The Alternate Current Transformer Vol. 2: The Utilisation of Induced Currents (1896) — part 31 of 36

The anihors oondade with the following remarks : — ''From these deflections the position of the node was estimated. It appears from the best results that we have obtained that the velocity of short electric waves traveUing along two parallel wires differs from the velocity of light by less than 02 per cent, of its value. It has been shown theoretically, that the velocity of such waves travelling along a single wire should be the velocity of light approximately. Our results, therefore, in a certain sense confirm the theory to an accuracy within their probable error. Theoretically, too, the velocity should be approximately equal to the ratio between the two systems of electrical units. The average of the best measurements of this ratio is 8001, which is nearer the average velocity obtained by us than it is to the velocity of Ught/'

f.T'

CHAPTER VL

I

THE INDUCTION COIL AND TRANSPOBMEB.

§ 1. General Description of the Action of tbe Xranafionner or Induction OoiL — In the preyioos chapters we have prepared the way, by a general study of the phenomena of the induction of electric currents, to enter upon a particular examination of the structure and action of the induction coil and trans- former. The most logical method of procedure would be to trace first the historical development of these appliances from the initial scientific principles and facts accumulated by the early investigators. It will, however, be more advantageous to the student to defer this historical survey to a later portion of this treatise, and to direct attention at present to the actual electrical and magnetic operations which go on in the induction coil and transformer.

The induction coil and transformer, or converter as it is sometimes called, consists essentially of two conducting circuits which are both linked with a third or magnetic circuit, the three circuits being called respectively the primary circuit, the magnetic circuit, and the secondary circuit. The magnetic circuit may consist wholly of materi^ having a magnetic permeability equal to that of air. A core of this kind may be obtained by winding the primary and secondary circuits on a ring of wood or on a paper tube, but whatever may be the exact material used, a transformer having a core made of a material, the magnetic permeability of which is equal to that of air, is generally called an air core transformer. Not very much interest attaches to the actions of an air-core transformer, for the reason that all pracLically-used transformers possess magnetic circuits con-

THE INDUCTION COIL AND TEAN8F0RMEB. 516

sisting eiiher partly or wholly of iron. If the magnetic oircTiit consists wholly of iron, the transformer is called a dosed-circuU transformer; and if it consists partly of iron and partly of air, or other material of unit permeability, it is called an open-circuit transformer. The ordinary induction coil is of this last type. It has a core formed of a bundle of iron wires, and the magnetic circuit lies partly through this core and partly through the air outside it. The two conduct- ing circuits consist generally of copper wires or bands insu- lated in various ways and wound on the core in sections or in overlying coils. In the chapter devoted to the practical con- struction of the transformer, the various methods of carrying this into effect will be described ; meanwhile it will suffice to state that the two circuits, which are called respectively the primary and the secondary circuits, are well-insulated con- ducting circuits, the several turns of which are insulated from each other, the two circuits as a whole being also carefully insulated. The number of convolutions of each circuit may be, and generally is, very different. These are briefly spoken of as the primary turns and secondary turns. The iron core is constructed of laminated iron or iron wire, and the thick- ness or diameter of this is most usually about 'Old or *014 of an inch. The object of this lamination is to prevent the pro- duction of local electric currents, called eddy currents, in the iron, which would represent an energy loss; but, as previously explained, this lamination does not, of course, prevent the hysteresis loss caused by the reversal of the magnetisation of the core.

Tbe general action of the transformer consists in the pro- duction of a current, called a secondary current, by means of the variation in the magnetic induction in a magnetic circuit linked with it, and this induction is produced by means of another current called a primary current, the variation of the primary current producing a change of magnetic induction in a core, or magnetic circuit, which in turn creates an electromotive force in the secondary circuit linked with it.

Assuming that periodic currents are employed, it is evident, also, that the relative number of primary and secondary turns will be an important fietctor in determining

ll2

516 THS INDUCTION COIL AND TRANSFORMBB.

the ratio between the mean-square value of the potential difference acroBS the primary terminals and that across the secondary terminals of the transformer, and that it is in our power to increase or diminish this ratio. It is of course obvious, also, from first principles, that there can be no creation of energy, but only a transformation, and we can only alter the potential difference of the terminals of the two circuits at the expense of a change of corresponding current strength.

The most fundamental and valuable quality of an induction coil or transformer is, then, that it enables us to increase or reduce electrical potential difference or current strength in a definite ratio, and it is this transformation of energy which gives the apparatus its name. Transformers may therefore be classified according to the nature of the change in the character of an electric energy supply they are intended to produce.

Transformers may be constructed to act as (1) constant- potential transformers, or (2) constant-current transformers, and these may furthermore be divided into step-up transformers or Btep-doum transformers, according as they are designed to increase or diminish in a certain ratio a potential difference or a current. Thus a transformer may be designed to work off a circuit of constant potential difference and to reduce that pressure in a certain ratio, called the transformation ratio. If it lowers the pressure it would be caUed a step- down constant-pressure transformer. In the same way a transformer may be employed to change a current strength in a certain ratio, or to convert from constant pressure to constant current.

The ordinary induction coil is a step-up transformer as generally used.

It is unnecessary to make any special classification depend- ing on the character of the change of current employed in varying the induction, but it will be obvious to the reader that a closed iron-circuit transformer can only be used with alternating currents, and that for use with interrupted currents, as in the case of the ordinary induction coil, an open iron circuit or air core transformer must be employed.

Hence we have the following classification of transformers, understanding by this term any electrical arrangement con-

THS INDUCTION COIL AND TRANSFORMER. 617

fiistizig of two ooDdactmg circnits, both linked with a magnetic

cirenif;, and in which the variation of current in one circuit

gives rise to a production of electromotive force in the other : —

1. Transformers may be —

(a) Iron core transformers, with core wholly or partly of

iron; (6) Air core transformers, with core wholly of non- magnetic substance. Iron core transformers may be —

(c) Closed iron circuit transformers, with core wholly of

iron;

(d) Open iron circuit transformers, with core partly of

iron.

Transformers may be used —

(e) To transform a potential difference in a constant ratio,

called constant-pressure transformers ; (/) To transform a current strength in a constant ratio,

called constant-current transformers. Transformers may be employed — (ff) To raise pressure or current, called then step-up

transformers ; (A) To lower pressure or current, called then step-down

transformers.

The above is not a complete or exhaustive classification, but is sufSdent to mark out broadly the various forms of the apparatus.

In whatever form it is used, the primary current must give rise to a variation of the magnetic induction in the core, and this in turn gives rise to an electromotive force in the secondary circuit.

The transformer or induction coil oaxi evidently be operated either by intermittent, continuous, or by alternating currents. Whichever mode is adopted, the instrument is, of course, essentially and merely an energy-translating device. The current passing through the primary circuit magnetises the core. The intermittance or reversal of the primary current causes a variation or reversal of the magnetisation of the core. The variation or reversal of the magnetic induction in the core creates an electromotive force in the secondary

618 fHB iNDUCTIOIf COIL AND TMAlfSFOUMEA.

circuit which is linked with it, and this sets up a secondary current in the secondary circuit if it is closed. The energy supplied to the primary circuit partly reappears in the secondary circuity and the difference is represented by energy losses, called the copper losses, caused by the resistance of the conducting circuits, and partly by energy losses in the core, called the iron losses, and due to the hysteresis and eddy-currents set np in it.

The complete examination of the transformer involves, therefore, a knowledge of the manner in which the variation of the primary current, the magnetic induction in the core, the secondary current, and the secondary terminal potential difference is taking place when a certain assigned and varying primary terminal potential difference is created. It involves, also, a knowledge of the magnitude of the energy losses above described, and of the efficiency of transformation or the ratio between the power given to the external secondary circuit and the power given to the primary circuit. In addition to this, the relation of the values of the primary and secondary currents, and the primary and secondary terminal potential differences, for various states of the transformer from no load to full load, has to be ascertained. The following is, then, a summary of the operations going on in the transformer or induction coil which must be known before we can consider the action as fully understood ; and the problem of transformer construction is to predetermine these variables from certain data, so as to foretell the result of construction. Given a potential difference created between the primary terminals of the transformer following any assigned law of variation, we shall have the following effects taking place as a conse- quence, and the practical problem is to determine or predeter- mine their mode and magnitude.

(1.) We have a primary current in the primary drouit following a certain mode of variation in strength, and causing a definite copper loss in the primary circuit ;

(2.) A magnetic induction in the core following a definite mode of variation, and having a certain magnitude at every instant, this magnetic induction causing, by its variation, iron core losses due to hysteresis and eddy currents in the iron core ;

THB INDUCTION COIL AND TRANSFORMER. 619

(8.) A secondary onrrent and secondary terminal potential difference following some definite law of variation and accom- panied by a copper loss in the internal secondary circuit dae to its resistance and a contribution of power to the external secondary circttit.

The transformer problem in all its completeness wonld be solved if we conld in all case predetermine the above effects from the known primary potential difference. This, however, is not capable of being effected in a perfect manner, for reasons presently to be stated.

In early discussions of the transformer problem it was customary to make arbitrary assumptions as to the mode of the variation of the currents, the induction and potential differences. These artificial assumptions did not, however, assist real knowledge. The only useful method is to endeavour, in the first place, to ascertain whafe does go on inside the transformer, and then, on the basis of this analysis, to construct as far as possible a true working theory of the transformer. We have accordingly abandoned all discussion of imaginary transformers with air cores and currents and inductions, which are simple sine functions of the time and base, for such theory as we are able to build up on an actual knowledge of what does take place in the transformer. The only scientific method of treating the problems involved is, we repeat, first to endeavour to ascertain what are the actions really taking place, and to make them the basis for further reasoning.

§22. The Delineation of Periodic Ourres of Current and Eleo* tromotlTe Force. — The method which has proved most fertile in enabling us to understand the operations taking place in the transformer is that which consists in graphically repre- senting the form and relative position of the curves of periodic current and electromotive force in the two circuits and deducing that of the magnetisation of the core. When the primary electromotive force is an alternating one, derived from a single alternating-current dynamo which is accessible, the method of obtaining a graph of the various periodic quantities required is some modification of the arrangement

620 THE INDUCTION COIL AND TRAN8F0EMEB.

first suggested by Joubert/ in which a oirouit is dosed £ar a very short bat assigned period daring the phase, and pats some electrical instrument intermittently into connection i;rith the circuit, so that it reads, not the mean-square value of the current or potential difference, but the ttutantaneous ffolue at the assigned instant. The modem method of delineating transformer curves is as follows :

. Let the ordinates of the periodic curve in the Fig. 168 represent the varying potential difference between two con- ductors connected to an alternator, and let a condenser be connected across the circuit in series with a switch. If the switch is permanently closed, the condenser has a flow of current into and out of it, and the potential difference of its

Fio. 16a

terminals varies periodically. If, however, the switch is closed intermittently at intervals which are equal to the periodic time of the alternator, then the condenser has a series of short con- tacts made with it, and its terminal potential difference is equal to the instantaneous value of the periodic potential difference of the circuit corresponding to the instant when the contact is broken. The difference of the potentials of terminals of the condenser has then to be determined. Several methods may be adopted. The condenser plates may be connected to the terminals of an electrostatic voltmeter, and the potential difference of the condenser plates thus determined. The con- denser may be discharged through a galvanometer, and the

• Comptes liendui of the Academy of Science, France, VoL XCL, July, 1880, p. 161.

THE INDUCTION COIL AND TBANSFOBMEB. 621

condenser plate discharge determined by the quantity of the charge fonnd in the condenser. We can thus charge the condenser by a series of contacts made with the circuit, for a very short time, always at the same position during the com- plete cycle or period of the varying potential difference. The condenser acquires, after a short time, a potential difference between its terminals which is exactly equal to that of the instantaneous value of the potential difference of the circuit at the instant, when the contact is made. If, instead of connect- ing the condenser across the circuit, it is connected to the extremities of a suitable non-inductive resistance inserted in any alternating -current circuit, we can obtain from its terminal potential difference the instantaneous values of the periodic current.

We have next to consider the various practical details of the process. If the alternator is accessible, or if a single alternator is providing the current, then the intermittent contact may be made by an apparatus fixed on the shaft of the alternator. If the alternator is not accessible, and if the primary potential difference is derived from a battery of alternators running in parallel, then it is necessary to operate an intermittent contact by means of a synchronous alterna- ting-current motor, driven from that part of the circuit which is accessible. As this last method is capable of so much more general application than that of the contact-maker driven off the shaft of the alternator, we shall describe in some detail the arrangement of a suitable motor and associated apparatus. In addition to the early experiments by Joubert above mentioned, the delineation of periodic curves of current and electromotive force by means of an intermittent contact on the shaft of the alternator was suggested and carried out by Dr. Louis Duncan, and experiments by this method were effec- tively conducted by Messrs. Duncan, Hutchinson and Wilkes, and also by Prof. Byan in the United States.* It has also been largely employed by Dr. J. Hopkinson, M. Blondel, by

^ See a Paper by Meeers. Duncan, Hutchineon and Wilkes in the Slectrical World of New York for March, 1888, referred to in The Electrician^ Vol. XXL, p. Ill, June 1, 1888. Aleo see Prof. Harris J. Kyan on Transformers in the Trantactions of the American Institute of Elec- trical Engineers, Vol. VIL, January, 1890.

622 TEB INDUCTION COIL AND TRANSFOMMER.

the author, and by many others for alternating current researches. The general arrangement of the apparatus required for the complete study of the periodic quantities in a transformer, under any circumstances, is as follows : — The principal instrument required is a small alternating-current synchronous motor. A suitable form has been devised and used by the author in investigations of this character,* and a view of it is shown in Fig. 169 on next page.

The general details of the machine are as follows: — The motor, as constructed by the author, consists of two sets of field magnets, MM, which are secured to two cast-iron discs. Between these field magnets revolves a small armature. A, the iron core of which is formed of a strip of very thin transformer iron, wound up into a ring, the armature coils being wound upon this ring. The armature coils are joined up in series with one another, so as to give a series of contrary polarities round the iron ring. The diameter of this armature is about Bin. The field magnets have eight poles, and the armature eight coils. The field-magnet cores are bobbins about 2in. long and l^in. in diameter, and when joined up in series in the proper manner the field magnets take a current of about 4 amperes to give them the proper amount of saturation. The armature is carried upon a hard wood boss fixed to a steel shaft. This steel shaft is carried through small ball bearings like bicycle bearings, the shaft being borne upon seven or eight balls carried in gun-metal cells. In order to prevent any side shake of the armature, there are at the opposite ends of the base cast iron pillars with a gun-metal screw at each end, against which the rounded end of the shaft bears. The shaft can thus be adjusted with great nicety, and runs with great freedom from friction. The ends of the armature coils are brought to two small insulated collars, fixed on the shaft, against which press two light brass brushes, marked B B, kept gently against the collars by means of an expanding steel wire, W. On the armature shaA is an ebonite disc, which carries a transverse steel slip let into it. Two insulated springs, S S, are carried upon a rocking arm, H ; the rocking arm can be traversed over through half a

"• See TUEUcirtoian, Vol. XXXIV., p. 460, Februaiy 15, 1895^ "On tli» DeUnefttion of Alternating Current Cunres."

THE INDUCTION COIL AND TRANBFOHMER. 623

circumference, and is centred upon the gnn-metal end screw, which prevents side shake in the shaft. A pointer and

graduated scale, G, enables the exact angular position of the contact springs, S S, to be determined.

624 THE INDUCTION COIL AND TRANSFOMMER

One of the BprlDgs, S, is carried on a small adjusting screw, so that one spring can be given a little lead over the other, and in this manner the duration of the contact made when the steel transverse piece passes underneath and electrically con- nects the springs S S is determined. By means of a set screw the springs can be lifted off from the ebonite disc, and their pressure also adjusted. This little synchronising motor with its attached contact-breaker forms the apparatus for determin- ing the form of the current and electromotive-force cui-ves. The motor is started in step with the alternating current flowing through the armature coils by passing round the end of the steel shaft which projects at the end opposite to the contact-breaker a tape or thin leather strap sprinkled with rosin. To start the motor the following arrangements are made: — The field magnets are excited by current obtained from a small secondary battery, or from any other constant source of continuous current. The armature circuit requires about 2 amperes to make it run properly. Let us assume that the potential difference curve is to be taken from two 100 volt alterjKating-current mains which come into a building. The armature of the motor is joined across these mains in series with two or three incandescent lamps placed in parallel. The field magnets being excited in the proper direction by a con- tinuous current from a few secondary cells, the operator passes the strap or tape half round the shaft, and by pulling on one side of the tape the motor can gradually be set in rotation with an increasing speed. If the frequency of the alternating current is, say, 100 no , then the 8-pole motor has to be brought up to run at something approaching to 1,500 revolutions per minute before it will drop into step; but at a certain speed the incandescent lamps in series with the armature begin to blink, and by a lit tie skill in adjusting the speed by suitable pulls on the tape the motor will drop into step and continue to run in synchronism with the circuits. If the springs are then put down gently upon the revolving contact piece, a contact is made from one spring to the other at an assigned position during the phase of electromotive force, depending on the position of the rocking arm. If the maximum electromotive force to be read does hot exceed 160 volts, then by far the most convenient instrument to employ

THE INDUCTION COIL AND TRAN8F0RMER. 625

for reading the eleotromotive force, and the one which has been constantly employed in these tests, is Lord Kelvin's vertical or horizontal pattern malticellnlar voltmeter. As these volt- meters only begin to read at about 60 or 80 volts, it is necessary to add a constant electromotive force in series with them, and this is done by employing a set of small secondary cells. About fifty cells in one tray form a convenient arrange- ment, provided they have contacts at every cell, so as to take off any required electromotive force. The battery is joined up in series with the electrostatic voltmeter, and the terminals of the voltmeter are short-circuited by a condenser having a capacity of about half a microfarad. This arrangement of volt- meter and battery is then connected across the two points between which the potential is to be determined through the two springs S S. The motor being started, the needle of the voltmeter takes a certain deflection, which is due to the electro- motive force of the cells, plus the value of the difference of potential between the mains at an instant depending upon the position of the rocking handle. By blocking up the voltmeter in this way, and using more or less cells as required, so as to add a known amount to the electromotive force to be measured, the electrostatic voltmeter can be employed to measure potential differences over the whole range varying from zero to 160 volts in either direction. These observations are taken at equal short intervals as the rocking arm H is swept over through a quarter of a circle. It is possible to thus measure the instantaneous values of the alternating potential difference between the two points at equi-distant instants throughout the phase. It has been found by experiment that this small alternating-current motor, when working on the circuits of any alternator of a size such as would be used in a generating station, does not sensibly affect the form of the curve of electro- motive force. The motor is only used as a means of making the contact with a voltmeter at an assigned instant during the phase. The current which passes through its armature is not in any way measured or taken account of ; the motor simply acts as a synchronising arrangement, which connects the contact-breaker electrically to the distant alternator.

The synchronising motor can, therefore, be set to run in step with any alternating-current circuit, and to make a

626 THE INDUCTION COIL AND TBAN8F0BMBB.

oontaot or close a oircuit for a short instant during every period at an assigned instant in the phase, which depencUi on the position of the rocking arm carrying the contact- making springs.

This apparatus may be employed to determine any of the curves of current or potential of a transformer, as follows : — Let us suppose the transformer is one intended to be operated with a primary terminal potential difference of 2,000 volts, and that the secondary terminal potential difference is 100 volts. Across the primary terminals of the transformer a non- inductive resistance is connected, which is divided into two sections in the ratio of 1 to 19, and in series with the primary circuit of the transformer is placed another non-inductive resistance having such a magnitude that, when traversed by the primary current of the transformer, it will create a fall of potential of about 100 volts. The synchronising motor is then suitably arranged to be operated from the same circuit which supplies the primary current for the transformer, and the armature circuit of the motor may be fed through a step- down transformer, which reduces this circuit pressure to a convenient magnitude.

The motor contacts are then arranged to close the circuit of a voltmeter, which is placed across one or other of the resistances. It is found necessary to connect a condenser across the termi- nals of the voltmeter to increase its capacity, or else the leakage of the voltmeter in the intervals between the moments when the contact is made causes irregularity and uncertain deflections of the instrument. The process of getting the complete set of curves of current and electromotive force of a transformer is then as follows : The curve of primary potential difference is obtained by connecting the voltmeter through the motor contacts across the smaller section of the divided resis- tance which bridges over the primary terminals. The motor being started, the voltmeter will read a potential difference, which is one-twentieth of the whole primary potential diffe- rence, and if the rocking arm of the motor is moved over step- by-step the indications of the voltmeter will successively give the values of this fraction of the primary potential differ- ence corresponding to the different intervals of the whole period.

TEE INDUCTION COIL AND TBAN8F0BMER. 527

In the same way the cnrve of primary current can be obtained by connecldng the voltmeter circuit across the termi- nals of the resistance inserted in series with the primary circuit of the transformer. The curves of secondary potentid difference and secondary current, if necessary, can be obtained by connecting the contact-maker and voltmeter across the secondary terminals of the transformer when closed by a known non-inductive resistance.

The curves of current and potential thus obtained can be set down in a chart, the horizontal absciss® in which repfe-

4000

4000L

Fia. 170. — Primary Current, Primary Terminal, Potential Diflferenoe^ and Induction Curves of Qanz 10 H.P. Transformer taken off a Eapp Alternator by the Alternating-current Curve Tracer.

sent fractions of the complete periodic time, and the vertical ordinates represent the instantaneous values of the potential differences or currents.

In Fig. 170 is shown a set of curves taken from a Ganz transformer connected to a Eapp alternator. The dotted curve marked voU curve is the curve of primary electromotive force,

^NIVEBsiTTj

628 THE INDUCTION COIL AND TUANSFOBMETL

or difference of potential at the primary terminals of the transformer. The dotted carve marked current curve is the carve of primary carrent, and the dots show the actual posi- tion of the observations. The figures on the horizontal line indicate degrees of phase. The scale on the left-hand side is the scale of volts, and that on the right-hand side the scale of current. From the curves of current and potential differ- ence we can obtain the curve of magnetic induction in the core as follows: — ^Let b be the induction density in the iron core — ^that is, the number of lines or unit tubes of induc- tion per square centimetre of cross-section of the core. This induction density will not in general be the same in all parts of the core or the same at full load as at no load. If, in the first place, we consider the case of the transformer when the secondary circuit is open, we have a definite relation between the current and the primary circuit, the magnetic induction in the core, and the primary terminal potential difference or electromotive force at any instant. Let t be the instantaneous value of the current in the primary circuit, e the instantaneous value of the primary potential difference, and b the induction density in the core. If Nj is the number of turns of the primary circuit, B the resistance of the primary circuit, and S the area of cross- section of the core, then from the ordinary current equation for inductive circuits we have the relation

at

or ft= [^ZVdt

fe-B,i

In most cases of closed iron circuit transformers, the second term or integral on the right-hand side of the last equation is a very small quantity compared with the first term, and may be neglected. Hence Yfhen/idt is small we can obtain the value of b by integrating the primary E.M.P. curve, or by finding the value oifedt between proper limits. Take, for

THE INDUCTION COIL AND TBANSFOBMEB, 629

instance, the case of the Ganz transformer, the oorves of which are given in Fig. 170. The resistance of the primary circuit B is 25 ohms and the maximum value of the primary current i is nearly 0-84 amperes. Hence the value of Bi never exceeds 085 of a volt. The value of e, the primary electromotive force, varies from 0 to nearly 8,000 volts, and hence at any instant, except very near the moment when e is zero, the value of B i is quite negligible compared with that of e.

In order to obtain the induction curve we have to integrate the curve of primary electromotive force, and to do this properly the following procedure must be followed. The whole area included by the primary E.M.F. curv« must be obtained, and the integration of the curve must be started from that point on the horizontal axis which corresponds with the bisection of the area of the E.M.F. curve. Starting from this point the area of the E.M.F. curve is obtained by successive increments, and corresponding to the limit of each increment an ordinate is set up whose length on some scale is proportional to the whole area of the E.M.F. curve measured from the abcissa corresponding to the semi-area of the curve to the limit considered. This ordinate will then be an ordinate of the curve of induction. In making this integra- tion the area of the E.M.F. curve below the time axis must be reckoned as negative. To obtain the absolute value of the induction at any point, the area of the E.M.F. curve must be reckoned out in volt-seconds and then divided by the value of Ni S, S being measured in square centimetres. The result must be multiplied by 10^ to reduce to O.O.S. measure and give the induction density in G.G.S. units.

In this manner the finn line curve which is marked induc- tion curve in Fig. 170 was obtained. If the curve of secondary terminal potential difference has been obtained, we can, by a similar integration of this curve, obtain another induction curve which is generally practically identical with that obtained from the primary E.M.F. curve if the transformer secondary circuit is unloaded, but which does not agree witjii it if the secondary circuit is closed and a secondary current is being produced therein. Into the causes of this we shall enter later. The fuU set of transformer curves for the currents,

KM

530 THE INDUCTION COIL AND TRANSFORMER.