GIFT OF 
 
Consulting Engineer 
 
ELECTRICAL ENGINEERING 
 
 AN ELEMENTARY TEXT-BOOK 
 
 SUITABLE FOR 
 
 PERSONS EMPLOYED IN THE MECHANICAL AND ELECTRICAL 
 ENGINEERING TRADES, FOR* ELEMENTARY S'lUDINIS 
 
 OF ELECTRICAL ENGINEERING, AND FOR 
 
 ALL WHO WISH TO ACQUIRE A KNOWLEDGE OF THE CHIEF 
 PRINCIPLES AND PRACTICE OF THE SUBJECT 
 
 BY 
 
 E. ROSENBERG 
 
 Chief Electrical Engineer at Mess*-*. Korting Bros., Hanover 
 
 TRANSLATED BY 
 
 W. W. HALDANE GEE AND CARL KINZBRUNNER 
 
 B.Sc. (Lond.), A.M.I.E.f-l. Lecturer on Electrical Engineering at the 
 
 Professor of Applied Physics at the Municipal Municipal School of Technology^ 
 
 School of Technology^ Manchester Manchester 
 
 AUTHORIZED EDITION 
 
 REVISED AND BROUGHT DOWN TO DATE FOR THE AMERICAN MARKET 
 
 BY 
 
 EDWARD B. RAYMOND, B.S. 
 
 Associate Member of the American Institute of Electrical Engineers 
 General Superintendent Schenectady Works of the General Electric Company 
 
 NEW YORK. 
 
 JOHN WILEY & SONS 
 
 43 AND 45 EAST NINETEENTH STREET 
 
 1907 
 
Copyiight 1903, 1906 
 
 JOHN WILEY & SONS 
 
 ROBFRT DRUMMONn. PRINTFR, NKW YOKK 
 
r* 
 
 Consulting Engineer ] 
 
 MASON STS 
 
 PREFACE 
 
 THIS book had its origin in a number of lectures which I delivered 
 two years ago to the workmen and the staff of a large electrical manu- 
 facturing firm. The circle of readers for which this book is intended 
 is, in the first place, the same as that to which my audience belonged. 
 It should give to workmen of electrical engineering works the knowl- 
 edge of the operation of machines and apparatus with which they are 
 concerned. I have endeavoured to use such language that people who 
 have only a general school education should be able to understand. 
 For this reason several matters have been dealt with very completely 
 which, to the mathematically educated man, could have been explained 
 in a few lines such as, for instance, Ohm's Law and resistance calcu- 
 lations. I have in these cases endeavouied to explain the matter 
 first without the help of mathematics, and then have finally, as a key- 
 stone after having worked some examples stated the formulae In 
 the part relating to the "output of a three-phase current system," 
 in a similar way, there are calculations and formulae, since it is 
 necessary for the student to be acquainted with these. Generally, 
 however, calculations and formulae are avoided, as in the case of the 
 whole chapter about dynamos. The book will not enable the reader 
 to calculate the parts and windings of dynamos, and he should not even 
 think that he is able to do so. For the elementary student, the wire- 
 man, for the engineer as well as for the general public who desire to 
 know something about electrical engineering, it is quite sufficient if 
 they understand the working of dynamos, their faults, and the reason 
 and the cure of the latter. 
 
 The book covers a wide area. It comprises, besides the funda- 
 mental phenomena of the electric current, dynamos and motors 
 for continuous, alternating, and three-phase current, then accumu- 
 
 vii 
 
vm PREFACE 
 
 la tors and their apparatus, measuring instruments and electric light- 
 ing. All these things must be known by an electrical engineer. It 
 was, of course, impossible to deal equally completely with all these 
 themes. It is not possible to describe all shapes of dynamos, and 
 many types of arc lamps and measuring instruments. Only the most 
 important types are dealt with, and their working is explained. 
 Further detail is impossible, having regard to the extent of the book, 
 and I do not consider it to be necessary. Every electrical firm pub- 
 lishes complete pamphlets about their special manufactures, and in 
 extraordinary cases the installer gets special diagrams of connections 
 and specification for the plant he has to erect. Besides that, any- 
 body who understands the machines and apparatus described in this 
 book will be able to make himself clear as to other types. 
 
 About laying mains and about installation material but little is 
 said in this book. The necessary information may be found else- 
 where. 
 
 Although this book is chiefly for electrical men and for those who 
 intend to become such, yet the general public desiring to get some 
 general information about Electrical Engineering will read the book 
 with advantage. I am also in hope that in some of the chapters 
 useful hints may be found by the educated electrical engineer. 
 
 My object has been a. practical one, and the arrangement of the 
 material has been made from a practical point of view. I have, there- 
 fore, departed in some cases from the historical order; for instance, in 
 the case of dynamos. Again, facts have been omitted which are 
 indeed necessary for the scientific, but not for the practical treatment 
 of the matter; for example, in describing accumulators and the 
 induction effects of alternating currents. 
 
 E. ROSENBERG. 
 
 HANOVER, 
 
 January, 1902. 
 
TRANSLATORS' PREFACE 
 
 WE have undertaken the translation of Herr Rosenberg's "Elektrische 
 Starkstrom-Technik" with the idea that the book will be distinctly 
 helpful to less advanced students of electrical engineering in the 
 English-speaking countries. 
 
 It is the work of an electrical engineer, and is writen from an 
 engineering standpoint. Its origin is explained in the Author's 
 Preface, and the opinion of German critics is seen to be favourable 
 from the Press Notices which are given at the end of the volume. 
 
 In one way the work is different to others of an elementary kind 
 in the space that is given to alternating current electrical engineer- 
 ing. This subject is usually practically ignored, or is treated so 
 mathematically that it is quite beyond the powers of most readers. 
 We feel sure that here the work will not only be of value for its clear 
 explanation of principles, but also for the useful practical hints relat- 
 ing to plant of this kind. In polyphase work, which is now becoming 
 of importance in England, the author has been especially careful to 
 make his explanations easy to follow. 
 
 Some change from the original has been inevitable in giving an 
 English dress to the work. The illustrations have been revised, and 
 a number from English firms have been added. We here tender our 
 thanks to those firms who have been so good as to provide us with 
 blocks. Their names are given with the illustrations. 
 
 We have also to thank our colleague Mr. Norman West, Demon- 
 strator in Electrical Engineering, and several of our third-year 
 students, for assistance in revising the final proofs. 
 
 W. W. HALDANE GEE. 
 CARL KINZBRUNNER. 
 
 MANCHESTER, 
 January, 1903. 
 
 ix 
 
REVISER'S PREFACE 
 
 IN revising this book for American readers, I have assumed that 
 not only will interest in American practice and apparatus in general 
 be equal to that in foreign, but also that the explanations on the 
 topics presented in the previous edition can in this be extended con- 
 siderably with complete understanding even by persons but little 
 versed in electricity, satisfying in addition thereby a large class of 
 readers with considerable electrical experience or preliminary train- 
 ing. Certain subjects essential to American practice have also been 
 added in various parts of the book. 
 
 E. B. RAYMOND. 
 
TABLE OF CONTENTS 
 
 CHAPTER I 
 
 FUNDAMENTAL PRINCIPLES 
 
 Electrical Phenomena 1 
 
 Electro-motive Force 3 
 
 Influence of the Electric Current on a Magnetic Needle 10 
 
 Electric Units Measurement of Currents Ohm's Law 14 
 
 The Calculation of Resistance 23 
 
 Other Forms of Ohm's Law 26 
 
 Internal Resistance Drop of Potential 27 
 
 Branching of Circuits 29 
 
 Cells in Series and Parallel 31 
 
 Voltmeters 33 
 
 Electrical Power 34 
 
 Equivalence of Electrical Mechanical and Heating Effects 37 
 
 Electric Mains 40 
 
 CHAPTER II 
 MAGNETS MAGNETIC LINES OF FORCE 
 
 Influence of a Magnet on an Electric Current Deprez Instruments 56 
 
 Influence of Electric Currents on each other The Electro-dynamometer. . 60 
 
 Electro-magnets 61 
 
 Induction 65 
 
 Electrical Machines 68 
 
 CHAPTER III 
 THE CONTINUOUS-CURRENT DYNAMO 
 
 The Ring Armature 74 
 
 Drum Armature. 79 
 
 Magnet System 85 
 
 Self-excitation Shunt Dynamo 91 
 
 Series Dynamo 94 
 
 Compound Dynamo 97 
 
 Types of Dynamos 98 
 
 xi 
 
xii TABLE OF CONTENTS 
 
 PAGE 
 
 Output of a Dynamo 103 
 
 Multipolar Dynamos 105 
 
 Armatures of Multipolar Dynamos 109 
 
 Sparking and Displaceme.it of Brushes 118 
 
 Methods lor changing Direction of Rotation 121 
 
 Causes 01 the Non-excitation of Dynamos. 124 
 
 AutOi...* ic 0111 nu Regulator 127 
 
 a. y of Dynamos . 129 
 
 CHAPTER IV 
 THE ELECTRIC MOTOR 
 
 The Shunt Motor 136 
 
 Speed Regulation 137 
 
 The Se: les Motor 139 
 
 The Co ro>md Motor 143 
 
 Direct i of Rotation of a Motor 145 
 
 Anna Reaction with Motors 149 
 
 Reve, V -naratus 150 
 
 Spaik. i Starters and Shunt Regulators 153 
 
 Mo o ; er. ain Purposes 159 
 
 Electrl -!. tion 164 
 
 The fepi; .e-General Electric Type M Control System 169 
 
 The Elect ric Brake 175 
 
 The Magnetic Blow-out : 177 
 
 Operating Troubles with Direct-current Motors 178 
 
 CHAPTER V 
 ACCUMULATORS 
 
 Machines for Charging Accumulators 187 
 
 Battery Switch 189 
 
 Accumulator Apparatus 191 
 
 Applications of Accumulators 195 
 
 CHAPTER VI 
 
 WORKING OF DIRECT -CURRENT DYNAMOS IN PARALLEL 
 Switching Dynamos in Parallel 201 
 
 CHAPTER VII 
 ELECTRIC LIGHTING 
 
 Glow Lamps 203 
 
 Arc Lamps 207 
 
TABLE OF CONTENTS xiii 
 
 CHAPTER VIII 
 ALTERNATING CURRENTS 
 
 PAGE 
 
 Properties of Angles concerned with Alternating Currents 217 
 
 Experiments with Alternating Currents , 221 
 
 Current Strength and Voltage of an Alternating Current 224 
 
 Induction Effects of an Alternating Current 225 
 
 1 ransformers 227 
 
 Shape of r l ransformers 229 
 
 Applications of Transformers 232 
 
 Ph se-difference 233 
 
 Vector Diagrams 239 
 
 Wattmeter Power-factor 244 
 
 CHAPTER IX 
 
 ALTERNATORS 
 
 Switching in Parallel of Alternating-current Machines Synchronizer 265 
 
 CHAPTER X 
 ALTERNATING-CURRENT MOTORS 
 
 Synchronous Motors 270 
 
 The Rotary Converter 276 
 
 Commutator Motors 281 
 
 CHAPTER XI 
 MULTIPHASE ALTERNATING CURRENT 
 
 Induction Motors Rotating Field 283 
 
 Three-pi ase Current 290 
 
 Actions in Induction Motors Squirrel-cage and Slip-ring Armatures 293 
 
 Slip 300 
 
 Single-phase Induction Motors 302 
 
 Phase-difference caused by Capacity 305 
 
 The Reversing of Alternating-current Motors 308 
 
 Faults with Alternating-current Motors 311 
 
 Transmission of Multiphase Currents 314 
 
 Power in a Three-phase System 322 
 
 Synchronizer for Multiphase Machines 324 
 
 CHAPTER XII 
 HIGH TENSION 
 
 Lightning Arresters 330 
 
 Switchboards 334 
 
 INDEX. . . 339 
 
ELECTRICAL ENGINEERING 
 
 CHAPTER I 
 
 FUNDAMENTAL PRINCIPLES 
 
 Electrical Phenomena 
 
 IP, in a gkss vessel, filled with water and dilute sulphuric acid, in 
 
 the proportion of about one part of concentrated acid to ten parts 
 
 of water, is placed a plate of zinc, Zn, (Fig. 1), and 
 
 a plate of copper, Gu, which are connected by a 
 
 wire, it will soon be observed that the wire gets 
 
 hot. If the contact at any point of the wire 
 
 be interrupted so as to make an air gap, a spark 
 
 will be observed at the moment when the break 
 
 is made. This is an electric spark. The name 
 
 "electricity" is derived from the Greek word 
 
 electron, meaning amber, because this substance, 
 
 when rubbed, produces electricity. The spark 
 
 produced with the apparatus of Fig. 1 is a very small one. But 
 
 if we connect a number of these galvanic cells in series, as shown in 
 
 FIG. 1. Galvanic 
 Cell. 
 
 -mmmmmmmmm- 
 
 FIG. 2. Galvanic Battery. 
 
 Fig. 2, taking care to connect the Zn of one cell with the Cu of the 
 other, then, on bringing together the last Zn and the last Cu, sparks 
 
ELECTRICAL ENGINEERING 
 
 of great length may be obtained, and long spirals of motal as 
 shown in the illustration may be raised to red heat, and may 
 even be melted. We call such an arrangement of cells a galvanic 
 battery. 
 
 From the heat produced in the circuit we might infer that some 
 kind of motion exists in the circuit. We know from experience that 
 whenever a body is set in motion by a force, part, or in some cases 
 the whole, of the force is expended in overcoming the frictional 
 resistance and producing heat. If, for example, some bricks slip 
 down an inclined plank, the latter becomes quite hot, especially if 
 the bricks follow each other very quickly. Again, when a train 
 
 E asses along the rails, the temperature of the rails is raised, and the 
 ister the train the greater the amount of heat. 
 
 In the case of the battery we may imagine a motion in the 
 connecting wire, and the faster this motion the hotter the wire be- 
 comes. We shall cs.ll this motion which cannot 
 be seen, and is known from its effects only, an 
 electric current. 
 
 Now let us wind the wire connected with the 
 battery in the form of a spiral, and slip it over a 
 rod of soft iron (technically called a core), as 
 shown in Fig. 3, then the iron becomes strongly 
 > _ magnetic. But if we break the battery circuit 
 anywhere, so that no current can flow, the core 
 
 . An Electro- will at once lose its magnetism. We conclude, 
 magnet. then, that the electrical current has the power of 
 converting an iron bar into a magnet. The 
 arrangement just described is termed an electro-magnet. 
 
 Again, if we take a bobbin wound with wire and pass a current 
 
 through it, we shall find that a light- 
 piece of iron will be attracted into the 
 interior of the bobbin (see Fig. 4). 
 We learn from this experiment that a 
 coil round which a current flows causes 
 even the space it encloses to be a 
 magnet. 
 
 Another important experiment is to 
 send the electrical current through 
 acidulated water. Immediately the 
 connection to the battery is made the 
 water behaves as if it were boiling, and 
 bubbles of gas rise up the wires and 
 escape into the air. With the apparatus shown in Fig. 5 the gases 
 may be collected. The tubes are first filled with water, and then 
 inverted over the wires, which should be of platinum. One gas is 
 oxygen, and it will be found that this escapes from the end of the 
 
 4. iron attracted by 
 Electro-magnet. 
 
FUNDAMENTAL PRINCIPLES 
 
 wire connected with the copper plate; the other gas is hydrogen, 
 
 and it will be double in volume 
 
 to the oxygen. Now collect both 
 
 gases in one tube, and bring a 
 
 lighted match to the mouth of the 
 
 tube, when the gases will instantly 
 
 combine with explosion, and form 
 
 water, which is composed of oxygen 
 
 and hydrogen. 
 
 From the various experiments, 
 we learn that the effects of the 
 current are threefold, namely 
 
 1. Heating and lighting effects. 
 
 2. Magnetic effects. 
 
 3. Chemical effects. 
 
 The student must try to re- 
 member that, technically, the upper 
 end of the copper plate is termed 
 the positive pole ; and that from 
 the wire connected to it oxygen FlG - ^-Decomposition of Water, 
 escapes, as shown in Fig. 5. The 
 
 zinc end is called the negative pole; from the wire connected to 
 it hydrogen escapes. 
 
 The electric current is supposed to travel from the positive to 
 the negative pole. 
 
 
 Electro=motive Force 
 
 From the foregoing experiments we come to the conclusion that 
 a force exists which drives a 
 current along the wire. This may 
 be readily illustrated by the help 
 of an analogy. Let there be two 
 tanks (Fig. 6) filled with water, but 
 to different levels, and connected 
 by a pipe. Then the force due to 
 the difference of heights of the 
 water will produce a motion of the 
 water in the pipe in the direction 
 from the high to the low level, 
 and will continue as long as there 
 is a difference of level. In the FIG. 6. Hydraulic Analogy, 
 case of electricity we must imagine 
 a similar difference of pressure to cause an electric current. This 
 
ELECTRICAL ENGINEERING 
 
 difference is termed a difference of electrical potential. When two 
 metals of the same kind are immersed in an acid solution no differ- 
 ence of electrical potential is produced, and therefore, when the metals 
 are connected by a wire, no current results. But if the metals are 
 of different kinds a potential difference is produced, and a current 
 will pass through a connecting wire. If the plates are of copper 
 and zinc, the potential of the copper plate is higher than that of the 
 zinc, and the current therefore flows from copper to zinc. The 
 force causing the difference of potential is usually called by electricians 
 the electro-motive force. 
 
 Again let us refer to the hydraulic analogy. The stream of 
 water, which we compared with the electric current, flows only as 
 
 long as there is a dif- 
 ference of levels. Now 
 let it be considered 
 what would have to be 
 done to produce a con- 
 tinuous stream of water. 
 If a pump be inserted 
 in the circuit (Fig. 7), 
 then the water may be 
 forced to an upper 
 tank, and will descend 
 through the opening 
 in the bottom, to be 
 again pumped up once 
 more. In the electrical 
 circuit we have a similar 
 action. When two dif- 
 ferent metals are im- 
 mersed in acid, and one 
 FIG. 7. Production of Water Current. of them is acted upon 
 
 by the acid, a continual 
 
 difference of potential is produced, the chemical action here producing 
 the necessary, as it were, pumping action. 
 
 An electro-motive force may be produced by other than chemical 
 means, and we shall show later that electro-motive force may be 
 obtained by mechanical power. The galvanic cell is only one 
 of several means of producing an electric current. 
 
FUNDAMENTAL PRINCIPLES 5 
 
 In electrical engineering galvanic cells, such as zinc -copper cells, 
 are hardly ever used as current-generators, but the current is gen- 
 erally produced by dynamos. A most important part of a dynamo 
 is the magnetic system, and we have therefore to deal in some detail 
 with the properties of magnets. 
 
 There are different forms of magnets, such as, for instance, bar 
 magnets (Fig. 8), horseshoe magnets (Fig. 9), magnetic needles 
 
 FKJ. 8. 
 Bar Magnet. 
 
 FIG. 9. 
 Horseshoe Magnet. 
 
 FIG. 10. 
 Magnetic Needle. 
 
 (Fig. 10). With a freely movable magnetic needle, such as shown 
 in Fig. 11, a characteristic property can be observed. If there are 
 
 FIG. 11. Pivoted Magnetic Needle. 
 
 no other forces acting on this needle, it sets itself in a certain direc- 
 tion, one of its ends pointing towards the north. This end is called 
 the north pole; the other end, pointing towards the south, is called 
 the south pole of the needle. 
 
 To distinguish the poles of a magnetic needle from each other, 
 the north end or half is generally marked or coloured. 
 
 We explain the above phenomenon by imagining that the earth 
 exerts upon the needle a force which, like the force of gravity, gives to 
 a suspended rod a certain definite direction, viz. vertically down- 
 wards. 
 
6 ELECTRICAL ENGINEERING 
 
 We can deflect the needle from the direction in which it has 
 settled under the influence of the earth, by placing near to it another 
 magnet. If we place the north poles of two freely movable needles 
 near each other, then they repel each other. The same is the case 
 with the two south poles. On the other hand, a north pole of one 
 and a south pole of another needle attract. Hence the rule, " Like poles 
 repel, unlike attract." 
 
 We can observe similar repelling and attracting effects by 
 approaching the magnetic needle to the poles of any magnet, and we 
 are able to determine its poles by the use of the above rule. 
 
 As is well known, a bar magnet attracts soft iron. If a soft iron 
 rod be attached to the north pole of a magnet, the rod behaves like a 
 magnet, i.e. it is able to attract small pieces of iron and hold them 
 fast. 
 
 By the aid of a magnetic needle we can convince ourselves that a 
 rod of iron adhering to a magnet possesses "polarity," the end of 
 the rod turned toward the north pole of the magnet being a south 
 pole, the other end being a north pole. 
 
 Magnets exert actions at a distance. They do not only held fast 
 pieces of iron which have been brought to them, but also attract 
 pieces of iron from some distance. The force of attraction becomes 
 smaller the greater the distance. By the aid of very exact experi- 
 ments the following law has been found : If the distance is doubled, 
 then the force exerted is not half, but the fourth part of what it was 
 previously; if the distance is three times as great, then the force is 
 only the ninth part; with a tenfold distance the force is only one- 
 hundredth, and so on. Hence we can say that the force exerted by a 
 magnetic pole decreases with the square of the distance. 
 
 Let us now take a large horseshoe magnet, and proceed to hold 
 near it a small magnetic needle in various positions. Fig. 12 shows 
 
 the poles of the magnet, 
 
 g seen from above, as well as 
 
 7J) 1 * son the different positions of the 
 
 needle. If the needle is 
 near the north pole, then we 
 
 n 2 _ ___ _ _ observe that it comes to rest 
 
 ^ s<s>n s<>n ( S )n<is with the south end pointing 
 
 5 V^W to the centre of the north 
 *** s pole, whereas the north end 
 FIG. 12. Magnetic Needle in Field of Large of the needle is repelled in 
 Magnet. the opposite direction (posi- 
 
 tions 1 and 2 in Fig. 12). 
 
 In the opposite position, on the right of the south pole, the needle 
 comes to rest in a similar way, so that its north end points right 
 to the centre of the south pole (positions 3 and 4). At all points 
 between the two poles (for instance, positions 5 and 6), the needle 
 
I 
 
 FUNDAMENTAL PRINCIPLES 7 
 
 settles parallel to a line joining the centre of the poles, its south 
 end, s, turning towards the north pole, N, of the magnet, whilst 
 the north end, n, is attracted towards the south pole, S. Why 
 these directions are taken up by the needle the student will easily 
 understand. 
 
 But if we now place the needle at any other point in the space 
 surrounding the poles, such as, fcr instance, at position 7, it will 
 again come to rest in a certain direction. 
 
 This position, however, will be such that neither the south 
 pole of the needle turns directly towards the north pole of the 
 magnet, nor the north pole turns directly towards the south pole 
 of the magnet. 
 
 To explain why this should be, we have to consider the action of 
 each of the magnet poles on each pole of our needle. 
 
 The south pole, s, of the needle is attracted by the north pole, N, in 
 the directi( n of the simple arrow r , and is also repelled in the direction 
 of the double-barbed arrow by the south pole, S. 
 
 These two forces are not equal to each other, since the centre 
 point of the needle is only half as far from the north pole as from the 
 south pole, the force exerted by N being therefore four times as 
 great as that by S. In the figure this is indicated by the length of 
 the respective arrows. 
 
 If there are two forces acting on a body, trying to pull it in 
 different directions, then the body can obviously follow neither of the 
 forces entirely. The direction which the body then will occupy will 
 be between the two forces, and that not exactly in the middle, but 
 more towards the greater force. 
 
 Accordingly, the magnetic needle will set itself in the direction 
 of the arrow with three barbs, with its south pole not pointing exactly 
 to the centre of the N pole. Exactly the same considerations may be 
 applied to the n pole of the magnetic needle. 
 
 In like manner, for each point in the space round the 
 magnet a certain line of direction of the magnetic force can be 
 determined. 
 
 A very good way to make this clear is the following one: 
 
 Let us take a horseshoe magnet, fix it vertically, and over it place 
 a sheet of thin cardboard. By means of a muslin bag filled with steel 
 filings, sprinkle the filings lightly and very uniformly over the card. 
 Then tap the card very gently. The steel filings will arrange them- 
 selves in beautiful curves (see Fig. 13). 
 
 Near the poles we observe rays, which are turned directly 
 towards the centre; between N and S there are formed straight 
 lines consisting of the filings, but at other places on the cardboard 
 the lines are fewer and form wide bent arcs, which run from pole 
 to pole. 
 
 These figures may be explained as follows : 
 
ELECTRICAL ENGINEERING 
 
 FIG. 13. Magnetic Curves. 
 
 Each iron filing, when it comes within the influence of the 
 
 strong magnet, becomes 
 
 a sm f n ma e ne > wh j^ 
 
 can turn round on the 
 card. 
 
 Hence each of the 
 iron filings will take 
 up a definite position, 
 according to its place 
 in the space, in the 
 same way that the 
 magnetic needle did. 
 This position will be 
 at the points 1 and 2 
 (Fig. 12), directly to- 
 wards the north pole; 
 at the points 3 and 4, 
 directly towards the 
 south pole; at the 
 
 points 5 and 6, along the line joining the poles: but at all the 
 other points the position will be an inclined one. 
 
 The small pieces of magnetized steel place themselves in a row, 
 and so form continuous lines. These lines are absolutely straight 
 in the space between the two poles, whereas beyond they are 
 curves. These curved lines hence consist of a great number of short 
 straight pieces (the single filings), and each of these short pieces 
 indicates the direction of the resultant force, which the combined 
 influence of the north and south pole produces at this point. These 
 lines are called lines of magnetic force. 
 
 The lines of magnetic force do not only indicate the direction 
 of the force at each point, but also give us a measure for the 
 magnitude of this force. We observe that there are many lines in 
 the immediate neighbourhood of the poles, and in the space between 
 the poles, whereas elsewhere the lines are weaker. How is this to 
 be explained? 
 
 As we know, the magnetic influence gets smaller with decreasing 
 distance. Hence the iron filings in the immediate neighbourhood of 
 the poles r.re acted en with great force, and some of them are pulled 
 up against the poles. The influence on the iron filings, which are at 
 a greater distance from the poles, is not sufficient to attract them, 
 perhaps not even enough to turn them, if the friction on the cardboard 
 is greater than the resultant force. 
 
 The easily movable iron filings only can follow this influence, 
 and the further outside we go, the fewer filings will set themselves 
 in the direction of the resultant force. Thus the lines of the mag- 
 
FUNDAMENTAL PRINCIPLES 
 
 netic force are getting weaker and less distinct the further we are 
 from the poles. 
 
 We therefore learn that the density of the lines at any point 
 gives us a measure of the magnitude of the force acting at this 
 point. 
 
 We can further attribute a third meaning to the lines of force. 
 Let us imagine that it were possible to have a small magnet with 
 north magnetism only. 
 (As a matter of fact 
 that is impossible, for, 
 if we divide a bar 
 magnet into as many 
 pieces as possible, each 
 of these pieces would 
 still have its north 
 and south pole.) If 
 the north magnetic 
 pole is brought into 
 the sphere of activity 
 of the two poles, which 
 is called the magnetic 
 field, then it would be 
 repelled by the north, FIG. 14. Field of Magnetic Force, 
 
 and attracted by the 
 
 south pole. If, now, this pole is freely movable, it would always 
 follow the course of one of the lines of magnetic force (as shown in 
 Fig. 14), and would travel from the north pole towards the south 
 pole. 
 
 In the position of 7, in Fig. 12, the particle of iron will lie in 
 the direction of the arrows with three barbs. If the iron be now 
 shifted the direction it indicates alters, and is that of the lines shown 
 in Fig. 14. The line at any particular place may be supposed to 
 enter the south pole of the little magnet, and to leave at its 
 north pole. 
 
 In order to measure magnetism as we do any other matter, it 
 becomes necessary to define a unit for it. The unit of magnetism 
 is an amount of magnetism which if concentrated in a point would 
 exert a unit force on a similar amount of magnetism also concen- 
 trated at a point and located a unit distance away; i.e., 1 centimetre 
 (2.54 centimetres equals 1 inch). The unit of force used to measure 
 these quantities is not the pound, but a small fraction of it. It is 
 called a dyne and is equal, approximately, to ^-gVjnJ of a pound. 
 Thus, units of magnetism at unit distance away exert a force of 1 
 dyne upon each other. 
 
 It is now assumed for convenience and uniformity that a unit 
 pole as described has emanating from it one line of force, as described 
 
10 ELECTRICAL ENGINEERING 
 
 above, per square centimetre (6.45 square centimetres equal 1 square 
 inch) at unit distance away. Thus, a unit pole has streaming out from 
 it as many lines of force as there are square centimetres in a sphere 
 of unit radius, or four times TT (the Greek letter TT is generally used for 
 convenience to represent the number 3.14159). Since at unit dis- 
 tance away a unit pole exerts a unit force on another unit pole, it 
 follows that one line of force per square centimetre means a unit 
 force on a unit pole at the point represented by this line of force 
 density. Thus, in a magnetic field, whenever the density of mag- 
 netic flow is equivalent to one line per square centimetre, a unit 
 pole there would be acted upon by a unit force of 1 dyne. In the 
 air-gap of a dynamo, for instance, the pull on a unit pole equals per- 
 haps 10,000, which means from the above definitions that in this 
 magnetic field a unit pole would be pulled by a force of 10,000 dynes 
 or AAA pounds. It means also that the lines of force in 
 this field are at a density of 10,000 per square centimetre or 
 10,000X6.45 per square inch. Faraday advanced these units and 
 hypotheses, and they have been used ever since. 
 
 Influence of the Electric Current on a 
 Magnetic Needle 
 
 Let us make the following experiment : Over a straight horizontal 
 wire hold a magnetic needle (see Fig. 15). If there is no iron in the 
 neighbourhood capable of deflecting the needle, the latter will set 
 itself in a direction lying north and south with the n pole pointing 
 towards the north. Now, taking care that the wire is parallel to 
 the needle, let a current be sent through the wire. We observe that 
 the needle is now deflected. If we increase the strength of the 
 current, the deflection will be greater, and with a very strong current 
 the needle will set itself nearly at right 
 angles to the direction of the wire as shown 
 in the figure. The deflection ceases imme- 
 diately if we stop the current, the needle 
 swings back, and after a few vibrations 
 comes to rest exactly in the original direc- 
 tion. Now let the direction of the current 
 be reversed by changing the connections 
 FIG. 15 Action of o f t h e w i res with the poles of the battery. 
 Current on Magnet. j f ^ n end Qf ^ needle were de fl ected 
 
 to the right hand in the first case, it will 
 now be deflected to the left hand. 
 
 Next let the needle be held underneath the wire instead of above 
 
FUNDAMENTAL PRINCIPLES 
 
 11 
 
 FIG. 16. Simple Galvanometer. 
 
 it, and we shall find that the deflections are now opposite to those 
 in the first case. Ampere, a celebrated French electrician, studied 
 these phenomena, and found a very simple rule by means of which 
 the direction of the deflection may always be predicted. "If we 
 imagine a man swimming in the wire with the electric current and 
 so as always to face the needle, then the north pole will be deflected 
 to the left hand of the swimmer." This rule is called Ampere's 
 Rule. 
 
 Hence, if we hold the needle above the wire, the swimmer 
 must swim on his back and face the needle, whereas if the needle 
 
 is under the wire the swimmer 
 must have his face downwards, 
 in order to apply the rule cor- 
 rectly. 
 
 As already stated, the de- 
 flection is greater the stronger 
 the current. It is also in- 
 creased by making the distance 
 between wire and magnet 
 smaller, and further by having 
 a number of windings round 
 the needle (Fig. 16). This 
 gives a number of conductors 
 above and below the needle, 
 
 the current flowing to the right in the upper conductors, and to 
 the left in the lower conductors. Applying Ampere's Rule, we find 
 that the action of the upper wires will deflect the n pole in towards 
 the paper; and in the case of the lower conductors, where the 
 swimmer must lie on his back, in the same direction. All the 
 twelve conductors therefore help each other in deflecting the 
 needle. 
 
 A number of windings arranged in this way is called a galvano- 
 meter coil, and the tendency of the current is to bring the needle into 
 the axis of the coil. When the deflecting force is far stronger than 
 the directing action of the earth's magnetism, so that the latter is 
 practically without effect, then the needle will be driven at right 
 angles to the coil. 
 
 The force of the coil depends on the strength of the current and 
 the number of turns on the coil. A coil having 10 turns with a 
 current of 1 amp. flowing through it has the same effect as a 
 coil of 100 turns and a current of T L- amp., or one with 1000 turns 
 and J-Q amp. Hence, the product of the two quantities is of great 
 importance, and is called the ampere-turns, which in the above 
 examples is equal to 10. 
 
 We have seen, in the previous section, that a freely movable 
 magnetic needle always points out the direction of the lines of 
 
12 
 
 ELECTRICAL ENGINEERING 
 
 magnetic force, and we must now come to the conclusion 
 that the electric current produces a magnetic field inside 
 the coil with lines of force in the direction of the axis of 
 the coil. 
 
 Returning to the experiment shown in Fig. 15, let 
 us move the magnetic needle to various positions round 
 the wire, and we shall find that the lines of force en- 
 circle the conductor (see Fig. 17). The action of the 
 conductor on the needle being more vigorous near the 
 wire, we infer that the lines of force are here more numer- 
 ous, and diminish as we are more distant from the wire. 
 As to the direction of the lines it is the same as the 
 handle of a corkscrew, if the current be assumed to flow 
 from the handle to the point of the screw. 
 
 In order to see the circular lines of force, place a 
 piece of cardboard horizontally and make a hole in the 
 centre. Through this hole pass a copper wire which is 
 held vertically. Sprinkle the card uniformly with fine 
 iron filings, and now send a strong current through the 
 wire. On tapping the card gently the filings will 
 arrange themselves in a series of circles, as shown in f 
 
 Fig. 18. I 
 
 If we wind wire in the form of a helix, or what is 
 
 generally called a solenoid, then the circular lines f Lines of Force 
 force around each part of the wire will combine in a round Cur- 
 way which will be understood from Figs. 1!) and 20. rent. 
 Here several windings are shown in cross-section. In 
 the upper part of the windings the current flows to us (marked by a 
 dot), whilst in the lower part it flows from us (marked by a cross). 
 Using the above rule, we find that the lines of force flow in the direc- 
 tions marked by the arrows. For the sake of simplicity, only a single 
 Circle is drawn round each wire. We notice that, when the lines of 
 force are directed upwards and downwards in the neighboring circles, 
 these destroy each other, and only those parts of the circles which 
 are situated inside or outside the coil are effective. We have, there- 
 fore, as a resultant only straight lines inside and outside the solenoid. 
 This is seen in Fig. 20. As a consequence, if a needle be placed within 
 the coil it will tend to set itself along the axis of the coil. 
 
 From the action of a current on a magnet as found by Ampere, 
 and from the fact that a free north pole moves in the direction of 
 a line of force, it follows that the lines of force are created by a flow 
 of electricity (or a current). It has been found that these lines of 
 force are concentric circles about the wire. A current in a wire 
 entering this page perpendicular to it creates lines of force in the 
 plane of the page circulating around in the direction of the hands 
 of a watch, or right-handed. Looking at the end of an electro-mag- 
 
FUNDAMENTAL PRINCIPLES 
 
 13 
 
 net, if the current circulates in the direction of the hands of a watch, 
 or right-handed, the pole is a south pole. If counter clockwise, or left- 
 handed, it is a north pole. From the fact that a free north pole moves 
 along the lines of force, it naturally follows that when a north pole 
 
 FIG. 18. Arrangement of Filings round Current. 
 
 of one magnet is approached to the north pole of another it is re- 
 pelled, since the lines of force come out from a north pole. Since 
 the lines of force go into a south pole, the north pole is attracted 
 
 FIG. 19. Lines of Force of Helix. 
 
 . 20. Resultant Field of Helix. 
 
 to a south pole. This, therefore, expresses the law of attraction 
 and repulsion of magnets, as well as the influence of currents on 
 magnets. It is only necessary to have in mind the direction of a 
 north pole when under the influence of a line of force. 
 
14 ELECTRICAL ENGINEERING 
 
 Another important law in connection with lines of force is that 
 concerning the production of electro-motive force. While current 
 and electro-motive force can be produced by a galvanic battery as 
 described, the method employed in dynamos is that of moving a 
 wire across a magnetic field. Faraday discovered this law of induced 
 currents. He found that if a wire were moved across a line of force pro- 
 duced by a magnet, that an electro-motive force was created in this wire, 
 and that the greater the density of lines of force and the faster the move- 
 ment the greater the electro-motive force created. 
 
 Electricians have defined the unit of electric pressure as that 
 resulting from cutting 100,000,000 lines of force per second. It is 
 called a volt. If 200,000,000 lines of force are cut per second, two 
 volts are created. If 200,000,000 lines of force are cut, but two 
 seconds are consumed in cutting them, one volt is produced. Thus 
 the rate of cutting of lines gives the resulting voltage produced. 
 In a closed circuit, therefore, the rate of change of lines of force in that 
 circuit gives the voltage produced. This is the essential principle of 
 the modern dynamo. The large iron circuit with its spools creates 
 the lines of force, and the armature with its revolving wires cuts 
 these lines of force and produces electro-motive force. Since motion 
 is relative, the same effect is produced if the armature stands still 
 and the poles or line of force producers revolves. This latter arrange- 
 ment is used for alternators where it is desirable, with the higher 
 voltages created, to have the wires in which the voltage is created 
 .stand still, and thus be easier insulated and free from damage due 
 to vibration of rotation. 
 
 Electric Units Measurement of Currents 
 Ohm's Law 
 
 It is not possible to measure an electric current directly, as in 
 the case of a stream of water we can measure its quantity. With 
 the electric current we must judge of its strength by observing of 
 the effects of the current. If, for example, we connect a piece of 
 wire with the poles of a cell, a'nd the wire does not get hot, and if 
 we now remove the wire and connect it across the poles of a battery, 
 .and the wire now is heated, it is quite clear that the current has 
 been stronger in the second case. Again, if a coil is connected 
 firstly with a single cell and it is found that only small pieces of 
 iron are attracted, whereas when the coil is connected with a battery 
 heavy pieces are attracted, we infer that the current is of greater 
 strength in the second case. A third example may be taken from 
 ~the decomposition of water: when an electric current passes through 
 
FUNDAMENTAL PRINCIPLES 15 
 
 it, the greater the evolution of gas the greater must be the strength 
 of the current. 
 
 In one of these ways the strength of an electric current may 
 readily be determined, if only we have a unit strength of current 
 with which to measure the effects. For this purpose the chemical 
 effect may be best used, because the quantity of the gas produced 
 c^n be measured by the help of a graduated tube. Electricians have 
 fixed upon that current as a unit which will liberate in one minute 
 10.4 cubic centimetres of mixed gas. This unit is called after an 
 e::.inent French scientific man an ampere (often abbreviated to 
 amp.). 
 
 If, then, we find that a certain current iuves 20.8 cubic centimetres 
 of gas per minute, we denote the current strength as of 2 amperes; 
 if, on the other hand, the amount of gas per minute be 104 c.c. then 
 the current is 10 amps.; and so on. 
 
 Another definition of unit current is as follows : Let the current 
 be considered as flowing in a wire of infinite length perpendicular 
 to and passing through this paper at some point. The magnetic 
 current of the lines of force from this current circles in the plane 
 of the paper. The density of the lines of force (or flux) is greater 
 the nearer to the wire they are located. Consider one of the cir- 
 cuits whose length is one centimetre. Imagine a unit pole on this 
 circle. A unit current can now be defined as that current in the 
 
 above circuit which will produce on this pole a force of ^ dynes, 
 or, what is saying the same thing, the density of the lines of force at 
 this point is ~ lines per square centimetre. This definition of unit 
 
 current is the same current as defined by the gas-freeing method. 
 It is called an ampere. An ampere denotes a flow. It means a 
 unit of electricity per second. This quantity passing per second 
 is called a coulomb. Thus, one ampere is one coulomb per second. 
 A coulomb can be moving or standing still. An ampere means motion. 
 From this definition of current an excellent proof of what is called 
 magneto-motive force can be deduced as follows : It has been stated 
 that ampere turns create magnetism or flux. The term magneto- 
 motive force or "driving power" for magnetism has been given to 
 ampere turns. The ampere turns per inch of magnetic circuit is 
 called magnetizing force. Thus, a bar 100 centimetres long, with a 
 thousand ampere turns acting upon it, has a magneto-motive force 
 
 1000 
 of 1000, and a magnetizing force of =10. 
 
 Thus, 
 
 ,, ... . magneto-motive force 
 
 Magnetizing force =-, 77; ? r : T- 
 
 length of magnetic circuit 
 
16 ELECTRICAL ENGINEERING 
 
 Return now to the definition of unit current. A un!t current 
 exerts a force of on a unit pole situated on a circle away from 
 
 the wire such a distance that the length of the circle is 1 cm. Thus, 
 in Fig. 21 the current of one ampere enters the paper and at right 
 -- f , angles to it at the point A. A unit 
 
 pole at b is acted upon by a force of 
 
 ; or the density of lines of force in 
 , / / \ \ air, which is expressed by the letter H 
 
 I / / \ 4JJ. 
 
 by electricians, is , which is saying 
 
 the same thing. Under the above con- 
 /c' ditions, b is located away from A by 
 
 X ,'' the distance instead of 1 cm. (since 
 
 " ^ _ - ' Zn 
 
 - ~ ~ V, the circumference of a circle = 2?r times 
 
 the radius when ^ = 3.14159). 
 
 If b were one centimetre away from A at b f (Fig. 21) the flux density 
 H would be less, since the circumference of the circle b'-c'-d' = In X 
 radius, or 2n X 1 = 2n, and the circumference of this circle is the length of 
 the magnetic circuit; for the lines of force produced by the current 
 flowing into the paper at A have their path in various circles in the 
 plane of this paper, and the larger these circles the less the flux, 
 as has been shown on page 12 by the iron-filings experiment. As 
 a matter of fact, the flow of flux in air with magnetic circuits follows the 
 
 , ., TT . ,. ,, ampere turns 
 
 law: flux density =H is proportional to -, = ~- : : r, . 
 
 length of magnetic circuit 
 
 Thus, referring again to Fig. 21, if the flux density (or, what is saying 
 the same thing, the force of dynes in a unit pole) at b with unit cur- 
 rent at A = j^r, the length of the magnetic circuit b-c-d being 1, it 
 
 would be T7v^-27r at point b', since the length of the magnetic cir- 
 cuit at b' = 2n. Hence the force with one ampere one cm. away 
 
 2 2 
 
 (at & / ) == TA or H at b' one cm. away=-rr. With / amperes the force 
 
 27 
 
 would be yrr, and, if N turns were interlinked with the flux, the force 
 
 2IN 
 would be -r^p one centimetre away. At a distance T, since the 
 
 length of the circles representing the magnetic circuits are propor- 
 
FUNDAMENTAL PRINCIPLES 17 
 
 tionate to the distance away from the point A, the flux per square 
 centimetre, or 
 
 Force H = ........ (1) 
 
 We have previously stated the following definition: 
 
 magneto-motive force 
 
 Magnetizing force = ; -- rr ? ~T. - = - ^r, ... (21 
 length of magnetic circuit 
 
 IN 
 
 or, at the distance T, as shown, magnetizing force = ^f) 
 
 IN 
 
 or, rearranging, T= - j -- . 
 
 2rX magnetizing force 
 
 Substituting this value of T in equation (1) we get 
 
 _2IN27rX magneti zing force 
 "~W ~IN~ "" 
 
 or H = rrX magneti zing force, 
 
 or H = 1.258 X ampere turns per unit length of magnetic circuit. This 
 formula expresses the law of production of flux, and is used in the 
 calculation of all magnetic circuits. 
 
 There is another value expressed by the Greek letter p, called per- 
 meability, which relates to magnetic circuits. The value H expressed 
 above refers to #ir magnetic circuits. It is a fact, however, that with 
 a given number of ampere turns acting upon a circuit more flux will 
 be produced with circuit of one material than another. Thus, iron 
 will produce perhaps 1500 times as much as air. The ratio of the 
 flux produced in a material to that produced if the material were 
 air is called the permeability of the substance, and is designated 
 by the Greek letter //. The JJL for air= 1. While H means flux density 
 per square centimetre (or force in dynes on a unit pole) with air 
 
 T> 
 
 only, B means the same thing with any material. Thus = /*. Thus B 
 
 means the flux density per square centimetre with any material 
 except air. Since iron is usually used for magnetic circuits, B usu- 
 ally means flux density in iron per square centimetre. 
 
 An apparatus for measuring the strength of an electric current by 
 
18 
 
 ELECTRICAL ENGINEERING 
 
 means of the chemical effects is called a gas voltameter, after Volta, 
 who was one of the first to study electric effects. 
 
 Although the voltameter will measure the strength of an electric 
 current it is rather troublesome in practice, and is therefore almost 
 exclusively used for laboratory work and for the testing of other 
 instruments, as we shall presently see. 
 
 Instruments depending on the magnetic effect, as described above 
 in the magnetic definition of unit currents, or the heating effect are 
 very frequently used. In Fig. 22 is shown a current-measurer of 
 the electro-magnetic type. It essentially consists of a coil of wire a, 
 into which dips a thin and easily movable iron core, e. A pointer, /, 
 is connected with a bent lever in such a way that when the core moves 
 downwards the counter revolves, and its position will be indicated 
 on a scale which is not shown in the figure. By the adjustment of 
 the little weights at d and d, which are movable along the screws 
 threaded on the short arms, the zero position of the index may be 
 adjusted. 
 
 If we send a current through the coil the core will be drawn into 
 the interior, and the pointer will therefore move to a new position; 
 
 J 
 
 FIG. 22. Electro-magnetic Ammeter. 
 
 and the stronger the current the greater will be the deflection of the 
 pointer. To find the value of the current corresponding to the 
 deflection it is necessary to submit the instrument to the process 
 called calibration. For this purpose it is necessary to connect the 
 instrument with the voltameter, in such a way as to cause the 
 current to flow simultaneously through both. If we now regulate 
 
FUNDAMENTAL PRINCIPLES 19 
 
 the current so as to ensure the liberation of 10.4 c.c. of gas per second, 
 
 FIG. 23. Hot-wire Ammeter. 
 
 FIG. 24 Electro-magnetic Ammeter 
 (The Electrical Company). 
 
 then we know that 1 amp. is passing, and the position of the pointer 
 can be marked accordingly. The 
 current can now be increased so 
 that 2 amps, pass and the new 
 position of the pointer be marked. 
 Let this process be continued un- 
 til the highest possible deflection 
 of the pointer is reached, when the 
 calibration will be complete. We 
 have now what is called an am- 
 meter. 
 
 In Fig. 23 is shown an ammeter 
 depending on the heating effect of 
 the current. It is known as of the 
 hot-wire type. It consists of a very 
 fine platinum-silver wire, hh, which 
 is fixed at the points 1 and 2, and 
 is connected at the middle point, 
 3, to another fine wire, d. This 
 
 FIG. 25. Electro-magnetic Ammeter 
 (Crompton & Co.). 
 
 latter is wound around a small 
 
 roller, r, and is kept continuously 
 
 strained by means of a spring, /. When a current is sent through the 
 
 wire hh gets heated and expands, and so enables the spring / to pull 
 
20 ELECTRICAL ENGINEERING 
 
 the wire d forward, which action rotates the roller and moves the 
 pointer z. The instrument can be calibrated in a similar way to the 
 electro-magnetic instrument. Complete instruments of the two kind* 
 are shown in Figs. 24, 25, and 26. 
 
 In Fig. 26 it will be seen that attached to the pointer and moving 
 with it is a disc of aluminium. The upper edge of this moA r es between 
 
 FIG. 26. Hot-wire Ammeter (Johnson and Phillips). 
 
 the poles of a horseshoe magnet. The purpose of this arrangement is 
 to damp the motion so that the pointer is speedily brought to rest and 
 the value of the current quickly known. 
 
 We must now examine the influence of the E.M.F. in producing 
 a current. For this purpose let us go back to the hydraulic analogy. 
 In the tube connecting the two tanks the water-stream will be the 
 greater, the greater the difference of the heights of the columns 
 of the liquid; but it does not depend on that alone. The resistance 
 of the tube must have a considerable effect. If the tube is very long 
 and the inside rough and the bore small, then but little water can 
 pass through it. If, on the other hand, the tube is short with a large 
 bore of polished material, then for the same water-driving force the 
 stream of water must be much greater. 
 
 Exactly the same is the case with the electric current, for the 
 
FUNDAMENTAL PRINCIPLES 21 
 
 strength of the electric current depends not only on the E.M.F., but also 
 on the resistance of the circuit. Through a short thick wire connecting 
 the poles of a cell a stronger current will pass than if a long and thin 
 wire be used. 
 
 This dependence of the intensity of the effect on the driving 
 force and the resistance is to be met with, not only in the case 
 of the water-stream and of the electric current, but also with many 
 things in daily life. For example, imagine there are two coun- 
 tries with a great difference of density of population, then many 
 people will emigrate from the country with the denser population 
 to the country with the smaller one. The more crowded country 
 produces, in a manner, a stronger pressure towards the other 
 country, and the pressure difference can be considered as the 
 driving force of emigration. But not only has this pressure 
 difference an influence on the emigration, but it is of great im- 
 portance what opposition is offered to the flow of the people 
 between the two countries. If the countries are near each other, 
 and a good and open road leads from one to the other, then the stream 
 of people which flows from one country to the other may be a very 
 great one; on the other hand, if the countries are separated by 
 means of high mountains, wide seas, etc., then the resistance to the 
 flow will be greater, causing a corresponding diminution of the emi- 
 gration. If, finally, the boundary happens to be an impassable one, 
 then, although a driving force may exist, yet the countries will be 
 insulated from each other. 
 
 In a like manner, there are some materials which offer a very 
 small, others which offer a very great resistance to the electric 
 current. When the resistance is extremely great the substance is 
 called an insulator. On the other hand, materials which do not much 
 impede the current are termed conductors of electricity. To this 
 latter class belong all metals: first of all, silver and copper; then gold, 
 aluminium, zinc, platinum, iron, tin, lead, German silver, the liquid 
 metal mercury; next carbon; and, finally, many solutions, such as 
 sulphuric acid mixed with water, and salt solutions. 
 
 Insulators, or non-conductors, include the following: dry wood, 
 silk, cotton, india-rubber, gutta-percha, asphalt, oil, porcelain, glass, 
 dry air, and so on. 
 
 It must be clearly understood that the term conductor or insulator 
 is not to be considered as an absolutely fixed one; this may be well 
 understood by the reference to the example of two countries. In 
 ordinary cases the impediment of great mountains will be sufficient 
 to stop the passage of people between the two countries, but cases 
 may arise in which the driving force may be so great that nearly 
 all impediments can be overcome. In a like manner, materials which 
 may insulate at lower voltages may become conductors at higher 
 pressures. If we cover a conductor with an insulating substance in 
 
ELECTRICAL ENGINEERING 
 
 order to prevent the escape of electricity, it follows, from what has 
 been just said, that it will be necessary to make the covering thicker 
 for higher than for lower voltages. For a wire which is connected 
 with a single cell, an insulation of a winding of cotton or silk is 
 quite sufficient, whereas a wire which is connected with a generator 
 of some 1000 volts must be covered with several layers of india-rubber 
 or other insulators. 
 
 The law which expresses the relation between current strength 
 and electro-motive force tells us that the current is stronger the 
 greater the E.M.F., and is smaller the greater the resistance of the 
 circuit. For resistance a unit is also required. This may be defi- 
 nitely fixed as follows : If we have a wire of any size, and of any 
 material through which flows a current of 1 amp., the difference 
 of potential between the beginning and the end of the wire being 
 equal to 1 volt, then the resistance is equal to the unit of resistance 
 which is called the ohm. A convenient abbreviation for the ohm is 
 the Greek letter a>. 
 
 The ohm can be made from any metal. To give a definite idea 
 of its value it may be mentioned that about 10 feet of a copper wire 
 of No. 33 S.W.G.,* which has a diameter of T J th of an inch, is 1 
 ohm in resistance. Again, 60 metres of copper wire 1 square 
 millimetre cross-section has a resistance of about 1 ohm. 
 
 To make a standard resistance copper is not used, for the reason 
 that it oxidizes in the air, and that it is difficult to obtain the metal 
 quite pure. The best material for the purpose is mercury. By 
 careful measurements it has been found that a column of mercury, 
 1 sq. mm. in section and 106.3 cm. long, has a resistance of 1 ohm. . 
 
 Since 1 volt produces in a circuit of 1 ohm a current of 1 amp., 
 therefore an E.M.F. of 10 volts will produce in the same circuit a 
 current of 10 amps.; or, in other words, the strength of a current 
 varies directly as the electro-motive force. 
 
 Let us now keep constant the pressure as 1 volt, and vary the 
 resistance of the circuit; then through a resistance of 2a> will flow 
 a current of J amp., through a resistance of lOaj will flow a 
 current of T \ amp., and so on; in other words, ^he strength of 
 a current varies inversely as the resistance. Let us next con- 
 sider what will be the strength of a current which is produced by 
 an E.M.F. of 10 volts, in a circuit of 2aj. If the resistance 
 is \w the resulting current is 10 amps.; but since the resistance is 
 twice as great, the strength of the current will only be one-half, or 
 5 amps. 
 
 An E.M.F. of 110 volts will produce in a circuit of 220 to a 
 current of i^# = i amp. It is obvious from these examples that 
 
 * S.W.G. is an abbreviation for the Standard, Imperial, or Board of Trade 
 Wire Gauge. 
 
FUNDAMENTAL PRINCIPLES 23 
 
 the number of amperes passing through a circuit is obtained by dividing 
 the number of volts by the number of ohms in the circuit, or 
 
 Current strength = Electro-motive force ^Resistance. 
 
 Expressed by the initial letters of these words, we may write 
 
 C = E-R 
 or in the form of a fraction 
 
 This last is the mathematical expression for the law that we have 
 expressed in words above. It is called, after its discoverer, the 
 Law of Ohm. 
 
 The Calculation of Resistance 
 
 To compare the electric resistance of different materials it is usual 
 to find the resistances of the substances when all are of the same 
 length and cross-section. If the materials are in the form of wires, 
 each of some specified length and cross-section, say 1 m. in length 
 and 1 sq. mm. in cross-section, then the number giving the resistance 
 in ohms is called in each case the specific resistance. 
 
 From the data previously given, it will be easy to find the 
 specific resistance of copper and mercury. We know that the 
 resistance of 60 m. of copper wire, 1 sq. mm. in section, has a 
 resistance of lo>, hence its specific resistance is -f^co. Again, a 
 mercury column of about 1.06 m. and of 1 sq. mm. cross-section 
 has a resistance of lo>, so the specific resistance of mercury is 
 
 The following numbers show approximately the specific resistance 
 of most of the important metals: 
 
 Ohms. 
 
 Silver ............................ 0.016 =-fa about 
 
 Copper ........................... 0.0167=^ " 
 
 Gold ............................. 0.02 =^ 
 
 Aluminium ........................ 0.033 =fa " 
 
 Brass ............................. 0.070 = T V " 
 
 Iron .............................. 0.10 = T V " 
 
 Lead ............................. 0.22 =| 
 
 German silver ..................... 0.25 = ^ 
 
 Nickelin .......................... 0.35 = J " 
 
 Mercury .......................... 0.94 =|f " 
 
24 ELECTRICAL ENGINEERING 
 
 The numbers given above are average numbers, and assume that 
 the temperature is about 60 F. In accurate work it is necessary to 
 state the temperature for the reason that the resistance varies with 
 the temperature. 
 
 The law of change of resistance with temperature is expressed 
 as follows: Let R =the resistance in ohms at the temperature of 
 Centigrade (to get Fahrenheit temperature from Centigrade, mul- 
 tiply the Centigrade reading by -f and add 32). 
 
 Let RT= resistance at temperature T; then RT=R (l + aT), 
 when a is a constant depending upon the material. For copper 
 a = .0042. 
 
 [The units, metre for length and sq. mm. for cross-section, are very convenient 
 for practical work, and are much used abroad. In England, where the metric 
 system has not yet been adopted, common units are the inch, foot, or yard for 
 length, and the square inch for sectional area. 
 
 A system in which centimetre measure is used is recommended for universal 
 use. To change the above numbers in accordance with this system it is neces- 
 sary to remember that 1 metre is equal to 100 centimetres, and that 1 sq, cm. is 
 equal to 100 sq. mm. ; then, from what follows, the above numbers must be 
 divided by 10,000 ; thus the specific resistance of silver in centimetre measure is 
 0.0000016. TRANSLATORS.] 
 
 The purity of the metal has an important influence on the spe- 
 cific resistance. With alloys the proportions of the metals will give 
 great changes in the resistance. For example, there are kinds of 
 copper which have a specific resistance of -^ and even greater; dif- 
 ferent kinds of the alloy nickelin have specific resistances ranging 
 from J up to J, while German silver may vary from \ to in specific 
 resistance. 
 
 The different materials used for resistances in workshops and 
 laboratories must therefore always be electrically tested, but for 
 approximate purposes the numbers given above may be used. 
 
 With carbon the specific resistance differs greatly with the nature 
 of the carbon. There are some kinds with a value of some hundreds 
 of ohms, whilst carbon prepared under high pressure may be only 
 12. 
 
 From the table which we have given we may readily calculate 
 the resistance of a wire of any of the materials, if we know the length 
 and cross-section of the wire. As we have learnt from the analogy 
 of water flowing through a tube, the resistance is greater the longer 
 the tube, and is smaller the bigger the area of cross-section. A 
 copper wire 10 m. long and 1 sq. mm. cross-section has a resistance 
 of 10X^F=-t&>. A copper wire 1 m. long and 2 sq. mm. cross- 
 section has only half as much resistance as the one of 1 m. and 
 1 sq. mm. area; thus ^~-2= T ^a>. We infer that a wire of 
 10 m. and 2 sq. mm. cross-section must have a resistance of 
 
 O=^w. 
 
 These results we may express in words as follows: 
 
FUNDAMENTAL PRINCIPLES 
 
 25 
 
 The resistance of a wire of certain cross-section and length is to be 
 found by multiplying the specific resistance by the number of metres 
 in length, and dividing by the number of square millimetres in cross- 
 section; or 
 
 Resistance = specific resistance X lengths area of cross-section. 
 This may be abbreviated to 
 
 where R = resistance in ohms; /= length of the wire; a = the cress- 
 section of the wire; K=the specific resistance. 
 
 EXAMPLES. 
 
 1. What resistance has a copper wire 1000 m. long and 4 sq. mm. cross- 
 section? 
 
 Applying the formula we have 
 
 2. There are resistance frames, as shown in Fig. 27, of frequent use in electro- 
 technical work, for the purpose of regulating the strength of an electrical cur- 
 
 FIG. 27. Resistance Frame. 
 
 rent. They are generally made with coils of metal having a high specific resist- 
 ance. Let us suppose that German silver be chosen as the kind of wire. The 
 
26 ELECTRICAL ENGINEERING 
 
 question is: How much wire, having a cross-section of 1 sq. mm., must be used 
 to give a resistance of 100<? 
 
 Since German silver has a specific resistance of }, then 4 m. of a wire of 
 1 sq. mm. will have a resistance of Ito, so that for lOOw we require 400 in. 
 To place such a length of wire in a comparatively small space necessitates its 
 winding in spirals as shown in the illustration. 
 
 3. If in the previous example we had used a wire of double the cross-section, 
 then it would be necessary to take 800 m. On the other hand, if the cross-section 
 had been halved the length would only be 200 m. 
 
 4. If a copper wire, 2000 in. long and 3 sq. mm. cross-section, be connected with 
 on E.M.F. ot 110 volts, find the current passing through the wire. 
 
 First we must find the resistance of the wire. This is 
 
 The current strength is now found by dividing the voltage by this resistance; 
 or 
 
 E 110 
 C =TJ=rrT = 9 - 9 am P s 
 
 Other Forms of Ohm's Law 
 
 We are now able to calculate the current, being given the voltage 
 and the resistance. It is, of course, also possible to calculate the 
 resistance of a circuit if the voltage and current strength be given. 
 If, for instance, the voltage be 10 volts, and the current flowing be 
 2 amps., then we may find the resistance of the circuit as follows: 
 If the resistance of the circuit be la>, then the number of the 
 amperes must equal the number of the volts. On the other hand, if, 
 as in our example, the amperes are smaller than the volts, it is 
 obvious that the resistance is greater than leu. A moment's thought 
 will show that it must be five times as great, or 5co, for the current 
 is one-fifth of the voltage. 
 
 In like manner, if the voltage be 110 and the current be J amp., 
 then the resistance of the circuit is -VV- = 220o>. 
 
 The general rule is, then 
 
 The resistance of a circuit is to be found by dividing the E.M.F. 
 by the current strength; or 
 
 Resistance = E.M.F. -f- Current strength; 
 
 There is still one other way of stating Ohm's Law. Suppose that 
 we know the current and the resistance, and require the voltage. 
 Given, for example, a resistance of 220 to and a current of J amp. 
 What is the voltage? Since to get with a resistance of la) a 
 current of J amp. a voltage of i volt must be used, we argue that 
 
FUNDAMENTAL PRINCIPLES 27 
 
 to get the same current with a resistance of 220oj we must increase 
 the volts 220 times, or the volts must be 0.5X220 = 110 volts 
 Writing this so as to apply in a general way, we say 
 
 E.M.F. = Current strength X Resistance ; 
 or E = CXR. 
 
 The three formulae: 
 
 C =l < 
 
 R = (2) 
 
 E-CXR (3) 
 
 really mean one and the same law in different forms convenient for 
 practical calculations. 
 
 Internal Resistance Drop of Potential 
 
 If we connect any apparatus, A (Fig. 28) by means of copper 
 wires with the battery B, our circuit consists of three parts, viz. the 
 battery, the main conductors, and the appara- 
 tus. Each of these has a certain resistance, 
 and that of the battery is called the internal 
 resistance. The sulphuric acid or other liquid 
 that may be used in the cells has, when 
 compared with metals, a very high specific 
 resistance, so that to prevent the internal 
 resistance becoming too great it is necessary 
 to have the cross-section of the liquid suit- PIG. 28. Simple Circuit 
 ably large. 
 
 Suppose that the internal resistance of the battery is la>, the 
 resistance of the main conductors 2cu, that of the apparatus 3a), 
 and the voltage of the battery 12. Then, since 1+2 + 3 = 6 is 
 the combined resistance, the current flowing in the circuit will be 
 ^=2 amps. ^ Hence a voltage of 12 is required to give 2 amps, in 
 the circuit with a total resistance of QOJ. If only the apparatus 
 of 3w had been in the circuit, then the pressure to produce the 
 2 amps, would have been but 3X2 = 6 volts. Now the conducting 
 wires having a resistance of 2aj, a pressure of 2X2=4 volts 
 will be required for them. Again, for the battery which has a 
 resistance of \a>, a voltage of 1x2 = 2 volts will be wanted. The 
 total voltage of 12 is thus consumed in the whole circuit, but only 
 6 volts are usefully employed on the Apparatus A. The 4 vclts 
 
28 ELECTRICAL ENGINEERING 
 
 consumed by the conductors and the 2 in the battery represent a 
 loss, or drop, of potential. 
 
 In consequence of the drop of potential in the battery, there is 
 available at the poles, not the whole 12 volts that the battery 
 produces, but a smaller number. If we measure, by the help of a 
 suitable instrument (to be described later on) the terminal voltage 
 of the battery, we shall find it to be 10 volts only when a current of 
 2 amps, flows through the circuit, giving under these conditions a 
 drop of pressure of 12 10 = 2 volts. 
 
 The drop of potential is larger if the external resistance becomes 
 smaller. Thus, if we replace A by an apparatus with leu instead of 
 3a>, since the combined resistance is now l+2 + l=4&>, the current 
 will be \ 2 =3 amps. This current will give a potential drop, in the 
 battery, of 3X1=3 volts; in the conductors, of 3X2 = 6 volts; 
 and for the new apparatus, 3X1 = 3 volts. The terminal voltage 
 of the battery is, in this case, 12 3 = 9 volts. 
 
 Again, if we suppose that the resistance of the external circuit, 
 consisting of the main conductors and the apparatus, be but laj, giv- 
 ing a total resistance of 2co and a current of ^- = 6 amps., this will 
 give a fall of volts in the battery of 6X1 = 6 volts, leaving only 6 
 as the terminal voltage. 
 
 The student will now perceive that, for a certain current that may 
 be needed for any purpose, it may be necessary to reduce the poten- 
 tial drop in the battery. How can this be done? Obviously by 
 decreasing the resistance of the battery. The largest part of the 
 resistance of the battery is usually due to the liquid. We may 
 diminish this by making the way through the liquid as short and its 
 cross-sectional area as great as possible. The path may be shortened 
 by placing the plates of the elements very near each other. The 
 cross-sectional area of the liquid may be enlarged by making the 
 plates that are immersed in the liquid as large as we may allow. It 
 may be remembered that cells with large plates have exactly the 
 same E.M.F. as cells with the smallest; but the drop of potential is 
 for the small plates greater for the same current than for the largest, 
 owing to the internal resistance. It will therefore follow that the 
 cell with the larger plates must have a greater terminal voltage 
 assuming the current is the same. To take the case of the 
 battery with an internal resistance of la> and an E.M.F. of 12 
 volts, at a current of 6 amps, there will be a potential drop of 
 6 volts, and therefore a terminal voltage of 6. Suppose, now, that 
 the plates of this battery be made double the size, causing the 
 internal resistance to be %(u; then, with a current of 2 amps., the 
 potential drop of the battery will be only 2Xi=l volt, leaving a 
 terminal pressure of 11 volts. On increasing the current to 6 amps, 
 the potential drop will become 6Xi = 3 volts, giving now a terminal 
 voltage of 9. 
 
FUNDAMENTAL PRINCIPLES 
 
 29 
 
 Cells with very large plates are seldom employed, because they 
 are difficult to manufacture and inconvenient in use. We shall 
 presently see how we may make one large cell from a number of 
 small ones. 
 
 Branching of Circuits 
 
 NWttttH 
 
 A, 
 
 o 
 
 FIG. 29. A Series Circuit. 
 
 We have so far dealt with a simple closed circuit, so that the 
 current coming from the battery had to flow through all parts of the 
 circuit. If in a circuit be connected 
 several pieces of apparatus A t , A 2 , A 3 , as 
 shown in Fig. 29, having respectively the 
 Resistances 2, 3, and lo>, then the current 
 depends on the sum of these values. 
 Suppose that the cross-sectional area of 
 the battery and of the conductors be so 
 great that their resistance is practically 
 nothing when compared with the resistance 
 of the rest of the circuit, as often is the 
 case in practice, then we can at once 
 
 obtain the current by dividing 6, which is the sum of the resistances, 
 into the pressure. If the E.M.F. be 24 volts, the current will therefore 
 be - 2 / =4 amps., which will be the same at all parts. 
 
 But we can group the apparatus in another way. We can, for 
 example, as shown in Fig. 30, connect A t , A 2 , and A 3 , all to the poles 
 of the battery. This is 
 called the method of con- 
 necting in parallel, whereas 
 the former way was in 
 series. 
 
 The problem now to be 
 considered is how to find 
 the current strength in each 
 apparatus. First we will 
 find the current through 
 Aj. Since this is connected 
 with a battery of 24 volts, 
 and assuming that the con- 
 necting wires and internal 
 resistance of the cells are 
 practically nil, then the current through A t will be obtained by 
 dividing 24, the E.M.F., by 2, the resistance of the apparatus; or, 
 writing this after Ohm's Law, 
 
 FIG. 30. Circuits in Parallel. 
 
30 ELECTRICAL ENGINEERING 
 
 0,=^-^ = 12 amps.; 
 
 C l denotes the current, and R x the resistance of 
 In like manner we can write 
 
 = - =- 2 3 4 - = 8 amps.; 
 
 amps. 
 
 The battery therefore has to deliver the current to all four 
 branches simultaneously, which amounts to 12 + 8 + 24 = 44 
 amperes. 
 
 We may now ask: If we had, instead of the three parallel con- 
 nected branches, a single outer resistance only, what must be its 
 resistance that we may get a current equal to 44 amps.? This 
 problem may be readily solved by means of Ohm's Law. To produce 
 a current of 44 amps, in a circuit with 24 volts requires resistance 
 in the circuit of 11 = 0.545^. This value is defined as the resultant 
 resistance of the three branches. It is smaller than any of the 
 three branch resistances. We may say, generally, that we make a 
 combined resistance smaller by connecting in parallel, whereas 
 the combined value of resistances in series is, of course, greater 
 than any of them. 
 
 To work out the value of any resistances in parallel we may 
 proceed as follows: Imagine any voltage, preferably that of a single 
 volt, to which the resistances are connected. Then, taking the three 
 branches of the above example with resistances of 2, 3, and 1 ohm, 
 the respective currents will be as follows : 
 
 C 1= = 1 = 0.500 amps. 
 C 2 = J = 0.333 " 
 C 8 =} = 1.000 " 
 
 Thus the total current flowing through the three branches 
 is 
 
 1.833 amps., 
 
 and the combined resistance to replace the three would be 
 
 which is the same value as that obtained in the previous calculation. 
 We may, then, state the law: 
 
FUNDAMENTAL PRINCIPLES 31 
 
 To find the resultant resistance of a circuit consisting of any 
 number of branches connected in parallel we imagine the branches 
 connected with a pressure of one volt, then calculate the current strength 
 of each of the branches, adding all these branch currents together. 
 The resultant resistance is then found by dividing 1 volt by the sum 
 of the currents. 
 
 The calculation becomes much simpler if the branch resistances 
 are equal. If we had, for instance, two branches, each with a resist- 
 ance of Wco, the current in each branch, if connected with a 
 voltage of 1, would be y 1 ^ amp., and the combined currents would 
 be -f$ =: z amp., giving by our rule a resultant resistance of 5co, which 
 is half that of the branches. In the same way, the combined resist- 
 ance of 10 branches, each with a resistance of 10o>, will be laj, i.e. the 
 tenth part of any one of them, and so on. 
 
 Cells in Series and Parallel 
 
 The cells of a battery may be connected in series or parallel, and 
 the effect on the internal resistance is exactly the same as we have 
 found for the external part of the circuit. When two cells are con- 
 nected in series the combined resistance is twice that of a single cell, 
 and when they are in parallel the resultant resistance is half that of 
 a single cell. 
 
 But in addition to the resistance, the effect on the E.M.F. must 
 be thought of. In Fig. 31 are shown two cells in series, the copper 
 of the second cell being connected with the zinc 
 pole of the first cell by means of a wire, the 
 external circuit being connected with the end 
 poles. The effect of this arrangement is to add 
 the pressures of the cells so that, if the cells are 
 qual in pressure and each of one volt, there will 
 fae two volts available for producing a current 
 through the circuit. In the same way a battery FIG. 31. Two Cells 
 of 100 cells would give a voltage of 100 times that in Series, 
 
 of a single cell. 
 
 We will next consider the grouping of the cells in parallel. 
 Fig. 32 shows two cells so arranged having the two copper plates 
 connected, and also the two zinc plates. On joining a wire from the 
 positive poles and the negative poles to an outer circuit, we have 
 the effect of a single cell of double size. 
 
 The voltage of this battery is 1, but the internal resistance is 
 half that of a single cell. If we connected 10 cells in parallel in 
 this way, all the zinc poles being joined together and the same with 
 all the copper poles, then the pressure will be as before, 1 volt; 
 
32 
 
 ELECTRICAL ENGINEERING 
 
 but the internal resistance will only be the tenth part of a 
 separate cell. 
 
 If no connection be made to an external circuit no current can 
 flow if the cells are equal in voltage. On looking at Fig. 33, where 
 
 FIG. 32 Two cells in 
 Parallel. 
 
 FIG. 33. Cells in 
 Opposition. 
 
 the arrows show the direction of the pressures in the two cells, we 
 see that the first cell tends to send a current in the direction of the 
 simple arrow, whilst the second cell would give a current as shown by 
 the feathered arrow. These arrows, pointing in opposite directions, 
 
 show that the E.M.F.'s of the cells 
 are in opposition, and therefore no 
 current can flow, just as a cart cannot 
 be moved by two equally strong 
 horses that are pulling in opposite 
 directions. 
 
 Let us now connect the poles with 
 an outer circuit; then, as shown in 
 Fig. 34, one cell tends to send a 
 current in the direction of the simple 
 arrow, and the second cell in the 
 direction of the feathered arrow, both 
 arrows having the same direction in 
 the outer circuit. Here the cells do 
 not oppose, but assist each other in 
 driving a current through the outside 
 circuit. If each cell supplies 5 amps., 
 then the current used in the external 
 circuit will be 10 amps. Again, if we 
 
 have 10 equal cells in parallel, the current in any of the cells will 
 be one-tenth part of the outer current. 
 
 When is it desirable to arrange the cells in parallel? The 
 
 FIG. 34. Cells jointly supplying 
 an Outer Circuit. 
 
FUNDAMENTAL PRINCIPLES 
 
 33 
 
 answer is derived from the following considerations: If the external 
 resistance is a great one, then a large E.M.F. is needed to produce 
 a current of a certain strength. To secure such a pressure it is 
 necessary to connect a number of cells in series. But in doing this 
 we have at the same time increased the internal resistance. If, 
 however, the external resistance is very great compared with the 
 internal resistance, we shall gain more from the increase of vol- 
 tage than we shall lose from the increase of the resistance of the 
 battery. 
 
 If, on the other hand, the outer resistance is comparatively small, 
 then we must dimmish the internal resistance as much as possible by 
 connecting the cells in parallel. 
 
 The strength of current given by any cell is, of course, limited. 
 If, for example, a cell has a pressure of 1 volt and a resistance of ^co, 
 then it is impossible to get a bigger current than 10 amp., even if 
 we short-circuit the cell by connecting its poles with a stout piece of 
 copper wire. Usually such a short-circuit current will destroy a cell 
 in a very brief time. 
 
 Even if we connect 100 such cells in series, we cannot get a 
 greater current than 10 amps.; for although the E.M.F. is 100 volts, 
 yet the internal resistance is lOOXiV lO^ giving on short-circuit 
 a current of 10 amps. 
 
 On the other hand, if these cells are arranged in parallel and 
 short-circuited, then the current obtainable will be 100X10 = 1000 
 amps. This result may be ob- 
 tained at once by remembering 
 that the resistance of the 100 
 branch circuits is i^-$X^ = 
 T oVs > an d with the 1 volt avail- 
 able the current will be -5-. /or = 
 1000 amps. 
 
 Voltmeters 
 
 The voltage can be measured 
 with a similar apparatus to the 
 ammeter. Fig. 35 shows one of 
 the electro-magnetic type. It 
 is, of course, very important 
 that the coil of such an instru- 
 ment take as little current as 
 possible. It accordingly consists 
 
 of many windings of a very fine wire, so that its resistance is a very 
 high one. Although the current is very small, yet owing to its 
 passing so often round the coil the magnetic effect may be as great 
 
 FIG. 35. Voltmeter, Electro-magnetic 
 Type. (The Electrical Company.} 
 
34 
 
 ELECTRICAL ENGINEERING 
 
 as in the case of the ammeter, which is supplied with a strong current 
 that passes round only a few times. 
 
 A hot-wire instrument may also be used as a voltmeter. But 
 here, owing to the very small resistance of the short and fine wire, 
 that expands when heated, it is necessary to put in series with it a 
 resistance. This resistance consists of very many windings of fine 
 wire. One or more of these resistance coils are placed either within 
 
 V 
 
 FIG. 36. Hot-wire voltmeter (Johnson and Phillips). 
 
 the instrument or in separate boxes. Fig. 36 shows a hot-wire 
 voltmeter. 
 
 The higher the voltage to be measured, the greater must the 
 resistance in the circuit of the voltmeter be made. 
 
 Electrical Power 
 
 Let the following experiment be made : Take a spiral of German- 
 silver wire of a certain length and a certain cross-section, and connect 
 
FUNDAMENTAL PRINCIPLES 
 
 35. 
 
 it with a source of, say, 10 volts. Then, if the voltage is sufficient, the 
 spiral will get hotter and hotter; and if the size of the wire be rightly 
 selected, a certain temperature will be reached which will remain 
 constant. We must, therefore, conclude that now the wire radiates 
 as much heat to the surrounding air as is produced by the electric 
 current. 
 
 The most common application of this principle is the electric 
 incandescent, or glow, lamp. 
 
 In a closed glass bulb exhausted of air a carbon filament is fixed, 
 and its ends are connected to two metallic contacts. Such a lamp is, 
 shown in Fig. 37. On arranging these con- 
 tacts so that they receive an electrical pressure 
 of a certain height, an electric current will 
 flow through the carbon and raise it to white 
 heat. 
 
 The most usual type of lamp has a candle- 
 power of sixteen, and is made for a supply at 
 110 volts. It has a resistance of 220 to, and 
 hence a curent of JJ-- = 0.5 amp. will flow 
 through it', which is just sufficient to keep the 
 filament at white heat. 
 
 If we wish two glow lamps to give light we 
 may connect them in series and provide a 
 voltage of 220, when through each will pass a 
 current of 0.5 amp. ; the resistance of the two 
 filaments being 440&>. 
 
 The same result can be produced in another 
 way. Let the two lamps be placed in parallel 
 with 110 volts. The resulting resistance will 
 now be 110&>, so that the total current will be 
 through each carbon will flow the necessary J amp. 
 
 On comparison of these two cases we see that the same heating- 
 effect is produced with 220 volts and 0.5 amp. as with 110 volts 
 and 1 amp. This shows that the heating effect is dependent, not 
 only on the voltage or current, but on the product of these two 
 quantities. This product, vo Its X amperes, is defined as the electrical 
 power. The unit volt-ampere, being rather long, is abbreviated to 
 Watt, after the name of the famous improver of the steam-engine. 
 
 The power of the current in the case of the lamps which we have, 
 been just considering is 
 
 110 voltsX0.5 amp. = 55 volt-amperes = 55 watts. 
 
 Generally we can write the equation 
 
 Power = Electro-motive force X Current strength ; 
 
 or Glow Lamp. 
 1 amp., and 
 
36 ELECTRICAL ENGINEERING 
 
 or, in symbols, 
 
 If it so happens that the voltage and the resistance only be known, 
 then it is easy to find the power by the aid of the preceding rule, first 
 finding the current from 
 
 r- E 
 R 
 
 hence 
 
 Or, if the voltage and resistance be given to find the power, we must 
 multiply the number of volts by itself, and divide the product by the 
 number of ohms. 
 
 To take the case of the glow lamp above, to find the power from 
 the voltage of 110 and the resistance of 220 we should have 
 
 P = 1 10 X 1 10 + 220 = 55 watts, 
 
 the same result as before. 
 
 Again, if the current and resistance be given, we can calculate the 
 power from the third form of Ohm's Law 
 
 By substituting this value of the E.M.F. in 
 
 P = EXC 
 we get 
 
 P=CXRXC 
 or 
 
 P=CxCXft 
 
 that is, we find the number of watts by multiplying the number of 
 amperes by itself, and then by the ohms. 
 
 Thus, for the case of the glow lamp, we have 
 
 0.5X0.5X220 = 55 watts, 
 
 this being the same result as we previously obtained, proving tht>_ 
 the three methods are equivalent. 
 
 For the value CxC the abbreviation C 2 (C squared or C raised 
 to the second power) is generally used. The exact meaning of this 
 will be understood from the following considerations: 
 
 If we have to determine the area of a square we can do so by 
 dividing its sides into equal parts, each of which is, say, 1 inch long; 
 then, by drawing lines through these marks horizontally and 
 
FUNDAMENTAL PRINCIPLES 
 
 37 
 
 vertically the whole area of the square will be split up into a number 
 of smaller squares, each having an area of 1 sq. inch. By counting 
 the number of these we find 
 that a square whose sides are 
 2 inches long has an area of 
 2X2=- 4 sq. inches, whilst 
 one of 3-inch side has an area 
 3X3 = 9 (see Fig. 38). To 
 multiply a number by itself 
 is thus the same as to deter- 
 mine the area of a square 
 whose side length is equal 
 to this number. The three 
 formulae given above may 
 therefore be written 
 
 FIG. 38. 
 
 P = EC 
 
 -I 
 
 P=C 2 R 
 
 (1) 
 (2) 
 (3) 
 
 The fact that the power depends both on the voltage and on the 
 current strength may be understood by reference to the hydraulic 
 analogy. The effect of flowing water depends, not only on the 
 pressure or water-driving force produced by a difference of levels, 
 but also on the quantity of water per second, which corresponds to 
 the strength of the electric current. A pound of water flowing down 
 a height of 10 feet will be able to do ten times the amount of work 
 such as driving a water-wheel as a pound of water flowing down 
 a height of one foot only. The effect of flowing water is thus pro- 
 portional to the product of the number of pounds of water and 
 number of feet. This product is called foot-pounds. The power of 
 a waterfall is estimated by the number of foot-pounds per minute or 
 second. 
 
 Equivalence of Electrical Mechanical and 
 Heating Effects 
 
 Heat can be produced both electrically and mechanically. A 
 vessel containing water may be heated by placing in it an insulated 
 spiral of wire which is connected with a current generator. Heating 
 of water may also be caused by dropping from a height a stone into 
 it; or a better example for our purpose would be the case of sand 
 flowing continuously into water. In this case there will be a gradual 
 
38 ELECTRICAL ENGINEERING 
 
 heating of the water, just as in the case of an electric current. After 
 some time the temperature of the water will cease to rise, showing 
 that the heat produced by the flowing $and is now equal to the heat 
 radiated to the surrounding bodies, sucn as the vessel, air, etc. 
 
 Seeing that both by mechanical and electrical means heat may be 
 produced, the question may be asked 1 : How many mechanical units 
 correspond to the electric unit, or ^ watt? This question may be 
 answered by the help of, the following experiment. Take two equal 
 vessels containing equal quantities of water, and let one be heated 
 electrically, and the other mechanically by the use of falling sand, 
 so that the amount of heat passing in is the same in the two cases as 
 shown by a thermometer. Let pe voltage and current, and the 
 rate at which the sand falls from a known height, be noted. By 
 means of these values, and experiments of a similar kind, it has 
 been found that 
 
 One foot-pound per second = 1.356 watts. 
 
 We shall, in subsequent pages, deal with machines which enable 
 us to convert mechanical into electrical power. We may, for 
 example, have an electrical machine driven by a water-wheel or a 
 turbine; then, if we know the height of the fall and the amount of 
 water per second, the electrical power can be estimated. If, for 
 instance, 1000 Ibs. of water falls down a height of 5 feet each second, 
 then the mechanical value- of this will be 5000 ft.-lbs. per second. 
 If it were possible to convert all the mechanical into electrical power, 
 or, in other words, if there were no losses, then the number of watts 
 produced would be 
 
 = 3687 watts. 
 
 As a matter of fact the number of watts would be considerably smaller 
 than this. 
 
 Being given the number of 3687 watts, we can calculate the 
 number of 16-candle-power lamps that could be lighted. If 
 each lamp required 55 watts, then the number would be -||- = 67 
 nearly. 
 
 It is not the general custom with engineers to state the output of 
 a machine, such as a turbine, or steam or gas engine, in foot-pounds 
 per second, because of the large numbers that would have to be 
 used. Instead, a much greater unit called the horse-power (abbre- 
 viated H.P.) is used. A horse-power is defined as that rate of doing 
 work that is equal to raising 33,000 Ibs. 1 foot high in 1 minute. 
 This is equal to an effort of s^op = 550 ft.-lbs. a second. This 
 unit was introduced by James Watt, and has since been used by 
 British engineers in stating the power of engines. It is supposed to 
 
FUNDAMENTAL PRINCIPLES 39 
 
 represent the power of a very strong horse when working very hard. 
 The equivalent electrical power is 
 
 550 X 1 .356 = 746 watts nearly. 
 
 If we know what is the output of any machine given in H.P., 
 then we can, if it is coupled to an electrical generator, calculate the 
 electrical output, providing that no losses be taken into account. 
 Thus a steam-engine of 1 H.P. would drive a dynamo giving an 
 output of 746 watts. The real electric output due to losses of friction, 
 etc., is, of course, less than this, and generally varies between 700 to 
 600 watts per H.P., according to the size of the machines. For small 
 machines it comes down to 500 watts. These numbers correspond 
 with from 9 to 13 lamps per H.P., if 55- watt lamps be used. 
 
 A small dynamo, for example, driven by a 5-H.P. steam-engine, 
 could feed about 5X10 = 50 lamps; whilst a large one of about 
 200 H.P. would supply 200X13 = 2600 lamps each of 16-candle- 
 power. On the other hand, if this 200-H.P. engine had been used 
 for driving a pump that was perfectly efficient, then it would lift 
 200X550 = 110,000 Ibs. of water per second, a height of 1 foot. 
 
 The relation that exists between the electric and heating effects 
 must now be studied. First, the unit of the quantity of heat must 
 be defined. This unit is called the calorie; it is equal to the 
 quantity of heat which is necessary to raise 1 grm. of water from 
 zero to 1 on the Centigrade scale. Hence, to raise 1 kg. of water 
 from zero to boiling-point (100 C.) will require 1000X100=100,000 
 calories. 
 
 The relation between the two effects can be determined by the 
 following experiment: A vessel containing a known quantity of 
 water is heated by means of an insulated wire through which a 
 current is flowing, and the temperature ascertained by means of 
 a thermometer before the current has been passed, and also after it 
 has been flowing for a known number of seconds. At the same time 
 the voltage and the current are noted. The quantity of heat, and the 
 watts, and time are thence known. The result is, that 1 calorie 
 corresponds to 4.2 watts per second; or the heat equivalent of 1 watt- 
 second [1 watt-second is called a " joule"] is 
 
 2^ =0.24 calories approximately. 
 
 We can now readily calculate the quantity of heat produced by 
 a 16-candle-power glow lamp in one hour. Since in one second 
 55 watt-seconds are used, then in one hour the number will be 
 55X60X60 = 198,000, so that the number of calories will be 
 198,000X0.24 = 47,520. 
 
 On flowing through the carbon filament the current causes 
 
40 ELECTRICAL ENGINEERING 
 
 a temperature which rises until a bright white heat is reached. 
 After this the temperature remains constant, because the heat is 
 radiated as fast as it is produced by the current; or ; in other words, 
 a stationary state is reached. 
 
 Let us next ascertain what will happen if we connect this lamp 
 with a voltage of 150 instead of 110. If we may make the supposi- 
 tion that the resistance of the lamp remains the same as before, the 
 current now will be 
 
 Jf =0.68 amps., 
 
 and its watts now become 
 
 150X0.68 = 102 watte. 
 
 The watts being nearly twice as great as previously, a double quantity 
 of heat will be produced per second, and the carbon will reach a far 
 higher temperature than before, and will give out more light. This, 
 however, will not last for a long time, because the effect of the high 
 temperature soon causes the filament to be broken. The "life" of 
 such a lamp will thus be far shorter than when the voltage is 
 normal. 
 
 Again, if we connect a 110- volt lamp with a voltage of 220 volts, 
 then the power required is four times as great as before, for 
 
 In this last case the heat produced is such as to ruin the filament 
 immediately on switching the lamp into the circuit. 
 
 As a matter of fact the power taken by the filament in the two 
 cases of 150 and 220 volts is far higher than our calculations indicate, 
 because the resistance of carbon decreases contrary to the case with 
 metals with a rise of temperature. 
 
 Electric Mains 
 
 To lead the electric current from the place of generation to the 
 place where it is used we require leads or mains. Generally we 
 want two mains, one leading in and one out, just as we must have 
 two channels, to lead water to a water-wheel and to lead it away 
 {see Fig. 39). 
 
 The first channel connects our water-wheel with the higher, the 
 other channel connects it with the lower level. In a like manner 
 the mains serve to connect the electric apparatus with the positive 
 and negative poles of the current-generator. 
 
FUNDAMENTAL PRINCIPLES 
 
 41 
 
 The water-channels, which are generally made from earthenware 
 or wood, always cause a loss of motive force. If the channel is 
 not tight enough, some of the water will leak through the cracks. 
 This part of the water will not reach the water-wheel at all, thus 
 involving a direct loss of water. Further, a part of the whole pressure 
 gets wasted in flowing through the channels; this loss is represented 
 in Fig. 39 by the line h { for the upper channel, and by the line h 2 
 for the lower channel. Thus, the total height difference of water 
 available for driving the water-wheel is not H, but H diminished by 
 Thus the heights h^ and h 2 represent pressure losses. 
 
 
 FIG. 39. 
 
 In like manner, we have with electric mains losses of current 
 and losses of voltage or potential drop. The former occur with 
 badly installed mains only. If, for instance, mains leading from 
 a central electric station to a group of lamps are in several places 
 in connection with the earth or with damp walls, then the current 
 will not only flow through the lamps, but a part of it will also 
 flow from the positive wire to the earth, and from the latter to 
 the negative wire, without going through the lamps. The current 
 lost in such a way will be greater the better is the connection 
 of each of the mains with the earth, and the nearer the bad points 
 of the positive main are to the bad points of the negative main. 
 If these be very near each other, then the resistance of the earth 
 
42 
 
 ELECTRICAL ENGINEERING 
 
 between them will be very small, and a comparatively large current 
 will leak away. 
 
 To avoid such losses, and the risks connected therewith, mains 
 have to be very well insulated. Mains are provided with a continuous 
 
 insulating covering, or they may be 
 left bare, but in this case they have 
 to be fixed on bell-shaped insulators, 
 as shown in Fig. 40. They are 
 made of porcelain or glass in the 
 shape of a bell, and fastened by 
 means of insulating cement to an 
 iron bracket. The latter is fixed on 
 a mast or to a wall. The conductor 
 is secured by wire to the groove of 
 the insulator. Porcelain and glass 
 are excellent insulating materials, 
 and the wires fixed to the insulators 
 are therefore entirely insulated from 
 
 the iron bracket. Owing to the special shape of the insulator even 
 raindrops cannot make an electric connection, because the bell-shaped 
 part is usually fixed in a vertical position. For extra high pressures 
 such as, for instance, 5000 or 10,000 volts, an insulator of the shape 
 described would not be srfe enough. In such cases double or triple 
 bells are employed, as shown in Fig. 41 . 
 
 FIG. 40. Porcelain Insulator 
 (General Electric Co.). 
 
 FIG. 41. High-tension Insulator. 
 
 This method of running mains is often used for overhead or aerial 
 lines. In fixing such lines, care must be taken to avoid contact 
 
FUNDAMEXTAL PRINCIPLES 
 
 43 
 
 of the wires with each other, and with other bodies. They must 
 not be placed near trees, because the branches and leaves might then 
 touch the wires, thus forming in damp weather a good connection 
 with the earth. 
 
 The mains installed in the streets of large towns, or within houses, 
 consist of insulated wires only. The method of insulation of these 
 mains depends, on one hand, on the voltage of the current which they 
 conduct, and, on the other hand, on the position in which they are 
 fixed. Mains for low voltages, installed in dry rooms, may be covered 
 with a thin layer of insulation only. In such a case it would, for 
 instance, be sufficient to cover the wires with a thin winding of cotton 
 or hemp, and to impregnate this winding with tar or asphalt. For 
 high voltages and damp rooms, the wires must be covered with india- 
 rubber, and several layers of cotton or hemp. 
 
 Cables laid in the earth or channels are exposed both to the 
 influence of moisture and acids, and are liable to mechanical injuries. 
 They have, therefore, to be protected in addition to the different 
 insulation layers with a lead covering, which is further covered with 
 an insulating layer. Protection against mechanical injuries is 
 frequently guarded against by an iron or steel armouring, which 
 latter may be protected against corrosion by a cotton or hemp network 
 impregnated with bitumen. 
 
 Fig. 42 shows a cross-section through a cable, containing both the 
 positive and the negative wire. The circles in the centre represent 
 one main surrounded by an 
 insulating layer. The second 
 main consists of a number 
 of thin copper wires arranged 
 in a circle. Next to these 
 wires is an insulation-layer, 
 then a lead covering, and, 
 finally, an outer casing. As 
 the inner and outer wires 
 form circles with the same 
 centre, the cable is called a 
 concentric one. Cables are 
 also manufactured in which 
 the single insulated wires are 
 stranded with each other. 
 
 Exact specifications relat- 
 ing to the insulation and 
 laying of cables may be 
 found in the Board of Trade 
 Regulations, the Rules of the 
 
 Institution of Electrical Engineers, and those of Fire Insurance 
 Companies. 
 
 The losses of current can be avoided by proper installation of the 
 mains. Losses of voltage cannot be avoided, because the mains have 
 
 FIG. 42. Section of Concentric Cable 
 (Siemens and Halske). 
 
44 ELECTRICAL ENGINEERING 
 
 in any case a resistance, and thus a voltage drop must occur in them, 
 which may be determined by Ohm's Law. 
 
 Let us now work out the following example : The distance between 
 a current generator and a room which is lighted by 20 lamps, each 
 of 16-candle-power, and connected with 110 volts, is 80 yards, 
 and the cross-sectional area of the wire is 0.04 sq. inch. What is 
 the voltage drop in the main, if all lamps are burning simultane- 
 ously ? 
 
 Since we have to consider both the positive and the negative main, 
 the total length of wire employed will be 160 yds. As is generally 
 the case for mains, the wire consists of copper, whose specific resistance 
 is T Q ire o- oh m s per yard per sq. inch; the total resistance of the mains 
 is thus 
 
 The current required for feeding 20 16-candle-power lamps is 20 X^ 
 = 10 amps. 
 
 Thus the voltage drop in this main which may be called e, to 
 distinguish it from E will be 
 
 To get the proper voltage of 110 at the lamps, we want in the 
 central station, say, 111 volts. The voltage drop in the main is thus 
 not quite 1 per cent., which may be allowed in any case. Even if 
 the pressure in the central station be only 110 volts, and the lamps 
 therefore burn with 109 instead of 110 volts, this would not be 
 any disadvantage, as the diminution of the light is not serious as 
 long as the voltage falls 2 or 3 per cent. only. 
 
 The power lost in the main is 1 voltXlO amps. = 10 watts; or, 
 using the formula 
 
 P=C 2 XR=10X10X T V=10 watts. 
 
 The total power given to the lamps is 
 
 EC = 110X10 =1100 watts. 
 
 Let us now assume that we have to transmit through a main of 
 equal cross-sectional area the same current a distance of 800 yards 
 (total length of the wire = 1600). Then the resistance of the wire 
 will be ten times as large as in the above example, viz. la>; the voltage 
 drop in the main will be 10, the power loss 100 watts. 
 
 These losses are comparatively very great. If in the central 
 station a voltage of 110 be maintained, then the lamps at the end of the 
 main would burn with 100 volts only, and would emit far less light 
 than they would do if connected with their proper voltage; further, 
 the loss in the main of 100 watts, that is more than 9 per cent, of 
 the total output, is a very high one. 
 
FUNDAMENTAL PRINCIPLES 
 
 45 
 
 We may hence lessen the voltage and the power loss by 
 diminishing the resistance of the main, i.e. by enlarging the cross- 
 sectional area of the copper wire. 
 
 If we, for instance, quadruple the cross-sectional area of the 
 copper wire, then its resistance becomes the fourth part only: 
 
 and then the voltage drop becomes one-fourth as well 
 e = HC = 0.25X10 = 2.5 volts. 
 
 The power lost in the main will thus be 
 P = C 2 R = 25 watts. 
 
 These values of voltage drop and power lost are allowable in 
 practice, but, as we have seen from the example, we get these 
 permissible losses only by employing wires having large cross- 
 sectional areas. If the distance were still longer than 800 yards 
 we must employ wires of still greater areas, and so the network of 
 lines would become exceedingly costly. 
 
 We have, however, other means of reducing the losses due to 
 voltage drop. Suppose we double the voltage in the central station, 
 and connect the lamps in ten parallel groups, each of 
 these groups consisting of two series connected lamps 
 (see Fig. 43). 
 
 The resistance of each of these groups is 2X220 = 
 44Qoj, and thus the current taken by each of the 
 groups, if connected with 220 volts, would be f}=0.5 
 amp. ; i.e. the same as taken by a single lamp before. 
 The 10 groups together require thus a current of 
 10X0.5=5 amps. 
 
 The power taken by the 20 lamps is obviously 
 now the same as before. It was 110 volts X 10 amps. 
 = 1100 watts in our first example, and is 220X5 = 
 1100 watts in this one. 
 
 For the mains we employ the same wires as in 
 the first example, with a cross-sectional area of 0.04 
 sq. inches. The voltage drop in this main, having 
 a resistance of \a), is 5 volts at the current of 5 amps. 
 These 5 volts are 2.3 per cent, of the voltage of 220 
 volts, thus being a permissible loss. The power lost 
 in the mains is 5 2 Xl = 25, i.e. again 2.3 per cent. 
 of the total load. By doubling the voltage we obtain, 
 thus, the same result as by quadrupling the area of the 
 cross-section. 
 
 FIG. 43. 
 Lamps in 
 Series and 
 Parallel. 
 
46 ELECTRICAL ENGINEERING 
 
 This reasoning explains why high voltages are employed when- 
 ever electrical energy is to be transmitted long distances. A 
 pressure of 110 to 150 volts is generally used for current supplied to 
 a single building only or to several buildings situated near each 
 other. For providing small districts with electrical energy a pressure 
 of 200 to 250, and for larger districts 400 to SCO volts is employed. 
 But even these voltages are not sufficiently high for mains spread 
 over large towns and districts. To get, in the latter cases, allowable 
 losses, nd yet not have too large a size of mains, voltages of 1,000 
 2,000, 5,000, 10,000 and up to 80,000 are employed. In laying 
 cables for such high voltages special care has to be taken to have 
 good insulation. The direct connection with a high-tension line, 
 or even through any substance which is not insulated perfectly from 
 the line, may have a fatal effect. . 
 
 At the end of this chapter a table is given, showing the approxi- 
 mate diameters and sectional areas of the wires and cables mostly 
 employed in practice. Their resistance in ohms per 100 yards 
 is also given. By means of this table we can calculate the sectional 
 area of a main, if its length and the current be given, and the 
 voltage drop has not to exceed a certain amount. This problem 
 has to be solved frequently by electrical engineers. 
 
 If the dimensions of all lines are not determined before laying 
 them, then it very often happens that the voltage drop is too large, 
 and the lamps give a poor light. 
 
 Further examples of installation calculations will now be given. 
 
 EXAMPLES. 
 
 1. A group of ten 16-candle-power 110-volt lamps is to be fed by means of a 
 cable whose single length is 100 yards; the voltage drop is not to exceed about 2 
 volts. What wire should be employed? 
 
 The current taken by the 10 lamps is 5 amps. The voltage drop in the line is 
 =CxR. Hence, since the current C=5 amps, and the voltage drop e=2 volts, 
 the resistance R of the line must be f = 0.4o, or less if a smaller voltage drop is 
 taken. The length of the lead and return is 200 yards, thus the resistance per 
 100 yards of the wire to be employed must not exceed 0.4Xi$$=0.2w. 
 
 As we see from our table, a cable of 7/18 S.W.G. has a resistance of 0.185o> per 
 100 yards. This cable is nearest to the one we want, whereas the resistance of 
 the next smaller wire, 7/20, is 0.329&>, and therefore far too high. We shall 
 prefer to employ a cable of 7/18 S.W.G. A glance at the table shows that the 
 maximum current allowed for this cable is 21 amps., giving a considerable margin 
 above the 5 amps, required for the lamps. 
 
 2. A current of 30 amps, at a voltage of 250 is to be conducted as far as 300 
 yards. The maximum voltage drop allowed is 3 per cent, of the total voltage, 
 
 i.e. 7.5 volts. Then the resistance of the line may be -^-=0.25(0. The total 
 
 length of the cable is 2X300=600 yards, the resistance allowable for 100 yards 
 is thus 0.25 X J{$= 0.0416. As we learn from the table, a cable of 19/16 S.W.G. 
 has to be employed in this case. 
 
FUNDAMENTAL PRINCIPLES 47 
 
 3. A current of 30 amps, is to be conducted 50 yards. The voltage drop 
 allowed is 6 volts. Then the resistance allowed for the total length of wire, viz. 
 100 yards, is /^=0.2w. From the table we learn that 100 yards of a cable of 
 7/18 S.W.G. has a resistance of 0.185w only. This cable would therefore be 
 sufficiently thick with regard to the voltage drop. Notwithstanding, we must not 
 use this cable, because the maximum current allowed for it is only 21 amps. We 
 must therefore take the nearest cable for which a current of 30 amps, is allowable, 
 i.e. 19/20 S.W.G. 
 
 Up to now we have considered in our calculations the voltage drop 
 and the power loss only. But another very important point, viz. 
 the heating of the line, must not be neglected. If we allowed for a 
 short, fine wire a loss equal to that in a long, thick wire, then the 
 former would be far more heated than the latter. To avoid exces- 
 sive heating of a line, the current strength of any cable has not to 
 exceed that value which is marked in our table as "Maximum Cur- 
 rent Allowable," and which has been fixed by the Institution of 
 Electrical Engineers. These maximum currents have been selected 
 so that the rise of temperature in the cables will be about 20 Fahr. 
 above the surroundings. 
 
 From a glance at the table, it will be noticed that for a sectional 
 area of 0.0019 sq. inch the maximum current is 4.4 amps., whereas 
 for a sectional area of 0.0198 not a current of 44, but only of 30 amps., 
 is allowed. For 0.19 sq. inch a current of 190 amps, is allowed, 
 instead of 440, as one should expect. One would imagine that a 
 cable of tenfold sectional area could also safely carry the tenfold 
 current; but, as a matter of fact, that is not so. The temperature 
 which a wire attains depends on the rate at which it can radiate the 
 heat produced in it to the surrounding bodies. To explain this fact, 
 let us consider a piece of copper wire, having a sectional area of 
 0.0019 sq. inch, which is situated in the centre of a thick wire with a 
 sectional area of 0.019 sq. inch. Suppose, now, that we send a cur- 
 rent of 44 amps, through the thick wire; then, in the centre-piece of 
 0.0019 sq. inch sectional area obviously an equal quantity of heat 
 will be produced as in the thin wire, having 0.0019 sq. inch cross- 
 sectional area. But the heat produced in the centre of the thick 
 wire cannot be led away as quickly as with the thin wire, because it 
 has to go through the whole thickness before arriving at the surface. 
 Thus a wire of 0.019 sq. inch sectional area carrying 44 amps, would 
 get much hotter than a wire of 0.0019 sq. inch area carrying 4.4 amps. 
 A wire with 0.19 sq. inch area, carrying 440 amps., would get so hot 
 that the insulation would be burnt away after a short time. 
 
 To prevent an excessive load on a wire, and thus its dangerous 
 heating, a fuse, or cut-out, is inserted in the main, the whole of the 
 current therefore flowing through it, but its cross-sectional area is 
 smaller than that of the line wire. It consists of an easily fusible 
 metal, such as lead, tin, or alloys of them, and sometimes of silver 
 and copper. 
 
48 
 
 ELECTRICAL ENGINEERING 
 
 With the normal current, with which the beatings of the mains is 
 hardly appreciable, the fuse, having a sectional area of the right size, 
 should be little more than the temperature of the hand. If the 
 current doubles in strength, then the heating of the fuse wire should 
 be such as to cause it to melt. By this means the main current is 
 broken, and no further heating can occur. 
 
 The double current does not involve any danger for the mains, 
 for, as mentioned above, the heat produced by the maximum allow- 
 able current does not raise the temperature of the wire more than 20 
 Fahr. The heat now produced by the double current will be a four- 
 fold one, the rise of temperature in the wire will thus not exceed 
 80 Fahr. Assuming a room temperature of 85 Fahr., the temper- 
 ature of the main would come to about 165 Fahr. All kinds of 
 insulating materials used for mains can stand this temperature, bufc 
 a higher one would be dangerous. 
 
 Fuses hence furnish an excellent means of preventing dangers 
 arising from electric mains. It is, of course, necessary to design 
 fuses so that they cannot give rise to any dangers themselves by 
 melting. They must be fixed on an incombustible base for instance, 
 marble or slate; and means have to be provided to prevent melted 
 metal from falling on inflammable bodies. Figs. 44 and 45 show 
 
 FIG. 44. Fuse or Cut-out 
 (British Schuckert Co.). 
 
 FIG. 45. Fuse or Cut-out for Large 
 Current (British Schuckert Co.). 
 
 two designs of fuses, or cut-outs, in which the fusible wire is within 
 a porcelain handle, enabling the cut-out to be also used as a switch. 
 In Fig. 44 the handle is intended to be removed directly, but in 
 Fig. 45 it may be hinged back. 
 
FUNDAMENTAL PRINCIPLES 
 
 49 
 
 Fig. 46 shows a form of fuse used extensively in America. It 
 consists of a stout tube of fibre capped at the ends with brass. In 
 this tube is the fuse, made of copper or lead-antimony alloy, packed 
 about solidly with some fire-proof powder like lime or clay, thus 
 excluding all air. The ends of the fuse wire are fastened to the 
 blade terminals of the fuse which project inside for this purpose. 
 When this fuse blows there is no sound whatever and no spark. 
 An ordinary fuse wire when it melts in the open air makes a loud 
 noise and a bad spark, particularly when inserted in a circuit of 
 500 volts. The result is that surrounding parts of apparatus, such 
 
 FIG. 47. 2300-volt Expulsion Fuse-block. 
 
 FIG. 46. Enclosed 
 Fuse. 
 
 FIG. 48. 2360-volt Expulsion-tube 
 Fuse-block. 
 
 as switchboards, terminal blocks, etc., are burned and injured in 
 appearance. In addition, actual injury can occur to individuals if 
 they should happen to be near at the time of the melting. Sparks 
 may fly about also and cause an actual fire. Thus the " enclosed 
 fuse " has a very wide use, and on circuits up to 750 volts and cur- 
 rents up to 400 amperes is a most satisfactory device to use. 
 
 Figs. 47 and 48 show two important types of fuse-block suitable 
 for higher voltages, up to 5000 volts. Here the fuse is in an enclosed 
 chamber as before, but it is not packed with any material. Instead 
 an opening is purposely left to the open air, but so placed that the 
 spark resulting from the interruption of the circuit from the melt- 
 ing of the fuse is directed in a proper and safe direction. The prin- 
 ciple of this fuse-block is that the gases from the melted fuse, being 
 
50 
 
 ELECTRICAL ENGINEERING 
 
 produced suddenly, " snuff out " the arc, the vapors shooting out 
 of the opening at the same time. The tube type is the more effective. 
 For higher currents and voltages, a device called a circuit-breaker is 
 used in America. Fig. 49 shows one of them built for 600 volts 
 and 300 amperes. The current is finally broken at the carbon points 
 shown at the top of the figure. Carbon has the ability of standing, 
 without particular injury, great heat. When the current is broken 
 the flash and the arc resulting are at a great temperature. Copper 
 is badly injured thereby, sometimes melting in drops. The current 
 
 FIG. 49. C. P. Circuit-breaker. 
 
 therefore is carried by the lower contacts in the figure, but the break- 
 ing is done at the carbon contacts, the copper and carbon being in 
 multiple, but the carbon leaving last. A coil as shown in the figure 
 acts as an electro-magnet. If the current gets excessive or above 
 a certain desired point, the magnet pulls a piece of iron called a 
 keeper, placed in front of it, which " trips " the breaker just as a 
 rat-trap is tripped. 
 
 Fig. 50 shows another form of circuit-breaker capable of break- 
 ing 10,000 amperes at 750 volts without being injured in the slightest. 
 The principle upon which this breaker acts is different from the other. 
 Here, in addition to the magnet which trips the breaker when the 
 current gets strong enough, there is another which is short-circuited 
 
FUNDAMENTAL PRINCIPLES 51 
 
 by brushes when the breaker is closed and carrying current. When 
 the breaker trips these brushes leave their contact before the con- 
 
 FIG. 50. K Breaker, Large Current. 
 
 tacts which open the circuit leave. In so doing they therefore 
 throw the current into the magnet which they short-circuited. The 
 magnet is placed so that its field or lines of force pass across the con- 
 tact at which the final break of current occurs. The result is that 
 the arc resulting from the break is " blown out" by the magnetism 
 and directed up a shute as shown in the figure, thus doing no harm.. 
 This blowing out of the arc is based upon Ampere's rule; the cur- 
 rent in the arc, or the arc itself, being deflected up by the magnetism. 
 
 This same principle of blowing out arcs by magnetism is used 
 in controllers, rheostats, etc. 
 
 A properly erected electric plant, which is always kept in order, can 
 hardly ever cause any danger of fire. The fuses do not only prevent 
 a permanent overload of a main and the excessive heating connected 
 with it, but they also act momentarily when a short circuit takes 
 place. If, for example, by any accident the positive and negative 
 wires be connected by a bare metal rod, the resistance of the main 
 becomes so small that a very great current, far exceeding double the 
 normal current, flows through the line, causing the fuses to melt at 
 once, and so preventing dangerous heating. 
 
52 ELECTRICAL ENGINEERING 
 
 It is a different matter with badly installed plants or such as are 
 not kept in order. If, for instance, due to a leakage to earth, 
 the current flowing through the main is greater than that taken 
 by the lamps, then a frequent melting of the fuses will, of course, 
 happen. If, now, the person in charge of the plant or a thoughtless 
 wireman puts thick metal strips instead of those of normal size into 
 the fuses, then they will no longer melt. This is just LS if any 
 one tied down a safety-valve of a boiler so that it could not work 
 when the pressure is excessive. The safety-valve will then no longer 
 be of any use, and the boiler may burst. A similar thing may 
 happen with electric mains when thin fuses are replaced by some 
 of too great sectional area. The danger is especially great when 
 increasing earth-currents are no longer indicated by the melting of 
 cut-outs. Eventually the insulation may become so defective, and 
 the earth-currents so strong, that the mains may themselves 
 actually melt. 
 
 It is not absolutely inadmissible to replace thin fuse-wires by 
 thicker ones. It may happen sometimes that the plant has been 
 but little loaded originally, and therefore thin fuse-wires have been 
 used, whereas the mains would have been able to carry a greater 
 load. If, then, another number of lamps be connected with the 
 mains, the replacement of the thinner fuse-wire by a thicker one is, 
 of course, allowable. But the maximum thickness of the fuse-wire 
 should always be limited by the fuse-current, which is given in the 
 table for the copper wires of different cross-sectional area (see Table 
 on next page). Thus the fuse-current of a wire, employed for a 
 cable of No. 15 S.W.G., for instance, should never exceed 16.4 amps. 
 What current is necessary to melt a particular piece of tin or 
 alloy used as a fuse-wire is best obtained by experiment, and should 
 be noted on a label attached to the bobbin of wire. 
 
 The circuit-breaker being designed to care for large currents is 
 used naturally in power stations and on switchboards where the 
 
 energy is great. They are used in 
 America on motor installations due 
 to the ease of reclosing the circuit 
 if it opens, which is accomplished 
 by merely closing the circuit- 
 breaker handle. Fuses are used usu- 
 ally on house circuit. The most 
 used form for this purpose is shown 
 FIG. 51. Plug Fuse. m Fig. 51, which consists of a 
 
 plug just like a lamp-socket in an 
 
 enclosure of which is located the fuse. Since they cost so little, 
 they are thrown away, plug and all, after blowing, being replaced by 
 a new one; thus no handling of the fuse proper is necessary. 
 
FUNDAMENTAL PRINCIPLES 
 
 53 
 
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54 
 
 ELECTRICAL ENGINEERING 
 
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FUNDAMENTAL PRINCIPLES 
 
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CHAPTER II 
 MAGNETS MAGNETIC LINES OF FORCE 
 
 Influence of a Magnet on an Electric Current 
 Deprez Instruments 
 
 We have learned that a freely movable magnetic needle is 
 deflected by a current flowing through a fixed conductor. If we 
 make the magnet stationary and the conductor 
 movable, we shall find that the latter will 
 move when a current is passed through it. 
 This can be well observed with the instrument 
 shown in Fig. 52. A strong magnetic field is 
 produced by the horseshoe magnet which is 
 provided with soft iron pole-pieces. Within 
 these is a fixed iron cylinder. By the action 
 of the poles of the magnet the cylinder also 
 becomes a magnet, with a north pole, n, and a 
 south pole, s. In the air gap between the 
 magnet poles and the cylinder a coil, consisting 
 of very fine wire, is arranged so as to be easily 
 movable. The ends of this coil are connected 
 by means of very fine spiral springs with the 
 two terminals of the instrument. These 
 springs hold the coil, to which a pointer is 
 attached, at a position of rest, and at the 
 same time give a means of leading a current 
 from the terminals of the instrument to the 
 coil. Hence, if we connect the terminals of the 
 instrument with a source of E.M.F., a current will flow through the 
 windings of the coil. It flows, for instance, in the left part of 'the 
 windings upwards, and in the parts to the right downwards. Now 
 let us consider what deflecting actions will take place between elec- 
 tric current and magnet. Imagine the swimmer of Ampere's Rule 
 in the part of the coil to the left hand of the reader. On this part 
 the north pole of the magnet and the south pole of the cylinder are 
 acting. Let the swimmer face the N pole, which would move towards 
 his left hand if it were not fixed, then the swimmer being himself 
 movable with the coil will be urged in the direction of the arrow 1. 
 The effect of the inner s pole will be the same, because we must now 
 suppose that the swimmer is facing this pole. The pole, being a 
 south one, is pressed towards his right hand, but, not being capable 
 of moving again, the coil must be driven to 1, just as if his right hand 
 were pressing in such a way as to move him along the face of the s 
 pole. Next considering the right-hand part of the coil, the swimmer 
 must proceed with his head turned towards the paper and from the 
 reader; then, exactly as before, we shall find the resultant action is to 
 
 56 
 
 FIG. 52. Deprez 
 Instrument. 
 
MAGNETS MAGNETIC LINES OF FORCE 
 
 57 
 
 drive the coil in the direction of the arrow 2. Thus the forces acting 
 tend to twist the coil round until they are balanced by the effort of 
 the springs to drive the coil in the opposite direction. 
 
 The stronger the current then of course the greater will be the 
 deflection, so that if a pointer be fastened to the coil, arranged so 
 chat it moves over a scale, the strength of the current can be inferred 
 from the amount of the deflection. Hence the new instrument with 
 which we have become acquainted may be used as an ammeter. It 
 is called after the inventor a Deprez instrument. 
 
 Important details of one make of a "moving coil instrument" (as 
 it is often called) are seen in Fig. 53. These instruments have very 
 
 FIG. 53. Construction of Moving Coil Ammeter 
 (Weston Electrical Instrument Co.). 
 
 useful properties. If the current be reversed, then the coil will 
 obviously be deflected in the opposite direction. We can therefore 
 furnish the instrument with a scale having a zero at the middle, and 
 reading both to the right and to the left. Then the pointer gives not 
 only a measure of the strength of the current, but also indicates its 
 direction. If, however, the instrument is furnished with cne scale 
 only, reading, say, from zero to the right, then the current must 
 always be sent through the instrument in a certain direction. The 
 terminals of such an instrument are therefore marked + and , 
 
58 
 
 ELECTRICAL ENGINEERING 
 
 telling us that the leads from the battery must be connected so 
 that the positive and negative poles are respectively at these ter- 
 minals. 
 
 A Deprez instrument may be used, it will be evident, as a pole- 
 finder. If the deflection is along the scale, then we know that the + 
 pole is connected to the + sign on the instrument; if otherwise, then 
 the sign is connected to the + pole. 
 
 A very fine wire being wound on the coil, the instrument can 
 only be used for feeble currents. If a thick wire coil were used, then 
 it might be serviceable for strong currents, but such an instrument 
 would be clumsy and not sufficiently sensitive. It is quite possible 
 to use the fine wire instrument for strong currents in the following 
 way: Suppose that a coil is only able to stand a current of T 1 Q-amp., 
 but that we had to measure a current of 1 amp. Then, if we connect 
 in parallel to the coil of the instrument, a resistance, called a shunt, 
 through which nine parts 
 of the whole current flow, 
 so that only one part passes 
 through the coil, the result 
 will be that the shunt will 
 take T 9 F and the coil T 1 - 
 amp. This ratio can be 
 obtained by making the 
 resistance of the shunt -^ 
 of the resistance of the 
 coil. Thus if the resist- 
 ance of the coil were 1&>, 
 then the shunt must be 
 
 |<w. 
 
 The same instrument 
 may be used to measure a 
 current of 10 amps., if we 
 make the resistance of the 
 shunt = T^W; again, if a 
 current of 100 amps, had 
 to be measured, we should 
 require a shunt of 7 i^, and so on for still greater currents. 
 
 The Deprez instrument is easily adapted as a voltmeter by con- 
 necting a sufficiently large resistance in series with the coil. If we 
 make the same assumption as before, and take as the maximum 
 allowable current through a certain instrument to be T V amp., and its 
 resistance Ico, then it follows that we must not connect the instru- 
 ment terminals with a voltage greater than -^. For measuring 
 higher pressures, say of 1 volt, we must place 9&> in series with the 
 instrument; this will give a total resistance of 10, and a current of 
 ^ amp. To measure 10 volts the total resistance must be 100<w, so 
 
 FIG. 54. Weston Ammeter (Weston Electrical 
 Instrument Co.). 
 
MAGNETS MAGNETIC LINES OF FORCE 
 
 59 
 
 that the extra resistance will have to be 99&>. In the same way a 
 voltage up to 100 will require 999&>, and a voltage up to 1000 will 
 require 9999& to be added. 
 
 The same methods are applied to hot wire and other instruments 
 in order to measure large voltages and currents. 
 
 In the case of voltages and currents that are not very great, the 
 shunt or resistance is placed within the instrument. 
 
 For technical purposes the instrument is graduated so that the 
 pointer directly shows the current or voltage of the circuit; thus in 
 Fig. 54 an illustration of an ammeter is given capable of measuring 
 up to 400 amps. The shunt for such an instrument would be like 
 that of Fig. 55, which, however, is only for 150 amps., and generally 
 
 FIG. 55. Shunt for Ammeter. 
 
 is made of a number of strips of 
 manganin, an alloy which changes 
 its resistance but little with rise 
 of temperature. 
 
 The Weston instruments, used 
 largely in the United States, oper- 
 ate under the same principle as 
 the Deprez. The General Electric 
 "astatic" instruments used on 
 switchboards have the movable 
 feature similar to the Deprez, 
 but instead of using permanent 
 magnets, a magnet produced by 
 an exciting current (i.e., an elec- 
 tro-magnet) is used, the source of 
 excitation being usually a storage 
 battery. This avoids the varia- 
 
 FIG. 56. Astatic instrument for 
 Switchboards. 
 
60 ELECTRICAL ENGINEERING 
 
 tion which may occur in residual or remanent magnetism. A picture 
 of such an instrument is shown in Fig. 56. This instrument has 
 a lamp behind a scale, thus illuminating it. 
 
 Influence of Electric Currents on each other 
 The Electro-dynamometer 
 
 We know that between an electric current and a magnet there is 
 an action capable of causing motion. This follows from the fact 
 that a pole tends to be forced along a "line of force," and currents 
 produce lines of force. For instance, a helix carrying current acts 
 precisely like a magnet, being influenced by magnets as well as by 
 currents. Hence it should be expected that two currents, each pro- 
 ducing lines of force, would act upon each other. To prove this exper- 
 imentally, use a fixed coil, consisting of a number of windings of in- 
 sulated copper wire. At right angles to this fixed coil is a movable 
 one. A current can be passed into the latter by means of wires 
 which dip into two cups containing mercury, as will be seen by exam- 
 ination of Fig. 57. Let currents be passed through the two coils, 
 when it will be found that the outer coil will be deflected, and on 
 reversing one of the currents either in the outer or inner coil the 
 deflection will be reversed. Careful tests with this apparatus will 
 prove that when the direction of the current is the same in the wires, 
 that is to say, both upwards^or^both downwards, then attraction results; but 
 when the direction of the currents is opposed, then repulsion takes place. 
 
 The influence of electric currents on each other is called electro- 
 dynamic action. The word dynamic is derived from the Greek word 
 dynamis, meaning force. 
 
 The arrangement just described may be used for the purpose of 
 measuring current strengths. Instruments of this type are called 
 electro-dynamometers, and usually have a pointer attached to the 
 movable coil. A wattmeter of this type will be described in later pages. 
 
 An interesting and valuable rule results from the action described 
 in Fig. 57. Let it be supposed for a moment that the coil B is 
 stationary and carrying current as shown by the arrows. Let it be 
 supposed that the coil A, carrying current as shown by arrows, can 
 freely move. ' According to the rule just explained, the side C of coil 
 A is attracted to side D t^coil B; or, in other words, the coils tend 
 to lie in the same plane. 'Consider the lines of force created by coil 
 B; they come up out of the paper inside of the coil. If the coil A 
 lies flat with B, it then contains all of the lines of force that it can. 
 In other words, a coil free to move under the action of electro-dynamic 
 force, tends to move so as to include the maximum number of lines of force. 
 
 If the current be reversed in A, the coil would tend to present its 
 
MAGNETS MAGNETIC LINES OF FORCE 
 
 61 
 
 other face to the reader, still following the above rule in so doing. 
 In considering this valuable rule, remember that the lines of force 
 
 D 
 
 J 
 FIG. 57, An Electro-dynamometer. 
 
 in one direction count positive (in the above figure, those coming up 
 out of the paper), and in the other negative. 
 
 Electro-magnets 
 
 Our early experiments have taught us that a piece of soft non- 
 magnetic iron placed within a coil becomes magnetic as soon as a 
 current is sent through the coil. Such an arrangement is called an 
 electro-magnet. The iron may have any shape; it is generally in 
 the form of a bar or horseshoe. 
 
 How the iron acquires its magnetism is not very easily explained, 
 but a comparison will enable us to understand what probably takes 
 place. Suppose that, instead of the iron bar, we had a tube filled 
 with a great number of exceedingly small magnetic needles. On 
 shaking the tube the needles will so set themselves that the tube shows 
 no or nearly no apparent magnetism. Place this tube within a coil 
 through which a current is passed. A directing action will now be 
 exerted on each of the magnetic needles, and they will attempt to 
 turn in a certain direction. But the magnetic needles not being freely 
 movable, hence offer a certain resistance to their rotation. If the 
 current, and therefore the directing force exerted on the needles, is 
 but small, then the resistance will prevent the needles from entirely 
 following the directing force. 
 
62 ELECTRICAL ENGINEERING 
 
 A certain amount of rotation of, at least, the easier movable 
 needles will, however, take place. The poles of the needles will no 
 longer be arranged in a confused manner, but their north poles are 
 directed more or less towards one end, say to the right, the south 
 poles more or less towards the left. The tubs will now be magnetic. 
 If we strengthen the current flowing through the coil, its directing 
 force on the needles becomes a greater one, and with a very large 
 current the directing force may overcome the resistance to motion 
 entirely, and all the needles group themselves in the direction of the 
 lines of magnetic force of the solenoid. The tube now shows very- 
 strong magnetism. If the current flows around the coil, as shown in 
 Fig. 20, a north pole will be formed to the right, and a south pole to 
 the left of the tube. If Fig. 20 be considered attentively, we are able 
 to deduce the following rule: 
 
 At that end at which (looking towards this end) the current flows 
 in a direction which is counter-clockwise round the coil there is a 
 north pole; and at that end at which (looking towards this end) the 
 current flows clockwise round a coil a south pole is formed. 
 
 From many other phenomena, not only of a magnetic nature, 
 which could not have been otherwise explained, it has been con- 
 cluded that the smallest parts of which a body consists are not 
 joined together rigidly, but possess a certain mobility. These 
 smallest parts, which cannot be made smaller by mechanical means, 
 are called molecules. 
 
 We have now to imagine that every molecule of the iron is a 
 diminutive magnet. If these, which we may call molecular magnets, 
 lie in confusion, then the iron bar will be like that within the tube 
 of the previous page, and have no apparent magnetism; but if we 
 exert a directive action on the iron, by, for example, bringing near 
 to it the north pole of a strong magnet, then all the south poles of the 
 molecular magnets will turn towards the strong north pole and the 
 north poles in the opposite direction, and the iron will now be 
 magnetized. If the cause of the directing force be removed, then the 
 molecular magnets return to their original position, either partially or 
 entirely. 
 
 We say partially or entirely, for if the molecular magnets are 
 difficult to move, a great force will be necessary to deflect them from 
 the position of rest. If, then, the deflecting force ceases, the particles 
 will not return to their original position because the resistance which 
 opposed the first motion will also resist any retrograde action. Iron 
 of this kind will show magnetic properties, even after the magnetizing 
 force ceases. Such magnetism is called residual magnetism or 
 remanence. 
 
 Iron having molecules that are easy to turn will be easy t3 
 magnetize., and will readily return to the non-magnetic condition. 
 
 Iron shows a different degree of remanence, according to its 
 
MAGNETS MAGNETIC LINES OF FORCE 63 
 
 hardness; hence it is rather difficult to magnetize hard steel, but its 
 residual magnetism is of a large amount. 
 
 Very soft, especially annealed wrought iron can be magnetized 
 very easily and strongly, and in a far higher degree than steel; its 
 remanence is, on the other hand, small, and very much less than that 
 of hard steel. 
 
 With steel we generally do not speak about a residual, but rather 
 of a permanent magnetism. 
 
 It follows from the molecular theory of magnetism that, if we 
 break a magnet into two parts, we cannot have one half containing 
 north, and the other half containing south magnetism only. Even 
 if we divide a magnet into exceedingly small parts, each of these will 
 have both a north and a south pole. 
 
 A field of magnetic force exists both in the space outside a 
 magnet and also in its interior; the lines are supposed to pass from 
 a north pole to a south pole in the external field, and then to travel 
 through the magnet from south to north pole, forming what is called 
 a magnetic circuit. This magnetic circuit is very analogous to the 
 electric one. 
 
 In our future discussions about magnets electro-magnets will be 
 chiefly considered, as these are of much greater technical interest 
 than permanent ones. 
 
 The exciting power of magnetism or magneto-motive force is rep- 
 resented by the effect of the current flowing through the solenoid. As 
 w r e are aware, this effect depends merely on the strength of the current 
 and the number of turns. Hence the number of ampere-turns is a 
 measure of the magneto-motive force. The greater the latter is the 
 larger is the number of lines of force, and the stronger the magnetic 
 flux. But this flux depends not only on the exciting force, but also 
 on the resistance which is opposed to the passage of the lines of 
 force. This resistance is quite analogous to the electric resistance, 
 and depends on the length of the path, the cross-section, and the kind 
 of material. 
 
 Iron offers a low resistance to the lines, whereas air and all non- 
 magnetic materials have a far higher resistance. Hence, if we wish 
 a strong magnetic flux, we must make the path through the bad 
 conductor (generally air) as short, and its sectional area as great, as 
 possible. 
 
 We shall now be able to understand why a horseshoe magnet 
 exerts a far stronger force than a bar magnet of equal strength of pole. 
 If we bring the piece of iron called the keeper near a horseshoe 
 magnet (see Fig. 58), the lines of force pass only a short distance 
 through the air. The greater part of their path is through iron, 
 either that of the magnet or of its keeper. Since the path through 
 the keeper has a lower magnetic resistance than that through the air, 
 nearly all the lines will go through the keeper, and only a com- 
 
64 
 
 ELECTRICAL ENGINEERING 
 
 paratively small number of them will take other paths through the 
 air. These last-mentioned lines are called stray lines. They are of 
 no utility, for only those lines which reach the keeper can cause 
 attraction. 
 
 If now we bring the same keeper, as in the previous example, to 
 the pole of a bar magnet, then we observe that all the lines have an 
 
 FIG. 58. Horseshoe Electro- 
 magnet 
 
 FIG. 59. Straight Electro- 
 magnet. 
 
 air path (see Fig. 59), and very few pass through the keeper, so that 
 the useful lines are very few. 
 
 Owing to the long air path the magnetic circuit here has a very 
 high resistance, and for the same magneto-motive force the number 
 of lines will be far less than in the case of the horseshoe magnet. 
 The flux into the keeper being small, the pull upon it by the magnet 
 will be correspondingly small. 
 
 The law connecting the flux with the exciting power and resistance 
 is known as Ohm's Law for magnetism. There is, however, a very 
 important difference between this law and the Ohm's Law for the 
 
INDUCTION 
 
 65 
 
 electric circuit. In the case of the electrical current a double E.M.F. 
 will cause a double current, and a pressure one hundred-fold will 
 give a current one hundred times as great through any constant 
 resistance. With the magnetic circuit this is by no means the case. 
 As we understood from the discussion about magnets, there is a 
 defined limit above which the magnetism cannot be further increased. 
 This maximum is reached when all the molecular magnets are pulled 
 into a straight line. Hence, as we approach this condition of satu- 
 ration any increase of magneto-motive force is practically useless. 
 Further, the magnetic resistance of such saturated iron is very 
 great. 
 
 Nevertheless, up to a certain point the magnetic is like the 
 electric circuit. A great increase of the electric pressure produces 
 a very strong current, which heats the wire and causes it to have a 
 higher resistance than before, preventing therefore the current from 
 becoming so great as it would be if the wire had been kept at the 
 original temperature. Compared with the corresponding increase 
 of the magnetic resistance this change of electric resistance is small. 
 
 Induction 
 
 We have learnt that an electric current flowing through a con- 
 ductor in a magnetic field is capable of producing motion of the con- 
 ductor or of the magnet. From the law of production of electro-motive 
 force by the cutting of lines of force, a volt being produced by the cutting 
 of 100,000,000 lines of force per second, 
 it follows that in a conductor which is 
 made to move in a magnetic field 
 an electric current is produced. 
 
 To prove this, let the following 
 experiment be tried: In front of the 
 poles of a horseshoe magnet move a 
 copper rod, which has its ends con- 
 nected by flexible wires to a sensitive 
 Deprez ammeter, as shown in Fig. 60. 
 If we move the copper rod in any 
 direction, say from left to right above 
 the north pole, the ammeter will show 
 a sudden deflection. If we move the 
 rod in the same way above the south 
 pole / the deflection will be in an 
 opposite direction. If now the rod be moved from right to left 
 the deflections will be opposite to the corresponding ones of the first 
 direction of motion for each pole. As the result of the motion, we 
 therefore produce an E.M.F. , whose direction depends both on the way 
 that the lines of force proceed through the conductor, and on the way 
 
 FIG. 60. A Conductor in a 
 Magnetic Field. 
 
C6 ELECTRICAL ENGINEERING 
 
 the conductor is moved. On stopping the movement of the conductor 
 the needle of the ammeter immediately comes to rest, proving that 
 motion is essential for the maintenance of the generated electrical 
 pressure. We shall further find that it is a matter of indifference 
 whether we move the conductor or the field. 
 
 If there is no closed circuit, a current cannot of course be pro- 
 duced, but an E.M.F. will exist immediately the conductor moves; 
 just as, hi the case of a galvanic cell, an E.M.F. is present even if 
 the poles are not connected. 
 
 It is of practical importance to determine in all cases the direction 
 of the current in the moving rod. 
 
 It will have been remarked that, in Nature, whenever a motion 
 takes place there exists some resistance which tries to bring the 
 moving body to rest. To overcome this resistance work has to be 
 done. The ground, for example, offers a resistance to the movement 
 of a vehicle. Such resistance is known as friction. If the moving 
 
 force be withdrawn, friction will 
 gradually cause the vehicle to 
 stop. It is exactly as if there 
 existed a force which acts in a 
 direction opposite to that of the 
 
 FIG. 61. -Action of Moving Boat. mo tion. If a boat is moved 
 
 on water (see Fig. 61), the 
 
 water is raised in front of the boat, which will try to drive the boat 
 in the opposite direction, and will really do so as soon as the moving 
 force ceases. 
 
 It is precisely the same with the moving conductor. As it is 
 made to travel in the magnetic field a current is 
 produced in such a direction as to oppose the 
 motion of the conductor. Ampere's Law will help 
 us here. An experiment (Fig. 62) will show that if 
 the conductor be moved to the right a current 
 will be produced so as to flow towards the spec- 
 tator (this direction is indicated by a dot in the 
 diagrams. Such being the direction of the current 
 FIG. 62. - the swimmer in the current may be thought of as 
 Direction pushing along the face of the fixed N pole with his 
 left hanc *' anc * hence he tends to drive the conductor 
 towards his right hand in the direction of the feathered 
 arrow. 
 
 To overcome this backward force of the current we must do 
 work to move the conductor .in the intended direction. The larger 
 the produced current and we may alter this according to the re- 
 sistance connected with the outer circuit the larger will be the 
 retarding force, and thus the greater must be the work which we 
 have to exercise to move the conductor. Hence it follows that we 
 
ELECTRICAL MACHINES 
 
 FIG. 63. The Hand 
 Rule. 
 
 do not get the current for nothing, but we must employ a certain 
 amount of mechanical effort. We therefore only transform mechan- 
 ical into electrical energy. 
 
 A rule will now be given by the aid of which it is possible to 
 determine the direction of the produced, or. as it is called, the induced 
 current in a much simpler way than employing 
 Ampere's Law each time. 
 
 Hold the palm of the right hand against the 
 lines of force, the thumb in the direction of the 
 motion, then the fingers point out the direction of 
 the induced current (see Fig. 63). 
 
 In the case of the example of Fig. 62 the 
 palm of the hand would have to be turned 
 downwards, against the north pole, since the 
 lines of force proceed upwards. On then 
 holding the thumb to the right, the fingers 
 point towards the spectator, indicating the 
 direction of the current as proved by ex- 
 periment. A very useful rule to bear in mind as to direction of 
 induced currents, of course dependent upon the same principles laid 
 down, ^s looking at an electric circuit in the direction of the lines of 
 force (i.e., in the direction a free north pole would tend to go), if the 
 lines of force are increasing, a current tends to flow in a counter-clock- 
 wise direction. If decreasing, in a clockwise direction. 
 
 There is another and important case of induction that mu^t 
 be studied. If we wind a wire round a core of soft iron, and 
 connect its ends with the terminals of a Deprez ammeter, we can 
 observe a deflection on the instrument immediately we approach a 
 magnet pole to the core. When the magnet comes to rest the 
 deflection at once ceases. If we take away the magnet from the 
 core a deflection is produced in the opposite direction. 
 
 The same effects can be observed by winding another coil on the 
 core which is connected with some source of E.M.F. As long as 
 the current in the new coil remains constant that is to say, as long 
 as the flux does not alter we cannot observe any current. But as 
 soon as we strengthen or weaken, start or stop the magnetizing 
 current we get a deflection of the ammeter which is greater the 
 greater is the variation of the magnetizing current, and the more 
 rapidly the alteration is caused. 
 
 Thus an E.M.F. is always induced in a winding surrounding 
 an iron core if the magnetism of the core is either strengthened or 
 weakened. 
 
 This law that the induced current is in such a direction that it 
 tends to stop the motion, as described in connection with Fig. 62, 
 is covered by what is now generally known as Lenz's law, which is. 
 that "in all cases of magnetic induction the induced currents are in. 
 
68 
 
 ELECTRICAL ENGINEERING 
 
 such a direction that their reaction tends to stop the motion that produced 
 them." 
 
 Electrical Machines 
 
 If we could by any special device move a conductor repeatedly 
 backwards and forwards in front of a pole of a magnet, we should 
 obtain a current which would change its direction with each alteration 
 of the direction of motion. This would be the simplest form of an 
 electrical machine serving for the transformation of mechanical into 
 electrical energy. 
 
 A motion backwards or forwards, or up and down, is called a 
 reciprocating motion, and is generally avoided from a mechanical 
 point of view. A rotating motion is much 
 more preferable and, as it is easy to construct 
 an electrical generator or dynamo with con- 
 ductors which rotate, this is the usual method 
 of construction. 
 
 In Fig. 64 is shown a horseshoe magnet, 
 which is similar to that of a Deprez instrument, 
 and is provided with pole-shoes of soft iron 
 having a circular bore. To give the lines of 
 magnetic force a very short air path, in the 
 Deprez instruments a fixed iron cylinder is 
 placed inside the circular space. Within the 
 air gap the conductors are movable. The same 
 device would serve as a dynamo too, but the 
 rotation of a fine wire coil in so small a space 
 is not a very practical construction, and would 
 be far too fragile for a dynamo. A more satis- 
 factory way is to fix the wire to the iron cylinder, 
 and make them revolve together. To this part 
 the name armature is given. 
 One method of making an armature is shown in Fig. 64, which is 
 called, after its inventor, a Siemens armature, or, after its method of 
 construction, a shuttle or H-shaped armature. It will be seen that 
 the iron cylinder has two slots in which the wire is wound. 
 
 The effect of the winding in cutting through the lines of force is 
 the same as in the case of the Deprez construction; for, the iron 
 armature is not a permanent magnet, but serves only for transmitting 
 the lines of force from the north to the south pole. Whether the 
 armature is rotating or not, the lines of force always keep in the 
 
 N 
 
 FIG. 64. A Magneto- 
 Generator. 
 
ELECTRICAL MACHINES. 
 
 same direction. They do not rotate together with the armature, but 
 always flow in a horizontal direction from the north to the south pole. 
 The direction of the current produced in the windings we may 
 easily determine by means of the various rules presented. Take, for 
 instance, the hand rule. Let us assume the rotation of the armature 
 to be clockwise. If we consider now those wires which pass the north 
 pole at this moment, then we have to hold the palm towards the 
 north pole (i.e. towards the left) and the thumb in the direction of 
 motion, or upwards. The fingers are then directed behind the plane 
 of the drawing. Hence in all wires to the left a current will be pro- 
 duced which flows from the spectator. (Marked, in Fig. 65, by 
 crosses.) We have now to consider the wires to the right, which are 
 near the south pole. The thumb in this case must be held down- 
 wards, because the armature with the wires moves downwards on 
 this side also, and the palm must be turned towards the north pole 
 as before. The fingers point towards the spectator, this direction of 
 the current being indicated, in Fig. 65, by dots within the circles 
 representing the wires. From the same diagram, which 
 shows also a plan of the windings, we learn that all 
 the induced E.M.F.'s add themseives. If, for instance, 
 there be 6 windings or 12 conductors on the armature, 
 the total E.M.F. produced in the latter will be 12 times 
 that induced in a single wire. If we wind 1000 turns 
 of a very fine wire on the armature, we may therefore 
 get a considerable voltage, especially if there is any 
 arrangement to make the armature rotate very quickly. 
 This may, for instance, be accomplished by a suitable 
 toothed wheel gearing. 
 
 Consider the production of electro-motive force by 
 the rule that looking along the lines of force if the flux 
 in the circuit is increasing, the electro-motive force is 
 induced which tends to produce a current in a counter- 
 clockwise direction. We must, in Fig. 64, look from 
 the north to the south pole, for that is the direction 
 of the lines of force. In the position shown in the 
 figure the armature coil is containing no lines of force, 
 being edgewise to them. As it turns clockwise it 
 commences to take lines; hence the lines are increasing, and a cur- 
 rent tends to flow counter-clockwise, or from the spectator on the 
 left and toward on the right. 
 
 To connect the armature with the outer circuit, we fix each of the 
 ends of the coil to a copper or brass slip-ring (see Fig. 66). 
 
 On these rings, metal springs or brushes press which may be 
 connected with the outer circuit. For it is, of course, impossible to 
 connect the wires of the outer circuit directly with the ends of the 
 coil and yet permit free rotation. 
 
 FIG. 65. The 
 Siemens Ar- 
 mature. 
 
70 
 
 ELECTRICAL ENGINEERING 
 
 After the armature has made a quarter of a revolution we observe 
 that the wires are neither within the influence of the north nor of the 
 south pole. They are exactly as far from the 
 north as from the south pole. Therefore at 
 this moment no E.M.F. at all is induced in 
 them. When the armature passes from this 
 position, the wires, which before have been 
 embraced by the north pole, come now to 
 the south pole, and viceversd, thus the direction 
 of the induced E.M.F. is altered. The current, 
 flowing through the outer connection, hence 
 alters its direction at each half revolution of 
 FIG. .66. Slip-Rings. the armature. 
 
 At the position of the armature shown in Fig. 64, the current has 
 its maximum value, for the lines of force are here being cut at 
 the maximum rate. Then it decreases gradually, and becomes nil 
 after a quarter revolution of the armature, and then gradually grows 
 to a maximum (but in a reversed direction), becomes again nil, and so 
 on. We shall be able to understand these changes better by drawing 
 a wavy line, such as shown in Fig. 67. A point, moving on this 
 wave line, has at a defined time its highest position, marked in the 
 figure by a; its height decreases then gradually, and becomes zero 
 at 6. Then the point descent^ beneath the horizontal line, till it 
 reaches its lowest position at c, which is exactly as far under the 
 
 FIG. 67. Alternating-Current Curve. 
 
 horizontal line as the point a is over the horizontal line. The point 
 now ascends until it reaches at d the horizontal line, and continues 
 to rise until at e its highest position is reached, which is equal to ths t 
 of a. From here the previous changes are repeated. 
 
 The current thus generated is quite different to that taken from 
 a galvanic cell, which is constant in strength and direction as long 
 
ELECTRICAL MACHINES 
 
 71 
 
 FIG. 68. Simple Commutator. 
 
 as we do not alter the resistance of the circuit or the connection of 
 the poles, and is therefore called a constant current. The current, 
 on the other hand, taken from the armature just described, is called 
 an alternating current, and each up-and-down change of the current, 
 as from b through c to d (Fig. 67), is called an alternation. If the 
 
 armature of such a two-pole or bipolar 
 dynamo makes 1000 revolutions per 
 minute, then the number of alternations 
 in the same time is 2000. 
 
 For certain purposes the employ- 
 ment of alternating currents is of great 
 advantage. It is, however, very often 
 desirable to obtain a rectified or con- 
 tinuous current. If the armature be 
 rotated very slowly, such a current may 
 be obtained by changing the wires going 
 to the slip-rings after each half revolu- 
 tion, at the moment the current is 
 reversing its direction. Changing of 
 the wires by hand is naturally im- 
 possible at the usual speed of rotation. A " commutator" enables 
 this difficulty to be readily overcome. It consists (see Fig. 68) of 
 two half rings, which are insulated from each other. One of these 
 half rings is connected with the beginning, the other with the end of 
 the armature coil. The brushes are opposite each other, one on the 
 highest, the other on the lowest point of the split ring. With the 
 brushes 1 and 2 the outer circuit is connected. The position of the 
 commutator shown in Fig. 68 corresponds with the armature position 
 of Fig. 64. Let the commutator revolve in the 
 direction of the arrow. Then Fig. 69 will show 
 its position a quarter of a revolution afterwards, 
 and, until reaching this position, an E.M.F. will 
 have been induced in a certain direction say, 
 so as to send a current from brush 1 to brush 2. 
 At the moment that the position of Fig. 69 is 
 reached the armature is short-circuited by. each 
 brush. This will be of no great disadvantage 
 because, as we know, at this position no E.M.F. 
 is induced. As the rotation of the commutator 
 proceeds, the half shown black in the diagrams 
 will come in contact with brush 2, and the white 
 half will touch brush 1. Now, it must be 
 remembered that the electrical pressure will 
 be in the reversed direction; but, at the same 
 time, the connections with the outer circuit have been changed, so 
 
 FIG. 69. Second 
 Position of Com- 
 mutator. 
 
72 
 
 ELECTRICAL ENGINEERING 
 
 that the current (as shown in Fig. 70) will again proceed from brush 
 
 1 to 2. No change in direction of the 
 
 induced current will follow until the 
 
 commutator from the position Fig. 69 
 
 has turned through half a revolution. 
 
 Reversal of the current, and the change 
 
 of brushes to rectify it, then takes 
 
 place. This is repeated at all subsequent 
 
 half turns. 
 
 The kind of current so produced is 
 not really a constant current, such as 
 can be obtained from a galvanic cell, 
 but it rises to a maximum and falls to 
 nothing repeatedly. The current is rep- 
 resented by the curve of Fig. 71, and 
 consists of half waves all directed up- 
 wards. The peak of each wave corresponds to an armature position 
 as shown in Fig. 64, and the zero positions show the absence of E.M.F. 
 at a quarter turn later. 
 
 The dynamo we have described is generally employed for the 
 generation of very small currents. As mentioned above, we can 
 produce in the small armature a comparatively great E.M.F., by 
 employing very many fine windings. Naturally, we obtain from 
 this armature only a very small current on account of the fine wire 
 used on the armature. 
 
 Tt is sometimes very useful to get a pressure of 100 volts from 
 
 FIG. 70. Third Position of 
 Commutator. 
 
 \A 
 
 FIG. 71. Rectified Current. 
 
 such a small portable machine. If a galvanic battery were used 100 
 cells would be required. This would be more bulky than a small 
 dynamo, and have the further inconvenience of requiring recharging 
 from time to time. 
 
 This simple form of a dynamo is therefore used as current gener- 
 .ator for certain tests, such as that of insulation. If, we require, for 
 instance, to examine if a line is well insulated from earth, we should 
 
ELECTRICAL MACHINES 
 
 73 
 
 connect one terminal of the dynamo with earth, and from the sec- 
 ond terminal lead a wire 
 to a galvanometer an in- 
 strument similar to an 
 ammeter. From the sec- 
 ond galvanometer termi- 
 nal a wire is led to the line 
 to be tested. Then the 
 armature is turned quick- 
 ly. If the line is well in- 
 sulated from earth, then 
 although the machine 
 produces an E.M.F., no 
 current results, and the 
 pointer of the galvanom- 
 eter remains in its posi- 
 tion of rest. If, on the 
 other hand, the insulation 
 is defective, the E.M.F. of 
 100 volts, produced in the 
 armature, will be able to 
 
 send a current through the circuit, and the pointer of the galvanome- 
 ter will be deflected. 
 
 We can, further, note from the force which has to be exerted for 
 turning the armature whether the machine is supplying any current 
 or not. In the former case it is rather difficult to turn the armature, 
 because electric power is produced in the machine. In the latter 
 case the turning is much easier, for, as there is no current, no electric 
 power can exist, although we have E.M.F. present. Thus, only such 
 power must be exerted as may be required to overcome the friction 
 of the armature and the gearing. 
 
 This machine, which is generally called a magneto, is also used 
 for ringing the kind of electric bells that are often used in connection 
 with telephones. A complete machine is shown in Fig. 72. 
 
 FIG. 72. Magneto-Electric Machine 
 (Berliner Telephone Manufacturers' Co.). 
 
CHAPTER m 
 
 THE CONTINUOUS CURRENT DYNAMO 
 
 The Ring Armature 
 
 WITH large dynamos, such as are employed for electric lighting or 
 power transmission, the Siemens armature is not used. In these 
 cases the ring armature invented by Gramme is sometimes employed, 
 particularly for arc lighting. 
 
 4- 
 
 FIG. 73. Ring Armature. 
 
 This will now be described. It is shown in Fig. 73, and it will be 
 seen that it consists of a ring-shaped iron core, which is not cast or 
 forged in one piece, but built up from a great number of thin sheets 
 
 74 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 75 
 
 of soft iron. Over the ring an insulated wire is wound in many turns. 
 The ends of the windings are soldered together, so that the whole 
 armature winding forms a circuit closed on itself. This is called a 
 closed-coil armature in opposition to the open-coil type, which is, 
 for instance, represented by the simple Siemens armature. If, now, 
 the armature rotates between the poles N and S, an E.M.F. will be 
 produced in each wire. We have to examine how the different 
 E.M.F.'s produced in the wires behave towards each other. 
 
 We must, first of all, be clear about the course of the lines of 
 force. To the latter, coming from the north, and going to the south 
 pole a way is offered through the armature. They can either make 
 a bend, and go through both halves of the iron core, or they can 
 
 FIG. 74. Lines of Force through Ring Armature. 
 
 go the shortest way, directly through the interior of the iron core, to 
 the south pole. The first way offers much the lower resistance, 
 because the path is only through iron having a small magnetic 
 resistance. Hence, most of the lines of force will pass round the iron, 
 and a small number only will go through the inside of the ring (see 
 Fig. 74). 
 
 We have next to carefully distinguish between the outer and the 
 inner wires of the armature winding. The outer wires cross the total 
 number of lines of force in the air gap in passing the north or south 
 pole. The result is that in the outer wires a considerable E.M.F. is 
 induced, the direction of which we can determine if the direction of 
 rotation is given. In all the outer wires passing the north pole an 
 E.M.F. in one direction, in the wires passing the south pole an 
 
76 ELECTRICAL ENGINEERING 
 
 E.M.F. in the opposite direction, is induced. From the inner wires 
 and the lateral parts of the windings very little E.M.F. is obtained, 
 because very few lines of force cross them. Hence the really effec- 
 tive portion of each winding is the outer wire, the other parts of the 
 winding serving for connecting each wire with the next one. By 
 means of these connections the pressures produced by the outer wires 
 within the embrace of the poles are placed in series. The wires in 
 the space between the upper tips and the lower tips of the poles, 
 called the neutral zone, are ineffective, and serve as connecting wires 
 only. 
 
 When this ring armature rotates no current will circulate through 
 its wires, for the E.M.F. produced by one pole is equal and opposite 
 to that produced by the other pole. Hence we have exactly the same 
 case as in Fig. 33, where we had two cells in parallel without any- 
 external connection, so that the cells were in opposition. Immedi- 
 ately we provide an outer path the two pressures combine, and send 
 a current in the same direction to feed the outside circuit, as shown 
 in Fig. 34. 
 
 With the ring armature we can get connection with an outer 
 circuit by removing the insulation from the portions of the wires 
 lying on the outside surface of the ring, and fixing brushes in the 
 neutral zone, which rub on the bared wire. On joining the + 
 and brushes (see Fig. 73) to lamps, etc., the right- and left-hand 
 windings now work in conjunction, and each supplies half the current 
 passing out from the brushes. 
 
 It is not very usual to collect the E.M.F. by baring the external 
 wires. It is much more usual to have a special part, the commutator. 
 
 FIG. 75. Ring Armature (British Schuckert Co.). 
 
 For this purpose every turn (or every second, third, or other turn, 
 according to circumstances) is connected by a soldered wire with a 
 bar or segment of hard copper. The single segments are of wedge 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 77 
 
 shape, and are insulated from each other by means of thin sheet-mica. 
 The segments are then fixed on a metal cylinder, from which they are 
 insulated. Along the surface of the segments brushes are so fixed 
 that between them and the segments there is not much friction. 
 The effect is just the same as if the wires were made to slide 
 on the brushes. Fig. 75 is an illustration of a ring armature 
 
 ZZL 
 
 FIG. 76. Section of Ring Armature and Commutator. 
 
 with commutator, and Fig. 76 gives a cross-section of such an 
 armature. 
 
 We have still to explain why the armature is not made from 
 solid iron, but is built up from a number of thin iron discs, and 
 provided with insulation, so as to 
 separate the iron discs from each 
 other. If we let a solid iron ring 
 rotate very quickly in a magnetic field, 
 it will, after a short time, become 
 exceedingly hot. This is explained by 
 the fact that the ring crosses magnetic 
 lines of force, hence inducing electro- 
 motive forces, which produce currents. 
 
 Let us now consider what would 
 happen, during the rotation in a mag- 
 netic field, in a solid piece of iron 
 cut from the armature along its axis. This piece would have a 
 shape similar to that of a commutator segment (see Fig. 77). 
 
 This armature sector would not cross the lines of force symmetri- 
 cally. The uppermost part crosses very many, the lowermost part 
 hardly any, and the intermediate parts always a less and less number 
 of lines of force, according to their distance from the surface. If 
 
 FIG. 77. Eddy Currents in Iron. 
 
78 ELECTRICAL ENGINEERING 
 
 we now imagine the sector to be divided into several strips (see 
 Fig. 78), these strips will represent electric conductors, which, on 
 rotation, cross lines of force, so that an E.M.F. 
 is induced in them, which will be greatest in 
 the uppermost strip, smaller in the second, 
 finally smallest in the innermost strip. If we 
 consider, for instance, the uppermost and the 
 7 8-~ Eddy Currents i owermO st strips, then, if the E.M.F. induced 
 in the former be, say, 1 volt, that induced in 
 the latter might be -J volt only. As all these 
 
 conductors form together one piece, they are connected electrically 
 with each other. Against the large E.M.F. of the uppermost strip 
 a small E.M.F. only of the lowermost strip will act. Thus in the 
 closed circuit a current will flow which is produced by the difference 
 of the two electro-motive forces. This difference is f volt in our 
 example. This E.M.F. can produce in a solid iron bar, which has a 
 very low electrical resistance, exceedingly strong currents. These are 
 transformed into heat, and may make a solid iron bar red hot after 
 a short time. 
 
 Hence the generation of strong eddy currents is prevented by 
 building up the iron core of thin sheet-iron discs and very thin 
 paper alternately (marked, in Fig. 76, by vertically hatching). The 
 single discs are insulated from each other by paper layers or insulation 
 painted upon the sheets of iron themselves. An E.M.F. is now, 
 of course, induced in each of the iron sheets. But if we, for instance, 
 assume that the number of discs required for building up the arma- 
 ture be 200, then the E.M.F. produced in the uppermost part of 
 each of the discs will be only the 200th part of that produced in 
 the full armature length, i.e. 3^ volt. Further, this far smaller E.M.F. 
 has to flow along a way, which offers to it a very high resistance. To 
 come to the innermost part of the armature disc, the current has to 
 flow through the very thin iron. As the resistance of the latter is 
 a very considerable one, the eddy currents will be far smaller than 
 those occurring with the solid iron armature. As a matter of fact, 
 the temperature rise of a well-designed armature over that of sur- 
 rounding air does not exceed 70 to 90 Fahr. This heating, however, 
 is to a considerable extent produced by the current in the armature 
 conductors; the eddy currents themselves alone cause a much less 
 rise of temperature. 
 
 The method of building up the armature out of single sheets of 
 iron is now generally followed. A slight difference in the construction 
 happens, inasmuch as, in some cases, the single discs are insulated, 
 not by thin paper, but by a coat of varnish or other special com- 
 pounds. The ring armature is particularly suited for high potentials, 
 .since wires having much difference of potential are not brought near 
 together. Thus such windings are used for arc dynamos, when the 
 voltage at the brushes may be as high as 6000 volts. It is well to 
 have the number of coils a multiple of the poles, so that the E.M.F. 
 between brushes will balance on both sides. 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 79 
 
 Drum Armature 
 
 The interior part of each winding of a ring armature is useless 
 for the generation of E.M.F., but is necessary for connecting every 
 conductor with the next one so that the E.M.F.'s do not act against 
 each other, but in the same direction. This series-connection of 
 the electro-motive forces may be obtained in another way, viz. by 
 connecting opposite conductors through a wire which is laid over 
 the surface of the armature, as with the Siemens armature. If we 
 consider Fig. 79, we shall see that 4 of the 12 armature wires are 
 
 11 10 
 
 FIG. 79. Drum Armature Connections. 
 
 situated within the reach of the north pole (viz. Nos. 12, 1, 2, and 3), 
 4 other wires are situated within the reach of the south pole (6, 7, 8, 
 and 9), and the remaining 4 wires in the neutral zone. Thus if in 
 the wires under the north pole an E.M.F. is produced which may 
 be directed from the spectator, in the wires under the south pole 
 an E.M.F is produced which is directed towards the spectator; 
 whereas in the wires 4, 5, 11, and 10 no E.M.F. at all is produced. 
 It is now clear that we get a proper series-arrangement of the 
 electro-motive forces if we connect the front end of wire 1 with the 
 front end of any of the wires 6, 7, 8, or 9. It would be the nearest 
 to connect wire 1 with the exactly opposite wire, 7. But this would 
 not give a proper, continuous armature winding; for, if we connect 
 
80 ELECTRICAL ENGINEERING 
 
 the back end of wire 7 with the opposite wire, we come back again 
 to wire 1. To get a continuous armature winding, we have, thus 
 to select as the pitch a number which is not exactly equal to half 
 the number of the wires. To come from wire 1 to wire 7, we have 
 to make six steps: the pitch is, therefore, said to be 6. Let us now 
 select as the pitch the next smaller number, viz. 5. We have then 
 to connect on the front wire 1 with 6. On the back of the armature 
 we have to connect 6 with 11. The connections at the front of the 
 armature are indicated in Fig. 79 by full, twice bent lines; those at 
 the back of the armature, by dotted, straight lines. In proceeding 
 with the connections, we come to a front connection from 11 to 4, 
 then at the back from 4 to 9, front from 9 to 2, back 2 to 7, front 
 
 7 to 12, back 12 to 5, front 5 to 10, back 10 to 3, front 3 to 8, back 
 
 8 to 1 that is, back to the point of departure. The closed circuit, 
 which is formed in such a way, comprises, thus, all the wires of the 
 armature. In the middle of each of the front connections a joint is 
 made with one commutator-bar. 
 
 This armature acts exactly like a ring-armature. Let us assume 
 the brushes to lie on the commutator-bars, which correspond to the 
 wires 4 and 11, and 5 and 10 respectively, and let us then follow the 
 course of the current. There are two ways going from the left-hand 
 brush one to wire 4, the other one to wire 11. On the first way we 
 come from 4 through the back connection to the wire 9, in which 
 an E.M.F. directed towards the spectator is induced (indicated by a 
 dot). If We proceed in this direction, we come through the front 
 connection to wire 2, in which an E.M.F. , directed from the spectator 
 (indicated by a cross), is induced. Thus this E.M.F. w is acting in 
 a like direction to the first one. We reach now, through a back 
 connection, wire 7, through a front connection wire 12, whereby all 
 the electro-motive forces add themselves, and come further through a 
 back connection to 5. Wire 5 is a neutral wire, in which no E.M.F. 
 is induced, and which is in direct connection with the second brush. 
 The current can thus flow from the second brush into the outer 
 circuit. This brush is, therefore; the positive one, whereas the left 
 brush is the negative one. 
 
 If we follow the second way, which is offered to "the current from 
 the left brush, we come from 11 to 6, from 6 to 1, from 1 to 8, from 
 8 to 3, in which wires the induced electro-motive forces add them- 
 selves, and at last from 3 to 10. On this second way we have as 
 many series-connected wires as in the first way, viz. 4 effective and 
 2 neutral conductors. The E.M.F. of the second half of the armature 
 is thus equal to that of the first one. Both halves of the armature are 
 connected in parallel, like the halves of the ring armature. Fig. 80 
 shows, further, a diagram of connections for an armature with 24 
 conductors. In this case the pitch is 11, and it can be seen that 
 the result is the same as with the armature with 12 conductors. 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 81 
 
 To get one continuous drum-winding, it is necessary to select 
 as pitch an odd number. If in the last example we made the 
 
 20 Jg 17 
 
 FIG. 80. Drum Armature Connections. 
 
 step = 10, then we go from 1 to 11, from 11 to 21, and so on, and 
 can never arrive at conductors with even numbers, but combine half 
 of the conductors, viz. the odd ones only, in a closed winding. 
 Instead of one continuous armature winding, we get in this way two 
 entirely separated windings. 
 
 On the other hand, it is not necessary to make the pitch just equal 
 . to a number smaller by one than the half of all wires. With 24 wires 
 we could make the pitch either 11 or 13. 
 
 The pitch on the front need not always be equal to that of the 
 back. If we had 22 wires, for instance (see diagram, Fig. 82), we 
 could make the front step = 11, the back step = 9, and would then get 
 a winding of quite the same kind as considered before. 
 
 It should be noted that in Figs. 79 and 80 the number of coils are 
 even (i.e. one-half number of conductors), and hence a wire on one 
 side of the armature* is not connected by the end connections to a 
 wire diametrically opposite. Thus, in Fig. 80, wire No. 1 cannot be 
 connected to wire No. 13 diametrically opposite, so that they do not 
 commutate under the brushes simultaneously. In the winding 
 shown in Fig. 82, the number of coils is odd, so that, if desired, wires 
 diametrically opposite could be connected by the end connecters. 
 
82 
 
 ELECTRICAL ENGINEERING 
 
 The method more often used in practice is not as shown in Figs. 79, 
 80, 82, but as shown in Fig. 81, where the coils are in two layers. 
 As shown in the figure, the outer layer is in multiple with the lower, 
 and due to this there may be a little unbalancing. To make this 
 perfect, in winding, instead of proceeding around in one layer, the 
 
 connections are made to wires first in the lower layer and then in 
 the upper. 
 
 The drum armature was first employed by Hefner-Alteneck. It 
 has certain advantages over the ring armature. Since no wires go 
 through the interior of the armature, for small armatures the iron 
 discs may be fixed directly on the armature shaft. The construction 
 of the armature becomes therefore cheaper, and the winding simpler. 
 Also for larger armatures the drum winding is preferred to the ring 
 winding. 
 
 Special means must be provided to prevent the conductors from 
 sliding on the smooth armature surface. ' For this purpose on the 
 surface of the armature sometimes grooves are made, and rods are 
 
THE CONTINUOUS CURRENT DYNAMO 83 
 
 put into these grooves, which prevent the conductors from sliding. 
 
 FIG. 82. Drum Armature Connections. 
 
 VtfUUl/1/7 
 
 These rods are called driving-keys. For smaller armatures about 4 
 to 10 driving-keys are required. 
 
 Generally this difficulty is overcome in quite another way, 
 viz. by placing the conductors themselves into slots, which are 
 
 either cut in the complete 
 armature core, or stamped 
 in the single discs which are 
 fixed on the shaft, so that 
 \ f \ f ^ey form continuous slots, 
 
 V / \ I into which the armature 
 
 U J ^ wires are laid. In this case 
 
 FIG. 83. Open FIG. 84. Nearly ' there are many slots, which 
 
 Slots. Closed Slots. are V ery near one another. 
 
 Between the single slots the 
 
 teeth or small bridges of the armature iron project In Figs. 83 and 
 84 portions of toothed discs are shown. Fig. 85 shows a toothed 
 drum armature without, and Fig. 86 one with its winding, which 
 lies well protected in the slots. These armatures are called toothed 
 armatures in opposition to the above described smooth armatures, 
 although the latter may have some slots for the keys. 
 
 Both the ring and the drum ; the smooth and the toothed armatures 
 
ELECTRICAL ENGINEERING 
 
 are capable of producing a continuous current of nearly the same kind 
 
 as that delivered by a battery. As a rule, the armature winding 
 
 consists of a great number of 
 
 conductors; and, by the action 
 
 of the commutator, at every 
 
 moment all wires within the 
 
 range of the lines of force act |p ^ V\ 
 
 in the same manner, and give 
 
 E.M.F.; whereas with the Sie- 
 
 FIG. 85. Armature-core, without 
 Winding. 
 
 FIG. 86. Armatures, Partially Wound. 
 
 mens armature the pressure oscillated at each half turn between 
 zero and its maximum value. Certain fluctuations do, however, take 
 
 FIG. 87. Finished Armatures, 
 place also with these armatures. It might happen that there are at 
 
 FIG. 88. Wound Drum Armature, 
 one time 20, whilst in the next moment there are 21 slots under the 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 85 
 
 pole-shoe, so that the E.M.F. induced is alternately a little smaller 
 and a little larger, but generally these fluctuations are unimportant. 
 
 The bipolar is manufactured in America for small dynamos and 
 motors up to about 10 kilowatts, and in a few installations made by 
 the old Edison Company they are in use up to 150 kilowatts. Also 
 
 FIG. 89. N. Y. C. Locomotive Motor Armature. 
 
 a recent type of locomotive motor, made by the General Electric 
 Company, has been adopted by the New York Central Railroad 
 Company, each armature being capable of delivering 600 H.P. Fig. 
 89 shows the general arrangement, the truck frame serving as 
 part of the magnetic circuit. A bipolar winding has the full potential 
 between layers, so that special care must be used in insulating, par- 
 ticularly on the ends. 
 
 Magnet System 
 
 Permanent steel magnets cannot be magnetized to as high a degree 
 as eleotro-magnets. Hence, for dynamos, electro-magnets are exclu- 
 sively employed. Fig. 90 shows a two-pole dynamo with horseshoe- 
 shaped magnets. Over the arms of the latter two coils are wound, 
 so as to drive all lines of force in the same direction through the mag- 
 net, and to make one pole north, the other one south, magnetic. 
 
 To get a strong magnetic field it is, as we know, essential to have 
 the lines of force going as far as possible through iron only, and to 
 make the way through air or any other non-magnetic materials as 
 short as possible. Hence the air gap between armature and pole- 
 shoes is kept very small. It is obvious that we can approach the 
 armature core nearer to the pole-shoe with a toothed armature than 
 with a smooth one, for with the latter the winding is arranged over 
 
86 
 
 ELECTRICAL ENGINEERING 
 
 the iron core. This is a further reason why, nowadays, toothed 
 armatures are generally employed for dynamos. 
 
 To get a current from the dynamo, it is necessary to "excite" the 
 magnet system, i.e. , to send a current through the coils, by which a 
 magnetic flux is produced. The current for exciting the magnet 
 coils may be taken from any current generator, as, for instance, a 
 
 FIG. 90. Magnetic Circuit of Bipolar Dynamo. 
 
 galvanic battery. We shall see, later on, that it is not necessary to 
 use an external current generator for this purpose. But as this case 
 is the simplest one, and easiest to be understood, we shall, first of all, 
 take this as a basis for our consideration. 
 
 If we send a current through the magnet coils, and let the 
 armature rotate, the dynamo will produce a definite voltage. The 
 E.M.F. induced in each conductor is larger the stronger the magnetic 
 field and the greater the speed of rotation of the armature. Since 
 the coils of each half of the armature are connected in series, the 
 E.M.F. of the whole armature will also increase with the number of 
 armature wires. If we turn a certain armature, firstly with a speed 
 of 500, and then with 1000 revolutions per minute, and leave un- 
 changed the strength of the magnet current, then the E.M.F. of the 
 armature will be twice as much in the second case as in the first 
 one. If we strengthen the magnetizing current, the E.M.F. of the 
 armature will also rise, but not quite in the same proportion. For, 
 as we know, there is a limit to the magnetization of iron, and if 
 we approach this limit, a great increase of the magnetizing ampere- 
 turns causes only a small strengthening of the magnetic field. By 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 87 
 
 means of a diagram we can make this clear as follows: Let us draw 
 a horizontal line (Fig. 91), and divide it into parts, each of say 
 1 cm. The length of 1 cm. represents, then, about 1000 ampere- 
 turns. Thus we mark the 
 first division with 1000, the 
 second one with 2000, the 
 third one with 3000, and so 
 on. Now let us draw a 
 vertical line from each of 
 these divisions. The length 
 of these lines we make 
 equal to that E.M.F. which 
 is produced by the armature 
 at a constant speed, if the 
 magnet arms be excited 
 with 1000 ampere-turns in 
 the first, with 2000 in the 
 second, 3000 in the third 
 case, and so on. A height 
 of 1 cm. of the vertical line 
 has to repre c ent about 20 
 volts. We observe that, 
 with 2000 ampere-turns, the 
 
 120 
 
 100 
 
 80 
 
 60- 
 
 40 
 
 20 
 
 Ampere [Turns 
 
 1000 2000 3000 4000 5000 
 FIG. 91. Magnetization Characteristic. 
 
 voltage is nearly double of 
 that with 1000 ampere-turns. 
 But as the excitation grows to 3000, the increase of the voltage is 
 slower. At still higher excitations the voltage, increases but less and 
 less. If we connect all the ends of the vertical lines by a line, we 
 get a bent line or a curve. This line is steep at its beginning, and 
 becomes rather flat at its end. This curve is called the electro-motive 
 force characteristic of a dynamo on open circuit. A simpler name is 
 the magnetization characteristic or saturation curve. 
 
 To obtain from the dynamo that we are considering a voltage of 
 110, we want on the magnet limbs about 4000 ampere-turns. We 
 can get this by sending through a coil of thick wire, with 40 windings, 
 a current of 100 amps., or by sending through a coil of fine wire, with 
 2000 windings, 2 amps., and so on. As, in the first case, the few 
 windings of thick wire have a small resistance, the voltage required 
 for this coil is small, say about two volts ; whereas, in the latter case, 
 the numerous windings of the fine wire have a very high resistance, 
 and therefore a much larger voltage about 100 volts is required. 
 The output in watts, however, which has to be spent for excitation 
 is practically the same in all cases. In our examples, for instance, 
 it would be 200 watts. 
 
 If the dynamo delivers current to an outer circuit, the voltage on 
 the brushes decreases. We have observed the same case with the 
 
ELECTRICAL ENGINEERING 
 
 loo 
 
 battery. The dynamo armature has a definite internal resistance. 
 If now through the armature a current is flowing, the resistance 
 consumes a certain voltage. Thus the terminal voltage, i.e. the 
 voltage of the brushes,, is smaller than the E.M.F. induced in the 
 armature. The larger the current the greater will, as we know, be 
 the voltage drop. . 
 
 But there is, in addition to the internal resistance, a further 
 reason which causes this voltage drop. The currents flowing in the 
 armature exert a reaction on the magnetic field, so as to weaken the 
 latter. We shall deal, later on, separately with the question of the 
 
 armature reaction. For the 
 
 120 present moment it is sufficient 
 
 to know that the armature 
 reaction has an effect similar 
 to the ohmic resistance of 
 the armature. Both cause a 
 voltage drop at an increased 
 load. 
 
 We can make this clearer 
 by means of a diagram. Let 
 us assume that the magnet 
 system be magnetized con- 
 stantly by 4000 ampere-turns, 
 and that the current taken 
 from the armature be 10, 20, 
 30 amps., etc., respectively. 
 1 cm. on the horizontal line 
 may represent 10 amps, (see 
 Fig. 92). On the vertical we 
 plot the voltages as before. 
 
 4-0 
 
 Amperes 
 
 10 
 
 30 
 
 FIG. 92. Closed Circuit Characteristic. Tr ,, , 
 
 It tne dynamo does not supply 
 
 any current, its pressure is 
 
 about 110 volts. If it supplies a current of 10 amps., the pressure 
 would go down to 109 volts, at a current of 20 amps, to 107 volts, at 
 30 amps, to 103 volts, and so on. By connecting all the ends of 
 the vertical lines we get a curve, which shows the decrease of the 
 voltage with increasing load. This curve is called the closed circuit 
 characteristic or load characteristic. 
 
 It is. of course, desirable in many cases to get a constant dynamo 
 voltage at a varying load. If, for instance, a dynamo supplies 
 current for a lighting plant, it would be very objectionable, if the 
 voltage fell from 110 to 103 volts, as we switched on more lamps. 
 To bring the voltage back to its normal value of 110 volts, it is 
 therefore necessary to increase the number of ampere-turns, so that, 
 for instance instead of 4000 ampere-turns, 4200, 4500, 5000, and 
 so on, ampere-turns may be produced. This can be effected by 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 89 
 
 means of a regulating resistance. To understand the action of 
 a regulating resistance, let us consider the following example. 
 Assume a dynamo, giving a voltage of 110, which requires for 
 excitation at no load 4000 ampere-turns. The magnet coils consist 
 of 2000 windings, having a resistance of 50&>. Hence, if we connect 
 these magnet coils with a voltage of 100, the current would be 
 2 amps., and the number of ampere-turns 2000x2 = 4000, i.e. just 
 what we want. But let us now connect the magnet coils with 
 110 volts; then the magnet current would be 2.2 amps., and the 
 number of ampere- turns 2.2x2000 = 4400. As we have previously 
 learned, an additional number of ampere-turns increases the voltage 
 so that in our case the pressure 
 would rise to, say, about 117 volts. 
 To get only a voltage of 110, we 
 have to connect a resistance of 5co 
 in series with the coils. Then the 
 current becomes again 2 amps., and 
 the number of ampere-turns 4000. 
 But if we now load the dynamo, its 
 voltage will come down to, say, 
 about 109 volts. By increasing the 
 number of ampere-turns, or, what 
 is the same, by increasing the mag- 
 netizing current, we can increase 
 the voltage. Thus we have to do 
 nothing but switch out a part of 
 the resistance connected in series 
 with the coils. This is effected by 
 approaching the lever of the adjust- 
 able resistance to its position of 
 short circuit. The greater the load 
 of the dynamo, the larger has to be 
 the magnetizing current, the more 
 resistance we have therefore to cut 
 out of the circuit. 
 
 Fig. 93 serves as an illustration 
 for a regulating resistance. The 
 
 connection with the coil and the circuit is made as follows: The 
 centre-point of the lever K is connected with one pole of the 
 battery, and the last contact on the left with one end of the coil. 
 The other end of the coil is connected directly with the second 
 battery pole. If the lever of the regulating resistance is over the last 
 contact on the left marked 0, the current can go from the battery to 
 the magnet coil directly, without flowing through the resistance spirals. 
 We say, then, that the resistance is short-circuited. The further we 
 
 FIG. 93. Regulating Resistance. 
 
90 
 
 ELECTRICAL ENGINEERING 
 
 move the lever to the right, the more resistance spirals the current 
 has to flow through. 
 
 FIG. 94. Enclosed Regulating Resistance (Berend & Co.). 
 
 Fig. 94 shows a regulating resistance covered with stamped sheet 
 iron. 
 
 Figs. 95 and 96 show two rheostats as made by the General 
 Electric Company, the first having a resistance of 20 ohms and a 
 carrying capacity of 10 amperes, and the second a resistance of 832 
 ohms and a carrying capacity of 75 amperes. The formula for the 
 E.M.F. of a direct current dynamo is derived as follows: Let n=the 
 number of coils in series between brushes = number of external con- 
 tacts divided by 4 in a bipolar. Let < = the flux going from pole 
 to pole being enclosed by the coils of the armature. Let N=the 
 number of revolutions of armature (in a bipolar) per second. In 
 connection with the production of E.M.F. in a dynamo, it has been 
 shown and illustrated in Fig. 65, page 69, that in each revolution 
 of the armature coil, starting say as shown in Fig. 65, the coil is 
 filled full of lines of force twice and emptied twice, thus four times 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 91 
 
 in each revolution does the coil cut all the lines of force <j>. Thus, 
 since a volt is produced by cutting 100,000,000 lines of force per 
 
 second, the average volts produced per coil= ^ ^. 
 
 iUU,UUU ; UUu 
 
 But there 
 
 FIG. 95. 
 
 FIG. 96. 
 
 Field Rheostats, 
 are n coils in series between brushes. Therefore the E.M.F. of a 
 
 D.C. dynamo having n coils on its armature = 1QQ QQ Q QQQ . On a two- 
 pole dynamo n=the external conductors divided by 4. Representing 
 the external conductors by C, the formula becomes ^. 
 
 Self=excitation Shunt Dynamo 
 
 As mentioned before, it is not necessary to use current from an 
 external current generator for exciting a dynamo. It would be the 
 simplest way to use the voltage of the dynamo armature, which was, 
 for instance, 110 volts in the last example, for feeding the magnet 
 coils, which had to be excited with 100 volts at no load, and with a 
 somewhat higher voltage at full load It is clear that, if the machine 
 is brought to its full voltage, the magnet coils may be connected 
 without any further difficulty with the armature, so that the machine 
 
92 
 
 ELECTRICAL ENGINEERING 
 
 3s able to work properly. The only question arising is now: How is 
 it possible to bring the machine up to its voltage without the use of 
 an external current generator ? 
 
 For this purpose, that property of the iron which we know as 
 residual magnetism is of great advantage. Iron which has once been 
 magnetized always retains some traces of magnetism. It follows, 
 therefore, that across the field of a non-excited dynamo a certain 
 although it may be a very small, number of lines of force pass. If in 
 this very weak magnetic field an armature is turned, a very small 
 E.M.F. is induced, and since the magnet coils are connected with the 
 armature, the small E.M.F. will send a defined, but small current 
 through the magnet coils. We hence get a few magnetizing ampere- 
 turns, which strengthen the residual magnetism. 
 The strengthened field now induces a larger E.M.F., 
 the latter again causing a stronger magnetizing 
 current, which produces a stronger magnetism, so 
 that, in this way, the machine, in a short time and 
 without an outer source of current, is brought to its 
 full voltage. 
 
 It is, of course, necessary to connect the magnet 
 coils properly with the armature, that is to say, in 
 such a way that 'the current produced by the 
 armature, and flowing through the coil, really 
 strengthens the existent magnetism. If we con- 
 nected the magnet coils with the armature in a 
 wrong way, then the current, flowing in the wrong 
 direction, would not strengthen the residual mag- 
 netism, but destroy it, and the dynamo would give 
 no voltage. 
 
 Werner Siemens was the first to find this prin- 
 ciple of self-excitation of dynamos. 
 
 Fig. 97 is a diagram of connections for a self- 
 exciting dynamo. To one side of the brushes is 
 connected the load (the small circles represent 
 lamps, connected in parallel), on the other side 
 are the magnet coils (marked by a zigzag line). 
 There might have been connected also a resistance 
 in series with the magnet coils, but this is not considered in the dia- 
 gram. The two circuits which branch off the armature terminals are 
 called, respectively, the main circuit -and the shunt circuit. The 
 magnet coil acts as a shunt to the main circuit. A machine, having 
 connections as shown in Fig. 97, is called a shunt dynamo. 
 
 How does such a shunt dynamo behave with various current 
 intensities in the main circuit? 
 
 Suppose, first of all, that the main circuit is disconnected. We 
 now have the circuit closed through the armature and the coils, and 
 
 FIG. 97. Shunt 
 Dynamo Con- 
 nections. 
 
THE CONTINUOUS CURRENT DYNAMO 93 
 
 the machine therefore comes to a defined voltage say, about 110 
 volts. Let us now switch on, in the main circuit, a number of lamps, 
 so that the dynamo has to supply a current of 10 amps. As a con- 
 sequence, the terminal voltage of the dynamo will fall, due to the 
 ohmic resistance and the armature reaction, to, say, 109 volts. But, 
 at this moment, the magnet coils are now no longer connected with 
 110, but only with 109 volts. The magnetizing current, and hence 
 the excitation, will therefore become smaller, and the dynamo voltage 
 will further fall. At a load of 20 amps, we had, with the separately 
 excited machine (page 88), a voltage of 107; whereas the voltage of 
 a self-excited dynamo at the same load will be only about 105 volts, 
 and at 30 amps, load about 100 volts (against 103 with the separately 
 excited dynamo). Thus the self -excited dynamo shows the property 
 of the separately excited machine the falling of the voltage with 
 increasing load in a far higher degree. Naturally, we may use here 
 the same auxiliary means a regulating resistance for keeping the 
 voltage constant, as before. 
 
 The magnet coils of a 110-volt dynamo are, for instance, wound 
 generally so as to get, at no load, a sufficient excitation for producing 
 in the armature a voltage of 110, even if the voltage, measured at the 
 magnet coil terminals, be 90 volts only The remaining 20 volts are 
 then absorbed by the shunt regulator. At an increased load, the 
 resistance switched into the shunt circuit has to be diminished. The 
 more the load increases, the more resistance of the shunt regulator 
 has to be short-circuited; 
 
 Fig. 98 shows the external characteristic of a shunt dynamo of 
 modern design. The line OB gives the current values delivered 
 
 A 
 
 9 10 1 
 
 1 12 
 
 
 
 1 
 
 *. 
 2 
 
 3 x 
 
 4 
 
 Tolts 
 
 
 
 
 
 
 
 
 
 
 
 
 R 
 
 Amps. 
 FIG. 98. Shunt Characteristic. 
 
 by the dynamo, and the line OA the voltage values. When one- 
 quarter load is put on at 5, the voltage drops at 1 below the normal 
 value shown by the line AC. If it is raised again by means of the 
 
94 
 
 ELECTRICAL ENGINEERING 
 
 field rheostat at 9, and another one-quarter load is put on, the voltage 
 will again fall, a little more this time, to 2, and so on up to full load. A 
 well-designed dynamo will drop at 2 about 5 per cent. Some machines 
 will entirely lose their magnetism when one-quarter load is put on 
 without any further adjustment of the field rheostat, particularly in 
 going from three-quarters to full current, the voltage dropping and 
 
 FIG. 99. External Characteristic. 
 
 the field following till no voltage at all is reached. This is called 
 unbuilding, but should not occur under ordinary conditions with 
 well-designed machines. The external characteristic without adjust- 
 ment of field rheostat is shown in Fig. 99. In this case the voltage 
 starting at a drops till it reaches b, when the machine unbuilds and 
 the voltage goes to O. 
 
 Series Dynamo 
 
 In the case of a shunt dynamo the magnet coils are excited with 
 nearly the full armature voltage, but only a small part of the 
 armature current flows through them. The greatest part of the 
 current goes through the main circuit. There is, however, possible 
 another method of connection of the armature with the magnet 
 coils. As we know, we can get any desired magnetic effect with a coil, 
 having but few windings, through which a strong current passes. 
 Hence we may send the whole dynamo current through a coil con- 
 sisting of a few windings of comparatively thick wire (see Fig. 100). 
 In this case the connections have to be as follows: One brush, say 
 the right one, has to be connected with the main circuit directly. 
 From the second brush no connection is made with the main circuit, 
 but the current has first to flow through the magnet coil, and then 
 enters the main circuit. The latter is again shown in the diagram 
 as consisting of lamps. Thus in this case the full or main current 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 95 
 
 flows through the magnet windings. This is called a series wind- 
 ing. 
 
 As the armature, the magnet coils and the main circuit are 
 connected in series, this dynamo is called a series dynamo. Let 
 us now consider the behaviour of such a machine with various 
 currents. 
 
 If we disconnect the main circuit, then the whole circuit is dis- 
 connected, and no current whatever can flow, neither in the mains 
 nor in the armature nor the magnet coils. Notwithstanding, a very 
 low E.M.F. is produced in the armature, originating in the residual 
 magnetism. No strengthening of this E.M.F. can, however, take 
 place. Thus the voltage of the machine is, with the main circuit open, 
 
 FIG. 100. Series Dynamo Connections. 
 
 a very low one, or, practically speaking, nothing at all. If we now 
 close the main circuit, a current produced by the small E.M.F. flows 
 through the magnet coils. As a consequence, a strengthening of the 
 field follows, with an increase of the E.M.F., producing a greater cur- 
 rent, and causing a further strengthening of the field as before. If 
 the main resistance happens to be a large one, the current (in this case 
 the main current as well as the magnetizing current) and the E.M.F. 
 will be small only. But if we diminish the main resistance (we may 
 do so, for instance, by connecting some more lamps in parallel with 
 
96 
 
 ELECTRICAL ENGINEERING 
 
 those which were already burning) the current flowing through the 
 whole circuit is increased. Thus the machine will be more strongly 
 magnetized, and hence produce a larger E.M.F. We thus learn that 
 the E.M.F. of a series dynamo will grow with an increasing load, quite 
 opposite to the case of the shunt dynamo. 
 
 But this growing of the E.M.F. at increasing load has naturally 
 an end. The strength of the magnetic field cannot increase continu- 
 ally, but remains constant, after having reached a certain value. 
 On the other hand, there is a voltage drop in the armature, which 
 is greater the larger the armature current. After the machine has 
 reached a certain voltage, it therefore follows that, since the strength 
 of the magnetic field cannot further be increased, the growing voltage 
 drop in the armature and armature reaction must cause the terminal 
 voltage to fall when we put a greater load on the dynamo. 
 
 As long as the load of a series dynamo is not too high, its 
 terminal voltage grows with an increasing load: if now the load 
 
 140T 
 
 120 
 
 100- - 
 
 80 
 
 60-- 
 
 40-- 
 
 20 
 
 50 
 
 100 
 
 .150 200 
 
 Amperes 
 
 250 
 
 FIG. 101. Closed Circuit Characteristic of Series Dynamo. 
 
 be further increased, the voltage remains constant for a certain 
 period; then if the load is increased once again, a fall of pressure 
 must ensue. 
 
 The diagram, Fig. 101, shows the characteristic curve for a series 
 dynamo. 
 
THE CONTINUOUS CURRENT DYNAMO 97 
 
 For feeding a variable number of glow lamps connected in parallel 
 a series dynamo cannot be employed, for the voltage of the dynamo 
 would vary with the number of lamps burning, and therefore the 
 lamps would from time to time vary in candle-power. If the number 
 of lamps burning can be kept constant, lighting with a series dynamo 
 is possible, although this is seldom done. We may compare the dif- 
 ference in the behaviour of the shunt and series dynamos with the 
 difference in the characters of two men. With one, his effort ceases 
 if he has to do more work, whereas with the other, a greater demand 
 strengthens his resolve and his power up to a certain point, beyond 
 which, if his task be increased, he also breaks down. 
 
 The series dynamo is particularly suited for series arc lighting, 
 where a constant current is desired and an increased voltage when the 
 number of lamps in circuit is increased. It is customary to work 
 such machines at the point D (Fig. 101) on the characteristic curve, 
 which results in a constant current with varying voltage. Even at 
 short circuit on such a machine, the current does not increase seriously 
 (at B, Fig. 101). The large armature reaction, to be explained later, 
 and high internal resistance, which naturally follow from a large 
 number of armature turns, and high voltage generated, hold the 
 increase of current in check. Sometimes series generators are used 
 as boosters, being connected to constant voltage supplies to increase 
 the voltage on a circuit in proportion to the load, tinder such con- 
 ditions the series dynamo would be operated on the characteristic 
 between C and A, the voltage under such conditions rising with the 
 load which is desired. In the latter condition, the iron of the mag- 
 netic circuit would be operated below saturation, so as to get a re- 
 sponse to the increase in ampere turns. Also the dynamo would 
 be designed with less armature reaction, so that the characteristic 
 would not tend to drop, as shown in Fig. 101, but would continue 
 in an approximate straight line. The designer, therefore, lays out 
 his machine to meet the conditions imposed. 
 
 Compound Dynamo 
 
 Generally, it is required from a dynamo to supply constant voltage 
 at varying loads. This may be effected, not only by a shunt dynamo 
 having an adjustable shunt resistance, but also by a suitable winding of 
 the magnet coils. For this purpose the dynamo is provided, besides 
 the shunt winding, with a series winding, which latter is wound 
 either over or under or beside the former. Care must be taken to 
 wind both windings in such a way that they may act in the same 
 sense. If the dynamo does not supply current, the series winding 
 
98 
 
 ELECTRICAL ENGINEERING 
 
 has no effect at all. The shunt coil only has to be considered, 
 and this brings the machine up to a certain 
 voltage say 110, for instance. If the machine 
 supplies current, its terminal voltage would 
 fall, if there were only a shunt coil; but 
 now, since the main current flows through the 
 series coil, the number of ampere-turns is in- 
 creased, and the magnetic field is strengthened. 
 With a proper proportion of the number of 
 the two windings, we can, up to a certain limit, 
 get a constant or nearly constant voltage at 
 varying loads. If, however, the current taken 
 from the dynamo exceeds the limit allowed, 
 then the voltage falls. The diagram of 
 connections for such a machine is shown 
 in Fig. 102. The shunt winding is indicated 
 by a fine, the series winding by a thick, zigzag 
 line. A dynamo whose magnets are wound 
 in the way described is called a compound 
 dynamo. 
 
 With a series coil, having a sufficient 
 number of turns, we may also obtain the 
 result that, at an increased current, the voltage 
 too is increased, so that the voltage drop in 
 the mains which grows with a larger current, 
 is compensated. In this way the pressure at a 
 place distant from the dynamo may be kept 
 constant. The dynamo in this case is said to FIG. 102. Compound 
 be over-compounded. Dynamo Connections. 
 
 Types of Dynamos 
 
 The essential parts of a dynamo are the magnetic frame, the 
 armature, and the commutator. The magnetic frame may assume 
 very many different shapes. For the actual working of the machine 
 only the field between the pole-shoes is of special importance. 
 But, to get a strong magnetic action, the connection between the 
 two poles must be made of iron. The shape of this part of the 
 magnetic circuit depends on the choice of the designer. 
 
 The magnetic frame may, for instance, have the shape of a horse- 
 shoe, with which we became acquainted in the previous chapter. 
 The horseshoe may have the yoke upwards, so that the magnet stands 
 in a manner on its poles (see Fig. 103). This type, which has been 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 99 
 
 employed by Edison, and is therefore called the Edison type, was very 
 
 common some time 
 ago, but is very sel- 
 dom built now-a-days. 
 When horseshoe mag- 
 nets are employed, 
 they are generally 
 built with the yoke 
 downwards, so that the 
 yoke may either be 
 used as base, or may 
 be cast together with 
 the base-plate. This 
 type (see Fig. 104) is 
 called the Kapp type, 
 after Kapp, who first 
 employed it. Fig. 105 
 shows a machine after 
 
 the Kapp type, but the construction of which differs somewhat with 
 
 regard to the arrangements of the bearings. 
 
 It is not necessary to employ two magnetizing coils. The 
 
 FIG. 103. Edison Type. FIG. 104. Kapp Type. 
 
 FIG. 105. Kapp Type. 
 
 magnetic flux may, of course, be obtained as well by one coil, having 
 double the number of ampere-turns. Fig. 106 shows a machine which 
 is also of the horseshoe type, but with the windings on one coil on; 
 that placed on the yoke. In this case the pole-shoes are one above 
 
100 
 
 ELECTRICAL ENGINEERING 
 
 the other, whereas they are arranged side by side in all the types we 
 have hitherto considered. 
 
 It is also not necessary to make the yoke between the two pole- 
 shoes in one piece, but it may be divided into two parts. We have 
 
 FIG. 106. C Type. 
 
 FIG. 107. Manchester Type. 
 
 then a magnetic circuit, split into two branches, similarly to the 
 branching of an electric circuit. 
 
 Fig. 107 shows a scheme of such a dynamo. The upper pole is 
 a north, the lower one a south pole. The lines of force go, then, 
 
 FIG. 108. Manchester Type. 
 
 from the north pole, through the air gap and the armature core, 
 downwards to the south pole, enter the yoke, and branch to the right 
 and to the left. They go upwards through the two vertical columns 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 101 
 
 bearing the coils, and join again in the upper yoke. The coils have, 
 
 of course, to be wound so that both 
 tend to generate a flux of lines of 
 force directed upwards. Thus the 
 current must, seen from one side, 
 flow in the same direction through 
 the coils. The machine described 
 here is called a Manchester machine. 
 Fig. 108 is an illustration of a com- 
 plete machine of this type. This 
 was the type tf magnetic circuit used 
 in the old Sprague dynamo of several 
 years ago. 
 
 The magnet coils may also be 
 arranged in another way. Fig. 109 
 shows a type of machine called the 
 Lahmeyer or semi-enclosed type. 
 With this machine the coils are 
 arranged over and underneath the 
 pole-shoe, and the yoke, which 
 
 PIG. 109. Lahmeyer type (British is split into two parts, partially 
 Schuckert Co.\ encloses the machine. Fig. 110 is 
 
 also built after this type. 
 
 FIG. 110. Lahmeyer Type (Mcschinenfabrik Oerlikori). 
 
102 
 
 ELECTRICAL ENGINEERING 
 
 The Gramme machine (see Fig. Ill) has another arrangement 
 of the magnetic field, which was employed in the early days of 
 dynamo building. Its magnetic field consists of a double magnetic 
 circuit, like that of a Manchester or Lahmeyer machine. With the 
 Gramme machine, however, each half is provided with two coils, one 
 at the top and one at the bottom. 
 
 There are, besides the types mentioned hitherto, a great number of 
 different magnet shapes, but which are very seldom used. The most 
 usual 2-pole machines are those after the Lahmeyer and the Kapp type. 
 
 FIG. 111. Gramme Dynamo. 
 
 As material for the construction of field magnets, formerly wrought 
 iron was in general use, because this can be magnetized more 
 strongly than other kinds of iron. Cast iron has a far lower magnetic 
 conductivity, not much more than half that of wrought iron. Hence 
 if we employ cast iron we have to make the cross-sectional areas of 
 the limbs and the yoke twice as big as with wrought iron, in order to 
 get the same number of lines of force. As, however, in spite of 
 the double cross-sectional areas, the cast-iron machines can be more 
 cheaply manufactured than the wrought-iron ones, the use of the 
 latter material has been abandoned. 
 
THE CONTINUOUS CURRENT DYNAMO 103 
 
 Lately, steel makers have succeeded in manufacturing by a process 
 of casting a material similar to wrought iron. This is generally 
 called cast steel. It is of quite another quality to the hard steel, 
 such as is used for tools and permanent magnets. The magnetic 
 properties of cast steel are very similar to those of wrought iron. 
 Hence, in making the magnet frame from cast steel, we can make 
 the cross-sectional areas as small as with wrought iron; that Is, 
 equal to about one-half of the cross-sectional areas of a cast-iron 
 frame. On the other hand, cast steel is more expensive than cast 
 iron. Hence none of these materials has come into exclusive use. 
 In some cases magnetic frames are made of cast steel, in other cases 
 cast iron is employed with advantage. 
 
 Output of a Dynamo 
 
 The strength of current which can be taken from a dynamo is 
 limited chiefly by the heating of the armature wires. From an 
 armature wound, for instance, with wire of No. 18 S.W.G. we cannot, 
 of course, take a current of 30 amps, for a long time. As the 
 armature has two parallel circuits, through each of them a current 
 of 15 amps, would flow, which would obviously heat this wire far too 
 much. 
 
 We must, however, not think that the table given at the end of 
 the first chapter is a standard for the maximum current allowable in 
 armature wires. For, firstly, the heating allowable for armature 
 conductors is a far higher one than it is with main conductors; and, 
 secondly, due to the quick rotation of the armature, the conductors 
 are cooled continuously by a draught of air. Thus the current 
 density used for armature wires varies between 650 and 4500 amps, 
 per square inch, and even more with very small machines. There is 
 no general rule with regard to current densities of armature wires, 
 for according to the special designs of armatures the draught of air 
 produced by them may be stronger or weaker. With regard to 
 the heating of a dynamo, a rise of the dynamo-temperature of 70 
 to 90 Fahr. over that of the surrounding air is generally considered 
 allowable. 
 
 Still, with any given type of dynamo, the maximum current allow- 
 able is determined by the thickness of the armature wires. Now, the 
 output of a dynamo is determined by the product of the number of 
 amperes by the number of volts. Hence we have to examine by 
 what factors the voltage of a dynamo is determined. 
 
 We know that the voltage is greater, the greater the number 
 of armature wires, the stronger the magnetic field or the number 
 
104 ELECTRICAL ENGINEERING 
 
 of lines of force leaving the pole, and the quicker the conductors are 
 moved. Now, the number of armature wires cannot be indefinitely 
 increased. On a given smooth armature, having a certain distance 
 from the pole-shoes, a limited number of wires only can be fixed. 
 Similarly, into the slots of a toothed armature a certain number of 
 wires only can be put. If, however, we employ a thicker wire in 
 order to get a stronger current from the armature, we can, on the 
 given space for winding, fix a smaller number of wires only; that is, 
 with given armature dimensions we can wind the latter either for a 
 smaller current and a larger voltage, or for a larger current and 
 a smaller voltage. Hence we may, for instance, wind an armature 
 so as to get from it either 10 amps, at a voltage of 110, or about 20 
 amps, at a voltage of 55. Thus the output of the armature remains 
 about the same in both cases, provided that the other determinative 
 factors are not altered. 
 
 These determinative factors are the number of lines of force, and 
 the number of revolutions of the machine. The number of lines 
 of force is greater the larger is the cross-sectional area of the 
 magnetic frame, and the more it is saturated. As a rule, the satura- 
 tion is never pushed to its limit, as in this case an extremely large 
 number of magnetizing ampere-turns, and thus very big magnet coils, 
 would be required Generally the iron is magnetized up to three- 
 quarters of its limit of saturation. Hence, if a field of a threefold 
 strength is required, and we cannot further substantially increase the 
 saturation, it will be essential to enlarge the cross-sectional area of 
 the magnet limbs about threefold, thus making the machine bigger 
 and heavier. 
 
 Naturally the number of revolutions of a machine is chosen as 
 great as possible. With smaller dynamos, up to an output of about 
 3000 watts, a speed of about 2000 revolutions per minute is generally 
 employed. If a machine be run at 1000 revolutions instead of 2000, 
 we get half the voltage, and hence only half the output. 
 
 Another factor in heating an armature is what is called hysteresis. 
 When the magnetism in the iron core is reversed, which occurs once in 
 a revolution in a two-pole machine, the molecules of iron in the core 
 tend to turn about with the magnetism. This naturally cannot 
 occur only very partially. The effort to do this causes rubbing of 
 the molecules one upon another and from this heating, due to the 
 friction resulting. Mr. Charles P. Steinmetz in a series of experiments 
 \ showed that the loss in hysteresis expressed in watt-seconds or joules 
 
 KB 1 ' 6 
 per cm 3 and cycle of magnetism^-- 7 , where B equals flux density 
 
 per centimeter, and K is a constant depending upon the quality of 
 the iron. 
 
THE CONTINUOUS CURRENT DYNAMO 105 
 
 This loss must be added to the loss in the copper and to the loss 
 due to eddy currents in copper and iron (previously discussed) to 
 give the total armature loss. The radiating surface must then be 
 found, from which the total loss in watts per square inch can be 
 determined. Knowing the loss per square inch, the temperature 
 can be accurately predicted, for the rise in temperature of any surface 
 is proportional to the loss of energy that must be radiated from that 
 surface. Thus from the surface of a spool a watt of energy from each 
 square inch will raise the temperature of the spool about 70 Cen- 
 tigrade. A loss due to friction and I 2 R of brush contact on a com- 
 mutator amounting to 1 watt per square inch of commutator surface 
 will raise the temperature of the commutator 10. 
 
 Multipolar Dynamos 
 
 Large machines cannot be built without considerable difficulty for 
 very high speeds, more especially if they have to be coupled directly 
 to steam or gas engines. In the latter cases, their speed must not 
 exceed 200 to 300 revolutions, and in a few cases only it may come to 
 400 to 500 revolutions per minute. With dynamos built in the way 
 we have already described, built with only two poles, the cross-sec- 
 tional areas of the magnetic frame would have to be made very large, 
 and the whole machine would become too bulky and expensive when 
 low speeds are required. 
 
 We may, however, design a machine from another point of view. 
 Suppose we let every conductor at each revolution pass, not two 
 only, but a row of several poles. Then we get, in spite of the lower 
 speed, as large a number of alternations as with a high-speed machine, 
 and the poles may then have a far smaller cross-sectional area. Fig. 
 112shjws,for instance, a 4-pole magnetic frame. The magnet coils 
 are wound so as to produce north and south po\es alternately. In 
 our example the upper pole would, for instance, be a north pole, the 
 one to the right a south pole, the lower one a north, and the one to 
 the left again a south pole. Hence the lines of force leave the upper- 
 most pole and enter the armature. Half of the lines go through 
 the armature to the right, enter from there the south pole to the 
 right, and come back again through the upper right part of the 
 yoke to the upper north pole. The other half of the lines pass the 
 
106 
 
 ELECTRICAL ENGINEERING 
 
 Fro. 112 Four-Pole Magnetic Circuit. 
 
 FIG. 133 Four-Pole Dynamo of American Manufacture 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 107 
 
 upper left quarter of the armature core, enter the left south 
 pole, and then come back through the left upper quarter of the 
 yoke to the upper north pole. In exactly the same way, we can 
 follow the course of the lines of force of all the single poles. 
 The yoke may be circular, or of polygon shape. Fig. 113 shows 
 a complete 4-pole machine. This type is generally used for 
 machines having an output of about 10 to 50 kilowatts; but these 
 limits are by no means always followed, and dynamos for a far smaller 
 output than 10 kilowatts, and sometimes for a higher output than 
 50 kilowatts, are made with four poles. But, as a rule, for machines 
 
 FIG. 114. Six-Pole Dynamo (British Schuckert Co.). 
 
 having a large output, 6-, 8-, and more, pole magnetic frames are 
 employed. Fig. 114 shows a 6-pole dynamo for an output of 
 about 100 kilowatts; Fig. 115 the 18-pole magnetic frame of a 
 dynamo, designed for direct coupling to a slow-speed steam-engine 
 and for an output of 400 kilowatts. 
 
 It is not absolutely necessary that with multipolar machines 
 every pole be provided with a magnetizing coil. In some cases there 
 is a magnet coil over every second pole only. Fig. 117 shows a 
 4-pole dynamo, having only two poles provided with magnet coils. 
 At a superficial glance, one might consider it to be a 2-pole machine. 
 
108 ELECTRICAL ENGINEERING 
 
 The essential difference, however, between such a machine and a 
 
 FIG. 115. Eighteen-Pole Magnet Frame (Korting Bros.}. 
 
 2-pole one is, that with the 
 latter the opposite poles are 
 different ones, for instance, a 
 north pole on the right and a 
 south pole on the left, whereas, 
 with the 4-pole machine the 
 opposite poles are alike, say, 
 for instance, two north poles. 
 If we follow the path of the 
 
 lines of force (see Fig. 116), 
 FIG. 116. Two-Coil Four-Pole Dynamo. we find that they ]eaye the 
 
 right north pole, and, after having passed the armature, one-half 
 
THE CONTINUOUS CURRENT DYNAMO 109 
 
 of them enter the upper, and the other half the lower of the poles, 
 having no coils. In a like manner, one-half of the lines, leaving 
 the left north pole, enter the upper, the other half the lower one 
 of the unwound south poles. Thus as many lines enter each of 
 the unwound poles as leave each of the wound north poles. The 
 strength of the unwound south poles is, therefore, as great as that 
 of the wound north poles. The former are called consequent poles. 
 There is naturally no saving of wire as we have one coil only for 
 
 FIG. 11T. Four-Pole Dynamo with two Coils. 
 
 each magnetic circuit against two with the usual pole arrangement, 
 and therefore each of the two coils has to have twice as many 
 ampere-turns as each of the usual four coils. 
 
 In a like manner we could wind a 6-pole machine with three 
 coils, and an 8-pole machine with four coils. This construction 
 is, however, very seldom employed. 
 
 The formula for the E.M.F. of a 4-pole dynamo remains the 
 same as for a 2-pole, but it must be remembered that the coils in 
 series between brushes on a 4-pole multiple winding equal the external 
 conductors divided by 8. With a series winding (described later) 
 the coils in series may equal the external conductors divided by 4, 
 as with a 2-pole machine. 
 
 Armatures of Multipolar Dynamos 
 
 The armatures of multipolar machines may be wound as ring or 
 drum armatures, like those of 2-pole machines. The ring armature 
 may even be used in quite an unchanged form for multipolar machines. 
 
110 
 
 ELECTRICAL ENGINEERING 
 
 The machine has then to be provided with as many brush- 
 holder studs as there are poles. In Fig. 118 a 4-pole ring armature 
 is shown. If we imagine it rotating clockwise, then in the outer con- 
 ductors, being under the influence of the north pole, currents are 
 induced which are directed from the spectator, whereas, in the wires, 
 being under the influence of the south pole, currents directed towards 
 the spectator are induced, which are marked in the diagram by crosses 
 and dots respectively. As we see now, the currents induced in the 
 upper quarter of the armature are directed towards the left upper 
 brush, thus making this a positive one, whereas the right upper 
 
 FIG. 118. Four-Pole Parallel Ring Armature. 
 
 brush becomes a negative one. The currents of the right quarter 
 flow from the right upper brush towards the right lower cne. 
 Hence the latter, too, is a positive brush. The currents of the 
 lower quarter flow from the left lower brush towards the right 
 lower one. Finally, the currents of the left quarter flow from the 
 left lower brush towards the left upper one. Hence every two 
 opposite brushes are of the same polarity. If we now connect two 
 opposite, corresponding brushes with each other, by a metal bridge, 
 such as, for instance, a bent copper strip, then we may connect the 
 mains with any point of the two bridges (see Fig. 119). The E.M.F 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 111 
 
 FIG. 119. Brush Arrangement of 4-Pole 
 Parallel Armature. 
 
 of the armature is produced here from one-fourth of the windings 
 being in series, and connecting these four quarters in parallel. Hence 
 the main current is four 
 times as great as the current 
 flowing in each armature 
 conductor. Such a winding 
 is called a parallel winding. 
 It is very suitable for com- 
 paratively low voltages and 
 large currents, for we may 
 even with large currents 
 employ relatively thin wires, 
 since each of the conductors 
 has to carry the fourth part 
 of the main current only, 
 and not the half of it, as was 
 the case with the 2 - pole 
 armature. For a 6-, 8-, 10- 
 pole machine, 6, 8, 10 brush- 
 holders are required, the cur- 
 rent flowing in every arma- 
 ture conductor being equal 
 to the 6th, 8th, 10th part of 
 
 the main current respectively. The E.M.F. of the armature is 
 produced by connecting in series the 6th, 8th ; 10th part of all 
 conductors. 
 
 For higher voltages, however, this winding would require, for 
 the reason just mentioned, a large number of windings. In these 
 cases it is preferred to wind the armature so as to get, similarly 
 to the 2-pole armature, but two parallel connected halves. The 
 E.M.F. is then produced by connecting in series the half of all wind- 
 ings. For this purpose it is necessary to connect the single sections 
 of the winding not directly with their neighbouring ones, but, by 
 means of copper bridges, with the opposite sections, which are always 
 under the influence of a like pole. A diagram of the connections 
 is given in Fig. 120. The armature winding, shown in this diagram, 
 consists of fifteen sections. Following the path of the current, we 
 find that it branches, after leaving the right (negative brush), one 
 part flowing through seven, the other one through eight sections, 
 the electro-motive forces of which are added. The two current 
 branches join again at the second (positive) brush. For this winding 
 two brushes only are required, which are distant from each other, not 
 the half, but the fourth part of the commutator circumference. This 
 is called series winding. 
 
 If a drum winding is used for multipolar dynamos, its pitch has 
 naturally not to be equal to about the half of the total number of 
 
112 
 
 ELECTRICAL ENGINEERING 
 
 armature wires, but has in the case of a 4-pole machine to be equal 
 to about one-fourth, with a 6-pole machine to about one-sixth of 
 the total number of wires. Then a proper series connection of the 
 electro-motive forces of the single wires will be secured. By the 
 selection of corresponding pitches, we may, as with the ring armature, 
 get either a series or a parallel connection of the armature. In Fig. 
 121 in which the front connections are marked by double bent lines 
 inside, and the back connections by single bent lines outside the 
 
 FIG. 120. Four-Pole Series Ring Armature. 
 
 armature the diagram of connections of a 4-pole armature, having 
 26 wires, is given. The pitch is 7 forwards and 5 backwards, and so 
 that, at the front, wire 1 is connected with wire 8, and then, going 
 backwards, wire 8 is connected with wire 3 at the back, at the front 
 3 with 10, at the back 10 with 5, and so on. In addition to being 
 denoted as parallel winding, this is also called loop winding. On 
 marking the brushes at 4 points, \ of the commutator circumference 
 distant from each other, we find that there are four parallel circuits, 
 which consist of nearly equal numbers of series-connected wires that 
 are effective in producing voltage. 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 113 
 
 Next let us consider the diagram shown in Fig. 122. With this 
 winding we do not go backwards in making the connections at the 
 back, but always proceed in one direction. Thus at the front, wire 
 1 is connected with 8, at the back 8 with 13, at the front 13 with 20, 
 at the back 20 with 25, and so on. In following the course of the 
 current we find, starting from the positive brush (there are two 
 collecting places only required with this winding), that there are 
 two ways to reach the negative brush. Each of these ways comprises 
 
 FIG. 121. Four-Pole Drum Parallel Armature. 
 
 half of all the armature windings; we have therefore two circuits con- 
 nected in parallel. This is called series or wave winding. Instead 
 of two, we may employ four brush-holder pins. In this case, two 
 opposite ones are of the same polarity, and there is between them 
 but one winding, consisting of neutral wires. This winding is short- 
 circuited by two brushes; but that is of no disadvantage. 
 
 It is, of course, impossible to deal here with all the winding 
 combinations which can be made by employing various pitches for 
 
114 ELECTRICAL ENGINEERING 
 
 multipolar machines. The few most important types of windings 
 which have been described above will suffice. 
 
 The arrangements of the winding on a drum armature may vary in 
 very many ways. With small armatures the connecting wires at both 
 armature ends may be wound one upon another, in a manner forming 
 a ball. Generally, however, the wires are arranged regularly, side by 
 side. This type of winding is used in nearly all cases in which, on 
 account of the large cross-sectional area, copper bars instead of wires 
 
 FIG. 122. Four-Pole Drum Series Armature. 
 
 are employed. Frequently the connections between the single con- 
 ductors are made by V-shaped copper strips, which are joined together 
 with the conductors. Figs. 123 and 126 show a type of winding 
 which is suitable for armatures, having thinner wires. The latter 
 are bent over wooden formers, and then placed in position on the 
 armature. These are called former wound. 
 
 From a glance at the figures, it may be seen that the pitch is in Figs. 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 115 
 
 123 and 126 smaller than one-half of the circumference of the respec- 
 tive armatures, and the latter therefore belong to multipolar machines. 
 The conditions to have in mind with parallel armatures of the drum 
 type, which type is the usual one in America, is that the total number 
 of conductors, counting each side of an armature coil as a conductor r 
 should be even, and that the pitch front and back should be odd, 
 
 FIG. 123. Former-wound Armature in course of construction. 
 
 differing by 2. Instead of being wound, as in Fig. 121, it is cus- 
 tomary to have two layers, the odd numbers being on top and the 
 even underneath, the location being as shown in Fig. 81 (p. 82). 
 Where the armature core has slots, several conductors can be put in 
 a slot. It is only necessary that the number of conductors be a 
 multiple of the number of slots. A very common arrangement in 
 dynamos is to have four conductors per slot, two on top of the other 
 two. Since the pitch is odd both front and back and differs by 2, 
 the average pitch is even. For winding series armatures a formula 
 
116 ELECTRICAL ENGINEERING 
 
 is convenient. If N equals the number of conductors on the armature, 
 and if y equals the pitch and p equals the number of poles, then 
 N = pi/+ or 2. y may be different at front or back, but must be 
 odd in each case. If same at front and back, y must be odd. If 
 different, the average pitch may be even. 
 
 Multiple windings are used on large apparatus, uusally above 
 150 Kw. Series windings above 150 Kw. sometimes give trouble 
 
 FIG. 124. Multiple-formed Armature Coil. 
 
 from unequal drawing off of current from the brushes in the various 
 studs. Theoretically, a series winding is independent of air-gap 
 variations when a multiple must be uniform in this respect, as all 
 
 FIG. 125. Series-formed Armature Coil. 
 
 the windings, being in multiple, must have equal voltages, else cross- 
 currents will tend to occur or the various parts of the armature will 
 do different amounts of work. The choice of series or multiple wind- 
 ings is one the designer has to carefully consider. Fig. 124 shows 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 117 
 
 a formed armature coil suitable for a multiple wound armature, and 
 Fig. 125 shows a formed coil suitable for a series-wound armature. 
 
 FIG. 126. Former-wound Armature. 
 
 It will be noticed that in the multiple armature the leads come out 
 near together for convenience of connecting to commutator. In the 
 coil for series connected armature the leads come out apart. 
 
118 
 
 ELECTRICAL ENGINEERING 
 
 Sparking and Displacement of Brushes 
 
 If the brushes on the commutator of a dynamo do not occupy 
 their proper position, we may observe a sparking or flashing at the 
 brushes. Sparking makes the commutator rough, and spoils the 
 latter as well as the brushes. By carefully displacing the brush- 
 rocker, we may easily find out a position for the brushes where the 
 sparking ceases. If now, after stopping the machine, we examine 
 to which armature wires those commutator-bars are connected which 
 are under the properly adjusted brushes, we find that these are situated 
 in the neutral zone, i.e. in the space between the two poles. They 
 
 are not exactly in the middle of the neutral zone, but generally dis- 
 placed a little forwards in the direction of rotation. 
 
 We are aware, that when a circuit is broken, sparking occurs. 
 Let us consider any winding of the 2-pole ring armature (Fig. 127), 
 then we find that through this winding, as long as it is on the right 
 side of the armature, a current is flowing in a certain direction 
 (marked by a dot). If then, on the rotation of the armature, the 
 winding we are discussing passes the lower brush, the direction of 
 the current is altered, for the current is flowing in the sense of the 
 cross in the left side of the armature. The brush is always wide 
 
THE CONTINUOUS CURRENT DYNAMO 119 
 
 enough to touch two commutator-bars simultaneously; hence the 
 winding remains short-circuited as long as the brush touches at one 
 time the two bars with which the ends of the windings are connected. 
 During this brief period the winding belongs neither to the left- nor 
 to the right-hand armature circuit. But the current in the short- 
 circuited winding does not stop suddenly. A car which is unlinked 
 from a running train does not stop suddenly, but follows a certain 
 distance. Similarly, in the short-circuited winding the current flows 
 for a certain, although it may be a very short, time in the same 
 direction as before, and then decreases gradually to nothing. If 
 during this period the armature be moved so much that the two bars 
 connected with the ends of the winding are no longer covered by the 
 brush, the current is interrupted, and there will suddenly flow through 
 the winding a current in the opposite direction, i.e. in that of the 
 current flowing in the wires on the left armature half, so that sparking 
 would appear. To prevent this, the current must be brought down to 
 nothing whilst the winding is still short-circuited; and immediately 
 afterwards, but yet whilst the winding is short-circuited, a current 
 must be induced in the latter which is in the same direction as the 
 current which will flow through the winding during its movement on 
 the left side. Then there is no sudden change of the current direction, 
 and thus no reason for sparking. We may bring this about by moving 
 the brushes from the middle of the neutral zone a little forwards io 
 the direction of the armature rotation. In this case the influence 
 of the forward pole induces in the short-circuited winding a small 
 E.M.F., but which is just sufficient to destroy the current in the coil. 
 
 If the armature current of a given machine is but small, then 
 we have to displace the brushes a very little from the middle 
 of the neutral zone forwards. If, however, the armature current 
 is a large one, for its sparkless collection a greater influence of the 
 forward pole, and hence a further displacement of the brushes, is 
 required. As a rule, we may note that, with an increasing load on 
 a dynamo the brush-rocker is to be moved forwards in the direction 
 of rotation, and with a decreasing load the rocker is to be moved 
 backwards. 
 
 When the brushes are at the neutral point it will be noticed (see 
 Fig. 127) that the magnetic influence of the armature is as shown 
 by the dotted lines ABC, A'B'C'; the armature thus strengthening 
 the pole tips at A and B' and weakening those at A' and B, since the 
 increase of ampere turns due to increase of density at two of the pole 
 tips is greater than the decrease of ampere turns due to lessening the 
 density at the other two pole tips (which follows from the shape of 
 the saturation curve as shown in Fig. 91). The result of this dis- 
 tortion is to add a necessity for increased ampere turns by the spools 
 of the dynamo. Thus the less the distortion the better the regulation 
 of the machine. The wires between the pole tips have no influence, 
 as the current flows one way in half of them, neutralizing those in the 
 
120 ELECTRICAL ENGINEERING 
 
 other half. (This latter effect is not shown in Fig. 127, since in this 
 figure the brushes have a forward shift.) If, however, the brushes 
 have a forward shift, as in Fig. 127, tho strengthening and weakening 
 of pole tips occurs as just described, but in addition the wires between 
 pole tips now act to actually demagnetize, thus pulling down the volt- 
 age. Thus, shifting the brushes tends to make the dynamo less excel- 
 lent in regulating properties. Looking at Fig. 127, just under the arrow 
 and similarly below, the turns oppose the flow of flux. The reader 
 can check this by remembering the rule of the production of mag- 
 netism from currents. In modern dynamos of say 500 Kw., these 
 cross ampere turns may be 6000 per pole. The gap density at the 
 pole face may be 60,000 per square inch, and the back ampere turns 
 between pole tips due to the load of the brushes may be 1800. The 
 total ampere turns required by the magnetic circuit, including gap, 
 teeth, magnet yoke, armature core, back ampere turns, may be 50,000. 
 In many cases it is of great advantage to employ carbon instead 
 of copper as the material for dynamo-brushes. Since the contact 
 
 FIG. 128. FIG. 129. 
 
 Carbon 
 Brush-h3lder. Sliding Type Carbon-holder. Swivel Type Carbon-holder. 
 
 resistance of carbon on the commutator is comparatively high, the 
 coils of the armature are not directly short-circuited, whilst the bars, 
 connected with the ends of the coil, are simultaneously in contact 
 with the brush. Thus, with a wrong position of the brushes, the 
 current produced in the short-circuited winding cannot become so 
 large as with copper brushes, and therefore the " commutation" is a 
 more gradual one with carbon than with copper brushes. Hence 
 it is no disadvantage with carbon brushes if they are rather wide 
 and touch more than two bars at one time. Several of the machines 
 shown in the illustrations are provided with carbon brushes. Some 
 types of carbon and other brush-holders are shown in Figs. 128-130. 
 
 Carbon brushes cannot, however, be employed in all cases. To 
 prevent the brushes from getting too hot, their number and contact- 
 surface have generally to be larger than with copper brushes; and 
 in many cases it is therefore impossible, with machines originally 
 designed for copper brushes, to afterwards furnish them with carbon 
 
THE CONTINUOUS CURRENT DYNAMO 121 
 
 brushes, because the commutator has usually not the width required 
 for this purpose. Nowadays very many machines are provided from 
 the first with carbon brushes, and these machines are generally less 
 sensitive with regard to changes of load than machines furnished with 
 
 FIG. 10. Copper Gauze Brush-Holder. FIG. 131. Brush-Holder with 
 
 Metal and Carbon Brushes. 
 
 copper brushes. Sometimes a combination of carbon and copper 
 brushes is employed (Fig. 131). 
 
 There are many modern machines not requiring a displacement 
 of brushes at all, and which, nevertheless, run at all loads practically 
 without sparking. The brush-rocker in this case must not be moved, 
 if cnce adjusted properly. Such machines are said to have a fixed 
 lead. 
 
 Methods for changing Direction of Rotation 
 
 We have learned, in the chapter about self -excitation, that we 
 must connect the magnet coils with the armature brushes in a par- 
 ticular way. so that the armature may send a current through the 
 '* magnet coils in such a direction as to strengthen the magnetism. 
 
 Let us imagine a machine excited separately (see Fig. 132). By 
 connecting the upper end, III., of the magnet coil with the positive, 
 rnd the lower end, IV., with the negative pole of the outer source of 
 current, the latter may flow through the coil in the direction shown in 
 the diagram. If now we turn the armature towards the right, then 
 brush I. may, for instance, become a positive, and brush II. a negative 
 one. (Whether that is really the case or not, does not depend on the 
 direction of rotation only, but also on the direction of winding of the 
 armature coils.) In altering the connections of the machine for 
 self-excitation, we have, naturally, to connect magnet terminal III., 
 
122 
 
 ELECTRICAL ENGINEERING 
 
 which was previously connected with the positive battery pole, now 
 with the positive armature brush I.; and terminal IV., in connection 
 
 FIG. 132. Separately excited 
 Machine Clockwise rotation. 
 
 FIG. 133. Shunt Dynamo Clockwise 
 rotation. 
 
 with the negative battery pole before, with the negative armature 
 "brush II., now. If, in addition, we use the necessary regulating 
 resistance, we get the scheme of Fig. 133. 
 
 Now let us reverse the direction of the armature rotation^ 
 
 FIG. 134. Separately ex- FIG. 135. Shunt Dynamo Counter-clockwise 
 cited Dynamo Counter- rotation, 
 
 clockwise rotation. 
 
 Then brush I., which was positive before, becomes now negative 
 
THE CONTINUOUS CURRENT DYNAMO 
 
 123 
 
 and brush II. becomes positive (Fig. 134). If, now, we connect 
 terminal III. with I., and IV. with II., as before, then the machine 
 
 FIG. 136 Series Dynamo 
 Clockwise rotation. 
 
 FIG. 137. Series Dynamo Counter- 
 clockwise rotation. 
 
 loses its magnetism immediately, for the current in the magnets 
 flows in the opposite direction. Hence, if we want the ma- 
 
 FIG. 138. Compound Dynamo FIG. 139. Compound Dynamo 
 Clockwise rotation. Counter-clockwise rotation. 
 
 chine to excite itself, we have to connect terminal III. of the 
 
124 ELECTRICAL ENGINEERING 
 
 magnets with brush II., and terminal IV. with brush I. (see Fie;. 
 135). 
 
 As with shunt machines, so also with series machines, the con- 
 nections must be altered for different directions of rotation of the 
 armature. This will be readily understood without further explana- 
 tion. The diagrams of connection for running a series dynamo 
 clockwise and counter-clockwise respectively are shown in Figs 
 136 and 137. 
 
 With compound machines, both the shunt and the series coil 
 connections have to be altered. Figs. 138 and 139 show the 
 respective schemes. 
 
 Although the left-hand brush is positive in the examples given in 
 Figs. 132-139, it must not be supposed that, with a clockwise 
 rotation, this is always the case. 
 
 Causes of the Non-excitation of Dynamos 
 
 The cause of a dynamo not exciting itself may very often be 
 found in a wrong connection, i.e. in one for the opposite direction 
 of rotation. In this case, the magnet terminals have to be changed, 
 as shown in the previous paragraphs. 
 
 With some types of dynamos, more especially with multipolar 
 machines, we may, instead of changing the magnet connections, 
 get the same effect by moving the brush-rocker. With 4-pole 
 machines we have to remove the brush-rocker one-fourth, with 
 6-pole machines one-sixth of the circumference. With 2-pole 
 machines this displacement of the brushes is not usual, as in this case 
 it would be equal to one-half of the circumference. Imagining the 
 brush-rocker in Fig. 132 removed one half turn, so that brush I., 
 which was before in the neutral zone to the left, comes now in the 
 neutral zone to the right (provided that the cables, forming the 
 connections between the terminals and the brush-holders, are of 
 sufficient length to allow this turning), it is clear that we get exactly 
 the same effect by displacing the brush-rocker as by changing the 
 magnet connections as in Fig. 134. 
 
 There may also be other reasons for a machine failing to excite. 
 In some cases the speed of the dynamo may, for instance, be too 
 small. To be clear about this, let us consider the following case. 
 Suppose that the magnets of a given dynamo had to be excited sepa- 
 rately with 90 volts, in order to get, at the normal speed, an armature 
 voltage of 110. (As we know in the case of self-excitation } the 
 remaining 20 volts would be consumed by the shunt regulating 
 resistance.) If this machine be run, not with its full speed, but 
 only with two-thirds of it, the armature voltage would be equal to 
 
THE CONTINUOUS CURRENT DYNAMO 125 
 
 two-thirds of 110, or about 73 volts. If, now, we switched over the 
 machine, from separate to self-excitation, after a very short time the 
 machine would lose its voltage; for, since the armature voltage is 
 smaller than that required for the proper excitation of the magnets, 
 the strength of magnetism will soon be decreased, and the armature 
 voltage will become still smaller, the smaller voltage will again cause 
 a weakening of the magnetism, and so on. 
 
 Exactly the same may happen at the proper speed of a dynamo, if 
 too large a resistance is connected in series with the shunt coils. Thus, 
 on starting a dynamo, the regulating resistance is short- or nearly 
 short-circuited, and, after observing that the machine has started 
 exciting itself, the resistance is gradually switched in. 
 
 Sometimes the resistance may also be increased by a bad contact 
 between brushes and commutator. This may especially happen with 
 carbon brushes, if they are not made to fit exactly the curved 
 surface of the commutator, thus making contact with the commutator 
 at a few points only. In this case, the contact resistance may be a 
 very considerable one. But this may easily be remedied by grinding 
 the carbon brushes so as to make them fit the surface of the com- 
 mutator, and polishing the latter a little with emery. 
 
 Non-excitation of a dynamo may further happen, if the brushes 
 are not in the neutral zone. If an armature is rotating, it is not 
 always possible to find out the neutral zone of its commutator, for, 
 very often, and especially with drum armatures, the armature wires 
 are not led straight to the commutator, but the connecting wires are 
 displaced by a certain part of the circumference say, for instance, J, J, 
 and so on. To be convinced that we have the proper position of the 
 brushes, we have, therefore, to examine with which commutator-bar 
 the wires in the neutral zone are connected. Non-excitation may 
 also be accounted for by the loss of the residual magnetism. This 
 may sometimes happen, if the polarity of the machine has been 
 changed by any accident. In such a case sufficient magnetism may 
 be regained by sending a current from a galvanic battery through 
 the magnet coils. 
 
 Finally, if there is any fault with the connections, or any 
 disconnection, either in the magnet or in the armature coils, 
 excitation will be prevented. There might, for instance, be con- 
 sequent magnet coils connected so as to give poles of the same name 
 side by side, instead of different poles alternating. By carefully 
 following the beginnings and the ends of the coils, such a fault may 
 easily be found out. If there is any source of current available, this 
 test may easily be made by sending a current through the coils, and 
 examining, by means of a magnetic needle, whether the neighbouring 
 poles are or are not of the same polarity. 
 
 Breaks in a circuit may sometimes be evident to the eye, but in 
 other cases can only be found out by the electric current. If we 
 
126 ELECTRICAL ENGINEERING 
 
 suppose a magnet coil to have a disconnection, then, by connecting 
 
 FIG. 140. 
 
 the magnet terminals with the two ends of a source of current, 
 no current will flow through this circuit (see Fig. 140). If now we 
 
 connect one pole of a glow lamp, the 
 terminals of which terminate in two 
 long wires (see Fig. 141) , with say the 
 positive pole of the source of current, 
 and touch with the other pole of the 
 lamp the terminals of the magnet 
 coils successively, we shall observe 
 the following: On touching the mag- 
 net terminal a, the lamp will glow, 
 as there is a connection between 
 terminal a and the second pole of 
 the source of current. The same will 
 be the case if we connect the terminals 
 b, c, d, e, respectively, as they are all 
 in connection with the negative pole. 
 But if, further, we come to /, the 
 lamp will no longer glow, showing that there is a disconnection 
 between e and /. For / is not in connection with the positive, but 
 
 FIG. 141. Testing Lamp. 
 
THE CONTINUOUS CURRENT DYNAMO 127 
 
 only with the negative pole, and, if we connect both terminals of a 
 lamp with the same pole, it can, of course, not glow. 
 
 A lamp of suitable voltage should be used for the test. When 
 the voltage is high we can employ several lamps connected in series, 
 instead of one lamp. 
 
 With machines that are not too small, and with a suitable voltage 
 of the outer source of current, a lamp serves as an excellent means 
 for finding out disconnections of coils. Instead of a lamp, we can 
 with greater certainty use a voltmeter. As long as the two ends of 
 the voltmeter are on different sides of the point of disconnection, 
 a deflection of the needle can be observed. The latter indicates the 
 full voltage. But as soon as the point of disconnection is passed 
 over, so that both terminals of the voltmeter are connected with one 
 side of the point of disconnection only, the voltmeter needle stops 
 at zero. 
 
 The magneto used for insulation tests (p. 73) may also be 
 employed for finding out the point of disconnection. 
 
 Automatic Shunt Regulator 
 
 A compound winding can keep constant the voltage of a dynamo, 
 provided that the latter is running with a constant speed. As some- 
 times the speed of the driving engine such as, for instance, the 
 steam-engine varies according to the variation of the load, and, 
 further, since the compound winding is not useful for all machines, 
 automatic shunt regulators are employed whenever a constant, 
 pressure is required, without having a man always present to alter 
 the shunt regulator as necessary. A simple construction for such 
 an apparatus is shown in Fig. 142. The wire spirals, forming the 
 regulating resistance, are arranged on an iron frame and supported 
 by porcelain insulators in the usual way. The wires coming from 
 the ends of the single spirals are not connected with the usual, 
 circularly arranged contact pieces, but are fixed vertically side by 
 side, and cut to different lengths. These wires dip into a movable 
 glass vessel, filled with mercury, and fixed on the top of an iron core. 
 The latter is suspended on one arm of a lever, and kept in balance 
 by a weight fixed on the other arm of the lever. With its lower 
 end the iron core dips into a fixed coil, consisting of very fine wire, 
 the ends of which are switched on to the full dynamo voltage. If 
 the dynamo pressure falls (for instance, through slower running of the 
 dynamo), the current flowing through the coil decreases, the iron core 
 
128 
 
 ELECTRICAL ENGINEERING 
 
 is therefore less attracted than before, and the counterweight is able 
 to lift the iron core a little, so that the glass vessel, and with it the 
 level of the mercury, is raised, touching some of the graduated wires, 
 and the respective resistance coils are short-circuited by the mercury. 
 Hence less resistance is now connected in series with the shunt, and 
 the voltage of the machine can rise to its proper value. 
 
 If, on the other hand, the voltage of the machine grows too great, 
 
 FIG. 142. Automatic Shunt Regulator (Voigt & Hdffner). 
 
 the current flowing through the coil will also become greater. The 
 iron core will then be pulled down a little, and the vessel with 
 the mercury is lowered, causing resistance to be again included in 
 the shunt circuit, the shunt current therefore decreases, causing the 
 dynamo voltage to be brought to its normal value. 
 
 There are, besides the apparatus described, many other ingenious 
 constructions of automatic shunt regulators, but which we cannot 
 deal with here. 
 
THE CONTINUOUS CURRENT DYNAMO 129 
 
 Efficiency of Dynamos 
 
 With every kind of work we have also to do things that are 
 -useless, in order to get the intended effect. For example, to convey 
 people or goods, it is also necessary to convey the carriage which is 
 employed for the conveyance. The work which has to be spent in con- 
 veying the carriage represents, in this case, in which the main pur- 
 pose is the conveyance of the people and goods only, a loss of mechani- 
 cal energy. A stove, only intended to warm the air of a room, also 
 heats a great quantity of air which does not remain in the room, but 
 escapes up the chimney, hence causing a loss. 
 
 In like manner there are losses in the transformation of mechani- 
 cal into electrical energy by means of a dynamo. These losses may 
 be classified as follows: 
 
 Firstly, work spent for excitation of the magnets. 
 
 Secondly, losses due to the resistance of the armature-winding. 
 
 Thirdly, due to eddy currents and to the varying magnetism of 
 the armature core. 
 
 And Fourthly, losses due to mechanical friction. 
 
 The current which has to be sent through the magnet coils of a 
 dynamo, in order to excite the magnets, is not available in the outer 
 circuit. If with a machine, giving 100 amps, at a voltage of 110, the 
 shunt current were 3 amps., then the loss due to excitation would be 
 equal to 330 watts, or 3 per cent, of the total dynamo output. 
 
 A further loss depends on the ohmic resistance of the armature, 
 including the resistance of the brushes and the connecting cables. 
 If, in our example, this resistance were 0.02o>, then the voltage drop 
 would be 0.02X100=2 volts, and the loss 2x100=200 watts that 
 is, nearly 2 per cent, of the total output. 
 
 There are, further, as we know, eddy currents in the armature. 
 By employing very thin iron discs, these eddy currents may be re- 
 duced to a very small value, so that the loss depending on them may 
 be but 1 per cent., or even less. 
 
 The continual reversal of the magnetism of the armature iron 
 also involves a certain amount of work. As we know, the molec- 
 ular magnets of the iron are not absolutely freely movable, but a 
 kind of friction has to be overcome to turn the molecular magnets 
 in the direction of the lines of force. For overcoming this resistance, 
 however, a certain amount of work is required, which causes like 
 all other losses a heating of the machine. This is generally called 
 the hysteresis loss. 
 
 Finally, there is to be considered the mechanical loss due to 
 friction in the bearings of a dynamo. These losses are, however, 
 not great, for dynamo bearings are generally very well oiled. 
 
130 ELECTRICAL ENGINEERING 
 
 Again, the air offers a certain resistance to the rapid rotation of the 
 armature, thus involving a further small loss. 
 
 The total amount of all these losses is not considerable. 
 With large dynamos it is equal to about 4 to 6 per cent., with dynamos 
 of medium size 10 to 15 per cent., and with small ones up to 30 per 
 cent, of the total output. Thus we get for mechanical power, sup- 
 plied to the dynamo, and corresponding to 100 watts, according 
 to the size and excellence of the machine, 96, 90, 85, 70 watts, re- 
 spectively, or the efficiency of the dynamo is 96, 90, 85, 70 per 
 cent. 
 
 Perhaps no other machine, employed for transforming one kind of 
 energy into another, is so efficient as a good dynamo. 
 
 If all the losses mentioned above = L, and the output of the dynamo 
 
 W 
 
 = W, then the efficiency =-KT . j This is usually called the com- 
 mercial efficiency. 
 
 To measure the efficiency of a dynamo, the losses are required. 
 They are, first, loss in the field windings. If R 8 = resistance of series 
 field, and RA= resistance of shunt field, the loss in them can be cal- 
 culated, since the current flowing in them is known. The loss equals 
 I 2 R, when I is the current and R the resistance. 
 
 If R a = resistance of armature and R& the resistance of brush con- 
 tact on the commutator, the loss in copper of the armature and in 
 contact brushes can be similarly calculated. The resistance of brush 
 contact with carbon brushes is about .028 ohm per square inch. 
 With copper brushes this resistance is about .003 ohm per square 
 inch. The density of current with carbon brushes is usually from 
 30 to -40 amperes per square inch; with copper brushes about 150 
 amperes per square inch. 
 
 There remains to be found the friction and hysteresis loss. A 
 convenient method to obtain this is to run the dynamo free until all 
 friction becomes uniform. Put upon the armature a voltage equal 
 to the operating voltage E at the terminals of the machine plus the 
 voltage drop in the armature when operating under full load. This 
 is equal to E + IR a . The dynamo armature should then be run at 
 full speed as a motor by adjusting the field current. Under these 
 conditions, since so little current would be flowing into the armature, 
 the back E.M.F. would be equal to the applied. But this is equal 
 to E + IRa, which is the voltage generated by the machine when 
 operating as a dynamo at full load. And since the speed is sot by 
 adjusting the field current till normal dynamo speed is obtained, the 
 flux must be the same as when the machine is operating as a dynamo. 
 And thus the hysteresis loss is the same as when running as a dynamo. 
 But the input measured by reading the current taken, I t , multiplied 
 by E+IR-o applied, measures all losses. Subtracting from this in- 
 
THE CONTINUOUS CURRENT DYNAMO 131 
 
 put the small amount of armature resistance loss, or Ii 2 R, created by 
 the " running light" current just mentioned (Ij equals a very small 
 percentage of the full load armature current), leaves the hysteresis 
 plus friction desired, and thus all the losses are determined. This- 
 is the stray power method first used by Dr. Hopkinson of England. 
 This same method of getting efficiency can be applied to a motor, but 
 in this case the voltage to apply to the commutator is E IR a , for 
 a motor when running under load creates a back E.M.F, of this amount, 
 
 ._, flux X external wires X revolutions 
 
 since E.M.F.- 100,000,000 ~ as haS been pre ' 
 
 viously shown. 
 
 If the same E.M.F. and revolutions are set, then the flux must 
 come the same, and if the flux and speed are the same, the hysteresis 
 must be the same. 
 
 Another method of measuring the iron loss or core loss of a dynamo, 
 as well as the friction, is to belt to the dynamo whose core loss is 
 desired, a motor of such a size as to be capable of handling the load 
 conveniently (say a motor of 10 per cent, of the size of the dynamo). 
 Separately excite the field of the motor and keep it constant through- 
 out the test. Thus the driving motor iron losses will remain constant 
 as far as the field is concerned. Apply enough voltage to the motor 
 armature to run the dynamo armature at normal speed, and measure 
 the input to the motor by multiplying the current taken by the motor 
 armature by the voltage applied. Repeat this reading with the normal 
 voltage in the dynamo whose core loss is being measured. Then 
 the difference between these two readings gives the loss due to putting 
 on the field and therefore obtaining the normal voltage of the dynamo. 
 To obtain the friction of the dynamo, subtract from the input of the 
 driving motor, when running the dynamo without field current, the 
 input of the driving motor with the belt removed (which thus records 
 losses of the driving motor itself) , and the remainder gives the fric- 
 tion of the dynamo. Knowing, therefore, the friction and the core 
 loss, and adding to the other losses, the efficiency is calculated as 
 before. 
 
 In reading the input to the motor, its speed, and that of the motor 
 having core loss taken, is kept constant by means of a tachometer 
 fastened to its shaft or to the dynamo shaft. Also it is proper for 
 a refinement to take out from each input reading to the motor its 
 own PR of armature and brush contact, since this value varies with 
 the different inputs, and, being the only variable, its subtraction 
 makes the remainder one of pure input transferred, barring the varying 
 loss in the bearings due to the varying belt pull, which may be neglected. 
 This last method is a very common method of measuring the core 
 losses of dynamo machinery. In taking the curve, the voltage on the 
 
132 ELECTKICAL ENGINEERING 
 
 driving motor should be approximately constant throughout the 
 test. If it is not, a slipping of the belt must be occurring, or the 
 internal drop of armature or brush contact resistance must be ex- 
 cessive. A driving motor fitted with copper brushes to reduce drop 
 in brush contact and only loaded under maximum conditions of core 
 loss on the dynamo to one-half load gives the best results. 
 
CHAPTER IV 
 THE ELECTRIC MOTOR 
 
 IF we send a current through the armature of a dynamo, whose 
 magnetic field is excited, the armature will be put into motion. 
 This will be at once expected from our study of the action of the 
 Deprez ammeter. With the dynamo armature there will, however, 
 take place not only a single movement, but a permanent rotation. 
 Owing to the action of the commutator, the current flows through 
 all wires on one-half of the armature, which are under the influence 
 of the north pole, in one direction, and through all wires which are 
 under the influence of the south pole, in the opposite direction; hence, 
 as long as a current from an outer source is sent through it, the arma- 
 ture will rotate. The machine now absorbs electrical and supplies 
 mechanical energy. In this case the machine is called an electric 
 motor or an electro-motor, which we may speak of simply as a 
 motor; whereas a machine by producing current is called a dynamo 
 or a generator. 
 
 The direction of rotation of the armature in Fig. 143 may easily 
 be determined by Ampere's Rule. The armature will rotate counter- 
 clockwise. 
 
 The scheme of the motor armature (Fig. 143) is strictly in accord- 
 ance with that of the dynamo armature (Fig. 73). In both cases 
 the pole to the left is a north pole, and the current in the left half 
 of the armature is directed from the spectator. We had to turn 
 the dynamo armature towards the right, in order to get a current in 
 the direction marked ; the motor armature will run towards the left, 
 if a current having the same direction flows through it. 
 
 We have seen, when considering the current direction in a dy- 
 namo armature, that in each armature conductor a current is pro- 
 duced which would turn the armature in an opposite direction, if 
 there did not exist any other force. In this case the induced cur- 
 rents produce an internal force in opposition to the external driving 
 force supplied to the armature. 
 
 With the motor we find the same action, but with a remarkable 
 difference. We know that in each wire, rotating in a magnetic field, 
 an E.M.F. is induced. With the electric motor we have an armature 
 
 133 
 
134 
 
 ELECTRICAL ENGINEERING 
 
 which rotates in a strong magnetic field. Naturally it does not 
 make any difference whether this rotation is effected by an electric 
 current, or by an outside driving force. In each wire on rotation an 
 E.M.F. is induced. To determine the direction of this E.M.F. we 
 have simply to compare this scheme with that in Fig. 73, in which 
 
 FIG. 143. Motor Ring Armature. 
 
 we had the same armature rotating towards the right. The direc- 
 tion of the induced E.M.F. was there marked by dots and crosses. 
 The lower brush was positive, the upper one negative. But now 
 the armature is rotating in the opposite direction, hence the direc- 
 tion of the current in the armature is reversed, the upper brush 
 becoming positive, the lower one negative; and we see, therefore, 
 that the E.M.F. produced by the rotation of the armature acts against 
 the current sent from the source of current into the armature. The 
 result is that the E.M.F. produced by the rotation would, if no other 
 E.M.F. existed in the circuit, cause a current to flow in a direction 
 which is opposite to that of the current sent into the armature by 
 the outer source. The E.M.F. produced by the armature of a run- 
 ning electric motor is therefore called the back electromotive 
 force or counter-electromotive force of the motor. It follows the 
 same law, of course, as the dynamo E.M.F. and therefore equals 
 
 ^ -, as has been shown. It causes the current flowing through 
 100,000,000 
 
 the armature to be far smaller than we should calculate it to be by 
 dividing the terminal voltage by the resistance of the armature. 
 
THE ELECTRIC MOTOR 135 
 
 If, for instance, we connected a stationary armature, having 
 a resistance of yf^w, suddenly with 110 volts, then, through the 
 armature, according to Ohm's Law, a current of i.H = 3666 amps, 
 would flow. This excessive current would instantly destroy the 
 armature, and melt both the brushes and the mains. If, however, 
 we do not connect the armature immediately with its full voltage, 
 but first interpose in series with it a resistance of about 5aj, then 
 a current of a little more than 20 amps, will flow through the 
 armature and the resistance. The armature then starts to rotate, 
 and produces by its rotation in the magnetic field a back electro- 
 motive force, which soon reduces the current to a smaller value. 
 The series resistance may now be reduced. The motor will then run 
 faster, its back electro-motive force will grow, and, if we gradually 
 short-circuit the series resistance, the motor will reach its full 
 speed. 
 
 A simple consideration will show us what this speed must be. 
 Obviously the motor will never run so fast as to produce a back 
 E.M.F., equal to the E.M.F. of the source of current, since in this 
 case no current would flow through the armature, and it would not 
 exert rotary power. But a certain amount of power is required 
 although it may be quite small for overcoming the friction in the 
 bearings and the resistance due to the air. Thus the current can 
 never become actually nothing, but must, for instance, with a motor 
 which is designed for 100 amps., be at no load about 3 to 5 amps. 
 If the outer E.M.F. or terminal voltage be 110, the back E.M.F. 
 will not be quite 110 volts, but at no load nearly as much, viz. 
 only some tenths of a volt less than 110. 
 
 If now we load the motor, for instance by putting a brake on, 
 or by making it drive a shaft by means of a belt, the small current 
 going through the armature at no load cannot exert sufficient power 
 to overcome the load. Thus the motor speed will decrease a little. 
 But as soon as the motor is running a little slower, say with 990 
 instead of 1000 revolutions, its back E.M.F. will decrease in the 
 same proportion. The balance of the outer above the inner voltage is 
 therefore greater, and the armature current can now grow to such an 
 extent as to produce sufficient rotary power to counterbalance the 
 load. The back E.M.F. will be in this case about 109 volts. If the 
 load be doubled, the motor will run still slower, until its back E.M.F. 
 falls to about 108 volts, the remaining difference of about 2 volts 
 sending a current double in strength through the armature. If the 
 load be removed, the motor will again run faster until its back E.M.F. 
 becomes nearly 110 volts. 
 
 We may, then, conclude that an electric motor regulates in a 
 perfect manner the absorption of electrical power according to the 
 work to be done. With steam engines, turbines, etc., the steam or 
 water supplied has to be regulated according to the load by means 
 
136 
 
 ELECTRICAL ENGINEERING 
 
 of complicated governors. The electric motor, on the other hand, is 
 self-governing. 
 
 The larger the armature resistance of a motor, the greater 
 must, for a definite load, be the difference between the terminal 
 voltage and back E.M.F., in order to get the necessary current to flow 
 through the armature, and the greater, therefore, must be the drop of 
 speed. 
 
 The Shunt Motor 
 
 In the above reasoning we have presumed that the magnetic field 
 of the motor is of constant strength. This may be effected by con- 
 necting the magnet coils directly to the outer source of pressure. To 
 the current two ways are then offered; one through the armature, 
 and the other through the magnet coils. The latter are in shunt with 
 the armature. This motor is called a shunt motor. About the 
 working of such a motor we have spoken already. With regard to 
 the speed of a shunt motor, we have just learned that the speed 
 decreases with increasing load. This fall of speed is, at a constant 
 voltage, small. It varies according to the type of motor, being 
 from T V to 5 per cent., unless the motors are small, when the variation 
 may be much greater. Practically speaking, the speed of a com- 
 mercial shunt motor may be considered as nearly constant with 
 varying loads. 
 
 It is most important to learn how a shunt motor should be started. 
 To get a proper start, the magnetic field has to be fully excited. 
 
 It is, therefore, necessary to switch 
 the magnet coils immediately on to 
 the voltage of supply, whereas, as 
 we have seen, with the armature a 
 resistance must be connected in 
 series at starting. To get both 
 connections simultaneously, starters 
 for shunt motors are constructed 
 as shown diagrammatically in Fig. 
 144. The centre of the starting 
 lever is connected with one main. 
 The lever slides over a row of 
 contacts, (which are connected with 
 the ends of the resistance spirals,) 
 and a slip-ring. The latter is con- 
 nected with one end of the magnet 
 winding, the last of the contact-pieces (on the left) with one 
 armature brush. The other brush and the other end of the magnet 
 ^winding are both connected with the second main. 
 
 FIG. 144. Shunt Motor with 
 Starting Resistance. 
 
THE ELECTRIC MOTOR 137 
 
 As long as the lever is in its extreme position to the right the 
 motor is at rest. Neither the slip-ring nor the contact-pieces, which 
 are in contact with the resistance spirals, are touched by the lever, so 
 that the motor is in connection with one (the negative) main only, 
 and, naturally, no current flows through it. In moving the lever a 
 little towards the left, it makes contact both with the slip-ring and 
 the first resistance contact. As the slip-ring is connected with one 
 shunt terminal, the magnet winding is immediately switched on the 
 full voltage, and the full magnetizing current flows. If, for instance, 
 the resistance of the shunt winding were 55w, then, at a voltage of 
 110, the shunt current would be 2 amps. Although the magnets in 
 this arrangement are fully excited, the armature is still in series with 
 the whole of the starting resistance, which may be about 5cu. Through 
 the armature a current of about 20 amps, therefore flows. It will 
 start to rotate, and, gradually the lever is moved to its extreme 
 position to the left, when finally the armature is switched on to the 
 full voltage of 110. During the whole time of starting the motor, 
 the magnets are fully excited. 
 
 Speed Regulation 
 
 The speed of a motor depends on the voltage of supply and the 
 strength of its magnetic field. As we have learned, the motor always 
 attempts to rotate so fast as to produce a back E.M.F. nearly equal 
 to the terminal voltage. Hence, by doubling the latter, the motor 
 will run with nearly double the speed. By decreasing the terminal 
 voltage, we decrease the speed of the motor. 
 
 A reduction of speed may therefore be effected by switching per- 
 manently a resistance in series with the motor, since, in this case, the 
 armature voltage will no longer be equal to the voltage of the outer 
 circuit. The series resistance will consume a definite part of the 
 voltage. If, for instance, the motor speed were 1000 revolutions at a 
 voltage of 110, then, if we switch a resistance of leu in series with 
 the armature, the terminal voltage, and with that the speed of the 
 motor, will vary according to the load, or what amounts to the 
 same thing according to the current strength required for over- 
 coming this load. If the armature current were 11 amps., in the 
 series resistance of leu a voltage of 11, i.e. the tenth part of the total 
 voltage, would be consumed. The motor w r ill therefore make 900 
 instead of 1000 revolutions per minute. If, due to an increasing 
 load, the armature current grows to 22 amps., then in the series 
 resistance 22 volts will be consumed. The motor speed will fall 
 
138 
 
 ELECTRICAL ENGINEERING 
 
 9y Shunt Regulator 
 
 down to about 800 revolutions per minute. At a current of 55 amps, 
 the speed will be equal to about one-half of the normal speed. Thus 
 we can regulate the speed of a motor by means of a series resistance 
 when it is required to run below the normal speed. 
 
 Another way to regulate the speed is to vary the shunt current. 
 If in the magnet circuit we arrange a shunt regulating resistance 
 
 (see Fig. 145), as we have done 
 with the dynamo, we may, by 
 switching in some resistance, 
 weaken the shunt current. Let 
 us now start the motor and 
 short-circuit the starter. To 
 produce in the weakened field 
 the same back E.M.F. as be- 
 fore, the motor has, naturally, 
 to run much faster. Thus, by 
 switching in some resistance in 
 the shunt circuit, the motor 
 speed may be increased above 
 its normal value say, for in- 
 stance, from 1000 to 1100, 
 1200, and even 1400 revolutions 
 per minute. 
 
 Care must, of course, be 
 
 FIG. 145. Shunt Motor with Starting taken, not to disconnect the 
 Resistance and Shunt Regulator for shunt circuit entirely, whilst 
 Speed Regulation. the armature is still in circuit. 
 
 In such a case the strength of 
 
 the magnetic field would be practically nil, since there would 
 only be the weak residual magnetism. Two things may then 
 happen. Either the motor reaches a dangerously high speed, in 
 order to produce a sufficient back E.M.F., with the weak magnetic 
 field, and in this case the excessive speed may cause the belt- 
 pulley, the commutator, or the armature winding to burst into 
 pieces; or, the motor is prevented from reaching such a high speed 
 by a heavy load, then it can produce but a small back E.M.F., and 
 consequently a current of so great a magnitude will flow through the 
 armature as to destroy the latter and melt the brushes, or, what 
 would be more desirable, to cause the fuses to go. 
 
 To prevent accidents of this kind, the shunt regulators for motors 
 are generally made so as to render a disconnection of the shunt circuit 
 impossible. The latter can then be switched off simultaneously with 
 the armature -circuit, by means of the starting lever, but in no other 
 way. 
 
 Shunt motors are usually used for power in factories, workshops, 
 etc., where constant speed under varying load is desired. 
 
THE ELECTRIC MOTOR 
 
 139 
 
 FIG. 146. Scries Motor with 
 Starting Resistance. 
 
 The Series Motor 
 
 Instead of exciting the magnetic field of a motor by many wind- 
 ings, which are switched directly on to the terminal voltage, this 
 may, similarly to the series wind- 
 ing of a dynamo, also be done by 
 providing the magnet coils of the 
 motor with comparatively few 
 turns of thick wire, connected in 
 series with the motor armature. 
 These motors are called series 
 motors. A diagram of connections 
 for a series motor, and the starter 
 belonging to it, is shown in Fig. 
 146. As can be seen from the 
 diagram, the starter is simpler than 
 that of a shunt motor, on account 
 of the omission of the shunt slip- 
 ring. 
 
 The properties of the series 
 motor are of quite another kind to 
 
 those of the shunt motor. With the latter we have a magnetic field 
 of constant strength, and the speed of the motor is practically con- 
 stant at varying loads. With the series motor the field is stronger 
 the larger the armature current of the motor, since the latter flows 
 through the magnet coils as well. If the motor is loaded but little, 
 and thus the armature current small, the magnetic field will be 
 weak. If now the motor is switched on to a constant voltage, 
 such as, for instance, the mains of a lighting plant, then it must 
 run with a very high speed, to produce in the weak magnetic field 
 a back E.M.F. corresponding to the outer voltage. If, on the other 
 hand, the motor is loaded very heavily, its magnetic field will also 
 be a very strong one. Thus the speed at which the motor pro- 
 duces a back E.M.F., corresponding to the outer voltage, will be 
 far lower than before. A series motor must never run light or 
 without load, for in this case its field would be very weak, 
 so that it would run with a dangerous speed, or, as it is called, 
 would " run away/' almost like a shunt motor the shunt circuit 
 of which is disconnected. Hence series motors are never em- 
 ployed where the load may be entirely removed. For driving by 
 means of belts, for instance, series motors are generally not em- 
 ployed, because a sudden release of load may cause the belt to be 
 ruptured or thrown off the pulley. On the other hand, they are 
 more frequently used for driving pumps, fans, and so on, by means 
 
140 ELECTRICAL ENGINEERING 
 
 of couplings, or for driving any machines by gearing. The latter 
 itself provides a certain load on account of its frictional resistance in 
 the toothed wheels and bearings. Very small motors may, even with 
 belts, be built as series motors, as their comparatively large frictional 
 resistance in the bearings represents in any case a certain, although 
 small, load, allowing the motor to reach a rather high, but not a 
 dangerous speed. 
 
 Series motors are for two reasons employed in some cases with 
 great advantage. A single line only proceeds from the starter to the 
 motor, so that, together with the direct return wire, two mains only 
 are required, whereas with the shunt motor there are two lines from 
 the starter to the motor, which, with the return wire, necessitates 
 the use of three mains. This offers an advantage and a saving of 
 cables when the distance between motor and starter is great. This 
 may, for instance, happen with a motor, coupled directly to a fan, 
 which is fixed on the ceiling of a very high room, and has to be con- 
 trolled by a starter, fixed below. Since the load of such fan motors 
 is constant, the speed of the series motor will also remain con- 
 stant. 
 
 As we know, the magnetic field of a series motor is stronger the 
 heavier its load. This makes it suitable for many special appli- 
 cations, such as lifting weights by means of cranes. A small 
 weight is more quickly lifted than a heavy one. If a series motor 
 is started under full load, it wants less current than a shunt motor 
 of the same size. For let us presume the magnets of a series motor 
 to be wound so as to produce, with a current of 25 amps., a mag- 
 netic field equal in strength to that of a corresponding shunt motor. 
 If, further, we assume that the motors have to start under a very 
 heavy load, so that the starting current grows to 50 amps., then it 
 is clear that the field of the series motor will increase as well, although 
 not to a double value. Naturally the armature of the series motor, 
 running now in a stronger magnetic field, is, with twice the current, 
 capable of developing more than double " torque." Thus the series 
 motor has a greater starting power than the shunt motor, since the 
 magnetic field of the latter remains constant at all loads, and its 
 armature can, therefore, with twice the current, overcome only twice 
 the load. 
 
 Hence the series motor will be able to overcome any given over- 
 load with a little less consumption of current than the shunt motor, 
 but will run a little slower than the latter, and, on starting, the series 
 motor will, with a given current, come sooner to its full speed than 
 the shunt motor. 
 
 For electrically driven cranes, as well as for electric railways and 
 motor cars, series motors are employed with great advantage. The 
 starting of an electric car can be effected more quickly with a series 
 than with any other motor. On gradients the car is running slower 
 
THE ELECTRIC MOTOR 
 
 141 
 
 Motor 
 
 and does not require so much current as one equipped with a shunt 
 motor, whereas on the level the series motor enables the car to run 
 with a far higher speed. 
 
 In all our discussions we have hitherto assumed that the series 
 motor is supplied with a constant voltage. If we want it to run 
 with a nearly constant speed at varying loads, we have to switch 
 the motor on a low voltage if it is loaded but little, and on a higher 
 voltage if it is loaded to a greater extent. This may be effected by 
 a series resistance, because with a small load we could switch in much, 
 and at a greater load less resistance. 
 
 This voltage regulation may be rendered quite automatic by 
 employing a series dynamo as source of current for the motor, an 
 
 arrangement which is 
 
 sometimes made for 
 power transmission to 
 long distances (see Fig. 
 147). The mains lead 
 in this case from the 
 dynamo to a single 
 motor only. A starter 
 is not required between 
 the t\\o machines, but 
 the starting of the 
 motor is done in the 
 following way : The 
 dynamo is run by the 
 steam - engine or the 
 turbine coupled to it, the motor being stationary and the circuit 
 closed, the resistance of the mains and of the motor alone being in 
 the external circuit of the dynamo. Since these resistances are 
 comparatively small, even when the dynamo runs slowly a large 
 current will flow through the circuit. This strong current in the 
 armature and the field of the motor will cause it to start, thus pro- 
 ducing a certain back E.M.F. The faster the dynamo runs the faster 
 the motor runs, and the speeds will always be in the same ratio, 
 for, since the same current is flowing through both dynamo and 
 motor, their magnetic fields are always of equal strength. Owing 
 to the loss of volts in the mains and the machines themselves, 
 the back E.M.F. of the motor will always be a little smaller 
 than the E.M.F. of the dynamo. Hence, if the machines are ap- 
 solutely alike, the motor will always run a little slower than the 
 dynamo. 
 
 When the load is small the motor only takes a small current, the 
 dynamo, through the coils of which this small current is also flow- 
 ing hence producing a small E.M.F. At an increased load the cur- 
 rent, and with it the voltage of the dynamo, increases. The speed 
 
 FIG. 147. Series Method of Power Transmission. 
 
142 
 
 ELECTRICAL ENGINEERING 
 
 of the motor is but little altered in this case, since it remains in a 
 nearly constant ratio to the dynamo speed. Thus, if the dynamo is 
 driven at a constant speed, that of the motor will, even at varying 
 loads, remain practically constant. 
 
 General Electric Co., 
 Engineering Depty, 
 
 \ i 
 
 1 \ 
 \ i 1 
 
 8 8 i 
 I 5 
 
 1400 
 1300 
 48 1200 
 44 1100 
 100 40 1000 
 90 36 900 
 60 32 800 
 70 28 700 
 60 24 600 
 50 20 500 
 40 16 400 
 30 12 300 
 20 6 200 
 10 4 100 
 00 
 
 c 
 
 9 may f904, 
 
 Ra Iway motor 
 GE-57-A-3 Chancter,sticno.2J( 
 
 50 H P. output at 93 Amp. input. 
 Volts at motor terminals 500. 
 Diameter of car wheel 33*. 
 Armature 3 turns, Field Spools 1 10 turns. , 
 Pinion 28, Gear 57, Ratio 2.04. 
 
 1 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 GE-57-A-3 
 
 20 40 60 60 100 120 40 160 160 
 Amperes 
 
 FIG. 148. Speed and Torque Curves, Series Motor. 
 
 The speed and torque curve of a series railway motor is shown in 
 Fig. 148, from which can be seen the variation of these factors with 
 amperes taken by the motor. Fig. 148 also shows the speed curve 
 of a shunt motor. 
 
THE ELECTRIC MOTOR 
 
 143 
 
 The Compound Motor 
 
 A compound winding may be used on motors for many different 
 purposes. If the current flows in the same direction through both 
 windings, then the effect of the series coil strengthens that of the 
 shunt coil. This strengthening is greater the larger the armature 
 current, i.e. the heavier the motor load. Thus the motor gets at 
 increasing load a stronger magnetic field, and will, therefore, if the 
 voltage remains constant, run slower than before. We hence 
 infer that, for a given current, the starting power of a compound 
 
 FIG. 149. Compound Motor started from a distant point. 
 
 motor will be greater than that of a shunt motor. With a decreas- 
 ing load the motor will run faster. A "running away," however, 
 cannot occur, because, even if the load be taken off entirely, the shunt 
 coil produces a magnetic field of sufficient strength. The compound 
 motor has, therefore, to a certain extent, the merits of the series 
 motor without its disadvantages. 
 
 By means of compound motors the starting at a distance with only 
 two mains may be effected, just as in the case of the series motor. 
 In Fig. 149 a diagram for such a connection is shown. If we imagine 
 
144 ELECTRICAL ENGINEERING 
 
 the motor without the shunt coil, then it is connected up exactly as 
 the series motor in Fig. 146. The current coming from the starter 
 enters the series coil in VI., flows through the series coil and leaves 
 it at V., flowing from there to the armature brush II., through the 
 armature to brush I., and from there through the second main back 
 to the generator. The shunt winding is connected directly with the 
 armature brushes I. and II., and gets at starting, therefore, a very 
 small voltage only, hence its field is nearly ineffective. But on account 
 of its series winding, the motor starts as a series motor. Obviously 
 such a motor will not develop a very large starting power, like a real 
 series motor, for, on account of the large space occupied by the shunt 
 coils, there is less space available for the series coils than with a series 
 motor. A compound motor may, however, even with this arrange- 
 ment, be easily got into motion, provided that the load on starting is 
 not too heavy. When once running the armature will produce a 
 back E.M.F., and the shunt coil will be supplied with nearly the full 
 terminal voltage. 
 
 This arrangement for starting at a distance may be employed in 
 cases in which the motor is not coupled directly to a pump, fan, 
 etc., but is driving the latter by means of a belt. Even if the belt 
 slips off the motor the latter cannot run away, as would be the case 
 with a series motor. 
 
 Sometimes it is wished to produce another effect with the com- 
 pound winding. As we know, the speed of a shunt motor does 
 not remain absolutely constant at all loads. Generally it decreases 
 a little at an increasing load. Now there are some cases in 
 which an absolutely constant speed is required, such as, for instance, 
 when driving spinning machines. This may be got by winding 
 over the shunt coil a series coil, consisting of a few windings only, 
 and which act in an opposite direction. The result is that, as the load 
 becomes heavier, the field of the motor is weakened, and the armature 
 runs faster. Since now, on the other hand, the motor would run slower 
 at an increasing load if it were a shunt motor only, this fall of the 
 speed is compensated by the action of the series winding. Thus a 
 compound winding is capable of giving a constant speed at all loads. 
 
 This statement is not absolutely true. There is a further 
 reason for the variation of speed, which cannot be compensated 
 by the series coil, namely, the gradually rising temperature of the 
 motor. The resistance of the shunt coils is greater when hot than 
 when cold, and if the coils are switched on to a constant voltage, 
 a larger current will flow through them if they are cold than if 
 they are hot. After the motor has been running for some time, 
 its magnetism will gradally become a little weaker. We may 
 therefore observe, with shunt motors, that the speed of the motor is 
 smaller immediately after starting, but grows gradually with the rise 
 of temperature. This increase of speed lasts only for a short time. 
 
THE ELECTRIC MOTOR 
 
 145 
 
 After some hours running, the motor does not get hotter, since it 
 gives the heat produced in it to the surrounding air. After the 
 motor has reached this state, its speed remains constant, providing 
 that there has been no change in the voltage. 
 
 This influence of the temperature may be done away with, by 
 connecting up in the shunt circuit of the motor a small regulating 
 resistance, as shown in Fig. 145. Before starting, when the resistance 
 of the coils is lower, some resistance is switched in the shunt circuit, 
 and, as the coils heat up and increase in resistance, the auxiliary 
 resistance is gradually short-circuited. 
 
 It must be added that compound windings are not much used for 
 running motors at constant speed. 
 
 Direction of Rotation of a Motor 
 
 To alter the direction of rotation of a motor we have either to 
 change the direction of the armature current, or to reverse the 
 
 polarity of the magnetic field. If 
 we reverse the armature current 
 and the polarity of the magnetic 
 field simultaneously, the direction 
 of rotation will naturally remain 
 the same as before. 
 
 Fig. 150 shows the diagram of 
 connections for a series motor, 
 which, seen from a certain side, 
 rotates counter-clockwise. The 
 current is flowing in the magnet 
 coils from terminal VI. to terminal 
 V., and in the armature from 
 brush II. to brush I. For re- 
 versing the direction of rotation, 
 we may either leave the direction 
 of the magnet current, and alter 
 
 that of the armature current by changing the two cables leading to 
 the brushes, thus connecting brush I. with magnet terminal V., and 
 brush II. with the second main, as shown in Fig. 151; or we 
 may, as shown in Fig. 152, leave the direction of the armature 
 current, and reverse that of the magnet current. 
 
 There would be no reversal of the motor if we changed the mains 
 leading to the starter and to the motor directly, since in this case 
 both the armature and the magnet current would be reversed. 
 
 Similar diagrams of connections for the shunt motor are shown 
 
 FIG. 150. Series Motor Counter- 
 clockwise rotation. 
 
146 
 
 ELECTRICAL ENGINEERING 
 
 in Figs. 153-155. In Fig. 153 the armature is rotating counter- 
 clockwise. The armature current is flowing from brush II. to brush 
 I., the magnet cunen* from terminal IV. to III. Fig. 154 shows 
 
 FIG. 151. Series Motor Clockwise 
 rotation. 
 
 FIG. 152. Series Motor Clockwise 
 rotation. 
 
 how the armature current may be reversed, whilst the magnet current 
 remains in the same direction, and Fig. 155 how the magnet current 
 may be reversed without changing the armature current. 
 
 Great care must be taken to always connect the magnet 
 
 FIG. 
 
 153. Shunt Motor Counter- 
 clockwise rotation. 
 
 FIG. 154. Shunt Motor Clockwise 
 rotation. 
 
 terminals so as to get the full terminal voltage on them as soon 
 as the lever touches the first contact piece. This full terminal 
 voltage has to remain on the magnets during the whole starting 
 
THE ELECTRIC MOTOR 
 
 147 
 
 period, and also when the starter has been short-circuited. If this 
 is not the case, the consequences may be serious. If, in changing the 
 armature cables as per Fig. 154, we had not connected magnet 
 terminal III. with the main, but had left it on brush I. (see 
 Fig. 156), then, at starting the motor the following would take place. 
 If we put the lever on the first contact, the current will flow through 
 the whole of the resistance, the latter consuming the greatest part of 
 the voltage. Magnet terminal IV. is connected by means of the 
 slip-ring directly with the main leading to the starter, whereas 
 terminal III. is connected, not with the return main, but with brush I., 
 a cable leading from this brush to the last contact piece of the 
 starter. As long as the motor is stopped, there is only a very 
 small voltage between I. and the return main II., the magnets are 
 
 FIG. 155. Shunt Motor Clockwise 
 rotation. 
 
 FIG. 156. Shunt Motor with 
 Wrong Connection. 
 
 on nearly the full voltage, the magnetic field will therefore have 
 nearly its full strength, and the motor will start to run. If, then, 
 the motor is running, it will produce a back E.M.F., and this 
 voltage, arising between I. and II., will gradually diminish the voltage 
 between I. and the main leading to the starter. But on this latter 
 voltage the magnets are connected. Thus the magnetic field will 
 become weaker in the same proportion as the motor runs faster. 
 If finally we, as is generally done, short-circuit the starter, then 
 the voltage between the two magnet terminals becomes nil, there 
 would be practically no magnetic field, hence the motor would either 
 "run away," or the fuses would go. It would also be wrong, to 
 connect the two magnet terminals directly with the two armature 
 brushes, or, what would be the same thing, to connect magnet 
 terminal III. with the armature brush I., and magnet terminal IV. 
 
148 
 
 ELECTRICAL ENGINEERING 
 
 with the short-circuiting contact of the starter, instead of connecting 
 it with the slip-ring (see Fig. 
 157). In this case the magnets 
 would at starting not get the full 
 voltage, but only that of the 
 armature; and since, due to the 
 starting resistance, the latter is 
 very small at the start, the 
 magnetic field would be a very 
 small one too. Thus the motor 
 can, if it is not loaded, start, 
 but will consume a very large 
 current in doing so. If, how- 
 ever, the motor is loaded, it will, 
 owing to the weak magnetic field, 
 not be able to start at all. If, 
 on the other hand, the motor is 
 running, producing hereby a back 
 
 FIG. 157 Shunt Motor with Wrong 
 Connection. 
 
 E.M.F., the voltage of the magnet 
 
 winding will gradually grow. When at last the starting resistance 
 is short-circuited, the magnet will be excited with full terminal 
 voltage. Thus a wrong connection, as described here, makes starting 
 impossible, or renders it at least very difficult, but if once started 
 the motor will run all right. The wrong connection described be- 
 fore, allows proper starting, but renders working of the motor im- 
 possible. 
 
 For all cases the following rule for connecting up a shunt motor 
 should be noted by the student: One pole of the mains to be con- 
 nected to a terminal common to the armature and the field; the second 
 pole of the mains to be led to the starter, and to be branched there 
 in such a way as to get at starting the full voltage on the second 
 magnet terminal, while there is still in use the whole starting resist- 
 ance in the armature circuit. The latter is then gradually to be short- 
 circuited during starting. 
 
 For reversing the direction of rotation of a compound motor we 
 have either to reverse the armature current or that of the shunt and 
 series coils simultaneously. If we changed the connections of the 
 shunt coil only, the motor would work quite differently. Consider, 
 for instance, those connections with which we have become acquainted 
 for starting at a distance (see Fig. 149): the following would happen: 
 In the beginning, when the series coil only acts, the motor would 
 start to run in a certain direction, but then the shunt coil, acting 
 oppositely, weakens the field so much as to cause the motor to run 
 away. 
 
 The reversal of direction of rotation may with many motors, 
 especially with multipolar ones, be done simply by moving the 
 
f 
 
 THE ELECTRIC MOTOR 149 
 
 brushes to another position, so that they are shifted the width 
 of a pole from their former position. This causes the direction 
 of the armature current to be reversed, and thus nothing further is 
 needed. 
 
 Armature Reaction with Motors 
 
 With motors there is an armature reaction of the same kind as 
 with dynamos, causing a weakening of the magnetic field. The 
 armature reaction is greater the stronger the current flowing through 
 the armature. Thus, with shunt motors under load, the field will 
 be somewhat weaker than at no load. The motor will, therefore, 
 due to the armature reaction, run somewhat faster under the 
 bigger load if it were not for an Ohmic voltage-drop in the armature. 
 Since this voltage loss tends to decrease the speed with increased 
 load, there is generally no action of the weakened field to be observed; 
 on the contrary, there generally occurs on loading the motor a de- 
 crease of its speed. 
 
 With motors having considerable armature reaction, it may 
 happen that the speed increases with the load; but in many cases 
 the action of the armature reaction and that of the Ohmic voltage- 
 drop compensate each other, so that the motor speed remains prac- 
 tically constant. 
 
 With series motors the armature reaction is of less consequence 
 because the main field is strengthened on increasing the load. 
 
 With motors which do not run without sparking at various loads 
 an adjustment of brushes is required as the load varies. This move- 
 ment of the brushes, the student should remember, has at an in- 
 creasing load not to take place in the direction of rotation as with 
 dynamos, but in an opposite direction. To reverse the current in 
 the armature coil that happens to be short-circuited by the brush, 
 we have to bring the latter within reach of a weak magnetic field, 
 which induces an E.M.F. opposite to that which was previously in- 
 duced in the coil. But, as we are aware, in each winding of the motor 
 armature under the influence of a pole an E.M.F. is induced, which 
 tends to produce a current in an opposite 'direction. Thus we have 
 only to short-circuit each winding before it comes beyond the in- 
 fluence of the magnet pole. We therefore have to displace the brushes 
 from the middle of the neutral zone backwards, and not forwards, 
 as with a dynamo (see Fig. 158). 
 
 In comparing Fig. 158 with Fig. 127, we see that the direction of 
 the current and displacement of brushes are the same as before; but 
 the direction of rotation of the motor has been changed. We thence 
 
150 
 
 ELECTRICAL ENGINEERING 
 
 note that the displacement of brushes has to be done in the direction 
 of rotation with a dynamo, but opposite to the direction of rotation with 
 a motor. 
 
 With motors which have to run in both directions (reversible 
 
 FIG. 158. 
 
 motors) it is naturally impossible to displace the brush-rocker at each 
 change of the direction of rotation. These motors have to be designed 
 so as to run without sparking, and without any displacement of the 
 brushes whatever being necessary. 
 
 Reversing Apparatus 
 
 In many cases such as, for instance, with lifts, cranes, electric 
 trams, and so on it is necessary to have the motors running at first 
 in one and then in the other direction. In such cases it is, of course, 
 impracticable to continually alter the position of the cables or the 
 brushes. 
 
 Quick reversal may be effected by means of a "double-pole, 
 throw-over" switch. This switch, the diagram of which is shown in 
 Fig. 161, and a general view in Fig. 160, consists of two levers 
 coupled to each other. The pivots of the levers form electric 
 contact-pieces; the levers themselves are made of metal, being insu- 
 lated from each other. By lifting the levers upwards, contact a 
 is connected with c, and b with d. On pushing them do WTI wards, 
 contact a is connected with e, and b with /. As shown in Fig. 159, 
 the contacts c and /, d and e are connected crosswise with each other. 
 
THE ELECTRIC MOTOR 
 
 151 
 
 Contact d is in connection with the positive, contact c with the 
 negative main, whereas the middle contacts a and b are in connection 
 
 FIG. 159. Shunt Motor with Change- 
 over Switch. 
 
 FIG. 160. Two-Pole 
 Change-over Switch. 
 
 o d 
 
 a 
 
 with the magnet terminals of the shunt motor. On putting the lever 
 upwards we connect magnet terminal IV. with the positive pole, and 
 
 terminal III. with the negative pole of the 
 mains, hence the current is flowing in the 
 magnet-coil in the direction from IV. to 
 III. On putting the levers downwards 
 we connect terminal IV. with the nega- 
 tive pole, and terminal III. with the posi- 
 tive pole, of the mains, thus reversing the 
 direction of current flowing in the coil, 
 and, as the current in the armature 
 always keeps the same direction, we there- 
 fore reverse the direction of rotation of 
 the motor. 
 
 Obviously we could also arrange the 
 throw-over switch in the armature circuit 
 instead of in the magnet circuit. 
 
 Such a reversing device would, of 
 course, be suitable for the purpose, but 
 
 it would be a dangerous one, for if we reversed the switch whilst the 
 starter is short-circuited, the sudden reversing of the motor might 
 cause its destruction. 
 
 e 
 
 O Of 
 
 FIG. 161. Two-Pole 
 Change-over Switch, 
 
152 
 
 ELECTRICAL ENGINEERING 
 
 To prevent accidents of this nature, the reversing switch is 
 generally rigidly connected with the starter, so as to render the 
 reversal only possible when the armature circuit is opened. Such an 
 apparatus is called a reversing and starting switch. The diagram of 
 connections for this apparatus is shown in Fig. 162. The left and 
 right half of the apparatus are quite symmetrical. The single resist- 
 ance spirals (marked by the vertically drawn zigzag line) are con- 
 nected both with the contacts 1, 2, 3, . . . 9 to the left, and with the 
 contacts 1, 2, 3, ... 9 to the right. Those marked 1 represent 
 
 FIG. 162. Starting and Reversing Switch for Shunt Motor. 
 
 the short-circuiting contacts. There are further some circularly 
 arranged half-rings ; the small ones that are innermost are connected 
 with the magnet terminals III. and IV., whereas the next wider ones are 
 connected with the positive and negative main respectively. Armature 
 brush II. is connected with the large outermost half slip-ring, whereas 
 armature brush I. ip in connection with short-circuiting contact 1. 
 On either side of the starting lever there are fixed brushes B t and 
 B 2 , which are insulated from each other, but each of which covers 
 simultaneously the three circles on either side. If now the lever be 
 
THE ELECTRIC MOTOR 153 
 
 put in the middle (vertically) neither of the two brushes will cover 
 any of the two current-leading rings (marked as + and ) because 
 these rings do not extend so far. By moving, however, the upper 
 part of the lever to the left into the position which is shown in 
 Fig. 162, the innermost half slip-ring and the starting contact 9 are 
 connected with the positive slip-ring. Thus the current will branch, 
 flowing on one hand directly to the magnet terminal III., on the other 
 hand to the contact piece 9, and from there through the whole 
 resistance to contact 1. which is connected with armature terminal I. 
 At the same time both the magnet terminal IV. and the second 
 armature terminal II. are connected by means of the lever brush B 2 
 with the negative main, and thus the motor can start to run. It may, 
 for instance, run to the left. If then we move the lever further to 
 the left, we gradually short-circuit the resistance, till finally we 
 come to contact 1 , when the armature is connected directly with the 
 positive main, and the motor running with its full speed. 
 
 If, however, we move the lever from its middle position towards 
 the right, instead of moving it to the left as before, the brush B 1? 
 covering the slip-ring marked - , connects the negative main both 
 with the magnet terminal IV. and, through the resistance, with 
 armature terminal I. At the same time the lower brush, B 2 , connects 
 the positive main with the magnet terminal III. and the armature 
 terminal II. Thus the current is flowing through the shunt coil in the 
 same direction as before, but in an opposite direction through the 
 armature. The motor will therefore run in the opposite direction. 
 
 To prevent the lever from being turned more than a quarter turn 
 on either side, there are arranged two stops, a, on the apparatus. 
 
 Other reversing and starting switches are designed so as to reverse 
 the magnet current, whilst the armature current remains in the same 
 direction. 
 
 Starting and reversing switches for series motors are constructed 
 in a very similar manner. 
 
 In Fig. 163 the construction of a simple reversing and starting 
 switch is shown. 
 
 Sparking with Starters and Shunt Regulators 
 
 When a shunt circuit is broken a much longer spark results than 
 in the case of a lamp circuit of equal current strength and voltage. 
 The reason of this strong sparking lies in a property of the electric 
 current, which is called self-induction, and with which we shall deal 
 later on, in a more detailed fashion. 
 
154 
 
 ELECTRICAL ENGINEERING 
 
 In a winding surrounding an iron core, an E.M.F. is induced as 
 soon as we alter the strength of magnetism of the iron core (see p. 67). 
 If, now, the strength of magnetism is changed by altering the current 
 flowing round the core, there will be produced an induction effect in 
 the coil resulting in a certain E.M.F. of " self-induction. " 
 
 If a rapid alteration of the current occurs for instance, on 
 breaking a circuit very quickly then at 
 this moment a far greater E.M.F. may be 
 induced than existed before. 
 
 The E.M.F. of the self-induction resists 
 any alteration of the current, it tends to 
 maintain the current at its original strength, 
 just as the inertia does not allow a moving 
 body to stop immediately the driving force 
 ceases. If a running vehicle is suddenly 
 stopped in its course by any impediment, 
 such as a wall or a door, then the sudden 
 stop will cause a force sufficient to destroy 
 the wall or door. Here a far greater force 
 is produced than had to be spent previously 
 in continuously moving the vehicle. 
 
 It is exactly the same on stopping an 
 electric current. The large E.M.F. of self- 
 induction produced on the sudden discon- 
 nection of a 110 volt shunt circuit sometimes destroys the insula- 
 tion of coils which could have withstood a voltage of even 500, 
 and might start an arc which the normal voltage would be un- 
 able to keep up. As a consequence, 
 the ends of the shunt slip-rings and 
 the corresponding contact brushes 
 of starters are generally burnt out 
 after a short time. 
 
 To avoid this we must adhere 
 to the rule of never breaking a 
 shunt circuit. Referring to our 
 analogy, the vehicle must not be 
 stopped suddenly, but allowed to 
 come to rest gradually. This may, 
 in .our case, be effected by making 
 the connections between motor and 
 starter according to the diagram in 
 Fig. 164. By this arrangement it 
 is possible to switch the motor off 
 the main without disconnecting the 
 shunt circuit. As may be seen from the diagram, the shunt slip-ring 
 is in connection with the first resistance contact. Starting the motor 
 
 FIG. 163. Motor Starting 
 Switch ( t ereingt e E. A. 
 G., Vienna). 
 
 FIG. 164. Starter with Inductionless 
 Break, having Shunt Slip-Ring. 
 
THE ELECTRIC MOTOR 155 
 
 has to be done as with the usual starter. When the lever is put from 
 the dead contact 6 to the first resistance contact 5, the shunt coils get 
 full voltage, for the slip-ring is connected with this contact. The 
 armature, as usual, is switched in series with the whole resistance. 
 If, then, we move the lever gradually to the left for instance, to 
 contact 3 the shunt coils remain connected with the full voltage, 
 because the lever always touches the slip-ring. The armature, how- 
 ever, is no longer in series with the whole of the resistance, but only 
 with the part, which is between contact 3 and 1. The resistance 
 spirals between 5 and 3 are without current. Contact 5. of course, 
 is, by means of the shunt slip-ring, in connection with the starting 
 lever, and thus with one main; but contact 3 is also in connection 
 with the lever and the main; hence this part of the resistance (viz. 
 that between 5 and 3) is connected at both ends with one pole only. 
 Between the ends of this part of the resistance there is no voltage, 
 and thus no current can flow through it. It will be exactly the same 
 if we gradually short-circuit the motor. Thus we see that there is no 
 difference whatever in starting by means of this apparatus compared 
 with starting by means of the usual apparatus. In starting, the motor 
 produces, as we know, a back E.M.F., which is nearly equal to the 
 voltage of the current. If now we switch out the motor quickly, we 
 do not interrupt the armature and the magnet circuit as we did with 
 the usual apparatus. We break, of course, the outer circuit, but 
 there is another closed circuit in the motor itself, viz. that from 
 armature brush II. through the whole resistance, from there over the 
 shunt slip-ring to magnet terminal IV., through the magnet coil, and 
 from magnet terminal III. back to armature brush I. Now the 
 armature has at the moment of the break, if this occurs quickly enough, 
 still its full speed, and thus its full back E.M.F. This latter produces, 
 if there is a closed circuit, a current opposite to the previous one. 
 Thus this current leaves brush II., flowing through the resistance from 
 1 to 5, the magnet coils from IV. to III., and enters the armature 
 again by brush I. The current flows through the magnets in the 
 same direction as before. As no interruption, and not even a sudden 
 alteration of the magnet current has taken place, there cannot be 
 produced a considerable E.M.F. of self-induction, and thus there will 
 be no sparking. 
 
 This starter, with " self -indue tionless break," has been further 
 simplified by omitting the shunt slip-ring, and connecting the 
 magnet terminal IV. directly with the first resistance contact 5 
 (see Fig. 165). There is obviously no alteration with regard 
 to the self-inductionless break when compared with the previous 
 case. On starting, however, there is an alteration. In putting 
 the lever on contact 5, the shunt coil gets full voltage as before. 
 But if we now bring the lever, for instance, to contact 3, magnet 
 
156 
 
 ELECTRICAL ENGINEERING 
 
 FIG. 165. Starter with Indue tionless 
 Break, without Slip-Ring. 
 
 terminal IV. is no longer connected directly with the starting 
 lever, but is in series with the resistance between the contacts 
 3 and 5. In putting the lever 
 on the short-circuiting contact 
 1, the magnet coil will be in f> 
 series with the whole starting 
 resistance, thus the magnet cur- 
 rent will be weakened. This is 
 of little importance, for, since the 
 resistance of the starting spirals 
 is very small, the voltage con- 
 sumed by the spirals, and thus the 
 weakening of the magnet current, 
 will be negligible. Suppose, for 
 instance, that the resistance of 
 the starting coils is 5&>, so that 
 the armature current, with the 
 lever on the first contact, is 
 with 110 volts about 20 amps., 
 the normal shunt current being 2 amps., and thus the shunt 
 resistance 55&>. In the diagram, Fig. 165, we have then, with a 
 short-circuited starter, a shunt resistance of 5 + 55 = 60&>, and thus 
 a shunt current of -y/ = 1.83 amps., against the 2 amps, previously. 
 This small weakening of the magnetic field will cause the motor 
 to run a little faster. 
 
 A sparkless breaking of the motor circuit can only be effected 
 if there is at the moment of switching out no ; or a very small, 
 pressure difference between the starting lever and the last contact. 
 Thus, to get a sparkless breaking of the motor circuit with a starter 
 such as Fig. 164 or 165, a rapid switching out is required. For, 
 if we moved the lever slowly from one contact to another one, the 
 speed, and with that the back E.M.F., would gradually decrease, so 
 that finally, if the back E.M.F. be only a very small one, we have to 
 break a large current at the full voltage, thus getting a long spark in 
 spite of the " self-indue tionl ess" connection. 
 
 Sometimes it is impossible to avoid the interruption of the 
 shunt circuit. Here we are generally helped by closing the shunt- 
 circuit on, itself whilst it is still being switched out, so that the 
 self-induction current may flow in the circuit so formed. For a 
 dynamo, this is shown in Fig. 166. With the exception of the 
 dead contact the arrangement of the shunt-regulator is quite a 
 normal one. The dead contact, however, which is usually without 
 any connection whatever, is now connected with the shunt terminal 
 III., and, since the latter is connected directly with the armature 
 brush I., also with this. Hence, if we come from the last resistance 
 
THE ELECTRIC MOTOR 
 
 157 
 
 contact to the dead one, the shunt is short-circuited on itself, and 
 
 the self-induction current pro- 
 duced on breaking flows in thi< 
 circuit. Since the lever covers for 
 a moment both the last resistance 
 and the dead contact, we get. 
 during this time, a current 
 from armature brush II. through 
 the resistance spirals and the 
 connecting wire to I., but that 
 is no disadvantage. 
 
 The switching out of shunt 
 regulators must not be done 
 suddenly like the switching out 
 of starters. It is, on the con- 
 
 Fio. 166. Shunt-Regulator, with Con- fraT _ advisable to IPSVP trip 
 nection for short-circuiting the Mag- , trary ' aavlsar> 
 net Coils in the "off" nosition. lever for some time on the last 
 
 resistance contact, in order that 
 
 net Coils in the "off" position. 
 
 PIG. 167. Starting and Reversing Switch with Connections for short- 
 circuiting the Magnet Coils in the "off" position. 
 
158 ELECTRICAL ENGINEERING 
 
 the voltage of the machine may meanwhile decrease. If the 
 resistance ^ of the shunt regulator be large enough, the machine 
 will lose its voltage almost entirely. In such a case, even with- 
 out the special connection between the dead contact and the second 
 magnet terminal, an injurious self-induction voltage and flashing 
 would not result. 
 
 A reversing and starting apparatus with "self-inductionless" 
 break is shown in Fig. 167. If, in switching out, the brush B t 
 leaves the wide slip-ring and the contact 9, the armature and the 
 magnets are still in connection. If, then, we place the lever in 
 the middle, the two shunt slip-rings are short-circuited. Care must 
 be taken when using this apparatus, not to move the lever too soon 
 over the middle position, for in this case the circuit of the short- 
 circuited magnet coils would be again interrupted before the self- 
 induction current had ceased, and consequently the self-induction 
 vvould cause considerable flashing. 
 
 Another important mode of control used for shunt or series motors 
 is called the Ward-Leonard system of control. In this system the 
 motor field may be separately excited. The armature of the motor 
 is connected directly to the armature of the generator without resist- 
 ance. If there is no field on the generator, no E.M.F. will be gen- 
 erated, and no current will flow to the motor. If now a little field 
 be put upon the generator, a small E.M.F. be generated in the gen- 
 erator, a current will flow at a few volts to the motor, and it will 
 slowly start. As the field of the generator is strengthened the voltage 
 continues to increase, and the motor continues to speed up until full 
 field and full voltage is being produced by the generator. Reduction 
 of speed can be effected by a reduction of field of the generator. 
 Thus, by manipulating the small field current of the generator, a 
 large armature current to the motor can be controlled. The con- 
 troller, therefore, since it handles such small currents, as compared 
 with the currents doing the work, is very many times smaller than 
 if it were located in the armature circuit. This method of control 
 has a very wide application. It is used on battleships, hoists, and to 
 control at a distance. A good generator will operate without sparking 
 under these low-voltage high-current conditions, for the voltage, 
 being low with consequently low volts per bar on commutator, gives 
 a very favorable sparking condition. As a matter of fact, a good 
 generator will take 25 per cent, over normal current down to volt 
 between brushes without trouble from sparking. 
 
THE ELECTRIC MOTOR 
 
 159 
 
 Motors for Certain Purposes 
 
 A dynamo can usually, without any alteration, be also used as 
 a motor, but, since motors are employed for so many different 
 
 FIG 168. -Enclosed Motor. 
 
 FIG. 169. -Enclosed Motor (Electromotors Company, Manchester}. 
 
160 
 
 ELECTRICAL ENGINEERING 
 
 purposes for which a special shape is desirable, a number of types 
 of motors have been designed. 
 
 A special form is the enclosed motor, which is employed for 
 damp and dusty rooms. The motor is entirely enclosed in a cast- 
 iron or steel case, which has doors near the commutator, through 
 which the latter may be inspected or cleaned. Figs. 168 and 169 
 are illustrations of enclosed motors. 
 
 For ventilating purposes there is sometimes, instead of a pulley, 
 a fan fixed on the shaft of the motor. With larger fans the motor 
 
 FIG. 170. Motor connected to Machine Tool. 
 
 is fixed on the case of the ventilator. Fig. 175 shows a big fan 
 combined with the motor. 
 
 Since smaller motors are generally built for high speeds, it is 
 sometimes necessary to reduce these speeds by means of reduction 
 gears. Even with belt driving it is sometimes desirable to reduce 
 the speed by gearing. Generally the reduction gear is built 
 together with the motor, and on the slow speed countershaft the 
 coupling or the belt pulley is fixed. 
 
THE ELECTRIC MOTOR 
 
 161 
 
 Motors in America are used for a wide range of work. 
 
 Fig. 170 shows an application of an electric motor to a machine 
 tool. 
 
 Fig. 171 shows an application to a pump. 
 
 Fig. 172 shows an application to an elevator. 
 
 Fig. 173 shows an application to a mine hoist. 
 
 On battleships, in America, a very extensive use of motors is 
 made. One battleship has over 200 motors installed upon it; the 
 turrets are turned by rnitor, the ammunition raised and pushed into 
 
 FIG 171. Motor connected to Pump. 
 
 the guns, boat cranes operated, ventilating blowers driven, and 
 rudder turned. 
 
 A special extra pole, or commutating pole motor, has recently been 
 developed. It has a magnetic circuit, as shown in Fig. 174. 
 
 This figure shows a 4-pole motor of usual magnetic circuit, but in 
 addition to the poles A, B, C, and D there are four other poles, a', 
 &', c', and d' ', which are wound with wire in series with the armature, 
 like a compound motor winding. The armature is wound for a 4-pole 
 motor, although there are actually eight poles. The four extra poles 
 are about half the size of the regular poles. The flux from these 
 poles is in such a direction that the current is reversed in the coil 
 which is short circuited under the brush without shifting brushes 
 to the proper pole to get such a flux (forward in a generator and 
 backward in a motor). Thus, such a motor runs without shift of 
 brushes and in either direction equally as well. The turns on the 
 
THE ELECTRIC MOTOR 163 
 
 commutating poles are chosen so that just the right amount of flux 
 
 \ 
 
 FIG. 173. Application to Mine Host. 
 
 is obtained to give exact reversal, and no more; hence, such motor 
 run sparklessly. They can be de- 
 signed on closer lines than an ordi- 
 nary motor, making their cost, there- 
 fore, less, since the commutating 
 poles are wound with series spools. 
 The balance of flux with reversing 
 requirements of the armature coils 
 when under the brush is the samo 
 at all loads. Such a motor, there- 
 fore, gives far better results under 
 overload than the usual design. 
 This extra pole application is just 
 as useful for generators, and from 
 present indications the commutat- 
 ing pole is to be rapidly extended 
 in dynamo design. 
 
 FIG. 174. Four-pole Inter-pole 
 Magnetic Circuit. 
 
164 
 
 ELECTRICAL ENGINEERING 
 
 Electric Traction 
 
 An important application of electric motors is that for rail ways , 
 especially street railways. In the latter case the current supply 
 device consists generally of a hard-drawn copper wire, which is 
 
 FIG. 175 Electrically driven Fan (Korting Brothers). 
 
 suspended by means of insulators supported either by posts or by 
 cross wires. The copper wire is in connection with the positive pole 
 of the central station dynamo, the negative pole of which is 
 connected with the rails. On the top of the motor car there is fixed 
 
THE ELECTRIC MOTOR 
 
 165 
 
 FIG. 176. Street-car Motor, closed. 
 
 either a metal bow, or an iron tube, the top of which is provided 
 with a little wheel, the "trolley." The bows or the trolleys are 
 pressed by a spring arrangement against the overhead wire, and 
 
 serve as the current supply 
 device. Both the bows and 
 the trolleys are very well 
 insulated from all the iron 
 parts of the car, and a cable 
 leads from them to the 
 motor starter and thence to 
 the motor itself. The latter 
 is fixed beneath the car, and 
 drives the car-axle by means 
 of a pinion and spur wheel. 
 The second pole of the motor 
 is connected with the car-axle, 
 and thus through the wheels and the rails with the negative pole 
 of the central station dynamo. Very often two motors are used 
 
 for one car, each of which drives one 
 axle. 
 
 The motors employed for driving elec- 
 tric cars have generally the character- 
 istic shape, as shown in Figs. 176 and 
 177. The motor is entirely enclosed 
 to prevent dust and moisture getting 
 into its interior. To be able to inspect 
 the commutator and the brushes, or to 
 take out the bearings or the armature, 
 the case is divided into two parts, 
 hinged to each other; the upper part 
 may be fixed and the lower one opened 
 downwards, or mce versa. Since it is 
 desirable to use as little space for the 
 motors as possible, the magnet coils 
 are not wound on separate bobbins, 
 but are, after they have been wound 
 on special wooden formers, and have 
 been well insulated with impregnated 
 cotton, mica and so on, pushed over the 
 cores. Since street railways are gene- 
 rally worked with a voltage of 500-600, 
 all the motor parts must be excellently 
 insulated. 
 
 About the working of these motors nothing special has to be re- 
 marked. They are four-pole motors with two brush-holder arms, each 
 of which is provided with one or two carbon brushes. The motors 
 
 FIG 177. Motor, open. 
 
166 
 
 ELECTRICAL ENGINEERING 
 
 are reversible, and, according to the position of the starting lever, 
 drive the car forwards or backwards. 
 
 The starter for street-car purposes is generally called a controller. 
 Since it has, like the motors, to be protected against dirt and dust, 
 it is entirely enclosed. Its internal construction (similar to that 
 shown in Fig. 178) is entirely different from that of the starters 
 with which we have hitherto become acquainted. The contact pieces 
 
 FIG. 178. Controller suitable for Single Motor (Royce, Manchester). 
 
 by which the different connections are effected, are not arranged 
 on a horizontal base, but fixed on the surface of a vertical cylinder. 
 On the contact pieces or contact rings there are sliding brushes or 
 contact levers which are fixed on a separate wooden plate, whereas 
 the cylinder with the contact pieces is movable. On the top of the 
 cylinder there is fixed a handle, by means of which the cylinder 
 may be turned to different positions. By rotating the cylinder the 
 
THE ELECTRIC MOTOR 167 
 
 connections of the contact rings with the contact levers are altered, 
 as with the usual starting apparatus. 
 
 A controller for a single motor-car is really little else than a 
 common starter. It has a position of rest, marked "stop," and a 
 number of starting steps. At the last step the whole of the starting 
 resistance is short-circuited, when the motor is switched on the full 
 voltage, and runs at full speed. The reversing of the motor for the 
 opposite direction of rotation is generally effected by a reversing 
 switch, which is separated from the controller, but mechanically con- 
 nected with the latter in such a way as to make reversing impossible, 
 unless the motor is stopped. 
 
 For cars which are provided with two motors, the controller becomes 
 more complicated. In this case there are two main working positions, 
 viz., firstly, a series connection of the motors, when each motor is 
 switched on half the voltage only; and, secondly, parallel connection 
 of the two motors, when each motor is switched on the full voltage, 
 thus running twice as fast as before. Starting the motors is effected 
 by connecting first of all the two series .connected motors in series 
 with some resistance, and then gradually short-circuiting this 
 resistance. The motors then run with half the voltage and a 
 corresponding speed. But if the connection is altered, and the 
 motors connected in parallel, they run with full speed. 
 
 Similar controllers are also employed for electric cranes. 
 
 Fig. 179 shows the cylinder of an American street-car controller 
 for two motors, developed on a flat surface to show the contacts 
 more easily. 
 
 Another form of controller mechanism is that known as the mul- 
 tiple-unit control system. In this case the controller is split up into 
 its component parts, each being separate from the other, but operated 
 from a master controller, which excites magnets, or contactors, located 
 upon the component parts just at the right time, so that they 
 take their turn in closing or opening the current circuits operating 
 the motors on the cars, as the master controller regulates. These 
 contactors, made to open heavy currents, may be placed under the car 
 out of the way where room is available, and where the big arc resulting 
 from their breaking large currents can give no trouble. The master 
 controller, on the other hand, having only to direct the small current 
 necessary to operate the magnets of the contactors, takes but little 
 room and can be placed conveniently to the motorman. In addition, 
 if several cars are connected together, all equipped with motors and 
 contactors, one master controller can operate them all simultaneously. 
 The figure shows the lines from the master controller to the contactor 
 for one car. As many cars can be connected, in parallel to this same 
 
168 
 
 ELECTRICAL ENGINEERING 
 
 controller, as desired. Thus, a whole train may be operated from 
 one or more master controllers, and every axle helps the train along. 
 
 17617 ||_ 
 
 ? 17618 
 
 - 
 
 '> - 153* H!*-- 
 |00 [^17619 
 
 
 
 17620 || (2 
 
 ) 17618 
 
 
 
 
 
 
 
 
 17619 >-*|0 0| I7<: 
 
 322-> 
 
 
 i*44'*i 
 
 *-- 69--* 
 
 
 I7794~[00 | 
 
 17619 
 
 
 17624 N ^ 51" ~ 
 
 U-44** 
 
 L 
 
 
 017621 
 
 
 
 * 94 --* 
 
 
 | 17617 
 
 17621 
 
 
 
 
 
 
 
 
 17617 
 
 I7733-". 
 
 
 *- 153" M 
 
 
 
 \ 
 
 > 17618 
 
 
 - 
 
 - I4O* * 
 
 
 
 
 ) 17618 
 
 j 
 
 1 
 
 I4O* - *- 
 
 
 FIG. 179. Cylinder Development of Street-car Controller (Two Motors). 
 
 In this way a very fast acceleration may be obtained. This system is 
 much used in America on elevated trains and on large surface cars and 
 locomotives, and is generally known as the multiple-unit control. By 
 its introduction a great stride was made in electric traction, as un- 
 limited power can be controlled by splitting it up into easily handled 
 units. 
 
 Following is a detailed description of the multiple-unit system 
 adopted by the General Electric Company of the United States of 
 America. 
 
THE ELECTRIC MOTOR 169 
 
 The Sprague=General Electric Type M Control 
 
 System 
 
 The Sprague-General Electric Type M Control is designed pri- 
 marily for the operation of a train of motor- and trail-cars, coupled in 
 any combination, and the whole operated as a single unit from any 
 controller on the train. The system may also be used to advantage 
 on individual equipments and locomotives. 
 
 The control apparatus for each motor-car may be considered as 
 consisting essentially of a motor controller and a master controller. 
 
 The motor controller comprises a set of apparatus usually located 
 underneath the car which handles directly the power circuits for 
 the motors, connecting them in series and parallel and commutating 
 the starting resistance in series with them. This motor controller is 
 operated electrically, and its operation in establishing the desired 
 motor connections is controlled by the motorman by means of the 
 master controller, which is similar in construction to the ordinary 
 cylinder controller, and is handled in the same manner. Instead of 
 effecting the motor combinations directly, however, this controller 
 merely governs the operation of the motor controller. 
 
 The master controller operates a number of electrically operated 
 switches or " contactors/ 7 which close and open the various motor 
 and resistance circuits, and an electrically operated "reverser" that 
 connects the field and armature leads of the motors to give the 
 desired direction of movement to the car. Both the contactors and 
 reverser are operated by solenoids, the operating current for which 
 is admitted to them by the master controller. 
 
 Each motor- and trail-car is equipped with train cable, consisting 
 of nine or ten individually insulated conductors connected to corre- 
 sponding contacts in coupler sockets located at each end of the car. 
 This train cable is connected identically on each motor-car to the 
 master-controller fingers and the contactor and reverser operating 
 coils, and is made continuous throughout the train by couplers between 
 cars, connecting together corresponding terminals in the coupler 
 sockets. 
 
 All wires carrying current supplied directly from the master con- 
 troller form the " control circuit"; those carrying current for the 
 motors form the " motor" or " power circuit." 
 
 Inasmuch as the motor-controller operating coils are connected 
 to this control train line, it will be appreciated that energizing the 
 proper wires by means of any master controller on the train will 
 simultaneously operate corresponding contactors on all the motor- 
 cars and simultaneously establish similar motor connections on all 
 cars. 
 
170 ELECTRICAL ENGINEERING 
 
 ADVANTAGES 
 
 The Sprague-General Electric Type M Control permits a train of 
 motor-cars and trailers to be operated as a single unit from any master 
 controller on the train. If desired, a master controller can be placed 
 on each platform of trail-cars, thereby providing for the operation of 
 the train from any platform. With this arrangement the motorman 
 can be always at the head of the train, regardless of the combination 
 of the cars. 
 
 The entire train, equipped with Type M Control, may thus be 
 regarded as a unit; the motorman has the same control over a train 
 that he would have over a single car with the ordinary cylinder con- 
 troller. 
 
 Should the motorman remove his hand from the operating handle 
 of the master controller, the current will be immediately cut off from 
 the entire train, thus diminishing the danger of accident in case the 
 motorman should suddenly become incapacitated. 
 
 The system will operate at any line potential between 300 and 
 600 volts, and the action of all contactors is absolutely reliable and 
 instantaneous. 
 
 On heavy equipments the effort of the motorman in operating the 
 master controller is so much less than that required to handle a large 
 cylindrical controller that he can give more attention to the air- 
 brakes and other parts of the equipment, especially in cases of emer- 
 gency. The ease with which it is operated also makes the Type M 
 Control particularly well suited for use on large locomotives. 
 
 The approximate total weight per motor-car of control equipments, 
 exclusive of supports, is as follows: 
 
 Aggregate H.P. of Motors. "Weight of Equipment in Pounds. 
 100 1500 
 
 200 2000 
 
 300 2500 
 
 500 4000 
 
 640 4500 
 
 The approximate weight of the apparatus for each trail-car, which 
 comprises train cable, coupler sockets and connection boxes, is 100 
 pounds. 
 
 In many cases it will be found advantageous to anticipate the 
 future growth of an interurban road by equipping each motor-car 
 with Type M Control. In these cases it will be easy to change from 
 single car to train service whenever warranted by traffic conditions. 
 
 The position of the handle on that master controller which the 
 motorman is operating always indicates the position of motor-control 
 apparatus on all cars. 
 
172 ELECTEICAL ENGINEERING 
 
 On account of the great flexibility of this system, it can be readily 
 adapted to many classes of service other than that of train operation. 
 The small space occupied by the master controller and the ease with 
 which the controller may be operated make this system for heavy 
 hoists desirable in some cases, or other classes of severe direct-current 
 service requiring a controller easily manipulated, or one which may 
 be located at a considerable distance from the motors. 
 
 All parts subject to wear are readily replaceable. 
 
 CONTACTORS 
 
 The contactors are the means of cutting in and out the various 
 resistances, of making and breaking the main circuit between trolley 
 and motors, and of changing from series to parallel connection. 
 
 Each contactor consists of a movable arm carrying a renewable 
 copper tip which makes contact with a similar fixed tip, and a coil 
 for actuating this arm when supplied with current from the master 
 controller. The contactor is so designed that the motor circuit is 
 closed only when current is flowing through its operating coil; and 
 gravity, assisted by the spring action of the finger, causes the arm 
 to drop and open this circuit immediately, when the control circuit 
 is interrupted. Each contactor has an effective and powerful mag- 
 netic blow-out, which will disrupt the motor circuit under conditions 
 far exceeding normal operation. In closing, the copper tips come 
 together with a wiping action, which cleans and smooths their surfaces. 
 
 All contactors in an equipment are practically identical, and the 
 few parts which are subject to burning and wear are so constructed 
 as to be readily replaceable. 
 
 In order to save space arid eliminate interconnections as much 
 as possible, several contactors are mounted on the same base. The 
 contactors should preferably be located under the car, and boxes are 
 therefore supplied which facilitate installation, protect the contactors 
 from brake-shoe dust and other foreign material, and provide the 
 necessary insulation. These boxes are built with perforated openings 
 for ventilation, but shields are supplied for closing these perforations 
 whenever desirable. 
 
 REVERSER 
 
 The general design of the reverser is somewhat similar to the 
 ordinary cylindrical motor reversing; switch, with the addition of 
 electro-magnets for throwing it to either forward or reverse position. 
 In general construction, the operating coils are similar to those used 
 on the contactors, but in order to secure absolute reliability of action 
 in throwing, the coil is given full line potential. The reverser is 
 
THE ELECTRIC MOTOR 173 
 
 provided with small fingers for handling control circuit connections, 
 and, when it throws, the operating coil is disconnected from the ground 
 and is placed in series with a set of contactor coils, thus cutting the 
 operating current down to a safe running value. These coils are 
 protected by a fuse, which will immediately open the circuit if the 
 reverser fails to throw. If the position of the reverser does not 
 correspond to the direction of movement indicated by the reverse 
 handle on the master controller, the motors on that car cannot take 
 current. While the motors are taking current the operating coil is 
 energized, and the electrical circuits are interlocked to prevent pos- 
 sibility of throwing. 
 
 MASTER CONTROLLER 
 
 The master controller is considerably smaller than the ordinary 
 street-car controller, but is similar in appearance and method of oper- 
 ation. Separate power and reverse handles are provided, as ex- 
 perience has led to the adoption of this arrangement in preference 
 to providing for the movement of a single handle in opposite directions. 
 
 An automatic, safety, open-circuiting device is provided, whereby, 
 in case the motorman removes his hand from the master-controller 
 handle, the control circuit will be automatically opened by means of 
 auxiliary contacts in the controller, which are operated by a spring 
 when the button in the handle is released. This device is entirely 
 separate and distinct in its action from that of the main cylinder. 
 Moving the reverse handle either forwards or backwards makes con- 
 nections for throwing the reverser to either forward or backward 
 position. The handle can be removed only in the intermediate or 
 off position. As the power handle is mechanically locked against 
 movement when the reverse handle is removed, it is necessary for 
 the motorman to carry only this handle when leaving the car. 
 
 When the master controller is ' thrown off, both line and ground 
 connections are severed from the operating coils of important con- 
 tactors, and none of the wires in the train cable are alive. 
 
 The current carried by the master controller is about 2.5 amperes 
 for each equipment of 400 H.P. or less. This small current carrying 
 capacity permits a compact construction, and the controller weighs 
 only 130 pounds. 
 
 MASTER-CONTROLLER SWITCH 
 
 A small enclosed switch with magnetic blow-out is used to cut off 
 current from each master controller, and is supplied with a small 
 cartridge fuse enclosed in the same box. When this switch is open 
 
174 ELECTRICAL ENGINEERING 
 
 all current is cut off from that particular master controller which it 
 protects. 
 
 CONTROL CABLE 
 
 A special flexible cable, made up of different colored individually 
 insulated conductors, is used for the train cable and, whenever pos- 
 sible, to make connections between the various pieces of control 
 apparatus. 
 
 CONNECTION Box 
 
 Connection boxes are provided for connecting the control circuit 
 cables at junction points without splicing, and small copper terminals 
 .are supplied for attaching to the ends of the individual conductors. 
 
 CONTROL COUPLERS 
 
 The master-control cables of each car terminate in sockets and 
 are interconnected by means of a short section of similar flexible 
 cable fitted with plugs. Each socket contains a number of insulated, 
 metallic contacts connected to the train wires, and the terminal 
 plugs of the coupler contain corresponding contacts. The parts 
 subject to wear are readily replaceable. 
 
 All coupler sockets are provided with spring catches which hold 
 the plugs in contact under normal conditions, and permit them to 
 automatically release in case two cars separate. 
 
 CONTROL CUT-OUT SWITCH 
 
 This is a switch, usually nine point, installed on each motor-car, 
 and is used to disconnect the operating coils of the contactors and 
 reverser from the train cable, and hence render them inoperative. 
 
 CONTROL FUSES 
 
 On each car several small enclosed fuses are placed in the control 
 circuit at such points as to effectively protect the apparatus. 
 
 CONTROL RHEOSTAT 
 
 During acceleration, tubes of a high-resistance rheostat are con- 
 nected in series with the contactor coils to cut down the operating 
 current to a value approximating that for the running positions of 
 the controller. This rheostat is enclosed in a sheet iron case for 
 protection. 
 
THE ELECTRIC MOTOR 175 
 
 CIRCUITS 
 
 The motor circuit is local to each car, and on the first point 
 the current on entering from the trolley or third-rail shoe passes 
 through the following pieces of apparatus in the order named: main 
 switch and fuse, contactors, resistances, reverser, motors; thence to 
 ground. 
 
 In the control circuit, the course of the current from trolley to 
 ground is through the master-controller switch and fuse, master con- 
 troller, connection box, to the cut-out switch. From the cut-out 
 switch the current passes through the control cable to the operating 
 coils of the reverser and contactors, and thence through fuses to 
 ground. 
 
 AUTOMATIC FEATURES 
 
 The apparatus described is used with the standard equipment for 
 hand control. If automatic features are desired, certain minor 
 changes will be entailed. 
 
 INSTALLATION 
 
 In order to insure economical operation, it is essential that the 
 apparatus should be so located under the car as to be easily inspected 
 and repaired. Attention should therefore be given to the disposition 
 of the apparatus. The best results are obtained by first locating the 
 contactors and reverser to the best possible advantage. The air- 
 brake apparatus can be placed in the remaining space. 
 
 The Electric Brake 
 
 With street railway-cars it is of the greatest importance to be able 
 to apply a brake quickly, especially in cases of danger, when, for 
 instance, people are in the way of the car. The usual mechanical 
 braking, as used for horse-cars, is not sufficient for the heavier and 
 more quickly running electric car. A very effective kind of braking 
 may be effected by disconnecting the motor from the mains, and then 
 connecting the armature brushes with each other through a resistance. 
 
 Let us consider, first of all, a shunt motor, assuming the shunt 
 coils to be connected with the outer mains during the whole running. 
 If now we disconnect the armature from the latter, connecting the 
 brushes to a resistance, the motor will, as long as it is rotating, 
 act as a dynamo. The armature will deliver a current into the 
 resistance, which current is greater the quicker the armature is 
 running, and the smaller is the resistance. It is quite clear that 
 for the production of this current mechanical work has to be spent. 
 Thus the live energy, which the car still has after switching off the 
 oiotors, is spent in generating a current, and will soon be consumed. 
 
176 
 
 ELECTRICAL ENGINEERING 
 
 The car will therefore run slower and slower, just as if the wheels 
 
 had been braked mechanically. This kind of braking is especially 
 
 effective at a very high speed of the car, 
 
 whereas at low speeds it is much less so. [ 
 
 For absolutely stopping a car, this kind 
 
 of braking cannot be employed at all, 
 
 since there is only a braking effect if a 
 
 current is really generated, and the latter 
 
 can only occur when the armature is 
 
 rotating. Each electric car has therefore, 
 
 besides the electric braking arrangement, 
 
 to be provided \vith a mechanically acting 
 
 one. 
 
 When a car is provided with series 
 motors, which is generally the case with 
 street railways, a reversal of the magnet 
 connections is required for getting a 
 braking effect. Imagine the motor to be 
 connected according to Fig. 181, the 
 current flowing in the armature in the direction from I. to II., in 
 the magnets from V. to VI. As we know, a back E.M.F. is 
 produced in the armature, which, after disconnecting the latter from 
 the mains, would tend to produce a current, leaving the armature 
 in I., and entering it again in II. Hence if, for the purpose of 
 braking the motor, we simply insert a resistance between I. and VI. 
 
 FIG. 181. Running Position 
 of Series Motor. 
 
 FIG. 182. Incorrect Connect : on FIG. 183. Correct Connection for 
 ' for Braking: Braking. 
 
 (see Fig. 182), then the current will leave the armature in I., flow 
 through the resistance, through the magnet coils in the direction 
 from VI. to V., and enter the armature at II. The current will 
 therefore flow through the magnet coil in a direction opposite to 
 that before. The magnetism of the machine will be destroyed, 
 no current production is possible, and the braking effect will 
 
THE ELECTRIC MOTOR 177 
 
 instantly cease. To get a braking effect, we must connect the 
 magnet coils so as to cause the armature current to flow through 
 them in the same direction as when the machine was working as 
 motor. The proper brake connections for a series motor are shown in 
 Fig. 183. 
 
 The brake connections are generally executed by means of the 
 controller. From the "stop" position to the left of the driver the 
 different running, and to his right the brake positions of the 
 controller handle, are generally arranged. 
 
 The Magnetic Blow-Out 
 
 A controller has generally far harder work to do than a common 
 starter, since it has continually to be operated for starting the motor, 
 altering its speed, and braking. To prevent the arcs and sparks, 
 arising from the frequent disconnections made with the controller, 
 from destroying the contact rings and the brushes, it is necessary to 
 blow out the sparks quickly. This may be done by magnetic blow- 
 outs. 
 
 If we bring a strong magnet near an electric arc, we observe 
 that the arc is deflected, being bent hi a large bow, and finally 
 extinguished. The arc is an easily movable conductor. It consists 
 of glowing metal or carbon vapour, through which the electric 
 current flows. As we know, each movable electric conductor is 
 deflected by a magnetic field, and therefore the deflection and 
 rupturing of the electric arc may be understood. 
 
 Each controller is provided with a strong electro-magnet, the 
 effect of its magnetic field extending over the fixed contact brushes. 
 The sparks arising between these contact brushes and the contact 
 rings are hence quickly extinguished. 
 
178 
 
 ELECTRICAL ENGINEERING 
 
 Operating Troubles with Direct=current Motors 
 
 Fig. 184 shows a contactor equipped with a magnetic blow- 
 
 FIG. 184 Contactor. 
 
 FIG. 185. Master Controller. 
 
 out to extinguish the arc, and Fig. 185 shows a master controller 
 operating many of these contactors. 
 
CHAPTER V 
 
 ACCUMULATORS 
 
 WE have learned in the first chapter of this book, that if with the help 
 of metal plates we pass a current through acidulated water, a decom- 
 position results, with the separation of hydrogen and oxygen. 
 
 Another phenomenon also takes place with which we have not yet 
 dealt. If the resistance of the voltmeter and the pressure at its ends 
 be first measured, and from the values so obtained we calculate the 
 current which ought to flow through the circuit in accordance with 
 Ohm's Law, we shall find that what is actually measured by an 
 ammeter is far smaller. 
 
 The explanation lies in the fact that in addition to the E.M.F. 
 driving the current through the liquid there is a back or counter 
 E.M.F. , just as we have learnt is the case with an electro-motor. 
 
 At the positive electrode, which is the place of entrance of the 
 current, the oxygen is liberated, whilst at the negative the hydrogen 
 is evolved. The current flows in the liquid from the positive to the 
 negative pole; the back E.M.F., on the other hand, is so directed 
 that it tends to send a current in the liquid from the negative to the 
 positive pole. Whenever a current passes through a liquid, as in the 
 case of galvanic cells, the development of gas at the plates produces a 
 back E.M.F., which tends to weaken the working pressure of the cell. 
 This effect is called electrolytic polarization. The simplest element 
 with copper and zinc in dilute sulphuric acid shows the property 
 of polarization in a very marked manner, and causes the E.M.F. of 
 such a cell to rapidly diminish when the cell is in use. 
 
 The separation of oxygen and hydrogen brings about a chemical 
 alteration of the immersed metal plates, unless they are made of 
 metals like platinum. For example, if we use as electrodes plates 
 of iron, then the oxygen liberated at the positive pole will cause 
 oxidation of the iron. We know that iron rusts on account of 
 the oxygen in the air which combines with the metal. The com- 
 pound so produced is known to chemists as oxide of iron. Exactly 
 the same thing happens during electrolysis, when the positive plate 
 is of iron. If we had used lead instead of iron a corresponding 
 change takes place. On the surface of the positive electrode a layer 
 of lead oxide would appear. 
 
 179 
 
180 ELECTRICAL ENGINEERING 
 
 At the negative electrode hydrogen is liberated, but does not, as a 
 rule, attack the electrode. If lead were used it would remain bright, 
 or, if it had previously been covered with a thin film of oxide, this 
 will now be destroyed, because the hydrogen, having an attraction for 
 oxygen, will decompose the oxide, producing water and liberating 
 lead. 
 
 After the passage of the current we have no longer two similar 
 electrodes, but at the positive pole we have the metal coated with 
 lead oxide, and at the negative a clean lead plate. Two different 
 metals in a liquid give, as we are aware, an E.M.F. The origin of 
 the back E.M.F. will now be self-evident. When gas is evolved, this 
 collecting on the plates gives a further difference between the plates. 
 Hence, to get a current through the liquid its voltage must be 
 sufficient to overcome the back E.M.F. The voltage of such a cell 
 may amount to more than 2 volts, i.e. twice as much as the E.M.F. 
 of a simple galvanic cell. 
 
 When a cell is coupled to an outside source of pressure, so as to 
 send a current through it, the process of charging is said to be in 
 progress. On stopping the current the evolution of gas immediately 
 ceases, but if we now connect the poles of the cell by a wire, 
 a current is obtained, the direction of which is opposite to the 
 charging current, and the cell is said to discharge. It may again 
 be charged by coupling it to a pressure supply, then discharged, and 
 the process may be repeated as often as may be desired. 
 
 An apparatus used in this way is called an accumulator that is, 
 a storage arrangement which is capable of accumulating energy and 
 giving it back when desired. 
 
 The accumulator that we have so far considered can supply 
 current for a short time only, for in charging it, only the surface of 
 the plates, which is in direct connection with the liquid, can be 
 chemically altered. When this is effected, further charging is use- 
 less. The liquid is then decomposed, but the oxygen formed on the 
 positive plate, after the whole surface has been oxidized, is unable to 
 further penetrate into the plate, and escapes, therefore, in the form of 
 bubbles. Hence, if the charging be continued, the only effect will be 
 to decompose the liquid. 
 
 If however, after the first charge, the accumulator is again 
 discharged, then the plate has been mechanically altered. The surface 
 of the lead plate has become spongy, and if we charge the accumu- 
 lator again, the chemical change can penetrate a little deeper into the 
 plate. If these charges and discharges be continued, the " capacity" 
 of the accumulator is gradually increased. 
 
 This process of forming the plates was first used by the French 
 experimenter Plante, and plates made in this way are called Plante 
 plates. The process of formation is a very lengthy one, and must be 
 continued for weeks, and even months if the cell is to have much 
 
ACCUMULATORS 
 
 181 
 
 capacity. The process is completed when one plate is covered to a 
 good depth with finely divided lead, and the other has a corresponding 
 amount of lead peroxide. 
 
 It was again a Frenchman Faure who showed how the tedious 
 process of forming might be much diminished in time. Instead of 
 using pure lead plates, he applied to the lead a mixture of the two 
 oxides of lead called minium. If as positive electrode a lead grid 
 filled with minium be employed, and as negative electrode a pure 
 lead plate, or a similar grid plate having the holes filled with spongy 
 
 
 
 mt%8s&$& 
 
 : ^ 
 
 FIG. 186. Accumulator Plate. 
 
 lead, then we have from the beginning two chemically different 
 electrodes, which have already an E.M.F., and do not require a 
 formation, or in some cases a formation lasting only a short time. 
 
 In the manufacture of accumulators it is extremely important 
 that the greatest care be taken that the pasted substance is se- 
 cured firmly to the lead grid. Various methods have been devised for 
 locking the material within the openings of the lead grids, one 
 example being shown in Fig. 186. 
 
 In order to give the Plante accumulators great capacity, it is 
 essential that the acting surface be as great as possible. Now, the 
 working surface of a plate of any particular size may be increased 
 
182 
 
 ELECTRICAL ENGINEERING 
 
 by providing it with very many ridges and cavities such as shown in 
 Fig. 187. 
 
 The chemical processes which take place in the accumulator are 
 by no means as simple as we have hitherto assumed them to be. We 
 have supposed that the water only in the cell is decomposed, and 
 that the acid serves merely for the purpose of improving the conduc- 
 tivity of the water. The sulphuric acid really has an influence on 
 the chemical process, and it has been found that the proportion of the 
 
 FIG. 187. Accumulator Plate. 
 
 quantity of water to that of the acid is of great importance. The 
 complete chemical changes which take place during the charging and 
 discharging of an accumulator are too complicated to be described 
 here. The main facts are these: During charging, spongy lead is 
 formed at one plate which is termed by cell makers the negative 
 plate; whilst at the other plate, called the positive, a dark red oxide 
 called lead peroxide is formed. On discharging the cell both the 
 lead and lead peroxide are changed to lead sulphate. 
 
 Under normal conditions an accumulator should never be dis- 
 
ACCUMULATORS 183 
 
 charged so that its E.M.F. falls lower than 1.80 to 1.83 volts. For 
 the purpose of charging, a higher pressure is necessary. Soon after 
 the beginning of the charge the E.M.F. of a cell rises to two volts, 
 and when completely charged becomes 2.5 to 2.7 volts. When gas is 
 evolved from both plates of the accumulator, the end of the charge 
 is indicated, and that no more oxygen is being absorbed by the 
 positive plate. 
 
 The rising of the voltage during charging is firstly due to the 
 chemical change of the plates, and then for two other reasons. There 
 are formed bubbles of hydrogen and oxygen, which increase the back 
 E.M.F. of the accumulator, and further the accumulator has of course 
 an ohmic resistance, to overcome which requires the expenditure of 
 certain E.M.F. 
 
 After charging, the voltage of the accumulator falls immediately, 
 down to 2.2 volts. If we connect the accumulator terminals with an 
 outer resistance, so that it supplies current, its terminal voltage 
 will fall still further, and more so the greater the current supplied. 
 This is partly due to the chemical alteration of the plates, and partly 
 due to the ohmic resistance of the cell. In charging we have to 
 make the terminal voltage larger than the E.M.F. of the accumulator 
 in order to overcome the ohmic resistance. But, in discharging, the 
 voltage drop caused by the ohmic resistance takes away part of the 
 E.M.F., so that the terminal voltage becomes smaller than the E.M.F. 
 of the cell. Hence the ohmic loss works to our disadvantage, both 
 in charging and discharging. 
 
 From an accumulator we cannot therefore get as much energy 
 as we put into it. The ratio between the quantity of energy which 
 we get in discharging to that energy which has to be spent for 
 charging is called the efficiency of the accumulator. With good 
 accumulators this is about 80 per cent. For 100 units of work put 
 into the accumulator we get about 80 units from the accumulator, the 
 remaining 20 units are transformed into chemical action and heat. 
 
 The accumulator is an excellent means for storing electrical 
 energy. If at any time there is electrical energy at liberty, we may 
 charge the accumulator, and afterwards obtain electrical energy from 
 it. We may, for instance, charge an accumulator during 10 hours 
 with 2 amps., and then take 20 amps, during nearly 1 hour; or we may 
 charge it with 20 amps, during 1 hour, and then obtain nearly 2 amps, 
 during 10 hours. The accumulator may be compared to a savings bank, 
 to which we may pay money from time to time in pence, and get back 
 in one payment a large sum: or to which we may pay at one trans- 
 action a large sum, to be withdrawn in small amounts as desired. 
 We may also, of course, charge and discharge the accumulator exactly 
 at the same rate. 
 
 This convenient transformation, of which we have just spoken, 
 is, of course, limited. The current passed into or taken out of an 
 
1S4 
 
 ELECTRICAL ENGINEERING 
 
 accumulator should never exceed a certain amount, or the plates will 
 
 be injured. The current an accumu- 
 lator can stand depends chiefly on the 
 size of the plates. To obtain large 
 currents with plates of reasonable size, 
 they are not made in one piece, but 
 consist of several plates connected in 
 parallel. Figs. 188 and 189 are 
 illustrations of accumulator cells, each 
 consisting of several plates. They are 
 placed side by side, so that any positive 
 plate lies between two negative ones, 
 and any negative (except the plates at 
 the two ends) between two positive 
 plates. Thick lead rods are used to 
 connect all the positive plates together, 
 and in a similar way all the negative 
 plates are in connection. The voltage 
 
 of the cell is, of course, equal to one consisting of two plates only. 
 
 FIG. 188. -Storage Cell. 
 
 FIG. 189. Storage Cell (General Electric Co.). 
 
ACCUMULATORS 
 
 185 
 
 To get the resistance as small as possible the plates are placed 
 very near each other. Direct contact of the plates is prevented by 
 glass or rubber rods placed between them. 
 
 Since in most cases far higher pressures than 2 volts are 
 employed, a number of accumulator cells are placed in series, forming 
 an accumulator battery. The single cells have then to be connected 
 so that the positive terminal of the first cell is connected with the 
 negative terminal of the second; and so on in series. Since any 
 other metal would be attacked and destroyed by the sulphuric 
 
 FIG. 190. Portable Storage Battery 
 
 acid or the spray arising from it, lead strips or rods are used for 
 connecting the poles. In Fig. 190 a battery is shown which 
 consists of four cells, mounted in a wooden box, which is lined 
 with celluloid. 
 
 If the battery has to feed 110 volt lamps, it must consist of about 
 60 cells ; for, as we have learnt, accumulators are discharged down to a 
 voltage of about 1.80 to 1.83, so that finally the voltage of 60 series- 
 connected cells becomes 110. 
 
 If, on the other hand, we had the lamps continuously switched 
 on the whole battery, this would be a great fault, for the voltage of a 
 single cell is at the beginning of the discharge more than two volte. 
 Thus the lamps at the beginning getting more than 120 volte, would 
 
186 ELECTRICAL ENGINEERING 
 
 be overrun, and have a short life only. Further, the lamps would 
 burn very brightly at the beginning, and darker later on. To prevent 
 this the lamps must at the beginning of the discharge be in connection 
 with a smaller number of cells. If the cell voltage be, for instance, 2, 
 
 then 55 cells at first are sufficient; 
 then, as the discharge continues, 56, 
 57, etc., cells are necessary to give 
 the voltage of 110. If the voltage 
 per cell falls to 1.83, then all the 
 60 cells will be required. 
 
 To easily secure this variation of 
 pressure, cell switches, as diagram- 
 matically shown in Fig. 191, are 
 employed. From each of the last 
 cells a cable leads to a number of 
 
 contacts, which are arranged in a 
 FIG. |191 Cell Switch. circle> and oyer which a metal lever 
 
 slides. The lamps are between cne 
 
 pole of the battery, and the lever of the battery switch. If the 
 latter is in its extreme left position, the lamps are connected to the 
 least number of cells. The cells on the right are then without effect, 
 they do not supply any current. In moving the lever to the right to 
 the next contact, one of the cells previously not in use is switched 
 into the circuit. As the battery voltage decreases the lever is moved 
 more and more to the right, and finally all the cells are in use. 
 
 As may be seen from the above, the end cells are not used so 
 much as the others, and therefore it is not necessary to charge them 
 as long as the main cells. The battery switch may therefore also be 
 used with advantage for charging the cells. First of all, the cells are 
 connected in series, the lever of the battery switch covering the last 
 contact. At the last cell, which is discharged but little, a strong 
 development of gas will soon be observed. If by means of a volt- 
 meter we examine the voltages of the single cells, the last will 
 probably show 2.5, whereas the voltage of the other cells will still be 
 lower, probably 2.3. The last cell is then switched off by removing 
 the lever of the battery switch to the last, contact but one. When 
 the last cell but one becomes fully charged, the lever is again moved 
 back one contact. Thus we see that on charging it is necessary to 
 move the lever gradually to the left, whereas on discharging we must 
 move the lever to the right as required. 
 
 A battery recently developed in America by Thomas A. Edison 
 has iron for its plates and for solution hydrate of sodium or caustic 
 soda. The chemical action here is that of oxidation, just as with 
 lead plates. The life of this battery is reported to be far longer than 
 the lead battery, and its weight per horse-power less. Many difficulties 
 have been met and overcome in manufacture, and while the sales 
 have not been large, those produced have given very great satisfaction. 
 
ACCUMULATORS 187 
 
 Machines for charging Accumulators 
 
 For charging a battery of 110 volts a pressure of 60 X 2.5 = 150 
 volts is required; sometimes the voltage per cell has to be raised up 
 to 2.72.75, bringing the voltage of the whole battery up to 165. A 
 dynamo employed for charging such a battery must therefore be 
 built for a far higher voltage than that used on the lighting 
 circuits. 
 
 For charging accumulator batteries, shunt dynamos are generally 
 employed. We know that with these machines by means of a shunt 
 regulator it is possible to alter the voltage within certain limits. 
 Machines for charging accumulators are now built so that by means 
 of a large shunt-regulating resistance their voltage can be varied 
 between 110 to 160. 
 
 Series and compound dynamos are practically never used for 
 charging accumulators. With a series dynamo the voltage increases 
 with the current. Hence if, by any means say by a resistance 
 inserted in the circuit instead of the cells we get such a voltage on 
 the dynamo as to get a certain current in the cells, then, on switching 
 in the latter, the E.M.F. of the accumulators will increase. The 
 result will be that the difference between the E.M.F. of the dynamo 
 and the back E.M.F. of the accumulators, and hence the current, will 
 decrease. Due to this smaller current, the E.M.F. of the series 
 dynamo will now fall, and it might then happen that a current will 
 flow back from the battery to the dynamo, and reverse its polarity. 
 The use of a series dynamo is therefore impossible. 
 
 Compound dynamos are, for similar reasons, also unsuitable for 
 directly charging accumulators. If, however, the voltage of the 
 compound dynamo be higher than the maximum accumulator voltage, 
 then with such a machine the cells may be charged by employing a 
 series resistance. In this case it is not to be feared that the 
 dynamo voltage may fall so low that a current will be sent back 
 from the accumulators through the resistance to the machine. 
 
 The most suitable and nearly exclusively employed machine .for 
 charging accumulators is the shunt dynamo. For, firstly, with 
 decreasing current its voltage increases, and it can therefore hardly 
 happen that its voltage should fall below that of the battery; and, 
 secondly, even if this should take place (for instance, through the 
 driving steam-engine running more slowly), a current would flow from 
 the battery into the machine in an opposite direction through the 
 armature only. The magnet coils are in this case traversed by a 
 current flowing in the same direction as before, the only difference 
 being that the current comes from the cells instead of from the 
 
188 
 
 ELECTRICAL ENGINEERING 
 
 Discharge 
 
 armature. Thus the polarity of the magnets is not reversed, and 
 the reversal of the current does not cause any subsequent injurious 
 effect, as it would do with series or compound dynamos. 
 
 With the means with which we have so far become acquainted, 
 it would not be possible to employ a dynamo for lighting and 
 
 charging accumulators simultaneously. 
 With two battery switches there is no 
 difficulty in doing this. In Fig. 192 it 
 is shown how the last few cells of the 
 battery are connected with the contacts 
 of two cell switches. The one which is 
 below in the diagram is called the 
 charging battery switch, the upper one 
 the discharging switch. 
 
 The machine is connected with the 
 first cell and the charging lever; the 
 lamps are connected with the first cell 
 and with the discharging lever. Now 
 we may produce with the machine a 
 voltage of 150, and charge with this 
 voltage the 60 series-connected cells. 
 The charging lever covers, for instance, 
 the last contact, but the discharging 
 lever is removed far to the left, so that 
 probably only 44 cells are switched on 
 the lamp mains. If the voltage of each 
 cell be 2.5, then the 44 cells will just 
 give the proper lighting voltage of 110. 
 The battery is in a manner fully 
 charged and simultaneously partly discharged. If, for instance, we 
 charge with 100 amps., and the lamps consume 10 amps, only, then 
 the full current of 100 amps, will flow through those cells only 
 which are between the charging and discharging levers. All the 
 other cells are charged with 100 - 10 = 90 amps. only. 
 
 As a matter of fact the lighting current of 10 amps, is not 
 supplied by the battery, but by the dynamo. W r e have in this case 
 a branched circuit. From the positive pole of the machine a current 
 of 100 amps, is flowing to the charging lever, giving a current through 
 the additional cells only. Through these the full current is flowing. 
 When the current comes to that cell which is connected with the 
 contact just covered by the discharging lever, the current has two ways 
 one through the whole battery to the negative pole, the other one 
 through the discharging lever and the lamps to the negative pole. 
 In the latter the branch currents are again combined, flowing back 
 to the negative brush of the dynamo. 
 
 This arrangement is only employed in cases where the current 
 
 FIG. 192. Double Cell 
 Switch. 
 
ACCUMULATORS 
 
 189 
 
 supplied to the mains during charging is small compared with the 
 charging current of the battery, otherwise a far larger current will 
 flow through the end cells than through the others. The former 
 must then be mad:) much 
 larger than the latter, or ; owing 
 to the larger currents, they 
 will be far sooner destroyed 
 than the other cells. 
 
 Sometimes, for raising the 
 voltage during charging a 
 special machine, a so-called 
 booster, is employed. The 
 main dynamo then supplies the 
 normal voltage, and it can 
 therefore, during charging, 
 supply current to the mains. 
 With the main dynamo the 
 booster is in series, by con- 
 necting the negative brush of 
 the latter with the positive 
 brush of the main dynamo (see 
 Fig. 193). The battery is 
 connected to the negative pole 
 of the main dynamo and the 
 positive pole of the booster. 
 The booster can be regulated 
 from a very low voltage up to about 50 volts (provided we have a 
 main voltage of 110, and 60 cells). 
 
 When we commence to change the cells the booster has to supply 
 a very low voltage, the excitation is very weak, but the charging 
 current might probably be very large. In such a case it may happen 
 that, owing to the armature reaction, which weakens the magnetism, 
 the polarity of the machine is changed. To prevent this, the booster 
 is generally separately excited. 
 
 FIG. 193. Connections for Booster 
 when charging. 
 
 Battery Switch 
 
 Fig. 194 shows one type of an accumulator battery cwitch. We 
 see from this illustration that the contact pieces are arranged in 
 a circle, .and that a lever with an elastic brush slides on them. 
 The lever is not quite as simple as are those of regulating resistances. 
 We observe a spiral of wire on it, and that there are fixed, not one, 
 but two contact brushes on the lever. If we had a single contact 
 
190 ELECTRICAL ENGINEERING 
 
 brush only, then there would be two possibilities : the brush may 
 be either narrower or wider than the insulating space between the 
 contact pieces. If we make the brush narrower, then in moving 
 the lever a break in the current takes place. This will cause a 
 violent sparking, and, if the motion of the lever be a slow one, the 
 lamps will be extinguished. Certainly with a quick motion of the 
 lever the current would not be entirely interrupted, nevertheless a 
 nickering of the lamps would be caused, and the contacts burnt by 
 the sparks. If, on the other hand, the contact brush be so wide 
 as to touch the second contact before it has left the first one, then the 
 flickering would, of course, disappear; but at the moment the brush 
 
 FIG. 194. Battery Switch (Voigt & Haffner). 
 
 covers two contact pieces, one of the cells is entirely short-circuited 
 thus causing a strong current, which would damage the cells. To 
 avoid this, near the narrow main brush which serves for taking 
 the current, another auxiliary brush is provided, which is con- 
 nected with the main brush by means of a resistance spiral of 
 German silver or nickelin, but is insulated from the lever. If 
 the lever stops, then only the main brush covers the contact, the 
 auxiliary brush is on the insulating piece between the contacts, 
 and thus is without effect. If we move the lever, in order to 
 switch in or off a cell, then, before the main brush has left the first 
 contact, the auxiliary brush covers the next one. The cell being now 
 between the two contacts is. of course, connected with a closed circuit, 
 
ACCUMULATORS 191 
 
 but is not ,s/ior/-circuited, for the resistance of the spiral is in the 
 circuit, and if this resistance be made sufficiently large, the current 
 produced by the cell will be only a moderate one. At the next 
 moment the main brush leaves the first contact, but since the 
 auxiliary brush now covers the second contact, there cannot be 
 a further interruption of the current, but the resistance spiral 
 is inserted in the outer circuit. If we left this resistance con- 
 tinuously switched in the circuit, energy would be wasted, for the 
 resistance spiral consumes nearly as much voltage as is supplied 
 by the last added cell. Hence we must remove the lever a little 
 more, until the main brush covers the contact piece, and the 
 auxiliary brush stands again on the insulating piece. In this way 
 both an interruption of the current and a short circuit is avoided, and 
 on moving the lever violent sparking is prevented. 
 
 The same effect may be obtained by connecting each cell with 
 two contact pieces, having one of them connected directly, and the 
 other one through a resistance spiral. A brush of double width 
 may then slide over the contacts. Battery switches of this construc- 
 tion have therefore twice as many contacts as those of the type 
 previously considered. 
 
 Charging and discharging switches may be combined in a single 
 apparatus, the double-cell switch. There is no difference in the 
 arrangement of the contact pieces between this and a single-cell 
 switch, but there are two levers insulated from each other, and 
 having arms of different length, sliding over the contact pieces. 
 This obviously produces exactly. the same effect as if we had con- 
 nected each of the end cells with the contacts of two different cell 
 switches. 
 
 In some cases it is desirable to save, during the time current 
 is taken from the battery, any attendance. In such cases an 
 automatic cell switch may be employed with great advantage. The 
 arrangement consists chiefly of a small motor, which, by a relay, is 
 switched in so as to run either clock- or counter-clockwise, according 
 to the voltage increasing above or decreasing below the normal value. 
 The contact brush of the cell switch is. by the motion of the motor, 
 then moved either upwards or downwards. When the normal voltage 
 is reached, the motor is switched off by the relay. 
 
 Accumulator Apparatus 
 
 We have previously learned that when machines and accumulators 
 work in parallel the current from the latter may possibly flow back 
 into the dynamos. We have further learnt that this danger is least 
 with shunt dynamos ; but in this case, also, a reversal of the current is 
 
192 
 
 ELECTRICAL ENGINEERING 
 
 liable to damage the accumulators. Assuming, for instance, that the 
 steam-engine driving the dynamo runs somewhat slower, then there may 
 come a moment in which the E.M.F. of the dynamo is smaller than 
 that of the accumulators. The dynamo will then consume current, 
 and run as a motor driving the steam-engine. Hence the accumulators 
 are discharged with a current, which may be dangerously large. 
 To prevent this minimum cut-outs are provided in the accumulator 
 circuit. Fig. 195 shows such an apparatus. There are two cups 
 filled with mercury, into which can dip a piece of metal, U-shaped 
 and fixed on a movable lever. The latter has an iron axle, with 
 which two small iron rods are connected, which project backwards. 
 These iron rods are connected by a brass strip, on which is fixed 
 
 FIG. 195. Minimum Cut-out (The Electrical Company}. 
 
 a counter-weight, pulling downwards the back part of the apparatus , 
 thus tending to lift the U-shaped piece out of the mercury. Over 
 the iron axle a copper spiral is wound, one end of which is connected 
 with the inner mercury-basin, the other end with one main terminal. 
 The outer mercury-cup is in connection with the second main 
 terminal. If a current flows through the copper spiral, both the 
 iron axle and, the two iron rods projecting backwards become 
 
ACCUMULATORS 193 
 
 magnetized, the whole arrangement representing then a horseshoe 
 electro-magnet. Imagine now one terminal to be connected with 
 the machine, the other one with the accumulators, then, in the 
 position of the apparatus shown in the figure, no current can flow 
 through the spiral. If, now, we wish to charge the accumulators, we 
 have to bring the dynamo to a voltage which is larger by a few volts 
 than that of the accumulators. Then we have to lift the counter- 
 weight of the minimum cut-out, so that the U-shaped metal piece 
 dips into the two mercury-cups, and the limbs of the electro-magnet 
 knock against the iron bar. In doing so we close the dynamo circuit. 
 The current coming from the dynamo flows from the right main 
 terminal to the mercury-cup, through the U -shaped metal bridge 
 (which is insulated from the lever) to the second mercury-cup, from 
 there through the copper spiral, and from the latter to the second 
 terminal of the apparatus and to the accumulators. The current 
 magnetizes the horseshoe-shapen iron pieces of the movable part, so 
 that it sticks to the iron bar above. With a current over a certain 
 value, this attraction is so great as to overcome the effect of the 
 counter-weight, and to keep the movable part in this position. 
 If, however, the dynamo voltage falls, then, at the moment in 
 which the E.M.F. of the accumulators is equal to the chanring 
 pressure, the current flowing in the circuit will be nil. and . the 
 electro-magnet of the movable part will lose its magnetism. It 
 no longer keeps fast the iron bar, and the counter-weight will 
 lift the iron bridge from the mercury-cups, thus disconnecting the 
 circuit. 
 
 If the electro-magnet touches the bare iron keeper, then, owing to 
 the remanent magnetism, the proper working of the armature is pre- 
 vented. To avoid this, two brass screws are provided, which project 
 over the electro-magnet, thus preventing the armature from direct 
 contact with the electro-magnets. 
 
 In some cases, instead of the minimum cut-outs, maximum cut- 
 outs are employed. These act when the current exceeds a certain 
 amount. The electro-magnet, excited by a current passing around 
 the spiral, causes, in this case, by attracting the iron armature, a break 
 in the circuit. 
 
 Maximum cutouts, both for accumulators and for motors, prevent 
 them from being heavily overloaded. As we know, fuses are generally 
 selected so that they will melt with a current of double the normal 
 value. Since, now, this increased current cannot do the mains any 
 harm, but may in some cases seriously damage the motors or cells, 
 a maximum cut-out is desirable, which is so adjusted that when 
 the allowable current is exceeded, the circuit is automatically dis- 
 connected. 
 
 We have still to mention the current indicators, which are 
 generally inserted in the accumulator circuit. When the current 
 
194 
 
 ELECTRICAL ENGINEERING 
 
 flows through the accumulators in such a direction as to charge 
 them, the pointer of the instrument indicates "charge;" if the 
 current flows in an opposite direction the pointer indicates " dis- 
 charge." See Fig. 196. 
 
 If in the accumulator circuit be 
 inserted a Deprez ammeter, with 
 the zero in the middle, as shown 
 in Fig. 197, a current indicator 
 becomes superfluous. 
 
 To examine the state of the 
 single cells, a little voltmeter is 
 used with a range of 3 volts, one 
 
 FIG. 196. Current Indi- 
 cator (General Electric 
 Co). 
 
 FIG. 197. Ammeter with Central 
 Zero Point (Berend & Co). 
 
 FIG. 198. Voltmeter for Cell Testing 
 
 (Berend & Co.}. 
 
 terminal of which terminates in a point; the other end is connected 
 with a short cable, the end of which also terminates in a point. The 
 two points are then pressed against the positive and negative 
 electrodes of the cell respectively, and so the cell voltage is measured. 
 An accumulator tester of this kind is shown in Fig. 198. 
 
ACCUMULATORS 195 
 
 Applications of Accumulators 
 
 Accumulators serve many purposes. In central stations for small 
 towns the current consumption is considerable only during a few hours 
 of the day, whereas during the remaining time comparatively few lamps 
 are in use. It is then very uneconomical to run the machines during 
 the whole day and night. If the machines run during the day, when 
 the demand for current is little, they are under a small load. 
 Dynamos and steam-engines running with a small load have a low 
 efficiency. Whilst, for instance, with a full-loaded steam-engine 
 plant the coal consumption per useful kilowatt-hour varies between 
 3J and 6 pounds (according to the size of the plant), this consumption 
 may with machines that are very little loaded increase up to 
 1220, and even more, pounds. But if an accumulator be used, this 
 may be charged during some hours of the day, enabling the machines 
 to run with greater load. During the whole time of small demand 
 of current the accumulator alone is sufficient. The machines are 
 stopped during this time, and are started again during the time of 
 maximum demand, when they can be assisted by the battery, so that 
 at the time of the maximum demand a larger current may be supplied 
 to the mains than could be delivered by the machines themselves. 
 
 In factories where electricity is used both for lighting and power 
 transmission, during the working hours much current is consumed by 
 the electric motors, and, in addition, in the evening a large current is 
 needed for lighting workshops, office-rooms, etc. After the work- 
 ing hours but little current is required for lighting special rooms, 
 corridors, yards, etc. This current is then supplied by the accumu- 
 lators, which may be charged again during the working hours. If 
 the battery is of sufficient capacity, even motors for driving small 
 lathes and other tools may, after the end of the general work, be 
 provided with current from the battery. 
 
 Excellent service may be rendered by accumulators as buffer 
 batteries when working in parallel with dynamos. In central electric 
 stations, both for lighting and traction purposes-, shunt dynamos 
 are frequently employed. These dynamos have, as we know, the 
 property, that with an increasing current the terminal voltage 
 decreases. Now, in all central stations in which electro-motors are 
 installed on the mains, sudden rushes of current occur, due to the 
 switching in or sudden loading of motors. This causes a sudden fall 
 of the voltage. Now the shunt regulator cannot be worked so 
 quickly as to prevent fluctuation of the lamps on the network. 
 The same thing takes place if the load is suddenly thrown off 
 the dynamos. In this case the voltage increases rapidly. Again, 
 
196 ELECTRICAL ENGINEERING 
 
 with central stations for traction purposes it may happen that the 
 current consumption increases for short periods to three, four, or 
 even five times the average demand, as when several cars start 
 simultaneously. This will cause the voltage of the dynamo sup- 
 plying the current to suddenly fall an undesirable amount. If, 
 however, there is a battery of suitable size working in parallel with the 
 dynamo, then it will supply current at these times of sudden demand, 
 and prevent the voltage of the mains from falling lower than that of 
 the battery. When a great number of amperes no longer is needed, 
 the E.M.F. of the dynamo will tend to rise, and the battery will 
 now be charged. In the opposite case when the current consumption 
 increases beyond the normal output of the dynamo, the battery will 
 again supply current to the mains. The battery thus is ready for 
 any sudden rushes, and acts just like a buffer between dynamo and 
 network. The dynamo will, therefore, if running in parallel with 
 a battery, work with a far steadier load, and thus prove more 
 economical than without the battery. 
 
 ^ If there be water power of comparatively small amount, then we 
 might accumulate in a battery energy during the whole day, and 
 during the evening take a comparatively large amount of energy 
 from the battery. This system is frequently used for lighting 
 purposes. 
 
 In the cases hitherto dealt with, the batteries have been stationary. 
 In many cases portable accumulators are employed, both for lighting 
 and power purposes. For lighting railway cars, for instance, accumu- 
 lators are charged at a terminus, and put in a special box be- 
 neath the car. From them glow lamps can be supplied with the 
 necessary current to light the car. 
 
 When it is wished to avoid trolley wires in streets, the car can 
 be provided with accumulators, which may be charged at special 
 charging stations. If only at certain parts of a line trolley wires are 
 not allowed to be used, a combined system is possible. The car 
 is then provided both with accumulators and trolley equipment. 
 On some parts of the line the current is taken from the overhead 
 trolley wires, and at the same time the accumulators are also 
 charged with this current. Along the other parts of the line the 
 accumulators supply the necessary electrical energy to the motors. 
 
 Since storage batteries with a great capacity have a considerable 
 weight, accumulator cars are generally far heavier than cars with 
 motors only. It may also happen that, if the battery is not 
 sufficiently charged, or the state of the street on account of dirt, 
 snow, etc., is very difficult for traction, the car may be brought to 
 a stop, because the battery is exhausted. For this reason neither the 
 accumulator alone, nor the combined system is very reliable, and it 
 is more satisfactory to have a special underground system whenever 
 the overhead system cannot be employed. 
 
ACCUMULATORS 197 
 
 Motor cars may also be driven electrically, and provide an 
 extended application for portable batteries. 
 
 The same remark applies to boats and launches. The spindle 
 of the screw is coupled directly with the motor, and the latter is fed 
 by a battery. 
 
 The accumulator has a considerable advantage over primary 
 galvanic cells. It can, by charging, be restored to its former state, 
 whereas with primary cells this is not possible. 
 
 The student must clearly understand that the accumulator does 
 not store electrical energy. It stores chemical energy which is con- 
 verted into electrical energy when the cell is discharged, the electrical 
 power depending on the rate of discharge. We must carefully 
 distinguish between electrical power and electrical energy. For the 
 former a convenient unit is the watt, for the latter the unit 
 mentioned on p. 195, the kilowatt-hour is in commercial use. 
 
CHAPTER VI 
 
 WORKING OF DIRECT-CURRENT DYNAMOS IN PARALLEL 
 
 ALTHOUGH dynamos are built of great output (2000 kilowatts and 
 over), it is seldom the case that there is only a single dynamo erected 
 for the whole output of a central station, but generally two or more 
 dynamos are used, each of which has to supply a part of the total 
 output. ^This division of the plant is for several reasons.- First of 
 all, continuity of service must be maintained. If an accident 
 
 FIG. 199. Shunt Dynamos ready for Switching in Parallel. 
 
 happens to one of the machines, there are still others to maintain, to 
 a certain extent, the demand. Secondly, it is possible to run one or 
 more machines nearly fully loaded, as required, and hence they will 
 work at the highest efficiency. 
 
 When several dynamos are used, it is usual to arrange them in 
 parallel on the same network. For this purpose shunt dynamos are 
 
 198 
 
WORKING OF DYNAMOS IN PARALLEL 199 
 
 suitable (see Fig. 199) as well as compound. It is almost impossible 
 to combine series dynamos in this way. 
 
 Imagine, for instance, two series dynamos working in parallel; 
 these would alter their voltage continuously, according to their load. 
 Assuming that their E.M.F. >s were, at a definite load, just alike; 
 then, with an increasing load, their voltages would rise. It is, however, 
 not at all certain that these pressures will rise equally. At 
 the double load, the voltage of one dynamo may increase 20 per 
 cent., whereas that of the other one perhaps only 15 per cent. But 
 at the same moment the latter machine supplies less current, hence 
 its armatures will instantly lose or nearly lose its voltage. The 
 result will be that the current flows through it from the other 
 dynamo, and, being in an opposite direction, reverses its poles. The 
 same thing may take place without a variation of the load if the 
 speed of one of the dynamos decreases. 
 
 With shunt dynamos we know that the voltage varies also accord- 
 ing to the load, but in an opposite way. The voltage increases on 
 decreasing, and decreases on increasing, the load. If, at a given 
 time, the load be equally divided between two machines and then 
 the load is suddenly decreased, it is also possible here that the E.M.F. 
 of one dynamo is greater than that of the other, so that the E.M.F. of 
 the first dynamo rises, say from 110 to 113, whereas that of the second 
 dynamo changes from 110 to 112 volts. This will, however, have 
 only the consequence that the first machine will supply more current 
 than the second one until the E.M.F. of the first machine is equal 
 to that of the second one. Even assuming that the second machine, 
 due to the slower speed of the driving engine, remains with its 
 E.M.F. so low as no longer to supply, but to consume, electrical 
 energy, this will only cause the other machine to be overloaded. 
 The dynamo taking current will now run as a motor, but a reversal 
 of the poles docs not occur, because the current flows around the 
 magnets in the same direction as before. 
 
 It is quite another matter with compound dynamos. If with 
 these a reversal of the current happened, great inconvenience would 
 arise, since the reversed current, flowing through the series-coil, 
 would weaken the magnetism, which is produced chiefly by the 
 shunt-coil. A means of avoiding this by the use of a so-called 
 equalizing wire (the connection of which with the poles of the 
 dynamos is shown in Fig. 200) has been devised. This equalizing 
 wire must be connected with those poles of the dynamos with which 
 are also connected the ends of the series windings. As long as both 
 armatures have the same voltage the current will not flow through 
 the equalizing wire, but only through the two mains. 
 
 Now let us consider what will take place if, by any accident, the 
 E.M.F. of one machine becomes lower than that of the other machine 
 so that it now consumes, instead of delivers, electrical energy. 
 
200 
 
 ELECTRICAL ENGINEERING 
 
 Assume that machine II. is taking, and machine I. is supplying, 
 current, then the current will flow from the negative terminal of 
 machine I. through the equalizing wire, through machine II. (in an 
 opposite direction to that when supplying current), and through the 
 positive bus-bar back to the positive brush of machine I. Thus 
 a current flows in an opposite direction through the equalizing 
 
 FIG. 300. Compound Dynamos ready for Switching in Parallel. 
 
 wire and the armature of machine II., but not through its series 
 coils. 
 
 When a dynamo is run in parallel with secondary batteries, the 
 shunt dynamo only need be taken into consideration. 
 
 If there are neither secondary batteries nor other dynamos it is 
 most suitable to employ the compound winding, since it gives a con- 
 stant voltage as long as the armature speed does not vary. 
 
 The series dynamo is suitable for supplying current for a 
 single circuit either for a large number of series-connected glow- 
 or arc-lamps, or for a single motor as a generator for power 
 transmission, as previously described, or for boosters to raise 
 
WORKING OF DYNAMOS IN PARALLEL 
 
 201 
 
 the voltage in proportion to the current. In some cases several 
 series dynamos have been connected in series, serving as generators 
 for a number of series motors connected in series. With this arrange- 
 ment of dynamos pressures of some thousands of volts have been 
 produced and used for long-distance power transmission. The cases 
 are, it must be added, quite exceptional. For power transmission 
 over long distances alternating currents are employed almost ex- 
 clusively. 
 
 Switching Dynamos in Parallel 
 
 When starting a dynamo which has to be run in parallel with 
 either a secondary battery or another dynamo, we have to be quite 
 certain that the leads are of the right polarity. For if we connected 
 the positive pole of one machine with the negative pole of another, 
 and vice versd, the two machines would not be connected in 
 parallel, but in series without any external resistance, giving a short 
 circuit supplied at double the voltage. 
 To make sure about the polarity, 
 proceed in the following way: Bring 
 both machines to the same voltage, 
 say 110 volts, then close the switch 
 
 1 (see Fig. 201), and connect the 
 ends of two series-connected 110 
 volt lamps with the contacts of 
 the switch 2, which must be kept 
 open. If the polarity of the two 
 machines is all right, no voltage 
 can exist between the poles of switch 
 2, and consequently the lamps cannot 
 glow. If, on the other hand, the 
 polarity of the two machines is 
 wrong, then there will be a double 
 voltage 220 in the case supposed 
 
 between the two contacts, and the two lamps will glow with their 
 normal intensity. The machines have then to be stopped, and the 
 cables of one machine reversed. 
 
 Instead of changing the cable connections the polarity of the 
 machines may be altered. For this purpose the brushes of machine 
 
 2 must be lifted off the commutator, and switches 1 and 2 closed. 
 By doing this machine II. is excited in the right direction. Now 
 
 FIG. 201. First Method of 
 securing Right Polarity. 
 
202 
 
 ELECTRICAL ENGINEERING 
 
 Voltmeter 
 
 open switch 2, and put the brushes on the commutator, when it will 
 be found that the polarity of the machine has been reversed. 
 
 If the machines are provided with two-pole switches instead of 
 single-pole ones, the procedure for finding the polarity just described 
 
 may be applied by connect- 
 ing temporarily the contacts 
 of one side of the switch of 
 machine II. by means of a 
 wire or strip of metal. 
 
 To ascertain the polarity 
 of a machine, Pole-finding 
 Paper is sometimes em- 
 ployed. It is made of paper 
 impregnated with a chemical 
 substance. When the paper 
 is wetted and included in a 
 circuit, the electrolytic action 
 
 ^ a ^ ensues causes the paper 
 at the end connected with 
 the positive pole to become 
 of one colour, whilst around 
 the negative end a different 
 colour will be noticed. 
 
 The examination becomes 
 simplest if there is a Deprez 
 
 /oltmeter 
 switch 
 
 FIG. 202. Second Method of securing 
 Right Polarity. 
 
 voltmeter provided with a 
 voltmeter switch fixed on 
 If on closing the voltmeter 
 
 the switchboard as shown in Fig. 202. 
 switch, so that it indicates the voltage of first one machine and then 
 the other, the pointer of the voltmeter is deflected in the same 
 direction in both cases, the machines are of the same polarity; if 
 otherwise, then the machines are of opposite polarity. 
 
 A similar method is used in putting compound dynamos in parallel, 
 but after throwing them together for the first time it must be found 
 that the series fields act together. It is necessary to close the 
 equalizer connection switch either before or simultaneously with 
 the other connections. Triple-pole switches are generally used for 
 this purpose. 
 
CHAPTER VTI 
 ELECTRIC LIGHTING 
 
 Glow Lamps 
 
 ONE of the first phenomena of the electric curreat with which we 
 became acquainted was the heating; of a wire through which a 
 current flows. The first idea was, therefore, to heat metal wires 
 by the electric current to such a high degree as to cause them to 
 glow and emit light. The common metals, however, alter their 
 nature when heated in the air, and therefore a metal which has the 
 property of not changing its nature, such as platinum, must be 
 employed. The incandescent or glow lamps, manufactured in this 
 way are very expensive, and, further, have a serious defect. Metals 
 do not grow bright until they are raised to a temperature which is 
 near to their fusing point. Hence, if through a platinum lamp a 
 current flows which is a little greater than the normal one, the 
 platinum filament will instantly fuse. 
 
 Fortunately there is a solid conductor which is neither a metal 
 nor fusible. This conductor is carbon. If in the open air we heat 
 a carbon filament to such an extent as to make it incandescent, it 
 will soon be burnt. Hence the electric heating of the filament must 
 be done in the absence of oxygen, a gas necessary for combustion. 
 This has been effected by enclosing the filament in a glass bulb from 
 which the air, and with it the oxygen contained in the air, has been 
 carefully exhausted. 
 
 The greatest practical difficulty consisted in finding out a method 
 of obtaining carbon strips of sufficiently small sectional area and 
 regular structure. This has been overcome by either carbonizing a 
 cotton or silk thread directly, or a filament formed by "squirting" 
 a solution of cellulose through a fine nozzle at high pressure. 
 Cellulose is the chief constituent of such vegetable substances as 
 cotton, linen, paper, etc. 
 
 The first to make carbon lamps practical were Edison and Swan. 
 The former used a carbonized and horseshoe-shaped fibre of bamboo, 
 
 203 
 
204 ELECTRICAL ENGINEERING 
 
 enclosed in a glass bulb from which the air was exhausted. The 
 connection between the carbon and the external conducting wires 
 was secured by short pieces of platinum wire fused through the glass 
 Since then, the manufacture of glow lamps has been very much 
 improved. Nowadays the filament is generally made from cellulose 
 in the way described above. 
 
 We shall now briefly deal with the method of making modern 
 glow lamps. The filaments of cellulose, having been dried, are cut 
 to about the desired length, sufficient margin being allowed for 
 making connections with the platinum wires, which pass through the 
 bulb to the external circuit. They are then subjected to the process 
 of "carbonizing/' which converts them into solid carbon filaments. 
 Each filament is then held by clips connected with suitable termi- 
 nals, by means of which connection can be made with a dynamo or 
 a secondary battery. Next, the suspended filament is placed in an 
 atmosphere of gas rich in carbon, and a current sufficiently strong 
 to raise it to a white heat is passed through the filament. If there 
 should be, as is generally the case, any inequality in the filament, 
 causing a variation in its resistance, one portion will be raised to a 
 higher temperature, and upon this hotter section a greater deposit of 
 carbon will take place. This process is therefore continued until 
 the filament is of equal thickness, that is to say, until it becomes 
 uniformly luminous throughout. 
 
 The glass in which the platinum wire with the carbon filament 
 is fixed is now fused to the bottom part of the bulb, and finally the 
 latter is exhausted of its contained air and moisture. 
 
 For connecting the filament to the external circuit many methods 
 are employed. The type of holder generally used in England is 
 
 known as the Swan or Bayonet 
 holder. The lamp is, in this case 
 (see Fig. 203), provided with an 
 insulated brass collar fixed with 
 cement, the filament being con- 
 nected to the two brass segments 
 FIG. 203. Brass Cap of Glow Lamp embedded in the cement. The collar 
 (The General Electric Co.). has two small side pins, which fit 
 
 into the "bayonet joint" holder. 
 
 There are, besides this, many other types of lamp-holders, such as, 
 for instance, the Edison, the Siemens, and others. 
 
 Generally glow lamps are tested by means of a photometer before 
 they are sent out, and their candle-power, as well as the voltage at 
 which they are to be used, is marked on them. If a lamp, designed 
 to give 16-candle-power at a voltage of 110, be connected with a 
 lower voltage, it will give out less than sixteen candles; if, however, it 
 is connected with a higher voltage, it will burn with greater candle- 
 power. The use of lamps on a higher voltage than that for which 
 
ELECTRIC LIGHTING 205 
 
 they are designed and tested, destroys them after a short time. The 
 life of an incandescent lamp, or the number of hours that it can 
 maintain illumination, varies considerably, but as an average period 
 for such lamps, which are used at the right voltage, about 1000 hours 
 may be taken. With lamps that have been in use for some time, 
 the vacuum deteriorates more or less, the carbon of the filament is 
 deposited on the interior of the bulb, thus diminishing its trans- 
 parency, till finally the filament is broken at its weakest point, and 
 the lamp becomes useless. 
 
 If a lamp is supplied with a higher than its normal voltage, this 
 reduction of the luminous effect and the destruction of the filament 
 takes place much more rapidly. 
 
 Tests on lamps burning on a higher pressure than the normal 
 voltage prove that their efficiency that is to say, the ratio of 
 the light emitted to the watts absorbed by the lamp is higher 
 than when the lamps burn at the proper voltage. The 16-candle- 
 power glow lamps usually employed consume about 50 to 55 watts (in 
 all the examples we have given in the first part of this book we have 
 assumed this consumption to be 55 watts =110 volts X 0.5 amps.), 
 whereas, lamps burning with a higher than their normal voltage 
 consume a greater number of watts, but the light emitted by them 
 is increased to a far greater extent than their watt consumption. 
 This fact has been taken advantage of when manufacturing glow 
 lamps of higher efficiency for instance, lamps which require 2J, 2, 
 or even less watts per candle-power. These lamps deteriorate far 
 more rapidly than those having a lower efficiency. Hence the 
 advantage of the lower cost of current when employing " high 
 efficiency" lamps is diminished by the necessity of frequent 
 renewals. The lamps which are most generally in use consume 
 about 3 to 3J watts per candle-power. 
 
 Until a few years ago, filaments for a higher voltage than 
 110 could not be satisfactorily manufactured. For a higher 
 voltage the filament has, naturally, to be longer and, at the same 
 time, thinner. Such a filament is, of course, very fragile, and for 
 a long time the difficulty of manufacture was insurmountable. By 
 improving the quality of the carbon, this difficulty has been over- 
 come, and nowadays glow lamps for 220, and even 250 volts are 
 manufactured almost of the same quality as 110- volt lamps. 
 
 A new system of electric incandescent lamps has been invented 
 by Professor Nernst, of Goettingen. He employs as filaments 
 second-class conductors that is to say, bodies which are insulators in 
 a cold state, but become conductors of the electric current when 
 heated nearly to redness. The filament has therefore to be heated 
 before the lamp can be made to glow. This may be effected either 
 by means of a small spirit-lamp or, automatically, by means of a 
 platinum coil, surrounding the filament. 
 
 The diagram of connections for a lamp in which the filament has 
 to be heated by a flame, is shown in Fig. 204. The automatic type 
 
206 
 
 ELECTRICAL ENGINEERING 
 
 is like Fig. 205. Here the current traversing the lamp from the + to 
 
 + -f 
 
 :FiG. 204 Nernst Lamp Fila- 
 ment Heated by Flame. 
 
 FIG. 205. Nernst Lamp with Electric 
 Heating. 
 
 the pole has two ways open : one 
 through the elastic armature of a small 
 electro-magnet to the platinum heating- 
 coil, and another one through the winding 
 of the electro-magnet, next through a 
 series resistance of iron wire (marked by 
 a zigzag line in the diagram), and then 
 through the short thick filament or rod 
 made of the special substance. On switch- 
 ing in the lamp, the rod is still cold, and 
 thus not conducting; the current can 
 therefore only flow through the armature 
 and the heating-coil. As soon as the rod 
 is made to glow by the heating effect of 
 the platinum coil, the current traverses 
 the second path through the winding of 
 the electro-magnet, the series resistance, 
 FIG. 206.-Six-Glower Outdoor and the rod itgelf . the electro-magnet is, 
 
 therefore, able to attract the _ elastic 
 
 armature, and disconnect the first circuit, the platinum coil being 
 then switched out of circuit. 
 
ELECTRIC LIGHTING 207 
 
 A resistance in series is required with both types of the Nernst lamp, 
 since the rod of special material is very sensitive to variations of the 
 current, and without this steadying resistance would melt at the slight- 
 est rise of voltage. The steadying resistance is made of iron wire, whose 
 resistance increases comparatively rapidly with rise of temperature. 
 
 The materials from which the glow-rods are made stand a far 
 higher temperature than platinum or carbon. The luminosity of a 
 source of light being greater the higher the temperature of the glowing 
 material, the Nernst lamp is more efficient than that of a carbon fila- 
 ment glow lamp. It consumes only about 1 watts per candle-power. 
 
 Since the materials employed for the Nernst lamp have, even 
 in a hot state, a far higher specific resistance than carbon, the glow- 
 rods are, for a given voltage and candle-power, far shorter and thicker 
 than the corresponding carbon filaments. Thus the rods are much 
 more solid, and can be manufactured for 220, 300, and even 400 volts. 
 
 Much thought has been given recently to produce a material which 
 will stand a higher temperature than carbon for filaments for incan- 
 descent lamps. There has been brought out a lamp having a filament 
 of titanium. This lamp will operate with a life of 1000 hours, con- 
 suming only 2 watts per candle instead of 3.1 taken by ordinary in- 
 candescent lamps. A filament made of uranium also gives excellent 
 results. Unfortunately, these metals are not so common as to make 
 the lamps cheap. Other metals are being tried, notably tungsten, 
 which gives even better economy with satisfactory life. When it is 
 realized how little of the energy delivered from a dynamo actually 
 appears as light, it can be understood how important any develop- 
 ments of improvements in incandescent lamps are to the electrical 
 industry. Undoubtedly, the time is not far off when lights will con- 
 sume far less energy than at present. 
 
 Arc Lamps 
 
 Whenever a circuit is broken a spark is produced. We must 
 now try to make clear why this should be. On opening a switch 
 or disconnecting a live main, the actual breaking of the current 
 requires a definite time. During this time the contact, originally 
 a very good one, becomes worse and worse, and the surface of the 
 touching parts becomes smaller and smaller. The result is that 
 resistance is introduced, and heat is produced. The temperature 
 becomes finally very great, so that the ends of the conductors 
 begin to glow, and emit glowing metal vapour, which, even after 
 some time, when the two conductors have been separated a little 
 distance from each other, may cross the gap, and form a conducting 
 luminous bridge, called the arc. 
 
 To obtain a continuous arc, metal rods are not suitable, because 
 they soon fuse and evaporate. Carbons are in every way preferable. 
 The carbons which are connected with the two mains are first of all 
 brought together, so that the current can flow from one carbon to 
 
208 ELECTRICAL ENGINEERING 
 
 the other. The contact surface offers a comparatively high 
 resistance, so that the carbon ends begin to glow. Then they are 
 removed some sixteenths of an inch from each other. The arc 
 that is formed continues, since the highly heated air and the carbon 
 vapour form between the two electrodes a conductor of very high 
 resistance. The arc itself does not emit the greatest part of the 
 light, but the carbon points, especially the positive, are the chief 
 source of light. 
 
 The two carbons are not, with a continuous current, consumed 
 at an equal rate, the consumption of the rod connected to the 
 positive pole of the dynamo being approximately twice as fast as 
 that of the other, or negative carbon. After burning some time, 
 the end of the positive rod becomes concave, forming a crater, and 
 in the hollow of this crater the most intense heat is developed, 
 making it, therefore, the chief source of light. The negative rod 
 is gradually consumed until its extremity is of a conical shape. 
 With continuous-current lamps the lower or negative carbon is 
 usually thinner than the upper one, the object being to make the 
 consumption of the carbons equal as regards length. 
 
 If we calculate the current strength of an arc lamp after Ohm's 
 Law, we arrive at an incorrect result, just as with the calculation 
 of the current in a liquid (see p. 179). The arc is, like a storage 
 cell, the seat of a back E.M.F., but which ceases immediately the 
 current stops. The back E.M.F. of the arc is very considerable, 
 and the current has also to overcome the ohmic resistance of the 
 arc. Hence the applied pressure must be above a certain value. 
 The voltage required for an ordinary arc is about 35 to 40 volte. 
 With special types, where the arc is formed in a partial vacuum, 
 and is very long, the voltage may be 80 or more. It is quite 
 impossible to get a continuous arc with a voltage of less than 30 to 35. 
 Glow lamps may, as we know, be built for any pressure, since they 
 have an ohmic resistance only, and thus the dimensions of the 
 filament may be made according to the voltage. There are, for 
 instance, glow lamps which require a voltage of but 2, and can 
 therefore be fed by a single accumulator cell. With arc lamps 
 this is impossible. 
 
 For an arc lamp a special mechanism is necessary. First of 
 all, the two carbon rods have to be placed in contact, and then 
 have to be separated, . so that an arc is formed between them. 
 Further, in order to maintain the arc, it is also essential that 
 some device should be provided for " feeding '' the carbons 
 together at a rate proportionate to their consumption. Generally 
 the electro-magnetic effect of the current is used to operate this 
 mechanism. 
 
 In Fig. 207 is shown one of the different types. The upper and 
 lower carbon holders are suspended from a flexible wire or a chain, 
 
ELECTRIC LIGHTING 
 
 209 
 
 passing over a roller. The upper carbon holder is provided with 
 an iron core, which moves within a fixed coil. The latter is con- 
 nected in series with the arc, so that the current forming the arc 
 also flows through this coil. If, now, the iron core is pulled up 
 by the action of the coil, the upper carbon holder is lifted, and 
 the bottom carbon is lowered, so that the carbons are separated. 
 If, on the other hand, the iron core is less attracted by the coil, it 
 will descend, causing the carbons to approach. 
 By selecting the weight of the iron core, then, 
 at a certain current strength, the attraction 
 of the solenoid and the weight of the core 
 are just balanced. Assuming now the arc 
 lamp, having a resistance in series, to be con- 
 nected to a constant voltage, then, after a 
 short time, due to the burning, the resistance 
 of the arc will be increased, and the current 
 will be decreased. With the weakened 
 current the attractive power of the coil 
 becomes smaller, the weight of the carbon 
 and its holder, therefore, causes the carbons 
 to be brought nearer together, until the 
 diminution of the arc resistance causes the 
 current to be increased to such an extent 
 as to again balance the weight of the carbon 
 holder. If, on the other hand, the iron core 
 falls too far down, so that the arc is shorter 
 than normal, then, the resistance being de- 
 creased, the current becomes greater, and the 
 attractive force of the solenoid overcomes 
 the weight of the iron core, with the result 
 that the carbons are separated a little. 
 
 The regulating solenoid being connected in series with the arc, 
 this lamp is called a series lamp. It tends, as we have seen, to 
 maintain a constant current, and is very suitable for use on a 
 constant voltage supply. 
 
 Let us now arrange a number of such lamps in series, and in 
 a circuit in which the current strength is kept constant by any 
 means, then it will be found that the regulating mechanism is abso- 
 lutely useless. For it is evident that as long as the current in the 
 circuit remains constant, the lamp will not regulate even if, owing 
 to the consumption of the carbon, the arc much exceeds its normal 
 length. Hence the voltage at the terminals of the lamp, which is 
 usually 40 to 45, may grow to 80, and even more. If, finally, the 
 resistance of the lamp becomes so great that the dynamo is unable 
 to supply this higher voltage at the normal current, then the latter 
 will decrease, thus causing all the solenoids to affect the length of 
 
 FIG. 207 Series Arc 
 Lamp. 
 
210 
 
 ELECTRICAL ENGINEERING 
 
 the arcs, although in some lamps the length of the arc might have 
 been the right one. 
 
 In such cases, instead of series lamps, shunt lamps may be 
 employed. A scheme of a shunt lamp is shown in Fig. 208. The 
 solenoid, consisting of many turn^ of a very fine wire, is arranged 
 so as to tend to lift the lower carbon holder. The solenoid 
 itself is connected with the two terminals of the lamp, thus being 
 in shunt with the arc; therefore the current traversing the 
 solenoid is greater the higher the voltage of the arc. At the 
 proper voltage of the arc ; the weight of the iron core is just counter- 
 balanced by the attraction of the solenoid. But if, due to a burning 
 of the carbons, the length of the arc is increased, its voltage will also 
 rise, provided that the current strength remains constant. Thus, if, 
 owing to the higher voltage, a larger current flows through the shunt 
 
 FIG. 208. Shunt Arc Lamp. 
 
 FIG. 209. Differential Arc Lamp. 
 
 coil, the action of the solenoid will preponderate, lift the bottom, and 
 lower the upper carbon holder, so that the arc is again shortened to 
 its right length. This lamp therefore tends to maintain constant 
 voltage. 
 
 A third kind of an arc lamp is the differential lamp, a scheme 
 of which is shown in Fig. 209. In this lamp we have two coils, 
 which act against one another. One of these coils is a series coil, 
 tending to lift the upper carbon holder, and to lengthen the arc; the 
 other a shunt coil, tending to lift the bottom carbon holder, and 
 hence to shorten the arc. 
 
ELECTRIC LIGHTING 
 
 211 
 
 This lamp will therefore combine the properties of the other two 
 types, and be suitable for use on 
 both constant current and constant 
 voltage circuits. 
 
 An essential requirement of an 
 arc lamp is a means of damping 
 the regulating mechanism, so as to 
 prevent any sudden or violent 
 movement of the carbons, causing 
 a flickering of the light. Several 
 devices have been arranged for 
 causing the mechanism to act in 
 a gradual manner. One of the 
 most usual consists of a roller, 
 over which passes a flexible cord 
 that carries the carbon holder. 
 This roller drives through several 
 toothed wheels a fan. The latter 
 is made to rotate at a great speed 
 against the resistance of the air. 
 Any sudden increase of the speed 
 of the roller is prevented, owing 
 to the great resistance that the air 
 offers to the increase of speed of 
 the fan. 
 
 Another damping arrangement 
 consists of a piston, moving within 
 a cylinder, with but little play, so 
 that the enclosed air is compressed 
 or expanded, preventing sudden 
 motion of the piston. 
 
 Fig. 210 shows the general 
 arrangement of the parts of an arc 
 lamp, and Fig. 211 shows the 
 principle of a Kfizik or Pilsen 
 differential lamp. To the explana- 
 tion given with the general scheme 
 of a differential lamp, we h,ve to 
 add the following for the Knzik 
 lamp : The iron cores, a and 6, are, 
 as may be seen from the dotted 
 lines in the figure, not cylindrical, 
 but conical. They are enclosed 
 within brass tubes, and by small 
 guiding rollers a true vertical 
 
 FIG. 210. Arc Lamp (The Elec- 
 trical Company). 
 
 motion is ensured. Over the iron cores the series coil g and the 
 
212 
 
 ELECTRICAL ENGINEERING 
 
 shunt coil / are respectively wound. When the lamp has been freshly 
 
 trimmed and the carbons are long, the core 
 of the upper carbon holder at its highest, 
 and that of the lower carbon is at its lowest 
 position, whereas at the end of the burning 
 hours the opposite would be the case. 
 With cores of a cylindrical shape, the 
 attractive power of the coil on the core 
 would vary according to the position of the 
 core to the coil. On the other hand, the 
 conical shape of the cores ensures that 
 the attractive forces will at these and other 
 positions be balanced, provided that through 
 both the series and the shunt coil the normal 
 current is flowing. 
 
 The roller c, over which passes the cord 
 bearing the carbon holders, is provided on 
 its circumference with fine teeth. Into the 
 latter a ratchet h interlocks, which is not 
 only movable about its axis, but, being 
 fixed within an oval hole, is also movable 
 for a short distance upwards. When the 
 lamp is not in circuit, the two carbons 
 touch each other, and the shunt coil is 
 short-circuited. On closing the switch, 
 the mains being connected to the terminals 
 marked + and , then the series coil moves its core 6 upwards. This 
 upward motion can, however, only take place so far as is allowed by 
 the length of the oval hole. This length is selected so as to give the 
 right length of the arc. The latter then has its normal voltage, and 
 the series and shunt coil are therefore counterbalanced. As the 
 lamp continues in use, the shunt coil / is able, without any im- 
 pediment from the ratchet, to lift the lower carbon holder and 
 shorten the arc, since this ratchet is arranged so as to stop a reverse 
 motion. 
 
 There are numerous other types of arc lamps, which the limits of 
 this book preclude us from describing. 
 
 It is of great importance to use a resistance in series with an arc 
 lamp. If we connected an arc lamp directly on to a 40-volt circuit, 
 then the variations of the current would be excessive, and quite 
 beyond the power of the regulating mechanism to control. On 
 switching on an arc lamp, the carbons are brought to directly touch 
 each other, whilst the lamp does not yet produce any back E.M.F. 
 Hence the lamp resistance is small, and the current therefore excessive. 
 Any, even the smallest, lengthening or shortening of the arc would 
 produce a great variation of the current, since any change of length 
 
 FIG. 211. The Kfizik Arc 
 Lamp. 
 
ELECTRIC LIGHTING 
 
 213 
 
 FIG. 212. Arc Lamp Resistance without cover. 
 (The Electrical Company}. 
 
 of the arc is followed by an increase or decrease of the back E.M.F. 
 Assuming, for example, the back E.M.F. to be 39 volts in one, and 
 36 volts in another case, then the difference between the terminal 
 voltage and back E.M.F. will be 1 and 4 volts respectively. The 
 ohmic resistance of the 
 lamp remaining the 
 same, the current will 
 be four times as much 
 in the second as in the 
 first case. In the feed- 
 ing of arc lamps from a 
 dynamo, the machine 
 voltage is therefore al- 
 ways made larger than 
 the lamp voltage should 
 be, and a constant resistance is kept in series (see Figs. 212 and 213), 
 which absorbs the superfluous voltage. The line voltage is then 
 best selected about 60 to 65, so that in the resistance 20 to 25 
 volts are absorbed. If, now, by any change of the length of the ^re 
 its voltage be varied, 
 say from 39 to 36, this 
 will cause only an un- 
 important rise of cur- 
 rent, because in the first 
 case the difference be- 
 tween the electro-motive 
 forces will be 65 - 39 
 = 26 volts, and in the 
 second case 65 36 = 
 29 volts. Hence, if the 
 total resistance be 3a>, 
 the current would be 8.6 amps, in the first, and 9.6 amps, in the 
 second case, the difference between these two currents being only 
 1 amp. Further, when the lamp is first connected with the current 
 supply, and the carbons are actually touching, nevertheless the 
 current cannot become too large. Its maximum value, of course 
 only for a brief period, will be 65 volts -r- 3a> = 21.6 amps. 
 
 The larger the series resistance the steadier the lamp will 
 burn. On the other hand, the resistance wastes electrical energy; 
 hence, for economical working, the series resistance should be 
 made as small as possible, i.e. just as small as will ensure good 
 regulation. 
 
 If on 110-volt mains single lamps are used, then about 70 volts 
 are absorbed by the resistance, i.e. about two-thirds of the total 
 energy is rendered useless. Hence, with 110-volt mains, two lamps 
 are often used in series, with a resistance absorbing about 30 volts; 
 
 FIG. 213. Arc Lamp Resistance enclosed. 
 (The Electrical Company). 
 
214 
 
 ELECTRICAL ENGINEERING 
 
 with 150-volt mains, the lamps are switched in groups of three in 
 
 series with a resistance; and with 
 220-volt, generally in groups of four. 
 There are also special connections, 
 where groups of three smaller arc 
 lamps are run on 110- or 120-volt 
 mains without a permanent resistance, 
 but merely with a starting resistance. 
 In these cases special precautions have 
 to be made; nevertheless, the lamps 
 can never burn as satisfactorily as in 
 groups of two. 
 
 Where the lighting is exclusively 
 by means of arc lamps, and the use of 
 glow lamps need not be considered, 
 connection of the lamps in series is a 
 frequent method. In this case there 
 is only one circuit, in which all the 
 arc lamps for example, 10, 20, or 
 30 are connected in series. The 
 voltage of the dynamo, or arc lighter, 
 has therefore to be high; viz., if for 
 one lamp with its regulating resistance 
 we assume a voltage of 50, then 30 
 lamps will need 1500 volts. Since 
 the extinguishing of one lamp would 
 prevent current supply to the re- 
 mainder, there must be provided an 
 alternative path in each lamp, which 
 either short-circuits it, or inserts a 
 compensating resistance. In this sys- 
 tem the current of the machine, gener- 
 ally provided by a series dynamo, has 
 to be kept constant, and its voltage 
 must be varied according to the 
 number of lamps running. This 
 method is not now so commonly in 
 use as it once was. 
 
 With arc lamps a more economical 
 lighting is effected than with glow 
 lamps. An arc lamp burning with 10 
 amps, has a luminous power of about 
 500 to 1000 candles. Since (including 
 
 the resistance) an arc lamp requires about 55 volts, we get for an 
 electrical power of 550 watts 500 to 1000 candles, from which 
 it follows that we have to spend only i to 1 watt per candle-power, 
 
 FIG. 214. Enclosed Arc Lamp. 
 (The Electrical Company'}. 
 
ELECTRIC LIGHTING 
 
 215 
 
 whereas a glow lamp requires 3 to 3J watts per candle. Arc lamps 
 are also constructed for 8, 6, 4, and 2 amps., sometimes for even less. 
 Smaller arc lamps, however, are not economical. Further, since arc 
 lamps require far more attendance than glow lamps, small arc lamps 
 are seldom used. On the other hand, for the lighting of streets, 
 squares, shops, etc., where much light is required, the use of large 
 arc lamps is common. 
 
 FIG. 215. Magnetite Arc Lamp. 
 
 A special application of the arc lamp is as a search light. Very 
 large arc lamps are used for this purpose, and the light is reflected by 
 means of parabolic mirrors, so that it can be directed on any object. 
 
 With the arc lamps hitherto considered, the arc is formed in the 
 
216 ELECTRICAL ENGINEERING 
 
 air, although for softening the exceedingly intense light glass globes 
 are always used. Now there exists another kind of arc lamp, whica 
 burns with the arc enclosed. Over the carbons there is a small glass 
 cylinder, so arranged that it fits round the carbons, making a small 
 and nearly air-tight chamber (see Fig. 214). This cylinder is, of 
 course, first filled with air, but, on burning for a short time, all the 
 oxygen contained in the air in the small cylinder is consumed. Hence 
 the carbons are consumed far less if burning in this enclosed manner, 
 and these lamps may be manufactured to burn a hundred hours and 
 longer, whereas the carbons of common arc lamps have generally a 
 burning time of only five to ten hours. 
 
 With " enclosed arc lamps " the length of the arc is generally 
 y to ", so that the voltage of the arc is equal to about 80. These 
 lamps may therefore be connected singly, and with only a small series 
 resistance, to 100- or 110- volt mains. 
 
 NEW TYPES OF LAMPS. 
 
 Dr. Auer, of Vienna, employs as a filament for a glow-lamp osmium, a material 
 which conducts when cold, and therefore does not require any preliminary heat- 
 ing. The efficiency of these lamps is said to be equal to that of the Nernst lamps. 
 On the other hand, owing to the low specific resistance of osmium, they can best 
 be used for voltages from 20 to 50. 
 
 In the Bremer arc lamp the carbons used have certain salts added to them, 
 with the effect of increasing the light, and, at the same time, making a great 
 change in its colour. 
 
 The Cooper-Hewitt lamp consists of a long tube, in which mercury vapour is 
 heated by an electrical current. It requires about half a watt per candle-power. 
 The light is especially rich in blue and violet rays. 
 
 The General Electric Company is now producing an arc lamp which has one 
 terminal of metal and the other a rod of magnetite. This gives a light quite 
 similar to the ordinary carbon, but with the same energy gives 60% more light. 
 It burns 150 hours, and does not have a glass enclosure around the arc with its 
 attendant cleaning and breakage. This lamp has recently been placed upon the 
 market. FIG. 215 is an illustration of this lamp. 
 
CHAPTER VIII 
 ALTERNATING CURRENTS 
 
 Properties of Angles Concerned with Alternating 
 
 Currents 
 
 ONE straight line intersecting another makes with it an angle. 
 Examine Fig. 216. 
 
 The line a-o intersecting the line b-o at o makes with it the angle 
 a-o-b] with c-o, the angle a-o-c etc. Take any given angle, a-o-6, 
 and from the point b drop a line 
 perpendicular to the line a-o, strik- 
 ing it at F ; then b-F-o is a right 
 angle. Take any point on the line 
 o-b, say gr, and drop a perpendic- 
 ular line g-h to o-a. It can be 
 easily shown by geometry that 
 the ratio of the line b-F to o-F is 
 the same as g-h to o-h, or the same 
 as any perpendicular to the base 
 line. Also the ratio of b-F to b-o 
 is the same as g-h to g-o, or the 
 same as any vertical to the amount 
 of the diagonal cut off by its inter- 
 section with the vertical. This ratio of the vertical to the per- 
 pendicular is in Fig. 216 -j or - , and is called the sine of the 
 
 angle o-o-F. Every angle has a definite sine. Thus, the sine of 30 
 degrees equals J. 
 
 The ratio of the horizontal part cut off by the perpendicular from 
 the diagonal to the opposite side, that is, the ratio in the triangle 
 o-F 6, of o-F to o-b is called the cosine. Every angle has a specific 
 
 value of the cosine. For 30 degrees it is -= . Thus, the sine of 
 
 an angle in a right-angle triangle is the ratio of the side opposite to 
 the longest side. The cosine, the ratio of the side adjacent to the 
 longest side. Consider in Fig. 216 the line o-d to be an edgewise 
 view of a coil revolving about o. Let l-l-l represent lines of force 
 
 317 
 
 <' h 
 
 FIG. 216. Properties of Angles. 
 
218 
 
 ELECTRICAL ENGINEERING 
 
 flowing through this coil. At the position o-d the coil contains the 
 maximum number of lines of force. As the coil turns less and less 
 lines of force go through the coil until, when it reaches the position o-a, 
 no lines of force go through the coil, being then edgewise to them. 
 The amount of area presented to the lines of force is represented by 
 the vertical lines c-e, 6-F, at the positions c and 6. 
 
 Let the length of the line o-d equal R; then, since the sine of the 
 
 ** p 
 
 angle c-o-e equals ^-, c-e equals R sine c-o-e. Also b~F equals R 
 
 sine 6-o-F. Thus, at any angle movement from the zero reference 
 line o-a, the amount of lines of force are equal to RXsine of the 
 angle away. At o-a the angle is o. Sine equals o, and the lines 
 of force equal R + o equals o, or no lines go through, which is evident, 
 since the coil is edgewise to the lines. At o-d the angle from o-a is 
 90 degrees. The sine of 90 degrees equals 1, since at 90 degrees the 
 perpendicular and the diagonal coincide and become one. If a curve 
 be plotted giving for 360 degrees at each angular position from the 
 value of the flux going through a coil, it will be as shown in Fig. 216, 
 or, what is the same thing, for all the values of the sine. The curve 
 will look like Fig. 217, having a maximum value at d. This curve 
 
 k 
 FIG. 217. Sine curve of E.M.F. and Current. 
 
 is called a sine curve, and represents how the flux varies in a coil 
 revolved in a uniform field of flux. Suppose the coil to be connected 
 to collector rings, as shown in Figs. 64, 65, 66. Since in the coil the 
 flux is changing, as shown in Fig. 216, a voltage must be generated , 
 since voltage is produced by a change in the number of lines of force 
 in a circuit, as has been shown. At d, Fig. 217, the flux is not changing 
 at all. Hence, here the voltage is 0, as shown by the dotted curve 
 h-l-j-k. At b, the flux is having a maximum rate of change; hence 
 the voltage is a maximum, as shown at I. If at each point of the 
 curve b-d-a-f-Qj the rate of change of flux be found, the corresponding 
 voltage can be determined, as shown in the dotted curve h-l-j-k. 
 
ALTERNATING CURRENTS 219 
 
 This curve is found to have the same shape as the other curve. Hence, 
 the rate of change of one curve gives another curve of the same shape. 
 This dotted curve is the voltage curve of an alternator. It is a sine 
 curve. This curve has certain important characteristics. If the 
 square root of the average of the squares of all the vertical lines l-m, 
 n-o, c-d, etc., be taken, the result equals the maximum value c-d+^/2 
 
 c-d 
 equals -- equals .707 c-d. This value is called the square root of 
 
 the mean square of all the values of the sine curve. The plain average 
 of all the verticals equals .637 c-d. Thus, the average value divided 
 
 by the square root of mean square value equals '-^-=- equals .90. 
 
 To return now to the alternator which with a single coil produces 
 the voltage as shown in the curve called a sine curve; to determine 
 the formula for the voltage we have shown previously that the average 
 
 E.M.F. of a coil revolving in a uniform field equals ^ , where 
 
 1UU,UUU,UUU 
 
 N equals the number of revolutions of armature (or coil) per second, 
 and (/) equals the number of lines of force threading through the 
 coil. But in a sine curve the maximum value equals the average 
 
 value multiplied by |- when n equals 3.14159, or the maximum volt- 
 
 4Ncf> X ^TT 27rX<i 
 
 age of a sme curve of E.M.F. equals 1 oo,ooo,000 X 2 100,000,000" 
 Also in a sine curve the square root of mean square value equals 
 the maximum value divided by \/2. Hence, the square root of 
 mean square value of the E.M.F. of an alternator with an armature 
 
 having one coil and with two poles equals ^ -f- \/2 equals 
 
 1 UU , UUu ; UUU 
 
 ' ^ . If there are n coils in series, the value is n times as 
 lUUjUUUjUuU 
 
 4.44Nn< 
 much ' or 100,000,000- 
 
 The revolutions per second of a 2-pole dynamo, or the equivalent 
 of a dynamo with more than two poles, are called the cycles of the 
 dynamo, or of the circuit which the dynamo is feeding. If the speed 
 
 per minute of a 2-pole alternator equals M, the cycles equal ^. If 
 
 OU 
 
 NX2 
 the speed of a 4-pole alternator equals N, the cycles equal . 
 
 Thus, the cycles of a P-pole alternator running at N revolutions per 
 
 P N 
 minute equal ~o^^7r- 
 
 All electrical measuring instruments record on their scale the 
 
220 ELECTRICAL ENGINEERING 
 
 square root of mean square values. Hence, when the voltage of an 
 alternator is read on an instrument the square root of mean square 
 value is read. From this thejnaximum value can, of course, be 
 calculated by multiplying by X/2*. Since the voltage of an alternator 
 varies as shown in Fig. 217, and since the current is always propor- 
 tional to the voltage, the current curve is the same as the voltage 
 curve, having its square root of mean square value, etc., just as the 
 voltage curve. Since heat from an electric current is proportional 
 to the square of the current, the square root of the mean square 
 value of current (called sometimes the effective or virtual current), 
 when squared and multiplied by the resistance through which it flows, 
 gives the same result as a direct current equal to the square root of 
 mean square current when squared and multiplied by the resistance 
 through which it flows. Hence, in calibrating A. C. instruments, 
 direct current may be used, each value of direct current equalling 
 in its effects on the instrument the square root of mean square, or 
 virtual, A. C. current. Referring once more to Fig. 217, if the line 
 b-g be divided into 360 parts, representing 360 degrees, or one com- 
 plete revolution, which gives a complete, cycle, as shown, the number 
 
 C 
 
 FIG. 218. E.M.F. and Current 90 apart in Phase 
 
 of degrees from a reference point b for any part of the curve is called 
 its phase. Thus, the phase of the point m is b-l; of the point o, o-n. 
 Thus, the point o differs in phase from the point m by the degrees 
 represented by l-n. Thus, the plus maximum of the full curve, that 
 is, c-d, differs in phase from the plus maximum of the dotted curve 
 by 90 degrees; that is, the distance a-c. Thus, the rate of change of 
 flux is 90 degrees lagging behind the flux itself. The sine curves, 
 therefore, must be plotted 90 degrees apart, if one represents flux 
 and the other the E.M.F. from that flux, the E.M.F. being later than 
 the flux. A convenient method of showing sine curves is to represent 
 them by their maximums or square root of mean square values. 
 Thus, the two sine curves shown in Fig. 217 can be represented as 
 shown in Fig. 218. Here c-d represents the maximum value, or square 
 
ALTERNATING CURRENTS 221 
 
 root of mean square preferably (either can be used, since one equals 
 V 2 times the other) of the flux, as shown in full line in Fig. 217, and 
 c-j shows the maximum value of E.M.F. resulting from this flux, 
 differing in phase by 90 degrees as shown. This method of plotting 
 alternating E.M.F. 's and currents is that generally used by electri- 
 cians and gives an eye picture of the relations of alternating values. 
 It is called the Vector Diagram method. 
 
 Experiments with Alternating Currents 
 
 THE first electrical machine with which we became acquainted pro- 
 duced alternating currents. On providing the Siemens H armature 
 with slip-rings, and rotating it in a magnetic field, we were enabled 
 to collect currents of an alternating kind. We then dealt with 
 devices for commutating the alternating current, which originally 
 is produced in any dynamo, into continuous current. We shall 
 now consider the properties of the unrectified alternating current,, 
 and the special types of machines designed as alternating current 
 dynamos. 
 
 First of all, let us try experiments similar to those we made with 
 a continuous current. Connect a wire resistance with the terminals 
 of an alternating current generator. On turning the armature the 
 wire is heated, and, if the current be sufficiently strong, the wire may 
 glow, and even melt. In like manner an alternating current will 
 cause an incandescent lamp to glow. We see, therefore, that the 
 heating effects of an alternating current are like those of a 
 continuous current. This is easily understood. The heating of a 
 conductor, traversed by an electric current, does not depend on the 
 direction, but merely on the strength of the current, and continues 
 therefore even if the current is continuously altering its direction. 
 
 On rotating the armature of our two-pole dynamo with a speed 
 of about 3000 revolutions per minute, thus getting 6000 alternations 
 per minute, or 100 per second, we observe a perfectly constant 
 illumination of the lamp. If, however, we turn the machine with 
 only the fourth part of this speed, and connect with it a lamp for 
 a correspondingly lower voltage, we observe that the lamp is not 
 giving a constant light, but flickers like a gas light supplied with 
 gas at a fluctuating pressure. This phenomenon is readily under- 
 stood. We know from our observations on page 69, that the 
 strength of an alternating current increases gradually from zero to 
 its maximum value, then decreases to zero, and, changing its 
 direction, again reaches its maximum value, etc., as shown in Fig. 67. 
 
222 ELECTRICAL ENGINEERING 
 
 Thus the carbon filament, traversed by the current, gets hotter and 
 then cooler, hence alternately glowing brighter and then darker. At 
 the moment when the voltage is zero, the lamp still gives out a 
 certain amount of light, since sufficient heat is stored up in the 
 filament to cause light to be emitted during the short period that 
 the current is zero. Nevertheless the flickering light resulting 
 would be very fatiguing to the eyes. When the alternations follow 
 one another very quickly, at least 50 times per second, the fluctua- 
 tions are not perceived, and a current of this periodicity can therefore 
 be used for electric lighting. 
 
 Our second experiment consists in bringing two wires, connected 
 with the terminals of an alternating current generator, into contact 
 and then separating them. A break spark will be produced, like that 
 with a continuous current, and, if we keep the two ends of the wire 
 sufficiently near together, we may get a continuous arc. Alternating 
 currents may therefore be employed, as well as continuous currents, 
 for feeding arc lamps. The same systems of regulation with 
 which we became acquainted in the continuous current lamps viz. 
 series-, shunt-, and differential-regulation may be applied to alter- 
 nating current lamps with almost equal success. There are, of 
 course, certain differences in the construction of the lamps, which 
 we shall deal with later on. 
 
 The property of the continuous current arc lamp, that the positive 
 carbon is sooner consumed than the negative, is naturally not found 
 with alternating current lamps, since the carbons are alternately 
 positive and negative, hence they are consumed at an equal rate. 
 The voltage required with the alternating current is lower (25-30 
 volts) than that necessary for the continuous current lamps. 
 
 The flickering of the light when the number of alternations is 
 too small occurs here far earlier than with the glow-lamp ; whilst 
 with the latter we get a fairly constant light at 50 alternations per 
 second, we can with an arc lamp hardly use a current of less than 
 SO alternations without getting a very unsteady burning of the 
 lamp. Thus in installations where arc lamps are employed, a 
 current of not less than 80, but generally 100, and in this country 
 frequently 200, alternations per second is employed. 
 
 With a continuous current a magnetic needle was deflected by a 
 current flowing through a wire, and an iron rod surrounded by a coil 
 was magnetized as soon as a current flowed through the latter. In 
 making the first of these experiments with alternating currents, we 
 are unable to observe any deflection of the magnetic needle. If we 
 watch the needle very attentively we find that it vibrates. On 
 reducing the speed of the machine which supplies the current, so as 
 to get but a few alternations per second, say two or three, we observe 
 that the needle swings from side to side. As often as the current 
 changes its direction, just as often the needle alters the direction of 
 
ALTERNATING CURRENTS 223 
 
 its deflection. If, however, the number of alternations is greater, say 
 twenty, thirty, or more, then the eye cannot any longer follow the 
 quicker and shorter oscillations of the needle, and only the small and 
 rapid vibrations of the needle about its position of rest can be 
 observed. 
 
 If we wind a coil of wire over an iron core and send through it 
 an alternating current, the core will be magnetized like a core 
 surrounded by a continuous current. It will also become able to 
 attract pieces of iron and keep them fast. In carrying out these 
 experiments with alternating currents we observe two secondary 
 phenomena, which we do not observe with continuous currents. 
 Firstly there is a loud humming noise, and secondly both the 
 magnetized and the attracted iron become strongly heated. The 
 heating we shall deal with later on. The noise may readily be under- 
 stood from the nature of an alternating current. At the moment the 
 strength of the current passes the zero line, the attractive force 
 ceases, and the iron pieces tend, and even begin, to fall off the core. 
 The falling very quickly ceases, since a very brief time afterwards 
 the current increases and the iron is again attracted. This pro- 
 ceeding, which is repeated as often as a change of the direction of the 
 current occurs, causes naturally a corresponding noise or a sound, 
 the pitch being higher or lower according to the number of the 
 alternations. 
 
 As it is with the magnetic, so it is with the electro-dynamic 
 effects. If through a fixed and a movable coil we send the 
 same alternating current, we observe the attraction or repulsion as 
 with a continuous current (see p. 61, Fig. 57). This is easily 
 understood. Assuming that, at any instant, the current in the two 
 coils have the same direction, then these coils attract each other. At 
 the same instant as the current changes its direction in one coil, 
 it will also do so in the other coil. Thus the two coils are again 
 traversed by currents in the same direction and attract each other. 
 
 If, on the other hand, we send through one of the two coils a 
 continuous, and through the other one an alternating current, then 
 we shall observe neither an attraction nor a repulsion, but only 
 a little vibration of the coils, since the first impulse of the 
 attraction is immediately followed by the opposite impulse of 
 repulsion, and these actions continue. 
 
 Next let us try to get a chemical effect with an alternating current. 
 For this purpose we have to connect the two electrodes of a voltameter 
 (see Fig. 5) with the slip-rings of an alternator. We can observe 
 then a production of gas at both poles, although not at one pole oxygen 
 and at the other hydrogen, as is the case with continuous current; 
 but at both electrodes equal quantities of the explosive gas, consisting 
 of a mixture of oxygen and hydrogen, are liberated. With an alter- 
 nating current each pole is first positive, and immediately afterwards 
 
224 ELECTRICAL ENGINEERING 
 
 negative, so that a bubble of oxygen, evolved from one pole, will 
 immediately be followed by a bubble of hydrogen, this by a bub- 
 ble of oxygen, and so on. 
 
 Although we thus get chemical effects with an alternating current, 
 it is impossible to separate the elements of a substance. For electro- 
 lytic purposes and for electro-plating, where, with the aid of the 
 electric current, we wish to separate metals from metal solutions for 
 instance, silver from a silver solution, or copper from a copper solution 
 alternating currents cannot be employed. It is also obvious that 
 alternating currents cannot be employed for charging accumulators. 
 
 No magnetic or chemical effects of an alternating current can 
 be observed if the number of alternations per second is extremely 
 great say, for instance, many thousands. Then the molecules of 
 iron or of the liquid have not sufficient time to follow the very 
 rapidly changing pulsations, which tend to drive them at one 
 instant in one direction, and at the next instant in the opposite 
 direction. 
 
 Current Strength and Voltage of an Alternating 
 
 Current 
 
 We can measure the strength of an alternating current by means 
 of its various effects; for instance, its heating or magnetic effects. It 
 is, however, necessary, before dealing with the different methods cf 
 measurement, to make clear the meaning that electrical engineers 
 attach to the strength of an alternating current, since the latter 
 varies between its maximum positive value, zero, and its maximum 
 negative value. In speaking about the current strength, we gene- 
 rally do not mean its maximum value. By an alternating current 
 of 1 amp. we understand a current which would cause the same 
 heating effect as a continuous current of 1 amp. This adopted value, 
 also called the effective or virtual current, is naturally a mean value 
 only. The maximum value of an alternating current is 1.41 times 
 as great as the mean value; or, in other words, the effective current 
 is equal to about two-thirds, or, speaking more exactly, to 0.707 of 
 its maximum value. 
 
 A hot-wire instrument shows the right current both for alter- 
 nating and continuous currents, since its reading depends on the 
 heating effect. This follows from the definition of the strength 
 of an alternating current; for the deflection of 1 amp. on the hot- 
 wire ammeter, tells us that the measured alternating current 
 produces in the instrument the same heating effect as a continuous 
 
ALTERNATING CURRENTS 
 
 225 
 
 current of 1 amp., with which the instrument has been cali- 
 brated. 
 
 Exactly the same meaning is attached to alternating voltage. By 
 effective or virtual voltage of an alternating current we understand the 
 voltage of an equivalent continuous current, which produces the same 
 heating effect, in a given ohmic resistance, as the alternating current. 
 A glow-lamp manufactured for 110 volts continuous current will 
 therefore glow with equal light if switched on to 110 volts alternating 
 current, although the instantaneous values of the alternating pressure 
 v^ry, at each half-alternation, from zero up to nearly 1J times the 
 effective voltage, that is, up to about 155 volte. 
 
 Induction Effects of an Alternating Current 
 
 All experiments hitherto carried out with alternating currents 
 have been similar to those with continuous currents. We must now 
 deal with effects produced by alternating currents, which are not 
 possible at all with continuous currents. 
 
 Let us wind over an iron core, consisting of a bundle of fine 
 wires or iron disks, a coil, so that the iron core projects beyond 
 
 the coil. Next let us lay on the top of 
 the coil a metal ring. As soon as an 
 alternating current passes through the 
 coil, the ring is knocked upwards as if 
 by an invisible hand (see Fig. 219). 
 It floats freely in the air, as if it had 
 no weight, and gets extremely hot. If 
 now we open the circuit, the ring falls 
 back on the coil, and gradually cocls 
 down. 
 
 From the heating and motion of the 
 ring, we conclude that an electric 
 current has been induced in it; from 
 the direction of the motion, we may 
 deduce the direction of the current. 
 We know, from the experiments with 
 
 the electro-dynamometer, that currents in the same direction 
 attract each other, and when in the opposite direction, repel each 
 other. Hence, we learn from the constant repulsion of the ring, that 
 a current is induced in the latter which is always opposite to that 
 of the coil. Representing this in a diagram, in Fig. 220, the full 
 line shows the curve of the original (inducing or primary] current, 
 and the dotted line the direction of the induced current. Fig. 220a 
 indicates the same thing in another way. If, at any moment, 
 the alternating current in the coil is directed upwards, then, at 
 
 Metal Ring 
 
 Coil 
 
 FIG. 219. Repulsion of Metal 
 Ring. 
 
22G 
 
 ELECTRICAL ENGINEERING 
 
 the same moment, the current induced in the ring is directed down- 
 wards; if the current in the coil changes its 
 direction, then the current in the ring does 
 the same. 
 
 If the coil were traversed by a continuous 
 current, one end of the iron core would be a 
 north, and the other end a south pole, and 
 the lines of force would therefore continuously 
 flow in one and the same direction through 
 the core. Since, however, the magnetizing 
 coil is traversed by an alternating current, 
 the magnetic field alters its direction re- 
 peatedly. Thus, through the interior of the 
 metal ring, which lies on the coil, lines of 
 force flow that continually change their 
 direction. These lines of force produce in 
 the ring, which represents a winding closed 
 on itself (see p. 67), an E.M.F., and hence 
 a current, of continually changing direction, 
 the number of alternations of which is 
 naturally equal to the number of alternations 
 of the primary current. 
 
 Exactly the same action which arises in 
 the metal ring or the secondary winding 
 prises also in the primary coil itself, even if 
 there is no secondary winding at all. Any 
 winding of the primary coil encloses a 
 magnetic field, the intensity and direction 
 of which is perpetually altered. Thus, in 
 each winding of the primary coil, there 
 must, as in the secondary metal ring, be 
 produced an E.M.F. which is opposite to 
 the original one; that is to say, there 
 exists a back electro-motive force, like that of the many examples 
 with continuous currents we have considered; as in the cases of 
 the electro-motor, the storage battery, and the arc lamp. The 
 back E.M.F., produced by the inducing effect of alternating currents 
 on their own circuit (thus, by self-induction), causes the current 
 flowing through the coil to become far smaller than would 
 be calculated by Ohm's law. Obviously the back E.M.F. can 
 never be equal to the primary E.M.F., since in this case no 
 current would flow through the coil, the iron core would 
 therefore not be magnetized, and at this moment the production 
 of the back E.M.F. would also cease. The back E.M.F. remains 
 
 FIG. 220. 
 
 0-2= 
 Primary 
 
 
 
 0-1= 
 
 Induced 
 
 Current 
 
 Current 
 
 FIG. 220. 
 
ALTERNATING CURRENTS 
 
 227 
 
 FIG. 221. 
 
 as in the electro-motor always a little less than the primary 
 E.M.F. 
 
 The heating of the secondary metal ring in 
 our experiments teaches us why we must employ 
 for this experiment a bundle of wires or disks 
 instead of a solid iron core, and why the solid core 
 we employed for demonstrating the electro-magnetic 
 effects of an alternating current got extremely hot. 
 Any portion of an iron core we may imagine as 
 consisting of many short-circuited iron rings (see 
 Fig. 221), and in all these iron rings currents are 
 induced as in the secondary metal ring. For this 
 
 reason the iron cores of all alternating-current apparatus have 
 like the armatures of continuous-current machines 
 to be made of insulated iron wires, cr from thin 
 iron disks, which are insulated from each other by 
 sheets of paper or by a layer of varnish (Fig. 222). 
 Since with alternating currents a far greater number 
 of alternations generally are employed than take 
 place within the armature of a continuous-current, 
 dynamo, the subdividing of the iron core has to 
 be carried out further with alternating- than with 
 continuous-current armatures. Whilst with the 
 latter, disks of 0.02 inch thickness are employed, 
 the thickness is generally reduced to 0.012 inch, 
 and even to 0.008 inch with alternating-current, 
 apparatus. 
 
 i Again, the bobbins for alternating-current electro- 
 magnets must never be complete metal bobbins. 
 
 Whenever metal bobbins are employed, they have to be made with 
 a slit (see Fig. 223), so that the bobbin itself 
 cannot serve as a short-circuited secondary 
 winding, and eddy, currents, and therefore 
 heating, is avoided. To entirely prevent the 
 production of these currents the bobbins are 
 in many cases made of insulating materials. 
 
 Transformers 
 
 FIG. 223. 
 
 The effects produced by alternating cur- 
 rents dealt with in the last chapter, are of the utmost importance in 
 practice. These effects enable us to produce, without using any 
 moving parts, an E.M.F. , in a secondary coil which is wound over 
 an iron core, providing that there is also a coil (the primary) 
 traversed by an alternating current, wound over the iron. The voltage 
 
 FIG. 222. 
 
228 
 
 ELECTRICAL ENGINEERING 
 
 Secondary 
 
 FIG. 224. Ring Transformer. 
 
 produced in the secondary coil may have any value, it may be larger 
 or smaller than, or equal to. the voltage of the primary coil. 
 
 The open iron core, employed in the experiment of Fig. 219, is 
 not employed in this case. Obviously we want to get a strong mag- 
 netic field with the smallest possible magnetizing current, and must 
 therefore provide for the lines of force a closed path through iron. 
 With the dynamo, having a movable part, an air gap in the magnetic 
 circuit cannot be avoided, 
 whereas with the trans- 
 former we may have an 
 entirely closed iron cir- 
 cuit, as, for instance, the 
 iron ring shown in Fig. 
 224. The ring looks like 
 a gramme armature. 
 Whilst, however, with the 
 latter the lines of force 
 enter the ring from out- 
 side, and the ring forms 
 only a part of the mag- 
 netic circuit, with the 
 transformer the lines of force are produced within the ring itself, 
 and are closed in the ring, without leaving it. The secondary 
 coil may be placed at any point of the ring. If the ends of the 
 secondary coil are disconnected, then an E.M.F. is induced, whereas, 
 if the ends are connected through an outer circuit say, for instance, 
 by lamps a current will flow both through the coil and the 
 circuit. 
 
 We have now to consider, how great the voltage produced in the 
 secondary coil will be. Let us assume the number of windings on 
 the primary coil to be 100, and that it is connected with an 
 alternating ^supply of 100 volts. Let the secondary be first of all 
 opened. Then, as we know, the back E.M.F. produced in the 
 primary coil will be nearly as much as 100 volts say, perhaps, 
 99 volts, or even a little more. For, since the lines of force are 
 flowing entirely through iron, we want only a small number of 
 ampere-turns for magnetizing the iron. Hence a very small pressure 
 difference between primary and back E.M.F. is required for sending 
 through the coil the magnetizing current for overcoming the ohmic 
 resistance. Since now in the 100 windings of the primary ceil 
 a back E.M.F. of nearly 100 volts is produced, the back E.M.F. cf 
 each winding will be nearly 1 volt. Any winding of the 
 secondary coil has, however, the same title to voltage as a winding 
 of the primary, since both are traversed by the same magnetic 
 flux. The voltage produced in any winding of the secondary 
 coil will, therefore, be equal to nearly 1 volt. If, for example, 
 
ALTERNATING CURRENTS 229 
 
 the secondary coil consist of 10 windings, then its voltage would 
 be about 10, with 100 windings about 100 volts, with 1000 windings 
 about 1000 volts, etc. The voltages of the secondary are to those 
 of the primary coil exactly, or nearly exactly, as the number of 
 windings on the two coils. 
 
 Now let us connect the ends of the secondary coil with an outer 
 circuit, so that the E.M.F. of the secondary coil may produce a 
 secondary current. Then the iron core will no longer be traversed 
 by the primary current only, but also by the secondary current. 
 The latter is. as we have learned from the experiment with the 
 metal ring, in an opposite direction to the primary current; it tends 
 therefore to demagnetize the iron core, and to weaken the flux 
 of lines of force. As soon, however, as there occurs the slightest 
 weakening of the flux, the back E.M.F. of the primary coil, which 
 was before nearly equal to the terminal voltage, will naturally 
 decrease. Even if the back E.M.F. decreases by 1 volt only, this 
 will, at the small ohmic resistance of the primary coil, cause a 
 considerable strengthening of the primary current. Thus through 
 the primary coil as much more current will flow as is necessary 
 to counterbalance the demagnetizing effect of the secondary coil. 
 If, for instance, we had 10 secondary windings, and the current 
 taken from them were 50 amps., then 500 secondary ampere- turns 
 would cause demagnetization. Instantly 500 primary ampere-turns 
 would result, and, since the number of windings of the primary coil 
 is 100, its current would be equal to |{J- =5 amperes. 
 
 Such an apparatus is called a transformer, because it enables 
 us to transform a current of high voltage and small amperage into 
 one of low voltage and great amperage, or vice versa. It regulates 
 its primary current consumption according to the current taken from 
 its secondary side, and is therefore quite as excellent an automatic 
 regulating apparatus as an electric motor. 
 
 Shape of Transformers 
 
 The ring, as shown in Fig. 224, is theoretically the best shape for 
 a transformer core. This shape has, however, the disadvantage 
 that the winding of the coils has to be done by hand, which is 
 rather troublesome and expensive work. Hence shapes are gener- 
 ally employed which enable us to use machine- wound coils. In 
 Fig. 225 such a shape is shown. The transformer consists of a 
 horseshoe-shaped main part, built up from thin iron disks with 
 
230 
 
 ELECTRICAL ENGINEERING 
 
 paper between them. The primary and secondary coils are pushed 
 over the limbs of this part. After fixing the coils on the open end 
 of the horseshoe, a straight piece, also consisting of iron disks and 
 paper, is pressed to the top of the limbs and fixed by means of screws. 
 We have now a closed magnetic circuit as before, but of a rectangular 
 shape. The iron core of Fig. 225 is obviously not quite as good as 
 that of Fig. 224. In the latter case, the magnetic circuit is entirely 
 through iron ; but in the core of Fig. 225 there is between the main 
 horseshoe part and the straight end piece a joint, which, though it 
 may be very small, still represents an air gap. This transformer 
 requires, therefore, a little more magnetizing current than a ring 
 
 FIG. 225. Transformer with Horseshoe- 
 shaped Iron Core. 
 
 FIG. 226. Transformer with Coils 
 subdivided. 
 
 transformer, and will also have a greater magnetic leakage. If with 
 this rectangular shape we fix a primary coil on one limb of the horse- 
 shoe, and the secondary on the other, then we are able to observe a 
 considerable difference in the voltage of any primary and secondary 
 winding. The reason for this is that all the lines of force produced 
 in one limb do not pass the other limb, but a considerable part of 
 them leaves the iron core at the edges and joints and flows through 
 the air. To prevent the disadvantageous effect of the magnetic 
 leakage, the primary and secondary coils are generally subdivided 
 into a number of smaller coils, alternately placed over the iron core, 
 as shown in Fig. 226. In this case we have four primary and four 
 secondary coils, which are fixed two at a time on each transformer 
 limb. Sometimes the internal diameter of one coil is larger than the 
 
ALTERNATING CURRENTS 
 
 231 
 
 outer diameter of the other coil, so that on each limb the secondary 
 coil may be pushed over the primary, or vice versa (see Fig. 227). 
 
 Another transformer shape is 
 shown in Fig. 228. Here the coils 
 are wound over the middle iron 
 core, which is then completed by 
 two U-shaped yoke-pieces. The 
 flux of lines of force is spread over 
 the two yoke-pieces. The working 
 of this transformer is obviously 
 quite the same as that of the trans- 
 formers described before. Practi- 
 cally it has the advantage that the 
 coils are protected by the two yoke- 
 pieces against mechanical injuries 
 and are enclosed as within a shell. 
 In no part of a transformer 
 which is exposed to the changing 
 
 FIG. 227. Transformer with Coils magnetic field must solid iron parts 
 wound one on the other. b e employed, because these would 
 
 be dangerously heated. Hence the 
 
 bobbins are generally made from insulating materials. Solid iron 
 bolts and castings must never be used in connection with the iron 
 
 FIG. 228. Shell Transformer. 
 
 core. For the constructive part, however, they may be employed, 
 but care has to be taken to prevent them from being traversed by a 
 considerable number of stray lines of force. In Fig. 229 the general 
 construction of a transformer (Ferranti type) is shown. 
 
232 ELECTRICAL ENGINEERING 
 
 Applications of Transformers 
 
 The transformer is of the utmost importance in the practical 
 applications of the alternating current. The facility of change of 
 pressure it affords has given it an important place in electrical 
 engineering. 
 
 We know, from what we have learnt about mains (see page 46), 
 
 FIG. 229. Westinghouse Oil-insulated Water-cooled Transformer 
 2250 K.W., 22,000 Volts. 
 
 the advantages high tension offers for the transmission of energy, but 
 we are aware on the other hand how dangerous a high-tension main 
 can become in inhabited rooms. It would, for instance, be possible 
 to generate with continuous currents voltages of some thousands, thus 
 
ALTERNATING CURRENTS 233 
 
 enabling an economical transmission of energy over distances of 
 several miles. Since, however, the consuming apparatus, such as arc 
 and glow lamps, can only be manufactured for comparatively low 
 voltages, a series connection of many lamps would be required. 
 Further, the dangers of high-tension circuits would have to be carried 
 into each room in which a lamp is used, involving the special pre- 
 cautions which are specified in connection with high-tension mains. 
 
 The alternating-current transformer allows the transformation of 
 high tension to any required low tension in a very simple and reliable 
 manner. It does not require any attendance, is self-regulating, and, 
 since in an apparatus in which the parts are ail stationary the insula- 
 tion between high- and low-tension coils can be made in a very 
 perfect manner, a transformer is safer than any rotating machine can 
 possibly be. From the secondary terminals of the transformer 
 only low-tension cables lead, with which the house mains are 
 connected: thus no special provisions have to be made in installing 
 lamps, etc. 
 
 If we wish to obtain a similar transformation with continuous 
 current, there is nothing left but to employ a high-tension con- 
 tinuous-current motor, which drives a generator supplying low- 
 tension current. Continuous-current converters require, it must 
 be remembered, since they are rotating machines, attendance and 
 regulation. Further, their efficiency is far lower than that of 
 stationary alternating-current transformers of the same output. 
 
 Sometimes alternating-current transformers are employed for the 
 transformation of low into high tension. Nowadays alternating- 
 current generators for 2000-5000, and even 10,000 volts, can easily 
 be manufactured. For power transmission on extremely long dis- 
 tances, however, voltages up to 30,000 and even more are employed. 
 It is then generally preferred to produce in the generators currents 
 of comparatively low voltages ; to transform these currents by means 
 of transformers into the high voltage required, lead this high tension 
 to the places of consumption, and there step it down again by 
 transformers to a pressure low enough to be used without danger 
 to life. 
 
 Phase-Difference 
 
 Not only in transformers, but also in all alternating-current 
 circuits, self-induction causes specific phenomena. We know that 
 any wire traversed by an electric current produces round it a 
 magnetic field, the lines of force being in circles (see Fig. 17). With 
 continuous currents this magnetic field is stationary and uniform 
 as long as the current does not alter its strength. The field of a 
 direct current, therefore, does not exert any reaction on the current 
 itself, since, as we know, we must have an alteration of the field 
 
234 
 
 ELECTRICAL ENGINEERING 
 
 intensity to produce induction effects. On the other hand, the field 
 produced by an alternating current changes its direction and strength 
 continually, thus inducing, both in the conductor itself and in all 
 neighbouring conductors, electro-motive forces. With straight con- 
 ductors, in the neighbourhood of which there is no iron, the electro- 
 magnetic whirl of force, and hence the E.M.F. of self-induction, is 
 comparatively small. If, on the other hand, there are coils in the 
 circuit, especially if they have cores of iron, the influence of the self- 
 induction on the circuit is considerable. The E.M.F. of self-induction 
 may, of course, be also represented by a wave line, like any alternating 
 current voltage and alternating current strength, but it does not 
 reach its maximum value at the same time that the current 
 strength reaches its highest value, and its zero occurs at a different 
 time to that of the current. 
 
 In Fig. 230 the course of an alternating current is shown by the 
 
 FIG. 230. Alternating-current Curve. 
 
 wave line. The magnetic field naturally reaches its maximum value, 
 its zero point, and its minimum (negative) value simultaneously with 
 the current by which it is produced. If, for instance, the current 
 reaches the zero line, then there are no magnetizing ampere-turns, and 
 no magnetic field can exist. When the magneti/.ing current reaches 
 its maximum value, then the strength of field also reaches its 
 maximum value. When, on the other hand, the magnetizing current 
 is in the opposite direction, then the direction of the lines of force 
 must also be in the opposite direction. The E.M.F. of self-induction 
 can only be produced with an alteration of the magnetic field. The 
 more rapid the alteration, the stronger the E.M.F. of self-induction 
 will be. Whether the field itself is strong or weak, or whether it is 
 directed in one or the other sense, does not make any difference at all; 
 the essential circumstance being only the rate of growth or decrease 
 of the field. 
 
ALTERNATING CURRENTS 235 
 
 The above figure represents the growth and decrease of the 
 magnetizing current, and therefore also the growth and decrease of 
 the strength of field. Considering the figure, we observe distinctly 
 that at a, c, and e that is, at the highest and lowest positions the 
 field for a moment does not alter its strength at all. Up to a the 
 current has grown, but at a the growing of the current stops for a brief 
 interval. From there it falls again, first slowly, then quicker and 
 quicker. The inclination of the wave line, and hence the decrease of 
 the current, is greatest at b, where the current passes the zero line. 
 On the current falling still further, the inclination of the wave line 
 becomes less steep, and the fall is slower, until the lowest point, c, is 
 reached. At this moment a point of rest occurs again for a moment, 
 then the field grows, first slowly, then more quickly up to d. 
 Thence it continues to grow up to the highest point e, but the 
 rate of increase is again a slow one. From this it is clearly 
 seen that the rate at which the current, or the field which it 
 produces, changes differs from point to point. When the current 
 reaches its maximum value there is no field alteration at all, and 
 when the current passes the zero line, the field changes at the 
 most rapid rate. 
 
 We may compare this with the differences in the length of day 
 and night at the different seasons. In winter and summer, when 
 the days last eight hours less or more respectively than the nights, 
 the alteration in length from one day to another is hardly perceptible; 
 whereas in spring and autumn, when the days and nights are almost 
 equal, the alteration in the length of consecutive days is very ap- 
 parent. 
 
 The E.M.F. of self-induction depends on the rate of the alteration 
 of the field. Hence it is greatest when the current passes the 
 zero line, decreases with an increasing current, and becomes nil as 
 the current reaches its maximum value. For determining the 
 direction of the induced E.M.F. we have only to consider that it is 
 always opposite to the alterations of the field, thus being positive 
 when the field decreases, and negative when the opposite is the 
 case. This rule enables us to draw a line in the form of a wave, 
 representing the E.M.F. of self-induction (see Fig. 231). A glance 
 at this diagram shows that the E.M.F. of self-induction is a quarter 
 of a wave or a quarter of a period behind the producing current. This 
 signifies that the current has at any definite moment a maximum 
 value, which is reached by the E.M.F. of self-induction a quarter of 
 a period later. 
 
 This is the case with the theoretical transformer, the secondary 
 circuit of which is open. The transformer is then not loaded, therefore 
 through the primary coil only a small magnetizing current flows, and 
 this lags a full quarter-period behind the impressed voltage. 
 
236 
 
 ELECTRICAL ENGINEERING 
 
 With continuous currents we calculated the watts required in 
 any circuit simply by multiplying voltage and current, or 
 
 Volts Xamps. = watts. 
 
 It is quite different with alternating currents. The power used 
 at any instant is still, of course, determined by the product volts 
 X amps, at this particular moment, but we must never forget to 
 multiply together the voltage and current that belong to each other. 
 In other words, the product of the simultaneous values of current 
 and voltage must be taken. 
 
 Now, just at the moment when the voltage has its maximum 
 value the current is zero, and when the latter has its maximum value 
 the voltage is zero, the product of voltage and current, the watts, 
 thus being at these times, in both cases, without value. 
 
 This fact can be made clearer by an example from daily life. 
 
 FIG. 231. 
 
 Imagine a workman who is sometimes diligent and at other times 
 lazy, in an untidy workshop, where the tools are frequently lost. If 
 he cannot find his tools just at the moment when he is most inclined 
 to work, or again, if he discovers them when he is inclined to be lazy, 
 he will not in either case do useful work. The phase-difference 
 between the possession of tools and inclination to work brings about 
 a working result of zero value, although there is sometimes inclination 
 to work and sometimes these are tools. If the "phase-difference" 
 is not quite as great that is to say, if the man finds the tools 
 just before he has lost his inclination to work, the result will not, 
 of course, be nil, but it will surely be smaller than if the possession 
 of tools and full inclination to work had been simultaneous. 
 
 Similiarly, the electrical effect, the watt output, is smaller when 
 a displacement of voltage, as regards the current, exists, and will be 
 smaller the nearer the phase-difference approaches to a quarter- 
 period. If the current has only magnetizing work to do, as is the 
 
ALTERNATING CURRENTS 237 
 
 case with a theoretically unloaded transformer, then there is no watt 
 output. 
 
 The magnetizing current which is displaced by a quarter-period 
 from the voltage is therefore called a wattless current. In the 
 case of a theoretical unloaded transformer, we have only wattless 
 current. 
 
 The reverse of a wattless current is a watt current that is, 
 a current which has no phase-difference from the voltage. If the 
 voltage reaches simultaneously with the current its highest, its zero, 
 and its lowest value, then we get the maximum of work that can 
 be done with these current and voltage values. The output may 
 then easily be calculated by multiplying the effective voltage by the 
 effective current. With a circuit without self-induction this is really 
 the case. If, for example, we measure the effective voltage as 100 
 volts, and the effective current as 40 amps., then the output is. 
 4000 watts, exactly as with a corresponding continuous current. 
 
 A circuit absolutely without self-induction does not exist, but, 
 frequently the self-induction is very smdl for instance, with glow 
 lamps. If with the secondary coil of a transformer we connect a 
 number of glow lamps, then through the secondary circuit nearly a 
 pure watt current flows. Thus to the wattless magnetizing current, 
 which was in the primary coil before, a watt current will be added. 
 The resulting current now flowing in the primary coil is, of course,, 
 neither absolutely in phase with the voltage nor displaced by a. 
 quarter-period. Its displacement becomes smaller the more the second- 
 ary of the transformer is loaded. With a fully loaded transformer 
 the small wattless magnetizing current is practically negligible when 
 compared with the large watt current, so that a phase-difference can 
 hardly be observed. Hence, if the fully loaded transformer takes. 
 300 amps, at a voltage of 100, this will correspond practically with 30 
 kilowatts. 
 
 Our discussions about an unloaded transformer have hitherto 
 referred to the theoretical case. With a commercial transformer the- 
 phase-difference is not really a quarter-period. We have learned 
 that only a wattless current that is, one which does not produce 
 any .effect, like the mere magnetizing current of the primary coil of 
 an unloaded transformer has a lag equal to a full quarter-period 
 behind the voltage. As a matter of fact, even in transformers with 
 an open secondary circuit, secondary currents are produced, since the 
 separate iron disks form closed circuits, and, even if they are very 
 thin and of high resistance, eddy currents flow through them. These 
 currents act like those produced in the secondary windings when 
 their circuit is closed. Now, whenever a current flows in the 
 secondary circuit a watt current enters the primary coil. It will 
 therefore be quite clear that through the primary coil of an unloaded 
 transformer a certain amount of watt current must flow. The 
 
238 ELECTRICAL ENGINEERING 
 
 transformer will always consume as much energy as is transformed 
 by the eddy currents in its core into heat. The phase-difference 
 between current and voltage of an unloaded transformer is therefore 
 always somewhat less than a quarter-period, and the watts taken 
 are always greater than zero, but far less than the product of voltage 
 and current. 
 
 The self-induction of a coil with an iron core may be used with 
 advantage in installations of arc lamps, so as to avoid loss of energy. 
 If we connect a single alternating-current lamp, requiring a voltage 
 of about 30, with 110-volt mains, we have to absorb about 80 volts 
 in a series resistance. An 8-amp. lamp consumes 8 amps. X 30 
 volts = 240 watts. In the series resistance, as much as 8 amps. 
 
 FIG. 232. Choking Coil (The General Electric Company). 
 
 X 80 volts = 640 watts would be lost! Thus the dynamo had to 
 supply 880 watts for this single arc lamp only. If. on the other 
 hand, we employ, instead of the series resistance, a ''choking coil" 
 that is, a coil wound over an iron core, similarly to a small 
 transformer, but with a single coil only (see Fig. 232) then in this 
 coil a back E.M.F. is produced, which causes a great phase-difference 
 between current and voltage. The current will, of course, in this 
 -case have to be again 8 amps., also the voltage of lamp and choking 
 coil together will be 110 volts; but the watts taken will be far less 
 than 880 perhaps not much more than the 240 watts required by 
 the arc lamp itself. Naturally this arrangement cannot be used with 
 continuous currents. 
 
 The property of self-induction and phase-difference between 
 (current and voltage is inherent in all alternating-current circuits, 
 especially in coils with iron cores. Hence electro-magnetic measuring 
 instruments show different deflections with continuous and alternating 
 currents of equal strength. If they be used for alternating-current 
 
ALTERNATING CURRENTS 
 
 239 
 
 work they must be calibrated with an alternating current of the 
 same number of periods. For the E.M.F. of self-induction is much 
 less with a current of 50 than with one of 100 periods. The instru- 
 ment will therefore be incorrect for any other periodicity than that 
 for which it has been calibrated. 
 
 Vector Diagrams 
 
 Let us draw the vector diagrams of the cases just cited. Take 
 the case of a voltage applied to a choking coil in series with an arc 
 lamp. Let us assume the arc lamp takes 8 amperes at 30 volts, the 
 current and voltage being in phase. Let us assume that the choking 
 coil is entirely inductance, having no resistance or iron loss. The 
 diagram would appear as in Fig. 233. 
 
 FIG. 233. Vector Diagram. 
 
 In this figure the line o-a represents in length and phase the 
 square root of mean square value of current, that is 8 amperes, as read 
 on an ammeter. The flux produced in the reactive coil is in phase 
 with the current, since current produces flux. 
 
 We have shown in Fig. 217 and in the text covering it, that the 
 E.M.F. produced by a flux is 90 degrees away from the flux. Thus, 
 o-d, 90 degress ahead of o-a, represents the E.M.F. produced by the 
 flux in the reactive coil, which in turn is produced by the current 
 flowing through the reactive coil. The line o-b represents in length 
 and direction the value of the E.M.F. at the arc lamp. This is in 
 phase with the current o-a, since it is assumed that the lamp is non- 
 inductive. Thus, the product of o-b and o-a gives the energy in 
 watts taken by the lamp itself. The voltage required to overcome 
 the voltage o-d, lost in the reactance, and the voltage o-b, required 
 by the lamp, is now to be determined. This voltage is not the arith- 
 metical sum of o-b and o-d, because they are out of phase with each 
 other, as shown in Fig. 233, and voltages or currents can only be added 
 
240 ELECTRICAL ENGINEERING 
 
 directly in alternating circuits when they are in phase. How, then, 
 should they be added? It can be shown that to add quantities out 
 of phase, it is necessary to -find the diagonal of the parallelogram whose 
 sides compose the two values to be added. Thus, in Fig. 233, the line 
 o-c represents their vector sum. Thus, o-c represents in phase and 
 amplitude the value of the E.M.F. necessary to put 8 amperes through 
 the arc lamp and reactance in series. It can be seen that this value 
 is much less than the actual sum of o-b and o-d. 
 
 This figure also shows that the E.M.F. o-c is out of phase with the 
 current o-a, by the angle c-o-a. This angle is called the lag of the 
 current o-a, behind the E.M.F. o-c. Inductive circuits cause, as 
 shown, a lag of current flowing into them behind the E.M.F. applied 
 to them. Take the case of the Fig. 219, but first without the 
 ring on the core. Let us assume that there is no loss in the iron of 
 the core or in the copper used around the core. Fig. 234 shows the 
 
 b 
 
 FIG. 234. Vector Diagram. FIG. 235. Vector Diagram of Trans- 
 
 Coil without Iron. former. 
 
 diagram of E.M.F. current. Here o-a represents the current flowing 
 into the coil and o-b, 90 degrees away, as has been shown, the applied 
 E.M.F. Now place the ring upon the core, and it is noticed that 
 the current into coil promptly increases. The current in the ring 
 must be supplied from somewhere; thus, each ampere in it must appear 
 in equivalent amperes (allowance being made for extra turns) in the 
 coil itself, since into it only can energy enter, the wire supplying 
 energy being connected only to the coil. 
 
 Consider Fig. 235. Let o-a equal in amplitude and phase the 
 flux. Let o-b in phase with the flux represent the current which, 
 when flowing into the coils of the magnet, produces the flux. We 
 assume no iron loss, so that the magnetizing current proper is only 
 considered. This flux produces, when alternating through the pri- 
 mary, an E.M.F. equal to o-f, and through the secondary an E.M.F. 
 equal to o-c; for, as has been shown, the E.M.F. from flux is 90 degrees 
 away from it, and the E.M.F. o-c in the secondary or ring appears in 
 
ALTERNATING CURRENTS 241 
 
 the primary or coil as o-g equal and opposite o-c (allowance being 
 made for the difference of turns between the ring and the coil) . Assum- 
 ing the ring itself to be non-inductive, the current flowing in it is the 
 result of the E.M.F. o-c and in phase with it, that is, o-d. This current 
 has its equivalent and just opposite to it in the primary or coil, 
 as has been shown, and hence appears in the diagram as o-f. Thus, 
 the ring E.M.F. appears in the coil as o-g, arid the ring current appears 
 in the coil as o-f. Therefore the two currents which must combine 
 as a single current, since only one current can flow in a wire at one 
 time, are o-b and o-f, and the two E.M.F. 's which must combine to 
 give the applied E.M.F. are o-g and o-i. The latter is the E.M.F. 
 of self-induction of the primary coil. This is at right angles, as has 
 been shown in the case of self-induction E.M.F., to the primary or 
 coil current o-e. But the combination of two vector quantities is 
 the resultant of the parallelogram with the two forces as sides; thus, 
 the vector sum of o-b and o-f is o-e, which gives the phase and ampli- 
 tude of the current flowing in the coil. And the vector sum of o-g 
 and o-i is o-h, which is, therefore, the applied E.M.F. upon the coil. 
 An inspection of the figures shows that the current flowing into the 
 coil o-e lags in phase behind the applied E.M.F. upon the coil o-h, 
 by the angle h-o-e, which now is less than the lag in Fig. 1966, where 
 the ring was not on the core. Thus, the energy given to the ring 
 brought the current and E.M.F. applied to the coil nearer in phase. 
 As a matter of fact, the energy now represented is the product of 
 the current o-e, and the proportion of the E.M.F. upon it o-k, for 
 energy means the product of current and E.M.F. when in phase. 
 Examining the triangle, k-o-h, shows the cosine of the angle k-o-h 
 
 equals -=-, as has been explained at the first of the chapter. Thus, 
 
 okXoe = ohXcos kohXoe, or energy equals product of E.M.F. and 
 current and cosine of angle of lag. This value cosine of angle of lag 
 of a circuit is called the power factor. When there is no lag the power 
 factor is unity, for the cosine of 0, as you know, is 1. With a lag 
 of 90 degrees, the power factor is 0, and the energy is 0, since the 
 cosine of 90 equals 0. This diagram, which has just been explained, 
 is that of the alternating transformer, the ring being the secondary 
 circuit and the coil the primary. It deserves careful study.* 
 
 In order to calculate in volts the value of self-induction of any 
 circuit, certain constants of that circuit must be known. In Fig. 235 
 the line o-i is drawn to show in volts the self-induction of the coil. 
 We will now proceed to show just how to calculate this voltage having 
 the coil. In any circuit there is a coefficient of self-induction denoted 
 "by electricians by the letter L. It is equal to the maximum of the 
 
 * For complete discussion of the design and operation of a transformer treated 
 itfith no calculus, see Chap. ITI, Raymond's " Alternating Current Engineering " 
 
242 
 
 ELECTRICAL ENGINEERING 
 
 flux wave times turns of the circuit divided by ampere times 
 
 ir^rkAnann max. flux X turns ,. . 
 
 100,000,000, or - vxmn nnnnnn This is expressed in a unit which 
 dmp. x iuu,uuu,uuu 
 
 has been given the name of Henry. When multiplied by 2?rN, when 
 TT equals 3.14159, and N equals cycles per second of the circuit, ohms 
 are obtained; thus, having L of a circuit, the ohms inductance equals 
 2?rNL and the volts inductance (o-i of Fig. 235) equals 2?rNLI, when 
 I equals the current flowing in the circuit. The calculation of L, 
 that is the flux times turns, is the same as the calculation of flux in 
 any circuit and must be done as shown in the first of this book, where 
 it was shown that flux equals 1. 258 X ampere turns per unit length of 
 circuit Xp, the permeability of the circuit. Consider another prob- 
 lem as follows: What would be the diagram of currents and E.M.F. 
 of a circuit consisting of an inductance in series with a resistance 
 having upon it an E.M.F. applied? The circuit would look like 
 Fig. 236. 
 
 -^MiffiSto 
 
 FIG. 236. Circuit with Inductance and Resistance. 
 
 Let the inductance equal LQ, and the resistance equal RQ. Let 
 the cycles of the circuit equal N cycles per second and the current 
 equal IQ. Then the E.M.F. consumed by the reactance LO equals 
 27rNLoIo> and by the resistance equals IR . We will now draw 
 the vector diagram of these voltages and current. 
 
 FIG. 237. Vector Diagram. 
 
 Draw o-a equalling in phase and amplitude the value of the 
 current flowing. Then the E.M.F. used up by resistance due to this 
 current is in phase with this current and represented by o-b (thus, 
 o&Xoa equals energy loss due to resistance). The E.M.F. of self- 
 inductance is at right angles to the current and is thus represented 
 by o-d. The total E.M.F. necessary to drive this current o-a through 
 the resistance and inductance in series is, therefore, the vector sum 
 
ALTERNATING CURRENTS 243 
 
 of o-b and o-d, or o-e. Any circuit can thus be analyzed and shown 
 diagrammatically by bearing in mind the laws which have been ex- 
 pressed. 
 
 To prove that 2;rNL equals ohms : 
 
 It has been shown that ~T= equals "square root of 
 
 mean square' 7 voltage of the sine curve of E.M.F. produced by an 
 alternator of one turn on its armature and of two poles. The back 
 E.M.F. of a coil of n turns having threaded through it an alternating 
 flux of maximum value of $ and turns n and cycles N (cycles N equal 
 revolutions per second of an alternator of two poles, as has been 
 shown) is 
 
 2?rNj> (max.)Xn ^ 
 \/2x 100,000,000 ~ 
 
 In this case the flux alters instead of remaining constant and the turns 
 revolving in it. Since motion is relative, the same formula held for 
 the E.M.F. produced by the flux alternation, as cycles equal N, 
 though turns equal n. 
 
 In this coil the coefficient of self-inductance, as has been shown, 
 equals 
 
 T = <t> (max.) n _ 
 
 amp. (max.) + 100,000,000* 
 
 From (1) 
 
 _, (j> (max.) n amp. (max.) 2;rN 
 amp. (max.) X 100,000,000 X V<f 
 
 Substituting (2) in (3) gives 
 
 ^ LXamp. (max.) 
 
 ~ 
 
 _. amp. (max.) , , , , . 
 
 But ^_ - equals, as has been shown, square root of mean 
 
 square amperes, as read on an ammeter. Hence, E (square root of 
 mean square) equals 2;rNLI, where I equals square root of mean 
 square amperes; since from Ohm's law volts equal current times 
 resistance, it follows from the equation E = 2;rNLX I that 2;rNL equals 
 ohms, which was to be proved. 
 
 Referring again to Fig. 237, the line o-e represents the opposite 
 of the flow of current by resistance and inductance. It is a fact that 
 in any right angle triangle the long side equals the square root of 
 the sum of the squares of the other two sides; thus, o-e equals the 
 
 square root of ob 2 + be* . But b-e equals o-d. Thus, o-e =^/2xnL 2 + R 2 . 
 
244 
 
 ELECTRICAL ENGINEERING 
 
 This value is called impedance and represents the opposition in 
 ohms to the flow of an alternating current in a circuit containing the 
 resistance R and the inductance 27rNL. 
 
 Wattmeter Power- Factor 
 
 For determining the watt consumption of an alternating circuit; 
 it is not sufficient to measure the effective voltage and current. For 
 this purpose it is therefore necessary to employ an instrument which 
 at any moment is influenced by the simultaneous values of current 
 
 FIG. 238. Wattmeter. 
 
 FIG. 239. Two-wire Single-ph* 
 Integrating Wattmeter. 
 
 and voltage, i.e. an instrument which directly indicates watts. Such 
 an instrument, the construction of which is shown in Fig. 238, is called 
 a Wattmeter. It is similar to the electro-dynamometer mentioned 
 on page 60, with the difference only that it is not, like the electro- 
 dynamometer, wound with wires of equal, but with wires of different 
 diameter. The wattmeter essentially consists of a fixed coil, of few 
 windings made of thick wire, through which (as with an ammeter) 
 the main current pases, and of a movable coil, with a few windings 
 of fine wire, which (like a voltmeter) is in series with a resistance, and 
 is directly connected on the full voltage. To prevent any phase-dif- 
 
ALTERNATING CURRENTS 245 
 
 f erence between the shunt-coil current and the voltage producing it, the 
 movable coil and the series resistance must have small self-induction. 
 Hence there must (1) be no iron in 
 the apparatus,, and (2) the coils of 
 the resistance in series with the coil 
 must be " doubly wound/' as is 
 shown in Fig. 240. A winding of 
 this kind prevents self-induction, 
 since to any winding tending to 
 produce a field in a definite direction 
 there is opposed a neighbouring FIG. 240. Spiral without Self- 
 winding tending to produce a mag- induction, 
 netic field in an opposite direction, 
 
 so that no magnetic field results. The shunt coil within the watt- 
 meter itself cannot, of course, be wound in this way, since it then 
 would be unable to exert a directive force. It possesses, therefore, 
 a certain, although small, self-induction, because the coil consists of 
 very few windings. The self-inductionless series resistance has, in 
 addition, an important influence in preventing lag, which depends 
 not only on self-induction, but also on the ohmic resistance of the 
 circuit. 
 
 To keep the fixed and the movable coils always at the same 
 position at right angles to each other, so that their repelling action 
 cannot be weakened, the movable coil has always to be turned back 
 to its original position. For this purpose in the centre of the dial 
 there is a milled head, with a spiral spring attached to it and to the 
 movable coil. The stronger the repelling force, the greater is 
 the angle we have to twist the spring through by using the milled 
 head in order to turn the movable coil back to its zero position. 
 The head has a pointer attached to it, so that we can read on the dial 
 how much we have turned the head and hence how great is the torsion 
 on the spring. The dial being usually divided into 360 degrees, it 
 is necessary to calibrate the instrument. This may be done with a 
 continuous current, by sending, for instance, a current of 10 amps, 
 through the main coil and connecting the shunt coil with its series 
 resistance to a source of 100 volts. If now, to bring the shunt 
 coil back to its zero position (to help in doing this a small aluminium 
 pointer is fixed to the shunt coil, and is bent up to reach the dial), 
 we had to turn the knob through 30, we then know that 30 
 correspond to 1 kilowatt, thus 1 corresponds to 33 J watts. 
 
 The force with which the movable coil is repelled or attracted by 
 the fixed coil depends with alternating current at any moment 
 on the instantaneous values of current and voltage. Since, as we 
 know, the product of instantaneous voltage X instantaneous cur- 
 rent really is equal to the instantaneous power in watts, the 
 wattmeter will at any moment indicate in a correct manner the 
 
246 ELECTRICAL ENGINEERING 
 
 output of, or the watts taken by, an alternating-current circuit. If 
 current and voltage are exactly in phase, as is, for instance, nearly 
 the case with a glow-lamp circuit, the reading on the wattmeter will 
 be exactly equal to the product of the voltage and current as indicated 
 by suitable instruments, such as a voltmeter and an ammeter of the 
 hot-wire type. If, for instance, in a glow-lamp circuit we read 
 on the voltmeter 100 volts and on the ammeter 10 amps., then the 
 wattmeter will indicate 1000 watts. If we had in circuit a phase- 
 difference of a quarter-period, the wattmeter would stop at zero. 
 The voltmeter would, for instance, show 100 volts, the ammeter 
 10 amps., and the wattmeter nothing. 
 
 The product of volts X amps, is called the apparent watts, that 
 indicated by the wattmeter is the real or effective watts. From the 
 ratio between the real and apparent watts we are able to cal- 
 culate the phase-difference. The ratio, that is the number we get 
 by dividing the real by the apparent watts, is called the power 
 factor. With an inductionless load the power factor is equal to 
 unity, with an inductive load it is smaller than unity, and with a 
 phase-difference of a quarter-period it is zero. Instead of the 
 expression "power factor," for mathematical reasons the expression 
 cos (/> is generally preferred (where <j> is the angle of lag and cos <j> 
 indicates the cosine of this angle). 
 
 If the power factor is known, we can even without a watt- 
 meter determine the real watts used. If, for instance, cos = 0.9, 
 then with a current of 10 amps, and a voltage of 100, the real watts 
 will be 100X10X0.9=900 watts. If with an unloaded trans- 
 former, consuming 100 volts and 40 amps, cos < = 0.3, then its real 
 consumption =100X40X0.3 = 1200 watts. With a fully loaded 
 transformer, consuming 100 volts and 300 amps., the power factor 
 (cos (f>) might be equal to 0.99, its real consumption being then 
 100X300X0.99=29,700 watts. 
 
 There are instruments for measuring directly the power factor, 
 which are, however, not often in use. They are called phasemeters. 
 
 Commercial wattmeters which read directly upon their dials the 
 reading of watts, just as ammeters or voltmeters, are sold by leading 
 manufacturers. 
 
CHAPTER IX 
 
 ALTERNATORS 
 
 THERE are many kinds of alternating-current generators or alter- 
 nators. The simplest we became acquainted with in the form of 
 the "magneto-electric machine." A Gramme ring may also be 
 employed as an alternator armature. Its construction is then still 
 simpler than that of a continuous-current armature. The commutr tor 
 can be omitted, and two opposite windings have to be connected by 
 wires with two slip-rings. This is shown diagrammatically in Fig. 241, 
 
 FIG. 241. Ring Armature with Slip-rings. 
 
 in which, for the sake of distinctness, the two slip-rings are indi- 
 cated by circles of different sizes. If the windings a and 6, with 
 which are connected the slip-rings, are situated just in the neutral 
 zone, then the conductors of the left half are in series, and also those 
 of the right half. The two halves are in parallel, and we get at this 
 moment the largest voltage, the same as would continuously appear 
 
 247 
 
248 ELECTRICAL ENGINEERING 
 
 if the armature were built for continuous current. If, however, the 
 windings a and b leave the neutral zone (as shown in the diagram), 
 then one part of the windings of each half is under the influence of 
 the north, the other part under the influence of the south pole, and 
 the voltage of each half becomes therefore smaller. If the windings 
 a and b are horizontal, then in each half there are as many wires 
 under the influence of the north as under the influence of the south 
 pole, and the momentary voltage becomes zero, whilst at the next 
 moment the voltage is reversed. As the armature continues to 
 revolve these changes of pressure are repeated, so that a regular 
 alternating-current pressure is produced between the two slip-rings. 
 
 Naturally in a four- or multi-polar magnetic frame, ring arma- 
 tures can also be employed for producing alternating currents, 
 provided that the series or parallel connections of the windings and 
 the connection with the slip-rings are made in a corresponding way. 
 
 Multi-polar machines are generally employed, since, to obtain the 
 usual periodicity of 100 per second, or 6000 per minute with a 2-pole 
 machine, a speed of 3000 revolutions per minute would be required, 
 whereas with a 4-pole machine but 1500, with a 6-pole machine 
 1000, and with an 8-pole machine 750 revolutions per minute are 
 necessary. 
 
 For exciting the field of an alternator, continuous current is essen- 
 tial, and is supplied either by an outer source of current or by a special 
 sniLll continuous-current machine, coupled directly to the alternator. 
 
 In cases in which a Gramme armature is employed as an alterna- 
 tor armature, besides the slip-rings there is frequently fixed on the 
 armature a commutator, enabling the machine to supply continuous 
 on one, and alternating current on the other side. The continuous 
 current may then be used for exciting the magnetic field. Such a 
 double-current machine is shown in Fig. 242. 
 
 Ordinary continuous-current drum armatures may also be used 
 in this manner and provided with slip-rings. The latter have 
 then to be connected with two armature wires, which are distant 
 by the width of one pole-shoe. 
 
 There are other drum windings, which are quite different from 
 those of continuous-current armatures, and only serve for producing 
 alternating currents. The simplest example of an alternating-current 
 drum armature is the Siemens H armature (see Fig. 64). This 
 armature is provided with a single slot per pole, and the windings 
 are wound as a coil through the two slots, which are opposite to 
 each other. With this armature all the conductors employed in 
 inducing E.M.F. have at any moment equal positions in the magnetic 
 field. All the wires are either in the neutral zone, or in any 
 position between the poles. Thus, with this winding in, all the 
 wires are at any moment either induced equal voltages, or none 
 at all. 
 
ALTERNATORS 
 
 249 
 
 We may also express this as follows: With a drum-winding, 
 which is wound .like a continuous-current winding, in the series 
 connected conductors, E.M.F.'s are induced, which are not in the 
 same phase, whereas with the two-slot alternating-current winding 
 
 FIG. 242. Rotary Converter (British Schuckert Co.). 
 
 the E.M.F.'s of all the conductors are at any time equal in phase. 
 Thus 100 conductors, wound as a continuous-current armature, will 
 not be as effective as 100 wires wound within the slots of a 2-slot 
 alternating-current armature. On the other hand, we can obviously 
 place more conductors on the whole arma- 
 ture circumference than in two slots. 
 
 Instead of a single slot per pole there 
 might as well be two or mere slots, as 
 shown in Fig. 243. But it is clear that 
 such a winding, even if there are many 
 slots, is very different to a continuous- 
 current winding. With the alternating- 
 current winding the armature is wound 
 so that all the coils, if traversed by a con- 
 tinuous current, would tend to magnetize 
 the armature in the same direction. 
 
 The winding diagram of a 4-pole machine, having a single slot 
 per pole, is shown in Fig. 244. Both coils must be connected in 
 series in a suitable manner. The windings shown in Figs. 64, 243, 
 
 FIG. 243. Armature with 
 Three Slots per Pole. 
 
250 
 
 ELECTRICAL ENGINEERING 
 
 FIG. 244 . Four-pole Armature 
 with Single Slot per Pole. 
 
 and 244 are all open windings, whereas the drum and ring 
 armatures have closed windings. 
 
 Since the alternating-current arma- 
 
 I I ture does not require a commutator, 
 
 the attention that it requires is much 
 
 U^^=^>J less, and the wear and tear of the slip- 
 
 rings is generally less, than that of a 
 commutator. Notwithstanding the 
 brushes of an alternating-current 
 dynamo also require attendance, since 
 they are worn through friction and 
 have to be readjusted from time to 
 
 r<^ -^1 time. Sparking might even take place 
 
 at the brushes if the contact with the 
 slip-rings is not sufficiently good. 
 
 With high - tension generators 
 brushes and slip-rings must be avoided 
 whenever possible. Now, with alter- 
 nating-current generators it is quite easy to build the armature 
 as the stationary, and the magnetic frame as the rotating, part. From 
 
 the stationary part the 
 alternating current may 
 then be taken without 
 slip - rings or brushes, 
 merely by means of fixed 
 terminals and cables. 
 A scheme of this type 
 is shown in Fig. 245. 
 The Gramme armature 
 is arranged on the out- 
 side, and in the interior 
 of it the magnet sys- 
 tem rotates. The arma- 
 ture is divided into four 
 quarters, and opposite 
 points are connected with 
 each other, exactly as is 
 the case with an ordinary 
 ring armature. There is 
 naturally no difference 
 .Fie. 245. Four-pole Inner-pole Generator in the inducing action 
 with Ring-armature. whether the armature or 
 
 the field rotates. 
 
 Very often a drum winding is used instead of a ring winding for 
 tthe reason that the fixing of the armature and its building up within the 
 casing is simpler. Fig. 246 shows the general construction of an 
 
ALTERNATORS 
 
 251 
 
 --,- 
 
 FIG. 246. Eight-pole Inner-pole Alternator 
 
 (Brothers Korting}. 
 
 8-pole machine, having two slots per pole. All the eight coils of the 
 armature are connected 
 in series, and wound 
 clock and counter- 
 clockwise alternately. 
 Since now the coils are 
 first under the influ- 
 ence of a north and 
 then of a south pole, 
 this winding will give 
 a proper series connec- 
 tion of all the induced 
 electro-motive forces. 
 
 Such inner -pole 
 machines with sta- 
 tionary armatures are 
 far more reliable than 
 machines with rotating 
 armatures. The arma- 
 ture wires are gen- 
 erally threaded through entirely closed mica or insulating press- 
 pahn tubes, which are embedded in the slots. The closed tubes 
 have a very high insulating power, so 
 that even with high voltages there is 
 no fear of their breakdown and the 
 leakage of electricity from the winding 
 to the iron part. The single wires do not 
 require very good insulation, since the 
 pressure difference between the wires is 
 comparatively small. 
 
 The rotating magnetic field must be 
 excited by a continuous current, it is 
 therefore provided with tw.O . slip-rings, 
 by means of which the continuous current 
 is supplied. For excitation a low-voltage 
 current of about 65 to 110 volts is 
 generally used. Thus, slip-rings and 
 their brushes do not present any 
 danger. 
 
 Shapes of slots generally used with 
 
 alternating-current machines are shown in Fig. 247. The slots 
 are here generally far larger than those of continuous-current 
 machines. They are either open or, more frequently, nearly closed, 
 and of rectangular, circular, or oval shape. With high-tension 
 generators entirely closed slots and insulating tubes are generally 
 used. 
 
 FIG. 247. Different Shapes 
 of Slots. 
 
252 
 
 ELECTRICAL ENGINEERING 
 
 Another method of machine construction is shown in cross-section 
 in Fig. 248. The magnet wheel consists of two halves On the 
 
 circumference of each half are provided tongue-shaped extensions. 
 
ALTERNATORS 
 
 253 
 
 arranged so that in the spaces of the right half the extensions 
 of the left half project, and vice versa. These tongue - shaped 
 extensions represent the poles. The single field coil is enclosed by 
 the two halves of the magnet, and thus rotates with them. It tends 
 to produce, in the direction of the axis of the magnet wheel, on one 
 side north, and on the other side south, polarity. Thus the 
 tongue-shaped extensions on the left become of north polarity, 
 those on the right of south polarity. Since now the extensions 
 
 FIG. 249. Inductor Type of Machine (Maschinenfabrik Oerlikon). 
 
 belong alternately to one and the other half, we have here a row of 
 alternating north and south poles, like the magnet wheels previously 
 described. Both these types of alternating-current machines belong 
 therefore to the " alternating pole type." 
 
 With both types the exciting current has to be led to the rotating 
 part by means of slip-rings. 
 
 The formula for the E.M.F. of an alternator has been shown to 
 
 be E.M.F. (virtual) = ^ QQQQQQ' wn ere N=cycles per second, n= 
 turns embracing flux <, which are connected in series, where N= 
 
254 
 
 ELECTRICAL ENGINEERING 
 
 revolutions of alternator per minute multiplied by the number of 
 pairs of poles and divided by 60. 
 
 A usual winding of a single-phase alternator armature is shown 
 in Fig. 250. 
 
 Each coil may have as many turns as desired to produce, when 
 all are in series, the proper number of n to give the desired E.M.F. 
 There are many forms of windings. Often, between the coils, as 
 shown in Fig. 250, another complete set of coils is inserted using the 
 same armature slots, the extra coils either being placed beside or 
 above in the slot of the other windings. These extra coils have to 
 
 FIG. 250. Armature Winding. 
 
 be wound left-handed, if the first are right, to put the E.M.F. induced 
 in them in series with the other E.M.F. By this \neans the armature 
 surface is more filled with wires and thus, for certain voltages, the 
 winding is more desirable. The poles, cycles, speed, amperes, and 
 volts regulate what type of winding should be used. Those shown 
 are common. Such a winding as shown in Fig. 250 gives a single- 
 phase alternating E.M.F., as shown in Fig. 217, page 218. The wind- 
 ing of Fig. 241, page 247, also gives the same wave of E.M.F., as 
 do the various alternator windings. Suppose in the alternator as 
 shown in Fig. 241, which from the collector rings as shown a wave 
 of alternating E.M.F. is obtained, taps to the winding are made at 
 c and d, at points at right angles or 180 degrees away from a and 6. 
 Suppose these taps are connected to two extra collector rings, what 
 E.M.F. would be obtained from these extra rings? Obviously an 
 
ALTERNATORS 
 
 255 
 
 alternating E.M.F. would result independent from the E.M.F. at the 
 rings, as shown in the figures. Obviously this E.M.F. would be alike 
 in value to that at the rings as shown in the figures. But this E.M.F. 
 differs in one important point. That is, it is out of phase 90 degrees 
 with the E.M.F. from the rings as shown. That is, when the E.M.F. 
 for ab taps is a maximum (which occurs when the taps ab are 
 vertical in the figure) the E.M.F. from c-d taps is (which occurs 
 when the taps c-d are horizontal). Thus, such an alternator produces 
 what is called a quarter-phase E.M.F. This machine is then of a class 
 called polyphase alternators. The E.M.F.'s are as shown in Fig. 217, 
 one phase producing the E.M.F. shown by the full line and the other 
 the E.M.F. shown by the dotted line, differing in phase from the 
 first by 90 degrees. Fig. 280, page 287, shows also the quarter- 
 phase relation of E.M.F.'s or currents. If the taps on armature 
 shown in Fig. 241 were at points 120 degrees apart instead of at 
 
 FIG. 252. Diagram Quarter-phase Alternator. 
 
 right angles to each other a three-phase E.M.F. would be produced, 
 the maximums of the three E.M.F.'s being 120 degrees apart in 
 phase. This is shown in Fig. 288, the three phases being represented 
 by a, b, and c, differing at their positive maximums by 120. A 
 polyphase (in this case a quarter-phase) is shown in Fig. 310. 
 
 Fig. 252 shows diagrammatically a quarter-phase alternator, the 
 two windings a and 6 being shown. 
 
 The E.M.F., E and E' are produced equal to each other and 
 reaching their maximum values 90 degrees apart. Fig. 253 shows 
 similarly a three-phase alternator, with coils A, B, and C set 120 
 degrees apart in phase, that is, to have any coil give the same E.M.F. 
 as the next the armature must turn 120 degrees. 
 
 Another way of showing this same thing diagrammatically is as 
 in Fig. 254. 
 
 As before, the coils A, B, C are 120 degrees apart, just as in 
 Fig. 253. There is, however, a different connection in the two 
 cases. In Fig. 253 each coil produces the full E.M.F. of the alter- 
 nator, while in Fig. 254 the E.M.F.'s are the resultants of the E.M.F. 
 of one coil with the next, the resultants being 120 degrees apart, as the 
 coils are. The windings of Fig. 253 are said to be connected delta, 
 
256 
 
 ELECTRICAL ENGINEERING 
 
 from their resemblance to the Greek letter A, and in Fig. 254 to be 
 connected Y or star-connected. The resultant of the E.M.F.'s A 
 and B (any two E.M.F. 's give the same result) made by the parallel- 
 ogram of forces, as has been shewn, equals V 3 times the E.M.F. 
 
 FIG. 253. Diagram Three-phase Alternator, Delta. 
 
 of _pne of them. Thus, the E.M.F. of a single coil A, of Fig. 253, is 
 \/3 (equals 1.732) times the E.M.F. of a single coil of Fig. 254 (assum- 
 ing, of course, the coils to be of equa_l^ turns). Thus, to change from 
 A voltage to Y voltages divide by \/3. 
 
 In Fig. 254 the current in the coils is the same as the current in 
 
 FIG. 254. Diagram Three-phase Alternator, Star. 
 
 the lines e e e running to the external circuit. This current is called 
 the Y current. The current in the coils of Fig. 253, however, com- 
 bines before going into the lines e e e. Thus, the current in the coils 
 of Fig. 253 is called the A current. The current in the lines is of 
 course as before the Y current; the Y current, being the combination 
 of the A currents, is larger. It is V 3 times A current. Thus, we 
 have in the three-phase circuits and machines Y currents and voltages 
 and A currents and voltages differing by the factor V3. A three- 
 phase winding is shown in Fig. 256. 
 
 The regulation of an alternator is influenced as in a direct-current 
 generator by the resistance drop of the armature and by the armature 
 
ALTERNATORS 257 
 
 reactance. The resistance drop is calculated just as in a direct- 
 current machine. The armature reactance, however, is a different 
 matter. In a direct-current machine the current is a constant, and 
 the flux produced by it is a constant, and no self-induction exists. 
 Also a direct-current generator has a commutator upon which the 
 brushes are shifted forward with their demagnetizing iniluence. With 
 an alternator, however, the armature current is variable (a sine- 
 curve current) ; hence this current must produce a variable flux 
 and therefore self-induction. Also, the current flowing from an 
 alternator need not necessarily be in phase with the E.M.F. created. 
 Hence, the maximum of the current may occur after the maximum 
 of the E.M.F. 
 
 The maximum E.M.F. is produced (see Fig. 241) when the taps 
 b-a are vertical. The current may not, if lagging, be a maximum 
 till later, as shown at b-a. When vertical, the demagnetizing action 
 of the armature is vertical between the poles, just as in a direct- 
 current machine with the brushes at neutral point. Thus, one pole 
 tip is strengthened and the other weakened not constant in value, 
 however, but naturally pulsating, due to the armature current pul- 
 sat ng. When the current lags, however, and the maximum comes 
 as in the position a-b, shown in Fig. 241, there is a component actually 
 opposing the flux, this being similar in effect to the shifting of the 
 brushes on a direct-current generator. Thus, we have to lower the 
 voltage of an alternator, the ohmic drop, the self-induction, and the 
 armature reaction. These must be overcome by extra field current. 
 If, instead of lagging current, the current is leading, then the armature 
 reaction tends to help the voltage, and the field current may have to 
 be lowered as the load comes on. Condensers and synchronous 
 motors with strong field excitation produce leading currents. 
 
 Due to the pulsating nature of the armature reaction of single- 
 phase alternators, the pole-pieces must be laminated to keep down 
 the eddy currents which would be produced by the alternating flux 
 of the armature currents near them. 
 
 To find the efficiency of an alternator, various losses must be 
 determined and added to the output. The ratio of the output to 
 the sum of the output and the losses gives the efficiency; that is, 
 the ratio of the useful output to the total energy generated. The 
 losses are, first, friction; second, core loss; third, I 2 R of field; fourth, 
 I 2 R of armature. The core loss should be determined exactly as 
 has been described for a dynamo, page 130. The normal core loss 
 corresponding to full load should be taken at a field in the alternator 
 to give the voltage of E + IR, when E equals the operating voltage 
 of the alternator and R equals the resistance. At first thought it 
 might be assumed that instead of R equalling the above, the impe- 
 dance, which is'V R 2 + 27rNL 2 , as has been shown, should be used. 
 But it must be remembered that the flux produced by the armature 
 
258 
 
 ELECTRICAL ENGINEERING 
 
 ampere turns, called armature reaction, and the flux produced by 
 the induction of the armature proper, combine with the main flux, 
 producing actually, therefore, but one flux. This flux need produce 
 but E + IR to give E at the terminals. The armature reaction and 
 inductance can be regarded as a tendency for pulling down the voltage 
 met by the field ampere turns. Having measured the resistance of 
 armature and field circuits, then PR losses are, of course, known, 
 from which the efficiency of the alternator is known, being equal 
 to output in watts divided by output in watts plus core loss, plus 
 I 2 R field, plus Ii 2 Ri armature. 
 
 The curve of voltage with load variation of an alternator is shown 
 in Fig. 257. As the load increases the voltage drops. 
 
 FIG. 256. 
 
 If the current of the field is increased to keep the voltage con- 
 stant, the curve of field current plotted against load appears as in 
 Fig. 258. As may be noted, the current in the field must be increased 
 as the load comes on. The two factors that tend to lower the voltage 
 as the load comes on are resistance and armature inductance. The 
 resistance can easily be measured. How, now, should the inductance 
 be measured? The inductance consists, first, of the demagnetizing 
 effect of the ampere turns of the armature. On a pure inductive 
 load of 90 degrees the armature ampere turns act directly, opposing 
 the field ampere turns and in the same magnetic circuit as act the 
 field ampere turns. Thus on 90 degrees lag the ampere turns of the 
 field spools must have subtracted from them the ampere turns of 
 
ALTERNATORS 
 
 259 
 
 the armature current. On no lag the ampere turns of the armature 
 act to produce a flux at right angles to the main flux flow of the 
 poles. Thus, as the lag is increased, the effect of the demagnetizing, 
 
 Load 
 FIG. 257. 
 
 ampere turns increases, swinging around until at 90 degrees lag the 
 armature ampere turns are actually opposing. Second, the pure 
 inductance of the armature windings themselves has its separate 
 influence to influence the E.M.F. The flux path of the ampere turns 
 is not the same as that of the main flux produced mainly by the 
 field spools, but it is a leakage circuit around the wires themselves. 
 If the wires of the armature are embedded in slots, this path would 
 be down one tooth, across underneath the slot, up the next tooth,, 
 across the gap, across the pole over the slot, across the gap again 
 to the starting-point, completing the circuit. As has been shown in 
 Fig. 237, the self-induction effect is at right angles to the current. If 
 the current from the alternator lags 90 degrees behind the alternator 
 E.M.F. , and if the E.M.F. of self-induction lags also 90 degrees behind 
 the current, under such conditions the self-induction E.M.F. would 
 exactly oppose the main alternating E.M.F. in its effect. Thus,, 
 just like armature reaction, the self-induction effect swings around 
 into exact opposition with increasing lag of alternator current. Due 
 to this similarity, a test to combine both effects has been suggested 
 by Charles P. Steinmetz, which most engineers now use to obtain 
 regulation. The method consists in short-circuiting the alternator 
 upon itself and increasing its field current until full current is flowing 
 in the armature. Note the ampere turns in the field. Under these 
 conditions these ampere turns are exactly opposing the armature 
 ampere turns as well as overcoming the exactly opposing induction 
 of the armature. This is true since when short-circuited the armature 
 current lags practically 90 degrees behind the small E.M.F. induced to. 
 
260 
 
 ELECTRICAL ENGINEERING 
 
 produce, through the short circuit, full-load current. Thus, we have a 
 direct measure in ampere turns of these values. Mr. Steinmetz gives 
 the name of synchronous reactance to this value. 
 
 Having now found this value for a given alternator (this holds 
 true for a single-phase or polyphase alternator), it should be used 
 as any value of reactance. Then consider the use of an alternator 
 on a non-inductive load. To calculate the regulation, let, in Fig. 259, 
 
 load 
 
 FIG. 258. 
 
 o-o equal the ampere turns to produce the normal E.M.F. of the 
 alternator E, plus the IR drop when running at normal speed no 
 load. Let o-d equal the ampere turns of synchronous reactance 
 determined as shown. Then the resultant of them equals o-c, which 
 equals the ampere turns necessary to produce normal voltage E at 
 full non-induction load. If, when this load be thrown off, the field 
 ampere turns be kept at o-c, the voltage would naturally rise above 
 E, since o-c is greater than o-b. The amount, then, the voltage 
 rises divided by E gives the regulation of the alternator. This 
 method, therefore, serves not only to determine the regulation, but 
 gives an opportunity to find out the necessary ampere turns of field 
 to give full load. Since it is not always practical to actually load 
 alternators when making tests of regulation, etc., this method is very 
 convenient and it is at the same time very accurate. 
 
 If the current flowing from the alternator be lagging, the diagram 
 of Fig. 259 appears as in Fig. 260, when the current o-a is shown 
 lagging by the angle a and behind the E.M.F. o-b. In such case, 
 plot 0-6 as before equal to the ampere turns necessary to produce 
 the voltage E+IR at no load; plot o-d as before, equal to synchronous 
 reactance ampere turns, but in this case plot them at right angles to 
 the current vector o-a, since induction is always 90 degrees away from 
 the current. Thus, in this case the resultant o-c is greater than in 
 Fig. 260, showing that under lagging load the ampere turns required 
 in an alternator are greater than under non-inductive load. 
 
ALTERNATORS 
 
 261 
 
 This same method is used to obtain regulation of transformers. 
 It is not practical to read direct the regulation of a transformer, so 
 instead the synchronous reactance is obtained similarly to a generator. 
 In the case of a transformer it is short-circuited upon itself, and the 
 
 FIG. 259 Vector Diagram. Regulation on A. C. Generator. 
 
 voltage necessary to put field current through the windings is read. 
 This voltage is then a measure of the inductance of both primary and 
 secondary added together. Knowing this and the resistance, and 
 remembering that inductance in vector diagrams must always be 
 plotted at right angles to the current and that resistance drop must 
 be plotted in phase with the current, the diagram under load can be 
 plotted just as has been shown in Figs. 259 and 260. 
 
 FIG. 260. Vector Diagram. Regulation on A. C. Generator Inductive Load. 
 
 Another type of alternating-current machines is represented by 
 the inductor type shown in Figs. 249, 261, and 262. As may 
 be seen from the illustration (Fig. 262), the magnet wheel has no 
 winding at all. It has on each side five (or with larger machines 
 more) pole pieces. By a stationary coil fixed in the casing 
 
262 
 
 ELECTRICAL ENGINEERING 
 
 FIG. 261. Armature and Exciting Bobbin of Inductor Machine (Maschinenfabrik OerLikon}. 
 
ALTERNATORS '263 
 
 the rotating iron part is magnetized, one side with its pole pieces 
 becoming north, the other side south, magnetic. The stationary 
 casing contains, besides the exciting coil, two armatures, which 
 
 FIG. 262. Magnet System of an Oerlikon Inductor Alternator. 
 
 are built up in the usual way with iron disks, forming rings 
 surrounding the pole pieces of the right and left sides respectively. 
 The two armatures are provided with windings in slots. A small 
 continuous-current dynamo, generally fixed beyond one of the bear- 
 ings, supplies the necessary exciting current. 
 
 The course of the lines of force in this machine is as follows : The 
 lines of force, produced by the stationary exciting coil, leave the pole 
 pieces in one, say the left, side, enter the left armature, and pass 
 through the case -which is generally made of cast steel, sometimes 
 of cast iron, flow through the right armature, and from there back 
 to the pole pieces of the right-hand side of the magnet wheel. Thus 
 with this machine the wires are not alternately under the influence 
 of a south and a north pole, but the wires of one half, say, for instance, 
 those of the left, are always acted upon by north poles, those cf the 
 other half always by south poles. Hence if with this machine we 
 connected the armature wires in the same way as we did with the 
 alternating-pole machines viz., always two wires which are distant 
 by the width of one pole the resulting E.M.F. would be nil; the 
 reason being that the two wires connected with each other would be 
 under the influence of a pole of the same name, and thus their E.M.F.'s 
 would act against each other. 
 
 To avoid this we must not lead the winding from one pole to 
 the next one, but must complete each coil by passing the winding 
 
264 ELECTRICAL ENGINEERING 
 
 through the space between poles of the same name. The separate 
 vCoils then may be connected in series as usual. 
 
 We want, therefore, with such a machine twice as many slots as 
 there are poles, and only half the wires are at any moment effective 
 in producing an E.M.F. From this it will be clear that this 
 machine is heavier, and thus more expensive, than one of the 
 :rotating-field type of equal output. It has, on the other hand, the 
 ;ad vantage of the absence of any rotating windings, and thus of any 
 slip-rings. Since, however, the rotating windings and the slip-rings 
 of a rotating-field machine do not give any trouble, this advantage is 
 not a very important one. 
 
 The right and the left half of a continuous-pole type represent, 
 in a manner, two separate machines, but we may as well connect 
 their windings in series and so get the double voltage. 
 
 The most up-to-date type of alternating-current generator is that 
 of the alternating-pole type with radial poles, which revolve. (See 
 Frontispiece.) 
 
B C 1 
 FIG. 263 Synchronizing Lamp Connections. 
 
 Switching in Parallel of Alternating-current 
 Machines Synchronizer 
 
 To run two alternating-current generators in parallel, several 
 conditions have to be fulfilled. The second machine must as in. 
 the case of continuous-current machines, be brought to the same 
 
 voltage as the first one; it 
 must run with exactly the 
 same speed; and it must, 
 at the moment of switching- 
 in parallel, be equal in phase 
 with the first machine. The 
 exact correspondence of 
 speed and phase is called 
 " Synchronism." 
 
 With mechanical speed- 
 measuring devices ta- 
 chometers and speed-counters it is impossible to determine the speed 
 as accurately as is necessary for this purpose. There is, however, a 
 very ingenious and simple device 
 which indicates electrically small dif- 
 ferences in the speeds. 
 
 In Fig. 263 the two double circles 
 represent two single-phase alternat- 
 ors, which can be connected by means 
 of a single-pole switch AA'. In par- 
 allel with the latter there is connected 
 a glow lamp which is able to stand 
 double the voltage of either of the 
 alternators. When the switch is open 
 there is a closed circuit, in which the 
 two machines and the lamp are con- 
 nected in series. If the two machines 
 were continuous-current machines, 
 there would be only two possibilities : 
 either they work in series, so that 
 their voltages are added, or they act 
 in opposition, so that the resulting 
 voltage is zero. If both machines were designed for 110 volts, thert 
 in the first case the lamp receives 220 volts, and burns with its normal 
 intensity. In the second case the lamp does not glow at all. On the 
 
 265 
 
 FIG. 264. Westinghouse Synchro- 
 scope. 
 
266 
 
 ELECTRICAL ENGINEERING 
 
 other hand, with alternating-current machines there are between these 
 two extremes many other possible cases. According to the phase- 
 difference between the two machines, tfjl voltages between double 
 and no voltage may be given to the lamp. 
 
 If now we want to switch the two machines in parallel, we 
 have to watch the lamp. Supposing that machine II. is running 
 a very little slower or quicker than machine I., then the lamp 
 will glow for one moment, and be dark the next. At the 
 instant, when the voltages of the two machines are equal in 
 phase, the lamp will remain dark, and at any other period, in which 
 the phases are displaced by half a period, the lamp will burn with 
 its maximum intensity. If two 60-pole machines differ in their 
 speeds by four revolutions per minute, the nickering of the lamp 
 will appear 240 times per minute. In this state the machines must 
 naturally not be switched in parallel, but the steam-engine of the 
 second generator must by some means say, for instance, by adjusting 
 the governor, be brought to the right speed. The nearer the alternator 
 approaches the right speed, the slower the flickering will become; 
 and when it is very slow, we can use the moment the lamp is dark 
 again to switch the machines in parallel. The machines are then in 
 the same phase, and will remain so, since if- one machine tends to 
 slow up it will be driven by the current of the other machine. 
 
 Instead of a lamp a voltmeter may be employed. As long as the 
 voltmeter pointer swings quickly backwards and forwards, the 
 machines must not 1 be switched in parallel, but if the vibrations 
 become very slow, the moment when the pointer is at zero may be 
 used for closing the switch. 
 
 The arrangement of Fig. 263 has a disadvantage: the machines 
 have to be switched in parallel at that moment when the lamp 
 indicates no pressure. This moment is rather difficult to determine, 
 since a 110-volt bmp becomes dark long before the voltage is 
 nothing, generally at Lbcut 
 
 A 
 
 B' 
 
 15 to 20 volts. Hence it 
 may happen with this 
 arrangement that the 
 machines are switched in 
 parallel, whilst there is 
 still a considerable differ- 
 ence between the two vol- 
 tages, and a sudden rush 
 of current be caused. 
 
 To obviate this an 
 arrangement is often 
 employed, which diagram- 
 matically is shown in Fig. 
 265. The machines, to be switched in parallel, are first separated 
 
 FIG. 265. Synchronizing Lamps cross-con- 
 nected. 
 
ALTERNATORS 
 
 267 
 
 by a 2-pole switch. Two glow lamps, each of the voltage of one 
 of the generators are in cross-connection with the two machines, 
 thus one lamp is connected with A and B', the second with B 
 and A'. The current flows from the terminal A of machine L, 
 through the upper lamp to terminal B' (b) of the other machine, 
 through this machine to terminal a (A'), from there through the 
 lower lamp to the second terminal B of the first machine. If both 
 machines are in phase, A is equivalent in voltage to A', and B to B'; 
 thus the lamp switched on A and B' will glow with the same voltage 
 that is, with a single generator voltage as if it were switched 
 on A and B. It is exactly the same with the second lamp. If the 
 machines happen to be exactly opposite in phase, then A is equiva- 
 lent to B', and B to A'; thus the lamps will remain dark. At any 
 other phase-difference the lamps will glow, but not as brightly 
 as when in phase. Hence the switching in parallel has, with 
 this arrangement, to be done at the moment when the lamps 
 are brightest, which point can be far better observed than when 
 they are dark. 
 
 The connections described can only be employed with low 
 
 voltages. For medium voltages, say 300-5GO. it will be necessary 
 
 to use, instead of single lamps, groups of 3-5 series connected lamps. 
 
 With still higher voltages this is inadmissible. Hence, with 
 
 high-tension generators, the lamps are not put in the high-tension 
 
 circuit, but small trans- 
 A' formers are employed, 
 
 to the low-tension side of 
 which the lamps are con- 
 nected. In Fig. 266 the 
 diagram of connections 
 is shown. If A is equal 
 in phase with A', then 
 the low-tension termi- 
 nals of the transformers, 
 viz. a and a', are equal in 
 phase. Since now a is con- 
 nected with 6', and a and 
 a' are in series with the 
 lamp, the voltages of the 
 low-tension coils of the 
 transformers are added, 
 
 and the lamp will glow with its maximum intensity. The trans- 
 formers are generally designed so as to produce a low-tension voltage 
 of 55. If, then, the machines are equal in phase, so that the low- 
 voltages of the transformers are added, a 110-volt lamp will just 
 burn with its normal intensity. The procedure for switching in 
 parallel is exactly the same here as with the previous arrangements. 
 
 FIG. 266. Arrangement of Synchronizing 
 Lamps for High-tension Circuits. 
 
268 
 
 ELECTRICAL ENGINEERING 
 
 The action of two alternators in parallel can be shown by Figs. 
 
 In Fig. 267 the lines 1-2 and 1-3 represented the E.M.F 's of 
 the two alternators in parallel. They are drawn beside each other 
 but in reality are exactly superimposed. The condition represented 
 by Fig. 267 is when the two alternators have the same wave shape, 
 the same voltage, and the prime movers (engines or water-wheels) 
 run at a constant speed throughout each revolution. Under these con- 
 ditions no cross-current flows between the alternators, but each does 
 its share of the work. Suppose the wave shapes are different. Then, 
 as the wave of one during its generation becomes bigger or smaller 
 than the other, a current will flow from one alternator across to the 
 other, since they are connected directly together, the path of the 
 
 FIG. 267. 
 
 current being thus through the armature of one machine across the 
 connecting wires between the two machines and then through the 
 armature of the second. This effect, while it may exist, is usually 
 negligible, and so will not be discussed here. The other, that is 
 variation in speed during a revolution, is more serious and frequent, 
 especially with engine direct-connected units. Under some circum- 
 stances the engine and generator may swing apart during a single 
 revolution. This effect is shown in Fig. 268. The two voltages 1-2 
 and 1-3 are now swung apart as described by the angle 2-1-3. This, 
 then, now equals the resultant voltage 2-3 (completing the triangle)! 
 which is free to create current through the windings of the two alter- 
 nators, circulating around through the cross-connecting or buss wires. 
 The line 1-3 'equals 2-3, drawing it as usual in either diagram to a 
 common centre (in this case, point 1). This represents the free 
 voltage. The current from this voltage equals the vector 1-3 divided 
 by the sum of the impedance of the alternator armatures in series. 
 This circuit is inductive, since the induction is much more than the 
 resistance. Thus, the current flowing lags much behind the E.M.F. 
 and the current for 1-3 equals 1-4, lagging behind it by the angle 
 4-1-3. But this brings, as can be seen, the current 1-4 apparently 
 in phase with the E.M.F. 's 1-2 and 1-3, and thus, since E.M.F. 's 
 and currents in phase represent energy, this exchange of current 
 
ALTERNATORS 269 
 
 represents energy, and thus there is a prompt tendency by the current 
 to pull the alternators together again. This is called synchronizing 
 action and is what keeps alternators in multiple from falling out of 
 step. 
 
 Suppose no swing action exists as just described, but one voltage 
 is greater than the other. This may be shown by Fig. 267 again, 
 where the vector 1-3' represents the difference between the two 
 E.M.F.'s in phase with them in this case, since exact synchronism is 
 assumed. Again, the current from these E.M.F.'s, as in Fig. 268, 
 lags about 90 degrees from it and can be shown by the vector 1-4. 
 This, however, is 90 degrees away from the voltage vectors 1-2 and 1-3, 
 and thus does not represent energy, since E.M.F.'s and currents in 
 phase represent energy, and 90 degrees apart represent no energy. 
 Thus, the current does not tend to pull the alternators together, rep- 
 resenting no energy. Hence, if alternators in parallel do not take 
 their respective portions of load, altering field will not usually help 
 matters, but the throttle and water (in case of water pans) must be 
 adjusted. Also in removing an alternator from the busses by pulling 
 the main switch, the current flowing cannot be cut down by lowering 
 the alternating field, since this may actually increase the current 
 flowing (being cross-current, not energy current, however). The 
 arc also from breaking such a lagging or leading current is much 
 worse than with an equal energy current, since with energy current 
 the E.M.F. and current pass through together, whereas with lagging 
 or leading current, if one is the other has value and hence gives more 
 sparks at whatever part of the wave the break of current may occur. 
 (With E.M.F. and current in phase, the arc is if the current happens 
 to be broken as the wave passes through 0.) 
 
 The way to withdraw one alternator from a group is to lower the 
 driving power until the current commences to lower, keeping the 
 alternators in phase (this takes care of itself) and the E.M.F.'s the 
 same as the other alternators. When due to lowering the driving 
 power by the throttle, the current dies down just as it reaches a very 
 small value, preferably 0, the switch can be pulled and the alternator 
 taken out of circuit. With high-tension machines, such as 10,000 
 volts, this method is desirable. 
 
CHAPTER X 
 ALTERNATING-CURRENT MOTORS 
 
 Synchronous Motors 
 
 ALTERNATING currents have the great advantage over continuous 
 currents that, in the stationary windings of a generator, high 
 voltages may be produced easily and without danger, and this high 
 pressure may be subsequently lt stepped down" by stationary 
 transformers to a conveniently low pressure. 
 
 There are different kinds of alternating-current motors. Our 
 first thought will naturally be, whether we cannot use an alternating- 
 current generator as a motor, as we are 
 accustomed to do with continuous-current 
 machines. Let us consider this case by 
 the aid of Fig. 269, which represents the 
 simplest type of an alternator, viz. the 
 Siemens armature with a single armature 
 winding rotating in a 2-pole field excited 
 by continuous current. If through this 
 winding we send by means of two slip- 
 rings, a current in the direction marked 
 by a dot and cross respectively, then the 
 armature will tend to rotate clockwise. IG< 
 
 Now the motor wants a definite time 
 
 for starting. But before it has started to move, the current has 
 already altered its direction; thus the armature now tends to rotate 
 in the opposite direction. With a current of 100 alternations per 
 second no rotation of the armature will take place, but merely a 
 vibration will be noticed, just as we have seen with a magnetic needle 
 surrounded by an alternating current. This motor cannot, therefore, 
 be made to start by an alternating current. 
 
 Assume now that we are able to keep the current in the direction, 
 as marked in Fig. 269 until the armature has started to rotate and 
 
 270 
 
ALTERNATING-CURRENT MOTORS 271 
 
 has made half a revolution. Whilst the wires are in the neutral 
 zone again, let us reverse the current. The armature now possesses 
 a certain amount of live energy, so that it can pass the dead 
 points which occur when the wires are in the neutral zone. After the 
 reversal of the current the wire which was previously under the 
 influence of the north pole will now be under the influence of the 
 south pole, and vice versd. Since, however, the current has altered its 
 direction, the rotation of the armature in the same direction will 
 continue, and the armature will therefore rotate more rapidly. 
 Obviously we must, just at the moment the wires pass the neutral 
 zone, alter the direction of the current, or the rotation cannot be 
 maintained. 
 
 To start a motor in this manner is naturally impossible, since 
 an alternating current supplied for driving a motor has its normal 
 periodicity from the beginning. Nevertheless we have learned from 
 this consideration that, if such a motor be once brought to its full 
 speed, it can be kept in rotation and do work. Thus we must 
 start the motor by some auxiliary power before switching it on the 
 mains, and bring it to its full speed that is to say, to that speed 
 which corresponds to the number of alternations of the current 
 supplied. If, for instance, the latter makes 6000 alternations per 
 minute, then we have to bring the armature to a speed of 3000 
 revolutions per minute, and after having made sure that the neutral 
 armature position coincides exactly with the change of direction 
 of the alternating current, i.e. that motor and generator are "syn- 
 chronous," we can switch the motor on the source of current. To 
 ascertain whether motor and generator are in synchronism we use 
 a synchronizer as described at the end of the last chapter. 
 
 "This type of motor is called a synchronous motor. Any alternate 
 ing-current generator can run as a synchronous motor. The speed of 
 a synchronous motor is quite a definite one, and may easily be found 
 from the number of alternations of the current and the number of 
 poles of the motor. A 2-pole machine will with a current of 6000 
 alternations per minute run with 3000 revolutions per minute, and 
 an 8-pole motor with 750 revolutions. If from any reason say, for 
 instance, a heavy overload of the motor its speed falls off but as 
 much as half the width of a pole, then the motor is almost instantly 
 stopped. For, while the armature conductors are still under the 
 influence of one pole, there are produced forces, due to the change 
 of the current which tend to drive the motor in an opposite 
 direction. Thus the motor is subjected to a powerful braking 
 action, and stopped in a short time, while consuming a large 
 current. 
 
 This type of motor has, therefore, two considerable disadvantages. 
 It requires an auxiliary power for starting, and is stopped if, 
 for any reason, the synchronism is destroyed. It may be compared 
 
272 ELECTRICAL ENGINEERING 
 
 to a novice in cycling. He cannot by himself get on a bicycle and 
 set it into motion, but once the machine is brought up to sufficient 
 speed, he is able to keep it from falling. If, however, he is impeded 
 by any obstacle in his run, he falls, and a new start has to be made 
 with the help of an assistant. 
 
 Hence, for many purposes, synchronous motors cannot be em- 
 ployed at all as, for example, for the purpose of driving shafts in 
 small workshops having no other power at liberty for starting the 
 motor. Likewise a synchronous motor cannot be employed in cases 
 where frequent starting, or a strong effort at starting, is necessary, 
 as is the case with cranes, lifts, and railways. 
 
 On the other hand, the synchronous motor has certain advantages. 
 First of all, the speed of the motor is very uniform, a property very 
 desirable in many cases. Further, the synchronous motor has a 
 decided advantage over all other alternating-current apparatus, in the 
 fact that no phase-difference between voltage and current is caused 
 by it. We shall later on deal with other alternating-current motors, 
 which do not require a field excited by a continuous current. 
 These motors, on the other hand, take a considerable amount of 
 wattless current. If a motor of this kind consumes 2000 effective 
 watts, its apparent watt consumption might be as much as 3000. 
 The generator has then to be designed for an output of 3000 watts, 
 and likewise the mains have to be calculated for a larger current, 
 much of which is useless for producing power. 
 
 Now, with a synchronous motor, the magnetization of which is 
 effected separately by continuous current, there is no phase-difference 
 as long as the excitation is correctly adjusted. Before switching the 
 motor on the mains it is brought to the same periodicity, voltage, 
 and phase as the alternating current with which it is supplied, and 
 therefore, after the motor is switched on the mains, there is no 
 magnetizing or wattless current flowing into the motor, the current 
 thus being in phase with the voltage. If the motor consumes 
 20 amps, at 100 volts, there are 2000 watts used. 
 
 If, on connecting the motor to the mains, the excitation is too 
 weak, so that its voltage is lower than that of the alternating current 
 supplied, then here a wattless current would appear, since the missing 
 magnetization has, as it were, to be supplied from an external source. 
 A wattless current, and therefore a phase-difference, also appears 
 when the magnetization of the motor is too strong. 
 
 It is easy to construct a vector diagram of the various values of 
 resistance, induction, and E.M.F. 7 s of a synchronous motor which will 
 illustrate why varying its field gives varying phase relation to its 
 incoming current. The E.M.F.'s in a synchronous motor are, first, 
 the IR drop; second, the inductance drop, which combine together 
 to give the impedance drop in the armature; third, the E.M.F. applied; 
 and, fourth, the E.M.F. created by the revolution of the armature in 
 
ALTERNATING-CURRENT MOTORS 
 
 273 
 
 the field, that is, back E.M.F. All these values are out of phase 
 with each other, but since all forces must balance with equilibrium, 
 they must form a closed triangle. 
 
 In Fig. 270, let o-b equal the current flowing into the synchronous 
 motor. The o-a equals the IR volts consumed by resistance, and 
 o-c equals the inductance volts consumed by induction. These two 
 combine into o-d, being the E.M.F. consumed by impedance. With 
 
 o as a centre, draw the circle e-g-h with a radius equalling the value 
 of volts applied to the rnotor. About d as a centre, draw the circle 
 i-e-j, intersecting the other circle at e. Connect e with o and d. 
 Then the triangle e-o-d contains all the voltages in a synchronous 
 motor. Draw o-f from o parallel and equal to d-e. The o-e equals 
 in value and in phase the applied E.M.F. o-b equals as drawn the 
 current, and o-f equals d-e, equals the back E.M.F. in value and phase 
 due to revolution of the armature. From this figure and with the 
 value of back E.M.F., the current o-b leads the E.M.F. applied to the 
 motor by the angle e-o-b. If now the back E.M.F. of the motor d-e 
 equals o-f be made smaller, it will be noticed that the current now 
 lags behind the applied E.M.F. Fig. 271 illustrates this. 
 
 Thus, as stated, the synchronous motor has, by means of field 
 excitation control, the means to alter the phase of the current entering 
 it. This holds true, of course, whether the synchronous motor is 
 single-phase or polyphase. Figs. 270 and 271 can be regarded as one 
 phase of a polyphase machine. A single-phase synchronous motor 
 has no tendency to start, but a quarter-phase or a three-phase machine 
 starts from rest with a considerable torque and will soon carry quite 
 a load. This is done by the reaction of the current induced in the 
 
274 
 
 ELECTRICAL ENGINEERING 
 
 pole-pieces and the field producing these currents. By Lenz's law 
 the armature tends to move in such a direction to prevent the induc- 
 
 . 271. 
 
 tion of the currents causing the motion. To add to this effect, poly- 
 phase synchronous motors have wound into the pole-pieces a regular 
 
 winding, which acts just like a " squirrel-cage" winding in the rotor 
 of an induction motor. Single-phase synchronous motors are rarely 
 used. Almost always three-phase motors are used, embodying the 
 advantages of a fair starting torque, less pole-piece losses, and tech- 
 nical designing features better than the single-phase arrangement. 
 
ALTERNATING-CURRENT MOTORS 275 
 
 Synchronous motors are particularly useful for large units. The 
 largest alternating motor in the United States to-day is a synchronous 
 motor. It delivers 9000 H.P. This feature of the synchronous 
 motor that at will by simple field control the phase of the incoming 
 current can be controlled sometimes results on transmission circuits 
 in the use of a motor running " light" solely for this purpose. A 
 plot at no load of the variation of incoming current with field strength 
 is shown in Fig. 272. 
 
 The curve c-d-e represents the plot of current. As may be noted, 
 at a field current of value o-g, the armature current is a minimum 
 at d. If the field current is reduced, the armature current commences 
 to rise until with field current o-f it reaches the full-load current value 
 a-b. Here the incoming current is lagging. If the field current is 
 now increased, the incoming armature current commences to fall till 
 it reaches its minimum at d. Further increase of current causes the 
 armature current to increase till full-load current is again reached, 
 but in this case on the leading side. The current taken at g is only 
 that necessary to supply the losses of the synchronous motor running 
 light, and is thus small in value. 
 
 The synchronous motor must have its field circuit excited by 
 direct current. For this purpose a small direct-current exciter is 
 belted or direct connected to the main motor. Since on starting 
 there is no field required on the synchronous motor, the exciter need 
 deliver no current to the field of the synchronous motor till it reaches 
 full speed, which therefore makes feasible the method of operating 
 the exciter from the synchronous motor field. Thus, even though 
 direct current is necessary, the unit is self-contained, requiring only 
 itself and the alternating energy to do its work. At starting with 
 the armature stationary, the field spools form a secondary of a trans- 
 former of which the armature is the primary, and since the field turns 
 are high as compared with the armature a voltage is induced in them 
 higher than the voltage applied to the armature. Thus, it is dan- 
 gerous to be near the terminals of the field at the instant of starting. 
 Deaths have occurred from this cause. To avoid trouble, the spools 
 may be split up at starting, or closed on the exciter, which entirely 
 annihilates the voltage. The latter method, however, reduces the 
 ability to start somewhat. Usually the field insulation is so designed 
 that it will stand the high voltage induced. These motors are very 
 generally used for a large variety of purposes in the United States. 
 
276 ELECTRICAL ENGINEERING 
 
 The Rotary Converter 
 
 For reasons already known to us, alternating currents are very 
 frequently employed for transmission of electrical energy. Now, 
 there are many purposes for which alternating currents are in- 
 applicable. They cannot be used for charging secondary batteries. 
 At alternating- current central stations it is therefore necessary, 
 even when there is a very small load during the daytime, to have 
 one or more generators running. Also the valuable "buffer effect" 
 of secondary batteries cannot be used in alternating-current central 
 stations. To combine the advantages of alternating currents with 
 those of continuous currents, the following scheme is employed in 
 many cases for transmission of energy to long distances: In the 
 central station alternating current is produced and is led to a 
 number of sub-stations distributed over the area of supply. In these 
 sub-stations the alternating current is transformed into continuous 
 current, and at the sub-station secondary batteries are generally 
 employed. For certain hours the secondary batteries in the sub- 
 stations are charged, thus providing current for the time of small 
 demand when the machines in the central station as well as in the 
 sub-stations are shut down. 
 
 At a sub-station, machines are required for transforming alternat- 
 ing into continuous current. For this purpose either two separate 
 machines, viz. one alternating-current motor coupled directly to a 
 continuous-current generator, which combination is generally called 
 a motor generator, or a single machine, with a rotating armature, 
 may be employed, having slip-rings on one side and a commutator 
 on the other. A machine of the latter type is generally called a 
 converter. 
 
 In both cases synchronous motors can be used without any 
 disadvantage, for the secondary battery installed at the sub-stations 
 will serve for exciting the synchronous motors. The procedure 
 is quite simple. In the case of the motor generator the con- 
 tinuous-current generator is started as a motor by means of the 
 secondary battery and its speed regulated until it is that required 
 for synchronism. Then the synchronous motor is excited and 
 the switch closed. The synchronous motor now drives the con- 
 tinuous-current machine, and, by more strongly exciting the latter, 
 its E.M.F. increases above that of the battery, so that the con- 
 tinuous-current machine supplies current to the battery; i.e., it is 
 working as a generator. Each of the two coupled machines may 
 be built for any voltage. For example, the synchronous motor 
 
THE ROTARY CONVERTER 277 
 
 might be built for a voltage of 2000 or 5000, and the continuous- 
 current dynamo for 110, 220, 500, or any other voltage. 
 
 The synchronous motor can also be started by itself, as has been 
 explained. Under these conditions there is a large drawing of current 
 at low-power factor (say double the normal operating current), so 
 that the voltage upon the line is affected considerably. If this is 
 troublesome, the starting from batteries by synchronizing can be 
 done, which cuts out all the trouble. Another method of starting 
 synchronous motor generator sets is to use a compensator, so that 
 just the required amount of current is given to the synchronous 
 motor, the line current being reduced in proportion to the ratio of 
 the compensator. After the motor is well started, throw one switch 
 within the compensator (or without), which gives normal voltage 
 again to the motor. Since the maximum current occurs with the 
 armature at rest, sometimes the motor is given a start by mechanical 
 means provided, such as a rod inserted in holes in the shaft. 
 
 With a converter the case is different. It is impossible to use 
 it for direct transformation of high-tension alternating into low- 
 tension continuous current. 
 
 Any alternating-current dynamo provided with a commutator and 
 slip-rings like that shown in Fig. 242 can be used as a converter. 
 The armature of the converter can have either a single winding 
 connected with slip-rings and commutator or two separate windings. 
 In this latter case one of them has to be connected with slip-rings, 
 the other with the commutator. 
 
 Both the motor generator and converter may be used for many 
 different purposes. They can be used as (1) a continuous-current 
 motor, (2) continuous-current generator, (3) a synchronous motor, 
 (4) an alternator, (5) a dynamo for continuous and alternating currents 
 simultaneously, (6) a continuous-current to alternating-current trans- 
 former, and (7) finally an alternating-current to continuous-current 
 transformer. 
 
 Since in all these cases of the use of a converter continuous and 
 alternating currents are either produced or transformed in one 
 armature, it is clear that there must exist a definite proportion 
 between the continuous and alternating voltage, and that, unlike the 
 motor generator, it is impossible with the converter to transform 
 alternating current into continuous current of any voltage. The 
 ratio between the two voltages may be determined by the help of 
 a simple consideration. We shall first of all consider an armature 
 with a single winding. 
 
 In dealing with the Gramme ring, as an alternating-current 
 armature (see p. 247), we learned that the maximum alternating 
 voltage is produced if the windings connected with the slip-rings 
 are just in the neutral zone. Now, this is the normal voltage of the 
 continuous current produced by the same ring, since in this case 
 
278 ELECTRICAL ENGINEERING 
 
 the brushes are always in the neutral zone. Thus we have the simple 
 equation: In a single-phase converter maximum alternating-current 
 voltage is equal to the normal continuous-current voltage. We have 
 learned that the measured or effective value of the alternating- 
 current voltage is equal to about 0.7 of its maximum voltage. Hence, 
 if with this converter a continuous voltage of 100 is produced, then 
 the effective voltage of the alternating current taken from the slip- 
 rings will be about 70. 
 
 Owing to the ohmic loss in the armature wires, the secondary 
 voltage of a rotary converter will be somewhat smaller than that 
 found by the above calculation. If the machine be used as a con- 
 tinuous- to alternating-current converter, we get, at a continuous 
 voltage of 100, not quite 70 volts on the alternating-current side, 
 but, according to the load of the machine, somewhat less say 69, 
 perhaps 68, volts only. If, on the other hand, we use the machine 
 as an alternating- to continuous-current converter, we shall for 70 
 volts alternating current get less than 100 volts continuous current, 
 perhaps only 98 or 97 volts. If there be two separate windings on 
 the armature, the winding connected with the slip-rings having 
 three times as many turns as that of the winding that is connected to 
 the commutator, then to a continuous current of 100 volts an alter- 
 nating current of 3X70 = 210 volts would correspond. In any case 
 there exists a definite relation between alternating and continuous 
 voltage which cannot be altered by the regulation of the continuous- 
 current excitation. If for charging cells we want to increase the 
 continuous voltage from 100 to 150, then we must increase the voltage 
 of the alternating current supplied to the slip ring by one-half. 
 
 Since to the rotating armature of a converter alternating current 
 has to be supplied, it is impossible to employ machines of this kind 
 for directly converting high-tension alternating into low-tension con- 
 tinuous current. For this purpose a further apparatus, an ordinary 
 or static transformer, is required, which first transforms the high- 
 tension alternating current of, say, 2000 volts into a low-tension 
 alternating current of, say, 70 volts. This alternating current may 
 then, by a rotary converter, be converted into a continuous current 
 of about 100 volts. 
 
 A converter is started in the same way as a motor generator. 
 The machine is first excited, then started as a continuous-current 
 motor, and, as soon as it is running in synchronism, it is switched 
 on the alternating-current circuit. 
 
 Often converters are started from the A.C. end when they are 
 not single-phase. As a matter of fact, single-phase converters are 
 rarely used in the United States. Three-phase converters are almost 
 universally used. Like the polyphase synchronous motor, a three- 
 phase converter will start from its own A.C. current. About 30 to 
 40 per cent, normal voltage is required. When this is applied the 
 
THE ROTARY CONVERTER 
 
 279 
 
 rotary will quickly come up to speed, drawing from the line about 
 double current. There is practically no sparking at the D.C. brushes 
 under these conditions. Sixty-cycle rotaries, due to their very small 
 armature reaction, draw more current from the line than do 25-cycle. 
 The ratio of A.C. to D.C. voltage in a three-phase rotary is different 
 than that of a single-phase. Consider Fig. 273. 
 
 FIG. 273. 
 
 Let the letters a, b, c represent the points where the A.C. taps 
 are connected to the winding, for, as has been stated, a three-phase 
 rotary converter consists simply of a D.C. generator (with commu- 
 tator and brushes) having taps in its winding at three equidistant 
 points which are connected to three collector-rings. Into these col- 
 lector-rings three-phase current is given, and out of the commutator 
 direct current is taken. 
 
 Let the armature be in the position shown. The D.C. brushes, 
 being at b and e, b-e equals the direct-current voltage and the maxi- 
 mum A.C. voltage of a single-phase converter. Thus b-d equals one- 
 half the D.C. voltage. In the triangle b-d-a, the value b-d equals 
 d-a is thus known, as well as the angle b-d-a, and the angles a-b-d 
 and d-a-b are equal. Thus the line b-a can be found, but b-a rep- 
 resents the three-phase voltage of the converter; i.e., the voltage 
 
 between collector-rings, b-a equals \/3Xb-d= Xb-e. But b-e 
 equals the maximum of the single-phase voltage. Thus, the virtual 
 
 E.M.F., or the square root of mean square voltage, b-a= X /=- 
 
 2 V2 
 
 since the ratio of maximum to virtual equals \/2, as has been shown. 
 Calling voltage 6-e=E the D.C. voltage, we get the A.C. voltage' 
 between collector-rings (equal a-b, Fig. 273) equals the D.C. voltage 
 
 v/o 
 E multiplied by^ ^. = 0.612E. Assuming the converter to be of 
 
280 ELECTRICAL ENGINEERING 
 
 100 per cent, efficiency, the input equals the output. _In a three- 
 phase circuit the input is, as will be shown later, ET\/3. The D.C. 
 output is, as has been shown previously, IE when E' equals the 
 alternating E.M.F. between collector-rings, V the current in the line 
 to the collector-rings, and E and I the D.C., E.M.K, and current. 
 
 Thus, E'lV3"=EI. But E' = EX Thus, = El, or 
 
 o 
 
 Since the efficiency is not 100, but nearer 94, the current I' in the 
 A.C. line has not only to supply the output but the losses. This 1' 
 is about 6 per cent, more than the above, or about equal to the D.C. 
 current. Thus, in a three-phase converter the A.C. and D.C. currents 
 are about alike. Since both the A.C. currents and the D.C. current 
 flow in the same wires in the armature, and since under such con- 
 ditions there cannot be two separate currents actually, it follows 
 that they must combine. Since also the A.C. currents act as driving 
 power and the D.C. as energy given out, it follows that these two 
 currents tend to flow opposite in direction and thus tend to neutralize 
 each other. We thus have in the windings of a rotary converter 
 D.C. and A.C. currents in opposition. It can be expected that since 
 one current has a sine wave in shape and the other a steady value 
 that this combination is rather complicated. Without covering the 
 matter in detail, it has been found that the resulting current in a 
 three-phase converter, when squared (this representing the heat 
 produced in the windings) is 58 J per cent, of the square of the D.C. 
 current. This value allows for the efficiency of the converter. From 
 this it can be at once seen that a rotary of a given size will heat less 
 than a D.C. machine of the same size, and thus a rotary is smaller 
 for the same heating and therefore cheaper than a D.C. machine, 
 which is true. In addition to this, it is apparent that since the A.C. 
 current flows in one direction and the D.C. in the other, that there 
 is no armature reaction, and thus no brush shift is required with 
 change of load. Thus, a rotary must be better in commutating char- 
 acteristics than an ordinary D.C. machine. As a matter of fact, 
 rotaries require no shift of brushes and will carry three times normal 
 load without difficulty. They are thus especially suitable for railway 
 lines when excessive load may momentarily come on. 
 
 Since the A.C. end of a rotary acts just like a synchronous motor, 
 it naturally follows that the phase of the entering current can be 
 altered by altering the field strength, a leading current resulting 
 from strengthening the field and a lagging from weakening it. Ad- 
 vantage is taken of this in rotaries to regulate the D.C. voltage, A 
 series field is placed on the rotary, and as the D.C. load comes on 
 the field is strengthened. As it strengthens the A.C. current comes 
 
COMMUTATOR MOTORS 281 
 
 more and more leading, holding up the voltage. To increase the 
 effect, inductance is inserted in the A.C. lines, and since A.C. current 
 in passing through inductance raises the voltage if the current is 
 leading, a combination of inductance and field strength may be 
 chosen, so that a constant or rising D.C. voltage will result. Thus, 
 rotaries can over-compound on their D.C. ends just as ordinary D.C. 
 machines. 
 
 Rotaries are extensively used in the United States for sub-stations 
 to supply lights or power. They are low in cost per kilowatt and 
 capable of large overloads and in general are very important adjuncts, 
 in electrical distribution of power. 
 
 Commutator Motors 
 
 The question may be asked, Is it possible to run a continuous- 
 current motor with alternating current? 
 
 We are acquainted with the fact that the direction of rotation 
 of a continuous-current motor remains the same if we change the 
 mains leading to the motor (p. 145), for the reason that both the 
 magnet field and the armature current change their direction. It 
 must hence follow that we are able to get motive power from a, 
 continuous-current motor supplied with an alternating current- 
 Naturally the magnet system of the motor must not be solid, but 
 must, like all cores of alternating-current magnets, consist of insulated 
 iron disks. Otherwise its construction is quite similar to an ordinary 
 continuous-current motor. Commutator motors are generally built 
 as series motors. 
 
 Let us now consider the starting of the motor. The motor has 
 to be switched on the alternating-current mains. Armature and 
 magnet coils are then traversed by the same current. The armature 
 wires in the magnetic field tend now to turn the armature in a definite 
 direction say, for instance, clockwise. The armature is therefore 
 turned a little, but before it has turned through one revolution the 
 direction of the armature current is altered. At the same instant 
 the direction of the magnet current is also altered. The effect after 
 the change of the current direction is the same as it was before; i.e., 
 the armature is turned again clockwise, and thus the motor will start. 
 Since, however, the armature windings short-circuited by the brushes 
 are traversed first by a negative, then by a positive current, these 
 motors, on starting, violently spark, and sparkless running is difficult 
 or impossible to obtain. 
 
 Alternating D.C. motors have characteristics similar to D.C. 
 motors, differing only in this fact, that the current lags behind the 
 applied E.M.F. to the motor, which condition cannot, of course, apply 
 
282 ELECTRICAL ENGINEERING 
 
 to D.C. motors. Thus, the line drop in the transmission is greater 
 than with D.C. motors, since, as has been shown, the line drop is 
 greater the greater the lag of current for a given condition of the 
 line. Also the generator must be large to furnish this lagging current 
 and must be better in regulation. In spite of the sparking tendency, 
 which is excessive at starting, this type of motor has been introduced, 
 and roads are now operating using them. In order to reduce the 
 sparking resulting from the pulsating flux through the armature coil 
 short-circuited by the brushes, the leads to the commutator are made 
 high in resistance, increasing the resistance of the circuit in which 
 the short circuit acts. By this means the motors are made operative. 
 More attention must be given, however, to the commutator to keep 
 it in good running condition. On railway lines a good deal of coasting 
 is done by the cars, during which time no current is flowing into the 
 motor. During this time the commutator gets polished up by the 
 brushes, partly or wholly, depending upon the condition of the injury 
 done by the sparking when current is flowing into the motors. 
 
CHAPTER XI 
 
 MULTIPHASE ALTERNATING CURRENT 
 
 Induction Motors Rotating Field 
 
 NEITHER of the two alternating-current motors described in the 
 last chapter is so simple in some respects as the continuous-current 
 motor. Whilst the alternating-current generator and transformer 
 are far simpler than the corresponding continuous-current appliances, 
 with the motor the contrary would seem to be the case. 
 
 It is important now to point out that we can, with alternating 
 currents, produce motion by availing ourselves of the effects of 
 
 induction. We have seen this with a 
 metal ring which was repelled by an alter- 
 nating current flowing through an electro- 
 magnet. On switching the coil in circuit 
 the ring was pushed upwards, on stopping 
 the current the ring fell down. 
 
 An up and down motion of this kind 
 is insufficient for a motor. What we 
 want is a means of producing rotating 
 motion. 
 
 The Italian electrician Ferraris found 
 that by two alternating currents differing in 
 phase a rotating field can be produced. 
 Fig. 274 shows two coils, A and B, whose 
 windings are at right angles to each other. 
 These coils are traversed by alternating 
 currents which differ in phase by 90. 
 Either of these coils in itself would produce a pulsating field, but 
 the two coils together produce a rotating field. 
 
 A simple experiment with a freely suspended stick, or, still better, 
 a stone suspended by a string, gives us a corresponding example, and 
 will make the matter clear. If we push such a pendulum from 
 its position of rest, then it will swing to and fro. A complete 
 
 283 
 
 X 
 
 FIG. 274. Production of 
 Rotating Field. 
 
284 ELECTRICAL ENGINEERING 
 
 movement from, say, the left to the right and back to the left is 
 called a period. If, from its position of rest, we push the stone 
 from us, it will then take up a swinging motion from front to back, 
 which differs from the first vibration in direction only, but not in the 
 kind of motion. If, now, we push the pendulum, firstly, from its 
 position of rest towards the right; and, secondly, after a quarter- 
 period that is, after it has made half an oscillation, being, therefore, 
 in its extreme position to the right we push it forwards, we shall 
 observe that the pendulum takes up a rotating motion. It swings 
 no longer in a single plane, but in a circle. The motion in a straight 
 line has been changed into a rotating motion. 
 
 It is essential for the second impulse to take place in a direction 
 which is at right angles to the first impulse, and also that the time 
 when the second impulse takes place is a quarter of a period later 
 than that of the first impulse. If, whilst the pendulum is swinging 
 from left to right, we strike it in the direction from front to back 
 just at the instant it passes its lowest position, we do not now get 
 a rotating motion of the pendulum, but it will swing in a direction 
 between the directions of the two impulses. 
 
 Similarly the two coils in Fig. 274, each of which alone is capable 
 of producing a pulsating field, are able to set up a rotating field, 
 provided that they are traversed by two 
 alternating currents, the phase-difference 
 between which is a quarter-period. We 
 know that the direction of a field is 
 that indicated by a freely movable north 
 
 pole. Let us now imagine a north pole ^- 
 
 under the influence of coil A (in Fig. 275 [BJ 
 
 the coils are shown more distinctly in 
 
 cross-section). The coil A tends to drive 
 
 the north pole at right angles to its plane 
 
 from left to right that is ; in the direction 
 
 of the single-barbed arrow. Coil B alone 
 
 will try to drive the pole from front FIG. 275. 
 
 to back in the direction of the arrow 
 
 with two barbs. If, now, the currents differ by a quarter-period, 
 
 exactly the same will take place as with the pendulum. The pole 
 
 will rotate. 
 
 What influence will this rotating field exert on a metal cylinder, C, 
 suspended in its interior? 
 
 A magnetic field rotating about a conductor produces, as we 
 know, in the latter an E.M.F., and if there is a closed circuit, electric 
 currents result. These currents have their direction so as to resist 
 any motion, following Lenz's law. In the metal cylinder C, in Fig. 
 274, such currents will be produced. These currents, first, tend to 
 weaken the primary field (just as the currents in the secondary coil 
 
MULTIPHASE ALTERNATING CURRENT 285 
 
 of a transformer do), and, secondly, they resist the motion of the 
 field, which will rotate as long as the primary currents differ in phase 
 by a quarter-period. The metal cylinder within this field will be 
 acted upon in a certain way. Consider for a moment what happens 
 to any one who tries to stop a heavy and fast-moving carriage by 
 taking hold of it. The attempt will be a failure, for he will be carried 
 along with the vehicle. In the same way, the armature C, which re- 
 sists the rotating motion of the field without being able to stop it, 
 will be taken with the rotating field, i.e., it will be turned round its 
 axis. 
 
 Hence there will be a tendency to turn the armature with the 
 same speed as that with which the field is rotating. This state can, 
 however, never be perfectly reached, for if the armature ran in 
 synchronism with the field, the effect on the armature would be the 
 same as if field and armature were at rest, and no current could be 
 induced in the armature. The result will be that the armature 
 can now no longer exert any force, and it will slow up owing to the 
 frictional resistances. As soon as this happens, the armature is again 
 crossed by lines of force, a current is again induced inside, it exerts 
 a force, and thus is able to overcome the frictional resistances. The 
 greater the load becomes, the slower the armature will run in 
 comparison with the speed of the rotating field. The consequence 
 will be that stronger currents are induced in the armature, enabling 
 it to overcome the heavier load. The armature currents have a 
 further important action they also tend to weaken the primary 
 field, and this will now, just in the same way as the primary 
 coil of a transformer, take more current when it is connected with a 
 source of constant voltage. Hence we observe that the behaviour of 
 this kind of motor is very similar to that of continuous-current shunt 
 motors. 
 
 Motors depending on this principle are called induction motors, 
 or asynchronous motors. They are called asynchronous because 
 their working principle depends on the fact that they do not run 
 synchronously; but their speed is less than the speed of synchronism. 
 
 The amount the armature speed of an asynchronous motor is 
 less than the speed of rotation of the field is called the "slip." 
 
 We shall now deal with the construction of a 2-phase induction 
 motor. To obtain sufficiently strong magnetic fields , both the outer a nd 
 inner parts have to be built up from the iron disks, and the windings 
 have to be laid in slots. We have here two circular parts, the cores 
 of which are built up like that of a continuous-current armature. 
 In its simplest form (see Fig. 276) the outer stationary armature, 
 called the "primary armature" or "stator," has four slots. Into 
 every two opposite slots, AA and BB, the coils are laid which cor- 
 respond to the first and second phase respectively. On the cir- 
 cumference of the inner, rotating armature, or so-called rotor, 
 there are a number of slots or holes through which wires are drawn. 
 
286 
 
 ELECTRICAL ENGINEERING 
 
 These wires can be connected with each other in many different ways. 
 One method of connection is shown in Fig. 277, which represents the 
 type called a squirrel-cage rotor. At the front and the back of the 
 armature all the wires are connected by copper rings. 
 
 If on the stator there were the coil A only, and this coil were 
 traversed by a continuous current in the direction marked in Fig. 278 
 by a cross Lnd dot respectively, it would produce a magnetic field as 
 shown in this figure. The lines of force leave the left part of the 
 stator and enter the right part, making the former a north and the latter 
 a south pole. If the coil has an alternating current passing through it, 
 then at a certain instant the left part will have the strongest north, 
 
 FIG. 276. Two-phase Motor. 
 
 FIG. 277. Squirrel Cage. 
 
 and the right part the strongest south, magnetism. The magnetism 
 will then gradually become weaker, until it is reduced to nothing, then 
 it will be reversed, and so on. Similarly coil B alone, if traversed by 
 a continuous current in the direction marked, would cause the lower 
 part of the outer armature to become a north, and the upper part a 
 south pole (see Fig. 279), whilst with alternating currents the polarity 
 would continually be reversed. 
 
 Now coils A and B are simultaneously supplied with alternating 
 currents, which differ in phase by a quarter -period. Hence, if the 
 current in A is a maximum, that in B will be a minimum or nothing. 
 It is as if coil B did not at this moment exist. A only produces 
 magnetism, say, for instance, a north pole on the left. Now, the 
 current in A decreases, whilst that in B increases (see the wave- 
 lines in Fig. 280). Hence A continues to produce a north pole 
 on the left, B tends firstly in a weak, but later in a stronger 
 manner to produce a north pole at the bottom. Both actions are 
 therefore combined, and there will appear a north pole at the left 
 
MULTIPHASE ALTERNATING CURRENT 
 
 287 
 
 lower quarter, which will be lower in position the stronger the 
 current is in B, and the weaker it is in A. After a quarter-period 
 
 FIG. 278. 
 
 FIG. 279. 
 
 the current in A becomes zero, whereas the current in B is now a 
 maximum, and thus the north pole is produced only by the latter. 
 The current in B now decreases, and the current in A has changed its 
 direction, and tends to produce a north pole at the right. By the com- 
 
 FIG. 280. Two-phase (or Quarter-phase) Current. 
 
 bined action of the coils A and B, the north pole will now travel from 
 the bottom to the right; and again, after a quarter-period, the north 
 pole will be produced on the right-hand side, since at this 
 moment the current in B becomes zero again. We have therefore 
 in the stationary outer armature a rotating magnetic field, which 
 makes a quarter of a revolution during each quarter-period of 
 
288 
 
 ELECTRICAL ENGINEERING 
 
 FIG. 281. Four-pole Two-phase Motor. 
 
 the current, and a whole revolution during each complete period. 
 This rotating field produces currents in the conductors of the 
 squirrel-cage, causing it to 
 revolve. 
 
 The speed of the rotor 
 differs but little from the 
 theoretical speed of the rotat- 
 ing field. If, for instance, 
 the current flowing in the 
 stator makes 6000 alterna- 
 tions that is, 3000 periods 
 or cycles per minute, then 
 the rotor will make nearly 
 3000 revolutions per minute. 
 If the motor is not loaded, 
 and the armature there- 
 fore has to overcome only 
 the frictional resistance in 
 the bearings, then even with 
 the most accurate speed- 
 counters no difference between the speed of the field and that of 
 the motor can be measured. On the other hand, if the motor is 
 loaded, its speed will fall 
 down to about 2900, 2800, 
 or even 2700. The motor 
 has then a slip of 100, 200, 
 or 300 revolutions, or ex- 
 pressed as a percentage of 
 3, 6, or 10 per cent. 
 
 The windings considered 
 have 2 poles. Two-pole in- 
 duction motors are seldom 
 used. Generally the motors 
 are, according to their size, 
 wound with four, six, eight, 
 or more poles. The diagram 
 of a 4-pole 2-phase motor 
 is shown in Fig. 281. The 
 least number of slots re- 
 quired in this case is eight. 
 We may then have two coils FIG. 282. Four-pole Two-phase Motor, 
 
 in each phase, as shown 
 in Fig. 281, or four coils 
 
 in each phase, as shown in Fig. 282, the former winding corre- 
 sponding to a consequent pole winding, as in the case of some 
 direct-current machines. The coils are wound so that each of 
 
MULTIPHASE ALTERNATING CURRENT 
 
 289 
 
 them tends to produce in the part it surrounds the same polarity, 
 say, for instance, each a north pole. Then, in the left- and right-hand 
 parts, north poles are produced by the coils, marked by full lines, 
 and therefore, as consecutive poles, south poles will appear in the 
 
 FIG. 283. Primary Ready for 
 Winding. 
 
 284. Primary Completely 
 Wound. 
 
 upper and lower quarters. The second phase (represented by the 
 winding which is marked by dotted lines) produces also a 4-pole 
 field, which here lags behind the first field by an eighth of the whole 
 
 FIG. 285. Secondary Complete. FIG. 286. Type C Motor Complete. 
 
 circumference. Thus, each quarter-period of the current will corre- 
 spond to J revolution of the rotating field. Two periods of the 
 
290 
 
 ELECTRICAL ENGINEERING 
 
 current correspond to one revolution of the field, and 3000 periods 
 to 1500 revolutions of the field, and nearly 1500 revolutions of the 
 armature. A 6-pole motor will run with a speed of nearly 1000, 
 an 8-pole with nearly 750, and a 12-pole with nearly 500 revolutions 
 per minute. 
 
 Fig. 283 shows the field of a 2-phase motor ready for winding. 
 Fig. 284 shows the field completely wound. Fig. 285 is the wound 
 rotor, and Fig. 286 is the complete machine. 
 
 Three-phase Current 
 
 A rotating field can also be produced in other ways than by 
 the method of two windings at right angles to each other, and 
 traversed by currents with a phase-difference 
 of a quarter-period. The most frequent 
 arrangement employed for producing rotat- 
 ing fields is that with three windings, with 
 an angle of 120 between each other (in a 
 2-pole field), when through these windings 
 three alternating currents are passed, each 
 of which has a phase-difference of one- 
 third of a period with reference to the 
 two other currents. 
 
 In Fig. 287 the three coils A, B, and 
 C, and in Fig. 288 the courses of the three FIG. 287. 
 
 respective currents a, b, and c are shown. 
 
 The dotted wave-line 6 is one-third of a period behind the full wave- 
 line a, but is in advance by one-third of a period of the wave c, 
 
 \ / 
 
 \/ 
 
 s* 
 
 ^ 
 
 yl 
 
 ^ 
 
 *' 
 
 \ y 
 \ / 
 
 '' 
 
 ^N. 
 
 \ / 
 \/ 
 
 ^ 
 
 ^ 
 
 \A 
 
 / 
 
 \ 
 
 \ 
 
 
 f 
 
 / 
 
 \ 
 
 
 2 
 
 / 
 
 \ 
 \ 
 
 
 3 
 
 / 
 
 \ 
 
 \ 
 
 
 
 / 
 
 \ 
 
 
 
 \ 
 \ 
 
 / 
 
 
 
 \ 
 
 / 
 
 
 
 \ 
 \ 
 
 / 
 
 
 
 \ 
 
 \ 
 
 \ 
 
 /' 
 
 
 
 \ 
 
 ^'' 
 
 
 
 --'' 
 
 A 
 
 \^ 
 
 ^ 
 
 A 
 
 
 j 
 
 /N 
 
 ^ 
 
 ^ 
 
 FIG. 288. Three-phase Current. 
 
 represented by a line made of up dots and dashes; hence a remains 
 behind c, but runs before b. At a definite moment 1 (see Fig. 288 
 
MULTIPHASE ALTERNATING CURRENT 
 
 291 
 
 the current will be strongest in A (see Fig. 287), thus tending to 
 produce a north pole above (south pole below). In B and C currents- 
 are also flowing, but these are not so strong as that in A. These 
 coils are arranged so that, at the same moment, B tends to produce 
 a north pole to the right above, and C also a north pole to the left 
 above. The action of these three coils is represented in Fig. 289 by 
 three arrows of different length. 
 
 We may compare this with a coach having three horses, the middle 
 the strongest, which pulls straight forward, whereas the weaker horses 
 also pull forward, but at the same time towards the right and left 
 respectively. The pulling of one horse to the right and the other one 
 to the left does not cause a deviation of the coach at all; yet the 
 vehicle will be drawn with greater force than if the middle horse, 
 although it is the strongest, had alone been in harness. Hence we 
 have at this instant the strongest north pole at, the top. 
 
 Next, the action of the coil C increases gradually, since, as we see 
 from the wave-line, the current c grows, whilst at the same time the 
 currents in A and B decrease. After a one-third period, when the 
 
 FIG. 289. 
 
 FIG. 290. 
 
 FIG. 291. 
 
 currents have the values as indicated in Fig. 288 by the vertical line 2 t 
 b has reached its maximum value. Since it flows now in an opposite 
 direction, it produces a north pole in the direction to the left, and 
 downwards (see Fig. 290). Also the current in phase A has changed 
 its direction, whereas the current in phase C has kept its direction. 
 Hence B, which now predominates, produces a strong north pole 
 below on the left, whereas A tends to produce a pole of the same 
 name right at the bottom, and, finally, C makes a north pole above 
 on the left side. It again resembles a vehicle with three horses. 
 The north pole will appear on the left below in the same strength 
 as it was previously above. 
 
 One-third of a period later (see Fig. 291) the north pole will have 
 wandered towards the right, and again, after one-third of a period, 
 upwards. Hence, in a third part of a period the field makes the 
 third part of a revolution, and in a complete period an entire 
 
292 
 
 ELECTRICAL ENGINEERING 
 
 FIG. 292. Four-pole Three-phase Motor. 
 
 revolution, exactly as in the case of the 2-phase rotating field. 
 The effect of a 3-phase field is, therefore, equal to that of a 
 2-phase field. The com- 
 bination of three currents, 
 the phases of which differ 
 by a third of a period, is 
 called a rotary or three- 
 phase current. 
 
 A rotating field may 
 also be produced with six 
 coils, which are arranged 
 so that the angle be- 
 tween any two coils is 
 60, and which are tra- 
 versed by currents dis- 
 placed by a sixth of a 
 period from each other. 
 
 As with 2-phase motors, 
 2-pole windings are very 
 seldom employed with 3- 
 phase motors. With a 4- 
 pole winding the dis- 
 tance between two coils 
 is naturally not equal to 
 the third, but to the sixth 
 part of the circumference. 
 The winding diagram of 
 a 4-pole 3-phase motor is 
 shown in Fig. 292. The 
 consequent-pole winding, 
 previously described, is 
 also employed here. The 
 two opposite coils AA 
 belonging to one phase are 
 connected with each other 
 in such a way as to pro- 
 duce, for instance, in the 
 part enclosed by the coils 
 north poles, and in the 
 parts not enclosed, conse- 
 quent south poles. 
 
 The squirrel-cage may FIG. 293. Four-pole Three-phase Motor, 
 obviously also be used as with Three Slots per Pole and Phase, 
 
 a rotor for 3-phase motors. 
 
 Generally 2- and 3-phase motors do not differ in their mechanical 
 construction, but merely in their winding. Without considering the 
 
 \ \ 
 
MULTIPHASE ALTERNATING CURRENT 293 
 
 winding and slotting of the stator, a 2-phase motor cannot be 
 distinguished from a 3-phase motor. 
 
 With regard to slots there is a difference between 2- and 3-phase 
 motors, inasmuch as with a 2-phase motor at least two, and 
 with a 3-phase motor at least three slots per pole are required. 
 The coils may, if desired, be laid into any larger number of slots; say, 
 for instance, 2, 3, 4, or more, as shown in Fig. 293. 
 
 Actions in Induction Motors Squirrel-cage 
 and Slip-ring Armatures 
 
 The induction motor armature as hitherto described is distin- 
 guished by utmost simplicity. A squirrel-cage rotor is little more 
 than a number of wires short-circuited on themselves. No current 
 has to be led to the rotating part from outside, consequently no slip- 
 rings are required. This is, of course, a great convenience. But 
 this type of armature has one great disadvantage: after it has once 
 been started it is found to work well, but in starting it causes trouble. 
 
 If by closing a 3-pole switch we connect the stationary winding 
 of a 3-phase motor with the mains, then the armature, whilst at rest, 
 corresponds to the secondary winding of a transformer, although 
 the actual construction is very different to that of an ordinary 
 transformer. Again, the field does not pulsate like that of a 
 common transformer, but rotates. This rotating field produces 
 electro-motive forces both in the stationary windings (primary 
 windings of the transformer) and in the rotor winding. The back 
 E.M.F. now produced in the stationary winding is, like that of 
 the primary coil of a transformer, nearly equal to the terminal 
 voltage supplied, so that only the magnetizing current flows in the 
 primary winding when the secondary windings are not shortr 
 circuited. 
 
 It may be remarked that the magnetizing current of a motor must 
 be far larger than that of a transformer. For the lines of force do 
 not here only flow through iron (see Figs. 278 and 279), but have 
 twice to pass air gaps. Now, although the space between rotor and 
 stator is kept as small as possible, a much greater number of 
 magnetizing ampere-turns is required than is the case with a 
 magnetic flux having a path entirely of iron. 
 
 On starting the motor the field rotates with full speed round the 
 still stationary armature. Hence, in the short-circuited armature 
 winding an excessive current will be produced, which reacts on the 
 
294 ELECTRICAL ENGINEERING 
 
 primary field of the stator with the effect of so weakening it that a 
 large current flows to the stationary windings from the mains. This 
 lasts only a short time, for the current flowing in the rotor winding- 
 causes the rotor to start with considerable turning effort, so that 
 it rotates very quickly. The quicker the rotor runs, the nearer it 
 approaches the speed of the rotating field, and the fewer lines of 
 force it will therefore cut. Consequently the E.M.F. and the current 
 induced in the armature decrease, the reaction on the field becomes 
 smaller, and the stator absorbs less current. 
 
 To give a numerical example, a motor which is designed for a 
 current of 30 amps, will, if running at full speed unloaded, absorb a 
 current (" no-load current") of about 10 amps. It must not be 
 thought that the motor really requires one-third of the maximum 
 energy for running without load. The phase-difference is (as with 
 the unloaded transformer) great, and the power factor is equal to 
 about 0.2 or 0.3, so that the watts taken by the unloaded motor 
 are about T V or -fa of the watts taken at full load. 
 
 At the moment of starting, the current will be large, perhaps 
 as much as 90 or 100 amps., which is about three times the normal 
 current. This is a great disadvantage in a motor with a squirrel- 
 cage armature. With very small motors up to about 1 H.P., 
 sometimes even with rather larger motors, such a sudden rush of 
 current might be allowable; on the other hand it is clear that if a 
 20-H.P. motor requires three times the normal current at starting, 
 this would be very objectionable, especially when lamps are also on 
 the motor mains. 
 
 To avoid these sudden rushes of current, we must not short-circuit 
 the windings of the rotor, but connect them with slip-rings, so that 
 resistance may be inserted in the rotor circuit. 
 
 With slip-ring motors the rotor winding is similar to that of the 
 stator. For a 3-phase motor it may be 2-, 3-, or poly-phase. 
 Supposing it to be 3-phase like the stator winding, then we can 
 connect the phases either in star or in mesh, and lead the ends to the 
 three slip-rings. The slip-rings are provided with brushes, and from 
 them cables lead to the three regulating resistances, which may be 
 connected either in star or mesh, and generally are switched in or out 
 by means of a single lever. If with an open rotor circuit we connect 
 the stator winding with the mains by using a 3-pole switch, then in 
 the rotor winding an E.M.F. is, of course, induced; but since the 
 rotor circuit is not closed, no current can be produced. Hence the 
 rotor does not exert a weakening reaction on the stator. Through 
 the latter, therefore, only the magnetizing current will flow that is, 
 a current equal to only | or \ of the normal current. 
 
 Round the armature a magnetic field say, for example, a 
 4-pole one is rotating with a speed of about 1500 revolutions per 
 minute. The effect is the same as if the rotor were the stationary 
 
MULTIPHASE ALTERNATING CURRENT 
 
 295 
 
 armature of a generator about which a 4-pole field rotates. In 
 the armature electro-motive forces, but no currents, are induced 
 until the outer circuit is closed. Immediately we close the switch 
 and connect the external circuit, currents flow through the armature 
 and the resistances, and these currents cause the armature to 
 rotate. The resistances ought, of course, to be so selected that 
 the currents in the armature and in the stator do not exceed the 
 normal currents. 
 
 When the armature is set into rotation, the E.M.F. induced in it 
 becomes smaller than that previously induced in the stationary arma- 
 ture, and if the regulating resistance be diminished the armature will 
 gain speed. We can then gradually diminish, and finally short- 
 circuit the resistance when the armature is nearly synchronous with 
 the field. Hence, when actually at work under a load there is no 
 difference between a "short-circuit" and a " slip-ring" armature, 
 since the windings of the slip-ring armature are short-circuited, 
 finally. Different windings of a 3-phase motor armature suitable 
 for use with slip-rings are shown in Figs. 294 and 297. 
 
 FIG. 294. Wound Rotor of Three-phase Motor (Korting Brothers). 
 
 The input of an induction motor can be measured by watt-meters 
 and the output by a proney brake. Thus, the ratio of input to output 
 is easy to obtain. These values can be obtained with the motor 
 
296 
 
 ELECTRICAL ENGINEERING 
 
 standing still, in which case the output is 0, since there is no motion, 
 but the torque, or tendency to turn, is a specific value which can be 
 measured. Thus, curves of current input, torque (or tendency to 
 turn) , and speed can be plotted all the way from rest up to synchro- 
 nism, as well as efficiency and power factor (i.e. cosine of angle of lag 
 of entering current, as has been explained), and maximum output 
 when running at normal speed. The curve of the torque from rest 
 to synchronism is shown in Fig. 295. Curve shown with 0.04 ohm 
 
 FIG. 295. 
 
 resistance in armature is so chosen that the torque at starting is the 
 maximum torque. The formula for torque of an induction motor is 
 
 Torque m pounds 
 
 when E = the applied E.M.F.; 
 
 p = number of field (an armature) circuits (thus 2 for quarter- 
 
 phase and 3 for three-phase) ; 
 s = slip of secondary (at standstill, s=l; at synchronism, 
 
 8 = 0); 
 
 R= resistance per circuit of armature in ohms; 
 n= cycles per second in primary; 
 HO = resistance per circuit of primary in ohms ; 
 LI = inductance per circuit of armature in ohms ; 
 LQ = inductance per circuit of field in ohms. 
 
 From this formulae the resistance for a given type can be calculated. 
 
MULTIPHASE ALTERNATING CURRENT 297 
 
 Curve with 0.005 ohm gives the torque values without resistance in 
 the armature. It can be seen at starting the torque is now less than 
 with resistance. A little farther the two curves cross and have the 
 same torque. At this point the resistance should be cut out and 
 the torque up to synchronism would then be on the curve higher. 
 Thus, under such conditions the complete torque curve would be as 
 in curve with 0.04 ohm resistance, the resistance being cut out at 
 the point where the two curves cross. From an inspection of the 
 formula for torque it can be seen that the applied voltage appears 
 in the numerator as the square. Thus, the torque of an induction 
 motor is proportional to the square of the applied voltage. Hence, 
 low voltage on an induction motor means more than lower starting 
 (or running) torque. The formula for the horse-power of an induction 
 motor is 
 
 746[(R 1 +SR ) 2 + S' 
 
 In this formula it can be seen that again the applied voltage EO 
 appears as the square, so that the output of an induction motor when 
 running on half-voltage is only one-quarter of the value when running 
 on full voltage. The formula for the maximum horse-power obtain- 
 able from an induction motor is 
 
 1492[(Ri + Ro) + (Hi + R ) 2 
 
 when Xi = 2xriLi 
 and XQ = 27inL Q . 
 
 The formula for maximum torque in pounds at one foot is 
 
 ^ 
 
 Maximum torque = 
 
 34.09[R + V R 2 
 
 From an inspection of this formula it can be seen that RI, the 
 resistance of the secondary, does not appear, from which it may be 
 concluded that the maximum torque of an induction motor is inde- 
 pendent of the secondary resistance, the presence of the latter only 
 determining at what per cent, of synchronism the maximum torque 
 will appear.* Curves of amperes and torque without resistance and 
 with armature short-circuited are shown in Fig. 295 ; as can be seen 
 when the amperes are 0; that is, running at exact synchronism, there 
 
 * See Raymond's "Alternating Current Engineering," pages 128-163, *'>! 
 proof of these formulae without using calculus. 
 
298 
 
 ELECTRICAL ENGINEERING 
 
 x is no torque; when standing still (or per cent, synchronism) the 
 current is a maximum. Thus, an induction motor standing still 
 without resistance in the armature takes its maximum current, 
 usually about ten or fifteen times its normal current. 
 
 The curves showing the efficiency, maximum output, etc., are 
 shown in Fig. 296. 
 
 It will be noticed that the output reaches a maximum in this 
 case at the line of about 195 per cent, output, beyond which no more 
 load can be put on the motor. If an attempt is made to do so, the 
 motor will slow down and, unless the load is relieved, come to rest. 
 Since under such conditions, that is, at rest, the motor takes, as has 
 
 FIG. 296. 
 
 been shown, many times normal full-load current, it will soon burn 
 up unless the current be taken off. Thus, in applying loads to in- 
 duction motors they must be chosen of such values that they are 
 always below the maximum output of the motor. 
 
 The power factor of a motor is the cosine of the angle of lag of 
 entering current, or the ratio of the real input to the apparent input, 
 obtained by multiplying volts and amperes together, not allowing 
 for any lag. 
 
 The commercial efficiency is the ratio of the actual energy given 
 out to the actual energy taken in. 
 
 The apparent efficiency is the ratio of the actual energy output to 
 the apparent input, obtained, as stated, by multiplying volts and am- 
 peres input together, not allowing for their phase difference. Thus, it 
 can be seen that the power factor equals the actual input divided by 
 the apparent input. A properly designed induction motor of about 
 50 H. P. should give from ordinary conditions a maximum output 
 of 100 H.P., a power factor at full load of 95, and efficiency of 92. In 
 spite of this limitation of maximum output, induction motors are 
 
MULTIPHASE ALTERNATING CURRENT 
 
 299 
 
 used very extensively indeed in the United States for all sorts 
 of power purposes, and are built in sizes up to 3000 horse-po\ver= 
 They are used in the same application exactly as our direct- 
 current motors. Thus, you will find them on hoists, cranes, street- 
 cars, pumps, driving shafting in mills, driving tools, the design of 
 output and torque being such as to properly meet the conditions 
 imposed. The advantage of the use of an induction motor over that 
 of a direct current is, in the former the commutator is dispensed 
 with. Thus, there are no brushes to attend to, nor any of the com- 
 mutator troubles which arise with direct-current apparatus. Also 
 since long-distance transmission is always alternating, the induction 
 motor can be used by stepping down from the line voltage to a safe 
 operating voltage, whereas with direct-current motors the A.C. 
 long-distance transmission voltage must be transformed into direct 
 current by rotaries or motor generator sets, with increased cost of 
 first installation and maintenance and attendance during operation. 
 These facts have made the use of induction motors very general. 
 
 m 
 
 FIG. 297. Triphase Rotor (Korting Brothers}. 
 
 By means of a voltmeter connected to two brushes, we may 
 observe that, before closing the starting switch, there is a considerable 
 voltage between two slip-rings, which nearly disappears after short- 
 circuiting the resistance. According to the number of wires on the 
 rotor, the voltage between the slip-rings will be smaller or larger, 
 and will be nearly equal to that of the stator, if the number of turns 
 on stator and rotor are alike. 
 
 The number of rotor windings will of course be selected so as to 
 avoid a dangerous voltage arising between the slip-rings whilst starting 
 
300 ELECTRICAL ENGINEERING 
 
 the motor. Generally the limit for this voltage is 200 to 300 volts. 
 But even this voltage might, under unfavourable circumstances, prove 
 dangerous, and therefore touching the slip-rings whilst starting the 
 motor should be avoided. After the motor has reached its full speed, 
 and the starter has been short-circuited, the slip-rings may without 
 hesitation be touched with both hands, since the voltage existing 
 between the slip-rings of a short-circuited rotor is very small. 
 
 Alternating-current motors only (both induction and synchronous 
 types) have the important advantage, that the feeding current is led 
 to a stationary winding. Hence these motors can, as well as 
 generators, be built for high tension. Large 3-phase motors for 
 2000 and even 5000 volts are frequently made. For these high 
 pressures the terminals and windings of the primary have, of course, 
 to be well protected to prevent danger to life, and the windings 
 must be excellently insulated. 
 
 Slip 
 
 If between a slip-ring and a starter terminal we place an 
 ammeter, we are then able, with a slip-ring induction motor, to make 
 some interesting observations. As long as the motor runs without 
 load, the ammeter indicates a very small current, but a very 
 evident and slow oscillation of the pointer of the instrument will be 
 noticed. If the load on the motor is increased, the deflection of the 
 pointer becomes greater and its oscillations occur more quickly. 
 The number of the swings gives us a direct measure for the 
 slip of the armature. If l^he armature runs synchronously with 
 the field, then as we know no currents at all can be 
 produced in the armature. On the other hand, if the armature 
 of a 4-pole motor (not loaded) remains by two revolutions per 
 second behind the field speed, then this has the same effect as if the 
 4-pole field rotated twice round the stationary armature. Hence, 
 in the armature an alternating current of eight alternations per 
 minute is produced. The ammeter pointer is then deflected eight 
 times from zero up to a maximum value. If the motor is fully 
 loaded, its slip then being 40 revolutions, we can observe 160 
 oscillations of the ammeter pointer per minute. Since in this case 
 the oscillations quickly follow each other, the pointer has no time to 
 return to its zero position. 
 
 This phenomenon becomes still more distinct if, instead of the 
 usual electro-magnetic or hot-wire instrument, we employ a Deprez 
 ammeter, with a zero in the middle of the scale. Then we see the 
 needle deflected from zero for instance, to the right, then to the left 
 and back again to zero, and so on. With a 4-pole motor a double 
 
MULTIPHASE ALTERNATING CURRENT 
 
 301 
 
 movement of this kind of the pointer corresponds to the slip of one 
 revolution. 
 
 The vector diagram of an induction motor is similar to that of 
 a transformer, since the former is really a stationary transformer 
 with a movable secondary, bearings being provided to permit revo- 
 lution. It may be that a vector diagram for an induction motor 
 cannot be drawn as in the case of a transformer, for in the former 
 case the secondary has the same frequency as the primary, while in 
 the induction motor the frequency in the rotor is the same as in the 
 primary when the rotor is standing, but is when running at syn- 
 chronism, and about 4 per cent, of full frequency when running at 
 full load. It must be borne in mind, however, that the revolutions of 
 the rotor plus the frequency in the rotor always equal exactly the pri- 
 mary frequency. From the nature of things, thus the rotation brings 
 around the secondary current, so that when they are at a maximum 
 they bear physically to the primary the same relation as if they had 
 full frequency and the rotor were standing still. In vector diagrams 
 and calculations, it is convenient to reduce the induction of sec- 
 ondary to the same number of turns as the primary by multiplying 
 the value of L and R by the square of the number of turns, reducing 
 similarly back again by dividing after the calculation is over. Fig. 
 
 FIG. 298. 
 
 293 is so drawn and gives the phase relations between the various 
 currents and the flux in an induction motor. 
 
 Let o-c equal in length and phase the flux (. Then the line o-a 
 equals the E.M.F. produced by the flux due to its pulsating in the 
 primary, and o-b the E.M.F. in the secondary when standing still. 
 Draw o-k ahead in phase to o-c an amount such that the product of 
 its projection upon o-a, the E.M.F., gives the losses in the motor due 
 to friction, hysteresis, eddy currents, etc. Thus, the product of 
 E.M.F. and current in phase represents energy. Let o-g represent 
 current in the armature. It lags behind the E.M.F. o-b by the angle 
 b-o-g. o-f in phase with the current represents the E.M.F. used up 
 in resistance. (The loss in resistance is always in phase with the 
 
302 ELECTRICAL ENGINEERING 
 
 current from Ohm's law.) o-e drawn 90 degrees away from the current 
 represents the E.M.F. consumed by induction. (Always 90 degrees 
 away from the current, as has been shown earlier in the book.) Then 
 o-d represents the E.M.F. at this load necessary to force the current 
 o-g through the rotor, since resistance drop and inductance drop 
 combine by the parallelogram of forces, as has been shown. The line 
 o-n equals the secondary current as it appears in the primary, since 
 the currents in the secondary always appear equal and opposite to 
 the primary (assuming, as in this case, a ratio of terms of 1:1). 
 Thus, the total primary current is the combination of the exciting 
 current o-k and this other component o-n, since there are no other 
 currents. Thus, o-p equals the primary current; this current flowing 
 through the primary windings consumes the E.M.F. o-i in phase with 
 itself, and the E.M.F. o-h at right angles with itself, or the combina- 
 tion of both, i.e., o-m. The primary applied E.M.F., therefore, has to 
 be of such a value and phase as to overcome the combination of the 
 primary impedance drop o-m and the E.M.F. produced in the wind- 
 ings by the flux pulsating through them, or o-a. The combination 
 by the parallelogram of force of o-a and o-m equals o-L, which rep- 
 resents, therefore, in amplitude and phase the applied E.M.F. Thus, 
 this diagram gives the value and phases of all the currents and E.M.F.'s 
 existing in an induction motor. 
 
 Single=phase Induction Motors 
 
 After the invention of the three-phase motor its simplicity 
 and its superiority over all other alternating-current motors soon 
 became known. Since then many alternating-current central 
 stations have been designed for the three-phase system, especially 
 when the motor load is important. There are, however, many older 
 central stations which work with single-phase current. For light- 
 ing purposes the single-phase system has an advantage over the 
 three-phase system, for with the latter it is rather difficult to 
 distribute the lamps so as to get equal loads on the three 
 phases. Further, with the 3-phase system three, but with the 
 single-phase system only two, mains are required. Engineers have, 
 therefore, given much attention to the design of single-phase induc- 
 tion motors, having the advantages of 3-phase motors. 
 
 If, whilst a 2-phase motor is running lightly loaded, we disconnect 
 one phase from the motor, we observe that the motor still continues 
 to run, and a considerable alteration takes place in the current 
 consumption, both in the connected phase and the armature, but, 
 
MULTIPHASE ALTERNATING CURRENT 
 
 303 
 
 what is most essential, the motor continues to do work. From this 
 observation we conclude that it is feasible to build single-phase 
 induction motors. 
 
 On stopping the motor, and trying to start it again, with only one 
 phase connected, the armature circuit being closed, a great difference 
 will be observed between this and an ordinary 3-phase motor. A 
 single-phase motor is not self -starting. 
 
 We can, after continued experiments, find out a position of the 
 lever of the starting resistance, so that, when the rotor is once 
 set in motion, it continues to run, runs quicker and quicker, till 
 finally, if we gradually short-circuit the resistance, the full speed is 
 reached. We may further observe that we can give the motor a 
 start either to the right or to the left, and that in both cases it will 
 continue to run in the direction in which we started the rotor, 
 whereas a 3-phase motor runs when the connections are made in 
 a definite way in one direction only. 
 
 With the single-phase motor we have originally not a rotating, 
 but merely a pulsating field. Hence there is no turning effort 
 
 on the stationary arma- 
 ture, but a force which 
 tends to pull the rotor 
 first in one and then in 
 the opposite direction. 
 Similar actions take 
 place in many other 
 machines. A steam-en- 
 engine furnishes a good 
 example : the piston 
 has a reciprocating mo- 
 tion, and piston-rod, 
 crank, and flywheel are 
 required to transform 
 this kind of motor into 
 a rotating one. The 
 bicycle and sewing- 
 machine are similar instances. If the crank (see Fig. 299) is at the 
 top or bottom, then the piston is unable to produce any motion, neither 
 by pulling nor by pushing. It is absolutely necessary that the crank 
 be turned past these dead points. If the crank reaches the position 
 shown in Fig. 300, on pushing the piston a further turning of the crank 
 in the direction indicated by the arrow will result. If, on the other 
 hand, we had turned the flywheel counter-clockwise (see Fig. 301) 
 instead of clockwise, then a thrust on the piston will cause a 
 motion of the crank to the left. Hence, if no other mechanism 
 prevents this, we can really turn as we please the flywheel of a 
 sewing-machine, either to the right or to the left. When 'the machine 
 
 FIG. 299. 
 
 FIG. 300. 
 
 FIG. 301. 
 
304 
 
 ELECTRICAL ENGINEERING 
 
 'Choking 
 Coil 
 
 Is once started, then we are assisted by the momentum of the 
 flywheel, which carries the crank over the dead points, thus giving us 
 the desired rotatory motion. 
 
 In a corresponding way the working of a single-phase motor may 
 be imagined. We have first of all to help the armature over the 
 dead points, in order to produce in the windings of the armature 
 (which, as with the 3-phase motor, might be wound as a squirrel- 
 cage, 2- or 3-phase), by its revolution in a pulsating field, currents 
 of different phases, which latter then produce a rotating field in 
 the armature. The armature wires then act like a flywheel, taking 
 up the reciprocating forces and producing rotating power. 
 
 Small motors may be started by hand, but this would be im- 
 possible with large motors. Such motors may be started by providing 
 them with a second phase 
 winding, obtaining by its 
 help a self-starting single- 
 phase motor. Through 
 this second phase, during 
 starting, a current is caused 
 to flow which differs in 
 phase from the current in 
 the main phase. We may 
 get a second phase from 
 a single-phase circuit by 
 dividing the main current 
 into two parts and insert- 
 ing in one branch, called 
 the ' auxiliary phase," a 
 choking coil. These con- 
 nections are shown diagrammatically in Fig. 302. The main 
 phase is connected to the mains directly, the auxiliary phase is in 
 series with a choking coil. In the latter circuit, due to the large 
 self-induction of the choking coil, a far greater phase-difference 
 between current and voltage is produced than in the main phase. 
 If the phase-difference becomes a quarter-period, then we get a 
 complete rotating field. The phase-difference will, however, here be 
 far smaller than a quarter-period, since in the main phase the current 
 already lags behind the voltage, owing to the main phase not 
 being free from self-induction. In the auxiliary phase the phase- 
 difference is, of course, larger, but in no case as much as a 
 quarter-period. Hence we do not get a true rotating field. The 
 effect may be compared to the swinging pendulum to the bob of 
 which we gave a lateral push before it reached its highest posi- 
 tion. Then the pendulum will not get a circular, but an elliptical 
 motion. 
 
 Hence the single-phase motor with an auxiliary phase is self- 
 
 FIG. 302. Single-phase Motor with Auxiliary 
 Phase with Self-induction. 
 
MULTIPHASE ALTERNATING CURRENT 305 
 
 starting, but the starting power so obtained is far smaller than that of 
 a 3-phase motor. Whilst the latter can start under full load and even 
 double the normal load, an ordinary single-phase motor with auxiliary 
 phase can only start with a part say about one-third to two-third's 
 of its normal load. By suitable means the starting power may be 
 increased, but then the current consumption is greatly increased. 
 
 Single-phase induction motors are therefore generally provided 
 with a loose pulley. Before starting the motor the belt is placed on 
 the loose pulley, so that, on starting, the motor has to overcome the 
 low frictional resistance of the loose pulley only. After the motor 
 has reached its full speed the auxiliary phase is switched out, and the 
 belt is removed from the loose on to the belt-pulley. Sometimes, 
 instead of a loose pulley, a friction coupling is employed. When 
 a coupling of this kind is fixed the motor starts without any load, and 
 after it has reached full speed the coupling is thrown into gear, either 
 by hand or automatically. 
 
 Phase= Difference caused by Capacity 
 
 We may now deal with another kind of phase-difference besides 
 that produced by self-induction. 
 
 Any cable and generally any conductor \vhich is connected with 
 a single pole of a source of alternating current, causes a so-called 
 "charging-current" to flow. Let, in Fig. 303, 
 the two circles I. and II. represent the slip-rings 
 of an alternating-current generator, the brushes of 
 which are connected with wires. The two wires 
 are not connected with each other. As long as 
 the machine is stopped, all conductors which are 
 considered here the armature conductors, the 
 brushes, and the two wires have the same 
 electrical potential. But when the machine is 
 working, in each of the two brushes an alternating 
 potential or pressure appears, sending a current 
 to the end of the wire, which is still at the 
 previous potential. If we had a continuous- 
 current dynamo, then this current would soon 
 FIG. 303. cease -the positive brush sending a current 
 
 through the wire connected with it until the 
 whole main had the potential of this part; similarly, the negative 
 brush would take current from the main connected to it till the 
 lattter came to the same low potential as the negative brush itself. 
 When this state is reached, the charging current will cease. 
 
306 
 
 ELECTRICAL ENGINEERING 
 
 If, on the other hand, we have a perpetually alternating potential 
 on the two brushes the case is different. As long as the voltage on the 
 brush increases, it will send a current to the end of the wire. The wire 
 is now charged, and will, when the voltage of the brush after reaching 
 itfe maximum begins to fall, return to the brush, like an honest 
 debtor, the amount previously borrowed. During the time the 
 voltage of the brush decreases from its maximum through zero to its 
 minimum value, the current comes back from the wire to the brush; 
 whilst during the time in which the voltage of the brush increases, 
 a current flows from the brush to the end of the cable; only at the 
 instant when the voltage has its maximum or minimum value 
 is the current zero. Hence the charging current has, exactly like the 
 magnetizing current, a phase-difference of a quarter-period from the 
 voltage. If the voltage has its maximum value the charging current 
 is zero, and if the charging current has its maximum value the 
 voltage is zero. The charging current is therefore a wattless current, 
 like the magnetizing current. It is a quarter of a period in advance 
 of the voltage, whereas the magnetizing current 
 is a quarter of a period behind the voltage. 
 
 The charging current will be greater the larger 
 is the " capacity" of the main connected with the 
 brush. A large capacity may be produced either 
 if the cables are very long or if they lead to very 
 large surfaces which are placed directly opposite 
 to each other, if for instance (see Fig. 304), the 
 wires I. and II. are connected to very large sheets 
 of tinfoil, which are fixed on opposite sides of a 
 glass plate or a sheet of mica. By this means the 
 capacity of the mains is increased considerably. 
 Such an apparatus is called a condenser. Instead 
 of using a single large sheet of glass or other 
 material, a number of smaller plates connected in 
 parallel may be employed with the same effect. 
 A type of condenser in ordinary use consists of a 
 large number of sheets of tinfoil, insulated from 
 each other by mica or sheets of paraffined paper; FIG. 304. Capao- 
 the alternate sheets of foil, say the 1st, 3rd, 5th, [i Y in Circuit, 
 etc., are connected to one terminal, whilst the 
 2nd, 4th, 6th, etc., go to a second terminal. When a condenser 
 is put across the mains supplied with alternating current, a current 
 flows in and out of it, although the two halves of the condenser are 
 insulated from each other. To get large currents from the mains 
 in this way necessitates the use of a very large number of sheets 
 of foil. 
 
 The effect of capacity may very well be observed with long, and 
 more especially with concentric, mains. Such cables cause a con- 
 
MULTIPHASE ALTERNATING CURRENT 
 
 307 
 
 Condenser 
 
 (Capacity) 
 
 FIG. 305. Single-phase Motor with Auxiliary 
 Phase having Capacity. 
 
 siderable current to flow from the alternator to the mains, even if 
 not a single lamp or apparatus is switched on. This current is, 
 however, as already mentioned wattless. 
 
 This capacity effect may be used instead of a choking coil for 
 producing a phase-difference in the auxiliary phase of a single-phase 
 
 induction motor (see 
 diagram, Fig. 305) . 
 Theoretically with this 
 arrangement it would 
 be quite possible to pro- 
 duce a perfect rotating 
 field; for the main phase 
 has a certain self-induc- 
 tion, and therefore a 
 phase retardation of the 
 current occurs in it, 
 whereas in the auxiliary 
 phase in which a con- 
 denser is inserted, a lead 
 and not a lag of the 
 current takes place. 
 With a condenser of the 
 
 right capacity a phase-difference of a quarter-period with the same 
 current could be effected, and thus a proper rotating field produced. 
 To secure that sufficient current passes to the condenser, it is 
 necessary to have much capacity, which means a very large and 
 expensive arrangement. With the usual construction of condensers 
 for these purposes a combined capacity and resistance effect is used. 
 The condenser plates are placed in a tank filled with a conducting 
 liquid, such as a solution of soda. The phase-difference is in 
 this case far less than a quarter of a period. It therefore follows 
 that motors provided with such condensers cannot start under a 
 heavy load. 
 
 Sometimes both induction and capacity are employed for starting 
 a single-phase motor, a choking coil being inserted in one, a condenser 
 in the other phase. 
 
 A phase-difference of the currents in the two windings may also 
 be produced by inserting in one winding ohmic resistance only. Then 
 in this phase the phase-difference between current and voltage will 
 not be as large as it is in the other phase. 
 
 Even without a resistance, choking coil or condenser, the starting 
 of a single-phase motor may be effected by providing the main and 
 the auxiliary phase with a very different number of windings and 
 switching them directly on the mains. Since the self-induction 
 in the two windings will then be different, the two currents will 
 also have different phase-differences against the outer voltage. 
 
308 ELECTRICAL ENGINEERING 
 
 All the means that have been described in which capacity or self- 
 induction effects help to create a rotating field, are used only for 
 starting single-phase motors. After this has been effected these 
 devices are switched out of the circuit. The motor is then really 
 running on a single phase only, but by the effect of the armature 
 conductors a rotating field is then produced. 
 
 How to build single-phase motors with considerable rotary power, 
 and which can be overloaded without stopping, like 3-phase motors, is 
 a problem that has yet to be solved in a satisfactory way. If it were 
 possible to design single-phase motors with the same good working 
 properties as 3-phase motors, then the 3-phase system would probably 
 be soon discarded. 
 
 The Reversing of Alternating=current Motors 
 
 For reversing single-phase motors without an auxiliary phase no 
 change of the connections is required. It has no inherent tendency 
 to rotate in a definite direction, the direction in which it is turned 
 by hand or by any auxiliary means decides the matter. This applies 
 both to single-phase synchronous motors and to single-phase induction 
 motors without an auxiliary phase. 
 
 The single-phase induction motor with auxiliary phase behaves 
 exactly like a 2-phase motor, and we shall therefore first of all 
 examine the behaviour of the latter. In considering the working of 
 a 2-phase motor we have assumed (see Figs. 278 and 279) that 
 phase A produces at one moment a north pole on the left; phase B, a 
 quarter of a period afterwards, a north pole below. Hence the field 
 rotates from the left downwards, then to the right, then upwards, i.e. 
 in counter-clockwise fashion. We may now reverse the direction of 
 rotation of the field in different ways : firstly by reversing the current 
 in phase A. The effect of this will be that phase A will not, at the 
 particular moment considered above, produce a north pole on the left 
 but on the right. Phase B, however, the connections of which have 
 not been altered, produces now, as before, a quarter-period later, 
 the north pole at the bottom. Hence we have now a rotation 
 from the right downwards, to the left, then upwards, i.e. a clockwise 
 rotation. 
 
 We. may, of course, get the same effect by leaving unaltered the 
 current direction in A, and reversing that in B. Reversing the current 
 both in A and B would, of course, be ineffective. 
 
 The third way to reverse the motor is to interchange the phases, 
 so that then phase B is traversed by that current, which has a lead 
 
MULTIPHASE ALTERNATING CURRENT 
 
 309 
 
 of one-fourth of a period. Then B will at first produce a north pole 
 below, and a quarter-period later A will produce a north pole to the 
 
 left, so that we now get a 
 clockwise rotation, i.e. we have 
 reversed the previous direction 
 of rotation. 
 
 We may hence alter the di- 
 rection of rotation of a 2-phase 
 motor having four terminals, 
 simply by changing the two 
 mains of one phase. With a 
 2-phase motor supplied with 
 three mains (the middle one 
 having about 1 times the area 
 of the outer mains) it is only 
 necessary to change the posi- 
 tions of the outer mains. 
 
 When we wish to reverse 
 the direction of rotation of a 
 
 single-phase motor with an auxiliary phase, either the two ends of the 
 main phase or those of the auxiliary phase must be changed. In Fig. 
 306 the diagram of connections for clockwise rotation is given. One 
 main is connected with end I/ of the main phase and with end II. of 
 
 FIG. 306. Connection of Single-phase 
 Motor for clockwise Rotation. 
 
 FIG. 307. FIG. 308. 
 
 Connections of Single-phase Motor for Counter-clockwise Rotation. 
 
 the auxiliary phase. The connection for counter-clockwise rotation 
 may then be made either according to Fig. 307 or to Fig. 308. In 
 the former case the direct main is connected with I. and II., whereby 
 the ends of the main phase are changed. In the latter case the direct 
 main is connected with I/ and II.', causing the ends of the auxiliary 
 phase to be changed. 
 
310 ELECTRICAL ENGINEERING 
 
 With a 3-phase motor the reversal of rotation may be effected 
 by changing the ends of any two of the three mains, it being a 
 matter of indifference whether the motor has either a star or a mesh 
 connection. Let us consider the motor to be star-connected, and the 
 rotating field to be produced as shown in Figs. 287, 289, 290, and 
 291. Let us further assume that the currents in the phases B and 
 C are interchanged; then we arrive at the following result: Previously, 
 the strongest north pole was produced first on the top, then below to 
 the left, next below to the right, etc., giving a field rotation in a 
 counter-clockwise direction. When now the change is made, the 
 north pole is produced firstly at the top, then below on the right, next 
 below on the left, and so on; hence the field is rotating clockwise. 
 The same result may be effected by changing the phases A and B or 
 C and A. 
 
 These alterations of connections refer of course only to that part 
 of the motor fed by the alternating current. Altering the con- 
 nections in the rotor of an induction motor or in the magnet 
 system of a synchronous motor are without effect on the direction of 
 rotation, because the armature of an induction motor or the magnet 
 system of a synchronous motor are always made to revolve with the 
 rotating field that is produced by the supplied alternating current. 
 
 In the United States slip-rings are used only in special cases. 
 Instead, the resistance is mounted within the armature itself, thus 
 
 FIG. 309. 
 
 revolving with it, and no slip-rings are required, the connection 
 between windings and resistance being direct. The resistance is cut 
 out by sliding contact brushes, which are pushed forward and back 
 on contacts mounted on the resistance (sometimes the contacts rest 
 upon the wire of the resistance itself) , by means of a collar mounted 
 upon the shaft. This collar has in it a groove in which the lugs 
 from the starting lever are located. Thus, the collar which revolves 
 
MULTIPHASE ALTERNATING CURRENT 311 
 
 with the shaft only makes contact with the lugs while they are pushing 
 the collar forward or back. The motion imparted to the collar is 
 thus transmitted to the brushes and the resistance altered while the 
 motor comes up to speed, being finally cut out at full speed. This 
 arrangement avoids all troubles from collector-rings. The latter are 
 used when the resistance needs to stay hi circuit all the time and 
 when, therefore, the resistance must be so large that there is not 
 enough room within the armature to place it. Fig. 309 shows an 
 induction-motor armature with internal resistance revolving with 
 the armature itself, thus avoiding collector-rings. 
 
 Faults with Alternating-current Motors 
 
 If we try to reverse the direction of a 3-phase motor by changing 
 the two ends of one phase as we did with the 2-phase and the single- 
 phase motor with auxiliary phase, the result will be of interest. The 
 motor is then no longer a 3-phase motor, and it either does not run 
 at all, or only with a third of its normal speed, consuming at the 
 same time an excessive current, and soon getting very hot. For in 
 this case phase A will, at the moment of its maximum strength, 
 produce a north pole at the bottom; a third period later phase 
 B, which is still connected as before, will produce a north pole below 
 to the left, and again, after a third period, phase C will produce a 
 north pole below to the right. 
 
 With a slip-ring motor we may, by observing the armature 
 voltage, perceive exactly whether the motor has a correct rota ting- 
 field connection or not. With a properly connected 2- or 3-phase 
 motor a lamp connected with the armature slip-rings will burn 
 regularly as long as the armature circuit is not closed (see p. 
 295). The position of the armature does not make any difference, 
 since the field rotates with a uniform speed about the stationary 
 armature. With a single-phase motor, however, we have no rotating 
 but merely a pulsating field, and thus, according to the position of 
 the respective armature coil in the pulsating field, the lamp will burn 
 either brightly or with little light or will not burn at all. The same 
 will occur with any irregular rotating field; thus, for instance, with 
 a single-phase motor with auxiliary phase, or with the erroneous 
 connection of a 3-phase motor just mentioned. Hence, if we con- 
 nect a lamp or a voltmeter with two slip-rings of the armature, and 
 with a slow rotation of the armature we observe that the voltage 
 between the two slip-sings varies considerably, then we infer that 
 there is something wrong with the 3-phase motor. If one of the 
 
312 ELECTRICAL ENGINEERING 
 
 3 phases is disconnected, so that the motor is on 2 phases, then 
 the same phenomenon may be observed as with the single-phase 
 motor. 
 
 There is another fault sometimes found in the working of 
 induction motors. If one phase of the armature is disconnected 
 for instance, if one of the brushes is not in contact with its slip-ring 
 then the motor may run at half speed. If this happens we can easily 
 make sure whether there is a disconnection in the armature itself, in 
 the brushes or in the starter circuit, by examining, with a lamp or a 
 voltmeter, firstly the voltage between each two of the three slip- 
 rings, then the three brush-holders, etc. If the disconnection is 
 within the armature, then, if the brushes are taken off and the 
 stator windings are switched in, no voltage can be observed between 
 one of the slip-rings and either of the two others; hence a lamp 
 connected with this slip-ring does not bum, and a voltmeter is not 
 deflected. 
 
 In the case of an induction motor without slip-rings, the fact 
 that one of the phases of a three-phase motor is connected in reversal 
 by mistake can be noted by the fact that the motor will not come 
 up to full speed. Then the three currents entering the motor are 
 not alike, as they should be, but one smaller and the other two con- 
 siderably larger, and often at starting a considerable humming will 
 be noticed. Another fault with induction motors is a sudden shut- 
 down and resulting blowing of fuses. Investigation may show all 
 circuits to be O.K. In fact, the motor may have been running satisfac- 
 torily for some time. Often the cause will be found to be due to the rui>- 
 bing of the field on the armature. Air-gaps of induction motors must 
 be as small as possible in order to get good power factors for the 
 magnetizing current, which, as has been shown, lags behind the 
 applied E.M.F. and thus lowers the power factor. The magnetizing 
 current is used principally in an induction motor in forcing the lines 
 of force through the air-gap, the iron parts of the circuit not counting 
 much. Thus, the smaller the gap the better. In motors as large 
 as 500 H.P. the gap is only .050 inch, and in small motors, such as 
 10 H.P., this may be only .015 inch. Thus, in the case of shut- 
 downs and blowing of fuses, the air-gap should be investigated. If 
 rubbing is occurring the H.P. consumed by the rubbing may be 
 such that, added to the regular H.P., the total may be beyond the 
 maximum output of the motor itself, thus causing the shut-down. 
 This touching when it first occurs is not noticeable, since it is slight. 
 As it increases a point may be reached where actual trouble results. 
 This rubbing is also most injurious to the windings ; since the energy 
 represented by it is shown as load, it may destroy the insulation, 
 introducing short-circuits, still further complicating matters. In a 
 plant using induction motors an examination of the air-gaps once a 
 month does not consume much time and guards against trouble. 
 Low voltage on the line is another cause of induction-motor trouble, 
 
MULTIPHASE ALTERNATING CURRENT 
 
 313 
 
 since, as has been shown, the output of an induction motor is pro- 
 portional to the square of the voltage. If the latter is low, a large 
 effect on the output results. Hence, if a motor has swings of load 
 carrying it up for a moment to something near its maximum out- 
 put, it may break down under such load conditions, if the voltage 
 be low. The same holds true in starting a motor; if the voltage is 
 low, a low starting torque results. Another cause of low voltage 
 
 FIG. 310. Forty-pole Armature of Tri-phaser (Korting Brothers}. 
 
 of a motor is unbalanced voltages, while a motor may give proper 
 output with balanced voltages. If these become much unbalanced, 
 the output is much reduced. 
 
 Finally, short-circuits in the fields are a source of considerable 
 bother with poor insulation. Such a short-circuited coil does, not 
 burn out at once, since by Lenz's law the current induced in it by the 
 pulsating flux opposes the flux. About three times normal E.M.F. 
 flews, but does not burn up the coil at once, but creates a local heating, 
 
314 ELECTRICAL ENGINEERING 
 
 which may affect other coils until finally the motor becomes in- 
 operative. In a plant using motors it is well to measure the insu- 
 lation resistance of all motors (and lines) once or twice a month to 
 locate such faults soon after they appear. 
 
 Transmission of Multiphase Currents 
 
 The two currents produced in a 2-phase generator may be 
 separately led through two pairs of wires to a 2-phase motor. In 
 this case four mains are required, each of which has to carry the 
 single current. This is shown schematically in Fig. 311. The zig- 
 
 FIG. 311. Two-phase System with four Mains. 
 
 zag lines represent the phase windings of the generator and motor 
 respectively. In order to indicate the 2-phase system, the zigzag 
 lines are at right angles to each other. 
 
 It is, however, possible to combine two of these mains. We can 
 join the ends I. and II. of the generator, and 1 and 2 of the 
 motor (see Fig. 312). Then each phase has one main for itself, but 
 the middle main is common to both phases, and through this 
 main the currents flowing in the two outer mains, return together. 
 Now one might think that the current flowing in the middle main 
 must be twice as great as that flowing in one of the outer mains. 
 This is not the case, since the two currents are different in phase, and 
 therefore do not arrive at their maxima simultaneously. This may be 
 seen from Fig. 313. On adding the values of the two wave-lines, by 
 first plotting the heights of one wave and above them the heights of 
 the second wave, we get a resultant wave, which is marked in the 
 figure by a thick line. We observe that this resultant wave is, of 
 course, of greater amplitude than either of the single waves, but only 
 1^ times or, as may be found by an exact calculation, 1.41 times as 
 great. Not only is the maximum value of the combined current 
 
MULTIPHASE ALTERNATING CURRENT 
 
 315 
 
 1.41 times as much as the maximum value of the single currents, but 
 this is also true regarding the effective value of the combined current; 
 that is to say, it is 1.41 times as large as the effective value of the 
 
 H 1 2' 
 
 FIG. 312. Two-phase System with three Mains. 
 
 single currents. For this reason the sectional area of the middle wire 
 must be made equal to about 1 times the area of the single wires. 
 
 We have now to consider what will be the voltage between these 
 two interlinked phases that is, between the terminals I' and II". 
 
 FIG. 313. Resultant of two Alternating Currents differing in Phase 
 by one-quarter of a period. 
 
 As will readily be understood, this voltage will not be twice as great 
 as the separate voltages, but also about 1.41 times as much. 
 
 This will be clearer from the following comparison. Suppose 
 a man walks from a (Fig. 314) 100 yards in a straight direction, 
 reaching a point o. At o he makes a quarter of a turn, and 
 then goes 100 yards up to 6. Although now the man has gone 
 
316 
 
 ELECTRICAL ENGINEERING 
 
 2 x 100 = 200 yards, he is not at a distance of 200 yards 
 from the starting-point, but, as can be found by measuring exactly 
 the connecting-line ab, a dis- 
 tance of 141 yards only. 
 
 This geometrical figure can 
 be used in another way. We 
 can measure on the line ab the 
 resulting voltage between the 
 outer terminals (the interlinked 
 voltage of the system) provided 
 that we make the length of the 
 other two sides of the triangle 
 correspond to the phase voltages. 
 If, for instance, for 100 volts 
 phase voltage we make the sides 
 oa and ob equal to 100 inches, 
 then the length of the line ab 
 will be equal to 141 inches 
 which means that the interlinked 
 voltage of this 2-phase system 
 is equal to 141. 
 
 With 3-phase machines we 
 may lead six mains from the 
 generator to the motor, as 
 shown in Fig. 315. The phases 
 of both machines are indicated 
 by zigzag lines at angles of 120. 
 
 
 
 100 
 FIG. 314. 
 
 Now we may, as we have done before, combine the returns. Thus 
 we have to connect the inner ends of the three phases of motor 
 and generator with each other, getting consequently three single 
 leads and one common return (see Fig. 316). 
 
 Next let us consider what will be the voltage between the outer 
 terminals and the current in the common return. The first of these 
 two questions may easily be answered by means of a drawing similar 
 to that shown in Fig. 314. Let us plot three straight lines, distant 
 from each other by 120 (see Fig. 318). The line oa represents 
 
MULTIPHASE ALTERNATING CURRENT 
 
 317 
 
 the voltage of the first phase, ob the voltage of the second, and oc 
 that of the third phase. To get the voltage between the outer 
 terminals of the nrst and second phase, we have only to connect a 
 
 and b by a straight line, and to measure the length of the latter. 
 If the lines oa and ob have a length of 100 inches, then the 
 dotted lines ah, cb, and ca, will have a length of 173 inches each 
 
318 
 
 ELECTRICAL ENGINEERING 
 
 / ^ 
 
 173 
 
 FIG. 318. 
 
 Thus between the outer terminals of a 3-phase generator so con- 
 nected a voltage of 173 will appear when the phase voltage is 
 100. The interlinked voltage is therefore equal to 1.73 times the 
 phase voltage. 
 
 With regard to the current flowing in the common return, 
 we arrive at a curious result. In Fig. 288, the three single currents 
 of a 3-phase system have been represented by three wave-lines, 
 
 a, b, and c respectively. To get 
 n the resultant current, we have, 
 
 as in the case of Fig. 280, to add 
 the three currents. Hence we 
 have to plot on each vertical line, 
 starting from the horizontal, first 
 the height of wave a at this 
 point ; and shall then according 
 as the waves 6 and c are directed 
 at this point, upwards or down- 
 wards plot their heights above 
 or below the zero line respectively. 
 In doing so, we are surprised to 
 find that we always arrive at the 
 horizontal middle line. If, for 
 instance, a reaches its maximum 
 
 upwards, then the waves b and c are directed downwards, each 
 having half the height of the wave directed upwards. It is now 
 obvious that if a person ascends a height of 100 yards, and then 
 descends twice 50 yards, he will come back to the level from which 
 he started; also that at any moment the waves b or c have their 
 upper- or lower-most position, and in any point between these 
 positions the same will occur. Thus through the middle wire no 
 current flows at all. 
 
 This seems very strange indeed, and one might ask what has 
 happened with the three currents which are flowing tin ~>ugh the 
 three outer mains. The answer is very simple. If the current 
 passing out of the phase OI of the generator (see Fig. 316) is just 
 at a maximum say, for instance, 100 amps., and is directed outwards, 
 then the currents in the two other phases have, as we have seen 
 from the wave-line in Fig. 288, an opposite direction and only half 
 the strength or 50 amps. Hence from I. to 1 a current of 100 amps, 
 is flowing, which passes through the phase 1 of the motor, then 
 branches in two parts, so that 50 amps, are flowing through each of 
 the two other phases, passing then through the two mains 2 II. and 
 3 III., coming back again along the generator phases II. and III. 
 0, to the common centre point of the generator. Sinre now4:he current 
 which passes outwards through one main comes back again through 
 the two other mains, the return O is useless; it is a neutral main, 
 
MULTIPHASE ALTERNATING CURRENT 
 
 319 
 
 and can be left out altogether. In Fig. 317 a 3-phase system 
 with three mains is shown, as is usually employed if the generator 
 is used for feeding motors only. Between any two of the three 
 mains the voltage is of the same value. 
 
 The neutral or middle wire is often employed in cases when 
 both motors and lamps are installed on the 3-phase mains (see 
 Fig. 319). The motors are then directly fed by the outer mains, 
 which might, for instance, have a voltage of 173; the lamps are 
 
 FIG. 319. Glow Lamps star-connected. 
 
 connected between one outer and the neutral wire, which have a 
 voltage of 100. 
 
 Very frequently the phase voltage is in such a system 110, the 
 interlinked voltage being then 110 X 1.73 = 190. 
 
 If one or several lamps are connected across a single phase only 
 (see Fig. 320), then obviously through this phase more current 
 
 
 
 FIG. 320. Glow Lamps on one Phase. 
 
 flows than through the two others. The balance is then disturbed, 
 and the neutral wire has to carry some current. If now this one 
 phase were loaded, and the two other phases were not loaded at all, 
 
320 ELECTRICAL ENGINEERING 
 
 then the neutral wire would have to carry the full current of the first 
 phase. This is also the case with currents, the waves of which are 
 irregular, when, even if the mains are equally loaded, current may 
 flow through the neutral wire. 
 
 The arrangement of the three phases of a 3-phase system so 
 far described is called the star method. It is essential with star 
 connections that the beginnings of the 3-phase windings are con- 
 nected together at one point. From this point, the neutral point, 
 the three phases radiate like the rays of a star. 
 
 The phases may also be arranged so that in turn the end of the 
 first is connected with the beginning of the second phase, the end of 
 the second with the beginning of the third, and the end of the third 
 with the beginning of the first phase, as shown in Fig. 321. We 
 get in this way the mesh or delta connections. Considering Fig. 321, 
 
 FIG. 321. Mesh Connection of Machine and Load. 
 
 one would expect that through these closed windings a strong 
 current must flow, even if there is no outer load at all. This is not 
 the case. As previously, with the star connections, the three currents 
 added together destroyed each other, so the three voltages adaed 
 together give no voltage. The matter is somewhat more complicated, 
 but similar to the phenomenon, which we have studied in the case of 
 the Gramme armature (see p. 76), where we have a closed circuit 
 in which electromotive forces are acting. These E.M.F.'s are equal, 
 but opposed to each other, so that their resultant is zero. 
 
 Any side of the triangle (Fig. 321), and therefore any phase, has 
 its voltage, and we may take current from the machine by connecting 
 between any two mains a conductor, for instance, a lamp. If the 
 lamps are equally distributed between the mains I. and II., II. and 
 III., III. and I., then through all the mains equal currents flow. 
 The current passing through any one of the mains will then of course 
 be equal to the sum of the currents flowing in the two phases, which 
 
MULTIPHASE ALTERNATING CURRENT 321 
 
 are connected with this main. Let us now suppose that the phase 
 currents are 100 amps. each. It might at first be thought that the 
 main will receive 200 amps. But it must be remembered that 
 between the currents in the single phases there is a considerable 
 lag, causing their resultant to be only 173 amps. 
 
 Hence with the mesh connection the voltage between two outer 
 mains is equal to the phase voltage, but the current in the outer 
 mains is 1.73 times as great as the phase current. 
 
 With the star connection the voltage between the outer mains 
 (the resultant voltage) is 1.73 times the phase voltage, but the 
 current in the outer mains is equal to the phase current. 
 
 Let us compare the amount of copper required to transmit a 
 certain amount of energy over a three-phase circuit with the amount 
 of copper for the same percentage loss using a single-phase circuit, 
 the condition of comparison being that the voltage between lines be 
 the same in each case. In the single-phase circuit let the percentage 
 line drop equal S, and the power transmitted equal P. Let L equal 
 length of line both ways, E equal volts between lines, and I the current 
 
 QTT 
 
 flowing into the line. Then the resistance of the line equals ^y-, and 
 
 QT? 
 
 the resistance per foot equals -yy. 
 
 LI 
 
 In the three-phase circuit the energy in watts per circuit equals 
 p 
 . We will imagine for a moment that there are three separate 
 
 o 
 
 -p 
 
 circuits, each having a voltage equal to -=, that is, the voltage between 
 
 any outside wire and the neutral. The current per circuit (thus 
 
 p "p 1 p 
 divided into three separate circuits) equals Ii equals + - = 
 
 Thus, Ii equals /=, when I is the single-phase current for power P. 
 
 For the same percentage drop, S, we have a voltage drop of 
 
 SE . SE I SE 
 
 -= or a resistance drop of ^=+== , and the resistance per foot 
 V3 \/3 V3 I 
 
 f SE L 2SE L . , . 
 
 of -- = . is used instead of L, since, as has been shown, 
 
 1 Z Ll L 
 
 on a three-phase circuit the return wires become unnecessary and 
 can be left out of the calculation. With the single-phase circuit the 
 
 QTT 
 resistance per foot was shown to be yp Thus, with the three-phase 
 
 circuit one of the wires has twice the resistance of one of the wires 
 in a single-phase circuit. Hence, since the resistance of w wire is 
 
322 ELECTRICAL ENGINEERING 
 
 usually proportional to its area or weight, the weight of the wire in 
 the three-phase circuit is one-half that of the single-phase circuit. 
 But there are three wires in the three-phase and only two in the 
 single-phase. Thus, if the weight of a single wire equals W in the 
 single phase, the total weight single-phase equals 2W, since there 
 
 3W 
 are two wires, and the total weight three-phase equals . Hence, 
 
 3W 
 
 the ratio of weight three-phase to weight single-phase equals -T- 2W 
 
 = |. Thus, for a given difference of potential between wires, it 
 takes only three-fourths the weight of copper at a given percentage- 
 line loss to carry the energy three-phase as compared with carrying 
 it single-phase. On this account it is customary in long-distance 
 transmission to carry the energy on three-phase transmission lines. 
 They have a further advantage that the self-induction with un- 
 balanced loads, while unbalancing the various voltages between the 
 various lines somewhat, does not do so to the extent of a quarter- 
 phase transmission with a common return wire. For, in the latter 
 case, as can be shown by plotting the vector diagram and remembering 
 that the induction is at right angles to the current, it is seen that 
 the inductance of the common wire boosts the voltage of one phase 
 and lowers the other. 
 
 Power in a Three-phase System 
 
 We are now in a position to determine the output of a 3-phase 
 generator, if the voltage and current be given. Assuming the phase 
 voltage to be 100, the phase current 10 amps., and assuming further 
 that the load consists of glow lamps, and is therefore inductionless , the 
 calculation becomes very simple. Each phase supplies 100 volts X 
 10 amps. = 1000 watts. Hence the generator supplies 3X1000 = 
 3000 watts. 
 
 The calculation becomes apparently more complicated if there be 
 given not the phase voltage and the phase current, but either (with 
 star connection) the resultant voltage and the phase current, or (with 
 mesh connection) the phase voltage and the resultant current. In 
 this case the calculation is not really difficult. Let the resultant 
 voltage be 173 and the phase current 10 amps., then to get the phase 
 voltage we have to divide the resultant voltage by 1.73. Next we 
 have to multiply the phase voltage by the phase current, thence 
 getting the output of one phase. This we have again to multiply 
 by 3 in order to find the output of the generator. 
 
 Thus |4|X10X3=100X10X3 = 3000 watts. 
 
MULTIPHASE ALTERNATING CURRENT 323 
 
 [It may be remarked here, that the phrases "resultant voltage" and "phase 
 current" are very seldom used. In speaking of the voltage of a star-connected 
 generator the resultant voltage, and in speaking of the current the phase current, 
 is generally understood.] 
 
 We might just as well have proceeded in another way, and firstly 
 have multiplied the current by the voltage, next by 3, and then 
 have divided the whole by 1.73. On dividing 3 by 1.73 we find 
 that we get 1.73. Hence, instead of first using the multiplier 3, 
 and afterwards the divisor 1.73, we can directly employ the factor 
 1.73. Our rule is then simply to obtain the product of volts, 
 amperes, and 1.73, getting thus the output of the generator in watts 
 when the load is free from self-induction. 
 
 EXAMPLE. 173 voltsXlO amps. X 1.73=3000 watts. (Exactly 2992.9, but 
 3000 is quite accurate enough.) 
 
 The same calculation applies to a mesh-connected generator. 
 Let the voltage be again 100 and the resultant current 1.73 amps. 
 Then we get the phase current by dividing the resultant current by 
 1.73. Next we have to multiply the phase current by the phase 
 voltage and again this product by 3. Instead of dividing by 1.73 and 
 Multiplying by 3, it is easier to use the factor 1.73 as before, getting 
 then the same result as above. Hence we may say: With an in- 
 ductionless load a 3-phase generator has an output in watts given by 
 the formula 
 
 1.73XEXC 
 
 where E is the voltage and C the current of the system. The formula 
 only applies if the three phases are equally loaded, otherwise it is 
 necessary to determine the output of each phase separately. 
 
 If the load of the generator is not free from self-induction if, 
 for example, the generator has to feed asynchronous motors then 
 we get by the above formula not the real, but the apparent watts, and 
 we have still to multiply the result by the power factor cos (f>, and 
 the formula becomes 
 
 Watts = 1 . 73 X voltage X current X power factor 
 = 1.73 EC cos </>. 
 
 A 3-phase motor which is supplied at 190 volts, taking a current 
 of 12 amps., and having a power factor of 0.8, consumes apparently 
 
 1.73 X 190 X 12 = 3944 (volt-amps.) 
 and really 
 
 3944 X 0.8 = 3155 watts. 
 
324 
 
 ELECTRICAL ENGINEERING 
 
 Synchronizer for Multiphase Machines 
 
 The connections of synchronizing lamps for 3-phase current are 
 similar to those for single-phase current. For low voltages three 
 lamps may be connected 
 
 between the terminals A 'A' 
 
 AA', BB', and CO' (see 
 Fig. 322). If all three 
 lamps become simulta- 
 neously dark or bright, 
 then the connections are 
 all right, and at an instant 
 of darkness the switch 
 may be closed. It might, 
 however, happen that, on 
 starting the machines, 
 or after any alteration on 
 the machine or switch- 
 board, the lamps do not 
 
 FIG. 322. Arrangement of Synchronizing 
 Lamps for Three-phase Circuit. 
 
 become bright or dark simultaneously, but one after the other. This 
 is a sign that the succession of the cables on the terminals of the 
 one machine does not correspond with the succession of cables on 
 the terminals of the other machine. In this case, any two of the 
 cables may be changed; for instance, that cable which was pre- 
 viously connected with the switch terminal A' might now be con- 
 nected with B', and that of B' with A'. 
 
 In still another way we can assure ourselves before starting that 
 the cables of the two machines are correspondingly connected with 
 the switch, viz., by switching a 3-phase motor first on the terminals 
 A, B, and C, and then in exactly the same way on A'B'C' (that ter- 
 minal of the motor which was on A before now to be connected with 
 A', B before now with B'). If the direction of rotation is the same 
 in the second case as it was in the first one, then the connections are 
 all right; if the direction of rotation is opposite, then two of these 
 cables must be changed as before. 
 
 About the proper connection of the two machines we have to 
 make sure once for all before switching them in parallel the first 
 time. In subsequent work it is quite sufficient that one phase of one 
 machine is synchronous with the corresponding phase of the second 
 machine, for in this case it is certain that also the two other phases of 
 the first machine are in synchronism with those of the second one. 
 Hence, for the normal working of the machine, three synchronizing 
 
MULTIPHASE ALTERNATING CURRENT 
 
 325 
 
 lamps are not required as is shown in Fig. 3'J2, but two or one respec- 
 tively, as with a single-phase system. 
 
 For high tension likewise, only a single-phase transformer is re- 
 
 *W\A< 
 
 FIG. 323 Arrangement of Synchronizing Lamps for High-tension Three-phase 
 
 Circuits. 
 
 quired, which then is switched between two mains. This arrange- 
 ment, shown in Fig. 323. corresponds to that of Fig. 266. 
 
 The connections of a 2-phase synchronizer are arranged in exactly 
 the same way. 
 
CHAPTER XII 
 HIGH TENSION 
 
 WITH alternating-current work, very frequently high-tension current 
 has to be considered, and we shall therefore now deal with the safety 
 appliances and arrangements which have to be provided both for 
 protecting human life and machines and apparatus against the 
 dangerous effects of high-voltage currents. 
 
 The windings of first-class machines are always insulated with 
 the very best insulating materials. Notwithstanding this, windings 
 may be spoiled if metal or carbon dust or damp is allowed to remain 
 on them. Hence the first condition is to keep the machines 
 always clean. To keep them dry is, however, not always possible. 
 Especially during erection or long disuse, dampness of the 
 windings in rooms that are not very dry can hardly be prevented. 
 Before a machine is started, it may therefore be necessary to 
 dry it thoroughly. This refers both to continuous-current and 
 alternating-current machines. 
 
 With alternating current generators drying is very easily effected. 
 For this purpose the machine has to be short-circuited, i.e. all the 
 terminals are directly and without any outer circuit connected ^\ith 
 each other, and an ammeter is inserted in either all or only in. cne 
 phase. Afterwards the machine is started, and the magnet field 
 feebly excited, so that the current produced by this weak field is 
 equal or somewhat larger than the normal current. The windings 
 get warmed, and if the machine be run for several hours, perfect 
 drying of the windings can be effected. 
 
 The voltage obtained with a short-circuited alternating-current 
 generator is negligibly small whenever the ends of each phase are 
 connected directly with each other. This is, for instance, the case 
 with a short-circuited single-phase generator, a 2-phase, a mesh-con- 
 nected 3-phase generator, and also with a star-connected generator, 
 the star-point of which is short-circuited together with the three 
 outer terminals. In all these cases there is practically no voltage 
 in the short-circuited machines. With a star-connected 3-phase 
 machine, on the other hand, in which the outer terminals only are 
 
 326 
 
HIGH TENSION 327 
 
 connected with each other, there might under certain circumstances 
 be considerable voltages. With such high-tension machines, even 
 when short-circuited, one must avoid touching the windings. 
 
 Synchronous motors may be dried in the same way as generators, 
 by short-circuiting the alternating-current terminals, driving the 
 machine as a generator, and exciting the field. 
 
 Induction motors may be heated by reducing the generator 
 voltage to a small value, short-circuiting the starting resistances 
 of the motor, and putting a brake on the armature. Then, despite 
 the low voltage, a considerable current will flow both through the 
 stator and the braked rotor. 
 
 In a corresponding manner static transformers may be treated 
 by short-circuiting the secondary winding and connecting the primary 
 (high-tension) winding with a voltage far lower than the normal 
 pressure (about 3 to 5 per cent, of the latter). The current flowing 
 through the coils is then about as much as, or a little greater than, 
 that for which they have been designed. 
 
 Machines and transformers may easily be manufactured so that 
 with proper management they remain in a proper state for a very 
 long time. But with ammeters and voltmeters and other instru- 
 ments it is very difficult to provide an insulation which can stand 
 voltages of several thousand volts with certainty. When ammeters 
 are employed in high-tensicn plants, they must always be mounted 
 on a very good insulating base. Voltmeters, measuring the voltage 
 directly between two high-tension terminals, are seldom employed. 
 Generally measuring transformers are used, the ratio of the number 
 of high- and low-tensicn windings being definitely fixed as required. 
 If, for instance, the primary winding has 100 times as many turns 
 as the seccndary, then at a primary voltage of 5000 on the terminals 
 of the low-voltage coil a voltage of 50 will appear. Generally the 
 scale of the reading instrument is not marked with the secondary, 
 but with the primary voltage, so that, for instance, that point to 
 which the pointer of the instrument is deflected with 50 volts is 
 marked 5000. 
 
 There are also measuring transformers for ammeters, the "current 
 transformers." Hence high-tension switchboards may be manufac- 
 tured without any high-tensicn apparatus at the front. Voltmeters, 
 ammeters, and wattmeters are inserted in low-tension circuits. The 
 measuring transformers, the high-tension fuses, and the high-tension 
 switches are placed behind the board, and nothing but the long 
 insulated handles of the switches project at the front of the board. 
 
 In connection with high-tension switches there are generally 
 employed special devices, so that on opening the switch there 
 are long air gaps between the contacts, hence the arc that is 
 produced on breaking a high- voltage current is destroyed (see 
 Fig. 324). 
 
328 
 
 ELECTRICAL ENGINEERING 
 
 For the same reason high-tension fuses are of special construction, 
 an d frequently of considerable length . Gen- 
 erally the fuse wires are enveloped in insu- 
 lated safety tubes, by which splashing of the 
 melted fuse wire is prevented (see Fig. 325). 
 
 An essential feature with which we 
 have still to deal, is the safety of persons 
 in charge of high-tension plants. It is a 
 matter of course that in no case two 
 terminals of different voltages must be 
 touched, but to touch even a single high- 
 tension terminal must also strictly be 
 avoided. This might have fatal conse- 
 quences for a man standing on an uninsu- 
 lated place if there existed anywhere an 
 earth connection with the second pole. 
 With alternating-current machines a fatal 
 shock may result, although the whole net- 
 work may be very well insulated ; for, as we 
 have learned from the effect of capacity, 
 there is even in a wire connected to a 
 sinrle pole a current continuously flowing 
 in and out. Hence, if a person in contact 
 with earth touches one pole, he will receive 
 a current flowing to earth, and even this 
 may prove fatal. 
 
 FIG. 324. High-tension 
 Switch. (The Brush Com- 
 pany). 
 
 FIG. 325. High-tension Fuse (The Brush Company). 
 
HIGH TENSION 329 
 
 Therefore, if it is necessary to touch working high-tension 
 machines, apparatus, or "live" mains, insulation of the operator is 
 necessary a position on a good uninjured india-rubber plate or dry 
 wood, the protection of the hands with good rubber gloves, and the 
 wearing of rubber shoes are necessary precautions. With high-tension 
 plants other dangers may arise in addition to those due to touching 
 the mains. Assuming that a high-tension generator or transformer is 
 fixed on an insulated foundation, and that one pole of the machine 
 has a connection with the frame. If the insulation of the windings 
 is uninjured, then obviously there can be no short circuit, and hence 
 no interruption of work will happen. But the frame of the machine 
 is now in connection with one pole, so that touching the frame is, 
 for a man standing on the earth, just as dangerous as touching one 
 high-tension terminal. If somewhere in the network there is another 
 pole earthed, then the attendant stands in a manner with his feet 
 on one pole of the high-tension service, touching with the hands the 
 second pole. It is, therefore, generally specified that such insulated 
 machines and transformers must be provided with an insulating 
 platform. 
 
 There is still another means of protection against such accidents 
 connect all machines and transformer frames with earth. This might 
 be done, for instance, by fixing a copper wire to a machine bolt, and 
 leading it to an earth plate or to the water pipe. If there is a good 
 connection between earth and the machine frame, then a considerable 
 potential difference cannot appear between them, and the frame 
 may be touched without any danger. If, now, one pole of the 
 machine at any time touches the iron frame, and the other pole has 
 anywhere in the network an earth connection, then, of course, an 
 interruption of the work will follow, due to the short circuit, but 
 without endangering human life. It is, notwithstanding, possible, 
 with a bad earth connection, for a considerable voltage to exist 
 between the iron frame and the earth, and therefore the very best con- 
 nection between earth and the iron part of the machine is the main 
 requirement with high-tension plants. Where a good earth con- 
 nection cannot be made, the machine should be insulated from 
 the earth, but in this case an insulated platform round the machine 
 is required. 
 
 The same refers also to switchboard apparatus either an 
 insulated platform in front of the switchboard in cases in which 
 the touching of all parts carrying current cannot be avoided, or earth 
 connection with all places which have to be touched must be made. 
 In the latter case all levers of the high-tension switches, the iron 
 frame of the board, the cores of the measuring transformers, etc., must 
 be connected with earth. If there are high-tension ammeters on the 
 switchboard, they are either enclosed in an insulating box, or covered 
 
330 ELECTRICAL ENGINEERING 
 
 with a metal box, which latter is then connected with earth. The 
 metal box has, of course, in the front, a glass window. 
 
 The limit at which the voltage becomes dangerous cannot be 
 stated exactly. Generally voltages up to 500 volts are not fatal, 
 whilst under especially unfavourable circumstances shocks of even 
 200 volts may have fatal results. 
 
 Lightning Arresters 
 
 With any overhead electric mains, whether they carry either high 
 or low tension, continuous or alternating currents, it is necessary to 
 arrange safety appliances to guard against lightning discharges. 
 Lightning consists of an electric arc which strikes either across two 
 clouds or a cloud and any object on the earth. Its potential is always 
 extremely high, and sometimes amounts to many millions of volts. It 
 is therefore able to break down any insulation, and to cross air gaps 
 in a way that would be impossible in the case of relatively low 
 voltages. Very frequently discharges of atmospheric electricity occur 
 which are invisible to the eyes (so-called dark discharges), but which 
 may damage electrical machines and apparatus. 
 
 From many observations it has been found that lightning does not 
 consist of a single discharge, but that, despite the short time of dis- 
 charge, a frequent alteration of the current direction takes place. 
 Lightning is hence really an alternating current with an extremely 
 high periodicity, many hundred times greater than that of the usual 
 alternating currents. Even if lightning lasts but a fraction of a 
 second, yet it changes its current direction in this short time many 
 thousand times. 
 
 Now, the effect of self-induction is far greater with a high 
 than with a low periodicity. With alternating currents of usual 
 periodicity we have observed strong induction effects on coils with 
 iron cores, whilst with currents of such a high periodicity as with 
 lightning, even on coils consisting of very few windings and having 
 no iron cores, we can observe strong self-induction effects. Since 
 a circuit having a large self-induction produces a back E.M.F. which 
 opposes the increase of the current-strength, this is equivalent to 
 adding a very large resistance to the circuit. Hence if we provide 
 for the lightning discharge two paths, one of them leading through a 
 coil, and the other one through an air gap to the earth, then by far 
 the greatest part of the discharge will flow through the air gap to the 
 earth, whereas, on account of the high inductive resistance, but a 
 small part will flow through the coil. This fact has been used to 
 prevent lightning from flowing through sensitive apparatus, and for 
 conducting it in a harmless way to the earth. 
 
HIGH TENSION 
 
 331 
 
 FIG. 326 Lightning 
 Arrester. 
 
 The simplest type of lightning arrester is shown in Fig. 326. It 
 consists of two horn-shaped thick copper wires 
 placed opposite each other and fixed on porce- 
 lain insulators. One horn is connected with 
 the supply line, from the other one ? cable is 
 led to an earth plate. The two horns do not 
 touch each other. The least distance between 
 them varies, according to their position 
 (whether fixed in rooms or outside), between 
 -fa to \ inch. The usual dynamo voltage can- 
 not bridge over this distance. 
 
 Before the aerial line is connected to any 
 machine, or to any transformer, a coil con- 
 sisting of several windings is interposed. This 
 latter does not offer any obstacle to continuous 
 currents, or for alternating currents of the 
 usual periodicity. If a discharge of atmospheric electricity into the 
 line takes place, then this discharge current will not flow through 
 the induction coil, which offers to it a very high resistance, but will 
 
 flash over the air-gap to the 
 second horn, where it finds a 
 direct way to the earth. 
 
 Since there is now formed 
 an arc between the two horns, 
 the dynamo current could also 
 take this way. But this bridg- 
 ing of the horns by means of 
 the arc lasts only a very short 
 time. By the action of the 
 stream of heated air the arc 
 rises upwards, becoming longer 
 (in this the special shape of the 
 horns assists), till finally, like 
 the arc of an arc lamp, the 
 carbons of which are separated 
 too far from each other, it is 
 extinguished. All this happens 
 FIG. 327. Westinghouse Lightning during a brief period, and after- 
 wards the plant remains with- 
 out injury. 
 
 A type of arrester for alternating current in use at the present 
 time by the General Electric Company consists of multi-air gaps 
 with shunt and series resistance. The air-gaps are carefully spaced 
 between cylinders of non-arcing metal, and part of the gaps shunted 
 by resistance. The whole is then connected in series with resistance. 
 This type of arrester for 2200 volts is illustrated in Fig. 328, which 
 
332 
 
 ELECTRICAL ENGINEERING 
 
 shows both the multiple and series connection. In Fig. 329 is shown 
 a 35,000-volt three-phase arrester with double-blade disconnecting 
 switches for Y-connected neutral grounded circuits. 
 
 FIG. 328. 
 
 The non-arcing character of the alloy used in the cylinders reduces 
 the number of gaps necessary, and aids the resistance in reducing the 
 
 FIG. 329 
 
HIGH TENSION 
 
 333 
 
 destructive effects and in opening the resulting arc. Not only the 
 high-potential, high-frequency discharges of lightning, but also the 
 smaller charges within the circuit, are discharged across the gap. 
 These surges within the system are caused by opening or closing of 
 feeder switches, switching in transformers, and sudden variations of 
 load. Records of discharge are made by inserting a small square of 
 paper between two adjacent cylinders in each line, the discharge 
 puncturing the paper. These are renewed regularly.. 
 
 In theory, the cylinders are charged electrostatically until the 
 voltage is high enough to break down the air-gaps in succession, 
 passing the charge along from cylinder to cylinder, thus discharging 
 th^ whole system 
 
 FIG. 330. Lightning Arrester used on Railway Apparatus. 
 
 An arrester used on railway apparatus is shown in Fig. 330. The 
 arrester has an adjustable spark-gap between two electrodes in the 
 field of an electro-magnet. One electrode is connected, through the 
 magnet windings and a small non-inductive resistance, to the ground. 
 The other electrode is connected to the positive side of the circuit. 
 Under normal conditions no current passes through the arrester coil, 
 but any arc established by a lightning discharge which jumps the 
 gap and is followed by current from the generator is blown out by 
 the magnetism induced by the coil of the blow-out magnet. 
 
 These arresters are used in connection with kicking or choke coils. 
 
334 
 
 ELECTRICAL ENGINEERING 
 
 When used on a feeder panel, the panel is equipped with a kicking 
 coil made of bare copper rod coiled and connected between the main 
 switch and the circuit-breaker. 
 
 It is desirable to isolate the arresters from the switchboard. 
 
 In addition to those just described, there are many other types 
 of lightning arresters. In one type many metal and mica plates are 
 arranged alternately one upon the other. The first metal plate is 
 connected with the line, the last one with the earth. This row is 
 practically an insulator for a low voltage, but a lightning discharge 
 glides over the outer surface of the mica and metal plates. 
 
 Every aerial line must be protected by a lightning arrester before 
 it is passed within any building. 
 
 Switchboards 
 
 The object of a switchboard is to concentrate (or concentrate 
 the means of controlling) all the energy developed or distributed in 
 a station for the purposes of control, distribution, measurement, and 
 protection. It is best located so as to give the operator full view 
 
 FIG. 331. A. C. Three-phase Generator and Feeder Panels. 
 
HIGH TENSION 335 
 
 of the machines and so as to keep the cable leads between the board 
 and machines as short as possible. Plenty of room back of the board 
 should also be provided in order that the attendant may safely in- 
 spect, repair, or adjust connections. 
 
 It is usually built up in sections or panels of a strong insulating 
 fire-proof material, upon which are mounted the various instruments 
 and devices. Marble and slate fulfil these requirements and are 
 most generally used. Slate, however, is not used for circuits above 
 1000 volts unless the high-potential carrying parts of the circuits are 
 insulated from the panel. This is due to the fact that slate is strat- 
 ified and is liable to have veins of lower insulating qualities. Slate 
 pa icls are finished with oil or black enamel. Natural black slate oiled 
 is very substantial, is easily retouched by the attendant in case it 
 becomes marred, and harmonizes with the finish of the devices mounted 
 on it. Black enameled slate gives an excellent polished surface, but 
 is difficult to retouch if scratched through the coating of enamel. 
 
 Marble is stronger and a much better insulator than slate. Any 
 kind of marble with a polished surface will show oil stains, which 
 renders it difficult to keep it looking neat. In order to overcome 
 this trouble, marble boards may be black enameled or given a dull 
 black finish. The latter finish is perhaps the more durable and is 
 also more easily repaired. 
 
 The panels are supported by bolting them to vertical pieces of 
 l|-inch gas-pipe by means of malleable iron clamps. The pipes are 
 held upright by mounting them in a cast-iron flange at the bottom 
 and by bolting to a horizontal brace at the top. This pipe- work 
 permits of many adjustments and is often used in supporting the 
 devices and connections. 
 
 In mounting the instruments and devices on a switchboard, 
 nothing should be used which has no other use than ornamentation. 
 Similar instruments and devices on the different panels are located 
 at the same heights, which tends to give the board a symmetrical, 
 uniform, and pleasing appearance. Circuit-breakers and fuses are 
 located near the top of the panel in order that any arc may rise with- 
 out injury to the adjacent devices or to the attendant. Just beneath 
 the breaker are located the instruments which must be of a type 
 not easily affected by a stray field. About the middle of the panel 
 are placed the rheostat hand-wheel, field switches, etc., with oil 
 switches, large recording wattmeters, and relays at the bottom, 
 depending upon the nature of the panel. In stations of large capacity 
 it is convenient to use electrically operated switches. In this case 
 a controlling board is used in the shape of an inclined table, with the 
 meters and instruments located on vertical panels back of the con- 
 trolling board. By this means an operator can stand in front of 
 the controlling board, with all of the controlling switches within 
 easy reach and with the various instruments in full view. 
 
336 
 
 ELECTRICAL ENGINEERING 
 
 The illumination of the board is best when it can be provided 
 for from that of the station. When necessary, however, lamps are 
 mounted at the top of the panel, but this is open to the objection 
 that it does not give uniform illumination and reflects in the attend- 
 ant's eyes. 
 
 The switchboard is a check upon the efficiency and economy of 
 the whole station. The various machines were designed to operate 
 under certain loads, and the board must be laid out with sufficient 
 indicating and recording instruments to determine if the machines 
 are working under proper load, and to obtain a record of the total 
 
 FIG. 332. 
 
 output. Sufficient protective devices must be provided in order to 
 protect the system and its various parts. Where these devices are 
 automatic, they should be reliable and kept in good order. Other- 
 wise they are liable to become a source of trouble, resulting in shut- 
 downs. 
 
 Switchboards are usually arranged so that similar panels are 
 together. This avoids crossing; of leads, with the liability to short- 
 circuit, fire risk, and shut-downs. Usually the generator panels are 
 at one end of the board and the feeder panels at the other. Fig. 331 
 shows a typical board of two alternating-current three-phase gen- 
 
FIG. 333. 
 
 337 
 
338 ELECTRICAL ENGINEERING 
 
 era tor panels and two feeder panels. Switchboard design and con- 
 struction has become so standardized that complete boards are made 
 up from standard panels to meet practically all conditions of gen- 
 eration and distribution of energy. Boards such as shown in the 
 above figure may be used on potentials up to and including 13,200 
 volts and may be of either slate or marble, since no parts carrying 
 high potential are mounted on the panels. 
 
 In laying out a station it is necessary to lay out both the board 
 and the cable runs carefully, in order to make proper provision for 
 the protection of the cables and the location of the board with 
 respect to the machines. Direct-current cables between the board 
 and the machines are usually supported on insulator racks under the 
 main floor. High-tension conductors from the alternator should 
 preferably be lead-covered, varnished cambric or paper cable laid in 
 ducts. 
 
 Isolated boards are made up for use in small plants where but 
 one or two machines are controlled. In this case the feeder switches, 
 instruments, circuit-breakers, or fuses are mounted on the same 
 panel as shown in Fig. 332. This shows an A. C. isolated board for 
 two machines and feeder circuits. Such boards are also largely 
 used for isolated motor control. 
 
 At the present time most sub-stations are used for railway work 
 and hence use railway converters in connection with high-tension 
 transforming apparatus. Such stations differ only in the number 
 and size of the units and use a board such as shown in Fig. 333. 
 Here the breaker is shown at the top of the panel, with the ammeter 
 just beneath. In the middle is located the handle for field rheostat, 
 feeder and field switches just beneath, and recording wattmeter at 
 the bottom. This does not differ materially from the standard 
 railway generator panel, while the feeder panels usually omit the 
 rheostat, field switch, and recording wattmeter. 
 
 The panels are connected in the positive side of the circuit, the 
 rotary converters having their series fields connected in the negative 
 side. No negative switches are required, as the negative side of the 
 board is connected directly to the ground. 
 
INDEX 
 
 Accumulator apparatus, 191 
 
 Accumulator battery, 185 
 
 Accumulator cars, 196 
 
 Accumulator cells, 184; end cells, 186 
 
 Accumulator plates, 181,182; negative, 
 182; positive, 182 
 
 Accumulators, 179, 180; Plante", 181; 
 efficiency of, 183; applications of, 
 195; cars provided with, 196 
 
 Action, electro -dynamic, 60 
 
 Alternating-current curve, 70, 234 
 
 Alternating-current generators, 247 
 
 Alternating-current motors, 270; im- 
 portant advantage of, 300; reversing 
 of, 308; faults with, 311 
 
 Alternating currents, 71, 217; angles 
 concerned with, 217; experiments 
 with, 221; magnetic, electro-dy- 
 namic, and chemical effects obtained 
 with, 223; current strength and 
 voltage of, 224; induction effects of, 
 225; differing in phase, 315 
 
 Alternation, 71 
 
 Alternators, 247; double-current, 248; 
 inner-pole, 250, 251; with single 
 lot at ing magnet bobbin, 252; in- 
 ductor type, 253, 261-264; alternat- 
 ing-pole type, 253 and frontispiece; 
 electro-motive force of, 253; quarter- 
 
 Ehase, 255; three-phase, 255; regu- 
 ition of, 256; efficiency cf, 257; on 
 a non-inductive load, 260; switch- 
 ing in parallel, 265; action of two 
 in parallel, 268; withdrawing one, 
 269; simplest type of, 270 
 Ammeter, electro-magnetic, 18, 19; 
 hot-wire, 19, 20, Deprez, 57, 133, 
 300; Weston, 58; shunt for, 59; 
 with central zero point, 194; 
 
 measuring transformers for, 327; 
 in high-tension plants, 327 
 
 Ampere, 15 
 
 Amperes, number of, passing through 
 a circuit, 23 
 
 Ampere's Rule, 11 
 
 Ampere-turns, 11, 16 
 
 Angles concerned with alternating 
 currents, 217 
 
 Apparent watts, 246 
 
 Applications of accumulators, 195 
 
 Arc, 207 
 
 Arc lamp, 207; current strength of, 
 208; series, 209; differential, 210; 
 shunt, 210; general arrangement of 
 parts of, 211; the Kfizik or Pilsen, 
 211, 212; enclosed, 214, 216; more 
 economical than glow lamp, 214; 
 magnetite (General Electric Com- 
 pany), 215, 216; used as a search- 
 light, 215; the Bremer, 216; the 
 Cooper-Hewitt, 216 
 
 Arc lamp resistance, without cover, 
 213; enclosed, 213 
 
 Arc lighter, 214 
 
 Armature, 68; Siemens, 69; Gramme, 
 74, 320; closed-coil, 75; toothed, 
 83; smooth, 83; partially wound, 
 84; finished, 84; locomotive motor, 
 85; of multipolar dynamo, 109; 
 former- wound, 115, 117; motor, 
 133; with three slots per pole, 249; 
 four-pole, with single slot per pole, 
 250; of inductor machine, 262; 
 primary, 285; slip-ring, 293; squir- 
 rel-cage, 293; short-circuit, 295; 
 induction-motor, diagram of, 310. 
 See also Drum armature and Rimj 
 armature 
 
 339 
 
340 
 
 INDEX 
 
 Armature coil, multiple-formed, 116; 
 series-formed, 116 
 
 Armature reaction, 88; with motors, 
 149 
 
 Armature resistance loss, 131 
 
 Armature winding, outer and inner 
 wires of, 75; of single-phase alter- 
 nator, 254; of three-phase alter- 
 nator, 258 
 
 Astatic instrument for switchboards, 
 59 
 
 Asynchronous motors, 285 
 
 Auer, Dr., of Vienna, 213 
 
 Automatic features of Type M Control, 
 175 
 
 Auxiliary phase of single-phase motor, 
 304, 307 
 
 Back electro-motive force, 134, 226 
 
 Bar magnet, 5 
 
 Battery, galvanic, 1, 2; storage, 185; 
 Edison, 186; buffer, 195 
 
 Battery switch, 189, 190 
 
 Bayonet holder, 204 
 
 Bicycle, diagrams of working of, 303 
 
 Bipolar dynamo, 71 
 
 Blow-out, magnetic, 177 
 
 Boat, moving, action of, 66 
 
 Bobbin, for alternating-current electro- 
 magnet, 227; exciting, of inductor 
 machine, 262 
 
 Boosters, 97, 189 
 
 Brake, electric, 175; Proney, 295 
 
 Branching of circuits, 29 
 
 Break spark, 222 
 
 Breaks in a circuit, 125 
 
 Bremer arc lamp, 216 
 
 Brushes, 69; sparking and displace- 
 ment of, 118; carbon, 120; copper, 
 120 
 
 Brush-holder, carbon, 120; copper 
 gauze, 121; with metal and carbon 
 brushes, 121 
 
 Buffer batteries, 195 
 
 Buffer effect, 276 
 
 C type of dynamo, 100 
 
 Cables, concentric, 43; control, 174 
 
 Calculation of resistance, 23 
 
 Calibration, 18 
 
 Calorie, 39 
 
 Carbon-holder, sliding type, 120; 
 
 swivel type, 120 
 Carbonizing, 204 
 Carbon lamp, 203 
 Cast steel, 103 
 Cell, galvanic, 1; storage, 184 
 
 Cell switches, 186 
 
 Cells, in series and parallel, 31; in 
 opposition, 32; jointly supplying 
 an outer circuit, 32 
 
 Cellulose, 203 
 
 Centigrade, 24 
 
 Centimetre measure, 24 
 
 Charging, 180, 194 
 
 Charging current, 305 
 
 Chemical energy, 197 
 
 Choking coil, 238, 304 
 
 Circuit-breaker, 50-52 
 
 Circuits, magnetic, 15, 63, 86, 106, 163; 
 simple, 27; series, 29; in parallel, 
 29; branching of, 29; characteristic 
 of closed, 88; main, 92; shunt, 92; 
 breaks in, 125; motor, 175; control, 
 175; with inductance and resistance, 
 242 
 
 Cleanliness, 326 
 
 Clockwise rotation, of separately 
 excited dynamo, 122; of shunt 
 dynamo, 122; of series dynamo, 123; 
 of compound dynamo, 123; of 
 series motor, 146; of shunt motor, 
 146, 147; connection of single- 
 phase motor for, 309 
 
 Closed circuit characteristic, 88 
 
 Closed-coil armature, 75 
 
 Coil, 225; primary, 227; secondary, 
 227; choking, 238, 304; without 
 iron, vector diagram of, 240 
 
 Commercial efficiency of a dynamo, 130 
 
 Commutator, simple, 71 
 
 Commutator motors, 281 
 
 Compound dynamos, 97; connections, 
 98, ready for switching in parallel, 
 199, 200 
 
 Compound motor, 143 
 
 Concentric cables, 43 
 
 Condenser, 306 
 
 Conductor in a magnetic field, 65 
 
 Conductors of electricity, 21 
 
 Connecting, in parallel, 29; in series, 
 29 
 
 Connection, correct and incorrect, 
 for braking motor, 176; earth, 329 
 
 Connection box, 174 
 
 Connections, for booster when charg- 
 ing, 189; of single-phase motor, 309; 
 star, 317; mesh or delta, 320 
 
 Consequent \ oles, 109 
 
 Constant current, 71 
 
 Contactors, 172, 178 
 
 Continuous current, 71, 72 
 
 Continuous-current dynamo, 75 
 
 Control cable, 174 
 
INDEX 
 
 341 
 
 Control couplers, 174 
 
 Control cut-out switch, 174 
 
 Control fuses, 174 
 
 Control rheostat, 174 
 
 Controller, street-car, cylinder develop- 
 ment of, 168; master, 173, 178 
 
 Converters, rotary, 249, 276 
 
 Cooper-Hewitt lamp, 216 
 
 Copper required to carry energy three- 
 phase and single-phase, 321 
 
 Copper wire, data on, 54, 55 
 
 Copper wires and cables, table of 
 sizes, resistances, and maximum 
 currents of, 53 
 
 Core, 2; iron, 225, 227 
 
 Core loss of a dynamo, 131 
 
 Correct connection for braking motor, 
 176 
 
 Coulomb, 15 
 
 Counter-clockwise rotation, of sepa- 
 rately excited dynamo, 122; of 
 shunt dynamo, 122; of series dy- 
 namo, 123; of compound dynamo, 
 123; of series motor, 145; of shunt 
 motor, 146; connections of single- 
 phase motor for, 309 
 
 Counter-electro-motive force, 134 
 
 Couplers, control, 174 
 
 Crater, 208 
 
 Current, electric, 2, 10, 60; measure- 
 ment of, 14; unit, 16; maximum, 
 47; induced, 66, 67, 226; constant, 
 71; rectified or continuous, 71, 72; 
 for exciting magnet coils, 86; run- 
 ning light, 131; effective or virtual, 
 224; inducing or primary, 226; watt- 
 less, 237, 307; field, curve of, 258, 
 261; at no load, 274, 275; multi- 
 phase alternating, 283; quarter- 
 phase or two-phase, 287; rotary or 
 three-phase, 290, 292; magnetizing, 
 293; no-load, 294; charging, 305; 
 multiphase, transmission of, 314; 
 phase , 323 
 
 Current indicator, 194 
 
 Current strength, 23, 29 
 
 Current transformers, 327 
 
 Currents, Eddy, 77, 78 
 
 Curve of field current plotted against 
 load, 258, 260 
 
 Curve of voltage with load variation of 
 an alternator, vector diagram of, 
 259 
 
 Curves, magnetic, 8; saturation, 87; 
 sine, 218 
 
 Cut-put, 47, 48; for large current, 48; 
 minimum, 192; maximum, 193 
 
 Cycles of a dynamo, 219 
 Cylinder development of street-car 
 controller, 168 
 
 Damping, 211 
 
 Dead points in working of bicycle and 
 sewing-machine , 303 
 
 Decomposition of water, 3 
 
 Delta connections, 320 
 
 Deprez instrument, 56, 57, 133; used 
 as an ammeter, 57; used as a pole- 
 finder, 58; used as a voltmeter, 58, 
 202 
 
 Diagrams, vector, 239, 259, 260, 261 
 
 Difference of electrical potential, 4 
 
 Differential arc lamp, 210 
 
 Direct-current dynamos in parallel, 
 198 
 
 Direct-current motors, operating troub- 
 les with, 178 
 
 Direction of rotation, methods for 
 changing, 121 
 
 Direction of rotation of a motor, 145 
 
 Discharging, 180, 194 
 
 Discs, toothed, 83 
 
 Double-cell switch, 188, 191 
 
 Double-current alternator, 248 
 
 Double-pole throw-over switch, 150 
 
 Driving-keys, 83 
 
 Drop of potential, 27; to reduce, 28 
 
 Drum armature, 79; connections, 79, 
 81, 82, 83; wound, 84; without 
 winding, 84; four-pole parallel, 113; 
 four-pole series, 114 
 
 Drying, 326 
 
 Dynamo, 68, 133; bipolar, 71; con- 
 tinuous-current, 75; shunt, 92; 
 self-excitation of, 92; series, 94; 
 compound, 97; types of, 98; over- 
 compounded, 98; Edison, 99; Kapp, 
 99; C type of, 100; Manchester, 
 100; Lahmeyer, 101; Gramme, 102; 
 multipolar, 105; four-pole, 106; 
 six-pole, 107; two-coil four-pole, 
 108, 109; non-excitation of, 124; 
 commercial efficiency of, 129, 130; 
 direct-current, 198; switching, 201; 
 cycles of a, 219 
 
 Dynamo armature, 74, 133 
 
 Dynamo-brushes, material for, 120 
 
 Dyne, 9 
 
 Earth connection, 329 
 
 Eddy currents in iron, 77, 78 
 
 Edison, Thomas A., 186; dynamo, 99; 
 
 battery, 186; carbon lamp, 203; 
 
 lamp-holder, 204 
 
342 
 
 INDEX 
 
 Effective current, 224 
 
 Effective voltage, 225 
 
 Effective watts, 246 
 
 Efficiency, of dynamos, 129; stray 
 power method of getting, 131; of 
 the accumulator, 183; of an 
 alternator, 257 
 
 Electric brake, 175 
 
 Electric current, 2; influence of, on 
 a magnetic needle, 10 
 
 Electric lighting, 203 
 
 Electric mains, 40 
 
 Electric motor, 133 
 
 Electric traction, 164 
 
 Electric units, 14, 38 
 
 Electrical energy, 197; the kilowatt- 
 hour a commercial unit for, 197 
 
 Electrical machines, 68 
 
 Electrical output, 39 
 
 Electrical phenomena, 1 
 
 Electrical potential, difference of, 4 
 
 Electrical power, 34, 197; the watt 
 a convenient unit for, 197 
 
 Electrical pressure, motion essential 
 for maintenance of, 66 
 
 Electrically driven fan, 164 
 
 Electro-dynamic action, 60 
 
 Electro-dynamometers, 60, 61 
 
 Electrolytic polarization, 179 
 
 Electro-magnetic ammeter, 18, 19 
 
 Electro-magnets, 2, 61; horseshoe, 63 
 64; straight or bar, 63, 64 
 
 Electro-motive force, 3 4, 27; back, 
 134, 226; of self-induction, 235, 
 236; of an alternator, 253 
 
 Electro-motor, 133 
 
 Enclosed arc lamp, 214. 216 
 
 Enclosed fuse, 49 
 
 Enclosed motor, 159, 160 
 
 Enclosed regulating resistance , 90 
 
 Energy, transformation of, 129; chemi- 
 cal, 197; electrical, 197 
 
 Engineering, electrical, fundamental 
 principles of, 1 
 
 Equalizing wire, 199 
 
 Equivalence of electrical mechanical 
 and heating effects, 37 
 
 Examples of installation calculations, 
 46 
 
 Expulsion fuse-block, 49 
 
 Expulsion-tube fuse-block, 49 
 
 Fahrenheit, 24 
 
 Fan, electrically driven, 164 
 
 Faraday, 14 
 
 Faure, 181 
 
 Ferranti transformer, 231, 232 
 
 Ferraris, 283 . 
 
 Field, magnetic, 9, 234, 235, 286, 287; 
 pulsating, 283, 303; rotating, 283 
 
 Field magnets, material for, 102 
 
 Field of magnetic force, 9 
 
 Field rheostats, 91 
 
 Filaments for incandescent lamps, 
 205-207 
 
 Filings round current, arrangement 
 of, 13 
 
 Fixed lead, 121 
 
 Foot-pounds, 38 
 
 Force, electro-motive, 3, 4, 27, 235, 
 236, 253; exerted by a magnetic 
 pole, 6; magnetic, lines and field 
 of, 8, 9; lines of, 12, 13, 56, 60, 
 75, 104; magnetizing, 15, 17; mag- 
 neto-motive, 15,63; back or counter- 
 electro-motive, 134, 226 
 
 Former-wound armature, 114, 115, 117 
 
 Forming, 180 
 
 Four-pole three-phase motors, 292 
 
 Four-pole two-phase motor, 288 
 
 Friction loss of a dynamo, 130, 131 
 
 Fundamental principles of electrical 
 engineering, 1 
 
 Fuse, 47, 48; for large current, 48; 
 enclosed, 49; plug, 52; control, 
 174; high-tension, 328 
 
 Fuse-block, expulsion, 49; expulsion- 
 tube, 49 
 
 Galvanic battery, 1, 2 
 
 Galvanic cell, 1 
 
 Galvanometer, 73; simple, 11 
 
 Galvanometer coil, 11 
 
 Gas voltameter 18 
 
 Generator, 133, 250; alternating-cur- 
 rent, 247; motor, 276, 277, 278 
 
 Glow lamps, 35, 203; brass cap of, 
 204; star-connected, 319; on one 
 phase, 319 
 
 Gramme, 74 
 
 Gramme armature, 74, 320 
 
 Gramme dynamo, 102 
 
 Gramme ring, 247, 277 
 
 Hand rule, 67 
 
 Heating effect, 35, 37 
 
 Heating of a line 47 
 
 Hefner-Alteneck, 82 
 
 Helix, lines of force of, 13; resultant 
 
 field of, 13 
 Henry, 242 
 High tension, 326 
 High-tension fuse, 328 
 High-tension insulator, 42 
 
INDEX 
 
 343 
 
 High-tension plants, safety in, 328 
 
 High-tension switch, 328 
 
 High voltages, 46 
 
 Holder, bayonet, 204; Swan, 204 
 
 Horse-power, 38 
 
 Horseshoe magnet, 5 
 
 Hot-wire ammeter, 19, 20 
 
 Hot-wire voltmeter, 34 
 
 Hydraulic analogy, 3 
 
 Hysteresis, 104 
 
 Hysteresis loss, 129, 130 
 
 Incandescent lamp, 35, 203; the 
 Nernst, 205-207 
 
 Incorrect connection for braking 
 motor, 176 
 
 Induced current, law of, 14; direction 
 of, 66, 67, 226; rule for determining, 
 67 
 
 Inducing or primary current, curve of, 
 226 
 
 Induction, 65 
 
 Induction motor, 283; two-phase, 285; 
 actions in, 293; input and output 
 of, 295, 298; formulae for torque of, 
 296, 297; curve of torque of, 296; 
 formulae for horse-power of, 297; 
 curves showing efficiency, maximum 
 output, etc., of, 298; power factor 
 of, 298; efficiency of, 298; advan- 
 tages of, 299; vector diagram of, 
 301; single-phase, 302 
 
 Inductionless load, 322 
 
 inductor alternator, Oerlikon, 263 
 
 Inductor machine, armature and ex- 
 citing bobbin of, 262 
 
 Influence of electric currents on each 
 other, 60 
 
 Installation calculations, 46 
 
 Installation of Type M Control, 175 
 
 Insulation, 327-329 
 
 Insulator, 21; high-tension, 42; por- 
 celain, 42 
 
 Interlinked phases, 315; voltage be- 
 tween, diagram of, 316 
 
 Internal resistance, 27 
 
 Iron core, diagrams of, 225, 227 
 
 Iron loss of a dynamo, 131 
 
 Joule, 39 
 
 Kapp type of dynamo, 99 
 Kilowatt-hour, 195, 197 
 Krizik arc lamp, 211, 212 
 
 L, use of, 241 
 Lag, 237 
 
 Lahmeyer type of dynamo, 101 
 
 Lamp-holder, 204 
 
 Lamps, incandescent, 35, 203; in 
 
 series and parallel, 45; testing, 126; 
 
 carbon, 203; new types of, 216; 
 
 synchronizing, 265 
 Law, of induced currents, 14; of 
 
 Ohm, 23, 26; of change of resistance 
 
 with temperature, 24; Lena's., 67, 
 
 274, 284 
 
 Lead, fixed, 121; of current, 307 
 Lenz's law, 67, 274, 284 
 Lighting, electric, 203 
 Lightning arresters, 330 
 Line, heating of a, 47 
 Lines, stray, 64 
 Lines of force, 60, 104; round current, 
 
 12; of helix, 13; magnetic, 56; 
 
 through ring armature, 75 
 Lines of magnetic force, 8; meanings 
 
 of, 8, 9 
 
 Load, inductionless, 322 
 Locomotive motor armature, 85 
 Loop winding, 112 
 Loss in field windings, 130 
 Losses in transformation of energy, 129 
 
 fJL, use of, 17 
 
 Machines, electrical, 68; magneto- 
 electric, 73, 247; for charging 
 accumulators, 187 
 
 Magnet, bar, 5; horseshoe, 5; poles 
 of, 13, 62; influence of a, on an 
 electric current, 56; molecular, 62; 
 field, 102 
 
 Magnet frame, eighteen-pole, 108 
 
 Magnet system, 85; of Oerlikon in- 
 ductor alternator, 264 
 
 Magnetic blow-out, 177 
 
 Magnetic circuit, 15, 63; of bipolar 
 dynamo, 86; four-pole. 106; four- 
 pole inter-pole, 1.3 
 
 Magnetic curves, 8 
 
 Magnetic field, 9, 286, 287; alteration 
 of, 234, 235 
 
 Magnetic lines of force, 56 
 
 Magnetic needle, 5, 6; pivoted. 5 
 
 Magnetism, unit of, 9; residual, 62; 
 permanent, 63 
 
 Magnetite arc lamp, 215, 216 
 
 Magnetization characteristic, 87 
 
 Magnetizing current of a motor, 293 
 
 Magnetizing force, 15, 17 
 
 Magneto, 73, 127 
 
 Magneto-electric machine, 73, 247 
 
 Magneto-generator, 68 
 
 Magneto-motive force, 15, 63 
 
344 
 
 INDEX 
 
 Magnets and magnetic lines of force, 
 56 
 
 Main circuit, 92 
 
 Mains, electric, 40 
 
 Manchester type of dynamo, 100 
 
 Master controller, 173, 178 
 
 Master switch, 173 
 
 Maximum current, 47 
 
 Maximum cut-outs, 193 
 
 Measurement of currents, 14 
 
 Measuring transformers, 327 
 
 Mesh connections, 320, 321 
 
 Metal ring, repulsion of, 225 
 
 Metric system, 24 
 
 Minimum cut-outs, 192 
 
 Minium, 181 
 
 Molecular magnets, 62 
 
 Molecules, 62 
 
 Motor, electric, 133; shunt, 136; 
 speed regulation of, 137; series, 139, 
 176; compound, 143; lor certain 
 purposes, 159; enclosed, 159, 160; 
 connected to machine tool, 160; 
 connected to pump, 161; commu- 
 tating pole, 161; connected to ele- 
 vator, 162; connected to mine 
 hoist, 163; street-car, 165; braking, 
 176; direct-current, 178; alternat- 
 ing-current, 270, 300, 308, 311; 
 synchronous, 270; polyphase syn- 
 chronous, 273, 274; commutator, 
 281; asynchronous, 285; two- 
 phase, 288; three-phase, 292, 295; 
 single-phase, 304. See also Induc- 
 tion motor 
 
 Motor armature, 133 
 
 Motor generator, 276, 277, 278 
 
 Motor starting switch, 154 
 
 Moving boat, action of, 66 
 
 Moving coil ammeter, construction of, 
 57 
 
 Multiphase alternating current 283 
 
 Multiphase currents, transmission of, 
 314 
 
 Multiple-unit control system, 167 
 
 Multipolar dynamos, 105 
 
 Needle, magnetic, 5, 6 
 
 Negative pole, 3 
 
 Nernst, 205; lamp, 205-207 
 
 Neutral point in three-phase wind- 
 ings, 320 
 
 Neutral zone, 76 
 
 No-load current, 294 
 
 Non-excitation of dynamos, causes of, 
 124 
 
 North and south poles of magnet, 13, 62 
 
 Number of amperes passing through a 
 circuit, 23 
 
 a), use of, 22 
 
 Oerlikon inductor alternator, magnet 
 
 system of, 263; lines of force in, 263 
 Ohm, 22 
 Ohm's Law, 23, 26; for magnetism, 
 
 64 
 Operating troubles with direct-current 
 
 motors, 178 
 Osmium as a filament for glow lamps, 
 
 216 
 
 Output, electrical, 39 
 Output of a dynamo, 103 
 Overload, 140 
 Oxide of iron, 179 
 
 TT, use of, 10 
 
 Paper, pole-finding, 202 
 
 Parallel winding, 112 
 
 Period, 284 
 
 Permanent magnetism, 63 
 
 Permeability, 17 
 
 Phase, quantities out of, 240; main, 
 
 304, 307; auxiliary, 304, 307 
 Phase current, 323 
 
 Phase -difference, 233; caused by 
 self-induction, 304; caused by ca- 
 pacity, 305; diagrams of capacity, 
 
 305, 306 
 
 Phase meters, 246 
 Phases, interlinked, 315 
 Phenomena, electrical, 1 
 Pilsen lamp, 211 
 Pitch, 80 
 
 Pivoted magnetic needle, 5 
 
 Plante", 180; plates, 180; accumu- 
 lators, 181 
 
 Plates, Plante, 180 
 
 Plot of current at no load, 274, 275 
 
 Plug fuse, 52 
 
 Point, neutral, 320 
 
 Polarity, 6; right, 201, 202 
 
 Polarization, electrolytic. 179 
 
 Pole-finder, 58 
 
 Pole-finding paper, 202 
 
 Poles, negative and positive, 3: north 
 and south, of magnet, 13, 62; con- 
 sequent, 109 
 
 Pole-shoes, 68 
 
 Polyphase synchronous motor, one 
 phase of, 273, 274 
 
 Porcelain insulator, 42 
 
 Portable storage battery, 185 
 
 Positive pole, 3 
 
 Potential, electrical, 4; drop of, 27 
 
INDEX 
 
 345 
 
 Power, electrical, 34, 197 
 
 Power factor, 241, 246 
 
 Power loss, 44, 45 
 
 Power transmission, series method of, 
 
 141 
 
 Presspahn tubes, 251 
 Pressure, electrical, 66 
 Pressure losses, 41 
 Primary armature, 285; ready for 
 
 winding, 289; completely wound, 
 
 289 
 
 Primary current, 226 
 Prone y brake, 295 
 Properties of angles concerned with 
 
 alternating currents, 217 
 Pulsating field, 283, 303 
 
 Quantities out of phase, 240 
 Quarter-phase alternator, 255 
 Quarter-phase current, 287 
 
 Raymond, 241, 297 
 
 Reactance, synchronous, 260 
 
 Real watts, 246 
 
 Rectified current, 71, 72 
 
 Regulating resistance, 89; enclosed, 90 
 
 Regulator, shunt, 127, 128, 157 
 
 Remanence, 62 
 
 Residual magnetism, 62 
 
 Resistance, 23, 25, 26; specific, 23; 
 calculation of, 23; internal, 27; 
 resultant, 31; regulating, 89 
 
 Resistance frames, 25 
 
 Resistance of a wire, 24 
 
 Resistances in parallel, 30 
 
 Resultant resistance, 31 
 
 Resultant voltage, 323 
 
 Reverser, 172 
 
 Reversing and starting switch, 152, 
 157, 158 
 
 Reversing apparatus, 150 
 
 Reversing of alternating - current 
 motors, 308 
 
 Revolutions, 104 
 
 Rheostat, control, 174 
 
 Right polarity, two methods of se- 
 curing, 201, 202 
 
 Ring, metal, 225; Gramme 247, 277 
 
 Ring armature, 74; lines of force 
 through, 75; with commutator, 76, 
 77; two-pole, 118; four-pole paral- 
 lel, 110, 111; four-pole series, 112, 
 motor, 134; with slip-rings, 247 
 
 Ring transformer, 228 
 
 Rotary converters or rotaries, 249, 276 
 
 Rotary or three-phase current, 292 
 
 Rotating field, 283; production of, 
 
 diagrams, 283, 284, 290, 291; speed 
 
 of, 288 
 Rotation, direction of, methods for 
 
 changing, 121; of a motor, 145. See 
 
 also Clockwise and Counter-clockwise 
 Rotor, 285; squirrel-cage, 286; speed 
 
 of, 288; wound, 289, 295; triphase, 
 
 299 
 
 Running light current, 131 
 Running position of series motor, 176 
 
 Safety in high-tension plants, 328 
 
 Saturation, condition of, 65 
 
 Saturation curve, 87 
 
 Search-light, 215 
 
 Secondary winding, 226 
 
 Self-excitation, 91; of dynamos, 92 
 
 Self-induction, 153, 226; electro-motive 
 force of, 235, 236 
 
 Series arc lamp, 209 
 
 Series circuit, 29 
 
 Series dynamos, 94; connections, 95; 
 closed circuit characteristic of, 96; 
 working in parallel, 199, 200 
 
 Series method of power transmission, 
 141 
 
 Series motor, 139; with starting resist- 
 ance, 139; speed and torque curves 
 of, 142; counter-clockwise rotation, 
 145; clockwise rotation, 146 
 
 Series winding, 95, 111, 113 
 
 Sewing-machine, diagrams of working 
 of, 303 
 
 Shell transformer, 231 
 
 Short-circuit armature, 295 
 
 Short-circuiting, 326 
 
 Shunt, 58 
 
 Shunt arc lamp, 210 
 
 Shunt circuit, 92 
 
 Shunt dynamos, 92; connections, 92; 
 external characteristic of, 93, 94; 
 ready for switching in parallel, 198, 
 199, 200 
 
 Shunt for ammeter, 59 
 
 Shunt motor, with starting resistance, 
 136; with starting resistance and 
 shunt regulator for speed regulation, 
 138; clockwise rotation, 146, 147; 
 counter-clockwise rotation, 146; 
 with wrong connection, 147, 148; 
 rule for connecting up, 148; with 
 change-over switch, 151 
 
 Shunt regulator, 157; automatic, 127, 
 128 
 
 Siemens, Werner, 92; armature, 69, 
 74, 75, 84, 221, 248, 270; lamp- 
 holder, 204 
 
346 
 
 INDEX 
 
 Simple circuit, 27 
 
 Simple commutator, 71; second posi- 
 tion of, 71; third position of, 72 
 
 Simple galvanometer, 11 
 
 Sine curve of electro-motive force and 
 current, 218; 90 apart in phase, 
 220 
 
 Single-phase induction motor, 302; 
 with auxiliary phase with self-in- 
 duction, 304; with auxiliary phase 
 having capacity, 307 
 
 Slip, 285, 300 
 
 Slip-ring armatures, 293 
 
 Slip-rings, 70, 310 
 
 Slots, different shapes of, 251 
 
 Smooth armatures, 83 
 
 Solenoid, 12 
 
 Spark, break, 222 
 
 Sparking and displacement cf brushes, 
 118 
 
 Sparking with starters and shunt 
 regulators, 153 
 
 Specific resistance ; 23 
 
 Speed and torque curves of series 
 motor, 142 
 
 Speed-measuring devices, mechanical, 
 265; electrical, 265 
 
 Speed regulation of a motor, 137 
 
 Spiral without self-induction, 245 
 
 Sprague-General Electric Type M 
 Control, 169 
 
 Square, to determine area of, 36 
 
 Squirrel-cage armatures, 293 
 
 Squirrel-cage rotor, 286 
 
 Squirrel-cage winding, 274 
 
 Standard Wire Gauge, 22 
 
 Star connections, 317, 321 
 
 Star method of arrangement of phases 
 of three-phase system, 320 
 
 Starter, 166; with inductionless break, 
 having shunt slip-ring, 154; without 
 slip-ring, 156 
 
 Starting and reversing switch for 
 shunt motor, 152, 157 
 
 Static transformer, 278 
 
 Stationary state, 40 
 
 Stator, 285 
 
 Steel, cast, 103 
 
 Steinmetz, Charles P., 104, 259, 260 
 
 Storage cells, 184 
 
 Stray lines, 64 
 
 Stray power method of getting effi- 
 ciency, 131 
 
 Street-car controller, 166-168 
 
 Street-car motor, closed, 165; open, 
 165 
 
 Sum, vector, 240 
 
 Swan, 203; holder, 204 
 
 S. W. G., 22 
 
 Switch, double-pole throw-over or two- 
 pole change-over, 150, 151; revers- 
 ing and starting, 152, 157, 158; 
 motor starting, 154; master, 173; 
 control cut-out, 174; cell, 186; 
 double-cell, 188, 191; battery, 189, 
 190; high-tension, 328 
 
 Switchboards, 334; astatic instrument 
 for, 59; high-tension, 327; insu- 
 lation of, 329 
 
 Switching dynamos in parallel, 201 
 
 Switching in parallel of alternating- 
 current machines, 265 
 
 Synchronism, 265 
 
 Synchronizer, 265; for multiphase 
 machines, 324 
 
 Synchronizing action, 269 
 
 Synchronizing lamps, connections, 265; 
 cross-connected, 266; for high-ten- 
 sion circuits, 267; arrangement of, 
 for three-phase circuit, 324; for high- 
 tension three-phase circuits, 325 
 
 Synchronous motors, 270; disad- 
 vantages of , 271 ; advantages of, 272 ; 
 electro-motive forces of, 272; field 
 insulation of, 275; use of, 276; 
 starting, 277 
 
 Synchronous reactance, 260 
 
 Synchroscope, Westinghouse, 265 
 
 System, metric, 24; Ward-Leonard, 
 158; multiple-unit control, 167; 
 train control, 171; two-phate, 314; 
 three-phase, 316 
 
 Temperature, change of resistance 
 with, 24 
 
 Tension, high, 326 
 
 Testing lamp, 126 
 
 Three-phase alternator, 256, 334 
 
 Three-phase current, 290 
 
 Three-phase motor, wound rotor of, 
 295 
 
 Three-phase system, with six mains, 
 316; voltage between outer ter- 
 minals, 316; with four mains, 317; 
 with three mains, 317; current in 
 common return, 318; power in, 322 
 
 Toothed armatures, 83 
 
 Toothed discs, open slots, 83; nearly 
 closed slots, 83 
 
 Traction, electric, 164 
 
 Train control system- 171 
 
 Transformation of energy, 129 
 
 Transformers, 227; ring, 228; shape 
 of, 229; with horseshoe-shaped 
 
INDEX 
 
 347 
 
 iron core, 230; with coils sub- 
 divided, 230; with coils wound one 
 on the other, 231; shell, 231; 
 Ferranti type, 231, 232; applica- 
 tions of, 232; vector diagram of, 
 240; regulation of, 262; static, 278; 
 measuring, 327 
 
 Triphase rotor, 299 
 
 Tri-phaser, forty-pole armature of, 313 
 
 Trolley, 165 
 
 Tubes, Presspahn, 251 
 
 Two -phase current, 287; motor, 286 
 
 Two-phase induction motor, 285, 286; 
 ready for winding, 289; wound, 
 289 
 
 Two-phase system, with four mains, 
 314; with three mains, 315 
 
 Two-pole change-over switch, 150, 151 
 
 Two -pole dynamo, 71 
 
 Type C motor complete, 289 
 
 Type M Control, 169; automatic 
 features of, 175 
 
 Types of dynamos, 98 
 
 Unit, of magnetism, 9; of electric 
 
 pressure, 14 
 Unit current, 16 
 Units, 24; electric, 14, 38 
 
 Value of resistances in parallel, 30 
 Vector diagram method, 221 
 Vector diagrams, 239, 259, 260, 261 
 Vector sum, 240 
 Virtual current, 224 
 Virtual voltage, 225 
 Volta, 18 
 
 Voltage, 14; high, 46; effective or 
 virtual, 225; square root of mean 
 square, 243; curve of, 259; in 
 three-phase rotary, 279: resultant, 
 323 
 
 Voltage drop, 44 
 
 Voltameter, 18 
 
 Volt-ampere, 35 
 
 Voltmeter, electro-magnetic type, 33, 
 202; hot-wire, 34; for cell testing, 
 194; switch, 202 
 
 Ward-Leonard system of control for 
 shunt or series motors, 158 
 
 Water, decomposition of, 3 
 
 Water current, production of, 4 
 
 Watt, 35, 197 
 
 Watt, James, 38 
 
 Wattless current, 237, 307 
 
 Wattmeter, 60, 244 
 
 Watts, apparent, 246; effective or 
 real, 246 
 
 Wave winding, 113 
 
 Westinghouse lightning arrester, 331 
 
 Westinghouse synchroscope, 265 
 
 Weston ammeter, 58 
 
 Winding, series, 95, 111, 113; loop, 
 112; parallel, 112; wave, 113; 
 field, 130; secondary, 226; squirrel- 
 cage, 274; three-phase, 320 
 
 Wire, resistance of a, 24; equalizing, 
 199 
 
 Working of direct-current dynamos in 
 parallel, 198 
 
 Zone, neutral, 76 
 
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 Lassar-Cohn's Application of Some General Reactions to Investigations in 
 
 Organic Chemistry. (Tingle.) i2mo, i oo 
 
 Leach's The Inspection and Analysis of Food with Special Reference to State 
 
 Control 8vo, 7 50 
 
 Lob's Electrochemistry of Organic Compounds. (Lorenz.) 8vo, 3 oo 
 
 4 
 
Lodge's Notes on Assaying and Metallurgical Laboratory Experiments. .. .8vo, 3 oo 
 
 Low's Technical Method of Ore Analysis , '. . . . .".'.". 8vo, 3 oo 
 
 Lunge's Techno-chemical Analysis. (Cohn.) izmo i oo 
 
 * McKay and Larsen's Principles and Practice of Butter-making 8vo, i 50 
 
 Mandel's Handbook for Bio-chemical Laboratory i2mo, i 50 
 
 * Martin's Laboratory Guide to Qualitative Analysis with the Blowpipe . . i2mo, 60 
 Mason's Water-supply. (Considered Principally from a Sanitary Standpoint.) 
 
 3d Edition, Rewritten 8vo, 4 oo 
 
 Examination of Water. (Chemical and Bacteriological.) I2mo, i 25 
 
 Matthew's The Textile Fibres 8vo, 3 So 
 
 Meyer's Determination of Radicles in Carbon Compounds. (Tingle.). . I2mo, 
 
 Miller's Manual of Assaying i2mo, 
 
 Cyanide Process i2mo, 
 
 Minet's Production of Aluminum and its Industrial Use. (Waldo.) . . . . I2mo, 
 
 Mixter's Elementary Text-book of Chemistry I2mo, 
 
 Morgan's An Outline of the Theory of Solutions and its Results i2mo, 
 
 Elements of Physical Chemistry i2mo, 3 co 
 
 * Physical Chemistry for Electrical Engineers.. . i2mo, 5 oo 
 
 Morse's Calculations used in Cane-sugar Factories i6mo, morocco, i 50 
 
 * Mu ; r's History of Chemical Theories and Laws 8vo, 4 oo 
 
 Mulliken's General Method for the Identification of Pure Organic Compounds. 
 
 Vol. I Large 8vo, 5 oo 
 
 O'Brine's Laboratory Guide in Chemical Analysis 8vo, 2 oo 
 
 O'Driscoll's Notes on the Treatment of Gold Ores 8vo, 2 oo 
 
 Ostwald's Conversations on Chemistry. Part One. (Ramsey.) i2mo, i 50 
 
 " " " " Part Two. (Turnbull.) i2mo, 2 oo 
 
 * Pauli's Physical Chemistry in the Service of Medicine. (Fischer.) .... i2mo, i 25 
 
 * Penfield's Notes on Determinative Mineralogy and Record of Mineral Tests. 
 
 8vo, paper, 50 
 
 Pictet's The Alkaloids and their Chemical Constitution. (Biddle.) 8vo, 5 oo 
 
 Pinner's Introduction to Organic Chemistry. (Austen.) i2mo, i 50 
 
 Poole's Calorific Power of Fuels 8vo, 3 oo 
 
 Prescott and Winslow's Elements of Water Bacteriology, with Special Refer- 
 ence to Sanitary Water Analysis I2mo, i 25 
 
 * Reisig's Guide to Piece-dyeing 8vo, 25 oo 
 
 Richards and Woodman's Air, Water, and Food from a Sanitary Standpoint..8vo, 2 oo 
 Ricketts and Russell's Skeleton Notes upon Inorganic Chemistry. (Part I. 
 
 Non-metallic Elements.) 8vo, morocco, 75 
 
 Ricketts and Miller's Notes on Assaying 8vo, 3 oo 
 
 Rideal's Sewage and the Bacterial Purification of Sewage 8vo, 4 oo 
 
 Disinfection and the Preservation of Food 8vo, 4 oo 
 
 Riggs's Elementary Manual for the Chemical Laboratory 8vo, i 25 
 
 Robine and Lenglen's Cyanide Industry. (Le Clerc.) 8vo, 4 oo 
 
 Ruddiman's Incompatibilities in Prescriptions 8vo, 2 oo 
 
 * Whys in Pharmacy I2mo, i oo 
 
 Sabin's Industrial and Artistic Technology of Paints and Varnish 8vo, 3 oo 
 
 Salkowski's Physiological and Pathological Chemistry. (Ornddrff.) 8vo, 2 50 
 
 Schimpf's Text-book of Volumetric Analysis i2mo, 2 50 
 
 Essentials of Volumetric Analysis i2mo, i 25 
 
 * Qualitative Chemical Analysis 8vo, i 25 
 
 Smith's Lecture Notes on Chemistry for Dental Students 8vo, 2 50 
 
 Spencer's Handbook for Chemists of Beet-sugar Houses i6mo, morocco, 3 oo 
 
 Handbook for Cane Sugar Manufacturers i6mo, morocco, 3 oo 
 
 Stockbridge's Rocks and Soils 8vo, 2 50 
 
 * Tillman's Elementary Lessons in Heat 8vo, r 50 
 
 * Descriptive General Chemistry 8vo, 3 oo 
 
 Treadwell's Qualitative Analysis. (Hall.) 8vo, 3 oo 
 
 Quantitative Analysis. (Hall.) 8vo, 4 oo 
 
 Turneaure and Russell's Public Water-supplies 3vo, 5 oo 
 
 5 
 
Van Deventer's Physical Chemistry for Beginners. (Boltwood.) . . . . i 2 mo, i 50 
 
 * Walke's Lectures on Explosives ................................. 8vo, 4 oo 
 
 Ware's Beet-sugar Manufacture and Refining .............. Small 8vo, cloth ' 4 oo 
 
 Washington's Manual of the Chemical Analysis of Rocks ........... .8vo, 2 oo 
 
 Weaver's Military Explosives ................................... . 8vo,' 3 oo 
 
 Wehrenfennig's Analysis and Softening of Boiler Feed-Water .......... 8vd, 4 oo 
 
 Wells's Laboratory Guide in Qualitative Chemical Analysis ............. 8vo, i 50 
 
 Short Course in Inorganic Qualitative Chemical Analysis for Engineering 
 
 I2mo> T SQ 
 
 Text-book of Chemical Arithmetic ............................ i2mo, i 25 
 
 Whipple's Microscopy of Drinking-water ............................ 8vo, 3 50 
 
 Wilson's Cyanide Processes ...................................... i2mo, i 50 
 
 Chlorination Process ....................................... i2mo, i 50 
 
 Winton's Microscopy of Vegetable Foods ........................... 8vo, 7 50 
 
 Wulling's Elementary Course in Inor^aLic, Pharmaceutical, and Medical 
 
 Chemistry .............................................. I2mo> 2 ^ 
 
 CIVIL ENGINEERING. 
 
 BRIDGES AND ROOFS. HYDRAULICS. MATERIALS OF ENGINEERING. 
 RAILWAY ENGINEERING. 
 
 Baker's Engineers' Surveying Instruments i2mo, 3 oo 
 
 Bixby's Graphical Computing Table Paper 19^X24^ inches. 25 
 
 Breed and Hosmer's Pr'.nciples and Practice of Surveying 8vo, 3 oo 
 
 * Burr's Ancient and Modern Engineering and the Isthmian Canal .... 8vo, 3 50 
 
 Comstock's Field Astronomy for Engineers 8vo, 2 50 
 
 Crandall's Text-book on Geodesy and Least Squares 8vo, 3 oo 
 
 Davis's Elevation and Stadia Tables 8vo, i oo 
 
 Elliott's Engineering for Land Drainage i2ir.o, i 50 
 
 Practical Farm Drainage 1200, i oo 
 
 *Fiebeger's Treatise on Civil Engineering 8vo, 5 oo 
 
 Flemer's Phototopographic Methods and Instruments 8vo, 5 oo 
 
 Folwell's Sewerage. (Designing and Maintenance.) 8vo, 3 oo 
 
 Freitag's Architectural Engineering. 2d Edition, Rewritten 8vo, 3 50 
 
 French and Ives's Stereotomy 8vo, 2 50 
 
 Goodhue's Municipal Improvements i2mo, i 75 
 
 Gore's Elements of Geodesy 8vo, 2 50 
 
 Hayford's Text-book of Geodetic Astronomy 8vo, 3 oo 
 
 Bering's Ready Reference Tables (Conversion Factors') i6mo, morocco, 2 50 
 
 Howe's Retaining Walls for Earth i2mo, i 25 
 
 * Ives's Adjustments of the Engineer's Transit and Level i6mo, Bds. 25 
 
 Ives and Hilts's Problems in Surveying i6mo, morocco, i 50 
 
 Johnson's (J. B.) Theory and Practice of Surveying Small 8vo, 4 oo 
 
 Johnson's (L. J.) Statics by Algebraic and Graphic Methods 8vo, 2 oo 
 
 Laplace's Philosophical Essay on Probabilities. (Truscott and Emory.) . i2mo, 2 oo 
 
 Mahan's Treatise on Civil Engineering. (1873.) (Wood.) 8vo, 5 oo 
 
 * Descriptive Geometry 8vo, i 50 
 
 Merriman's Elements of Precise Surveying and Geodesy. 8vo, 2 50 
 
 Merriman and Brooks's Handbook for Surveyors i6mo, morocco, 2 oo 
 
 Nugent's Plane Surveying 8vo, 3 50 
 
 Ojden's Sewer Design I2mo, 2 oo 
 
 Parsons's Disposal of Municipal Refuse 8vo, 2 oo 
 
 Patton's Treatise on Civil Engineering 8vo half lealher, 7 50 
 
 Reed's Topographical Drawing and Sketching 4to, 5 oo 
 
 RideaPs Sewage and the Bacterial Purification of Sewage .8vo, 4 oo 
 
 Siebert and Biggin's Modern Stone-cutting and Masonry 8vo, i 50 
 
 6 
 
Smith's Manual of Topographical Drawing. (McMillan.) 8vo, 2 50 
 
 Sondericker's Graphic Statics, with Applications to \ russes, Beams, and Arches. 
 
 8vo, 2 oo 
 
 Taylor and Thompson's Treatise on Concrete, Plain and Reinforced 8vo, 5 oo 
 
 * Trautwine's Civil Engineer's Pocket-book i6mo, morocco, 5 oo 
 
 Venable's Garbage Crematories in America 8vo, 2 oo 
 
 Wait's Engineering and Architectural Jurisprudence 8vo 6 oo 
 
 Sheep, 6 50 
 
 Law of Operations Preliminary to Construction in Engineering and Archi- 
 tecture 8vo, 5 oo 
 
 Sheep, 5 50 
 
 Law of Contracts 8vo, 3 oo 
 
 Warren's Stereotomy Problems in Stone-cutting 8vo, 2 50 
 
 Webb's Problems in the Use and Adjustment of Engineering Instruments. 
 
 i6mo, morocco, i 25 
 
 Wilson's Topographic Surveying 8vo, 3 50 
 
 BRIDGES AND ROOFS. 
 
 Boiler's Practical Treatise on the Construction of Iron Highway Bridges. .8vo, 2 oo 
 
 * Thames River Bridge 4to, paper, 5 oo 
 
 Burr's Course on the Stresses In Er.dgcs and Roof Trusses, A/ched Ribs, and 
 
 Suspension Bridges 8vo, 3 50 
 
 Burr and Falk's Influence Lines for Bridge and Roof Computations 8vo, 3 oo 
 
 Design and Construction of Metall.c Bridges 8vo 5 oo 
 
 Du Bois's Mechanics of Engineer.ng. Vol. II Small 4to, 10 co 
 
 Foster's Treatise on Wooden Trestle Bridges 4to, 5 oo 
 
 Fowler's Ordinary Foundations 8vo, 3 50 
 
 Greene's Roof Trusses 8vo, i 25 
 
 Bridge Trusses 8vo, 2 50 
 
 Arches in Wood, Iron, and Stone 8vo 2 50 
 
 Howe's Treatise on Arches 8vo, 4 oo 
 
 Design of Simple Roof-trusses in Wood and Steel 8vo, 2 co 
 
 Symmetrical Masonry Arches 8vo, 2 50 
 
 Johnson, Bryan, and Turneaure's Theory and Practice in the Designing of 
 
 Modern Framed Structures Small 410, 10 oo 
 
 Merriman and Jacoby's Text-book on Roofs and Bridges : 
 
 Part I. Stresses in Simple Trusses 8vo, 2 50 
 
 Part II. Graphic Statics 8vo, 2 50 
 
 Part III. Bridge Design 8vo, 2 50 
 
 Part IV. Higher Structures 8vo, 2 50 
 
 Morison's Memphis Bridge 4to, 10 oo 
 
 Waddell's De Pontibus, a Pocket-book for Bridge Engineers. . i6mo, morocco, 2 oo 
 
 * Specifications for Steel Bridges I2mo, 50 
 
 Wright's Designing of Draw-spans. Two parts in one volume 8vo, 3 50 
 
 HYDRAULICS. 
 
 Barnes's Ice Formation 8vo, 3 oo 
 
 Bazin's Experiments upon the Contraction of the Liquid Vein Issuing from 
 
 an Orifice. (Trautwine.) 8vo, 2 oo 
 
 Bovey's Treatise on Hydraulics 8vo, 5 oo 
 
 Church's Mechanics of Engineering 8vo, 6 co 
 
 Diagrams of Mean Velocity of Water in Open Channels paper, i 5O 
 
 Hydraulic Motors 8vo, 2 oo 
 
 Coffin's Graphical Solution of Hydrr.ulic Problems i6mo, morocco, 2 50 
 
 Flather's Dynamometers, and the Measurement of Power i2mo, 3 oo 
 
 7 
 
Folwell's Water-supply Engineering 8vc, 4 co 
 
 Frizell's Water-power. 8vo, 5 oo 
 
 Fuertes's Water and Public Health i2mo, i 50 
 
 Water-filtration Works. i2mo, 2 50 
 
 Ganguillet and Kutter's General Formula for the Uniform Flow of Water in 
 
 Rivers and Other Channels. (Hering and Trautwine.) 8vo, 4 oo 
 
 Hazen's Filtration of Public Water-supply 8vo, 3 oo 
 
 Hazlehurst's Towers and Tanks for Water- works 8vo, 2 50 
 
 Herschel's 115 Experiments on the Carrying Capacity of Large, Riveted, Metal 
 
 Conduits 8vo, 2 oo 
 
 Mason's Water-supply. (Considered Principally from a Sanitary Standpoint.) 
 
 8vo, 4 oo 
 
 Merriman's Treatise on Hydraulics 8vo, 5 oo 
 
 * Michie's Elements of Analytical Mechanics 8vo, 4 oo 
 
 Schuyler's Reservoirs for Irrigation, Water-power, and Domestic Water- 
 supply Large 8vo, 5 oo 
 
 * Thomas and Watt's Improvement of Rivers 4to, 6 oo 
 
 Turneaure and Russell's Public Water-supplies 8vo, 5 oo 
 
 Wegmann's Design and Construction of Dams 4to, 5 oo 
 
 Water-supply of the City of New York from 1658 to 1895 4to, 10 oo 
 
 Whlpple's Value of Pure Water Large i2mo, i oo 
 
 Williams and Hazen's Hydraulic Tables 8vo, i 50 
 
 Wilson's Irrigation Engineering Smau 8vo, 4 oo 
 
 Wolff's Windmill as a Prime Mover 8vo, 3 oo 
 
 Wood's Turbines 8vo, 2 50 
 
 Elements of Analytical Mechanics 8vo, 3 oo 
 
 MATERIALS OF ENGINEERING. 
 
 Baker's Treatise on Masonry Construction 8vo, 5 oo 
 
 Roads and Pavements 8vo, 5 oo 
 
 Black's United States Public Works Oblong 4to, 5 oo 
 
 * Bovey's Strength of Materials and Theory of Structures 8vo, 7 50 
 
 Burr's Elasticity and Resistance of the Materials of Engineering 8vo, 7 50 
 
 Byrne's Highway Construction 8vo, 5 oo 
 
 Inspection of the Materials and Workmanship Employed in Construction. 
 
 i6mo, 3 oo 
 
 Church's Mechanics of Engineering 8vo, 6 oo 
 
 Du Bois's Mechanics -of Engineering. Vol. I Small 4to, 7 50 
 
 *Ecke,'s Cements, Limes, and Plasters 8vo, 6 oo 
 
 Johnson's Materials of Construction Large 8vo, 6 oo 
 
 Fowler's Ordinary Foundations 8vo, 3 50 
 
 Graves's Forest Mensuration f vo, 4 co 
 
 * Greene's Structural Mechanics 8vo, 2 50 
 
 Keep's Cast Iron 8vo, 2 50 
 
 Lanza's Applied Mechanics 8vo, 7 50 
 
 Marten's Handbook on Testing Materials. (Henning.) 2 vols 8vo, 7 50 
 
 Maurer's Technical Mechanics 8vo, 4 oo 
 
 Merrill's Stones for Building and Decoration 8vo, 5 oo 
 
 Merriman's Mechanics of Materials 8vo, 5 oo 
 
 * Strength of Materials I2mo, i oo 
 
 Metcalf's Steel. A Manual for Steel-users i2mo, 2 oo 
 
 Patton's Practical Treatise on Foundations : 8vo, 5 oo 
 
 Richardson's Modern Asphalt Pavements 8vo, 3 oo 
 
 Richey's Handbook for Superintendents of Construction i6mo, mor., 4 oo 
 
 * Ries's Clays: Their Occurrence, Properties, and Uses 8vo, 5 oo 
 
 Rockwell's Roads and Pavements in France i2mo, i 25 
 
 8 
 
Sabin's Industrial and Artistic Technology of Paints acd Varnish 8vo, 3 oo 
 
 Smith's Materials of Machines . . . i2mo, i oo 
 
 Snow's Principal Species of Wood 8vo, 3 50 
 
 Spalding's Hydraulic Cement i2mo, 2 oo 
 
 Text-book on Roads and Pavements 12 mo, 2 oo 
 
 Taylor and Thompson's Treatise on Concrete, Plain and Reinforced 8vo, 5 oo 
 
 Thurston's Materials of Engineering. 3 Parts 8vo, 8 oo 
 
 Part I. Non-metallic Materials of Engineering and Metallurgy 8vo, 2 oo 
 
 Part II. Iron and Steel 8vo, 3 50 
 
 Part III. A Treatise on Brasses, Bronzes, and Other Alloys and their 
 
 Constituents 8vo, 2 50 
 
 Tillson's Street Pavements and Paving Materials 8vo, 4 oo 
 
 Waddell's De Pontibus. (A Pocket-book for Bridge Engineers.). . i6mo, mor., 2 oo 
 
 * Specifications for Steel Bridges i2mo, 50 
 
 Wood's (De V.) Treatise on the Resistance of Materials, and an Appendix on 
 
 the Preservation of Timber 8vo, 2 oo 
 
 Wood's (De V.) Elements of Analytical Mechanics 8vo, 3 oo 
 
 Wood's (M. P.) Rustless Coatings: Corrosion and Electrolysis of Iron and 
 
 Steel 8vo, 4 oo 
 
 RAILWAY ENGINEERING. 
 
 Andrew's Handbook for Street Railway Engineers 3x5 inches, morocco, i 25 
 
 Berg's Buildings and Structures of American Railroads 4to, 5 oo 
 
 Brook's Handbook of Street Railroad Location i6mo, morocco, i 50 
 
 Butt's Civil Engineer's Field-book i6mo, morocco, 2 50 
 
 Crandall's Transition Curve i6mo, morocco, i 50 
 
 Railway and Other Earthwork Tables 8vo, T 50 
 
 Dawson's "Engineering" and Electric Traction Pocket-book. . i6mo, morocco, 5 oo 
 
 Dredge's History of the Pennsylvania Railroad: (1879) Paper, 5 oo 
 
 Fisher's Table of Cubic Yards Cardboard, 25 
 
 Godwin's Railroad Engineers' Field-book and Explorers' Guide. . . i6mo, mor., 2 50 
 Hudson's Tables for Calculating the Cubic Contents of Excavations and Em- 
 bankments 8vo, i oo 
 
 Molitor and Beard's Manual for Resident Engineers 1 6mo, i oo 
 
 Nagle's Field Manual for Railroad Engineers i6mo, morocco, 3 oo 
 
 Philbrick's Field Manual for Engineers i6mo, morocco, 3 oo 
 
 Searles's Field Engineering i6mo, morocco, 3 oo 
 
 Railroad Spiral i6mo, morocco, i 50 
 
 Taylor's Prismoidal Formulae and Earthwork 8vo, i 50 
 
 * Trautwine's Method of Calculating the Cube Contents of Excavations and 
 
 Embankments by the Aid of Diagrams 8vo, 2 oo 
 
 The Field Practice of Laying Out Circular Curves for Railroads. 
 
 i2mo, morocco, 2 50 
 
 Cross-section Sheet Paper, 25 
 
 Webb's Railroad Construction i6mo, morocco, 5 oo 
 
 Economics of Railroad Construction Large i2mo, 2 50 
 
 Wellington's Economic Theory of the Location of Railways Small 8vo, 5 oo 
 
 DRAWING. 
 
 Barr's Kinematics of Machinery 8vo, 2 50 
 
 * Bartlett's Mechanical Drawing 8vo, 3 oo 
 
 * " " Abridged Ed 8vo, i 50 
 
 Coolidge's Manual of Drawing. 8vo, paper, i oo 
 
 9 
 
Coolidge and Freeman's Elements of General Drafting for Mechanical Engi- 
 neers Oblong 4to, 2 50 
 
 Durley's Kinematics of Machines 8vo, 4 oo 
 
 Emch's Introduction to Projective Geometry and its Applications 8vo, 2 50 
 
 Hill's Text-book on Shades and Shadows, and Perspective 8vo, 2 oo 
 
 Jamison's Elements of Mechanical Drawing 8vo, 2 50 
 
 Advanced Mechanical Drawing 8vo, 2 oo 
 
 Jones's Machine Design: 
 
 Part I. Kinematics of Machinery 8vo, i 50 
 
 Part II. Form, Strength, and Proportions of Parts 8vo, 3 oo 
 
 MacCord's Elements of Descriptive Geometry 8vo, 3 oo 
 
 Kinematics; or, Practical Mechanism 8vo, 5 oo 
 
 Mechanical Drawing 4to, 4 oo 
 
 Velocity Diagrams 8vo, i 50 
 
 MacLeod's Descriptive Geometry Small 8vo, i 50 
 
 * Mahan's Descriptive Geometry and Stone-cutting 8vo, i 50 
 
 Industrial Drawing. (Thompson.) 8vo, 3 50 
 
 Meyer's Descriptive Geometry 8vo, 2 oo 
 
 Reed's Topographical Drawing and Sketching 4to, 5 oo 
 
 Reid's Course in Mechanical Drawing 8vo, 2 oo 
 
 Text-book of Mechanical Drawing and Elementary Machine Design. 8vo, 3 oo 
 
 Robinson's Principles of Mechanism 8vo, 3 oo 
 
 Schwamb and Merrill's Elements of Mechanism ..8vo, 3 oo 
 
 Smith's (R. S.) Manual of Topographical Drawing. (McMillan.) 8vo, 2 50 
 
 Smith (A. W.) and Marx's Machine Design 8vo, 3 oo 
 
 * Titsworth's Elements of Mechanical Drawing Oblong 8vo, 25 
 
 Warren's Elements of Plane and Solid Free-hand Geometrical Drawing. i2mo, oo 
 
 Drafting Instruments and Operations i2mo, 25 
 
 Manual of Elementary Projection Drawing i2mo, 50 
 
 Manual of Elementary Problems in the Linear Perspective of Form and 
 
 Shadow i2mo, oo 
 
 Plane Problems in Elementary Geometry i2mo, 25 
 
 Primary Geometry. I2mo, 75 
 
 Elements of Descriptive Geometry, Shadows, and Perspective 8vo, 3 50 
 
 General Problems of Shades and Shadows 8vo, 3 oo 
 
 Elements of Machine Construction and Drawing 8vo, 7 50 
 
 Problems, Theorems, and Examples in Descriptive Geometry 8vo, 2 50 
 
 Weisbach's Kinematics and Power of Transmission. (Hermann and 
 
 Klein.) 8vo, 5 oo 
 
 Whelpley's Practical Instruction in the Art of Letter Engraving. ...... 12 mo, 2 oo 
 
 Wilson's (H. M.) Topographic Surveying 8vo, 3 50 
 
 Wilson's (V. T.) Free-hand Perspective 8vo. 2 50 
 
 Wilson's (V. T.) Free-hand Lettering 8vo, i oo 
 
 Woolf's Elementary Course in Descriptive Geometry Large 8vo, 3 oo 
 
 ELECTRICITY AND PHYSICS. 
 
 * Abegg's Theory of Electrolytic Dissociation. (Von Ende.) i2mo, i 25 
 
 Anthony and Brackett's Text-book of Physics. (Magie.) Small 8vo 3 oo 
 
 Anthony's Lecture-notes on the Theory of Electrical Measurements. . . . i2mo, i oo 
 
 Benjamin's History of Electricity 8vo, 3 oo 
 
 Voltaic Cell 8vo, 3 oo 
 
 Classen's Quantitative Chemical Analysis by Electrolysis. (Boltwood.).8vo, 3 oo 
 
 * Collins's Manual of Wireless Telegraphy i2mo, i 50 
 
 Morocco, 2 oo 
 
 Crehore and Squier's Polarizing Photo-chronograph 8vo, 3 oo 
 
 * Danneel's Electrochemistry. (Merriam.) I2mo, i 25 
 
 Dawson's "Engineering" and Electric Traction Pocket-book. i6mo, morocco, 5 oo 
 
 10 
 
Dolezalek's fheory of the Lead Accumulator (Storage Battery). (Von 
 
 Ende.) izmo, 2 50 
 
 Duhem's Thermodynamics and Chemistry. (Burgess.) 8vo, 4 oo 
 
 Flather's Dynamometers, and the Measurement of Power i2mo, 3 oo 
 
 Gilbert's De Magnete. (Mottelay.) 8vo, 2 50 
 
 Hanchett's Alternating Currents Explained i2mo, i oo 
 
 Bering's Ready Reference Tables (Conversion Factors) i6mo morocco, 2 50 
 
 Holman's Precision of Measurements 8vo, 2 oo 
 
 Telescopic Mirror-scale Method, Adjustments, and Tests. . . .Large 8vo, 75 
 
 Kinzbrunner's Testing of Continuous-current Machines 8vo, 2 oo 
 
 Landauer's Spectrum Analysis. (Tingle.) 8vo, 3 oo 
 
 Le Chatelier's High-temperature Measurements. (Boudouard Burgess.) i2mo, 3 oo 
 
 Lob's Electrochemistry of Organic Compounds. (Lorenz.) 8vo, 3 oo 
 
 * Lyons' 3 Treatise on Electromagnetic Phenomena. Vols. I. and II. 8vo, each, 6 oo 
 
 * Michie's Elements of Wave Motion Relating to Sound and Light 8vo, 4 oo 
 
 Niaudet's Elementary Treatise on Electric Batteries. (Fishback.) i2mo, 2 50 
 
 * Parshall and Hobart's Electric Machine Design 4to, half morocco, 12 50 
 
 Reagan's Locomotives: Simple, Compound, and Electric. New Edition. 
 
 Large i2mo, 3 50 
 
 * Rosenberg's Electrical Engineering. (Haldane Gee Kinzbrunner.). . .8vo, 2 oo 
 
 Ryan, Norris, and Hoxie's Electrical Machinery. Vol. 1 8vo, 2 50 
 
 Thurston's Stationary Steam-engines 8vo, 2 50 
 
 * Tillman's Elementary Lessons in Heat 8vo, i 50 
 
 Tory and Pitcher's Manual of Laboratory Physics Small 8vo, 2 oo 
 
 Hike's Modern Electrolytic Copper Refining 8vo, 3 oo 
 
 LAW. 
 
 * Davis's Elements of Law 8vo, 2 50 
 
 * Treatise on the Military Law of United States 8vo, 7 oo 
 
 * Sheep, 7 5<> 
 
 * Dudley's Military Law and the Procedure of Courts-martial . . . Large i2mo, 2 50 
 
 Manual for Courts-martial i6mo, morocco, i 50 
 
 Wait's Engineering and Architectural Jurisprudence 8vo, 6 oo 
 
 Sheep, 6 50 
 
 Law of Operations Preliminary to Construction in Engineering and Archi- 
 tecture 8vo 5 oo 
 
 Sheep, 5 5<> 
 
 Law of Contracts 8vo, 3 oo 
 
 Winthrop's Abridgment of Military Law I2mo, a 50 
 
 MANUFACTURES. 
 
 Bernadou'S Smokeless Powder Nitro-cellulose and Theory of the Cellulose 
 
 Molecule i2mo, 2 50 
 
 Bolland's Iron Founder i2mo, 2 50 
 
 The Iron Founder," Supplement I2mo, 2 50 
 
 Encyclopedia of Founding and Dictionary of Foundry Terms Used in the 
 
 Practice of Moulding i2mo, 3 oo 
 
 * Claassen's Beet-sugar Manufacture. (Hall and Rolfe.) 8vo, 3 oo 
 
 * Eckel's Cements, Limes, and Plasters 8vo, 6 oo 
 
 Eissler's Modern High Explosives 8vo, 4 oo 
 
 Effront's Enzymes and their Applications. (Prescott.) 8vo, 3 oo 
 
 Fitzgeralc'.'s Boston Machinist .- I2mo, i oo 
 
 Ford's Boiler Making for Boiler Makers i8mo, i oo 
 
 Hopkin's Oil-chemists' Handbook 8vo, 3 oo 
 
 Keep's Cast Iron 8vo, 2 50 
 
 11 
 
Leach's The Inspection and Analysis of Food with Special Reference to State 
 
 Control. Large 8vo, 7 50 
 
 * McKay and Larsen's Principles and Practice of Butter-making 8vo, i 50 
 
 Matthews's The Textile Fibres 8vo, 3 50 
 
 Metcalf's Steel. A Manual for Steel-users: i2mo, 2 oo 
 
 MetcalfeV Cost of Manufactures And the Administration of Workshops. 8vo, 5 oo 
 
 Meyer's Modern Locomotive Construction 4to, 10 oo 
 
 Morse's Calculations used in Cane-sugar Factories i6mo, morocco, i 50 
 
 * Reisig's Guide to Piece-dyeing. 8vo, 25 oo 
 
 Rice's Concrete-block Manufacture 8vo, 2 oo 
 
 Sabin's Industrial and Artistic Technology of Paints and Varnish 8vo, 3 oo 
 
 Smith's Press-working of Metals 8vo, 3 oo 
 
 Spalding's Hydraulic Cement i2mo, 2 oo 
 
 Spencer's Handbook for Chemists of Beet-sugar Houses i6mo morocco, 3 oo 
 
 Handbook for Cane Sugar Manufacturers i6mo morocco, 3 oo 
 
 Taylor and Thompson's Treatise on Concrete, Plain and Reinforced 8vo, 5 oo 
 
 Thurston's Manual of Steam-boilers, their Designs, Construction and Opera- 
 tion 8vo, 5 oo 
 
 * Walke's Lectures on Explosives 8vo, 4 oo 
 
 Ware's Beet-sugar Manufacture and Refining Small 8vo, 4 oo 
 
 Weaver's Mil'.tary Explosives 8vo, 3 oo 
 
 West's American Foundry Practice i2mo, 2 50 
 
 Moulder's Text-book i2mo, 2 50 
 
 Wolff's Windmill as a Prime Mover 8vo, 3 oo 
 
 Wood's Rustless Coatings : Corrosion and Electrolysis of Iron and Steel. .8vo, 4 oo 
 
 MATHEMATICS. 
 
 Baker's Elliptic Functions. 8vo, i 50 
 
 * Bass's Elements of Differential Calculus, i2mo, 4 oo 
 
 Briggs's Elements of Plane Analytic Geometry I2mo, oo 
 
 Compton's Manual of Logarithmic Computations i2mo 50 
 
 Davis's Introduction to the Logic of Algebra 8vo, 50 
 
 * Dickson's College Algebra Large i2mo s 50 
 
 * Introduction to the Theory of Algebraic Equations Large i2mo, 25 
 
 Emch's Introduction to Projective Geometry and its Applications 8vo 50 
 
 Halsted's Elements of Geometry 8vo, 75 
 
 Elementary Synthetic Geometry, 8vo, 50 
 
 Rational Geometry I2mo, 75 
 
 * Johnson's (J. B.) Thrte-place Logarithmic Tables: Vest-pocket size. paper, 15 
 
 100 copies for 5 oo 
 
 * Mounted on heavy cardboard, 8X 10 inches, 25 
 
 10 copies for 2 oo 
 
 Johnson's (W. W.) Elementary Treatise on Differential Calculus . . Small 8vo, 3 oo 
 
 Elementary Treatise on the Integral Calculus SmalfSvo, i 50 
 
 Johnson's (W. W.) Curve Tracing in Cartesian Co-ordinates, i2mo, i oo 
 
 Johnson's (W. W.) Treatise on Ordinary and Partiaf Differential Equations. 
 
 Small 8vo, 3 50 
 
 Johnson's (W, W.) Theory of Errors and the Method of Least Squares. i2mo, i 50 
 
 * Johnson's (W. W.) Theoretical Mechanics i2mo, 3 oo 
 
 Laplace's Philosophical Essay on Probabilities. (Truscott and Emory.) . i2mo, 2 oo 
 
 * Ludlow and Bass. Elements of Trigonometry and Logarithmic and Other 
 
 Tables 8vo, 3 oo 
 
 Trigonometry and Tables published separately Each, 2 oc 
 
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 Barr's Kinematics of Machinery 8vo, 2 50 
 
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 * " " " Abridged Ed 8vo, i 50 
 
 Benjamin's Wrinkles and Recipes I2mo, 2 oo 
 
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 Cromwell's Treatise on Toothed Gearing i2mo, i 50 
 
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 Flather's Dynamometers and the Measurement of Power. i2mo, 3 oo 
 
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 Gill's Gas and Fuel Analysis for Engineers i2mo, i 25 
 
 Hall's Car Lubrication i2mo, i oo 
 
 Bering's Ready Reference Tables (Conversion Factors) i6mo, morocco, 2 50 
 
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 Jones's Machine Design: 
 
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 Leonard's Machine Shop, Tools, and Methods 8vo, 4 oo 
 
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 MacCord's Kinematics; cr, Practical Mechanism 8vo, 5 oo 
 
 Mechanical Drawing 4to, 4 oo 
 
 Velocity Diagrams 8vo, i 50 
 
 13 
 
MacFar land's Standard Reduction Factors for Gases 8vo, i 50 
 
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 Berry's Temperature-entropy Diagram I2mo, i 25 
 
 Carnot's Reflections on the Motive Power of Heat. (Thurston.) i2mo, i 50 
 
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 Goss's Locomotive Sparks 8vo, 2 oo 
 
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 Hemenway's Indicator Practice and Steam-engine Economy I2mo, 2 oo 
 
 14 
 
Button's Mechanical Engineering of Power Plants 8vo, 5 oo 
 
 Heat and Heat-engines 8vo. 5 oo 
 
 Kent's Steam boiler Economy 8vo, 4 oo 
 
 Kneass's Practice and Theory of the Injector 8vo, i 50 
 
 MacCord's Slide-valves 8vo, 2 oo 
 
 Meyer's Modern Locomotive Construction 4to, 10 oc 
 
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 Valve-gears for Steam-engines 8vo, 2 50 
 
 Peabody and Miller's Steam-boilers 8vo, 4 oo 
 
 Pray's Twenty Years with the Indicator Large 8vo, 2 50 
 
 Pupin's Thermodynamics of Reversible Cycles in Gases and Saturated Vapors. 
 
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 Compton's First Lessons in Metal-working I2mo, i 50 
 
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 Du Bois's Elementary Principles of Mechanics: 
 
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 Vol. II. Statics 8vo. 4 oo 
 
 Mechanics of Engineering. Vol. I Small 4to, 7 50 
 
 Vol. II Small 4to, 10 oo 
 
 Durley's Kinematics of Machines 8vo, 4 oo 
 
 15 
 
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 Flather's Dynamometers, and the Measurement of Power i2mo, 3 oo 
 
 Rope Driving i2mo, 2 oo 
 
 Goss's Locomotive Sparks 8vo, 2 oo 
 
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 * Greene's Structural Mechanics. . . 8vo, 2 50 
 
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 Holly's Art of Saw Filing i8mo, 75 
 
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 Part II. Form, Strength, and Proportions of Parts 8vo, 3 oo 
 
 Kerr's Power and Power Transmission 8vo, 2 oo 
 
 Lanza's Applied Mechanics 8vo, 7 50 
 
 Leonard's Machine Shop, Tools, and Methods 8vo, 4 oo 
 
 * Lorenz's Modern Refrigerating Machinery. (Pope, Haven, and Dean.). 8vo, 4 oo 
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 * Martin's Text Book on Mechanics, Vol. I, Statics 121110, i 25 
 
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 * Elements of Mechanics I2mo, i oo 
 
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 * Parshalland Hobart's Electric Machine Design 4to, half morocco, 12 50 
 
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 * Iles's Lead-smelting i2mo, 2 50 
 
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 Boyd's Resources of Southwest Virginia 8vo, 3 oo 
 
 Map of Southwest Virginia Pocket-book form 2 oo 
 
 Douglas's Untechnical Addresses on Technical Subjects I2mo, I oo 
 
 Eissler's Modern High Explosives c 8 4 no 
 
 Goesel's Minerals and Metals : A Reference Book . . i6mo, mor. 3 oo 
 
 Goodyear's Coal-mines of the Western Coast of the United States i2mo, 2 50 
 
 Ihlseng's Manual of Mining 8vo, 5 oo 
 
 * Iles's Lead-smelting I2mo, 2 50 
 
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 * Walke's Lectures on Explosives 8vo, 4 oo 
 
 Weaver's Military Explosives 8vo, 3 oo 
 
 Wilson's Cyanide Processes i2mo, I 50 
 
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 Treatise on Practical and Theoretical Mine Ventilation i2ino, i 25 
 
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 * Outlines of Practical Sanitation i2mo, i 25 
 
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 Fuertes's Water and Public Health i2mo, i 50 
 
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 ence to Sanitary Water Analysis i2mo, 
 
 * Price's Handbook on Sanitation 12010, 
 
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