 
 Chapters X. to XIII. touch upon the mechanical construction of Commutators ; and Chapter 
 XIV. to XXVI. deal with Commutator Sparking, with which has become necessarily involved tb 
 whole subject of what are known as " armature reactions." The book closes with a chapter on tfc 
 Taper of Commutator Segments. 
 
 The various subjects are treated without, or almost without, the use of mathematics. But th> 
 author ventures to think that, in this case, the absence of mathematics is far from unjustifiable. 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London, E.G. 
 
"The Electrician" Printing and Publishing Co., Ltd., 11 
 
 " THE ELECTRICIAN " SERIES continued. 
 Over 300 pages, 106 illustrations. Price 10s. 6d., post free. 
 
 The ART of ELECTROLYTIC SEPARATION of METALS. 
 
 (THEOKETICAL AND PEACTICAL.) 
 By GEORGE GORE, LLJX, F.R.S. 
 
 THE ONLY BOOK ON THIS IMPORTANT SUBJECT IN ANY LANGUAGE. 
 
 SYNOPSIS OF CONTENTS. 
 HISTORICAL SKETCH. 
 
 Discovery of Voltaic and Magneto-Electricity First Application of Electrolysis to the 
 Refining of Copper List of Electrolytic Refineries. 
 
 THEORETICAL DIVISION. 
 
 Section A. : Chief Electrical Facts and Principles of the Subject. Electric Polarity and 
 Induction, Quantity, Capacity, Potential Electromotive Force Electric Current Conduction 
 and Insulation Electric Conduction Resistance. 
 
 Section B. : Cliief Thermal Phenomena. Heat of Conduction Resistance Thermal Units, 
 Symbols, and Formulae. 
 
 Section C. : Chief Chemical Facts and Principles of the Subject. Explanation of Chemical 
 Terms Symbols and Atomic Weights Chemical Formulae and Molecular Weights Relation of 
 Heat to Chemical Action. 
 
 Section D.: Chief Facts of Chemico- Electric or Voltaic Action. Electrical Theory of 
 Chemistry Relation of Chemical Heat to Volta Motive Force Volta-Electric Relations of 
 Metals in Electrolytes Voltaic Batteries Relative Amounts of Voltaic Current produced by 
 Different Metals. 
 
 Section 2. : Chief Facts of Electro-Chemical Action. Definition of Electrolysis Arrange- 
 ments for Producing Electrolysis Modes of Preparing Solutions Nomenclature Physical 
 Structure of Electro-Deposited Metals Incidental Phenomena attending Electrolysis Decom- 
 posability of Electrolytes Electro- Chemical Equivalents of Substances Consumption of Electric 
 Energy in Electrolysis. 
 
 Section F. : The Generation of Electric Currents by Dynamo Machines. Definition of a 
 Dynamo and of a Magnetic Field Electro-Magnetic Induction Lines of Magnetic Force. 
 
 PRACTICAL DIVISION. 
 
 Section G. : Establishing and Working an Electrolytic Copper Refinery. Planning a Refinery 
 Kinds of Dynamos Employed Choice and Care of Dynamo The Depositing Room The Vats 
 The Electrodes The Main Conductors Expenditure of Mechanical Power and Electric 
 Energy Cost of Electrolytic Refining. 
 
 Section H. : Other Applications of Electrolysis in Separating and Refining Metals. Elec- 
 trolytic Refining of Copper by other Methods Extraction of Copper from Minerals and Mineral 
 Waters Electrolytic Refining of Silver Bullion and of Lead Separation of Antimony, of Tin, of 
 Aluminium, of Zinc, of Magnesium, of Sodium and Potassium, of Gold Electrolytic Refining of 
 Nickel Electric Smelting. 
 
 Appendix. Useful Tables and Data. 
 
 Second Edition, price 2s., post free. 
 
 ELECTRO-CHEMISTRY. 
 
 By GEORGE GOEE, LL.D., F.E.S. 
 
 This book contains, in systematic order, the chief principles and facts of electro-chemistry, 
 and is intended to supply to the student of electro-plating and electro-metallurgy a scientific basis 
 npon which to build the additional practical knowledge and experience of his trade. A scientific 
 foundation, such as is here given, of the art of electro -metallurgy is indispensable to the electro- 
 depositor who wishes to excel in his calling, and should be studied previously to and simul- 
 taneously with practical working. As the study of electro-chemistry includes a knowledge not 
 only of the conditions under which a given substance is electrolytically separated, but also of the 
 electrolytic effect of a current on individual compounds, both are described, and the series' of 
 substances are treated in systematic order. An indispensable book to Electro-Metallurgists. 
 
 1, 2, and 3, Salisbury Court, Fleet Street, London, E.G. 
 
12 "The Electrician" Printing and Publishing Co., Ltd. 
 "THE ELECTRICIAN" SERIES continued. 
 
 Electrical Laboratory Notes & Forms. 
 
 ARRANGED AND PREPARED BY 
 
 I>r. J. A.. FJLESIMIIVO, M.A., FJR.S. 
 
 Professor of Electrical Engineering in University College, London. 
 
 These " Laboratory Notes and Forms " have been prepared to assist Teachers, Demonstrators 
 and Students in Electrical Laboratories, and to enable the Teacher to economise time. They 
 consist of a series of (about) Twenty Elementary and (about) Twenty Advanced Exercises 
 in Practical Electrical Measurements and Testing. For each of these Exercises a four-page Report 
 Sheet has been prepared, two pages of which are occupied with a condensed account of the theory 
 and practical instructions for performing the particular Experiment, the other two pages being 
 ruled up in lettered columns, to be filled in by the Student with the observed and calculated 
 quantities. Where simple diagrams will assist the Student, these have been supplied. These 
 Exercises are for the most part based on the methods in use in the Electrical Engineering 
 Laboratories of University College, London ; but they are perfectly general, and can be put into 
 practice in any Electrical Laboratory. 
 
 Each Form is supplied either singly at 4d. nett, or at 3s. 6d. per dozen nett (assorted or 
 otherwise as required) ; in sets of any three at Is. nett ; or the set of (about) Twenty Elementary 
 (or Advanced) Exercises can be obtained, price 5s. 6d. nett. The complete set of Elementary 
 and Advanced Exercises are price 10s. 6d. nett, or in a handy Portfolio, 12s. nett, or bound in 
 strong cloth case, price 12s. 6d. nett. 
 
 Spare Tabulated Sheets for Observations, price Id. each nett. 
 
 Strong Portfolios, price Is. each. 
 
 The very best quality foolscap sectional paper (16in. by 13in.) can be supplied, price Is. 
 per dozen sheets nett. 
 
 ^Yoit' ready.) 
 
 1. The Exploration of Magnetic Fields. 
 
 2. The Magnetic Field of a Circular Current. 
 
 3. The Standardization of a Tangent Galvanometer by the Water Voltameter. 
 
 4. The Measurement of Electrical Resistance by the Divided Wire Bridge. 
 
 5. The Calibration of the Ballistic Galvanometer. 
 
 6. The Determination of Magnetic Field Strength. 
 
 7. Experiments with Standard Magnetic Fields. 
 
 . The Determination of the Interpolar Field of an Electromagnet with Varying Lengths of Air Gap. 
 
 9. The Determination of Resistance and Temperature Coefficients with the Post Office Pattern of Wheatstpne's 
 
 10. The Determiuation of Electromotive Force by the Potentiometer. [Bridge. 
 
 11. The Determination of Current Strength by the Potentiometer. 
 
 12. A Complete Test of a Primary Battery. 
 
 13. The Calibration of a Voltmeter by the Potentiometer. 
 
 14. A Photometric Examination of an Incandescent Lamp. 
 
 15. The Determination of the Absorptive Powers of Semi-Transparent Screens. 
 18. The Determination of the Reflective Powers of Various Surfaces. 
 
 17. The Determination of the Electrical Efficiency of an Electromotor by the Cradle Method. 
 
 18. The Determination of the Efficiency of an Electromotor by the Brake Method. 
 
 19. The Efficiency Test of a Combined Motor Generator Plant. 
 
 20. Test of a Gas Engine and Dynamo Plant. 
 
 ADYACTGIED SERIES.- (Ready shortly.) 
 
 21. The Determination of the Specific Electrical Resistance of a Sample of Wire. 
 
 22. The Measurement of Low Resistances by the Potentiometer. 
 
 23. The Measurement of Armature Resistances. 
 
 24. The Standardization of an Ampere-meter by Copper Deposit. 
 25. The Standardization of a Voltmeter by the Potentiometer. 
 
 26. The Standardization of an Ammeter by the Potentiometer. 
 
 27. The Determination of the Magnetic Permeability of a Sample of Iron. 
 
 28. The Standardization of a High Tension Voltmeter. 
 
 29. The Efficiency Test of a Transformer. 
 
 30. The Delineation of the Curves of Current and Electromotive Force of a Transformer 
 
 SI. The Photometric Examination of an Arc Lamp. 
 
 32. The Measurement of Insulation and High Resistance. 
 
 33. The Examination of a Secondary Cell by the Potentiometer. 
 
 34. The Efficiency Test of an Alternator. 
 
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 ELECTRIC MOTIVE POWER. 
 
 By ALBION T. SNELL, Assoc.M.lNST.C.E., M.I.E.E. 
 
 The rapid spread of electrical work in collieries, mines, and elsewhere has created a demand for a practical 
 fcook on the subject of transmission of power. Though much had been written, there was no single work dealing 
 with the question in a sufficiently comprehensive and yet practical manner to be of real use to the mechanical 
 or mining engineer ; either the treatment was adapted for specialists, or it was fragmentary, and power work 
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 reference to collieries and coal-getting, and the results of the extensive experience gained in this field are 
 -embodied. 
 
 In general, the Author's aim has been to give a sound digest of the theory and practice of the electrical 
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 which lie in the province of the specialist, and elementary proofs which properly belong to text-books on 
 electricity and magnetism. 
 
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 MOTIVE POWER AND GEARING 
 
 FOR EJL.ECTRICAT, IVI ACMHSTER Y. 
 
 BY E. TEEMLETT CARTER, C.K 
 
 {COPIOUSLY ILLUSTRATED WITH SCALE DRAWINGS & NUMEROUS PLATES.) 
 
 The purpose of this work is the explanation of the principles and practice of modern mechanical motive 
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 tion are such that a special study of the mechanical plant is necessary. Just as marine or locomotive steam 
 practice is treated in a special manner in works on the subject ; so the Author has endeavoured to hold in 
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 "MOTIVE POWER AND GEARING" is adapted equally to the needs of the practical engineer and of the 
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 This work constitutes the only existing treatise on the Economics of Motive Power and Gearing for Electrical 
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"The Electrician" Printing and Publishing Co., Ltd., 15 
 
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