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 REESE LIBRARY 
 
 UNIVERSITY OF CALIFORNIA. 
 
 c %ecerct'J /^fa/t^2s^^ ,i8t)/. 
 
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 ^Accessions No. (#(#*/ /. . CA/.w No. 
 
ALTERNATING AND INTERRUPTED 
 ELECTRIC CURRENTS. 
 
ALTERNATING AND INTERRUPTED 
 ELECTRIC CURRENTS. 
 
 BY 
 
 PROF. G. FORBES, M.A., F.R.S., F.R.S.E., 
 
 ETC. 
 
 
 
 Based upon Three Lectures delivered to the Members 
 of the Royal Institution. 
 
 ILLUSTRATED. 
 
 f OF THE ^V 
 
 (UNIVERSITY) 
 \w Qf j 
 
 BIGGS AND CO., 139-140, .SALISBURY COURT, FLEET STREET, E.G. 
 
IP IR, E F .A. C 
 
 The present book is the outcome of a course of 
 three lectures delivered before the Royal Institution 
 of Great Britain in 1895. The verbatim report of 
 these has been revised and corrected, and forms the 
 substance of the book. The lectures were primarily 
 intended for an intelligent audience, but one not 
 well versed in electrical science ; hence, technicalities 
 have, in general, been avoided, and the colloquial 
 form of expression has been retained in the book. 
 The attention of telegraphists may, however, be 
 drawn to the mechanical torsion model of a sub- 
 marine cable described in the first and second 
 lectures as illustrating in a novel and remarkably 
 accurate manner the various phenomena observed 
 in working with an actual cable. It is believed, 
 too, that many of the other mechanical analogies 
 described, which formed a chief feature of the 
 course, will be found of interest to others besides. 
 
 My special thanks are due to Mr. H. E. Mitchell 
 for his assistance in preparing these lectures for the 
 
 press. /^eSE uwfcs, 
 
 f OF THE \ 
 
 (TGrNIVERSITYl GEORGE FORBES. 
 
 34, Great George-street, Westminster, S.W., 
 December, 1895. 
 
CONTENTS 
 
 INTRODUCTORY . ... 9 
 
 ANALOGIES TO ELECTRIC PROPAGATION ... ... ... 11 
 
 OBVIOUS AND NON-OBVIOUS FACTS ... ... ... 13 
 
 FARADAY'S DISCHARGE EXPERIMENT 19 
 
 ANALOGUE OF AN ELECTRIC CIRCUIT ... 19 
 
 CAPACITY 21 
 
 MECHANICAL ANALOGIES ... 25 
 
 SUBMARINE CABLES ..... ... ... '2S 
 
 WORKING TORSION MODEL OF CABLE 35 
 
 DEDUCTIONS FROM MECHANICAL ANALOGIES 38 
 
 ELECTRICAL UNITS AND THEIR MECHANICAL ANALOGIES 
 
 COMPARED ... ... ... ... ... ... 41 
 
 OERSTED'S DISCOVERY ... ... ... 43 
 
 FARADAY AND INDUCTION 43 
 
 SELF-INDUCTION AND INERTIA .... ... 44 
 
 MUTUAL INDUCTION ... ... ... ... ... 50 
 
 DANGERS OF SUDDENLY BREAKING CERTAIN CIRCUITS ... 55 
 
 TRANSFORMER ON OPEN CIRCUIT 60 
 
 PRACTICAL UTILITY OF ANALOGIES ... ... ... 70 
 
 FURTHER ALTERNATING-CURRENT PHENOMENA 71 
 
 INDUCTION WITH HIGH FREQUENCY ... . ... % 73 
 
 SKIN RESISTANCE ... .. 79 
 
 ELECTRICAL RESONANCE 88 
 
 ELECTRICAL OSCILLATIONS ... ... ... ... ... 92 
 
 EXPERIMENT BY LORD ARMSTRONG 95 
 
OF THE 
 
 UNIVERSITY 
 
 ALTERNATING AND INTERRUPTED 
 ELECTRIC CURRENTS. 
 
 INTRODUCTORY. 
 
 When I was asked by Sir Frederick Bramwell 
 to give a short course of lectures here, I enquired 
 of him what subject he would like me to lecture 
 on, and he said, " Some physical subject." I am 
 aware that at the Royal Institution it is generally 
 the custom to deal with principles rather than 
 applications of science. 
 
 Now it happened that in the course of the 
 work upon which I have been engaged during 
 the last few years I have had occasion to try and 
 find some simple means of expression to convey to 
 my own mind, and also to the minds of others, 
 certain somewhat obscure electrical phenomena which 
 take place especially when using alternating or 
 interrupted currents, and I arrived at a method of 
 producing mechanical analogies to the various cases 
 of the propagation of electricity which was of such, 
 very great assistance to myself, and which enabled me 
 so much the more easily to convey some intelligent 
 idea of these curious electrical actions to others, that I 
 
 B 
 
10 ALTERNATING AND INTERRUPTED 
 
 thought it might be well to incorporate them in 
 the present lecture. 
 
 Prof. Dewar told me yesterday that you would be 
 expecting me to talk about the Niagara Falls, and 
 that you would be disappointed if I did not. I 
 can only say that I cannot imagine why any such 
 opinions should have been held, and I can only 
 express regret if I am disappointing those present 
 by taking up a different subject. However, I have 
 this to say, that the present lecture does deal with 
 some of the work which has been going on there r 
 while it deals far more directly with submarine 
 telegraphy. The next two will deal almost entirely 
 with problems which have been cropping up in the 
 work of utilising those great falls and distributing 
 the power in the neighbourhood, so that I trust 
 the course will not be altogether a disappointment 
 to those who had expected a different one. 
 
 Before beginning my subject I have also to say 
 that I have a good many experiments to show you 
 to-day, and in the following lectures, which I do 
 not wish you to think for one moment are original 
 many are somewhat striking, but they have been 
 evolved after very patient work by others than 
 myself. I particularly mention the name of Mr. 
 Campbell Swinton, who has placed at my disposal 
 some extremely beautiful apparatus which I shall 
 have frequent occasion to refer to. Others have 
 assisted me much. Dr. Muirhead has been invalu- 
 able to me in the assistance he has given me in 
 illustrating the present lecture by enabling me 
 to produce before you some of the phenomena 
 
ELECTEIC CURRENTS. 11 
 
 which occur in submarine cables. This is a subject 
 which has hitherto been wrapped up in a certain 
 amount of obscurity of mathematical symbols ; and 
 while dealing with that it is my hope that I may 
 be able to divest the treatment of the subject of 
 those difficulties, and make the phenomena which 
 appear in submarine cables generally intelligible to 
 anyone. This I hope to do by means of a working 
 model which I devised, and which Dr. Muirhead 
 has constructed for me. 
 
 Analogies to Electric Propagation. 
 
 When we speak about the propagation of electric 
 currents through an Atlantic cable, we know that 
 while the electricity is passing into the cable a 
 charge of electricity is being imparted to the outside 
 of it the cable is being charged, and that prevents 
 the rapid propagation of the current through it. 
 Now, it is all very well to talk about it in that 
 kind of language, but it is not very intelligible, and 
 for this reason, that it is difficult to understand. 
 We do not understand exactly what we mean by 
 electricity. All the time we are using such a phrase, 
 we are trying to grasp some mechanical analogy 
 which shall make it intelligible to us. 
 
 For instance, in dealing with the propagation of 
 electricity, it has been very common in the past 
 to treat of the subject by the analogy of a flow of 
 water through pipes, and an excellent analogy it is. 
 The pressure of the water is the electromotive 
 force which compels the water to move, the velocity 
 
 B2 
 
12 ALTERNATING AND INTERRUPTED 
 
 of the water acquired in the pipe is the current, 
 and the friction of the pipe represents the resistance 
 of the wire. If you have a pipe in a complete 
 circle, with a centrifugal pump at one part, the 
 power being expended in overcoming the friction of 
 the pipe, you have there the analogue of a battery 
 with a certain pressure, on a closed circuit carrying 
 a current, with a certain amount of resistance ; where 
 the velocity of the water increases and increases 
 until the backward force caused by the friction of 
 the pipe is equal to the motive force, or the pressure 
 of the water, in the centrifugal pump. That is the 
 usual analogy employed for the most ordinary elec- 
 trical phenomena. 
 
 We might extend that analogy, and instead of 
 using up the energy of the centrifugal pump in 
 overcoming the friction of the pipe uniformly along 
 its length, which is equivalent to heating the con- 
 ductor, we might place a very narrow piece of pipe 
 at one part of it. The energy would then be 
 almost entirely consumed in that narrow pipe at 
 the far end. Now, that is analogous to the incan- 
 descent lamp circuit of this room. The centrifugal 
 pump is the dynamo in the central station, the 
 pipes that go round are the wires that go to and 
 from the central station, the narrow piece of pipe 
 at the far end where the power is used up is the 
 analogue of the incandescent lamps. Instead of 
 placing that small pipe at the end, we might have a 
 little water-motor, such as a turbine, and we should 
 then have the central pump, a limited amount of 
 whose power is used up in friction in the pipe, 
 
ELECTEIC CUEEENTS. 13 
 
 driving the motor, which might be doing useful work. 
 There we have the source of power in a central 
 station, in which the centrifugal pump represents 
 the dynamo, the pipe representing the wires along 
 which a current is flowing with a certain amount of 
 resistance offered, and the turbine at the far end 
 representing the electric motor which is driving the 
 machinery. 
 
 That is a very useful and simple sort of analogy, 
 but when we come to some more obscure points 
 this analogy fails to be quite so easily understood, 
 though in many cases it is a very good one indeed. 
 I am speaking about the difference between ordinary 
 cases which are easily understood, and cases which 
 are somewhat more obscure. In the ordinary ways 
 of electric lighting in a house or from a central 
 station the method of the propagation of electricity 
 is comparatively simple, and any person who has 
 played with a battery and wires is able to see at a 
 glance what will happen when you connect the 
 wires to the dynamo and then to the lamps. But 
 there are other phenomena which come in later, 
 especially when using alternating currents, which 
 are not so easily understood. 
 
 Obvious and Non-obvious Mechanical Facts. 
 
 As I am going to adopt mechanical analogies to 
 show the electrical phenomena, I may well here 
 adopt a mechanical analogy to show you the 
 different class of phenomena which I wish to 
 illustrate. There are many mechanical phenomena 
 
14 ALTERNATING AND INTERRUPTED 
 
 which are perfectly obvious to any intelligent person 
 when looking at them. If you have a steelyard 
 with a heavy weight supported at the short arm 
 and a light weight at the long arm, it seems 
 perfectly natural that the heavy weight may be 
 supported by the light weight, without going into 
 the mathematics of it or measuring the quantities 
 and proportions ; the result is a very obvious one 
 to every person. When I exert pressure upon a 
 screw press, I, with a very little pressure and a 
 large motion of my hand, produce a very small 
 downward motion, but a huge pressure upon a 
 copying-book, for instance: that is a transformation 
 of mechanical energy which is perfectly obvious. 
 And so with the different mechanical powers the 
 lever, the wheel and axle, the screw any of 
 these are perfectly obvious. 
 
 I will now show you one or two phenomena not 
 quite so obvious, and the first which will occur to 
 everyone of you here is simply the gyrostat, which 
 everyone of you know, but which I wish to show 
 you because it illustrates what I am working at 
 (Fig. 1). All I wish to say with respect to the 
 gyrostat is that it takes some time and some thought 
 to realise why it is that the wheel does not twist 
 in the direction in which the obvious forces are 
 impelling it. That is a thing requiring a con- 
 siderable amount of thought to enable you to arrive 
 at a correct conclusion. Why does it twist round a 
 vertical axis, and in which direction does it twist ? 
 These are things which are not perfectly obvious, 
 and which do require a considerable amount of 
 
ELECTKIC CURRENTS. 
 
 15 
 
 thought to make them clear. If I were able to 
 show you a mechanical analogy which would make 
 them clear, it would be a great step. In the 
 
 FLg.1. 
 
 ease of the electrical phenomena I think I shall be 
 able to show you some analogies which are very 
 complete. 
 
16 
 
 ALTEENATING AND INTERRUPTED 
 
 Here, again (Fig. 2), I have a chain caused to 
 revolve by means of an electric motor, where the 
 motions which take place are those which are not 
 perfectly obvious even to the very well-trained 
 
 FIG. 2. 
 
 mechanic. [Experiment.] When I draw this chain 
 towards me it acts somewhat like a rigid bar. If I 
 impress a motion on it, it retains that motion ; and 
 if I draw the chain out for a considerable distance 
 
ELECTEIC CUEEENTS. 
 
 17 
 
 and let it go, it goes through a series of motions- 
 which are somewhat difficult to understand, which 
 
 Front Elevation, 
 
 Section 
 
 you would not have foreseen if you had been asked 
 to say what the action was to be. 
 
 I will not delay you with these any longer ; a 
 
18 ALTERNATING AND INTERRUPTED 
 
 series of beautiful experiments may be done with 
 them. I will show you another example, however, 
 where the ordinary intelligent man with mechanical 
 knowledge does not immediately realise what is 
 going to happen when a certain experiment is going 
 to be performed. I have a disc of ordinary cartridge 
 paper loaded at the circumference by means of 
 weights, and J shall give a considerable speed of 
 rotation to the disc, and I shall bend this disc 
 while it is in rotation, and I do not think that the 
 result will be exactly as everyone would have 
 expected (Fig. 3). Now we leave it alone, and I do 
 not think that everybody here would have expected 
 it would retain the same shape continuously for so 
 long a time. I go faster, and you see that there 
 is a force which is tending to lead it back to the 
 vertical. In time it would come back to the vertical 
 position. 
 
 These are all phenomena dependent upon inertia, 
 and a great number of those in the electrical 
 experiments which I wish to show you are equally 
 dependent upon what is called electrical inertia. 
 Electrical inertia usually goes by the name of 
 4t self-induction," "and I shall have frequent occasion 
 to use it ; and as you have seen the effect in these 
 cases, you will remember inertia is the analogue 
 of self-induction. These experiments also involved a 
 certain pliability (e.g., of the chains), flexibility, 
 ability to move, and a natural tendency to return if 
 not affected by other causes. 
 
 Many of the electrical experiments T shall have 
 to show you depend for their action upon electrical 
 
ELECTRIC CURRENTS. 19 
 
 flexibility or yielding, which usually goes by the 
 name of electrical capacity. These two things, 
 capacity and self-induction, are the two most 
 interesting things which come into the course 
 of these lectures. I shall have to deal with them 
 pretty largely. 
 
 Faraday's Discharge Experiment. 
 
 An experiment of Faraday's may be mentioned as 
 a fitting opening to this course of lectures. He 
 discharged a large number of Leyden jars, and tried 
 to discharge them through a lengthy wire ; but the 
 discharge preferred to cross from one end of the 
 wire to the other end instead of going round 
 the circuit, although that metallic conducting circuit 
 was there, and you would have thought it would go 
 round it. In that experiment is involved nearly the 
 whole of the principles of the various things I wish 
 to bring before you in the course of these lectures. 
 Instead of bringing all things grouped together in 
 one experiment, and then separating them out, we 
 shall now proceed to take them separately. 
 
 Analogue of an Electric Circuit. 
 
 I wish to show you before we go on what is the 
 mechanical analogue which I wish in these lectures 
 to follow up in the study of an electric circuit. The 
 water analogy which I have spoken of is very 
 useful, but I am anxious to draw your attention to 
 .another mode of illustrating it, and I will just do 
 
20 ALTEENATING AND INTEEEUPTED 
 
 so by means of a model here. I have an electric 
 circuit represented here, by means of this piece of 
 apparatus (Fig. 4). My hands represent a central 
 station at the top, and there is a factory at the 
 bottom where the power is going to be used in 
 lifting weights. These are suspended on the vertical 
 indiarubber ring I hold. I have to apply an electro- 
 motive force. The force is purely a mechanical 
 one a twisting force. I then give a certain twist. 
 By applying the force up here I apply a certain 
 twist to the indiarubber tube, which is transmitted 
 
 to the bottom, and the weights rise. That is a very 
 simple thing, and therefore it is all the better for 
 illustrating what I wish to say. We may use a 
 continuous current, in which case I always wind 
 up in the same direction, or we may use an alter- 
 nating current, when I twist in opposite directions. 
 The resistance in this case is represented by the 
 distortion of this indiarubber, but any other form 
 of friction would do just as well. For example, if 
 I take one of these strips of telegraphic paper 
 and twist it round at the top, there is a certain 
 
ELECTRIC CURRENTS. 21 
 
 amount of air friction resisting the twisting ; or I 
 might suspend in water or oil anything I wish to 
 twist, when the friction between the surface of 
 the object and the fluid would exemplify electrical 
 resistance. 
 
 Capacity. 
 
 Now, one of the first things I wish to give a little 
 idea about is capacity. Of course, as I mentioned, 
 an Atlantic cable has a certain amount of capacity, 
 a considerable capacity, and it shows itself by a 
 charge upon the outride of the cable. To illustrate 
 this more clearly, I will just go through the 
 experiment of charging a Ley den jar and then 
 discharging it, just to show you what I mean by 
 capacity and what I mean by something which 
 will hold a charge. [Experiment.] Now, an Atlantic 
 cable consists of a wire surrounded by insulating 
 material. The wire is the one conductor we will 
 call it the internal coat and the sea is the outer 
 one, and this is to be charged to the potential 
 or pressure which you are using. Now, to charge 
 any portion of it, involves the passage of a certain 
 quantity of electricity through the cable. But 
 the current cannot pass instantaneously through 
 the whole length of the wire, because the condenser 
 action is demanding a certain supply for every 
 length of cable as you go from one side of the 
 Atlantic to the other, and the resistance of the 
 wire will not allow it to flow more rapidly than 
 at a certain rate with the number of batteries you 
 happen to be using. That is the foundation of 
 
22 ALTEENATING AND INTEEEUPTED 
 
 the difficulty that arises in dealing with submarine 
 cables. 
 
 I wish to illustrate this question of capacity to 
 make it a little more clear, and will do so at the 
 risk, perhaps, of confusing you a little ; but I will 
 try to avoid that. I have here an arrangement by 
 means of which I can chnrge and discharge a 
 condenser with extreme rapidity. The amount of 
 electricity which a Leyden jar can hold is extremely 
 small. Faraday used to say that in decomposing 
 a drop of rain there is far more electricity used 
 than there is in a lightning flash. 
 
 The little Leyden jar which I used just now is 
 capable of holding but a very small quantity of 
 electricity, but the condenser which I am going 
 to make use of immediately is capable of holding 
 still less. It holds very little electricity indeed. 
 But by using a current interrupted with extreme 
 rapidity, and reversing it, I am able to supply the 
 current at . very rapid intervals, so that the total 
 quantity which passes in one discharge is very 
 small, but the total quantity which passes in the 
 course of a second is very large indeed owing to 
 the enormous number of discharges which take 
 place in the course of a second. In the experi- 
 ment which I am going to show you, by using 
 two pairs of tin plates we have two condensers of 
 far less capacity than that of the Leyden jar which 
 you saw just now, and yet their capacity will be 
 sufficient with the enormously high pressure I am 
 going to use, and the very rapid alternations, to 
 provide a current sufficient to light an incandes- 
 
ELECTRIC CURRENTS. ZO 
 
 cent lamp, which is really a considerable quantity. 
 I now take the two top plates of the con- 
 densers and place them over the others, and you 
 see a light glowing (Fig. 5). I can place them 
 still lower, and you get a brilliant illumination 
 even using that extremely low capacity, simply 
 owing to the hundreds and thousands of volts 
 we are using and the enormous frequency of 
 alternation. 
 
 Oscillator 
 Fig. 5. 
 
 While I have this apparatus I will show j you' 
 the effect in a more exaggerated manner. I am 
 going to use simply two points separated by a 
 glass plate. The capacity of this arrangement is 
 something almost infinitesimal, and yet by using 
 these enormous electric pressures and the high 
 frequency which we are employing, we are able 
 to get electricity enough to light a lamp. I lean 
 a glass plate against one of the terminals, and to 
 the other terminal I attach an incandescent lamp 
 with a wire leading up to the glass plate. Now, 
 the capacity of that condenser is rp ally almont 
 
 "UNIVERSITY 
 CALIFORNIA- 
 
144 ALTERNATING AND INTERRUPTED 
 
 infinitesimal, and yet there is a sensible amount 
 of electricity passing into and out of it during the 
 
 ^01 ass 
 
 -- 
 
 Oscillator 
 Fiq- 6. 
 
 time I am exhibiting it (Fig. 6). This is a very 
 striking experiment, and as I have got the appa- 
 ratus here I will show you what the capacity of 
 
 my own body is. I think you may take it that 
 ihis ground is completely insulated, and that there 
 
ELECTEIC CUEEENTS. 25 
 
 is no cuirent capable of going through me to it. 
 Therefore, since the lamp is only attached to 
 one of the terminals, the current has no com- 
 plete circuit to pass round, and when I touch this 
 it is only by charging my body and then discharg- 
 ing my body that the lamp is able to light up 
 (Fig. 7). There is the lamp lighted, the charging 
 and discharging of my body being utterly imper- 
 ceptible to myself. 
 
 Mechanical Analogies. 
 
 Now, capacity is what I am going to deal with 
 chiefly to-day, and most of the rest of the lecture 
 will be devoted to illustrating an Atlantic cable. 
 But I will first show you the kinds of models that 
 I have found instructive, and they are of the rudest 
 and most elementary character ; and as soon as 
 you realise what you can do by thinking over these 
 simple models, and the perfect analogies you can 
 get to the electrical phenomena, I am sure it will 
 be a very great help to you in working out any 
 electrical problems or in thinking about what is 
 going on in connection with any. 
 
 Well, in the first place, I am dealing altogether 
 with twisting of materials. I consider that the 
 electromotive force is the force with which I twist 
 them. I can measure the force with which I twist 
 anything by winding a wire round it and letting it 
 go over a pulley and putting a weight on it (Figs. 8, 
 9, 10, and 11). That force is the electromotive 
 force. Here (Fig. 8) I have a central station with 
 
 c 
 
26 
 
 ALTERNATING AND INTERRUPTED 
 
 a twisting force, here I have the line going to a 
 distant place, and there I have a resistance, and 
 a resistance eventually such as to equalise the force 
 I am exerting. There is also a definite velocity of 
 rotation attending, which is analogous to the 
 current. Therefore, in these models, twisting force 
 
 copper 
 w/re \ 
 
 Paper 
 vane 
 
 Rubber 
 
 Fig. 8. 
 
 Fig. 3. 
 
 is the electromotive force, friction is resistance, and 
 velocity of rotation is electric current. In such a 
 station I can have either a continuous current, 
 ever going round in the same direction, or I can 
 have an alternating current, going first in one 
 direction and then in the other. In either case, 
 
ELECTRIC CURRENTS. 
 
 27 
 
 if the wire is perfectly stiff, it is pretty easy to 
 follow the phenomena which are going on. 
 
 I have a number of other examples. You may take 
 any mechanical devices for these illustrations, and 
 
 i) 
 
 Cham 
 
 Paper 
 
 vanes 
 
 Rubber 
 
 Tig. 11. 
 
 you can find electrical analogues. Here is a case 
 (Fig. 9). Suppose I twist that elastic string at 
 the top and keep it fixed at the bottom what is 
 that equivalent to? It is equivalent to a cable of 
 
 c2 
 
28 ALTERNATING AND INTERRUPTED 
 
 large capacity which is insulated from the earth 7 
 and not free. In that case there is a certain charge 
 given to it with any electromotive force in other 
 words, there is a certain twisting given to it, with 
 an ounce weight, for example. Therefore we are 
 able to conceive of capacity as a yielding in the 
 same direction, and the charge is the amount of 
 that yielding. A twisted, flexible thing like this 
 is a charged cable. I have here (Fig. 11) some- 
 thing which represents a combination of flexibility 
 in the cable, which is capacity, and of mass or inertia 
 at the far end, which is self-induction, and of air 
 friction, which is resistance. If I pass alternat- 
 ing currents through that, you will notice that the 
 progress is in a sort of wavy motion down the 
 length of the line. Exactly the same thing happens 
 in the electrical case, provided you have your units 
 in the two cases proportionate to each other. 
 
 Here (Fig. 10) I have a heavy chain. When I 
 twist this chain round backwards and forwards it 
 does not all rotate at the same time ; that is due 
 partly to its having pliability, which is capacity, 
 and also inertia, which is self-induction. The 
 inertia of the mass prevents the motion being 
 communicated to the whole length at the same 
 time. 
 
 Submarine Cables. 
 
 And now I wish to draw your attention to sub- 
 marine cables. I will first simply show you how 
 a deep, narrow trough of water is an analogy of 
 a submarine cable. This trough has capacity at 
 
ELECTRIC CURRENTS. 29 
 
 every part of it because the level can rise, it 
 has resistance all along because there is friction 
 between the water and the sides ; in every way 
 the propagation of a wave along the trough is 
 analogous to the propagation of a wave in a sub- 
 marine cable in fact, it is a very accurate analogy 
 indeed. To illustrate it, I will simply make a few 
 waves, and you will see the propagation of the 
 waves along the whole length of the water surface, 
 the change of level taking some time to reach the 
 far end. That is what happens in the case of a 
 submarine cable it takes some time to reach the 
 far end. 
 
 I wish now to show you a most curious thing. 
 I have here a thread, suspended from a spring, 
 with a set of vanes attached, and two nozzles for 
 blowing in either direction against the vanes. The 
 thread is immersed in the glycerine and water in 
 this long tube, and at the bottom I have a mirror 
 attached, from which I can reflect a beam of light. 
 The thread might be attached to the bottom of the 
 tube by a spring, but instead of having a spring 
 I have used the obvious method of attaching a 
 small magnet to the mirror, and using controlling 
 magnets. Let me refer you to this diagram (Fig. 
 12). The top spring acts as a condenser, and the air- 
 blast and the vanes take the place of the battery. 
 The spring below takes the place of the other con- 
 denser. There we have mechanical apparatus taking 
 the place of electrical as used in sending messages 
 through an Atlantic cable. But before using it I 
 wish to show you the phenomenon of the retar- 
 
30 
 
 ALTERNATING AND INTERRUPTED 
 
 dation of the current in an Atlantic cable that is to 
 say, by means of those artificial cables which are 
 made by Dr. Muirhead, who has assisted me so 
 much. Dr. Muirhead's artificial cables are made 
 
 SPRIMC 
 
 AIR BLAST 
 
 THREAD 
 
 VANES 
 
 CONDENSER' 
 
 SPRING 
 
 CABLE 
 
 1 CONDENSE* 
 
 A a 
 
 FIG. 12. A B represent Earth. 
 
 of tinfoil, cut in these shapes (Fig. 13) so as to* 
 have high resistance and a proportionate capacity. 
 These two kinds of plates are placed alternately 
 side by side and separated by paraffin, the 
 plain sheets being connected with the earth. He 
 builds up his cable that way, and thus makes an 
 Atlantic cable. Now, to show the retardation of 
 
ELECTRIC CURRENTS. 
 
 31 
 
 current. This galvanometer is connected with the 
 sending end ; the current will then pass through 
 the whole length of the cable and come out at 
 the other end, and, passing through the galvano- 
 meter there, deflect it. The movements of these 
 galvanometers will be seen by spots of light shining 
 on the screen. Now, Mr. Edgar is going to send 
 
 Fig. 13 
 
 a signal through fche cable, and what you will 
 have to observe is that the sending galvanometer 
 is deflected immediately, and that it takes a sensible 
 time before the other is affected. [Experiment.] 
 You can see the interval of time between the 
 sending signal and the arriving signal, and you 
 also see how gradually the arriving signal creeps 
 up. Exactly the same is noticed when I rotate 
 the vanes at the top of the thread hanging in the 
 tube of glycerine that I have just described to you, 
 
32 ALTEENATING AND INTEEBUPTED 
 
 and the deflection of the mirror at the bottom is 
 noted. This, I have to say, is really, if you choose 
 your units, a perfect representation of the Atlantic 
 cable. I would like to devote a little more time to 
 show how, by measuring the twist, you can get the 
 capacity and the resistance. Time will not permit 
 ,me to do that, but I may perhaps have more to 
 say about that on another occasion. 
 
 I will now show you some further illustrations 
 with these. I will begin by using the suspended 
 thread in order that we may see and foretell what 
 we ought to expect from the cable. We will use the 
 suspended thread first without a curb. Let me tell 
 you what a curb is. In the Atlantic cable it is 
 found that if you simply send a positive current 
 through, the signal arrives in a certain manner 
 which is perfectly satisfactory, but it is much 
 improved if, after having given a positive current, 
 you give it a short time afterwards a negative 
 current. I will put it in another way. If you 
 take a whip with a long lash, and you want 
 to make a crack you may give a forward motion 
 to the lash, and you get a certain amount of 
 cracking ; but you get a much sharper crack if 
 you follow it with a backward motion. So here, 
 curbing with my cable means giving it first a twist 
 in one direction and then a twist in the other. 
 Not curbing simply means giving a twist in one 
 direction only. We will signal the letter S that 
 means three dots to show you what takes place 
 in a real cable. You will follow it by means of 
 the spot of light you see on the screen. I want 
 
tTNIVERSIT 
 
 ELECTRIC CURRENTS. 
 
 to show you the difference between curbing and 
 not curbing. When you are telegraphing quickly, 
 the signals bank up on the top of each other, and 
 cause the spot of light to travel further and further 
 away as the second and third dots come. With 
 curbing this does not occur. NOTT, by depressing 
 first the positive key and then the negative we can 
 produce the equivalent of curbing, and Mr. Edgar 
 will be able to show you that by means of the 
 reflection on the screen. [Experiment.]* 
 
 In conclusion, I wish to draw your attention to 
 the beautiful apparatus which is used in the 
 operation of transmitting through an Atlantic cable. 
 In the first place, the message is punched out on 
 a piece of paper, and it is punched so as to send 
 the current in either a positive or negative direction, 
 according as the holes punched are on one side or 
 other of the strip. The strip of paper is afterwards 
 passed through the transmitter, and as it is passed 
 through contact is made through the holes, and 
 the message is sent. We will now transmit a 
 message through this. [Experiment.} The messages 
 which are sent through the cable are received on 
 an instrument called a syphon-recorder, which was 
 originally invented by Lord Kelvin. There is in the 
 curb sender I have on the table a most beautiful 
 device, for while it is working I can, by turning 
 this screw, vary the amount of curbing in signalling 
 through the cable, and by varying it you are able 
 to adjust the signals so as to get the most beautiful 
 
 * See note at end of lecture for more detailed description of the 
 model. 
 
34 
 
 ALTEENATING AND INTEEEUPTED 
 
 trace of the recorder possible ; and also by this 
 means, not using a condenser, but by direct signal- 
 ling, you are able to get an increase of speed in 
 
 C H 
 
 FIG. 13A. 
 
 some cables of about 25 per cent., which is a matter 
 of great importance. I do not think there is time 
 
ELECTEIC CUEEENTS. 35 
 
 to go into much more, but I am able to show you 
 a few specimens on cards of some signals made in 
 three different ways : first, with a condenser ; second, 
 without a condenser ; and third, without a condenser 
 and with a curb, and the third is the best. I 
 wanted a word with a good many special letters 
 like " h" and " s " in it, and so I selected the name 
 of a member of this institution who has had a great 
 deal to do with telegraphs, and there (Fig. 13A) are 
 the marks exactly as they came out drawn by the 
 recorder itself "Hughes." 
 
 NOTE. 
 Working Torsion Model of Submarine Gable. 
 
 For the benefit of telegraphists, I wish to say a 
 few more words about the model illustrating sub- 
 marine cables. 
 
 If you hang a thread in a vertical tube filled 
 with glycerine and water in suitable proportions, 
 you have all the essentials for understanding the 
 propagation of electric pressure through a submarine 
 cable. But the measurement of the twisting force 
 applied at the top is difficult. Any mechanism 
 having mass or inertia is objectionable, as this intro- 
 duces the analogue of a heavy self-induction, which 
 does not exist in the electrical problem. For this- 
 reason, I suspend the thread, and support on it 
 vanes, which can be blown round by a constant 
 pressure of air to represent the electromotive force. 
 The spindle of the vanes is solid, and goes down 
 
36 ALTERNATING AND INTERRUPTED 
 
 into the liquid to the point where the thread 
 representing the cable is attached. The vanes and 
 air-jet represent the battery, and I find it convenient 
 to have two jets, to rotate the vanes in opposite 
 directions. These are actuated by two keys, + and - , 
 which admit the air to one or other of the 
 nozzles. If there be no sending condenser to be 
 represented in the model, the vanes must be sup- 
 ported by a silk fibre without torsion. Of course 
 the vane spindle is held laterally by bearings. Things 
 being so arranged, we may assume that the air 
 pressure used represents the electromotive force of 
 the sending battery, say 100 volts. If the cable 
 which we wish to imitate has 800 microfarads 
 capacity, the permanent charge on it would be 
 
 800 
 
 6 x 100 = 0'08 coulomb. Fixing now the lower 
 
 end of the thread, apply the air-jet and observe 
 the twist. Suppose it is ten revolutions. Then one 
 revolution means O'OOS coulomb, and one revolu- 
 tions per second means 0*008 ampere. If now the 
 resistance of the cable be 6,000 ohms, the 
 
 permanent current would be - ampere. This 
 
 60 
 
 should be represented by = = two 
 
 60 x 0-008 0-48 
 
 revolutions per second. Leave now the lower end 
 free, and apply the air-jet at the top. If you get 
 two revolutions per second, you have the right 
 resistance in the model. If not, add more water 
 or glycerine till you get it right. 
 
 You cannot, however, get mirror signals thus, as 
 
ELEJTEIC CUBEENTS. 3T 
 
 velocity indicates current, and the zero would 
 move. If, however, you put a mirror at the lower 
 end of the thread, and restrain its motion either 
 by a spring or by magnets, you are putting in a 
 receiving condenser, and the deflection indicates the 
 potential at that point of the cable, just as a 
 recorder does. "You ought to add here, below the 
 mirror, a vertical wire to represent the resistance 
 of the recorder. If now, instead of having a tor- 
 sionless fibre at the top, you attach the vanes to- 
 a spring, this is equivalent to a sending con- 
 denser, and you have a perfect representation of the 
 signals received by the special cable which you. 
 have imitated. 
 
38 ALTERNATING AND INTERRUPTED 
 
 LECTURE II. 
 
 In my last lecture I indicated my desire during 
 the course to illustrate a good many of the pheno- 
 mena that occurred in using interrupted and alter- 
 nating electric currents by means of mechanical 
 analogies; and I think you will have already seen 
 that, in certain cases at least, these mechanical 
 analogies are of very great help to us in under- 
 standing what is going on. I propose to carry on 
 the same method through the present lecture at 
 ;any rate. I concluded the last lecture by showing 
 you by means of such mechanical analogies that we 
 were able to form a clearer conception, as it seemed 
 to me, of what is going on in a submarine cable 
 than we would be able to without such mechanical 
 analogies. 
 
 Deductions from Mechanical Analogies. 
 
 Now, I am very anxious to prevent anyone here 
 from being led to draw too much from these 
 mechanical analogies, and I warn you that a 
 considerable amount of care is required in dealing 
 with them. In going a step further, sometimes one 
 is apt to miss the complete analogy, and so to be 
 led to wrong deductions. This is not the case 
 with the submarine cable which I showed you. In 
 
ELECTRIC CURRENTS . 39 
 
 that case the analogy is, I may say, almost abso- 
 lutely complete. There is only one defect in that 
 analogy, and it is a trivial one the resistance to the 
 current is represented by the friction between the 
 glycerine and water in this tall tube, the thread 
 inside which is being twisted, and the current is 
 represented by the velocity of rotation of that 
 thread. Now, the force opposing the motion in 
 the case of this glycerine varies as the square of 
 the velocity. In the analogy the resistance ought 
 to vary as the velocity. The opposing force varies 
 as the square of the current instead of as the 
 current, and that is the only defective part of the 
 analogy ; and it is really of very little consequence. 
 As you all saw last day, it does not interfere with 
 the very perfect representation of what is going on 
 in a submarine cable. In fact, those who are accus- 
 tomed to read the signals for a submarine cable 
 agree that they cannot be distinguished from the 
 signals given by this twisted thread. I said last 
 lecture that these analogies would enable me to 
 dispense with mathematical formulae in explaining 
 some obscure phenomena to you, but I would also 
 wish to say that I think that these analogies are 
 of more use really to the mathematician than to 
 anybody else, because such analogies enable the 
 mathematician to gain a clearer grasp of the quan- 
 tities he is dealing with, and to treat them more 
 easily than he would be able to do otherwise. In 
 fact, I may say that Clerk Maxwell, in the whole 
 of his great work on electricity, was merely 
 working with mechanical analogies, although he 
 
40 ALTERNATING AND INTEERUPTED 
 
 does not say so in the book. It is obvious to 
 anyone who reads that book that he was work- 
 ing out mechanical problems the whole time, 
 knowing that the mechanical results at which he 
 arrived would apply equally well to the electrical 
 phenomena, and that instead of putting mass, 
 momentum, and all those mechanical quantities, 
 he put the electrical equivalents. That is perfectly 
 obvious, and if any proof of it was wanted it is in 
 his invention of that term, " electric displacement," 
 which is a little puzzling to the electricians ; but 
 as soon as we realise that it is a mechanical 
 equivalent the difficulty disappears.* 
 
 Now, I was a little hurried last week at the end 
 of my lecture in the demonstration of the appa- 
 ratus I have just referred to, and it is such a 
 very complete reproduction of what goes on in an 
 Atlantic cable that I wish to make it a little more 
 clear. At the top of this thread I have a spindle, 
 with guides or bearings, the whole being supported 
 by a spring. On the spindle is a disc with vanes. 
 There are also two brass jets, through either of 
 which I can send a current of air to twist the 
 thread in one direction or the other. These 
 
 * As an example of the importance of mechanical analogies to the 
 philosopher, see the remarks by Prof. Larmor, F.R.S., in his paper, 
 " A Dynamical Theory of the Electric and Luminiferous Medium," 
 Phil. Trans., 1894, Part II., p. 719: "The credit of applying with 
 success the pure analytical method of energy to the elucidation of 
 optical phenomena belongs to MacCullagh ; he was, however, unable 
 to discover a mechanical illustration such as would bring home to the 
 mind by analogy the properties of his medium, and so his theory has 
 failed rather into neglect from supposed incompatibility with the 
 ordinary manifestations of energy as exemplified in material struc- 
 tures." 
 
ELECTEIC CUEEENTS. 41 
 
 currents of air are manipulated from below. I 
 may blow into the pipe, or I may use a reservoir 
 of air, as I did on the last occasion, and the 
 rotation is given to right or left by the two keys 
 011 the table, which open the air-valves and let the 
 air pass in. The string itself is so chosen that it 
 represents a long Atlantic cable. The way in which 
 the measurements are made is this. To see that it 
 represents a cable, you first, with a definite pressure 
 of air, allow the string to rotate at a steady speed. 
 Suppose it makes a revolution in a second. Then, 
 with the same pressure, holding the bottom part of 
 the string, or restraining it by means of magnets, 
 apply the same air pressure and see how much 
 twist it gets. It gets, perhaps, ten revolutions. 
 Divide that ten revolutions by the one revolu- 
 tion, and that ten gives you a constant of the 
 cable ; it gives you the ratio of the charge to the 
 current, or the product of the capacity into the 
 resistance ; that defines the quality of the cable 
 exactly, and the number of turns that the top 
 of the string can acquire defines the capacity of 
 the cable as a condenser. I will not delay you 
 further to explain the ways in which this is an 
 extremely close analogy, because it will become 
 tedious. Those of you who have followed me so 
 far, and have any interest in submarine cables, will 
 see the very close analogy which there is. 
 
 Electrical Units and their Mechanical Analogies 
 Compared. 
 
 I have placed in the accompanying table a number 
 
 D 
 
42 
 
 ALTERNATING AND INTERRUPTED 
 
 of electrical units which can be taken, and their 
 analogies. I have simply tabulated them as a 
 means of reference. I have given examples of 
 certain units which may be assumed ; for instance, 
 volt and ampere. From these all the other units 
 will follow. I have assumed 1 volt to mean 1 ounce 
 driving at a radius of 1 inch, and 1 ampere one revo- 
 lution per second, A coulomb is equivalent to one 
 revolution ; there is a farad when it makes one revolu- 
 tion with 1 ounce of pressure ; a henry, if you have 
 a current of 1 ampere gradually reduced during one 
 second of time by its own self-induction to zero. 
 
 -Electric 
 
 -Mechanical- 
 
 Potential. 
 
 Charge. 
 
 Capacity. 
 
 Current. 
 
 Volt. 
 
 Coulomb. 
 
 Farad. 
 
 Twisting force. 
 
 Twist. 
 
 Twist per unit force. 
 
 Ampere. Rotation per second. 
 
 loz. at lin. radius. 
 1 revolution. 
 1 revolution with 
 loz. at lin. 
 1 revolution per 
 second. 
 Self-induction. Henry. Moment of inertia. 62oz. at lin. radius. 
 
 The pressure that is in that interval of time being 
 constantly exerted is the equivalent of 1 volt and 
 that is obtained by the momentum of a cylinder of 
 1 inch radius weighing 62 ounces. That is all I 
 need say about that at the present time. Of 
 course, these units might be varied, and when we 
 are dealing with submarine cables we will take 
 very different ones from these. On the other hand, 
 we might take, for instance, an ampere to be three 
 hundred revolutions per second. So, when we are 
 dealing with transmission of power from one place 
 
ELECTRIC CURRENTS. 43 
 
 to another, if we wanted to make an analogy we 
 would use other units from these. In each par- 
 ticular case we select our own units. 
 
 Oersted's Discovery and Possible Deductions 
 Therefrom. 
 
 The primary discovery in electromagnetics was 
 made by Oersted in 1820, when he showed that 
 an electric current would deflect a magnet. From 
 that simple experiment the philosopher might have 
 retired to his chamber and worked out the whole 
 of the electromagnetic phenomena which we are 
 at present acquainted with. He could have seen 
 that the north pole tends to rotate round the wire 
 carrying a current in one direction and the south 
 in the other. He would have foreseen Faraday's 
 experiment, in which a pole was made to rotate ; he 
 would have seen that in producing this mechanical 
 action some power was used, and therefore the 
 current was diminished and a counter electromotive 
 force set up in the wires ; he would have seen that 
 a wire acted as a magnet, and would consequently, 
 when properly wound round a core, induce mag- 
 netism in a piece of soft iron. 
 
 Faraday and Induction. 
 
 But of all these different phenomena, the one I 
 wish to direct your attention to most to-day is the 
 work of Faraday. I am going to show it in its 
 crudest and most simple and elementary form. I 
 
 D2 
 
44 
 
 ALTERNATING AND INTERRUPTED 
 
 have here (Fig. 14) a single wire, which I shall con- 
 nect with a battery through which I can suddenly 
 send an electric current. In its neighbourhood, but 
 not touching it, I have another wire which is not 
 connected with any battery, but is connected with 
 the galvanometer at the far end of the room. What 
 I wish to show you is that at the moment of 
 making an electrical contact in the one wire,, a 
 current in the opposite direction is generated in 
 the second one, and on breaking it a current in 
 the same direction as the primary current is 
 
 created ; that is to say, we ought to get a deflec- 
 tion of the galvanometer connected with the second 
 wire in one direction when we make the circuit, and 
 in the other direction when we break it. Remember 
 the galvanometer shows a current generated in the 
 wire which has no battery connected with it. 
 
 Self-induction and Inertia. 
 
 This phenomenon of induction is somewhat equi- 
 valent to that of inertia; it may be considered as 
 
ELECTEIC CUEEENTS. 45 
 
 its equivalent in its effect on the current in 
 the primary wire. It sets up a counter electro- 
 motive force in the wire through which the electric 
 current is sent. It is just like the effect produced 
 when we set a mass in motion there is at first an 
 opposition to its motion, and that is due to its 
 inertia, and is a good analogue of the induced 
 current. 
 
 Now, you will see we have a spot of light upon 
 the screen perfectly steady. I will send a positive 
 current in one direction through one wire, and the 
 spot of light, which indicates the current which goes 
 through the other wire, is deflected to the right. 
 I will now break the circuit, and the spot of light 
 goes in the opposite direction, showing that the 
 current is induced in the opposite direction when we 
 break the circuit. Now, a certain amount of power 
 is being generated in the second wire, which is, of 
 course, being taken out of the primary wire, through 
 which the current is going that is to say, momen- 
 tarily an electromotive force is acting against the 
 current in the primary wire. That I have said is 
 analogous to the effect of inertia. For example, 
 here (Fig. 19) I have a rod with a heavy flywheel 
 at the base of it. If I try to give a rotation to 
 it with my finger there is at first an opposition to 
 the motion there is a sensible opposition, and it 
 takes some little time to acquire the steady rotary 
 motion which it eventually obtains. That resistance 
 which my finger experiences is the counter electro- 
 motive force. But once I have started the thing, 
 and it has acquired a steady rotation, there is no 
 
46 ALTERNATING AND INTERRUPTED 
 
 such counter electromotive force acting at all. If I 
 try to stop it I feel the effect of the flywheel trying 
 to pull my finger round, and trying to continue its 
 motion in the same direction. That gives us a 
 clear idea of the self-induction that is taking place. 
 
 G/ass rod 
 
 Application of Foregoing Experiments to Engineering 
 Work. 
 
 The application of these experiments and pheno- 
 mena really belongs to great engineering works. In 
 these we have frequently to do with alternating 
 currents instead of continuous, and you will be able 
 to see, by the analogy, that we have the induction 
 of the current producing a very remarkable difference 
 in its effects in the two cases. After I have once 
 
' ELECTEIC CUEEENTS. 
 
 47 
 
 got the flywheel started by applying a very slight 
 force at the top, the flywheel has nothing to do 
 with the rotation. But now suppose I try to 
 produce an alternating current with the same force 
 backwards and forwards. I find that I am not able 
 to get up the same velocity, that the inertia of 
 the flywheel interferes most materially with the 
 freedom of rotation, and I am not able to get the 
 same speed or the same current through my circuit 
 by that means as I did with the continuous current. 
 
 That that is the case, I am going to show you very 
 simply by means of a lamp, which is a very good 
 indicator of the strength of a current. I have here 
 a coil of wire, which is really a Gramme ring, and 
 I will pass a current of electricity through this coil 
 of wire and then through the lamp (Fig. 15). First, 
 I will use a continuous current, and show you that 
 the lamp at once acquires its full brightness. I will 
 then use an alternating current, and show that the 
 
 OF THE 
 
 UNIVERSITY 
 
48 ALTERNATING AND INTERRUPTED 
 
 behaviour is very different, because here in the 
 self-induction of this coil we have the inertia 
 of the current acting in the same way as the 
 inertia of the flywheel that I showed you. We will 
 now try a continuous current. Now the current is 
 simply passing through the coil of wire and through 
 the lamp, and being a continuous current, there is 
 no self-induction and no counter electromotive force 
 generated. Consequently, when I short-circuit the 
 coil you see no preceptible increase in the light of 
 the lamp. The resistance of that coil is very slight 
 indeed, and produces no effect. I will now put on 
 an alternating current. I cut out the self-induction 
 coil, and I have now the alternating current passing 
 through the lamp directly, and you see it gives the 
 same brightness as with the continuous current 
 using 100 volts as I did before. I will now cut out 
 this short-circuit and allow the current not only to 
 pass through the lamp, but also to pass through the 
 self-induction coil acting as the flywheel to which 
 we are trying to give oscillating movements, and the 
 lamp is immediately greatly diminished in brilliancy, 
 as the effect of the reduction in the current pro- 
 duced by the coil. 
 
 One of the greatest helps given to electrical 
 engineering by the discoveries of Faraday has been 
 the invention of the induction coil, or, as it is some- 
 times now called, the transformer. This can only be 
 used with alternating currents. I have a specimen 
 of a transformer on the table here. It consists of a 
 core, of a bundle of iron wires, through the centre, 
 with a coil of wire wound round it ; around that 
 
ELECTEIC CUEEENTS. 49 
 
 again there is a separate coil of wire, and presently 
 I shall show you that when we change the 
 strength of the current in the one wire a current 
 is generated in the other wire. In fact, the trans- 
 former gives really a magnified effect of the 
 induction of two wires upon each other. That we 
 were only able to detect by means of the galvano- 
 meter. The action of the transformer is much 
 more powerful. The iron core is introduced, and 
 instead of having a single wire, a number of coils 
 are wound round, and both increase the effect to 
 a great extent. The transformer has been and is 
 of much use to electric light engineers, because 
 it enables us to convert a current of one pressure 
 into a current of another pressure. If we have a 
 large number of turns of wire on one of these 
 coils, and a small number on the other, we may 
 convert a high-pressure current into a low-pressure 
 current, or vice versa. That has been of great use 
 in electric light engineering, enabling us to carry 
 a high-pressure current to great distances over a 
 thin wire, and to transform it at the point of 
 consumption, where the users of electric light 
 desire to transform the current to one of low 
 pressure, such as is suitable for electric lamps, and 
 perfectly safe to handle. 
 
 But if that has been of use in electric light 
 engineering, it has been infinitely more useful in 
 electric power distribution. When I first had to 
 investigate the problem of transmitting energy from 
 a power station at the Niagara Falls to different 
 -consumers who might be near at hand or far away, 
 
50 ALTERNATING AND INTERRUPTED 
 
 the first thing that struck me was the enormous 
 difference in the character of the electric currents 
 that different people wanted. Some people wanted 
 it for lighting, some wanted it for power, some 
 wanted to generate electric currents for electrolytic 
 operations ; some wanted continuous currents and 
 some wanted alternating ; some wanted 100 volts- 
 and some 50 volts. I do not say that it would 
 have been impossible to have supplied these demands 
 by means of a continuous current, but there would 
 have been very great difficulty in doing so, and in 
 every single case it would have been necessary to- 
 introduce a motor driving a dynamo. The expense 
 and the trouble of such a distribution would have 
 been simply enormous. And although some very high 
 authorities I may mention Lord Kelvin among 
 others, he almost alone now persistently said that 
 the continuous current would have been the best 
 for the purpose, I am glad to say that in that great 
 work the alternating current has been used, and it 
 is a matter for congratulation to everybody concerned 
 that the further we proceed with the work the more 
 we see its adaptability to all the different circum- 
 stances that arise. 
 
 Mutual Induction. 
 
 Now, with regard to this mutual induction which 
 takes place in the transformer, I am going to show 
 you the effect in a more striking way than by using 
 the alternating current. I am going to show you 
 that, with the continuous current, when you break 
 
ELECTEIC CURRENTS. 
 
 51 
 
 the one circuit you are able to get a very power- 
 ful electromotive force in the other circuit. I shall 
 pass a continuous current at 100 volts through the 
 fine coil of this transformer (Fig. 16) ; I shall then 
 break the circuit suddenly, and you will be able to 
 see that, at the moment of breaking, an induced 
 current is created in the secondary coil which will 
 
 Continuous 
 ,. Current Supply 
 
 ^^ '"WO Volts 
 
 be sufficient to light up the lamp in circuit with 
 it. I will first show you this without producing 
 any effect that is, with the electromagnet unexcited. 
 I have now closed the primary circuit, and when 
 I break the circuit it will not be sufficient to light 
 the lamp. The induced current is not sufficient to 
 light that lamp, although I break this circuit as 
 sharply as possible. The reason is that an arc is- 
 formed, as you perceive, where I break the circuit. 
 
-52 ALTERNATING AND INTERRUPTED 
 
 This is equivalent to putting in a graduated resist- 
 ance which prevents a sudden break to the current 
 -causes the break in the circuit to be gradual and 
 slow ; whereas it is the suddenness of the break 
 that causes the illumination of the lamp. 
 
 I will now excite the electromagnet, and you 
 will see that when I break the circuit at a point 
 between the poles of the electromagnet, the electro- 
 magnet will not allow an arc of light to remain. 
 The tendency is for the current of electricity to 
 be deflected. That arc of light is the current of 
 electricity, and the arc will be blown out almost 
 instantaneously, and this enables me to get a 
 sudden break instead of the gradual break which I 
 had before. Now I have made the contact, and 
 :as soon as I break it you will notice that a much 
 sharper spark is produced than before, and that the 
 lamp now lights up brilliantly; and by having a 
 very powerful magnet it is possible to produce a 
 very sudden break indeed, and consequently a high 
 electromotive force in the secondary circuit. 
 
 These different phenomena of induction, pleasing 
 as they are, and useful as they are to the electrical 
 engineer, are matters that require considerable cau- 
 tion in their handling. There are some troubles 
 which would arise if sufficient attention were not 
 paid to them, and I am going to deal it may 
 seem at an unnecessary length with some of the 
 analogies to show you what happens when we 
 suddenly break a circuit possessing much self- 
 induction. 
 
ELECTRIC CURRENTS. 53 
 
 Clerk Maxwell's Model Illustrating Mutual Induction. 
 
 Before doing that, I wish to show you a model, 
 which I am enabled to do by the kindness of ProL 
 J. J. Thomson, which was introduced by Clerk 
 Maxwell to illustrate this phenomenon that I 
 have been describing to you the phenomenon of 
 induced currents in a secondary wire or coil. It 
 is an instructive experiment to look at, and 
 it is an interesting one, because it shows how 
 thoroughly Clerk Maxwell's ideas were connected 
 with mechanical phenomena when he was work- 
 ing out his electrical theories. 
 
 Here you will notice that I have two wheels 
 (Fig. 17) one which I am turning, and a second 
 one which is not turning at all. The one which 
 I am turning at this moment represents the 
 primary circuit, and the one which you see is 
 not turning represents the secondary circuit. These 
 wheels are connected, not by a rigid axis, but by 
 means of three bevelled wheels in the centre one 
 keyed on each of the two horizontal shafts, and 
 the third running loosely on a shaft, at right 
 angles to the others, forming one of four cross- 
 arms, having heavy weights at their extremi- 
 ties, and therefore possessing considerable inertia, 
 and so resisting any tendency to set them in 
 motion round the horizontal shaft of the hand- 
 wheel, to a sleeve running on which they are 
 attached. The consequence is that, if I suddenly 
 try to turn this handle round, the inertia of 
 
54 
 
 ALTERNATING AND INTERRUPTED 
 
 the intermediate gearing will prevent it from turn- 
 ing round at first, and the secondary wheel will 
 turn in the opposite direction to that in which 
 I am turning the first i.e., if I do it suddenly 
 enough. 
 
 FigJ7 
 
 But now the intermediate gearing is rotating, 
 and if I suddenly stop the hand-wheel, then the 
 secondary circuit goes on rotating in the same 
 direction in which my hand has been going. This 
 is a very beautiful analogy, but, of course, you 
 must not look upon it as more than a rough sort 
 of analogy. It is not an explanation. it were 
 
ELECTRIC CURRENTS. 55 
 
 an explanation we should have to say, I suppose, 
 that this beautiful gearing and these masses at 
 the end of the cross-arms are parts of the ether. 
 In the true case a motion is given to the ether 
 in the one direction, and the moment the ether has 
 motion it transmits it to the other wire. That it 
 could be an explanation is, of course, quite out of 
 the question. But a mathematician finds a very 
 great help from getting a mechanical thing to illus- 
 trate what he wants to arrive at in his theories. I 
 remember that in the celebrated Baltimore lectures 
 which Lord Kelvin gave in 1884, he was trying to 
 work out a theory of an ether and vibrating mole- 
 cule giving light, and he worked out then and 
 subsequently a view of the ether in which every 
 part of the ether consisted of an assembly of gyro- 
 stats. I remember perfectly well his saying : "I 
 do not believe ether is made up of gyrostats, but 
 I am never contented with any theory until I can 
 get a mechanical analogy to satisfy me. After we 
 have the mechanical analogy which satisfies the 
 conditions, we can then work on to get something 
 in the ether which will be analogous to it in the 
 method, though different to it in the i mechanical 
 conception." 
 
 Dangers of Suddenly Breaking Circuits Possessing 
 Heavy Self-induction. 
 
 I want now to show you some of the analogies 
 which help to show us the enormous dangers which 
 may be incurred in practice by suddenly breaking our 
 
56 ALTERNATING AND INTERRUPTED 
 
 circuits which have very large self-induction. In an 
 alternating-current system like that which has just 
 been put in at Niagara Falls, the self-induction of 
 the transformers is something very large indeed. 
 And if we were to permit insulated cables to be 
 laid in the earth over considerable distances, the 
 capacity would also be very considerable of these 
 cables. The combined effects of the capacity and 
 of the self-induction would lead to a great deal of 
 trouble, and it has been my effort to reduce the 
 capacity of the cables, and to lay them in such a 
 way that there should be as little capacity as 
 possible, so that that source of trouble should be 
 as much obliterated as could be. Also I introduced 
 the plan of lowering the frequency of the alterna- 
 tions as low as possible, which also assists largely 
 to get over troubles due to capacity. I need not 
 go into that question of the reduction of the fre- 
 quency at the present moment ; but I may say 
 that the further we have gone in the matter 
 the more thankful we are that we realised the 
 necessity in time, and have lowered the frequency 
 to a figure that has never been reached before. It 
 is not only an enormous safeguard to us in count- 
 less other ways, but it enables us to use apparatus, 
 of a simplicity which would have been impossible 
 with the higher frequencies. 
 
 Now, neglecting the question of the capacity, let 
 us deal simply with the question of the self-induc- 
 tion. It is remarkable what great difficulty I have 
 had in getting people to realise the danger that 
 there is in suddenly breaking these circuits contain- 
 
ELECTEIC CUREENTS. 57 
 
 ing enormously powerful self-inductions ; but I have 
 insisted throughout that it is indispensable to the 
 success of that great work at Niagara that no circuit 
 in which these powerful effects of self-induction 
 are operative shall ever be suddenly broken. I 
 found manufacturers relying simply upon their past 
 experience, and not looking into the new conditions 
 caused by the enormous size of the machinery. I 
 found able manufacturers unable to realise this 
 point. They had noticed, when they broke the 
 circuit of an alternating current with some self- 
 induction, large sparks at the switch. Several had 
 the notion that, if they could get rid of these, that 
 was all they had to do. Their desire was to make 
 the break at the switch as sudden as they possibly 
 could, so as to present sparks and a train of fire 
 from appearing there. As a matter of fact, by doing 
 so they were endangering the whole system by 
 introducing an enormous electromotive force of self- 
 induction tending to break down the whole system 
 in one part or the other. I will show you one or 
 two mechanical analogies to illustrate this ; and, 
 first, I will adopt the analogy of flowing water 
 which I spoke of last time, which is an extremely 
 useful analogy. 
 
 I have here (Fig. 18) a pipe through which I am 
 able to allow water to flow downwards. There is 
 in the middle of the suspended part an opening, 
 which is at present closed. A little brass tube 
 projects from the main tube, and that brass tube 
 is closed simply by means of a membrane of paper, 
 which prevents the water from escaping there. 
 
 E 
 
58 
 
 ALTERNATING AND INTERRUPTED 
 
 That piece of paper in this analogy represents the 
 insulation on the cable, and what we are going to 
 test just now is the dielectric strength that is to 
 say, the breaking strain of the paper. If that 
 
 GlassRod 
 
 Fig. 18 
 
 paper gives out when we suddenly stop the water, 
 thenithe analogy is, that when we stop the electric 
 current we shall tend to break down the insulation 
 of our cables and any other part of our machinery. 
 
ELECTEIC CUEKENTS. 
 
 Now I turn on the electric current. By turning the 
 stop-cock I am able to stop it suddenly, and imme- 
 diately you notice the water escapes through that 
 side tube. If I now open the stop-cock there will 
 naturally be a small leak, through which a very 
 little water trickles ; but if I suddenly stop the 
 main flow of water, then the pressure is obviously 
 increased momentarily to a very large extent, pro- 
 ducing a spurt of water followed by a steady stream. 
 That is an instructive illustration of the high elec- 
 tromotive force which is generated when we suddenly 
 break the circuit. 
 
 I have been introducing to your notice another 
 analogy that of twisting bodies as illustrating a 
 line for the transmission of power or for any other 
 purpose. I will show you now what the dielectric 
 strength of this glass rod is. I have here (Fig. 19) 
 a flywheel, suspended by the glass rod, to which 
 I give a somewhat rapid rotation and then 
 a sudden stop [illustrating] , and you see that 
 the dielectric strength of my conductor was not 
 sufficient to stand the strain, and it has broken 
 down. The strain is produced by the inertia 
 or momentum of the flywheel that is, by the 
 self-induction of, say, the transformer. I will 
 now put in another rod, and show you how we 
 oan prevent these disastrous effects from happen- 
 ing, and that is simply by never breaking the 
 circuit suddenly. By putting in a gradually increas- 
 ing resistance, the momentum of the flywheel is 
 not able to exert its full impulse instantaneously, 
 .and we have a gradual slowing down, and the 
 
 E 2 
 
60 ALTEENATING AND INTEBEUPTED 
 
 dielectric is able to stand the' strain. To illustrate 
 this, after I have the wheel in rotation, I simply 
 have to grasp it with the fingers and gradually 
 bring it to rest in about a second of time. I have 
 brought it to rest without the slightest undue 
 strain upon the insulation. 
 
 That means that, with large and powerful plant,. 
 in order to prevent the too sudden breaking of 
 the circuit, you ought to have a switch which will 
 enable you to gradually insert a resistance before 
 you break the circuit altogether. Then the whole 
 danger to the insulation of the system would be 
 done away with entirely. 
 
 Now, I have given you these two analogies 
 because my own experience is that the more 
 analogies we have, the greater the help it is to us. 
 I know myself that I have gained the greatest 
 assistance from always putting such problems into- 
 a mechanical form ; it assists one to foresee results 
 that may happen, and enables one to apply one's 
 mathematics to it. I do not wish it to be under- 
 stood that by these analogies one can get rid of 
 mathematics. One can illustrate the qualitative 
 results extremely well, but when we come to the 
 quantitative results our true servant is mathe- 
 matics. 
 
 Transformer on Open Circuit. Lag of Current. 
 
 When we are sending an alternating current 
 through a transformer there are two cases that 
 may be considered : one where the power is all 
 
ELECTEIC CTJEKENTS. 61 
 
 being used up, the secondary of the transformer 
 having a full supply of lamps upon it, for instance; 
 and the other where there are no lamps on it, 
 where the secondary is not using up the current at 
 all. In the latter case there is still a sensible current 
 going through the primary of the transformer, and 
 the question is, what becomes of it ? Are we using 
 up a large amount of power in the transformer 
 when the lamps are not on ? No, we are not using 
 a large amount ; we are using a certain amount. 
 The counter electromotive force created by the 
 self-induction of the primary of the transformer 
 mitigates this effect when the transformer is doing 
 no work, as then the electromotive force that is 
 applied to the primary is in opposite phase to the 
 opposing electromotive force of self-induction. The 
 current, which follows very nearly the phase of 
 the impressed electromotive force when the trans- 
 former is doing full work, now, no work being 
 done, lags behind the electromotive force, which 
 still further reduces the power consumed in the 
 transformer on open circuit. 
 
 What is meant by the lag of the current behind 
 the electromotive force? I have a model here 
 (Fig. 20) which might have been better designed if 
 I had had more time to do it, but it will illustrate 
 what I want to show you. I have a wheel sup- 
 ported on an axis, and I have two weights, one 
 of which in descending causes the wheel to turn 
 in one direction, the other in the opposite direction. 
 What I wish to show you is that when I stop 
 allowing one of these forces to act, the momentum 
 
62 
 
 ALTERNATING AND INTERRUPTED 
 
 of the wheel will still carry it round in the 
 same direction in which it was going ; although an 
 opposing electromotive force is acting, the wheel 
 continues to rotate in the same direction. You will 
 see the analogy which I wish to be conveyed to 
 you. Now I apply the electromotive force, or 
 weight, on. the right, and it sets the wheel in 
 motion. Now I stop it, and you notice that the 
 other weight still goes up for a sensible time that 
 
 is to say, the current lags behind the electromotive 
 force. I might put an alternating electromotive 
 force on first in one direction and then in the other, 
 but the momentum of the flywheel would always be 
 pulling up during a portion of the time against the 
 weight. 
 
 Now, I have a very interesting experiment to 
 show you illustrating this lag of the current. It is 
 one which was designed in Prof. Ayrton's laboratory 
 a few years ago by Mr. E. W. Smith, and is a very 
 beautiful experiment. I have here (Fig. 21) two 
 
ELECTEIC CURRENTS. 
 
 63 
 
 55-volt lamps connected in series, which will be fed 
 from an alternating source of 100 volts. I shall 
 pass an alternating current through them, and you 
 will see that they light up rather dimly. After 
 that I shall put this high-resistance self-induction 
 coil as a shunt upon one of the lamps, and I shall 
 put on the other lamp a high-resistance non- 
 inductive shunt. The difference between the two 
 conditions of the experiment is this : In the first 
 
 place, I pass an alternating current through the 
 two lamps in series, and I get a very low illumi- 
 nating power. I then give a by-path across the 
 one lamp containing self-induction, and a non- 
 inductive by-path across the other, and you will 
 see that the lamps increase in brightness. [Experi- 
 ment.] The reason is that we have to deal with 
 an additional source of electromotive force besides 
 the main town supply which I am using. This 
 electromotive force is also tending to set up electric 
 currents, and lags behind the phase of the electro- 
 
64 ALTERNATING AND INTERRUPTED 
 
 motive force which is supplied from the mains ; 
 consequently we will have these two lamps lighted 
 up by two currents in different phases, and one 
 will attain its maximum when the other is near 
 its minimum. Of course, we do not see this, 
 because the maxima and minima come a hundred 
 and fifty times a second. So that really by 
 affording by-paths to these two lamps we are able 
 to produce a greater illumination than when the 
 whole current is going directly through them. Of 
 course, we are drawing much larger currents from 
 the mains in this case than we were before, but 
 by introducing these coils we are actually able to 
 increase the brightness of the lamps. 
 
 The lag of the current behind the electromotive 
 force is a cause of many curious phenomena, which 
 have been worked out by Prof. Elihu Thomson, of 
 America. Here I have an electromagnet, which I 
 am going to magnetise with an alternating current. 
 It consists of a bundle of iron wire surrounded by 
 a coil. I will show you that the electromagnet 
 produces a repulsion upon conducting bodies 
 brought into its neighbourhood. Of course, the 
 variations of the electromagnetism due to the alter- 
 nating current will clearly set up induced currents 
 in a ring or any piece of metal brought near to it. 
 But one would have naturally thought that these 
 currents would be strongest just at the moment 
 when there is no magnetism at all in the core, 
 and that therefore they would act sometimes 
 positively and sometimes negatively, that sometimes 
 there would be repulsion and sometimes attraction, 
 
ELECTRIC CURRENTS. 65 
 
 and of equal amounts; and that would be the case, 
 if there were no lag in the current. That the 
 reverse is the case, however, is because the current 
 lags behind the electromotive force induced by the 
 lines of force from the electromagnet. The attrac- 
 tion or repulsion, say, on a ring, at any moment, 
 being proportional to the product of the strength 
 of the magnetic field into the strength of the 
 induced current, these former would be of equal 
 amount in an alternation, if there were no self- 
 inductiou in the ring, and hence no lag, the induced 
 currents being 90 degrees later in phase than the 
 magnetic field. But if there is self-induction in 
 the ring, as of necessity there must be, then the 
 induced currents occur more than 90 degrees later 
 in phase than the magnetic field, and then the 
 products of the strengths of the two at every 
 moment during an alternation show that there is a 
 marked preponderance of repulsive over attractive 
 impulses ; the attraction has become comparatively 
 insignificant. The most striking experiment is with 
 an aluminium ring, because of its good conductivity 
 and lightness (Fig. 22). You see, as soon as I put 
 on the current it throws the ring into the air to a 
 considerable height. 
 
 I will now show you the same experiment with 
 a copper ring, which is much heavier, but is an 
 extremely good conductor. You will see that it will 
 not rise quite so high as the aluminium ring 
 did. [Experiment.'] 
 
 I will next show you that with worse conducting 
 materials the effect does not take place to the same 
 
66 
 
 ALTERNATING AND INTERRUPTED 
 
 extent. This is a zinc ring, and you will find that 
 it will not rise to so great a height. [Experiment.] 
 I have here a brass ring, which is even a worse 
 conductor still, and it may possibly be able to 
 topple over the edge of the electromagnet, but that 
 is about all. [Experiment.'] 
 
 Here is a ring (Fig. 26) held by two threads to 
 a piece of wood, which I can keep floating above 
 the magnet. [Experiment.] The ring, of course, 
 gets intensely hot in a very short space of time, 
 owing to the enormous power that is being 
 expended in it. 
 
 There are one or two other experiments I can 
 show you. Here (Fig. 23) we have a gyrostat, which 
 consists of an iron wheel, with a thick copper 
 tyre, which becomes magnetised by the induced 
 currents. The axis is supported on a copper frame- 
 
ELECTBIC CUEKENTS. 
 
 67 
 
 work that surrounds it, and that causes the lag of 
 the current to become operative, and a continuous- 
 rotation to be set up in the gyrostat. Here, then, 
 we have an electro-gyrostat driven by this curious 
 force of Prof. Thomson's. He has devised a great 
 number of different methods of illustrating this, 
 from which different results may be obtained. I 
 am able to illuminate this lamp (Fig. 24), but I 
 can screen it off by means of this plate of copper. 
 
 It is difficult to pass it between the two coils- 
 because the repulsive force is so great. [Experi- 
 ment.] If that had been of a bad conducting 
 material it would not have done it so well, but 
 being a good conductor it screens off the lines 
 of magnetic force, and prevents them going through 
 the coil. The lines of magnetic force, instead of 
 going straight up, are bent round. 
 
 Finally, I will show you the effect of trying to 
 
68 ALTERNATING AND INTERRUPTED 
 
 bring"* a plate of copper upon the pole (Fig. 25). I 
 cannot strike the pole : it always rebounds from 
 
 it owing to the repulsion caused by the induced 
 currents. [Experiment.] 
 
ELECTRIC CURRENTS. 69 
 
 LECTURE HI. 
 
 Sometimes at the close of a discourse the lecturer 
 finds that he has miscalculated his time, and has 
 to hurry through things more rapidly than he 
 intended, so to prevent this to-day I shall begin 
 by saying a few words which I should have said 
 at the end of the lecture but for the reason just 
 stated. In the course of these lectures I have 
 been able to show you some interesting experi- 
 ments in different branches of electricity, and also 
 certain curious phenomena connected with them. 
 I wish, in the first place, to say that in all these 
 experiments there is nothing original or new that 
 they are simply illustrating well-known principles ; 
 and, secondly, that I have to express my great 
 indebtedness to all those who have assisted me 
 in the matter and especially I feel that I owe 
 a great deal to Mr. Campbell Swinton, who has 
 put at my disposal this induction coil and much 
 of the apparatus which you see before you, some 
 of which I shall be using to-day. I wish, also, to 
 express my indebtedness to Dr. Alexander Muir- 
 head for the use of the different illustrations in the 
 matter of the submarine cable, and also to my 
 assistant (Mr. Mitchell), Mr. Stanton, and Mr. 
 Edgar, who have made the work of preparing these 
 
70 ALTERNATING AND INTERRUPTED 
 
 lectures quite contrary to what it usually is a 
 pleasure instead of a labour. 
 
 If in any of the illustrations that I have given 
 you of the analogies between electrical phenomena 
 and mechanical phenomena I have introduced any- 
 thing different from what you have been accus- 
 tomed to, it is simply the result of a natural interpre- 
 tation of the mathematical formulae that represent the 
 phenomena. The same mathematics apply equally 
 well to mechanical phenomena and to electrical 
 phenomena, and it seemed to me a suitable thing 
 on this occasion not to use mathematics to illus- 
 trate electrical phenomena, but to use mechanical 
 phenomena to do so. 
 
 Practical Utility of Analogies. 
 
 Now about these analogies which I have used. 
 I should like to impress upon you before we part 
 that there is more in these analogies than the mere 
 rough resemblance which I have tried to point out 
 to you in the course of these lectures, and those 
 who are more deeply interested in the applications 
 of electricity will easily see that there is room for 
 a great deal of thought in carrying out these 
 analogies, and in making clearer to their minds the 
 processes which are going on in certain electrical 
 phenomena. 
 
 In the table which I showed you last lecture I 
 gave you an example of the methods of measuring 
 the actual units when we are using the twisting 
 of any rod or other mass to represent electric 
 
ELECTEIC CUERENTS. 71 
 
 currents, and I wish to say that if in the case 
 of any electrical problem occurring to an engineer 
 such, for example, as the propagation of electri- 
 city along a cable from Niagara Falls to Buffalo 
 he wishes to see whether troubles will arise in 
 the electrical phenomena, if he will make an imita- 
 tion cable on the lines of assuming those units 
 which are in the table, he will be able to imitate 
 the action of what will go on in the real cable, and 
 he will be able to see whether any peculiar 
 mechanical effects are produced, and to trace their 
 analogy with the electrical effects, and so be able 
 to see what kind of troubles he is likely to meet 
 with. And this I consider to be a matter of very 
 great importance, but it was with the object more 
 of throwing out suggestions than of working out 
 the analogies in a complete form that I introduced 
 them in the present course of lectures. The work- 
 ing of them out in a complete form would not 
 have been a suitable subject for these lectures, 
 but I hope I may have time on another occasion 
 to work some of them out more fully, and so to 
 render some assistance to those who are engaged 
 in such work. 
 
 Further Alternating-Current Phenomena. 
 
 In the first lecture I was dealing chiefly with 
 capacity and the influence which it has upon 
 alternating or interrupted currents, and the most 
 interesting illustrations of the effects of capacity 
 were naturally found in the submarine cable. Last 
 
72 ALTERNATING AND INTERKUPTED 
 
 lecture I was dealing chiefly with self-induction, 
 and I showed you that there were certain troubles 
 which were likely to arise in cases in which 
 we used self-induction, and I explained to you 
 that this would be so with both kinds of currents 
 whether we were using continuous currents or 
 alternating currents ; that in either case, if you 
 have much self-induction in the circuit, by break- 
 ing the circuit suddenly you are able to get far 
 larger electromotive forces than those you are 
 impressing upon the system, and that these may 
 be destructive to the insulation of the system. 
 The only difference is that, in the case of alter- 
 nating currents, the self-inductions that we have 
 for example, in the transformers are enormously 
 greater than those existing when using con- 
 tinuous currents ; and, therefore, the importance of 
 this question becomes clearly greater in the prac- 
 tical application of alternating currents, and espe- 
 cially in the practical application of them on a 
 gigantic scale, as in the case of the utilisation of 
 the Niagara Falls, where the transformers are of 
 enormous magnitude, and the self-induction very 
 great indeed. In such a case as that, it would be, 
 I might almost say, criminal to suddenly break 
 the circuit, the danger would be so great. 
 
 Now, to-day, passing from these two subjects, I 
 am going to deal with three other peculiarities 
 which arise in the transmission of electric currents, 
 especially alternating or interrupted currents skin 
 resistance, resonance, and oscillations. All of these 
 are interesting phenomena from a purely scientific 
 
ELECTKIC CUEBENTS. 73 
 
 point of view, and are of far more importance from 
 an engineering point of view. I mentioned in the 
 course of the last lecture that a great many of 
 the difficulties presenting themselves in discussing 
 an engineering problem can be got over by reduc- 
 ing the frequency of the alternations. When I 
 first reported to the people who are utilising 
 the Niagara Falls, in the year 1890, now some 
 five years ago, as to the proper means of utilis- 
 ing them, I urged the adoption of the system 
 we are now introducing ; and on noticing those 
 various troubles that might arise, I gave the best 
 solutions of them that seemed available. Since 
 that date the enormously valuable suggestion 
 occurred to me of lowering the frequency of the 
 alternations, and that, together with the other 
 precautions that have been introduced, has taken 
 away all the danger of any trouble that we might 
 otherwise have expected from the use of alternating 
 currents with such a system. 
 
 Induction with very High Frequency and Electro- 
 motive Force. 
 
 When we come to very high frequencies, effects 
 are greatly increased ; and by means of Mr. 
 Campbell Swinton's apparatus which I have been 
 employing I am able to get very high fre- 
 quencies indeed. By using an oscillatory discharge 
 (about which I shall speak more fully later on) 
 which oscillates at very rapid frequency, and 
 also by using high electromotive force in this 
 
 F 
 
74 
 
 ALTERNATING AND INTERRUPTED 
 
 discharge, I am able to get effects which could not 
 be got by an ordinary alternating current. At the 
 same time, by observing the effects with these 
 very high frequencies, we are able to grasp what is 
 going on at the lower frequencies, though not to 
 such a serious extent. 
 
 ' I will now show you the effects of mutual 
 induction between two circuits, which can be 
 obtained by means of very high frequencies and by 
 very high electrical pressure great voltage. I am 
 going to employ the two copper wire hoops (Fig. 27), 
 measuring about 6J or 7 feet in diameter, which 
 are standing in front of me, and I am going to 
 pass through one of them an oscillatory discharge 
 of very high frequency indeed and of very high 
 
ELECTEIC CUEEENTS. 75 
 
 electrical pressure. Now, everyone who is accus- 
 tomed to seeing cases of mutual induction will be 
 perfectly well aware that if we are simply passing 
 an alternating current through one of these wires, 
 it would be impossible to induce much current in 
 the other wire. I wish to show what a considerable 
 amount of power can be developed when the 
 electromotive force is sufficiently high and the 
 frequency sufficiently great. And that is what 
 obtains when we use the oscillatory discharge of 
 Mr. Campbell Swinton. I take away from the appa- 
 ratus one of the hoops, to which I have attached 
 an incandescent lamp at the top. I will immediately 
 cause a high-frequency oscillatory discharge to pass 
 round the other, and I wish to show you that 
 even at a considerable distance the current which 
 passes through the wire which I am holding is 
 sufficient to light the incandescent lamp at the top. 
 [Experiment.] At the present moment the illumi- 
 nation is not visible. I approach the hoop nearer 
 to the other. At this distance it is very dull indeed, 
 but preceptible. I now approach to within a 
 couple of feet, and you see it rises to its full 
 brilliancy. That is simply due to the induced 
 current which is going round this wire of a 
 diameter of 7 feet. I can twist it round in different 
 directions, almost at right angles to the fixed one, 
 and still get a certain amount of illumination, but 
 when it is completely at right angles it goes out. 
 When I turn further round and the two wires are 
 once more parallel, the brightness is as great as it 
 was before. 
 
 UNIVERSITY 
 
76 ALTERNATING AND INTERRUPTED 
 
 Lighting Lamp by Current passing through a 
 Person's Body. 
 
 Now, I intend to take some of the same appa- 
 ratus which I have been using here to show you 
 another very remarkable effect of these very high- 
 voltage experiments. When we use very high 
 electric pressures we require a much smaller current 
 to produce the energy, which lights up an electric 
 lamp, for instance; and if we send very high electric 
 pressures through a lamp with intervals of rest, 
 when the current is not passing, we are able to 
 produce a very large current for a small fraction of 
 time, after an interval another very large current, 
 and on the whole the total quantity of electricity 
 which has passed through to produce the lighting of 
 the lamp will not be so great as the total quantity 
 which would have passed through if we had had an 
 ordinary continuous or alternating current passing 
 through the lamp. The effect which takes place on 
 the lamp is measured by the square of the current. 
 Let us take two cases first, of a lamp with the 
 current continuously going through it : we have a- 
 certain amount of electricity passing through the 
 lamp; in the second case, if we pass it for one-tenth 
 of a second, each second of time, and pass ten 
 times as much current through it, we would get one 
 hundred times as much effect during that tenth of 
 a second : therefore you would get ten times as 
 much effect on the whole. It follows, then, that 
 when we increase the maximum current during a 
 
ELECTRIC CURRENTS. 77 
 
 short period we are increasing the energy which 
 is supplied to the lamp during that period far more 
 than we are increasing the current. It appears, 
 therefore, that by acting in this way we are able 
 to pass a far smaller sum total of current through 
 the lamp, and yet to get the same amount of energy 
 consumed in the lamp. This is probably the reason 
 why an electric lamp can be lighted by a current 
 passing through a person's body, as is possible with 
 such apparatus as is before you. The amount of 
 current which I am able to stand without injury 
 from hand to hand is certainly less than l-100th of 
 an ampere ; and if I can show that a lamp can be 
 lighted through my body, I know that there is not in 
 the energy more than 1 -100th of an ampere passing 
 through the lamp, and yet if I bring the lamp up 
 to incandescence a lamp that ought to require 
 one-fifth of an ampere I must assume that the 
 energy is the same, but that the total quantity of 
 electricity is not the same ; and that is what I 
 wish to show you is the case with this apparatus. 
 Now, this is an experiment which everybody does 
 not like to try, so I will ask Mr. Campbell 
 Swinton, who has arranged it, if he will assist 
 me in taking the current through the body. There 
 is really not the slightest trouble, or danger, or 
 inconvenience in doing it. I showed you in the 
 first lecture some experiments of the same sort. 
 There it was perfectly obvious that the electricity 
 which was lighting the lamp was not passing 
 through the body, but in this case it is perfectly 
 obvious that the electricity is passing through the 
 
78 
 
 ALTEENATING AND INTEEEUPTED 
 
 body (Fig. 28). [Experiment.] The electricity 
 coming from that coil is certainly going through 
 both of our bodies, and we can lower it to any 
 amount we please, or raise it up to its full incan- 
 descence, by cutting out more or less of the impe- 
 dance coil, shown across the centre of the diagram. 
 Whether the explanation which I have given you, 
 and which Mr. Campbell Swinton supports, is the 
 
 Operator 
 
 \ 
 
 Q 
 
 Sc.p. 100 r 
 T 
 
 Operator 
 
 i 
 
 Fig. 28. 
 
 //jcfuct/on Co/7 
 
 complete explanation or not I cannot tell you. It 
 seems to go a long way towards explaining it. 
 Passing the current through the body is a very 
 different thing to receiving it, using yourself as a 
 Leyden jar, as I did in the first lecture. There,, 
 there is no sensation whatever ; you do not feel the 
 current at all. In this case both Mr. Campbell 
 Swinton and I felt the current passing through 
 our bodies and it was nearly as much as one 
 
ELECTRIC CURRENTS. 79 
 
 cared to take comfortably, and one feels a certain 
 amount of warmth, but I do not think that that is 
 the true calorific value which is imparted owing to 
 the energy used up in one's body ; but it certainly 
 does warm one. 
 
 Skin Resistance. 
 
 I now wish to introduce to your notice a subject 
 which has attracted considerable attention during 
 the last eight or nine years of the use of alter- 
 nating or interrupted currents. I speak of what 
 has gone by the name of " skin resistance." The 
 fact has certainly been proved and it could not be 
 otherwise, from the mathematical treatment of the 
 question that, when we pass a high-frequency alter- 
 nating current through a conductor, the flow of 
 current is not uniform throughout the section of 
 the conductor. It is, partially at least, confined to 
 the exterior layers of the conductor. At the centre 
 there is not so much current passing as there 
 is through an equal section of the outside, and 
 therefore the resistance of the conductor is com- 
 posed more of the resistance of the external parts 
 than of the central part. If we make the 
 frequency high enough, we can cause the current to 
 cease flowing through the central part entirely. In 
 that case, the material which is offering resistance 
 is simply a tubular conductor and not a solid con- 
 ductor. That is the meaning in which the term 
 " skin resistance " must be read. 
 
80 ALTERNATING AND INTERRUPTED 
 
 It was in the presidential address to the Institu- 
 tion of Electrical Engineers in, I believe, the year 
 1886, that Prof. Hughes first introduced this 
 seriously to the notice of electricians. I do not 
 mean to say that some of the effects had not been 
 observed before, and I do not say that it was not 
 known that there might be difference of flow in 
 different parts of the section of the conductor, but 
 that was the first date when this resistance was 
 proved to consist of a pure, definite resistance, and 
 was measured in ohms as a resistance by Prof. 
 Hughes. Up to that time people had been per- 
 fectly well aware that, in using the Wheatstone 
 bridge, if you kept the key of the galvanometer (a 
 deflection of which indicates divergence from the 
 resistance) down, and interrupted the circuit from 
 the battery, you could not get a true reading. 
 Generally, this was ascribed to self-induction, which 
 at the time of breaking the circuit to the electric 
 battery made itself apparent, but which to a far 
 greater extent, except when you were using coils, 
 was often due to what Prof. Hughes showed to 
 be really an increase of the resistance ; and that 
 increased resistance is due to the electric current 
 being at the outside instead of being uniformly 
 distributed through the section of the conductor 
 which is carrying the current. Well, the ex- 
 planation of this is not difficult to seek. You 
 remember in the last lecture I had two wires of 
 about 7 feet long parallel to each other. I 
 passed an electric current through one, and at 
 the moment of making that circuit the other, 
 
ELECTRIC CURRENTS. 81 
 
 which was completed through the galvanometer, 
 showed an induced electric current, and at the 
 moment of breaking the primary current the circuit 
 connected with the galvanometer again showed an 
 induced current, but in the opposite direction to 
 that previously exhibited. Thus, with an alternating 
 current, at the moment when a change in the 
 direction of the electric current takes place, there 
 is a reverse effect due to mutual induction. Now, 
 suppose that this rod on the table (which is merely 
 for illustration) consisted of a bundle of wires, and 
 I insulated and separated out the central wire, 
 and also insulated and separated out one of the 
 external wires, and then sent through the whole 
 bundle an electric current, there would be an 
 induction current in both of those which I have 
 selected and insulated ; there would be in those an 
 opposing electromotive force, a force tending in the 
 opposite direction to that in which the primary 
 current was going. But if you will notice the posi- 
 tion of all the wires in the bundle relatively to 
 one of them on the outside, and also notice the 
 position of them relatively to the central wire, you 
 will see that the central one has far more wires 
 within a short radius of it than the external one 
 has. Therefore the opposing force on the central 
 wire is much greater than the opposing force 
 on the external one, so that, when I suddenly 
 pass an alternating current through a bundle 
 of wires, at each change in the direction of the 
 current there is an opposing force in both these 
 wires, but the opposing force in the central wire 
 
82 ALTERNATING AND INTERRUPTED 
 
 will be much greater than that in an outside 
 wire. That is to say, there is an opposing force 
 in each of the wires of the bundle, but a greater 
 opposing force in the central ones, and therefore 
 the current will not flow so powerfully there a& 
 it does at the outside. If, therefore, we have the 
 whole bundle grouped into a solid conductor, the 
 central line has always an opposing electromotive 
 force which is greater than the opposing electro- 
 motive force at any other portion of the section. 
 Therefore the current is mainly passing through 
 the external part of the conductor, and that causes- 
 an increase of resistance, which was actually 
 measured by Prof. Hughes by means of the 
 ingenious apparatus he produced on the occasion 
 to which I have referred. 
 
 Here is a brass tube, with which, however, it 
 would take some time to show you the effect,, 
 because it needs a very sensitive galvanometer,, 
 and it takes some time to bring it to rest. Prof. 
 Hughes showed that the additional resistance pro- 
 duced by an iron conductor was much higher than 
 a conductor made of any other material. There- 
 fore, I will proceed by using an iron tubular con- 
 ductor (Fig. 29) with a wire supported through 
 its centre, and another insulated wire lying on it& 
 surface, and I wish to show you the different 
 effects of induction on these two. I will begin 
 with the outside wire, which is subjected to- 
 the induction at varying distances up to the 
 diameter of the tube. There will only be a deflec- 
 tion in this case of two or three divisions of the 
 
ELECTRIC CUEEENTS. 8$ 
 
 scale on the screen, so you will please watch 
 carefully and make the measurements for y ourselves. 
 [Experiment.} There, you see, it goes nearly up 
 to the second division. I passed the current through 
 this mass of iron, and the induced current on the 
 outside wire was sufficient to deflect it through 
 about one and a half divisions of the scale. Now 
 change the connections, and you will see the great 
 difference there is. [Experiment.] This time you 
 see that the deflection passed along nearly the 
 whole length of the scale. That is the deflection 
 
 obtained with the wire which passes through the- 
 centre of the tube, showing the enormous differ- 
 ence of the electromotive force in the two parts. 
 This enormous effect in the case of the iron is 
 due to the fact that the iron becomes magnetised 
 by the passage of the electric current through it 
 it becomes circularly magnetised. Lines of force 
 cut the wire which is suspended in the middle 
 of the tube, and in cutting it they exert a 
 force greater than the electromotive force that 
 there would be if there had been no iron for 
 
84 ALTERNATING AND INTERRUPTED 
 
 example, if the iron had been replaced by copper, 
 through which you cannot get so great an amount 
 of magnetism so easily. This illustrates the 
 general principle of skin resistance. I may 
 say that in actual practice an engineer is 
 seldom troubled with this except when he is 
 coming to very large works where the cables are 
 very thick indeed. With all the cables that we 
 have been accustomed to in industrial work in 
 this country there should be no trouble whatever ; 
 and it is only when you come to cables of one 
 or two inches in diameter that the trouble would 
 -come to be serious. But when once you lower 
 the frequency as I have done on the other side 
 of the Atlantic, this trouble disappears entirely. 
 Up to any size that we wish to use for our 
 conductors there, there is not the slightest trouble 
 from the extra resistance produced in this way. 
 
 I can bring before you some other illustrations 
 of the same sort to show you that the skin resist- 
 ance may become very high indeed, and may even 
 almost prevent an electric current from passing 
 through a good solid conductor. This is really an 
 explanation of the experiment which was made by 
 Faraday so long ago, in which he had a wire 
 going round the room and separated by a few 
 inches where the two ends came together, into 
 which an electrical discharge from a powerful 
 Ley den jar was sent at the two ends, when the 
 discharge preferred to cross the air-space rather 
 than to travel round the copper conductor, although 
 that was an excellent conductor. That was owing 
 
ELECTRIC CURRENTS. 
 
 85* 
 
 to the suddenness of the discharge, and the experi- 
 ments I am going to show you are illustrative of 
 the same fact. To show you it in a more complete 
 manner, perhaps, I will take a copper wire arch 
 (Fig. 30) about 6 feet high this is one of Mr. 
 Campbell Swinton's arrangements for which I am 
 indebted to him and I will send into it an oscilla- 
 tory discharge. I have the means of producing 
 very rapid oscillations in the apparatus before 
 
 
 
 Kg. 3a 
 
 The terminals will be connected with the two* 
 ends of the arch at the base, and there is an 
 incandescent lamp stretched across the two vertical 
 pillars which form the sides of the arch. Any 
 person looking at this in relation to the ordinary 
 method of using currents would say that, when we 
 applied the current, it would all flow through the 
 thick arch and none would flow across this very 
 high-resistance lamp ; but, as a matter of fact, the- 
 
'86 ALTERNATING AND INTERRUPTED 
 
 resistance opposed to these very high oscillations 
 becomes so great that it is very difficult to get 
 the current to go up a short length of this 
 wire, and still more difficult to get it to go right 
 round ; and instead of going right round by that 
 thick conductor, which would offer a certain amount 
 of skin resistance, a great portion of the energy 
 prefers to go across that thin filament, where the 
 skin resistance is not so predominant a feature. 
 [Experiment.] At this height, you see, compara- 
 tively little current is able to go through 
 the lamp. When we bring the lamp lower down, 
 nearer to the terminals, we get more and more of 
 the current able to pass through the conductors 
 up as far as the lamp, and less and less able to 
 go round the rest of the copper arch, and when 
 we bring it down low enough we are able to get 
 the lamp up to its full incandescence. 
 
 Now that I have explained to you the nature 
 of skin resistance, and I have explained how it was 
 produced owing to there being a solid conductor, 
 in order to demonstrate this to you better I will 
 now take a strip of copper instead of a piece of 
 solid circular section. This strip of copper is of 
 exactly the same weight per unit length, it is of the 
 same section, as the wire, and it is of the same 
 conductivity. And what I wish to show you here 
 is that the induction of one part on another is 
 not such a serious matter in the case of the strip, 
 where the distances from the centre of the various 
 parts in a section are greater than in the case 
 of the wire, where they are all very close to the 
 
ELECTRIC CURRENTS. 87 
 
 
 
 central part of the section. We will now put on 
 the current. There, you see that the current is 
 able to pass through the copper strip much more 
 easily at the top than it was before. The light 
 is very dim, but as I gradually lower the lamp, 
 gradually there is more light. The lamp, how- 
 ever, is not yet at its full incandescence, and it 
 is only when I bring it down near to the base 
 of the arch that I am able to get sufficient current 
 through the lamp to bring it to its full brilliancy. 
 The difference between the copper strip and the 
 solid copper rod is most marked. 
 
 Next I will show you the effect with an iron rod, 
 and in this case I have chosen an iron rod of such 
 a section that it has exactly the same conductivity 
 as either the copper rod which I first used or the 
 copper strip which I used in the second place. 
 And here the opposition to the passage of a 
 current is very much greater than in either of 
 the cases which we had before, as will be obvious 
 as soon as I put the thing into action. [Experi- 
 ment.] Gradually, as we lower it, it is coming 
 towards its full brilliancy ; but, you see, when the 
 lamp reaches the same level at which in the first 
 case it arrived at its full brilliancy, it has not 
 arrived at its full brilliancy in this case, simply for 
 the reason that the iron between the lamp and the 
 terminals offers such an amount of opposition that 
 there is very little current able to pass through 
 the lamp even at that height, and we have to 
 lower it very low indeed before it arrives at its full 
 brilliancy. These three experiments illustrate in 
 
88 ALTERNATING AND INTERRUPTED 
 
 
 
 an extremely good way the general character of 
 the phenomena of skin resistance, which not only 
 from a purely scientific point of view are of great 
 interest, but also require careful consideration in 
 dealing with engineering problems. 
 
 Electrical Resonance. 
 
 Passing now from skin resistance, I wish to talk 
 a little I cannot very well show you electrical 
 experiments, although I shall show you experi- 
 ments illustrative of electrical oscillations about the 
 phenomena of electrical resonance, such as the rise 
 in pressure noted some time ago on the conductors- 
 running from Deptford to London, owing to the 
 considerable capacity of the mains used. You will 
 see the distinction I draw between effects of 
 resonance and effects of oscillations. Let me introduce 
 the subject by saying a few words about ordinary 
 resonance. I have here (Fig. 33) a spiral spring,, 
 suspended from the ceiling, to which I can apply 
 a force at regular intervals. And I will show 
 you that when I apply a very small force to the 
 spring in a vertical direction at regular intervals it 
 accumulates the energy, and eventually gives rise 
 itself to a much greater force than that which I 
 had been imparting to it at each individual motion. 
 And also you will see that the velocity of motion 
 after I have repeatedly given an impulse to the 
 spring becomes very much greater than it would 
 be with the same force applied at a single effort.. 
 The reason of this is that the spring is capable 
 
ELECTEIC CURRENTS. 
 
 89 
 
 of vibrating in a perfectly definite period. The 
 period of vibration depends upon two things it 
 depends, first, upon the elasticity of the spring, 
 which tends to draw it back when once it has 
 been displaced; and, secondly, it depends upon the 
 mass or inertia of the spring, which resists 
 
 Fm. 33. 
 
 the movement. These are the only two things that 
 come into play in this spring. There is no friction. 
 It is not in a vessel of oil or anything of that 
 nature which would offer resistance. The electrical 
 analogy is simply a circuit in which there is 
 capacity that is, the elasticity part of it and self- 
 induction that is, the inertia part of it. Just 
 as with the mechanical spring, when we know the 
 
 G 
 
90 
 
 ALTERNATING AND INTERRUPTED 
 
 mass and elasticity of it, we can calculate the 
 period, so, substituting for mass and elasticity the 
 electrical units which I have given in the table 
 (page 42), you can calculate what the period of 
 an electrical vibration is also. [Experiment.] I 
 exert a very slight force upon it at each stroke, 
 and, you see, before I have taken more than two 
 or three strokes its motion is increased to a very 
 considerable extent. The vibration of the spring 
 now in its own period is very violent indeed that 
 
 Timing Fork 
 
 is to say, the current that is flowing through that 
 electrical system is very much greater than that 
 which is due to the electromotive force. That is 
 the real interpretation of the increased velocity of 
 the spring. 
 
 I will now give you another illustration of elec- 
 trical resonance. I take for that purpose a jar 
 (Fig. 31) containing a little water, and also a certain 
 amount of air. The air is capable of vibrating, and 
 if the length of the jar is fixed it is capable of 
 vibrating in a perfectly definite period. When I 
 
ELECTRIC CURRENTS. 91 
 
 cause a tuning-fork to vibrate, and hold it over 
 the mouth of the jar, it sets the air at the upper 
 part of the jar vibrating ; this vibration communi- 
 cates itself to the air in the lower part, and it is 
 then reflected from the bottom just in exactly the 
 same way as that spring was from the ceiling. 
 The air at the top of the jar represents the free 
 end, and the water, being fixed, corresponds to the 
 ceiling. I will now show you that the air in the 
 jar will resound to this tuning-fork, and greatly 
 increase its sound. The period that it will resound 
 in depends upon the length of the column of air. 
 [Experiment.] 
 
 I used then a tuning-fork of low note. Here I 
 have one of a higher note, and you will see that 
 this column of air will no longer resound to it, 
 and we must find a column of air of different 
 length which will do so. I will cause the tuning- 
 fork to vibrate, and then get Mr. Mitchell gradually to 
 fill up the jar with water. As the water is poured 
 in you will find that the sound gradually increases, 
 until when we reach a certain length the resonance 
 is complete. [Experiment.] Knowing the elasticity 
 of the air, and knowing the weight of the air, we 
 can calculate the period just the same as we can 
 in the case of the spring, and so we can in any 
 electrical system where we know the farads and the 
 henrys. As in the cases of the spring and the air, 
 the period depends only on the elasticity and 
 inertia, so in the electrical phenomenon it 
 depends on the capacity and self-induction, and 
 not upon the resistance. The resistance may tend 
 
92 
 
 ALTERNATING AND INTERRUPTED 
 
 to dim the resonance, but does not alter its 
 period. 
 
 As an illustration of ordinary resonance, I will 
 show you an apparatus (Fig. 32) which I got out 
 some time ago for measuring the amount of fire- 
 damp in a coal mine. It consists of a tube, 
 whose length I can vary by means of a wheel, and 
 a tuning-fork which resounds to it. [Experiment.] 
 
 Fy.32 
 
 There is no resonance now ; there is now ; there is 
 none now. I pass through a maximum of reson- 
 ance. I adjust the length of the tube by turning 
 the wheel till I get the maximum, and I read by 
 the index that there is firedamp, or there is not, 
 as the case may be. The elasticity of the firedamp 
 is the same as the elasticity of the air, but the mass 
 that is to be moved is not so great, and therefore it 
 will move more rapidly than a column of air of the 
 same length. 
 
 Electrical Oscillations. 
 Leaving the question of electrical resonance now, 
 
UNIVERSITY 
 
 ELECTKIC 
 
 the last feature of interrupted and alternating 
 currents which I wish to show you is oscillations. 
 In order to illustrate the phenomena, I have pre- 
 pared a model to carry out our analogies. The 
 oscillators which I am going to use consist of a 
 capacity and a self-induction which are so chosen 
 as to give a very rapid period of oscillation, so that 
 effects are produced by an alternating current with 
 
 Compressed air bottle 
 
 FIG. 34. 
 
 these oscillators which could not be produced by 
 the alternating currents alone. I will now show 
 you by means of a simple model what the character 
 of these is. I have a spring which you see here 
 (Fig. 34). It has a natural period of oscillation. I 
 have applied a strip of paper to the piece of 
 clock-spring, so that I can blow against it and 
 produce a disturbance. The velocity of the motion 
 represents the amperes of the current ; and what 
 
94 ALTERNATING AND INTERRUPTED 
 
 I want to show you is that I can get far more 
 powerful currents from this by a broken current 
 than I can by using an undulatory current. I have 
 arranged it so that in the first place I move a jet 
 of air up and down so as to be undulatory in its 
 character. [Experiment.] There, now notice the 
 amount of the motion and the amount of the 
 velocity. You notice the spring moves backwards 
 and forwards as the air jet is moved, and the 
 velocity is comparatively slow. 
 
 Now, I am going to use the same jet of air, 
 but instead of passing it so as to be continuously 
 pressing upon it, but with more or less effect, I 
 am going to pass it across the spring so as to 
 be acting upon the spring only for a minute 
 space of time, and then leave it to its own oscilla- 
 tions. [Experiment.] There, you see the velocity is 
 one hundred times as great as it was before, 
 although the amount of pressure which I am 
 applying summed up over a second is not nearly 
 so great, and yet I get enormously more velocity 
 than I did before, when I was applying an undu- 
 latory current. In the electrical analogy, if I have 
 an oscillating system I am able to produce a far 
 greater current by allowing it freely to oscillate 
 than if I apply a gradually increasing and diminish- 
 ing current to the system. 
 
 As an illustration of this I will show you an 
 experiment with the oscillatory discharge. These 
 oscillatory discharges of high voltages have been 
 used so frequently of late years that I suppose 
 most people have seen them. At the same time 
 
ELECTRIC CURRENTS. 
 
 95 
 
 they are very instructive experiments, and it will 
 take but a few moments to arrange them. I 
 just wish to show you the lighting up of a tube 
 {Fig. 35). The experiment was first shown in this 
 room extremely well by Mr. Nikola Tesla, of 
 America, some few years ago, and it has justly 
 gone by his name ever since. The oscillations 
 are transmitted through my body and are able to 
 light up the tube. [Experiment.] Here, you see 
 the tube lights up through my body, and the 
 oscillations are shown by the intervals of light. 
 
 Osciltator 
 
 FIG. 35. 
 
 It lights up only when the discharge has taken 
 place, and you are thus enabled to see the nature 
 of the oscillatory discharge. 
 
 Experiment by Lord Armstrong. 
 
 I have now come to the end of the experi- 
 ments which illustrate my subjects, and I feel 
 sure that those of you who have listened to these 
 
96 ALTERNATING AND INTERRUPTED 
 
 lectures, and have seen the admirable arrangements 
 which Mr. Campbell Swinton has made, will not 
 be disappointed if I show you another experiment,, 
 using the same apparatus, but not connected with 
 the subject of the lecture. The experiment is a 
 very remarkable one, and was originally due to Lord 
 Armstrong. He first found it out when using 
 a hydro-electric machine in which he got great 
 currents at high pressures by means of a jet of 
 steam. In the arrangements which I am showing 
 you I get the high electromotive force, or rather 
 Mr. Campbell Swinton has obtained it, by using 
 an induction coil and using a continuous current 
 through the induction coil. In most of the 
 experiments which I have been showing before 
 we have used an alternating current, because if we 
 had used a continuous one we could not have 
 interrupted it rapidly enough. In the present 
 instance we are using a continuous current. From 
 experiments I have shown you before, you know 
 that when we make contact with the primary there 
 is an induced current in the secondary, and a 
 reverse one at break. We are able to almost 
 eliminate the current in one direction, and to thus- 
 secure the secondary current almost entirely in one 
 direction; and this is emphasised by having a spark 
 gap which the current has to leap over, and which 
 is wide enough to prevent the spark passing at the 
 make, but not too wide to prevent it passing at the 
 break. This is really an experiment on continuous- 
 currents, and not on either alternating or interrupted 
 currents. 
 
ELECTEIC CUEEENTS. 
 
 97 
 
 In this experiment the apparatus (Fig. 36) is not 
 shown on the screen inverted ; it is shown in the 
 same position as it is on the table in front of the 
 lamp. Here, in a square glass trough, I have a 
 glass bulb, open at the top and open at the 
 bottom, with a piece of cotton wick coming through 
 the narrow neck-like opening at the bottom of the 
 bulb. The wick is at present coiled up in the 
 bulb, and a short end is projecting into the liquid 
 below. The high-pressure electric current is taken to- 
 
 High pressure 
 Continuous current 
 
 FIG. 36. 
 
 the water outside by one terminal, the other terminal 
 being in the water inside the bulb. According as I 
 pass the current in the one direction or the other, 
 so the cotton wick will be seen to move out of the 
 bulb, or to be retracted again into the bulb r 
 and the most remarkable feature about this is that 
 at the same time that the cotton wick is moving 
 in one direction the water is moving in the other, 
 and by the projection on the screen you will 
 actually be able to see the direction in which the 
 water is moving. In order to carry out this experi- 
 
ALTERNATING- AND INTERRUPTED 
 
 ment, the purest distilled water is necessary, and 
 provided everything goes well we ought to see the 
 motions easily. 
 
 I may say that Lord Armstrong has shown this 
 experiment to a few of his friends, and Mr. Camp- 
 bell Swinton has shown it to me and others, but 
 very few persons have seen it. I can only say, in 
 explanation, that it is probably due to the same 
 phenomena as electrical endosmose. If you have 
 a vessel of liquid with a septum of porous material, 
 the terminals of a battery being immersed in the two 
 compartments, there is an action which tends to draw 
 the liquid through the septum, it being probably a 
 mutual action between the porous material and the 
 water. Probably the same effect is exhibited in this 
 experiment ; there is a mutual action between this 
 wick and the water outside, and the mutual 
 action causes the one to be drawn downwards and 
 the other to be pushed upwards. [Experiment.] 
 Now you will notice that the water is rising in 
 the bulb, and the wick is coming out at the 
 bottom. We will reverse the current, and as 
 soon as we do that you will see that the action 
 takes place in the opposite direction, and the 
 wick is going upwards while the water is going 
 out at the bottom. 
 
I 1ST 13 IE IXI. 
 
 A. 
 
 Alternating Currents, 22, 50, 57, 60, 
 71, 75, 81, 93 
 
 Frequency 23, 56, 73-75, 84 
 
 Phenomena 72 
 
 Aluminium Ring Experiment 65 
 
 Ampere, Mechanical Analogy 
 of 42,93 
 
 Analogies Ampere . 42,93 
 
 Atlantic Cable 11, 25, 40, 41 
 
 Coulomb 42 
 
 Electrical Oscillations 93 
 
 Electric Circuit 19 
 
 Henry . . 42 
 
 Incandescent Lamps, etc. . 12 
 
 Lag 61-65, 67 
 
 Mechanical 25-28 
 
 Mechanical, Deductions 
 
 from 38. 39 
 
 Mutual Induction 53-55 
 
 Pressure 12 
 
 Self-induction 55-59 
 
 Submarine Cables 28-33 
 
 Utility of 9 11, 13, 70, 71 
 
 Volt 42 
 
 Armstrong's, Lord, High-Pres 
 sure Experiment 96 
 
 Brass Ring Experiment 
 Breaking Circuit 
 
 59 
 
 C- 
 
 Cables Current through 11 
 
 Submarine 11 
 
 Submarine, Analogy of 28-33 
 Submarine, Working Tor- 
 sion Model of 35, 37 
 
 Capacity 19, 21-25, 28 
 
 Chain, Rotating.... 16, 18 
 
 Clerk Maxwell's Work... 39, 40, 53 
 
 Condensers 22 
 
 Copper Ring Experi- 
 ment 65 
 
 Coulomb Mechanical Analogy 
 
 of 42 
 
 Counter Electromotive Force, 
 
 61, 82 
 
 Curb Sender 33,34 
 
 Currents Alternating, 22, 50, 57, 
 60, 71, 75, 81, 93 
 
 Broken 94 
 
 Continuous 20,50,51,96-98 
 
 Induced 66-68,81 
 
 Undulatory 94 
 
 D. 
 
 Discharge Experiments, 
 Faraday's 19 
 
 Discharge, Oscillatory, 
 
 73-75, 94, 95 
 
 Disc, Rotating, Experi- 
 ment .. 18 
 
INDEX. 
 
 Electrical Resonance 88-92 
 
 Electricity, Propagation of 9, 11, 71 
 Electromagnetics, Primary 
 
 Discovery of 43 
 
 Electromagnetism, Variations 
 
 of 64 
 
 Electromotive Force, Counter 
 
 or Opposing, 61, 82 
 
 Energy, Transmitting 49 
 
 Experiments Armstrong's, 
 
 Lord 98 
 
 Capacity 21, 27 
 
 Curbing 32 
 
 Current, Retardation of ... 30 
 Current Lag, etc., 62, 63, 65 67 
 Electrical Oscillations . 94, 95 
 
 Electrical Resonance 90, 92 
 
 Faraday's 43, 84 
 
 with Gramme Ring 47 
 
 with Gyrostats 15,66 
 
 High- Voltage 75-79 
 
 Oersted's 43 
 
 Skin Resistance 83-88 
 
 Smith's, Mr. E. W 62 
 
 Syphon-Recorder 33 
 
 F. 
 
 Faraday, Work of, etc., 22, 43, 
 48, 84 
 
 Farad, Mechanical Analogy of 42 
 Force, Electromotive, Illus- 
 tration of 59 
 
 Frequency 23, 56, 73-75, 84 
 
 G. 
 
 Gramme Ring Experiment ... 47 
 
 Gyrostat, Experiments with, 14. 
 
 15, 66 
 
 Henry, Mechanical Analogy of 42 
 
 Hughes, Prof 34, 80, 82 
 
 Hydro-Electric Machine 96- 
 
 I. 
 
 Incandescent Lamp, Analogue 
 
 of 12 
 
 Experiments with, 23 47 63, 
 75-78- 
 
 Induction 43. 44 
 
 Coil 51, 52 
 
 Mutual 50-55, 74, 75, 81 
 
 Self 18, 44-48,56, 57, 65 72 
 
 Inertia 44 45 
 
 Electrical.. 18- 
 
 K. 
 
 Kelvin's, Lord, Ether Theory 55 
 Views on Power Transmis- 
 sion at Niagara Falls 50 
 
 Lag, Current 61-65, 67 
 
 Lamps, Incandescent, 12 23, 47, 
 
 63, 75-78 
 
 Light, Arc, Experiment with 52 
 
 M. 
 
 Mechanical Analogies, Use- 
 fulness of 13 
 
 Muirhead's, Dr., Cables etc., 10, 
 
 30,69 
 
 Mutual Induction . .. 50-55 
 
N. 
 
 INDEX. 
 
 Syphon-Kecorder 
 
 Niagara Falls... 10, 49, 50, 56, 57, 
 71-73 
 
 0. 
 
 Obvious Mechanical Facts ... 13 
 
 Oersted' s Discovery 43 
 
 Oscillations, Electrical 92 
 
 P. 
 
 Potential, Analogy of Unit of 42 
 Pumps, Centrifugal, Analogue 
 of Pressure by 12 
 
 Resistance Experiment ... 82, 83 
 
 Skin 79, 86-88 
 
 Resonance, Electrical 88-92 
 
 S. 
 
 Self-induction, 18, 44 48, 56, 57, 72 
 
 Shunts 63 
 
 Smith's, Mr. E. W., Experi- 
 ment 62 
 
 Spring, Period of Vibration 
 
 of 88, 89 
 
 Swinton's, Mr. Campbell, Ap- 
 paratus, 
 
 10, 69, 73, 75, 77, 78, 85, 96 
 
 ill. 
 33 
 
 Tesla'e, Mr. Nikola, Experi- 
 ment 95 
 
 Thomson's, Prof., Force ... 64, 67 
 
 Transformers 48, 49, 60, 61 
 
 Transmission 72 
 
 Transmitter, Cable ... 33 
 
 U. 
 
 Units, Electrical Analogies of, 
 Compared with Mechanical, 
 
 41-44 
 Measuring ... 70 
 
 Volts, Mechanical Analogy of 42 
 
 W. 
 
 Water Analogue of Electrical 
 
 Pressure 12 
 
 Wheatstone Bridge 80 
 
 Zinc Ring Experiment ........ . 66 
 
 UNIVERSITY 
 
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 Alternating and Interrupted Electric Currents. 
 
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 Economics of Iron and Steel. By H. J. SKELTON. 
 
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 IN THE PRESS. 
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 Central - Station Management. By J. HESKETH r 
 
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MUNICIPAL ENGINEERS' SERIES. 
 
 Refuse Destructors, with Results up to Present Time. 
 
 Second and Revised Edition. A Handbook for Municipal Officers. 
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