GIFT OF 
 E.P.Lewis 
 
 tuWfil 
 
MODERN VIEWS 
 
 OF 
 
 ELECTRICITY 
 
NArURE SERIES 
 
 MODERN VIEWS 
 
 OF 
 
 ELECTRICITY 
 
 BY * i ' < r " J ' . 
 
 OLIVER ]. L^ODGE, D.Sc., LL.D, F.R.S. 
 
 Professor of Experimental Physics in University College, Liverpool 
 
 WITH ILLUSTRATIONS* 
 
 Jtonbon 
 MAC MILL AN AND CO. 
 
 AND NEW YORK 
 1889 
 
 The Right of Translation and Reproduction is Reserved 
 
> o / 6 
 
 *s 
 
 Ri5HA%i> CL!Y AND SONS, LIMITED 
 
 
 lto>ft>ON AND BUNGAY. 
 
ADVERTISEMENT. 
 
 THE object of this work is to explain without technicalities, and 
 to illustrate as far as possible by mechanical models and analogies, 
 the position of thinkers on electrical subjects at the present time. It 
 deals particularly with that view of electrical theory which is specially 
 associated with the names of Clerk-Maxwell and Sir William Thomson, 
 and it aims at going as far as possible into the most recondite portion 
 of the subject, explaining what is known of the nature of electricity, 
 but entirely without the use of mathematics. The subject is divided by 
 the author into four parts : (i) Electricity under strain, or Electro- 
 statics ; (2) Electricity in Locomotion, or current Electricity ; (3) 
 Electricity in Whirling Motion, or Magnetism ; (4) Electricity in 
 Vibration, or Radiation, commonly called Light. The propagation of 
 electro -magnetic waves through space, the nature of what is called 
 self-induction, the physical meaning involved in the terms magnetic 
 permeability and specific inductive capacity, are all fully treated and 
 explained by mechanical illustrations. The discoveries of Faraday, 
 of Kerr, and of Hall ; the velocity of electro-magnetic disturbances ; 
 the new discoveries relating to the transmission of electrical energy 
 through the insulating medium instead of through a conductor, and 
 other similar views associated to some extent with the names of 
 Poynting and Heaviside ; the magnetic observations of Ewing, the 
 most recent discoveries of Hertz ; are all treated and explained in 
 a more or less full and popular manner. The treatment is adapted 
 to that large class of persons who, having some acquaintance with the 
 ordinary facts and phenomena of electrical science as detailed in the 
 usual text-books, find some difficulty in perceiving the -theoretical 
 bearing of the whole, or in reading the works of the masters of science. 
 The book begins by assuming an elementary knowledge of facts, 
 gradually develops the " incompressible-fluid" idea of electricity, and 
 thence leads on slowly to some of the most recent speculations and 
 opinions concerning the structure of Ether, the nature of Light, the 
 conceptions of Electricity, of Elasticity, and of Matter, and the rela- 
 tionship existing between them. It thus aims at placing its readers on 
 a higher platform whence they can follow the still further progress 
 which in our own day is being so rapidly accomplished in these difficult 
 branches of Natural Science. 
 
 6G5408 
 
PREFACE. 
 
 THE doctrine expounded in this book is the etherial 
 theory of electricity. Crudely one may say that as 
 heat is a form of energy, or a mode of motion, so 
 electricity is a form of ether, or a mode of etherial 
 manifestation. 
 
 This doctrine is led up to by gradual stages, and 
 the explanations in Part I. do not aim at the same 
 fulness of detail as those in Part III. or IV. Since 
 the book is intended to be useful to the higher class 
 of students it seemed very permissible to adopt a 
 method which I always use in teaching, viz. to 
 begin by giving some ideas at first, and to gradually 
 polish them up later, rather than by attempting a too 
 highly finished statement ab initio to overburden and 
 depress, and possibly to confuse, a student. Because 
 of this progressive arrangement I may be permitted 
 
viii PREFACE. 
 
 to urge students to read the book through before 
 proceeding to dip into it by help of the index 
 and before taking notice of references forward, which 
 subsequently it is hoped will prove useful. 
 
 Persons who are occupied with other branches of 
 science or philosophy or with literature, and who have, 
 therefore, not kept quite abreast of physical science, 
 may possibly be surprised to see the intimate way 
 in which the ether is now spoken of by physicists, 
 and the assuredness with which it is experimented 
 on. They may be inclined to imagine that it is still 
 a hypothetical medium whose existence is a matter 
 of opinion. Such is not the case. The existence of 
 an ether can legitimately be denied in the same 
 terms as the existence of matter can be denied, but 
 only so. The evidence of its existence can be 
 doubted or explained away in the one case as in 
 the other, but the evidence for ether is as strong 
 and direct as the evidence for air. The eye may 
 indeed be called an etherial sense-organ, in the 
 same sense as the ear can be called an aerial one, 
 and somewhat in the same sense as the hand and 
 muscles may be called a sense-organ for the apprecia- 
 tion of ordinary matter. 
 
 Some of the details of my explanations may fce 
 wrong (though I hope not), and all must be capable 
 
PREFACE. ix 
 
 of ultimate improvement, but as to the main doctrine 
 concerning the nature of electricity, though I call it a 
 " view," it is to me no view but a conviction. Few 
 things in physical science appear to me more certain 
 than that what has so long been called electricity is a 
 form, or rather a mode of manifestation, of the ether. 
 Such words as "electrification," "electric," may re- 
 main ; " electricity" may gradually have to go. It can 
 be noticed that whereas in the earlier part of the 
 book the word electricity occurs frequently and the 
 word ether seldom, in the later portion this order of 
 frequency is inverted. 
 
 A rough and crude statement adapted for popular 
 use is that electricity and ether are identical ; but that 
 is not all that has to be said, for there are two opposite 
 kinds of electricity, and there are not two ethers. But 
 there may be two aspects of one ether rf just as there 
 are two sides to a sheet of paper, or two aspects of a 
 transparent clock face ; and similarly may positive and 
 negative electricity be two aspects, or, as I have some- 
 times called them by chemical analogy, " components," 
 of the ether. Anything which can be sheared (and 
 ether is sheared by every electromotive force applied 
 to it) must consist of two parts sufficiently different to 
 travel or to be displaced in opposite directions. 
 
 If this statement is vague, it is because our present 
 
x PREFACE. 
 
 knowledge of the structure of the ether is vague ; not 
 because the relationship of electricity to ether is un- 
 certain, or will be anything but definite so soon as we 
 know the constitution of the ether more precisely. 
 Vague at present our knowledge of the ether is, but 
 not so vague as these lines may suggest. 
 
 That which has now to be investigated is not the 
 nature of electricity, but the nature of the ether. Ex- 
 planation always progresses by stages ; no explanation 
 is ultimate ; every explanation is a step up, a removal 
 of a thing from a lower to a higher category. Thus 
 comets at one time might have been anything ; they 
 have been shown to be a form (or swarm) of meteo- 
 rites. Meteorites, again, have been shown to be lumps 
 of common matter usually iron or rock. There 
 remains the question, What is iron or rock, or any 
 form of matter ? Heat was once thought to be a 
 form of matter ; it is now known to be a form of 
 energy. There remains the question, What is energy ? 
 Electricity has been thought to be a form of energy ; 
 it has been shown to be a form of ether. There 
 remains the question, What is ether ? 
 
 And a question it is indeed : the question of the 
 physical world at the present time. But it is not un- 
 answerable : it is, in my belief, not far from being- 
 answered. And it is probably a simpler question than 
 
PREFACE. xi 
 
 the supplementary and next subsequent question, What 
 is matter ? It is simpler, partly because ether is one, 
 while matter is apparently many ; partly because the 
 presence of matter so modifies the ether that no com- 
 plete theory of the properties of matter can possibly 
 be given without a preliminary and fairly complete 
 knowledge of the properties and constitution of un- 
 disturbed ether in free space. When this has been 
 attained, the resultant and combined effect we call 
 matter may begin to be understood. 
 
 If a continuous incompressible perfect fluid filling 
 all space can be imagined in such a state of motion 
 that it will do all that ether is known to do ; if, 
 simply by reason of its state of motion, it can be 
 proved capable of conveying light and of manifesting 
 all electric and magnetic phenomena which do not 
 depend on the presence of matter ; and if the state 
 of motion so imagined can be proved stable and 
 such as can readily exist, the theory of free ether 
 is complete. 
 
 The latest contribution towards such a theory appears while I 
 write in a letter to Nature by G. F. Fitzgerald (May Qth, 1889). 
 The fluid structure there imagined, a liquid in turbulent or 
 vortex motion consisting of interlaced vortex filaments like a 
 sponge, is proved to be capable of doing all that is required of 
 free ether ; and the motion is believed with great probability 
 to be a stable and possible state. The Fitzgerald ether may 
 
xii PREFACE. 
 
 consist, for instance, of an assemblage of columnar vortices 
 threading each other in three cardinal directions in square (or 
 cubical) order ; adjacent vortices rotating opposite ways like 
 the cells in my sectional diagrams, Figs. 37 and 46, in which 
 the clockwise whirls are positive electricity and the counter- 
 clockwise are negative electricity. Until we come into the 
 neighbourhood of matter no further distinction but exact 
 opposition of properties is existent. A somewhat similar idea 
 concerning the ether has been worked at by Mr. Hicks, see 
 156 ; and Sir William Thomson proved in a famous paper at 
 the British Association in 1887, that a laminar arrangement of 
 vortices could transmit transverse vibrations (i.e. light), though 
 with some absorption and therefore partial opacity (Phil. Mag. 
 October 1887). Fitzgerald has now gone a step further and 
 devised a fibrous ether which is not only optically, but also 
 electrically, sufficient. If no flaw appears, if it stand the test 
 of criticism and further development, the theory of free ether is 
 far more than begun. 
 
 The theory of bound ether and of matter must next 
 follow, and thereby, in addition to all optical and 
 electrical phenomena, gravitation and cohesion must 
 be explained too. Then must be attacked the specific 
 differences between various kinds of matter, and the 
 nature of what we call their " combinations." When 
 this is accomplished the complex facts of chemistry 
 will have been brought under a comprehensive law. 
 
 The next fifty years may witness these tremendous 
 victories in great part won. 
 
 UNIVERSITY COLLEGE, LIVERPOOL. 
 May i$th, 1889. 
 
CONTENTS. 
 PART I. 
 
 INTRODUCTION AND ELECTROSTATICS. 
 
 
 
 CHAPTER I. 
 
 p 
 
 FUNDAMENTAL NOTIONS 
 
 CHAPTER II. 
 THE DIELECTRIC . l8 
 
 CHAPTER III. 
 CHARGE AND INDUCTION 3 
 
PAGE 
 
 xiv CONTENTS 
 
 PART II. 
 CONDUCTION. 
 
 CHAPTER IV. 
 METALLIC AND ELECTROLYTIC CONDUCTION '55 
 
 CHAPTER V. 
 CURRENT PHENOMENA . 
 
 CHAPTER VI. 
 
 CHEMICAL AND THERMAL METHODS OF PRODUCING CURRENTS. 
 CONDUCTION IN GASES 106 
 
 PART III. 
 MA GNETISM. 
 
 CHAPTER VII. 
 RELATION OF MAGNETISM TO ELECTRICITY 131 
 
 CHAPTER VIII. 
 NATURE OF MAGNETISM 147 
 
CONTENTS. xv 
 
 CHAPTER IX. 
 
 PACK 
 
 STRUCTURE OF A MAGNETIC FIELD 168 
 
 CHAPTER X. 
 MECHANICAL MODELS OF A MAGNETIC FIELD 177 
 
 CHAPTER XT. 
 
 MECHANICAL MODELS OF CURRENT INDUCTION 193 
 
 PART IV. 
 RADIA TION. 
 
 CHAPTER XII. 
 RELATION OF ETHER TO ELECTRICITY .....* 219 
 
 CHAPTER XIII. 
 CONSTANTS OF THE ETHER 234 
 
 CHAPTER XIV. 
 ELECTRICAL RADIATION, OR LIGHT 246 
 
 CHAPTER XV. 
 
 ELECTRO-MAGNETIC AND ELECTROSTATIC EFFECTS ON LIGHT . 276 
 
xvi CONTENTS. 
 
 APPENDED LECTURES 
 
 LECTURE I. 
 
 PAGE 
 
 THE RET ATION BETWEEN ELECTRICITY AND LIGHT 311 
 
 LECTURE II. 
 
 THE ETHER AND ITS FUNCTIONS 327 
 
 LECTURE III. 
 THE DISCHARGE OF A LEYDEN JAR 359 
 
 APPENDIX 387 
 
 INDEX 411 
 
PART I. 
 
 INTRODUCTION AND ELECTROSTATICS. 
 
CHAPTER L 
 
 FUNDAMENTAL NOTIONS. 
 
 i. IT is often said that we do not know what elec- 
 tricity is, and there is a considerable amount of truth 
 in the statement. It is not so true, however, as it 
 was some twenty years ago. Some things are begin- 
 ning to be known about it ; and though .modern views 
 are tentative, and may well require modification, 
 nevertheless some progress has been made. I shall 
 endeavour in this essay to set forth as best I may the 
 position of thinkers on electrical subjects at the 
 present time. 
 
 I begin by saying that the whole subject of elec- 
 tricity is divisible for purposes of classification into 
 four great branches. 
 
 (i) Electricity at rest, or static electricity : wherein 
 are studied all the phenomena belonging to stresses 
 and strains in insulating or dielectric media brought 
 
 B 2 
 
4 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 about by the neighbourhood of electric charges or 
 electrified bodies at rest immersed therein ; together 
 with the modes of exciting such electric charges and 
 the laws of their interactions. 
 
 (2) Electricity in locomotion, or current electricity : 
 wherein are discussed all the phenomena set up in 
 metallic conductors, in chemical compounds, and in 
 dielectric media, by. .the passage of electricity through 
 them ; together with the modes of setting electricity 
 in continuous motion and the laws of its flow. 
 
 (3) Electricity in rotation, or magnetism : wherein 
 are discussed the phenomena belonging to electricity 
 in whirling or vortex motion, the modes of exciting 
 such whirls, the stresses and strains produced by them, 
 and the laws of their interaction. 
 
 (4) Electricity in vibration, or radiation : wherein 
 are discussed the propagation of periodic or undu- 
 latory disturbances through various kinds of media, 
 the laws regulating wave velocity, wave-length, re- 
 flection, interference, dispersion, polarization, and a 
 multitude of phenomena studied for a long time 
 under the heading " Light." Although this is the 
 most abstruse and difficult portion of electrical 
 science, a certain fraction of it has been known 
 to us longer than any other branch, and has been 
 studied under special advantages, because of our 
 happening to possess a special sense-organ for its 
 appreciation. 
 
CHAP. I.] FUNDAMENTAL NOTIONS. 5 
 
 Now in order to be able to get through a survey 
 of these four great and comprehensive groups in 
 moderate compass, it will be necessary for me to 
 assume acquaintance with all the elementary facts 
 and proceed at once to their elucidation. 
 
 2. The great names in connection with our progress 
 in knowledge as to the real nature of electricity, irre- 
 spective of a mere study and extension of its known 
 facts, are 
 
 FRANKLIN, CAVENDISH, FARADAY, MAXWELL. 
 
 To these, indeed, you may feel impelled to add 
 the tremendous name of THOMSON ; but one 
 has some delicacy in attempting to estimate the 
 work of living philosophers, and as Maxwell has 
 been very explicit in acknowledging his indebted- 
 ness to his illustrious contemporary, whose work 
 will in the course of nature have to be criticized 
 and appraised by far abler hands than mine and 
 by the philosophers of generations yet unborn, 
 we may well afford to abstain from minute 
 considerations and accept for the present the 
 name of Maxwell as representative of the great 
 English school of mathematical physicists, under 
 whose influence, Cambridge, in the pride of 
 having reared them, is awaking to new and 
 energetic scientific life, and whose splendid achieve- 
 
6 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 merits will shine out in the future as the glory of this 
 century. 
 
 The views concerning electrification which I shall 
 try to explain are in some sense a development of 
 those originally propounded by that most remarkable 
 man, Benjamin Franklin. The accurate and acute 
 experimenting of Cavendish laid the foundation for 
 the modern theory of electricity ; but, as he worked 
 for himself rather than for the race, and as moreover 
 he was in this matter far in advance of his time, 
 Faraday had to go over the same ground again, with 
 extensions and additions peculiar to himself and 
 corresponding to the greater field of information at 
 his disposal three-quarters of a century later. Both 
 these men, and especially Faraday, so lived among 
 phenomena that they yielded up their hidden secrets 
 to them in a way unintelligible to ordinary workers ; 
 but while they themselves arrived at truth by pro- 
 cesses that savour of intuition, they were unable 
 always to express themselves intelligibly to their 
 contemporaries and to make the inner meaning of 
 their facts and speculations understood. Then comes 
 Maxwell, with his keen penetration and great grasp 
 of thought combined with mathematical subtlety and 
 power of expression ; he assimilates the facts, sym- 
 pathizes with the philosophic but untutored modes 
 of expression invented by Faraday, links the theorems 
 of Green and Stokes and Thomson to the facts of 
 
CHAP. I.] FUNDAMENTAL NOTIONS. 7 
 
 Faraday, and from the union there arises the young 
 modern science of electricity, whose infancy at the 
 present time is so vigorous and so promising that we 
 are all looking forward to the near future in eager 
 hope and expectation of some greater and still more 
 magnificent generalization. 
 
 3. You know well that there have been fluid or 
 material theories of electricity for the past century ; 
 you know, moreover, that there has been a reaction 
 again'st them. There was even a tendency a few 
 years back to deny the material nature of electricity 
 and assert its position as a form of energy. This was 
 doubtless due to an analogical and natural, though 
 unjustifiable, feeling that just as sound and heat and 
 light had shown themselves to be forms of energy so 
 in due time would electricity also. If such were the 
 expectation, it has not been justified <by the event. 
 Electricity may possibly be a form of matter it is 
 not a form of energy. It is quite true that electricity 
 under pressure or in motion represents energy, but the 
 same thing is true of water or air, and we do not 
 therefore deny them to be forms of matter. Under- 
 stand the sense in which I use the word electricity. 
 Electrification is a result of work done, and is most 
 certainly a form of energy ; it can be created and 
 destroyed by an act of work. But electricity none 
 is ever created or destroyed, it is simply moved and 
 strained like matter. No one ever exhibited a trace 
 
8 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 of positive electricity without there being somewhere 
 in its immediate neighbourhood an equal quantity of 
 negative. 
 
 The simplest proof of this statement consists in 
 making experiments inside a closed conducting in- 
 sulated room or shell ; it may be the size of a living 
 room, or the size of a beer-tankard, whichever is most 
 convenient. All known electrical experiments being 
 performed inside such a room, bodies electrified 
 strongly, moved about, sparks taken, &c., &c., a 
 sensitive electroscope connected to the room shall not 
 show the slightest permanent effect. In other words 
 the room will not become in the slightest degree 
 charged. I say no permanent effect, because it is just 
 possible that small transitory effects may occur during 
 the rapid rearrangement of internal charges. I do 
 not feel sure whether such transitory effects are or 
 are not really possible, but whether they are or not, 
 they have nothing to do with our present argument. 
 All the electrification that have been going on will 
 not have resulted in the creation of the minutest 
 quantity of electricity ; the only way to charge the 
 room is to pass a charge in from some other body 
 outside. 
 
 This is the first great law, expressible in a variety 
 of ways : as, for instance, by saying that total 
 algebraic production of electricity is always zero ; 
 that you cannot produce positive electrification with- 
 
CHAP, i.] FUNDAMENTAL NOTIONS. 9 
 
 out an equal quantity of negative also ; that what one 
 body gains of electricity some other body must lose. 
 
 Now, whenever we perceive that a thing is pro- 
 duced in precisely equal and opposite amounts, so that 
 what one body gains another loses, it is convenient and 
 most simple to consider the thing not as generated in 
 the one body and destroyed in the other, but as simply 
 transferred. Electricity in this respect behaves just like 
 a substance. This is what Franklin perceived. 
 
 4. The second great law is that electricity always, 
 under all circumstances, flows in a closed circuit, the 
 same quantity crossing every section of that circuit, 
 so that it is not possible to exhaust it from one region 
 of space and condense it in another. 
 
 Another way of expressing this fact is to say that 
 no charge resides in the interior of a hollow conductor, 
 but that every trace of charge is on the, outer surface 
 and penetrates, to no appreciable depth. 
 
 Another is to say that total induced charge is 
 always equal and opposite to inducing charge. 
 
 This second law can also be illustrated by the 
 insulated room or conducting cavity already men- 
 tioned. Having found that internal electrification 
 produces no effect on an outside electroscope connected 
 to the walls, proceed to pass a charge in to the cavity 
 through a temporarily opened window or lid. Instantly 
 the chamber has become charged by a definite amount, 
 viz. by the precise amount which has then been intro- 
 
10 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 duced into its cavity. The charge need not be in any 
 way communicated to the chamber, all that is necessary 
 is that it shall be wholly inside. Moving the charge 
 about, or letting it spark to the walls of the chamber, 
 makes not the slightest difference to the electro- 
 scope outside. It may be watched through a micro- 
 scope at the instant the spark occurs and it will not 
 show the slightest twinkle. Both these experiments 
 of the hollow chamber were made by Faraday, and the 
 latter is well known and often quoted as his " ice-pail " 
 experiment, because he happened to use an ice-pail 
 sometimes as his insulated chamber. 
 
 Another mode of illustrating the same series of facts 
 is afforded by an insulated parrot cage with an electro- 
 scope inside it connected by a wire to the bars of the 
 cage. The cage may now be charged strongly, its 
 potential may be changed from a million volts positive 
 to a million volts negative, sparks of any length may 
 be taken from it ; but provided the meshes are close 
 enough for it to be regarded as a really closed vessel 
 the electroscope inside is wholly unaffected. In 
 making this experiment the electroscope must be 
 connected with the bars of the cage, for if it be de- 
 tached it can be affected by charged air blown through 
 the meshes. Directly a charge gets inside the cage it 
 can affect the electroscope easily enough unless there 
 is a wall-connexion, and then it is powerless. 
 
 The open-work of a cage is objectionable on this 
 
CHAP. I.] FUNDAMENTAL NOTIONS. 11 
 
 account, that it allows the permeation of electrified 
 air, just as it might allow an electrified pith ball to be 
 thrown in. Making the meshes close is no guarantee 
 against this source of disturbance : I have blown elec- 
 trified air from a point through very fine earth- 
 connected copper gauze and affected strongly an 
 electroscope on the other side. 
 
 No doubt a solid metal walled room is secure against 
 this cause of disturbance but then it is difficult to see 
 the electroscope. Faraday however constructed a room 
 for the purpose big enough to get into himself, and 
 thus performed the experiment quite satisfactorily. 
 
 But perhaps the most rigorous mode of examining 
 the precise truth of the property of electricity which 
 lies at the bottom of all these experiments is that 
 adopted in the famous Cavendish experiment : some- 
 times referred to in French books by the name of Biot. 
 This consists in charging strongly an insulated sphere 
 provided with a couple of hemispherical caps which 
 can afterwards be taken off mechanically, and connect- 
 ing a delicate electroscope immediately after to the 
 disclosed ball. Not the faintest trace of charge is 
 found upon it. This experiment has been repeated 
 by Clerk Maxwell in the Cavendish Laboratory, Cam- 
 bridge, using a Thomson Quadrant Electrometer and 
 all modern appliances, with absolutely negative result. 1 
 
 1 See Maxwell's " Electrical Researches of Cavendish," pp. 104 and 
 417. An interesting little experiment with soap bubbles, made by Mr. 
 
 
12 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 This series of experiments are most vital, and give 
 us most fundamental information regarding elec- 
 tricity : let us consider how we can best express their 
 teaching. 
 
 5. When we thus find that it is impossible to charge 
 a body absolutely with electricity, that though you can 
 move it from place to place it always and instantly 
 refills the body from which you take it, so that no 
 portion of space can be more or less filled with it 
 than it already is, that it is impossible by any rise ol 
 potential to squeeze a trace of electricity into the in- 
 terior of a cavity, and that if a charge be introduced a 
 precisely equal quantity at once passes through the 
 walls to the outside ; it is natural to express the phe- 
 nomenon by saying that electricity behaves itself like 
 a perfectly incompressible substance or fluid, of which 
 all space is completely full. That is to say, it behaves 
 like a perfect and all- permeating liquid. Understand 
 I by no means assert that electricity is such a fluid 
 or liquid ; I only assert the undoubted fact that it 
 behaves like one, i.e. it obeys the same laws. 
 
 It may be advisable carefully to guard oneself 
 against becoming too strongly imbued with the notion 
 that because electricity obeys the laws of a liquid 
 
 Vernon Boys, illustrates the fact that the depth to which a charge 
 penetrates is less than the diameter of a few molecules, for one soap 
 bubble inside another is entirely screened from such electrostatic forces 
 as can be applied. 
 
CHAP, i.] FUNDAMENTAL NOTIONS. 13 
 
 therefore it is one. One must always be keenly on 
 the lookout for any discrepancy between the behaviour 
 of the two things, and a single contradictory dis- 
 crepancy not a mere difference but a real opposition 
 of properties will be sufficient to overthrow the fancy 
 that they may perhaps be really identical. Till such 
 a discrepancy turns up, however, we are justified in 
 pursuing the analogy more than justified, we are 
 impelled. And if we resist the help of an analogy 
 like this there are only two courses open to us : either 
 we must become first-rate mathematicians, able to live 
 wholly among symbols and dispensing with pictorial 
 images and such adventitious aid ; or we must remain 
 in hazy ignorance of the stages which have been 
 reached, and of the present knowledge of electricity 
 so far as it goes. I need hardly say that by " modern 
 views " I do not mean ultimate views ; nor do I mean 
 that I can give an account of all the speculations 
 and ideas floating in the minds of some two or three 
 of our most advanced thinkers. All I attempt is to 
 give an account of the stage which has certainly been 
 attained, to indicate the directions in which immediate 
 progress is probable, and to ask you to take for granted 
 that the next quarter of a century will see as great 
 advances made upon these views as they are superior 
 to the doctrines inculcated by the ordinary run of text- 
 books. 
 
 6. Imagine now that we live immersed in an 
 
14 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 infinite ocean of incompressible and inexpansible all- 
 permeating perfect liquid, like fish live in the sea : how 
 can we become cognizant of its existence ? Not by 
 its weight, for we can remove it from no portion of 
 space in order to try whether its has weight. 
 
 We can weigh air, truly, but that is simply because 
 we can compress it and rarefy it. An exhausting or 
 condensing pump of some kind was needed before 
 even air could be weighed or its pressure estimated. 
 
 But if air had been incompressible and inexpansible, 
 if it had been a vacuum-less perfect liquid, pumps 
 would have been useless for the purpose, and we should 
 necessarily be completely ignorant of the weight and 
 pressure of the atmosphere. 
 
 How then should we become cognizant of its 
 existence ? In four ways : 
 
 (i) By being able to pump it out of one elastic 
 bag into another [not out of one bucket into another : 
 if you lived at the bottom of the sea you would 
 never think about filling or emptying buckets the 
 idea would be absurd ; but you could fill or empty 
 elastic bags], and by noticing the strain phenomena 
 exhibited by the bags and their tendency to burst 
 when over full. [Water (or air) was here pumped 
 out of one elastic bag into another, and the analogy 
 with an electrical machine charging two conductors 
 oppositely, i.e. pumping electricity from one into the 
 other, was pointed out.] 
 
CHAP. I.] FUNDAMENTAL NOTIONS. 15 
 
 (2) By winds or currents ; by watching the effect 
 of moving masses of the fluid as it flows along pipes 
 or through spongy bodies, and by the effects of its 
 inertia and momentum. [A hanging vane arranged 
 in a tube so as to be deflected by a stream of water 
 was here likened roughly to a galvanometer ; also the 
 effect of suddenly stopping a stream of water, as in 
 a water ram, was mentioned as analogous to self- 
 induction.] 
 
 (3) By making vortices and whirls in the fluid, and 
 by observing the mutual action of these vortices, 
 their attractions and repulsions. [Whirlwinds, sand- 
 storms, waterspouts, cyclones, whirlpools.] 
 
 (4) By setting up undulations in the medium : i.e. 
 by the phenomena which in ordinary media excite in 
 us through our ears the sensation called " sound!' 
 
 In all these ways we have become acquainted with 
 electricity, and in no others that I am aware of. 
 They correspond to the four great divisions of the 
 subject which I made above. 
 
 7. But there are differences, very important dif- 
 ferences, between the behaviour of a material liquid 
 ocean such as we have contemplated and the be- 
 haviour of electricity. First it is doubtful whether 
 electricity by itself and disconnected from matter has 
 any inertia. It is by no means certain that it has 
 not : the experiments made by Maxwell with a 
 negative result ( 39) need only prove either that its 
 
16 MODERN VIEWS OF ELECTRICITY. [PART l. 
 
 speed of flow is very small, or that an electric current 
 consists of equal opposite streams of equal momentum. 
 The laws of electric flow in conductors are such as 
 indicate no inertia, ( 48) and this fact would be con- 
 clusive were it not that a recent brilliant paper by Prof. 
 Poynting explains the reason of it completely other- 
 wise, and leaves the question of inertia quite open ; 
 on the other hand, the facts of magnetism seem 
 definitely to require inertia, or something corres- 
 ponding to it. Leaving this for the present as an 
 open question, there can be no doubt but that when in 
 connection with insulating or dielectric matter tJie com- 
 bination most certainly possesses inertia ( 38 and 39). 
 8. A more serious and certain difference between 
 the behaviour of electricity and that of an incom- 
 pressible fluid comes out in the fourth category that 
 concerned with wave-motion. In an incompressible 
 fluid the velocity and length of waves would both be 
 infinite, and none of the phenomena connected with 
 the gradual propagation of waves through it could 
 exist. Such a medium therefore would be incapable 
 of sound-vibrations in any ordinary sense. On the 
 other hand, it is quite certain that the disturbances 
 concerned in light-radiation take place at right angles 
 to the direction of propagation they are transverse 
 disturbances and such disturbances as these no body 
 with the entire properties of a fluid can possibly 
 transmit. Remember, however, that the medium 
 
CHAP. I.] FUNDAMENTAL NOTIONS. 17 
 
 which transmits light is the ether and notsimply electri- 
 city. We have nowhere asserted that electricity and 
 the ether are identical. If they are, we are bound to 
 admit that ether, though fluid in the sense of enabling 
 masses to move freely through it, has a certain 
 amount of rigidity for enormously rapid and minute 
 oscillatory disturbances. If they are not identical 
 we can more vaguely say that ether contains elec- 
 tricity as a jelly contains water, but that the rigidity 
 concerned in the transverse vibrations belongs not to 
 the water in the jelly but to the mode in which it is 
 entangled in its meshes. However all this is a great 
 and difficult question into which we shall be able to 
 enter with more satisfaction twenty years hence 
 (Chap. XII.). 
 
 Provisionally we will accept as a temporary working 
 hypothesis the idea of the ether consisting of electri- 
 city in a state of entanglement similar to that of 
 water in jelly ; and we are driven to this view by the 
 exigencies of mode i ( i and 6), the electrostatic or 
 strain method of examining the properties of electri- 
 city, because otherwise the properties of insulators are 
 hard to conceive. If it turn out that space is a con- 
 ductor, which seems to me highly improbable, not to 
 say absurdly impossible, then we must fall back upon 
 the other view that it is rigid only for infinitesimal 
 vibrations, and fluid for steady forces. 
 
CHAPTER II. 
 
 THE DIELECTRIC. 
 
 9. RETURN now to the consideration of electro- 
 statics. We are to regard ourselves as living immersed 
 in an infinite all-permeating ocean of perfect incom- 
 pressible fluid (or liquid), as fish live in the sea ; but this 
 is not all, for if that were our actual state we should have 
 no more notion of the existence of the liquid than deep- 
 sea fish have of the medium they swim in. If matter 
 were all perfectly conducting, it would be our state : 
 in a perfectly free ocean there is no insulation no 
 obstruction to flow of liquid : it is the fact of insula- 
 tion that renders electrostatics possible. We could 
 obstruct the flow and store up definite quantities of a 
 fluid in which we were totally submerged by the use 
 of closed vessels of course. But how could we pump 
 liquid from one into another so as to charge one 
 positively and another negatively ? Only by having 
 the walls elastic : by the use of elastic bags, and 
 elastic partitions across pipes. And so we can represent 
 a continuous insulating medium (like the atmosphere 
 
CHAP. n.J THE DIELECTRIC. 19 
 
 or space) by the analogy of a jelly, through which 
 liquid can only flow by reason of cracks and channels 
 and cavities. 
 
 Modify the idea of an infinite ocean of liquid into 
 that of an infinite jelly or elastic substance in which 
 the liquid is entangled, and through which it cannot 
 penetrate without violence and disruption ; and you 
 have here a model of the general insulating atmo- 
 sphere. Our ocean of fluid is not free and mobile like 
 water, it is stiff and entangled like jelly. 
 
 Nevertheless bodies can move through it freely. 
 Yes, bodies can, it is the liquid itself only which is 
 entangled. How we are to picture freely and naturally 
 the motion of ordinary matter through the insulating 
 medium of space it is not easy to say. It is a difficulty 
 not fatal, but sensible, and due to an imperfection 
 in our analogy. 
 
 Insulators being like elastic partitions or impervious 
 but yielding masses, conductors are like cavities, porous 
 or spongy bodies perfectly pervious though with more 
 or less frictional resistance to the flow of liquids 
 through them. Thus, whereas bodies easily penetrable 
 by matter are impervious to electricity, bodies like 
 metals which resist entirely the passage of matter, are 
 quite permeable to electricity. It is this inversion of 
 ordinary ideas of penetrability that constitutes a small 
 difficulty at the beginning of the subject. 
 
 However, supposing it overcome, let us think of 
 
 C 2 
 
20 MODERN VIEWS OF ELECTRICITY. [PART I. 
 
 these insulated spheres and cylinders on the table 
 connected by copper wire as so many cavities and 
 tubes in an Otherwise continuous elastic impervious 
 medium which surrounds them and us, and extends 
 throughout space wherever conductors are not. All, 
 however, cavities as well as the rest of the medium, 
 are completely full of the universal fluid. The fluid 
 which is entangled in insulators is free to move in 
 conductors ; whence it follows that its pressure or 
 potential is the same in every part of a conductor in 
 which it is not flowing along. For if there were any 
 excess of pressure at any point, a flow would im- 
 mediately occur until it was equalized. In an insulator 
 this is by no means the case. Differences of pressure 
 are exceedingly common in insulators, and are natur- 
 ally accompanied by a strain of the medium. 
 
 It is instructive now to think over a number of 
 ordinary electrical experiments from this point of view : 
 to think of the fluid as flowing freely through con- 
 ductors and settling down to a state of equilibrium or 
 uniform pressure in them, but straining insulators as 
 high pressure water might strain elastic walls or bound- 
 aries, straining them even to bursting if the partitions 
 be made too thin. 
 
 10. There have been, as you know, two ancient 
 fluid theories of electricity the one-fluid theory of 
 Franklin, and the two-fluid theory of Symmer and 
 others. A great deal is to be said for both of 
 
CHAP. II.] THE DIELECTRIC. 21 
 
 them within a certain range. There are certainly 
 points, many points, on which they are hopelessly 
 wrong and misleading, but it is tJieir foundation 
 upon ideas of action at a distance that condemns them , 
 it is not the fluidity. They concentrate attention 
 upon the conductors ; whereas Faraday taught us to 
 concentrate attention on the insulating medium 
 surrounding the conductors the " dielectric " as he 
 termed it. This is the seat of all the phenomena : 
 conductors are mere breaks in it interrupters of its 
 continuity. 
 
 To Faraday the space round conductors was full of 
 what he called lines of force ; and it is his main 
 achievement in electrostatics to have diverted our 
 attention from the obvious and apparent, to the in- 
 trinsic and essential, phenomena. Let us try and seize 
 his point of view before going further. If: is certainly 
 true as far as it goes, and is devoid of hypothesis. 
 
 Take the old fundamental electric experiment of 
 rubbing two bodies together, separating them, and 
 exhibiting the attraction and repulsion of a pith ball, 
 say ; and how should we now describe it ? Something 
 this way : 
 
 Take two insulated disks of different material, one 
 metal, say, and one silk, touch them together ; the 
 contact effects a transfer of electricity from the metal 
 to the silk ; rub slightly to assist the transfer, since 
 silk is a non-conductor, then separate. As you 
 
22 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 separate the disks the medium between them is thrown 
 into a state of strain, the direction of which is mapped 
 out by drawing a set of lines, called lines of force, 
 from one disk to the other, coincident with the direc- 
 tion of strain at every point. As Faraday remarked, 
 the strain is as if these lines were stretched elastic 
 threads endowed with the property of repelling each 
 other as well as of shortening themselves ; in other 
 words, there is a tension along the lines of force and 
 a pressure at right angles to them. When the disks 
 are near, and the lines short, they are nearly straight 
 
 FIG. i. Rough diagram of the state of the medium near two oppositely charged 
 disks when close together. 
 
 (Fig. i), but as the distance increases they become 
 curved, bulging away from the common axis of the 
 two disks and some even curling round to the back of 
 the disk (Fig. 2), until when the disks are infinitely 
 distant as many lines spring from the back of each as 
 from its face ; and we have a charged body to all 
 intents existing in space by itself. 
 
 The state of tension existing in the medium between 
 the disks results in a tendency to bring them together 
 again, just as if they were connected by so many 
 elastic threads of no length when unstretched. The 
 
CHAP, ii.] THE DIELECTRIC. 23 
 
 ends of the lines are the so-called electrifications or 
 charges, and the lines perpetually try to shorten and 
 shut up, so that their ends may coincide and the strain 
 be relieved. If one of the disks touch another con- 
 ducting body, some of its lines instantly leave it and 
 go to the body ; in other words, the charge is capable 
 of transference, and the new body is urged towards 
 
 FIG. 2. Rough diagram of the s'ate of the medium near two oppositely charged 
 disks when separated. 
 
 the other disk, just as the disk was from which it 
 received the lines. If this new body completely stir- 
 rounds the disk, it receives the whole of its lines, and 
 the disk can be withdrawn perfectly free and inert. 
 [Faraday's " ice pail " experiment ( 4).] 
 
 ii. Now take the two charged disks, facing one 
 another, and let, say, a suspended gilt pith ball hang 
 between them. Being a conductor there is no strain 
 
24 MODERN VIEWS OF ELECTRICITY. [PART I. 
 
 inside it, and so it acts partially as a bridge, and 
 several of the lines pass through it or, rather, they 
 end at one side of it and begin at the other : thus it 
 has opposite charges on its two faces it is under 
 induction (Fig. 3). Let it now be moved so as to 
 touch one of the disks, the lines between it and the 
 disk on that side have shut up, and it remains with 
 
 FIG. 3. Rough diagram of the medium between two disks disturbed by the presence 
 of an uncharged metal sphere. The two halves of the sphere are oppositely 
 charged "by induction." 
 
 those only which go to the other disk. In other 
 words, it has received, unbalanced, some of the lines 
 which previously belonged to the touched disk. 
 These will pull it over to the far disk and there shut 
 themselves up. From that disk it receives more ; and 
 it travels with their ends back to the first disk, and so 
 
CHAP. II.] 
 
 THE DIELECTRIC. 
 
 on (Fig. 4), perpetually receiving lines and shutting 
 them up, until they are all gone and the disks are 
 discharged. 
 
 This mode of stating the facts involves no hy- 
 pothesis whatever it is the simple truth. But the 
 
 FIG. 4. Rough diagram of the medium near two oppositely charged disks between 
 which a metal carrier ball is oscillating, having just touched the right-hand disk. 
 (Discharge by "alternate contact.") 
 
 " lines of force " have no more and no less existence 
 than have "rays of light." Both are convenient 
 modes of expression. 
 
 12. But so long as we adhere to this mode ol 
 
26 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 expression we cannot form a complete mental picture 
 of the actually occurring operations. In optics it is 
 usual to abandon rays at a certain stage and attend 
 to the waves, which we know are of the essence of 
 the phenomenon, though we do not yet know very 
 much about their true nature. 
 
 Similarly in electricity, at a certain point we are 
 led to abandon lines of force and potential theories, 
 and to try to conceive the actual stuff undergoing its 
 strains and motions. It is then we get urged towards 
 ideas similar to those which are useful in treating of 
 the behaviour of an incompressible fluid. 
 
 In an utterly modified sense, we have still a fluid 
 theory of electricity, and a portion of the ideas of the 
 old theories belong to it also. 
 
 1 hus Franklin's view that positive charge was 
 excess, and negative charge was a deficit, in a cer- 
 tain standard quantity of the fluid which all bodies 
 naturally possessed in their neutral state, remains 
 practically true. His view that the fluid was never 
 manufactured, but was taken from one body to give 
 to another, so that one gained what the other lost- 
 no more and no less remains practically true. Part ' 
 also a less part of the two-fluid theory likewise 
 remains true, in my present opinion ( 90) ; but this 
 is not a branch of the subject on which I shall cnter 
 in the present part. It will suffice for the present to 
 fix our attention on one fluid only. 
 
CHAP, ii.] THE DIELECTRIC. 27 
 
 You are to think of an electric machine as a pump 
 which, being attached to two bodies respectively, 
 drives some electricity from the one into the other, 
 conferring upon one a positive and upon the other 
 a precisely equal negative charge. One of the two 
 bodies may be the earth, in which case the charge 
 makes little or no difference to it. 
 
 13. But, as has been objected before, if electricity 
 is like an incompressible and inextensible fluid, how 
 is it possible to withdraw any of it from one body 
 and give it to another ? With rigid bodies it is not 
 possible, but with elastic bodies it is easy. 
 
 The act of charging this sphere is therefore 
 analogous to pumping water into this elastic bag, or 
 rather into a cavity in the midst of an elastic medium, 
 whose thick walls, extending in all directions and 
 needing a great pressure to strain them,, better repre- 
 sent the true state of the case than does the thin 
 boundary of a bag like this. 
 
 Draw a couple of such cavities and consider fluid 
 pumped from one into the other, and you will see 
 that the charge (i.e. the excess or defect of fluid) 
 resides on the outside. 
 
 If the fluid is exactly incompressible, not the least 
 extra quantity will be squeezed by the pressure into 
 the space originally occupied by the cavity. This is 
 the moral of the Cavendish experiment ( 4) : it 
 proves that electricity is precisely incompressible. 
 
28 MODERN VIEWS OF ELECTRICITY. [PART 1. 
 
 You may also show that when both cavities are 
 similarly charged the medium is so strained that 
 they tend to be forced apart ; whereas when one is 
 distended and the other contracted they tend to 
 approach. 
 
 Further you may consider two cavities side by side, 
 pump fluid into (or out of) one only, and watch the 
 effect on the other. You will thus see the phenomena of 
 induction, the near side of the second cavity becoming 
 oppositely charged (i.e. the walls encroaching on the 
 cavity), the far side similarly charged (the cavity 
 encroaching on the walls), and the pressure on the 
 fluid in the cavity being increased or diminished in 
 correspondence with the rise or fall of pressure in the 
 charged or inducing cavity. In other words, con- 
 ductors rise in potential when brought near a 
 positively charged body ; and their charge, if any, 
 though not altered in quantity becomes redistributed. 
 
 The actual changes in volume necessary to the 
 strain of these cavities are a defect in the analogy. 
 To avoid this objection, one will have to accept a dual 
 view of electricity a sort of two-fluid theory, which 
 many phenomena urge one to accept, but about which 
 I will say nothing at present. It is sufficient at first 
 to grasp the one-fluid ideas ( 90). 
 
 14. Return Circuit, Sometimes a difficulty is felt 
 about electricity flowing in a closed circuit as, for 
 instance, in signalling to America and using the earth 
 

 CHAP, ii.] THE DIELECTRIC. 29 
 
 as a return circuit : the question arises, How does the 
 electricity find its way back ? 
 
 The difficulty is no more real than if a tube were 
 laid to America with its two ends connected to the 
 sea and already quite full. If now a little more sea- 
 water were pumped in at one end, an equal quantity 
 would leave the other end, and the disturbed level 
 of the ocean would readjust itself. Not the same 
 identical water would return, but an equal quantity 
 would return. That is all one says of electricity. 
 One cannot label and identify electricity. 
 
 To imitate the inductive retardation of cables, the 
 tube should have slightly elastic walls ; to imitate the 
 speed of signalling, the water must be supposed quite 
 incompressible, not elastic as it really is, or each 
 pulse would take three-quarters of an hour to go. 
 
CHAPTER III. 
 
 CHARGE AND INDUCTION. 
 
 15. Condensers. Returning to the subject of charg- 
 ing bodies electrically, how is one to consider the 
 fact that bringing an earth-plate near a conductor 
 increases its capacity so greatly, enabling the same 
 pressure to force in a much larger quantity of fluid ? 
 how is one to think of a condenser, or Leyden jar ? 
 
 In the easiest possible way, by observing that 
 the bringing near an earth-connected conductor is 
 really thinning down tJie dielectric on all sides of the 
 body. 
 
 The thin-walled elastic medium of course takes 
 less force to distend it a given amount than a thick 
 mass of the same stuff took ; in other words it has 
 much more capacity. Remember that capacity of 
 elastic cavities cannot satisfactorily be measured as 
 the capacity of buckets is measured, by the maximum 
 quantity they will hold when full, they are never 
 
CHAP, in.] CHARGE AND INDUCTION. 31 
 
 " full " till they burst ; and the amount required to 
 burst them measures rather their strength than their 
 capacity. The only reasonable definition of capacity 
 in such cases is the ratio of any addition to their 
 contents to the extra pressure required to force it in : 
 and this is exactly the way electrical " capacity " is 
 denned. A Leyden jar is like a cavity with quite 
 thin walls in other words, it is like an elastic 
 bag. 
 
 But if you thin it too far, or strain it too much, the 
 elastic membrane may burst : exactly, and this is the 
 disruptive discharge of a jar, and is accompanied by 
 a spark. Sometimes it is the solid dielectric which 
 breaks down permanently. Ordinarily it is merely 
 the air ; and, since a fluid insulator constitutes a 
 self-mending partition, it is instantaneously as good 
 as new again. 
 
 There are many things of interest and importance 
 to study about a Leyden jar. There is the fact that 
 if insulated, it will not charge : the potential of both 
 inner and outer coatings rises equally ; that, in order 
 to charge it, for every positive spark you give to the 
 interior an equal positive spark must be taken from 
 the exterior. There is the charging and the discharg- 
 ing of it by alternate contacts, as by an oscillating 
 ball ; and there are the phenomena of the spark- 
 discharge itself. 
 
 But, as you know, all charging is really a case of a . 
 
32 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 Leyden jar. The outer coat must always be some- 
 where the walls of the room, or the earth, or 
 something you always have a layer of dielectric 
 between two charges the so-called induced and 
 the inducing charge. You cannot charge one body 
 alone ( 5). 
 
 1 6. To illustrate the phenomena of charge, I will 
 now call your attention to these diagrams which 
 less completely but more simply than hydraulic 
 illustrations, serve to make the nature of the 
 phenomena manifest. 
 
 First you have an inextensible endless cord circu- 
 lating over pulleys ; this is to represent electricity 
 flowing in a closed circuit. Electromotive forces are 
 forces capable of moving the cord, and you may 
 consider them applied either by a winch, or by a 
 weight on the hook W. A battery cell corresponds to 
 a small weight ; an electric machine to a slow but 
 powerful winch. Clamping the cord with the screw S 
 corresponds to making the resistance of the circuit 
 infinite. Instead of the cord, clamp, and driving 
 pulley, one might consider an endless pipe full of 
 liquid with a stop-cock and a pump on it, but for 
 many purposes the cord is sufficient and more simple. 
 In Fig. 5, the only resistance to the motion is friction, 
 and there is no tendency to spring back. Fixed 
 beads are sho\i4fc on the cord to typify atoms of 
 matter, and they may be more or less rough to 
 
CHAP, in.] CHARGE AND INDUCTION. 
 
 33 
 
 represent different specific resistances. If the cord be 
 moved, heat is the only result. 
 
 Now pass to Fig. 6. Here the cord is the same as 
 before, but the beads are firmly attached to it, so that 
 if it moves they must move with it. They represent, 
 therefore, the particles of an insulating substance. 
 
 FIG 5. Mechanical analogy of a metallic circuit. 
 
 Nevertheless, their supports are not rigid they do 
 not prevent the cord moving at all ; they allow what 
 is called electric " displacement" not conduction ; 
 they can be displaced a little from their natural 
 position, but they spring back again when the 
 disturbing E.M.F. is removed. The beads in this 
 
 D 
 
34 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 figure are supposed to be supported by elastic 
 threads : if the cord were replaced by a closed pipe 
 full of water the beads and threads would be replaced 
 by elastic partitions. The specific inductive capacity 
 of the dielectric is represented by the stretchability, or 
 inverse elasticity, of the elastic threads. The stiffer 
 the threads are to pull out, the less is the inductive 
 
 FIG. 6. Mechanical analogy of a circuit partly dielectric : for instance, of a 
 charged condenser. A is its positive coat, u its negative. 
 
 capacity of the medium ; because evidently a greater 
 E.M.F. is needed to cause a given displacement. 
 
 Apply a given E.M.F. to this cord, as for instance 
 the weight W, and a definite displacement is produced. 
 One side, A, gets more cord than usual it is 
 positively charged ; the other side, B, gets less it is 
 
CHAP, in.] CHARGE AND INDUCTION. 35 
 
 negatively charged. If the applied E.M.F. exceeds a 
 certain limit the strain is too great ; the elastics break, 
 and you have disruptive discharge with a spark. But 
 even when the strain is only moderate some of the 
 supports may yield viscously, or be imperfectly elastic 
 and permit a gradual extra displacement of the cord, 
 known to telegraphists as " soaking in." 
 
 When discharge is now allowed, it will not at once 
 be complete ; a large portion of the displacement will 
 be at once recovered, but the rest will gradually " soak 
 out " and cause residual discharges. 
 
 If the dielectric is at all stratified in structure, so 
 that some of the beads allow cord to slip through 
 them or yield more than others then this residual 
 charge effect will become very prominent. 
 
 17. These are matters which it is easy to thoroughly 
 understand, and Fig. 7 illustrates different stages 
 sufficiently. In Diagram I. are represented 8 strata, 
 each displaced from its normal position by an amount 
 3. The restoring force being proportional to the dis- 
 placement, the total restoring force can be called 24. 
 The diagram represents, therefore, a Leyden jar or 
 other dielectric strained by an applied E.M.F. of 24 
 units. If every stratum insulates perfectly that is, if 
 every bead is quite firmly attached to the cord nothing 
 further happens so long as this force is kept applied. 
 This state of things may be maintained in two ways, 
 either by keeping on the weight W that is, by keeping 
 
 D 2 
 
3 6 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 the condenser permanently connected to the battery ; 
 or by clamping the cord and thus making the resistance 
 infinite that is, by insulating the terminals- of the 
 condenser. 
 
 But now suppose that some of the strata are not 
 perfectly insulating ; let some of the beads slowly slide 
 back along "the cord towards their zero position. Then 
 we shall witness different phenomena according to 
 whether the weight has been left on, or the cord has 
 been clamped. 
 
 Take first the case of leaving the weight on that 
 is, keeping the battery connected. If every bead 
 slide equally, all we get is a continuance of the state 
 represented in Diagram I., combined with a slow oozing 
 forward of the cord that is, a slow and steady leak 
 through the condenser. But suppose every bead does 
 not slide equally, suppose some do not slide at all ; 
 then the slipping of some throws extra strain on the 
 others, and the cord moves forward, but more and more 
 slowly until the insulating strata ultimately have to 
 bear all the strain, and the cord asymptotically comes 
 to rest. 
 
 This process is observed in Atlantic cables and 
 Leyden jars ; one is liable to it with all condensers 
 except air condensers, and it is called " soaking in " ; it 
 is accompanied by the development of internal charges, 
 because plainly the original normal length of cord 
 between the beads is no longer maintained ; some 
 
CHAP, in.] CHARGE AND INDUCTION. 37 
 
 layers have acquired an extra length that is, are 
 positively charged others are negatively charged. 
 
 The strain is distributed very unequally, but its 
 total amount, in this case, continues constant. 
 
 Remove the electromotive force now : we get first 
 a quick discharge, and then a slow leaking out or 
 reverse motion of the cord as it is propelled by the 
 still displaced insulating strata through the now 
 oppositely displaced badly conducting ones ; the 
 time of " soaking out " being comparable with that 
 permitted to the soaking in. 
 
 It is important to see how these phenomena are 
 entirely reconcilable with the incompressible character 
 of electricity that is, the unyielding character ot 
 the cord. The ordinary notion of the positive and 
 negative charges of a Ley den jar crawling into the 
 glass to meet each other, and then crawling out again, 
 is quite erroneous ; and the actual process which 
 simulates the effect of this impossible process is 
 perfectly clear. 
 
 So much for the case when the battery is left 
 connected ; now attend to the case when the terminals 
 are insulated. 
 
 Having got the dielectric into the state represented 
 by I., screw down the clamp and wait. If some of 
 the beads slide, while some do not, we shall shortly 
 arrive at the state represented by II. Beads Nos. 3, 
 5, and 6, have slid partially back, and the total strain 
 
38 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 on the cord has been reduced to 17. The jar will 
 appear to have partially discharged itself by internal 
 
 FIG. 7. Stages in the discharge of a stratified condenser, with some of its layers of 
 imperfect insulating power ; showing one way in which the phenomena of 
 "residual charge," "internal charge," and "soaking out" are produced. 
 I. represents a recent charge, of E. M. F. 24. 
 
 II. represents the same after lapse of time, reduced 1017 by partial internal leakage, 
 and shows internal charge. The circuit itself is supposed to have been perfectly 
 insulating all the time; the charge on the plates therefore remains constant. 
 III. shows the first discharge. 
 
 II II shows the state attained after again waiting, viz. a residual charge with an 
 E.M. F. 3 in the old direction. 
 
 leakage, and yet not the slightest motion of the cord 
 has been permitted. Internal charges have appeared 
 
CHAP, in.] CHARGE AND INDUCTION. 39 
 
 positive between Nos. 3 and 4 apd between 5 and 6, 
 negative between 2 and 3 and between 4 and 5. The 
 charges on the coatings at A and B have remained 
 constant the jar has apparently increased in capacity, 
 because the same charge is maintained by a less 
 electromotive force. All these effects may present 
 themselves at first sight as irreconcilable with the 
 behaviour of an incompressible fluid ; but the dia- 
 gram clearly says otherwise. 
 
 Now unclamp the cord momentarily, i.e. discharge 
 the condenser and insulate again. At the instant of 
 discharge a rush of electricity takes place, and the 
 force falls to zero. The state of the discharged jar 
 at the first instant after discharge is represented in 
 Diagram III. The surface charges have not wholly 
 disappeared ; the internal charges have been un- 
 affected ; the displacement of none of the strata is 
 zero. The insulating ones remain with some of their 
 original displacement, the leaking ones have been 
 forced into a position of inverse displacement, so as 
 just to reduce the force on the cord to zero. The 
 most slippery one, No. 5, has now been most dis- 
 placed in the inverse direction. But not long do they 
 thus remain. They at once begin to slowly ooze 
 back, and before long they will have got into the 
 state represented by IV., where the now almost un- 
 balanced stress of the insulating strata exerts on 
 the cord a force 3 in the original direction. This is 
 
40 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 known as the residual charge, and on unclamping 
 the cord, the first residual discharge can be obtained. 
 Not even yet, however, is the jar wholly discharged. 
 Waiting again, another but feebler residual charge 
 makes its appearance, and so on, almost without 
 limit, until the sum of all the residual discharges plus 
 the original discharge make up exactly the charge 
 originally imparted to the jar make it up exactly 
 if any one of the strata has declined to leak. If all 
 leaked more or less, then of course there will be 
 some deficiency. 
 
 FIG. 7A. Cf. also Fig. 46. 
 
 1 8. The only thing needful to guard one's self 
 against in following out this mechanical analogy is 
 the idea that there need be any mechanical dis- 
 placement of the atoms of matter accompanying 
 the electric displacement. Manifestly a model con- 
 taining fixed beams for the attachment of the beads 
 cannot really correspond very closely to electrical 
 facts. To make the model imitate facts more closely 
 we should have to take a number of rows of beads, 
 each row threaded on its cord and attached crosswise 
 by elastic threads, as in Fig. /A. 
 
 If these cords are simultaneously displaced alter- 
 
CHAP. III.] CHARGE AND INDUCTION. 41 
 
 nately in opposite directions, and if they be con- 
 sidered as representing positive and negative electricity 
 alternately, while the beads represent the electro-posi- 
 tive and the electro-negative elements of the material 
 substance, then perhaps something more like the 
 actual state of things may be imagined. There is 
 here no- displacement of the molecule as a whole, 
 but there is a displacement of its constituent atoms ; 
 there is a shearing stress applied to each molecule, 
 which, if strong enough, may result in electrolytic 
 disruption. I certainly regard disruptive discharge 
 as being of this electrolytic character ( 112). 
 
 19. Return, however, to the simple discharge, and see 
 how it occurs. Will it take place as a simple sliding 
 back of the beads to their old position ? Yes, if the 
 resistance of the circuit is great, but not otherwise. 
 If the. cord is fairly free the beads will fly past their 
 mean position, overshooting their mark, then re- 
 bound, and so, after many quick oscillations, will 
 finally settle down in their natural position. Thus 
 is represented the fact that the discharge of a Leyden 
 jar is in general oscillatory ; the apparently single and 
 momentary spark, when analyzed in a very rapidly 
 rotating mirror, turning out to really consist of a 
 series of alternating flashes rapidly succeeding one 
 another, and all over in the hundred-thousandth of a 
 second or thereabouts. These oscillatory currents 
 were predicted and calculated beforehand by Sir 
 
42 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 William Thomson ; they were first observed experi- 
 mentally by Feddersen. 1 The oscillations continue 
 until the energy stored up in the strained medium 
 has all rubbed itself down into heat. The existence 
 of these oscillations proves distinctly that electricity 
 in conjunction with matter possesses inertia. The 
 rapidity of these oscillations is something tremendous : 
 it may reach as high as a hundred million per second, 
 or it may be as slow as ten thousand per second, ac- 
 cording to the capacity and inertia of the circuit. 
 
 The rapidity of oscillation, and its rate of dying 
 away, as well as the circumstances which change the 
 recovery from an oscillatory one into a simple one- 
 directioned leak, are precisely analogous to those 
 which regulate the recovery of a bent spring suddenly 
 let go. If the spring is in a very viscous medium, or 
 if it has but small inertia, it will not oscillate, but will 
 merely return to its natural position. Under ordinary 
 circumstances, however, it will make many oscillations 
 before its energy is all rubbed into heat ( 123 et seq^}. 
 
 Fig. 8 shows part o % f an actual model of the kind. 
 
 20. To make the model represent charge by induction 
 all that has to be done is to immerse a conductor into 
 the polarized dielectric in other words, to make one 
 or more of the beads of the fixed and slippery con- 
 ducting kind, the other beads on the cord being of 
 
 1 We now find that they were experimentally discovered with con- 
 siderable clearness by Joseph Henry of Washington. (See p. 369.) 
 
CHAP, in.] CHARGE AND INDUCTION. 
 
 43 
 
 the elastic and adhesive or insulating kind. Then, 
 when the displacement occurs, it is plain that a de- 
 ficiency of cord will exist on one side of the metallic 
 layer arid a surplus on the other, as shown in Fig. 9. 
 
 FIG. 8. Partial model of a dielectric. 
 
 This state of things corresponds exactly to the 
 equal opposite induced charges on a conductor under 
 induction, as in Fig. 3. 
 
 FIG. 9. Metallic layer in the midst of a polarized dielectric, showing opposite 
 charges " induced" on its surfaces. (Compare Fig. 3.) 
 
 If the strain on one side be relieved by letting the 
 beads on that side slip back on the cord, that corre- 
 sponds to touching the conductor to earth, as in Fig. 
 4. The other side has now to withstand the whole 
 
44 MODERN 'VIEWS OF ELECTRICITY. [PARTI. 
 
 E.M.F., consequently the strain there and the charge 
 there will have increased. Remove now the applied 
 E.M.F., and the negative charge appears on both 
 sides of the metal partition, either equally, or more 
 markedly on that side which has fewest beads, i.e. 
 which is nearest to other conductors. 
 
 21. This being a matter which it is desirable 
 thoroughly to understand, a series of figures illus- 
 trating the various stages of the process are appended, 
 in Fig. 9 A. 
 
 I. represents an ordinary polarized dielectric, say 
 air, between two oppositely charged bodies, A and B, 
 maintained at constant difference of potential. For 
 simplicity the field is taken of uniform strength, i.e. 
 with its lines of force parallel straight lines, so that A 
 may be considered as a large positively charged plate, 
 and B an earth-plate facing it. The difference of 
 potential between A and B is called 60, and is dis- 
 tributed among 8 strata or units of thickness, each of 
 which therefore bears the strain ' 7 J, and is displaced 
 J of the width of a square from its normal position. 
 The charges on A and B we may call + 3 respectively. 
 
 An insulated metal plate two units in thickness is 
 now introduced, replacing a couple of the dielectric 
 strata. The remaining 6 have therefore more strain 
 thrown upon them, viz. 10 on each, and accordingly 
 each is now displaced a whole square-width from its 
 normal position, the charge of the plates A and B has 
 
CHAP, in.] CHARGE AND INDUCTION. 
 
 45 
 
 risen to + 4, and the effect is the same as if the thickness 
 of dielectric had been reduced in the ratio of 4 to 3- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 ! 
 
 
 \ 
 
 
 \ 
 
 
 I 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 
 T , 
 
 
 A 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 B 
 
 
 f,r\ 
 
 1 
 
 i 
 
 3 
 
 / 
 
 
 / 
 
 
 / 
 
 
 
 
 
 
 
 
 f 
 
 
 l 
 
 -3 
 
 
 
 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 1 
 
 
 / 
 
 
 / 
 
 
 1 
 
 
 I 
 
 
 
 
 
 
 
 / 
 
 
 / 
 
 
 7 
 
 
 1 
 
 
 / 
 
 
 / 
 ' 
 
 
 I 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 \ 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 A 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 
 
 fto 
 
 
 + 
 
 4 
 
 / 
 
 
 / 
 
 
 / 
 
 
 / 
 
 -4 
 
 
 
 -H 
 
 4 
 
 / 
 
 
 / 
 
 4 
 
 
 
 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 
 
 
 
 i 
 
 
 / 
 
 
 
 
 
 
 
 / 
 
 
 7 
 
 
 f 
 
 
 I 
 
 
 
 
 
 
 1 
 
 
 / 
 
 
 
 
 
 
 
 \ 
 
 
 \ 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 A 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 B 
 
 
 
 m. 
 
 
 
 \ 
 
 
 V 
 
 
 \ 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 60 
 
 
 + 
 
 6 
 
 
 7 
 
 
 / 
 
 
 / 
 
 
 /(, 
 
 
 
 c 
 
 
 
 
 c 
 
 > 
 
 
 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 IV 
 
 -1-4 
 
 FIG 9A. Stages during the charge of a metal by induction and contact. Numbers 
 preceded by + or - represent charges; numbers affixed to an arrow-head 
 represent E.M.F. In this series the E.M.F. applied is supposed constant. 
 
 The metal partition introduced has also a charge on 
 
46 MODERN VIEWS OF ELECTRICITY. [PARTI. 
 
 its surface, viz. 4 on the side facing A, and + 4 on 
 the side facing B. See Diagram II. 
 
 The next stage is to connect the metal momentarily 
 to earth. The effect of this is to entirely relieve the 
 strain on the B side by replacing the dielectric with 
 metal, which allows the cord to freely slip through. The 
 cord makes another bound forward, and all the strain is 
 now thrown upon 4 strata, which each have to bear 15, 
 and are displaced ij from their natural position. Re- 
 storing the dielectric (i.e. removing the temporary earth 
 connection) makes no further change, but leaves every- 
 thing as shown in Diagram III. The charge on one side 
 of the metal partition is now 6, and on the other side 
 is nothing. 
 
 Finally remove the constant E.M.F. which has been 
 acting all this time. The cord makes a bound back, 
 its strain becomes nothing ; the 2 strata on the right 
 have to balance the 4 strata on the left, and accord- 
 ingly their displacements are I and J respectively. 
 The charges on the faces of the partition are 2 and 
 4 ; both negative. The charges on A and B are 4- 2 and 
 + 4 respectively, although they are at the same potential. 
 The state of things is shown in IV., and the metal 
 partition has been charged negatively by means of 
 induction. Of course it may have been charged 
 equally on its two faces ; that is a mere matter of the 
 relative proximity of adjacent objects, A and B. 
 
 If instead of maintaining A at constant potential 
 
CHAP, in.] CHARGE AND INDUCTION. 47 
 
 it possessed a constant charge, the series of operations 
 would differ in a slight and easily appreciated manner. 
 The resultant tension on the cord would then be zero 
 all the time, and the series of operations would be 
 practically the electrophorus series, such as go on 
 rapidly and continuously in all inductive machines 
 and replenishers. It will, however, be worth while to 
 sketch this electrophorus series more particularly ; 
 the process of working out what is happening in any 
 given case will then be sufficiently illustrated. 
 
 Electropiiorus. 
 
 22. Diagram I., Fig. QB, shows the cake excited 
 negatively, resting on its sole. The negative charge 
 on surface of cake is called 13 units; of these, 12 
 are what is sometimes called "bound". by the sole, 
 and I is free. In other words, the strain due to 12 
 units of charge is thrown on the layers of the cake, 
 the remaining small strain is thrown on the atmo- 
 sphere above. The strain in the atmosphere is small 
 because it is so much thicker than the cake there 
 are so many layers in it that a very small displace- 
 ment of each suffices to balance the stress in the 
 cord. One unit of charge is induced on the ceiling 
 and walls of the room by the electrified cake. We 
 now bring down the insulated metal cover of the 
 electrophorus. If it is any appreciable thickness it 
 
4 8 
 
 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 displaces a few of the strained layers, and thus there 
 is a little extra strain on the others ; but this effect is 
 
 (Ceiling) 
 
 Atmosphere 
 
 Lowest position l 
 of Cover J 
 
 Air film 
 
 Cake -I 
 
 Sole 
 
 Fig. 98. The electrophorus series of operations. Double lines mean rigid rods 
 supporting smooth beads, and represent metal. Gripping beads supported by 
 elastics represent dielectric. Numbers preceded by + or - represent charges. 
 The charge excited on the surface of the electrophorus cake remains constant all 
 the time. The sole is supposed connected with the floor and walls of the room. 
 Numbers in parentheses at the top represent the charge induced on ceiling and 
 walls. The whole thickness of atmosphere does not pretend to be represented. 
 It must be thick enough in I. precisely to balance the stress in the cake. The 
 resultant stress in the cord is in each case zero. 
 
 I. shows the charged cake, with cover either offer in position but insulated. 
 II. shows effect of connecting cover and sole. 
 III. and IV. show effect of gradually raising cover. 
 
 I. again shows effect of having removed cover and discharged it. 
 
 extremely small, and it is quite unessential. We 
 may therefore take the cover as of no thickness, and 
 
CHAP, in.] CHARGE AND "INDUCTION. 49 
 
 bring it down into what is marked in the diagram as its 
 lowest position ; the stress passes through it, and nothing 
 is affected except the one layer whose place it takes, 
 Diagram I. will serve to represent the cover thus put 
 on, so long as it is insulated. The dotted lines show it 
 in position. It does not make intimate contact with 
 the cake ; a film, either of air or of the substance of the 
 cake itself, intervenes between it and the negatively 
 charged surface, and this is exhibited in the diagram. 
 
 The next thing is to connect the cover and the 
 sole together. This immediately brings about the 
 state of things represented by Diagram II. 
 
 A charge of 9 units has rushed round from sole to 
 cover, making with the charge I which previously 
 existed on the walls of the room a total of ro. 1 The 
 strain above the cover is entirely relieved, and the 
 whole excitement is now internal between cover 
 and sole. The strain in the cake is considerably 
 relieved, but the work of balancing what remains in 
 it is thrown on the very thin film between cover and 
 top of cake. This, therefore, is highly strained. 
 
 We now raise the again insulated cover. As it as- 
 cends fresh layers of dielectric intervene between it and 
 the cake, and receive some of the strain. The effect 
 
 1 If the sole had been insulated, and connection between it and cover 
 also made in an insulated manner, then this unit on the walls of the 
 room would stay there ; the cover would only acquire a charge 9, and 
 the slight strain above it shown in I. would continue to exist unaltered. 
 
 E 
 
50 MODERN VIEWS OF ELECTRICITY. [PART I. 
 
 of this is threefold. First, they partially relieve .the 
 strain in the original very thin layer ; next, they 
 increase the strain in the cake ; and thirdly, they 
 put a little strain on the air above the raised cover. 
 The sole therefore receives 5 units instead of 3 ; the 
 cover retains its charge 10, but part of this is on its 
 upper surface ; the induced charge - 2 makes its 
 appearance on the walls of the room. The state 
 is shown in Diagram III. 
 
 Diagram IV. continues the process of raising, 
 until ultimately when the plate is removed to 
 infinity, its charge above and below is equal, 
 being 5 on each, and the cake and cover have 
 returned to their original state I., ready to begin 
 again. The cover having now a charge 10, the 
 walls of the room wherever it is will have a charge 
 10, and it may be discharged whenever one pleases 
 without affecting the cake at all. Having discharged 
 it, one can put it on, as in I., and perform the 
 cycle again. 
 
 If one chooses to put the cover on before dis- 
 charging it, the cycle of operations is just reversed, 
 from IV. to II. 
 
 It is instructive to mount an electrophorus on 
 an insulating stand and connect its sole to earth 
 through a delicate galvanometer ; then the rush out 
 of it when the cover is touched, and the flow back 
 again as the cover is raised, can be easily watched. 
 
CHAP, in.] CHARGE AND INDUCTION. 5-1 
 
 23. There is one more thing which is so important 
 to see clearly that an illustration of it is desirable, 
 and that is the effect of inserting not a metal but 
 a slice of some other perfectly insulating dielectric, 
 with a different inductive capacity, in the midst of a 
 polarized medium. Thus, for instance, between the 
 plate of a charged condenser insert a thick slab of 
 glass. The effects will differ according as the con- 
 denser plates are charged each with a given quantity, 
 or are maintained at a constant difference of potential . 
 
 Refer to Fig. QC ; the 8 similar strata are sup- 
 posed to be displaced with a total E.M.F. 24, the 
 tension in the cord (negative electric potential) ac- 
 cordingly rises by a step 3 at each layer. Diagram 
 I. shows this initial state. Clamp the cord, to represent 
 a constant charge on the plates A and B, and now 
 introduce a slab of glass that is, re'place the 4 
 middle layers by elastics only half as stiff (see II.). 
 The stress in the cord steps up now by only I at each 
 of these layers, and the total difference of potential, 
 instead of being 24, is now only 18. Meanwhile the 
 charges remain the same, and there is no charge on 
 the surface of the glass ; the capacity of the whole 
 condenser has increased in the ratio of 4 to 3. 
 
 There is no charge on the surface of the glass ; 
 but the resultant effect is very much the same as if 
 there were. The effect on the cord will be precisely the 
 same as if the replaced elastics were still of the same 
 
 E 2 
 
MODERN VIEWS OF ELECTRICITY. [FART i. 
 
 T 
 
 + 
 
 3 
 
 \ 
 
 \ 
 
 \ 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 \ 
 
 \ 
 
 \ 
 \ 
 
 \ 
 
 \ 
 \ 
 
 \ 
 
 \ 
 \ 
 
 \ 
 
 \ 
 
 -3 
 
 
 
 2 4 
 
 
 i 
 
 L 
 
 / 
 
 / 
 
 7 
 
 / 
 
 / 
 
 7 
 
 X 
 
 / 
 
 / 
 / 
 
 / 
 
 / 
 / 
 
 X 
 
 / 
 / 
 
 / 
 
 / 
 / 
 
 / 
 
 / 
 
 / 
 
 X 
 
 / 
 
 I 
 
 3 
 
 
 
 + 
 
 3 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 x 
 
 \ 
 
 \ 
 
 ^ 
 
 \ 
 
 \ 
 
 X 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 \ 
 
 s. 
 
 \ 
 
 \ 
 
 -3 
 
 
 
 18 
 
 
 
 
 x 
 
 x 
 
 7 
 
 x 
 
 x 
 
 ,X 
 
 / 
 
 / 
 
 / 
 
 '/ 
 
 
 J 
 
 
 
 / 
 
 x 
 
 x 
 
 / 
 
 ^ 
 
 
 
 
 
 
 4- 
 
 3 
 
 \ 
 
 \ 
 
 \ 
 
 x 
 
 \ 
 
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 \ 
 
 \ 
 
 \ 
 
 "jx 
 
 \ 
 
 \ 
 
 \ 
 
 -3 
 
 
 
 
 
 / 
 
 X 
 
 / 
 
 x 
 
 / 
 
 ? 
 
 / 
 
 
 / 
 
 / 
 
 / 
 
 A 
 
 'V 
 
 / 
 
 / 
 
 / 
 
 
 
 
 4 
 
 4 
 
 N 
 
 ^ 
 
 ^ 
 \ 
 
 X 
 
 x 
 
 v v 
 
 X 
 
 "v v 
 "X 
 
 S N> 
 
 N \ 
 
 x 
 
 x 
 
 X x 
 x x 
 
 ^ 
 
 ~"^, 
 
 \ 
 
 ^ 
 
 X 
 
 IN 
 
 \ 
 
 X 
 
 ^ 
 
 
 "> 1 
 
 
 
 
 / 
 
 / 
 
 / 
 
 / 
 
 7 
 
 X 
 
 x 
 
 / 
 
 :: 
 
 ' 
 
 X 
 
 X 
 
 ^ 
 
 s 
 
 f f' 
 
 / 
 
 / 
 
 1 
 
 ^ 
 
 _/ 
 
 
 
 --T 
 
 \ 
 
 V 
 
 24 
 
 FIG. 90 Real and apparent effects of introducing a glass slab between the plates 
 of an air condenser. 
 
 I. shows the original condenser, of capacity \. 
 II. shows the effect of inserting a slab of half the whole thickn 
 
 ess, arn^of 
 
 specific inductive capacity 2, the charge being kept constant. The 
 capacity has risen to \. 
 
 shows a spurious imitative mode of obtaining the same effect, without any 
 change of inductive capacity, by help of surface charges. 
 
 IV. shows the effect of introducing the slab into the condenser when it is 
 
 supplied with a constant E.M.F. The capacity hs again become \. 
 
 V. shows a spurious imitation of this effect by help of surface charges. 
 
 III 
 
CHAP, in.] CHARGE AND INDUCTION. 53 
 
 strength, but as if their beads had slid half-way back, 
 into the positions shown in III., where surface charges 
 exist as indicated by numbers. This, I repeat, is not 
 the state of things caused by the glass, but it is so 
 like it in effect as to be difficultly distinguishable 
 from it ; and one sometimes speaks of the spurious 
 or virtual charges set up on the glass surface, meaning 
 the charges in Diagram III., which so exactly imitates 
 the resultant effect of II. 
 
 So much for the effect of constant charge ; now take 
 the case of constant potential. 
 
 Diagram IV. shows the effect of replacing some of 
 the elastics by weaker ones in this case. The E.M.F. 
 is kept constant, so the strong elastics have more 
 strain thrown on them than before ; no internal 
 charge is possible so long as the substances insulate 
 perfectly, so all the beads are pulled forward equally. 
 The step of potential is now 4 at all the stiffer (or 
 air) strata, and 2 at all the weaker (or glass) strata, 
 making up the total E.M.F., 24. The charge on the 
 plates A and B has increased from 3 to 4 in 
 accordance with the increase of capacity, the rate 
 of increase of which is still 4 : 3. Here again the real 
 effect shown in IV. may be simulated by spurious 
 surface charges without any change of inductive 
 capacity, as is sufficiently indicated by Diagram V., 
 wherein all the elastics are supposed of the same 
 strength. 
 
54 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 24. Hydraulic Model of a Leydeii Jar. So much 
 for t]jie cord model, but I will now describe and ex- 
 plain an hydraulic model which illustrates the same 
 sort of facts ; some of them more plainly and directly 
 than the cord model. Moreover, since all charging 
 is essentially analogous to that of a Leyden jar, 
 let us take a Leyden jar and make its hydrostatic 
 analogue at once. 
 
 The form of jar most convenient to think of is one 
 supported horizontally on an insulating stand, with 
 pith ball electroscopes supplied to both inner and outer 
 coatings. Or one may use, as I commonly do, in con- 
 junction with the hydraulic model, a vertical coated 
 pane, with a pith ball connected to each coating ; but 
 if the electroscopes were of such a kind as to show a 
 difference between positive and negative potential, 
 they would do better. 
 
 To construct its hydraulic model, procure a thin 
 india-rubber bag, such as are distended with gas at 
 toy-shops ; tie it over the mouth of a tube with 
 a stop-cock A, and insert the tube by means of a 
 cork into a three-necked globular glass vessel or 
 " receiver," as shown in the diagram, Fig. 10, 
 
 One of the other openings is to have another 
 stop-cock tube, B ; and the third opening is to be 
 plugged with a cork as soon as the whole vessel, both 
 inside and outside the bag, is completely full of water 
 without air-bubbles. 
 
CHAP, in.] CHARGE AND INDUCTION. 
 
 55 
 
 This is the insulated Leyden jar : the bag represents 
 the dielectric, and its inner and outer coatings are the 
 spaces full of water. 
 
 Open gauge-tubes, a and b, must now be inserted 
 in tubes A and B, to correspond to the electroscopes 
 supplied to the jar; and a third bent tube, C, con- 
 
 FIG. ID. Skeleton diagram of hydraulic model of a Leyden jar. 
 
 necting the inner and outer coatings, will correspond 
 to a discharger. Ordinarily, however, of course C 
 will be shut. 
 
 A water-pump screwed on to A will represent an 
 electric machine connected to inner coating ; and the 
 outer coating, B, should open into a tank, to represent 
 the earth. The pump will naturally draw its supply 
 of water from the same tank. 
 
56 MODERN VIEWS OF ELECTRICITY. [PART I. 
 
 The bag being undistended, and the whole filled 
 with water free from air, the level of the water in the 
 two gauge-tubes will correspond with that in the 
 tank ; and this means that everything is at zero 
 potential, i.e. the potential of the earth. 
 
 Now, C being shut, shut also B, open A, and work 
 the pump. Instantly the level in the two gauges rises 
 greatly and equally : you are trying to charge an 
 insulated jar. Turn an electric machine connected to 
 a real jar, and its two pith balls will similarly and 
 equally rise. 
 
 Now open B for an instant, the pressure is relieved, 
 and both gauges at once fall, apparently both to zero. 
 Repeat the whole operation several times however, and 
 it will be found that, whereas b always falls to zero, a 
 falls short of zero each time by a larger amount, and 
 the bag is gradually becoming distended. This is 
 charge by alternate contact. It may be repeated exactly 
 with the real jar : a spark put into the inner coating, 
 and an equal spark withdrawn from the outer coat- 
 ing each time ; and unless this outer spark is so 
 withdrawn, the jar declines to charge : water (and 
 electricity) being incompressible. 
 
 If B is left permanently open, the pump can be 
 steadily worked, so as to distend the bag and raise the 
 gauge a to its full height, b remaining at zero all the 
 time, save for oscillatory disturbances. 
 
 Having got the jar charged, shut A, and remove 
 
CHAP, in.] CHARGE AND INDUCTION. 57 
 
 the pump, connecting the end of A with the tank 
 directly. 
 
 Now of course by the use of the discharger c the 
 fluid can be transferred from inner to outer coat, the 
 strain relieved, and the gauges equalized. But if this 
 operation be performed while the jar is insulated, i.e. 
 while A and B are both shut, the common level of the 
 gauges after discharge is not zero, but a half-way 
 level ; and the effect of this is very noticeable if 
 you touch an insulated Leyden jar after it has been 
 discharged. 
 
 Instead of using the discharger c, however, we can 
 proceed to discharge by alternate contact, and the 
 operation is very instructive. 
 
 Start with the gauge b at zero, and the gauge a 
 at high pressure. Open stop-cock A ; some water is 
 squeezed out of inner coating, and the a gauge falls to 
 zero, but the suck of the contracting bag on the outer 
 coat pulls down the gauge b below zero, the descent 
 of the two gauges being nearly equal. 
 
 Next shut A and open B ; a little water flows in 
 from the tank to still further relieve the strain of the 
 bag, and both gauges rise ; b to zero, a to something 
 just short of its old position. 
 
 Now shut B and open A again : again the two gauges 
 descend. Reverse the taps, and again they both rise ; 
 and so on until the bag has recovered its normal size. 
 This is discharge by alternate contact, and exactly 
 
58 MODERN VIEWS OF ELECTRICITY. [PART I. 
 
 imitates the behaviour of an insulated charged Leyden 
 jar whose inner and outer coats are alternately touched 
 to earth. Its pith balls alternately rise with positive 
 and with negative electricity, indicating potentials 
 above and below zero. 
 
 FIG. ii. First actually constructed model Leyden jar, with mercury gauges or 
 electrometers ; the whole rigged up with things purchasable at a plumber's, 
 except the pump. The glass globe contains an elastic bag, which swells as 
 water is pumped into it. The tank is kept full of water, and its level represents 
 the potential of the earth. Flexible tubes full of water effect the desired 
 earth-connectibns when wished. (The gauges a and b represent electroscopes 
 connected to inner and outer coats of the jar respectively. 
 
 Figs, ii and 12 are taken from photographs of 
 apparatus I have made to use as just described. The 
 glass globe with the partially distended bag inside it, 
 the pump, the tank, the gauges a and b, the stop-cocks 
 
CHAP, in.] CHARGE AND INDUCTION. 59 
 
 A B C, will be easily recognized. Two extra stop-cocks, 
 A' and B', leading direct to tank, are extra, and are to 
 
 FIG. 12. Latest form of hydraulic model of a Leyden jar with "water gauges ; the 
 whole arranged vertically to be more conspicuous. The pump is a force-pump 
 with a communi.cation between top of barrel and tank to get rid of stray water. 
 The parts are labelled to correspond with the skeleton diagram, Fig. 10, as well 
 as with Fig. n. 
 
 save having to disconnect pump and connect A direct, 
 when exhibiting the effect of " discharge by alternate 
 
60 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 contact." But the tank is not sufficiently tall in Fig. 
 1 2 ; I have doubled its height since. The full height 
 of the gauge-tubes is barely shown. 
 
 In any form of apparatus it is essential to fill the 
 whole with water pipes, globe, everything before 
 commencing to draw any moral from its behaviour. 
 It is rather difficult to get rid of a large bubble of air 
 from the top of the globe of Fig. 11, and though 
 it is not of very much consequence in this place, the 
 stop-cock in Fig. 1 2 is added to make its removal 
 easy. The gauges in Fig. 1 1 may be replaced by 
 others arranged as a lantern-slide, and connected by 
 flexible tubing full of air. 
 
 25. I have explained thus fully the hydraulic illus- 
 tration of Leyden jar phenomena, because these 
 constitute the key to a great part of electrostatics. 
 The illustration is not indeed a complete or perfect 
 one by any means, but by combining with it a con- 
 sideration of the endless cord models, and of what I 
 have endeavoured to explain concerning conduction 
 and insulation in general, a distinct step may be 
 gained. 
 
 Think of electrical phenomena as produced by an 
 all-permeating liquid embedded in a jelly ; think of 
 conductors as holes and pipes in this jelly, of an 
 electrical machine as a pump, of charge as excess or 
 defect, of attraction as due to strain, of discharge as 
 
CHAP, in.] CHARGE AND INDUCTION. 61 
 
 bursting, of the discharge of a Leyden jar as a 
 springing back or recoil, oscillating till its energy has 
 gone. 
 
 By thus thinking you will get a more real grasp of 
 the subject and insight into the actual processes 
 occurring in Nature unknown though these may still 
 strictly be than if you employed the old ideas of 
 action at a distance, or contented yourselves with no 
 theory at all on which to link the facts. You will 
 have made a step in the direction of the truth, but 
 I must beg you to understand that it is only a step ; 
 that what modifications and additions will have to be 
 made to it before it becomes a complete theory of 
 electricity I am unable fully to tell you. I am con- 
 vinced they will be many, but I am also convinced 
 that it is unwise to drift along among a host of 
 complicated phenomena without guide other than 
 that afforded by hard and rigid mathematical 
 equations. 
 
 The mathematical theory of potential and the like 
 has -insured safe and certain progress, and enables 
 mathematicians to dispense for the time being with 
 theories of electricity and with mental imagery. Few, 
 however, are the minds strong enough thus to dispense 
 with all but the most formal and severe of mental aids ; 
 and none, I believe, to whom some mental picture of 
 the actual processes would not be a help if it were 
 safely available. 
 
62 MODERN VIEWS OF ELECTRICITY. [PART i. 
 
 Such a representation I have endeavoured par- 
 tially to lay before you ; and I hope, if I have 
 succeeded in making myself at all intelligible, that 
 students of electricity will find it of some use and 
 service. 
 
PART II. 
 
 CONDUCTION. 
 
CHAPTER IV. 
 
 METALLIC AND ELECTROLYTIC CONDUCTION. 
 
 26. WE have now glanced through electrostatic 
 phenomena, and seen that they could be all compre- 
 hended and partially explained by supposing electri- 
 city to be a fluid of perfect incompressibility in other 
 words, a liquid permeating everywhere and every- 
 thing ; and by further supposing that in conducting 
 matter this liquid was capable of free locomotion, but 
 that in insulators and general space it was as it were 
 entangled in some elastic medium or jelly, to strains 
 in which electrostatic actions are due. This medium 
 might be burst, in a disruptive discharge, but easy flow 
 could go on only through channels or holes in it, 
 which therefore were taken to represent conductors ; 
 and it was obvious that all flow must take place in 
 closed circuits. 
 
 To-day I want to consider the circumstances of this 
 flow more particularly : to study in fact, the second 
 
 F 
 
66 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 division of our subject (see classification on page 4), 
 viz. Electricity in locomotion. 
 
 I use the term " locomotion " in order to eliminate 
 rotation and vibration : it is translation only with 
 which we intend now to concern ourselves. 
 
 Consider the modes in which water may be made to 
 
 move from place to place ; there are only two : it may 
 
 be pumped along pipes, or it may be carried about in 
 
 jugs. In other words, it may travel through matter, 
 
 or it may travel with matter. Just so it is with heat 
 
 also ; heat can travel in two ways : it can flow through 
 
 matter, by what is called " conduction," and it can 
 
 travel with matter, by what is called "convection." 
 
 There is no other mode of conveyance of heat. You 
 
 frequently find it stated that there is a third method, 
 
 viz. " radiation " ; but this is not truly a conveyance 
 
 of heat at all. Heat generates radiation at one place, 
 
 and radiation reproduces heat at another ; but it is 
 
 radiation which travels, and not heat. Heat only 
 
 naturally flows from hot bodies to cold, just as \vater 
 
 only naturally flows down hill ; but radiation spreads 
 
 in all directions, without the least attention to where 
 
 it is going. Heat can only flow one way at any given 
 
 point, but radiation travels all ways at once. If water 
 
 were dissociated on one planet into its constituent 
 
 gases, and if these recombined on another planet, it 
 
 would not be water which travelled from one to the 
 
 other, neither would the substance obey the laws of 
 
CHAP, iv.] METALLIC CONDUCTION. 67 
 
 motion of water water would be destroyed in one 
 place, and reproduced in another ; just so is it with the 
 relation between radiation and heat. 
 
 Heat, then, like water, has but two direct modes of 
 conveyance from place to place. For electricity the 
 same is true. Electricity can travel with matter, or 
 it can travel through matter ; by convection or by 
 conduction, but in no other known way. 
 
 Conduction in Metals. 
 
 27. Consider, first, conduction. Connect the poles of 
 a voltaic battery to the two ends of a copper wire, and 
 think of what we call the " current." It is a true flow 
 of electricity among the molecules of the wire. If 
 electricity were a fluid, then it would be 'a transport of 
 that fluid ; if electricity is nothing material, then a 
 current is no material transfer ; but it is certainly a 
 transfer of electricity, whatever electricity may be. 
 Permitting ourselves again the analogy of a liquid, we 
 can picture it flowing through, or among, the mole- 
 cules of the metal Does it flow through or between 
 them ? Or does it get handed on from one to the 
 next continually ? We do not quite know ; but the 
 last supposition is often believed to most nearly repre- 
 sent the probable truth. The flow may be thought of 
 as a perpetual attempt to set up a strain like that in a 
 
 F 2 
 
68 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 dielectric, combined with an equally perpetual breaking 
 down of every trace of that strain. If the atoms be 
 conceived as little conductors vibrating about and 
 knocking each other, so as to be easily and com- 
 pletely able to pass on any electric charge they may 
 possess, then, through a medium so constituted, electric 
 conduction could go on much as it does go on in a 
 metal. Each atom would receive a charge from those 
 behind it, and hand it on to those in front of it, and thus 
 may electricity get conveyed along the wire. Do not, 
 however, accept this picture as anything better than 
 a possible mode of reducing conduction to a kind 
 of electrostatics an interchange of electric charges 
 among a series of conductors. If such a series of 
 vibrating and colliding particles existed, then certainly 
 a charge given to any point would rapidly distribute 
 itself over the whole, and the potential would quickly 
 become uniform ; but it by no means follows that the 
 actual process of conduction is anything like this^ 
 Certainly it is not the simplest mode of picturing it for 
 ordinary purposes. The easiest and crudest idea is to 
 liken a wire conveying electricity to a pipe full of 
 marbles or sand conveying water ; and for many 
 purposes, though not for all, this crude idea suffices. 
 
 Leaving the actual mode of conveyance as un- 
 known, let us review how much is certainly known 
 of the process called conduction in homogeneous 
 metals. 
 
CHAP, iv.] METALLIC CONDUCTION. 69 
 
 This much is certainly known : 
 
 (1) That the wire gets heated by the passage of a 
 current. 
 
 (2) That no trace of a tendency to reverse discharge 
 or spring back exists. 
 
 (3) That the electricity meets with a certain amount 
 of resistance or friction-like obstruction. 
 
 (4) That this force of obstruction is accurately 
 proportional to the speed with which the electricity 
 travels through the metal that is, to the intensity of 
 the current- per unit area. 
 
 28. About this last fact a word or two must be said. 
 The amount of electricity conveyed per second across 
 a unit area is called the intensity of current, 1 and 
 experiment proves, what Ohm originally guessed as 
 probable from the analogy of heat conduction, that 
 this intensity is accurately proportional to the slope 
 of potential which causes the flow ; or, in other words 
 (since action and reaction are equal and opposite), 
 that a current in a conductor meets with an obstruc- 
 tive electromotive force exactly proportional to itself. 
 Or, quite briefly, a current through a given conductor 
 is proportional to the E.M.F. which drives it. The 
 particular ratio between slope of potential and cor- 
 responding intensity of current depends upon the 
 particular material of which the conductor is composed, 
 
 1 Often called "density" of current, but "intensity" is the natural 
 and proper expression for the purpose. 
 
70 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 and is one of the constants of the material, to be deter- 
 mined by direct measurement. It is called its " speci- 
 fic conductivity " or its " specific resistance " according 
 to the way it is regarded. 
 
 The law here stated is called Ohm's law, and is one 
 of the most accurately known laws there are. Never- 
 theless it is an empirical relation ; in other words, it 
 has not yet been accounted for it must be accepted as 
 an experimental fact. Undoubtedly, it is one of vast 
 and far-reaching importance : it asserts a connection 
 between electricity and ordinary matter of a definite 
 and simple kind. Using the language of hydraulic 
 analogy, it asserts that when electricity flows through 
 matter the friction between them is accurately as the 
 first power of the velocity for all speeds. 
 
 29. Now if we think of this opposing electromotive 
 force as analogous to friction, it is very easy to think 
 of heat being generated by the passage of a current, 
 and to suppose that the rate of heat-production will 
 be directly proportional to the opposing force and 
 to the current driven against it as in fact Joule 
 experimentally proved it to be. 
 
 But if we are not satisfied with this vague analogy, 
 and wish to penetrate into the ultimate nature of heat 
 and the mode in which it can be generated, then we can 
 return to the consideration of a multitude of oscil- 
 lating and colliding particles, moving with a certain 
 average energy which determines what we call the 
 
CHAP, iv.] METALLIC CONDUCTION. 71 
 
 " temperature" of the body. If now one or more of 
 these bodies receives a knock, the energy of the blow 
 is speedily shared among all the others, and they all 
 begin to move rather more energetically than before : 
 the body which the assemblage of particles constitutes 
 is said to have " risen in temperature." This illustrates 
 the production of heat by a blow or other mechanical 
 means. But now, instead of striking one of the balls 
 give it an electric charge ; or, better still, put within its 
 reach a constant reservoir of electricity from which it 
 can receive a charge every time it strikes it, and at 
 the same time put within the reach of some other of 
 the assemblage of particles another reservoir of infi- 
 nite capacity which shall be able to drain away all the 
 electricity it may receive. In practice there is no 
 need of infinite reservoirs : all that is wanted is to 
 connect two finite reservoirs, or " electrodes," as one 
 might now call them, with some constant means of 
 propelling electricity from one to the other, i.e. with 
 the poles of a voltaic battery or a Holtz machine. 
 
 What will be the result of thus passing a series of 
 electric charges through the assemblage of particles ? 
 Plainly the act of receiving a charge and passing it on 
 will tend to increase the original motion of each par- 
 ticle ; it will tend to raise the temperature of the 
 body. In this way, therefore, it is possible to picture 
 the mode in which an electric current generates heat. 
 
 But although this process may be used as a possible 
 
72 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 analogy, it cannot be a true and complete statement of 
 what occurs ; for it is essentially the mode of propaga- 
 tion of sound. Sound travels at a definite and known 
 velocity, being a mechanical disturbance handed on 
 from particle to particle in the manner described. 
 But heat, being some mode of motion, must also be 
 handed on after some analogous fashion, so that when 
 heat is supplied to one point of a mass it spreads or 
 diffuses through it. It is difficult to suppose the con- 
 duction of heat to be other than the handing on of 
 molecular quiverings from one to another, and yet it 
 takes place according to laws altogether different from 
 those of the propagation of the gross disturbance 
 called sound. The exact mode of conduction of heat is 
 unknown, but, whatever it is, it can hardly be doubted 
 that the conduction of electricity through metals is 
 not very unlike it, for the two processes obey the same 
 laws of propagation : they are both of the nature of a 
 diffusion, they both obey Ohm's law, and a metal 
 which conducts heat well conducts electricity well 
 also. 
 
 Conduction in Liquids. 
 
 30. Leaving the obscure subject of conduction in 
 metals for the present, let us pass to the consideration 
 of the way in which electricity flows through liquids. 
 By " liquids," in the present connection, one more par- 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 73 
 
 ticularly means definite chemical compounds, such as 
 acids, alkalies, salt and water, and saline solutions 
 generally. Some liquids there are, like alcohol, tur- 
 pentine, bisulphide of carbon, and possibly water, 
 which, when quite pure, either wholly or very nearly 
 decline to conduct electricity at all. Such liquids as 
 these may be classed along with air and gases as more 
 or less perfect dielectrics. Other liquids there are, 
 like mercury and molten metals generally, which con- 
 duct after precisely the same fashion as they do when 
 solid. These, therefore, are properly classed among 
 metallic conductors. 
 
 But most chemical compounds, when liquefied 
 either by heat or by solution, conduct in a way 
 peculiarly their own ; and these are called " electro- 
 lytes." 
 
 31. The present state of our knowledge enables 
 us to make the following assertions with considerable 
 confidence of their truth : 
 
 (1) Electrolytic conduction is invariably accom- 
 panied by chemical decomposition, and in fact only 
 occurs by means of it. 
 
 (2) The electricity does not flow through^ but wit/i, 
 the atoms of matter, which travel along and convey 
 their charges something after the manner of pith 
 balls. 
 
 (3) The electric charge belonging to each atom of 
 matter is a simple multiple of a definite quantity of 
 
74 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 electricity, which quantity is an absolute constant 
 quite independent of the nature of the particular 
 substance to which the atoms belong. 
 
 (4) Positive electricity is conveyed through a liquid 
 by something equivalent to a procession of the electro- 
 positive atoms of the compound, in the direction called 
 the direction of the current ; and at the same time 
 negative electricity is conveyed in the opposite 
 direction by a similar procession of the electro- 
 negative atoms. 
 
 (5) On any atom reaching an electrode it may be 
 forced to get rid of its electric charge, and, combining 
 with others of the same kind, escape in the free state ; 
 in which case visible decomposition results. Or it may 
 find something else handy with which to combine say 
 on the electrode or in the solution ; and in that case 
 the decomposition, though real, is masked, and not 
 apparent. 
 
 (6) But, on the other hand, the atom may cling to 
 its electric charge with such tenacity as to stop the 
 current : the opposition force exerted by these atoms 
 upon the current being called polarization. 
 
 (7) No such opposition force, or tendency to spring 
 back, is experienced in the interior of a mass of fluid : 
 it occurs only at the electrodes. 
 
 32. The first three of these statements constitute a 
 summary of Faraday's laws of electrolysis. These 
 laws are of far-reaching importance, and appear to be 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 75 
 
 accurately true. The first is called the " voltametric 
 law," and asserts that the amount of chemical action 
 electrolytically produced in any given substance is ex- 
 actly proportional to the quantity of electricity that 
 has passed through it. The vague phrase " chemical 
 action " is purposely used here to include decomposi- 
 tion or recomposition or liberation or deposition or 
 dissolution, or any other effect that can be brought 
 about in either elements or compounds by the pas- 
 sage of an electric current. The weight of substance 
 acted on measures the quantity of electricity which 
 has passed ; hence a decomposition cell can act as a 
 voltameter, and the law is called the voltametric law. 
 Its truth enables us to make the first of the above 
 statements ; which many qualitative facts concerning 
 the details of electrolysis modify into statement No. 2. 
 The second of Faraday's laws is called the law of 
 electro-chemical equivalence, and asserts that, if the 
 same current be passed through a series of voltameters 
 for the same time, the amount of chemical action in 
 each substance acted on is exactly proportional to its 
 ordinary chemical equivalent ; not to its atomic 
 weight merely, but to its atomic weight divided by 
 what is called its valency, or atomicity, or quanti- 
 valence ; this being its real chemical equivalent. 
 Thus an atom of oxygen weighs 16 times as much as 
 an atom of hydrogen, and is equivalent to two such 
 atoms in combining power ; hence the law asserts that 
 
76 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 * 
 
 8 grammes of oxygen are liberated for every gramme 
 of hydrogen. Again, an atom of silver is 108 times as 
 heavy as an atom of hydrogen, and is equal to it in 
 combining power ; hence 108 grammes of silver are 
 deposited in a silver voltameter while I gramme of 
 hydrogen is being liberated by the same current in a 
 gas voltameter. Once more, an atom of gold weighs 
 as much as 197 atoms of hydrogen, and is able to re- 
 place three of them in combination ; hence 65*7 
 grammes of gold are deposited by the same current 
 in the same time, and so on. 
 
 Now this law plainly means that the same number 
 of monad atoms is liberated by the same quantity of 
 electricity no matter what their nature may be ; half 
 that number of dyad atoms ; one third that number of 
 triad atoms. Hence, assuming statement (2), that the 
 current flows by convection each atom conveying 
 electricity it follows that every monad atom carries 
 the same quantity, whether it be an atom of hydrogen 
 or of silver or of chlorine, or a complex radicle like 
 NO 3 ; that each dyad atom carries twice as much, 
 whether it be an atom of oxygen or of zinc or of 
 copper, or a complex dyad radicle like SO 4 ; that each 
 triad atom carries three times as much, and so on. 
 And this is what is laid down in the third of the 
 above statements. 
 
 True, it is possible that every atom may have a 
 specific charge of its own with which it never parts ; 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 77 
 
 but about such nothing is known ; we can only make 
 experiments on the charge it is willing to part with at 
 an electrode, and there is no doubt that this is 
 accurately the same for all substances, up to a simple 
 multiple. And this quantity, the charge of one 
 monad atom, constitutes the smallest known portion 
 of electricity and is a real natural unit. Obviously 
 this is a most vital fact. This unit below which 
 nothing is known has even been styled an " atom " of 
 electricity ; and perhaps the phrase may have some 
 meaning. I have ventured to suggest one or two 
 effects which would result from the hypothesis that 
 this unit quantity of electricity were really in fact an 
 absolute minimum, and as indivisible as an atom of 
 matter. 1 The natural unit of electricity is exceedingly 
 small, being about the hundred-thousand-millionth 
 part of the ordinary electrostatic unit ; or less than 
 the hundred-trillionth of a coulomb. 
 
 The charge of each atom being so small, its potential 
 is not high. Something between I and 3 volts is a 
 probable difference of potential for two oppositely 
 charged atoms. But they are so near together that 
 even this small difference of potential causes a strong 
 electrostatic attraction or " chemical affinity " between 
 the oppositely charged atoms. 
 
 This electrical force between two atoms at any 
 
 1 See paper on " Electrolysis " at Aberdeen (Reports of the British 
 Association for 1885, p. 763). 
 
78 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 distance is ten thousand million billion billion times 
 greater than their gravitative attraction at the same 
 distance. The force has an intensity per unit mass 
 (and therefore is able to produce an acceleration) nearly 
 a trillion times greater than that of terrestrial gravity 
 near the earth's surface. 
 
 These are undoubtedly the forces with which 
 chemists have to do, and which they have long called 
 chemical affinity. 
 
 33. But it may be asked, If the atoms in each 
 moleculecling together by their electrostatic attractions, 
 and there are an enormous number of atoms between 
 two electrodes, how comes it that a feeble E.M.F. can 
 pull them apart and effect decomposition ; moreover, 
 how can the E.M.F. needed to effect decomposition help 
 varying directly with the thickness of fluid between 
 the plates ? It does not depend on anything of the 
 kind ; the length of liquid between the electrodes is 
 absolutely immaterial. This proves that throughout 
 the main thickness of liquid no atoms are torn asunder 
 at all. Probably they frequently change partners, one 
 pair of atoms not always remaining united but occa- 
 sionally getting separated and recombined with other 
 individuals. During these interchanges there must be 
 moments of semi-freedom during which the atoms are 
 amenable to the slightest directive tendency, and it is 
 probably these moments that the applied E.M.F. makes 
 use of. 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 79 
 
 The reality of such a state of continual interchange 
 between molecules has been forced upon chemists by 
 the facts of double-decomposition such facts as the 
 interchange of atoms between strongly combined salts 
 when their solutions are mixed so as to form very 
 much weaker compounds ; the proof that such com- 
 pounds are formed being very clear in the case when 
 they happen to be insoluble. The fact that if a 
 precipitate is insoluble enough it" is bound to form, 
 really proves that some small quantity of the corre- 
 sponding compound is always formed in every case, 
 whether it happens to be insoluble or not. 
 
 The state of continual interchange results in a 
 perfect sensibility to the migratory tendency of ex- 
 tremely weak forces, so that even the faintest trace 
 of an electromotive force is able^o affect the charged 
 atoms, on the average assisting the pbsitive atoms 
 down the slope of potential, and the negative atoms up 
 the slope. The fact that the most infinitesimal force 
 is sufficient to effect its due quota of decomposition 
 has been proved most clearly and decisively by the 
 experiments of Helmholtz. 
 
 Sometimes the term dissociation is used to signify 
 this practical freedom of atoms to locomotion ; and, 
 as stated originally by Prof. Clausius, the idea of 
 dissociation was certainly involved. It was thought 
 that a certain percentage of atoms existed in the 
 liquid in an uncombined state, wandering about 
 
8o MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 seeking partners, that it was these loose atoms on 
 which the electromotive forces acted, and that the 
 procession of these conveyed the current. But we now 
 see that the addition of the idea of double-decompo- 
 sition and interchange to the original hypothesis of 
 Grotthus explains all that is required by the facts, viz. 
 a virtual or potential dissociation, a momentary state 
 of hovering and indecision, without the need for any 
 continuous and actual dissociation. 
 
 34. I will now try and make the process of 
 electrolytic conduction clearer by reverting to our 
 mechanical analogies and models. 
 
 Looking back to Figs. 5 and 6, we see illustrations 
 of metallic conduction and of dielectric induction. In 
 each case an applied electromotive force causes some 
 m jvement of electricity ; but, whereas in the first case 
 it is a continuous almost unresisted movement or 
 steady flow through or among the atoms of matter, in 
 the second case it is a momentary shift or displace- 
 ment only, carrying the atoms of matter with it, and 
 highly resisted in consequence : resisted, not with a 
 mere frictional rub, which retards but does not check 
 the motion, but by an active spring back force, which 
 immediately checks all further current, produces what 
 we call " insulation," and ultimately, when the pro- 
 pelling force is removed, causes a quick reverse motion 
 or discharge. But the model is plainly an incomplete 
 one ; for what is it that the atoms are clinging to? 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 81 
 
 What is it ought to take the place of the beam in the 
 crude mechanical contrivance ? Obviously another 
 set of atoms, which are either kept still or urged in the 
 opposite direction by a simultaneous opposite displace- 
 ment of negative electricity ; as in Fig. /A. We are 
 to picture two or any number of rows of beads, each 
 row threaded on its appropriate cord ; the cords 
 alternately representing positive and negative elec- 
 tricity respectively, and being simultaneously displaced 
 in opposite directions by any applied E.M.F. The 
 beads threaded on any one cord have, in a dielectric, 
 elastic attachments to those on some opposite cord, 
 and thus continuous motion of the cords in opposite 
 directions is prevented : only a slight displacement is 
 permitted, followed by a spring back and oscillation 
 after the fashion already described. 
 
 Very well ; now picture the elastic connections be- 
 tween the beads all dissolved, and once more apply a 
 force to each cord, moving half of them one way and the 
 alternate half the other way, and you have a model 
 illustrating an electrolyte and electrolytic conduction. 
 The atoms are no longer attached to each other, but 
 they are attached to the cord. In the first respect, an 
 electrolyte differs from a dielectric ; in the second, it 
 differs from a metal. 
 
 Moreover, electrolytic conduction is perceived to 
 be scarcely of the nature of true conduction : the 
 electricity does not slip through or among the 
 
82 MODERN VIEWS OF ELECTRICITY. [PART 11. 
 
 molecules, it goes with them. The constituents of 
 each molecule are free of each other, and while one 
 set of atoms conveys positive electricity, the other 
 set carries negative electricity in the opposite direction ; 
 and so it is by a procession of free atoms that the 
 current is transmitted. The process is of the nature 
 of convection : the atoms act as carriers. Free loco- 
 motion of charged atoms is essential to electrolysis. 
 
 35. In order to compare with Figs. 5 and 6, so as to 
 bring out the points of difference, Fig. 13 is drawn. 
 The beads representing one set of atoms of matter 
 are tightly attached to the cord, no trace of slip 
 between them being permitted, but otherwise they 
 are free, and so are represented as supported merely 
 by rings sliding freely on glass rods. The only 
 resistance to the motion, beside the slight friction, 
 is offered at the electrode, which is typified by the 
 spring-backed knife-edge, Z. This is supposed to be 
 able to release the beads from the cord when they are 
 pressed against it with sufficient force. The cling 
 between the bead and cord (i.e. between each atom 
 and its charge) is great enough to cause a perceptible 
 compression of the springs, and accordingly to bring 
 out a recoil force in imitation of polarization. 
 
 The piece of cord accompanying each bead on its 
 journey (i.e. the length between it and the next bead) 
 represents the atomic charge, and is a perfectly con- 
 stant quantity : the only variation permissible in it is 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 83 
 
 that some kinds of atoms have twice as much, or are 
 twice as far apart on their cord, and these are called 
 by chemists dyad atoms ; another kind has three times 
 as much, another four, and so on ; these being called 
 triad, tetrad, &c. 
 
 3 4 
 
 FIG. 13. Crude mechanical analogy, illustrating a few points in a circuit partly 
 electrolytic. 
 
 If the cord be taken to represent positive electricity, 
 the beads on it may represent atoms of hydrogen, or 
 other monad cation^ travelling down stream to the 
 cathode. Another cord representing negative electricity 
 may be ranged alongside it, with its beads twice as 
 
 G 2 
 
84 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 far apart, to represent the atoms of a dyad anion^ 
 like oxygen. If the cords are so mechanically con- 
 nected that they must move with equal pace in 
 opposite directions, we have a model illustrating 
 several important facts. The number of oxygen 
 atoms liberated in a given time will be obviously 
 half the number of hydrogen atoms set free in the 
 same time, and will therefore in the gaseous state 
 occupy but half the volume. For any element what- 
 ever, the number of atoms liberated in any time is 
 equal to the number of atoms of hydrogen liberated 
 in the same time, divided by the "valency" of the 
 element as compared with hydrogen. This law was 
 discovered by Faraday, and appears to be precisely 
 true ; and inasmuch as the relative weight of every 
 element is known with fair accuracy, it is easy to 
 calculate what weight of substance any given current 
 will deposit or set free in an hour, if we once determine 
 it experimentally for any one substance. 
 
 We may summarize thus : 
 
 If we apply E.M.F. to a metal we get a continuous 
 flow, and the result is heat. 
 
 If we apply it to a dielectric we get a momentary 
 flow or displacement, and the result is the potential 
 energy of " charge." 
 
 If we apply it to an electrolyte we again get a con- 
 tinuous flow, and the result is chemical decomposition. 
 
 36. There are a large number of important points 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 85 
 
 to which I might direct your attention in the mode by 
 which an electric current is conveyed through liquids, 
 but I will specially select one, viz. that it is effected 
 by a procession of positively charged atoms travelling 
 one way, and a corresponding procession of negatively 
 charged atoms the other way. 
 
 Whatever we understand by a positive charge and 
 a negative charge, it is certain that the atoms of, say, 
 a water molecule, are charged, the hydrogen positively t 
 the oxygen negatively ; and it is almost certain that 
 they hang together by reason of the attraction 
 between their opposite charges. It is also certain 
 that when an electromotive force i.e. any force 
 capable of propelling electricity is brought to 
 bear on the liquid, the hydrogen atoms travel on 
 the whole in one direction, viz. down hill, and the 
 oxygen atoms travel in the other direction, viz. up 
 hill ; using the idea of level as our analogue for 
 electric potential in this case. The atoms may be 
 said to be driven along by their electric charges 
 just as charged pith balls would be driven along ; 
 and they thus act as conveyers of electricity, which 
 otherwise would be unable to move through the liquid. 
 
 Each of this pair of opposite processions goes on 
 until it meets with some discontinuity either some 
 change of liquid, or some solid conductor. At a 
 change of liquid another set of atoms continues 
 the convection, and nothing very particular need be 
 
86 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 noticed at the junction ; but at a solid conductor the 
 stream of atoms must stop : you cannot have loco- 
 motion of the atoms of a solid. The obstruction so 
 produced may stop the procession, and therefore the 
 current, altogether ; or on the other hand the force 
 driving the charges forward may be so great as to 
 wrench them free, to give the charges up to the elec- 
 trode which conveys it away by common conduction, 
 and to crowd the atoms together in such a way that 
 they are glad to combine with each other and escape. 
 
 Now notice the fact of the two opposite processions. 
 One cannot have a procession of positive atoms 
 through a liquid without a. corresponding procession 
 of negative ones. In other words, an electric current 
 in a liquid necessarily consists of a flow of positive 
 electricity in one direction, combined with a flow of 
 negative electricity in the opposite direction. And if 
 this is thus proved ,to occur in a liquid, why should it 
 not occur everywhere ? It is at least well to bear the 
 possibility in mind. 
 
 Another case is known where an electric current 
 certainly consists of two opposite streams of electricity, 
 viz. the case of the Holtz machine. While the 
 machine is being turned, with its terminals somehow 
 connected, the glass plate acts as a carrier conveying 
 a charge from one collecting comb to the other at 
 every half revolution ; but, whereas it carries positive 
 electricity for one half a rotation, it carries negative 
 
CHAP, iv.] ELECTROLYTIC CONDUCTION. 87 
 
 for the other half. The top of the Holtz disk is 
 always, say, positively charged, and is travelling 
 forward, while the bottom half, which is travelling 
 backward at an equal rate, is negatively charged. 
 
 In the Holtz case the speeds are necessarily equal, 
 but the charges are not. In the electrolytic case the 
 charges are necessarily equal, but the speeds are not. 
 Each atom has its own rate of motion in a given liquid, 
 independently of what it may happen to have been 
 combined with. This is a law discovered by Kohl- 
 rausch. Hydrogen travels faster than any other kind 
 of atom ; and on the sum of the speeds of the two 
 opposite atoms in a compound the conductivity of the 
 liquid depends. Acids, therefore, in general conduct 
 better than their salts. 
 
 37. The following table gives the rates at which 
 atoms of various kinds can make theif way through 
 nearly pure water, when urged by a slope of potential 
 of i volt per linear centimetre : 
 
 II . . I'oS centimetre per hour. 
 
 K . 0-205 
 
 Na 0-126 
 
 Li ... 0-094 
 
 Ag . . . 0-166 
 
 C 0-213 
 
 I ... 0-216 
 
 NO 3 ... 0-174 
 
CHAPTER V. 
 
 CURRENT PHENOMENA. 
 
 Electrical Inertia. 
 
 38. RETURNING now to the general case of conduc- 
 tion, without regard to the special manner of it, we 
 must notice that, if a current of electricity is anything 
 of the nature of a material flow, there would probably 
 be a certain amount of inertia connected with it, so 
 that to start a current with a finite force would take a 
 little time ; and the stoppage of a current would also 
 have either to be gradual or else violent. It is well 
 known that if water is stagnant in a pipe it cannot 
 be quite suddenly set in motion ; and again, if it be 
 in motion, it can only be suddenly stopped by the 
 exercise of very considerable force, which jars and 
 sometimes bursts the pipe. The impetus of running 
 water is utilized in the water-ram. It must naturally 
 occur, therefore, to ask whether any analogous phe- 
 nomena are experienced with electricity ; and the 
 
CHAP, v.] CURRENT PHENOMENA. 89 
 
 answer is, they certainly are. A current does not start 
 instantaneously : it takes a certain time often very 
 short to rise to its full strength ; and when started 
 it tends to persist, so that if its circuit be suddenly 
 broken, it refuses to stop quite suddenly, and bursts 
 through the introduced insulating partition with 
 violence and heat. It is this ram or impetus of the 
 electric current which causes the spark seen on break- 
 ing a circuit ; and the more sudden the breakage the 
 more violent is the spark apt to be. 
 
 The two effects the delay at making circuit, and 
 the momentum at breaking circuit used to be called 
 " extra-current " effects, but they are now more 
 commonly spoken of as manifestations of " self- 
 induction." 
 
 We shall understand them better directly ; mean- 
 while they appear to be direct consequences of the 
 inertia of electricity ; and certainly if electricity 
 ^cvere a fluid possessing inertia it would behave to a 
 superficial observer just in this way. 
 
 39. But if an electric current really possessed inertia, 
 as a stream of water does, it would exhibit itself not 
 only by these effects but also mechanically. A con- 
 ducting coil delicately suspended might experience a 
 rotary kick every time a current was started. or stopped 
 in it ; and a coil in which a steady current is main- 
 tained should behave like a top or gyrostat, and 
 resist any force tending to deflect its plane. 
 
90 MODERN VIEWS OF ELECTRICITY. [PART 11. 
 
 Clerk Maxwell has carefully looked for this latter 
 form of momentum effect, and found none. He took 
 a bar electro-magnet, mounted it on gimbals so that it 
 was free to rotate if it wished, and then spun it rapidly 
 about an axis perpendicular to the magnetic axis. If 
 there had been the slightest gyrostatic action, the 
 magnet would have rotated about the third perpen- 
 dicular axis. But it did nothing of the kind. One may 
 say, in fact, that nothing like momentum has yet been 
 observed in an electric current by any mechanical mQ&t 
 of examination. A coil or whirl of electricity does not 
 behave in the least like a top ( 185). 
 
 Does this prove that a current has no momentum ? 
 By no means necessarily so. It might be taken as 
 suggesting that an electric current consists really of 
 two equal flows in contrary directions, so that 
 mechanically they neutralize one another completely, 
 while electrically i.e. in the phenomena of self-induc- 
 tion or extra-current they add their effects ( 89). 
 Or it may mean merely that the momentum is too 
 minute to be so observed. Or, again, the whole 
 thing the appearance of inertia in some experi- 
 ments and the absence of it in others may have to 
 be explained in some altogether less simple manner, 
 to which we will proceed to lead up. 
 
CHAP, v.] CURRENT PHENOMENA. 91 
 
 Condition of the Medium near a Circuit. 
 
 40. So far we have considered the flow of electricity 
 as a phenomenon occurring solely inside conductors ; 
 just as the flow of water is a phenomenon occurring 
 solely inside pipes. But a number of remarkable 
 facts are known which completely negative this view 
 of the matter. Something is no doubt passing along 
 conductors when a current flows, but the disturbance 
 is not confined to the conductor ; on the contrary, it 
 spreads more or less through surrounding space. 
 
 The facts which prove this have necessarily no 
 hydraulic analogue, but must be treated suorum 
 generum^ and they are as follows : 
 
 (1) A compass needle anywhere near an electric 
 current is permanently deflected so long as the 
 current lasts. 
 
 (2) Two electric currents attract or repel one 
 another, according as they are in the same or 
 opposite directions. 
 
 (3) A circuit in which a current is flowing tends to 
 enlarge itself so as to inclose the greatest possible 
 area. 
 
 (4) A circuit conveying a current in a magnetic 
 field tends either to enlarge or to shrink or to turn 
 part way round according to the aspect it presents to 
 the field, 
 
92 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 (5) Conductors in the neighbourhood of an electric 
 circuit experience momentary electric disturbances 
 every time a current in it is started or stopped or 
 varied in strength. 
 
 (6) The same thing happens even with a circuit 
 conveying a steady current if the distance between it 
 and a conductor is made to vary. 
 
 (7) The effects of self-induction, or extra-currents, 
 can be almost abolished by doubling a covered wire 
 conveying the current closely on itself, or better by 
 laying a direct and return ribbon face to face ; 
 whereas they may be intensified by making the 
 circuit inclose a large area, more by coiling it up 
 tightly into close coil, and still more by putting a 
 piece of iron inside the coil so formed. 
 
 Nothing like any of these effects is observable with 
 currents of water ; and they prove that the pheno- 
 mena of the current, so far from being confined to 
 the wire, spread out into space and affect bodies at 
 a considerable distance. 
 
 41. Nearly all this class of phenomena were dis- 
 covered by Ampere and by Faraday, and were called 
 by the latter " current-induction." According to his 
 view, the dielectric medium round a conducting circuit 
 is strained, and subject to stresses, just as is the same 
 medium round an electrically charged body. The 
 one is called an electrostatic strain, the other an 
 electro-magnetic or electro-kinetic strain. 
 
CHAP, v.] CURRENT PHENOMENA. 93 
 
 But whereas electrostatic phenomena occur solely in 
 the medium conductors being mere breaks in it, 
 interrupters of its continuity, at whose surface charge- 
 effects occur, but whose substance is completely 
 screened from disturbance that is not the case with 
 electro-kinetic phenomena. It would be just as 
 erroneous to conceive electro-kinetic phenomena as 
 occurring solely in the insulating medium as it would 
 be to think of them as occurring solely in the conduct- 
 ing wires. The fact is> they occur in both not only 
 at the surface of the wires, like electrostatic effects, but 
 all through their substance. This is proved by the 
 fact that conductivity increases in simple proportion 
 with sectional area ; it is also proved by every part of 
 a conductor getting hot ; and it is further proved in 
 the case of liquids by their decomposition. 
 
 But the equally manifest facts of current attraction 
 and current induction prove that the effect of the 
 current is felt throughout the surrounding medium as 
 well, and that its intensity depends on the nature of 
 that medium ; we are thus wholly prevented from 
 ascribing the phenomenon of self-induction or extra- 
 current to simple and straightforward inertia of 
 electricity in a wire like that of water in a pipe. 
 
 We are thus brought face to face with another 
 suggestion to account for these effects, viz. this : Since 
 the molecules of a dielectric are inseparably connected 
 with electricity, and move with it, it is possible that 
 
94 MODERN VIEWS OF ELECTRICITY. [PART H. 
 
 electricity itself has no inertia at all, but that the 
 inertia of the atoms of the displaced dielectric confer 
 upon it the appearance of inertia. Certainly they do 
 sometimes confer upon it this appearance, as we see 
 in the oscillatory discharge of a Leyden jar. For a 
 displaced thing to overshoot its mean position and 
 oscillate till it has expended all its energy is a pro- 
 ceeding eminently characteristic of inertia ; and so, 
 perhaps, the phenomena of self-induction are similarly, 
 though not so simply, explicable ( 98). 
 
 Further consideration of this difficult part of the 
 subject is, however, best postponed to Part III. 
 ( 48 and 88). 
 
 Energy of the Current. 
 
 42. I have now called attention to the fact that the 
 whole region surrounding a circuit is a field of force 
 in which many of the most important properties of the 
 current (the magnetic, to wit) manifest themselves. 
 But directly we begin thus to attend to the whole 
 space, and not only to the wires and battery, a very 
 curious question arises. Are we to regard the current 
 in a conductor as propelled by some sort of end-thrust, 
 like water or air driven through a pipe by a piston or 
 a fan, or are we to think of it as propelled by side 
 forces, a sort of lateral drag, like water driven along a 
 trough by a blast of air or by the vanes of paddle- 
 
CHAP, v.] CURRENT PHENOMENA. 95 
 
 wheels dipping into it ? Or, again, referring to the 
 cord models, Figs. 5, 6, and 13, were we right in 
 picturing the driving force of the battery as located 
 and applied where shown in the diagrams, or ought 
 we to have schemed some method for communicating 
 the power of the battery by means of belts or other 
 mechanism to a great number of points of the 
 circuit ? 
 
 Prof. Poynting has shown that, on the principles 
 developed by Maxwell, the latter of these alternatives, 
 though apparently the more complicated, is the true 
 one ; and he has calculated the actual paths by which 
 the energy is transmitted from the battery to the 
 various points of a circuit, for certain cases. 
 
 We must learn, then, to distinguish between the 
 flow of electricity and the flow of electric energy : they 
 do not occur along the same paths. Hydraulic ana- 
 logies, at least hydraulic analogies of <a simple kind, 
 break down here. When hydraulic power or steam 
 power is conveyed along pipes, the fluid and its 
 energy travel together. Work is done at one end of 
 the tube in forcing in more water, and this is pro- 
 pagated along the tube and reappears at the distant 
 end as the work of the piston. But in electricity it is 
 not so. Electric energy is not to be regarded as 
 pumped in at one end of a conducting wire, and as 
 exuding in equal quantities at the other. The elec- 
 tricity does indeed travel thus whatever the travel of 
 
96 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 electricity may ultimately be found to mean but the 
 energy does not. The battery emits its energy, not 
 to the wire direct, but to the surrounding medium ; 
 this is disturbed and strained, and propagates the 
 strain on from point to point till it reaches the wire 
 and is dissipated. This, Prof. Poynting would say, is 
 the function of the wire : it is to dissipate the energy 
 crowding into it from the medium, which else would 
 take up a static state of strain and cease to transmit 
 any more. It is by the continuous dissipation of the 
 medium's energy into heat that continuous propaga- 
 tion is rendered possible ( 107). 
 
 The energy of a dynamo does not therefore travel 
 to a distant motor through the wires, but through the 
 air. The energy of an Atlantic cable battery does 
 not travel to America through the wire strands, but 
 through the insulating sheath. This is a singular and 
 apparently paradoxical view, yet it is well founded. 
 
 Think of a tram-car drawn by an underground 
 rope, like those in the streets of Chicago or Hamp- 
 stead Hill. A contact piece of iron protrudes from 
 the bottom of the car and grips the moving rope, 
 which is thus enabled to propel the car. How does 
 the energy of the distant stationary engine reach the 
 car ? Via the rope and the iron connector, un- 
 doubtedly. They both have to be strong, and are 
 liable to be broken by the transmitted stress. 
 
 Next, think of an electric tram-car driven by means 
 
CHAP, v.] CURRENT PHENOMENA. 97 
 
 of a current taken up from an underground conductor, 
 like that of Mr. Holroyd Smith at Manchester, or at 
 the late Inventions Exhibition. A contact piece of wire 
 rope protrudes from the bottom of the car and drags 
 a little truck along the conductor, which is thus 
 enabled to supply electricity to the electro-magnetic 
 motor geared to the wheels. How does the energy of 
 the distant dynamo reach the car in this case ? Not 
 via the wire connector ; not even via the underground 
 conductor. It travels from the distant dynamo 
 through the general insulating medium between cable 
 and earth, some little enters the conductor and is 
 dissipated, but the great bulk flows on and converges 
 upon the motor in the car, which is thus propelled. 
 All the energy of the conducting wire is dissipated 
 and lost as heat : it is the energy of the insulating 
 medium which is really transmitted and utilized. 
 
 The manner in which the transmission of energy 
 goes on we will attend to further in Part III. ( 105 
 et seq.}. 
 
 Phenomena Peculiar to a Starting, or Stopping, or 
 Varying Current. 
 
 43. There is a remarkable fact concerning electric 
 currents of varying strength, which has been lately 
 brought into prominence by the experimental skil}, 
 of Prof. Hughes, viz. that a current does not start or 
 
 H 
 
98 MODERN VIEWS OF ELECTRICITY. [PART IT. 
 
 stop equally and simultaneously at all points in the 
 section of a conductor, but starts at the outside first. 
 This fact is naturally more noticeable with thick wires 
 than with thin, and it is especially marked in iron wires, 
 for reasons which in Part III. will become apparent ; 
 but the general cause of it in ordinary copper wires 
 can very easily be perceived in the light of the views 
 of Prof. Poynting just mentioned. 
 
 For, remember that a current in a wire is not 
 pushed along by a force applied at its end, so as to be 
 driven over obstacles by its own momentum combined 
 with a vis a tergo ; but it is urged along at every point 
 of its course by a force just sufficient to make it over- 
 come the resistance there, and no more, the force being 
 applied to it through the medium of the dielectric 
 in which .the wire is immersed. A lateral force it is 
 which propels the electricity ; and it naturally acts 
 first on the outer layers of the wire or rod, only acting 
 on the interior portions through the medium of the 
 outside ( 102). 
 
 44. To illustrate this matter further, rotate a 
 common tumbler of liquid steadily for some time and 
 watch the liquid ; dusting powder perhaps over it to 
 make it more visible. You will see first the outer 
 layer begin to participate in the motion, and then the 
 next, and then the next, and so on, until at length the 
 whole is in rotation. Stop the tumbler, and the liquid 
 also begins gradually to stop by a converse process : 
 
CHAP, v.] CURRENT PHENOMENA. 99 
 
 the outside stopping first, and then gradually the 
 central portions. 
 
 If the liquid sticks together pretty well, like treacle, 
 the motion spreads very rapidly : this corresponds to 
 a poor conductor. If the liquid be very mobile, the 
 propagation of motion inward is slow : this corre- 
 sponds to a very good conductor. If the liquid were 
 perfectly non-viscous, it would correspond to a perfect 
 conductor, and no motion would ever be commu- 
 nicated to it deeper than its extreme outer skin. 
 
 Think now of an endless tube full of water, say 
 the hollow circumference of a wheel, or the rim of 
 a top, and spin it : the liquid is soon set in rotation, 
 especially if the tube be narrow or the liquid viscous ; 
 but it is set in motion by a lateral not an end force, 
 and its outer layers start first. 
 
 Just so is it with a current starting in a metal wire. 
 If the wire be fine, or its substance ba'dly conducting, 
 it all starts nearly together ; but if it be made pretty 
 thick, and of well conducting substance, its outer 
 layers may start appreciably sooner than the interior. 
 And if it were infinitely conducting, no more than the 
 outer skin would ever start at all (see Chap. X. and 
 
 In actual practice the time taken for all the 
 electricity in an ordinary wire to get into motion is 
 excessively short something like the thousandth 
 of a second so that the only way to notice the 
 
 H 2 
 
ioo MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 effect is to start and reverse the current many times 
 in succession. 
 
 45. If the hollow-rimmed wheel above spoken of were 
 made to oscillate rapidly, it is easy to see- that only 
 the outer layers of water in it would be moved to and 
 fro ; the innermost water would remain stationary ; 
 and accordingly it would appear as if the tube contained 
 much less water than it really does. The virtual 
 bore of the pipe would, in fact, for many purposes be 
 diminished. So is it also with electricity ; the sectional 
 area of a wire to a rapidly alternating current is 
 virtually lessened so far as its conducting power is 
 concerned ; and accordingly its apparent resistance 
 is higher for alternating than for steady currents. 
 The effect is, however, too small to notice in practice, 
 except with thick wires and very rapid alternations. 
 
 By splitting up the conductor into a bundle of 
 insulated wires, thus affording the dielectric access to 
 a considerable surface of conductor, the force is applied 
 much more thoroughly, and so the effect spoken of is 
 greatly lessened. The same thing is achieved by 
 rolling out the conducting-rod into a flat thin bar. 
 Making the conductor hollow instead of solid offers 
 no particular advantage, beyond the gain of surface 
 per given weight, because no energy travels via the 
 hollow space, it still arrives only from the outside ; 
 unless, indeed, the return part of the circuit is taken 
 along the axis of the hollow like a telegraph cable. 
 
CHAP, v.] CURRENT PHENOMENA. 101 
 
 In this last arrangement all the energy travels via the 
 dielectric between the two conductors, and none 
 travels outside at all. It will be perceived therefore 
 that, as in static electricity, the term " outside " must 
 be used with circumspection : it really means thait 
 side of a conductor which faces the opposite conductor, 
 across a certain thickness of dielectric.- . . -;.:': 
 
 46. We learn from this that, whereas in the case of 
 steady currents the sectional area and material of a 
 conductor are all that need be attended to, the case is 
 different when one has to deal with rapidly alternating 
 currents, such as occur in a telephone, or, again, such 
 as are apt to occur in a Leyden-jar discharge (see 
 Part I., p. 41), or in lightning. 
 
 In all these cases it is well to make the conductor 
 expose considerable surface to the propelling medium 
 the dielectric else will great portions of it be 
 useless. 
 
 Hence, a lightning-conductor should not be a round 
 rod, but a flat strip, or a strand of wires, with the 
 strands as well separated as convenient. 
 
 47. I might go on to say here that iron makes an 
 enormously worse conductor than copper for rapidly 
 alternating currents. So it does for currents which 
 alternate with moderate rapidity a few hundred or 
 thousand a second like those from a dynamo or 
 a telephone ; but, singularly enough, when the rapidity 
 of oscillation is immensely high, as it is in Leyden-jar 
 
102 MODERN VIEWS OF ELECTRICITY. [PART ir. 
 
 discharges and lightning, iron is every bit as good as 
 copper, because the current keeps to the extreme outer 
 layer of the conductor, and so practically does not find 
 out what it is made of. 
 
 The Question of Electrical Momentum again. 
 
 48. We are now able to return to the important 
 question whether an electric current has any mo- 
 mentum or not, as it would have if it were a flow of 
 
 FIG. 14. Stream-lines of water flowing through a pipe with an obstruction in it. 
 
 material liquid. Referring to Part I. 7, a hint will 
 be found that the laws of flow of a current in con- 
 ductors the shape of the stream-lines, in fact are 
 such as indicate no inertia, or else no friction. Now 
 Ohm's law shows that at any rate friction is not 
 absent from a current flowing through a metal ; hence 
 it would appear at first sight as if inertia must be 
 absent. 
 
 The stream-lines bear upon the question in the 
 
CHAP, v] CURRENT PHENOMENA. 103 
 
 following kind of way. If an obstacle is interposed 
 in the path of a current of water, the motion of the 
 water is unsymmetrical before and behind the obstacle. 
 The stream-lines spread out as the water reaches the. 
 obstacle, and then curl round it, leaving a space full 
 of eddies in its wake (Fig. 14). 
 
 But if one puts an obstacle in the path of an electric 
 current say by cutting a slit in a conducting strip of 
 tinfoil the stream-lines on either side of it are quite 
 symmetrical, thus 
 
 FIG. 15. Electrical stream-lines past an 6bstacle. 
 
 And this is exactly what would be true for water 
 also, if only it were devoid either of friction or of 
 inertia, or of both. 
 
 49. Is not this fact conclusive, then ? Does it not 
 prove the absence of momentum in electricity ? 
 
 Plainly the answer must depend on whether there is 
 any other possible mode of accounting for this kind of 
 flow. And there is. 
 
 For suppose that water, instead of being urged by 
 
104 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 something not located at or near the obstacle instead 
 of being left to its own impetus to curl round or shoot 
 past as it pleases suppose it were propelled by a force 
 acting at every point of its journey, a force just able 
 to drive it at any point against the friction existing 
 at that point and no more ; then the flow of water 
 would take place according to the electrical stream- 
 lines shown in Fig. 15. 
 
 An illustration of such a case is ready to hand. 
 Take a spade-shaped piece of copper wire or sheet, 
 heat it a little, and fix it in quiescent smoky air ; 
 looking along it through a magnifier in a strong light 
 you will see the warmed air streaming up past the 
 metal according to the stream-lines of Fig. 15; and 
 this just because the moving force has its location at 
 the metal surface, and not in some region below it. 1 
 One cannot indeed say that it is propelled at every 
 point of its course, but it is propelled at the critical 
 points where the special friction occurs, and this 
 comes to sufficiently the same thing. 
 
 We learn, therefore, that stream-lines like Fig. 15 
 prove one of three things, not one of two ; and the 
 three things are : (i) that the fluid has no friction ; 
 or (2) that it has no inertia ; or (3) that it is propelled 
 at every point of its course. 
 
 If any one of these is true of electricity, there is no 
 need to assume either of the others in order to explain 
 
 1 See Lord Rayleigh, Nature, vol. xxviii. p. 139. 
 
CHAP, v.] CURRENT PHENOMENA. 105 
 
 the actual manner of its flow. Now we have just 
 seen in 42 that, according to Prof. Poynting's inter- 
 pretation of Maxwell's theory, the third of the above 
 is true electricity is propelled at every point of its 
 course ; consequently, as said in Part I. 7, the 
 question of its inertia so far remains completely 
 open ( 88, 89, and 98). 
 
CHAPTER VI. 
 
 CHEMICAL AND THERMAL METHODS OF PRODUCING 
 CURRENTS. CONDUCTION IN GASES. 
 
 Voltaic Battery. 
 
 50. Leaving this singular mode of regarding the 
 subject for the present, to return to it in Chap. X. 
 Part III., let us proceed to ask how it comes about 
 that a common battery or a thermopile is able to 
 produce a current (read Chapter IV. again), 
 
 If we allow ourselves to assume the existence of an 
 unexplained chemical attraction between the atoms 
 of different substances, an explanation of the action 
 of an ordinary battery cell is easy. You have first 
 the liquid containing, let us say, hydrogen and oxygen 
 atoms, free or potentially free that is, either actually 
 dissociated, or so frequently interchanging at random 
 from molecule to molecule that the direction of their 
 motion may be guided by a feeble directive force ( 33). 
 Each of these atoms in the free state possesses a charge 
 of electricity the hydrogen all a certain amount of 
 
CHAP, vi.] PRODUCTION OF A CURRENT. 107 
 
 positive electricity, the oxygen twice that amount of 
 negative. Into this liquid you then plunge a couple 
 of metals which attract these atoms differently : for 
 instance, zinc and copper, which both attract oxygen, 
 but zinc more than copper ; or, better, zinc and 
 platinum, the latter of which hardly attracts it at all ; 
 or, better still, zinc and peroxide of lead, one of which 
 attracts oxygen, the other hydrogen. 
 
 Immediately, the free oxygen atoms begin moving 
 up to the zinc, the free hydrogen atoms to the other 
 plate. 
 
 51. When one speaks of the plates attracting the 
 atoms, it is not necessary to think of their exerting a 
 force on all those in the liquid, distant and near : all 
 that is necessary is to assume a force acting on those 
 which come within what is called " molecular range " 
 of its surface a distance extremely minute, and 
 believed to be about the ten-millionth part of a 
 millimetre. If the zinc plate removes and combines 
 with all the oxygen atoms which come within this 
 range, they will be speedily replaced by others from 
 the next more distant layer by diffusion, and these 
 again by others, and so on. And thus there will be 
 a gradual procession of oxygen atoms all through the 
 liquid towards the zinc, the rate of the procession 
 being regulated by the force acting, and by the rate 
 of diffusion possible in the particular liquid used. All 
 the atoms which reach the zinc neutralize a certain 
 
io8 MODERN VIEWS OF ELECTRICITY. [PART IT. 
 
 / 
 
 portion of its electricity by means of the positive 
 charge they carry, and thus very soon it would 
 become positively electrified enough to neutralize 
 its attractive power on the similarly charged oxygen 
 atoms, and everything would stop. But if a channel 
 for the escape of its electricity be provided by leading 
 a wire from it .to the copper plate, the circuit is 
 completed, the electricity streams back by the wire, 
 and the procession goes steadily on. The electricity 
 thus imparted to the copper, or platinum, neutralizes 
 any repulsion it exerted on the negatively charged 
 hydrogen atoms, and make them in a similar way 
 begin a procession towards it, deliver up their charges 
 to it, combine with each other, and escape as gas. 
 
 Without going into all the niceties possible, this 
 mode of thinking of the matter at least calls attention 
 to some of the more salient features of a battery. 
 
 52. If, instead of two different plates, plates of 
 the same metal be immersed, they will need to be 
 oppositely electrified by some means before they 
 are able to cause the two opposite processions, and 
 so maintain a current in the liquid. This plainly 
 corresponds to a voltameter. 
 
 53. Taking advantage of the known fact that the 
 atoms are charged, Helmholtz avoids the necessity 
 for postulating any chemical (non-electrical) force 
 between zinc and oxygen, by imagining that all 
 substances have a specific attraction for electricity 
 
CHAP, vi.] PRODUCTION OF A CURRENT. 109 
 
 itself, and that zinc exceeds copper and the other 
 common metals in this respect. 
 
 He would thus think of the zinc attracting, not the 
 oxygen itself, but its electric charge ; and so would 
 liken a battery cell still more completely to a 
 voltameter. The polarization or opposition force 
 acting at the hydrogen-evolving plate he would 
 account for by the attraction of hydrogen for negative 
 electricity, and the consequent repugnance of the 
 hydrogen atoms to part with their charges. 
 
 Voltas so-called Contact Force. 
 
 54. It may be convenient to append to this account 
 of the action of a battery a statement of the way in 
 which the electric charges observed on plates of zinc 
 and copper which have been put into contact and 
 separated are brought about. It is a very simple 
 matter, though a great deal has been written about it. 
 
 Plates of zinc and copper immersed in air are 
 under precisely the same chemical conditions as if 
 they were immersed in water. The only difference is 
 that, whereas water is a conductor, air is an insulator. 
 Until the plates of zinc and copper (or other pair of 
 metals) are made to touch, nothing happens in either 
 case, because the chemical tendency is uniform all 
 over both plates ; and though the attraction of the 
 
no MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 zinc for oxygen is pretty strong it would be impossible 
 for it to combine with many atoms, receiving their 
 charges, without becoming so negatively charged as 
 to repel them electrically as much as it attracts them 
 chemically. This, indeed, may be considered as the 
 state of equilibrium, which is instantaneously attained. 
 
 But directly metallic contact between the two 
 metals is effected, all the oxygen atoms at this point 
 are swept away, and a clear passage is opened from 
 the zinc to the copper for the flow of electricity. 
 Unless, therefore, there is some E.M.F. at their junction 
 which we have good reason ,for asserting there is not, 
 of any magnitude worth speaking of an immediate 
 rush of negative electricity from zinc to copper, or of 
 positive the other way, occurs. The copper therefore 
 becomes negatively charged, the zinc becomes positive. 
 So far everything goes on just the same whether the 
 plates are in acidulated water or in common air. 
 
 What happens next depends upon the difference 
 between water and air in conducting power that is, 
 in the existence of the potentially free or dissociated 
 atoms necessary to electrolytic conduction. Acidulated 
 water possesses these ; air does not. Accordingly, 
 in the case of the water the negative charge is being 
 continually carried back from the copper to the zinc 
 by a procession of oxygen atoms, which continually 
 are drawn up against the zinc and combine with it, 
 as already explained ; whereas in the air nothing 
 
CHAP, vi.] PRODUCTION OF A CURRENT. in 
 
 further happens except the slight electrostatic strain 
 into which the air is thrown by the quantity of 
 electricity accumulated upon the metals, positive on 
 the zinc, negative on the copper, and which has no 
 vent or outlet. Ordinarily these charges will be 
 extremely small, the electromotive force producing 
 them being rather under than over I volt, and accord- 
 ingly the electrostatic strain in the air near a couple 
 of zinc and copper plates in contact is extremely 
 minute. By suspending a delicate aluminium needle 
 highly charged near such a junction, however, Sir 
 William Thomson has been able to observe the 
 state of strain : the needle if positively charged moving 
 perceptibly towards the copper. The more usual 
 method of rendering the phenomenon conspicuous, and 
 the one originally used by Volta, is to increase the 
 capacity of the arrangement by bringing two carefully 
 ground plates very close together. Although the 
 E.M.F. is small (just the same as with a mere point con- 
 tact), yet now the capacity is so great that quite a rea- 
 sonable quantity of electricity can be stored in the two 
 opposing metals, opposing each other across an air film 
 and only really touching at a few points ; so that when 
 they are neatly separated sufficient charge is found in 
 them to affect even a common gold-leaf electroscope. 
 
 55. The mistake which has been, and still frequently 
 is, made with regard to this simple and not very 
 important experiment, has been to regard the charge 
 
ii2 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 as evidence of a peculiar E.M.F., at the point of contact, 
 causing a difference of potential in the two metals. 
 And this fictitious contact E.M.F. has then been 
 appealed to to explain the voltaic battery. 
 
 The right way of regarding the matter is to con- 
 sider the battery first, and explain its action chemically 
 so far as it is possible to explain it at present ; and 
 then to point out that similar things will occur in air 
 (an air battery, in fact), with the slight difference that 
 since air is a dielectric instead of an electrolyte no 
 continuous current is possible, but merely a slight 
 electric displacement 
 
 56. The effective cause of the whole phenomenon 
 in either case is the greater affinity of oxygen for zinc 
 rather than copper. This by itself would cause a 
 greater strain of negative electricity towards zinc a 
 slackening of the negative cords in it, to speak in the 
 language of the cord model and a consequent rise of 
 negative potential. A piece of isolated zinc is there- 
 fore some 1*8 volt below the potential of the atmo- 
 sphere. The same sort of thing is true for copper 
 except that the intensity of strain is less ; as evidenced 
 by the less heat of formation of CuO compared with 
 ZnO ; and accordingly a piece of isolated copper is 
 about o - 8 volt below the potential of the atmosphere. 
 
 Directly the two metals touch they necessarily 
 become of the same potential all parts of a conductor 
 are at one potential unless there are disturbing internal 
 
CHAP, vi.] PRODUCTION OF A CURRENT. , 113 
 
 forces and the equalization of potential is effected 
 by the rush of electricity across the junction, whereby 
 the zinc receives a positive charge and the copper a 
 negative charge, until their potential is equalized. In 
 air the equalization is effected in an instant. In water 
 it is a matter of eternity. That is all the difference. 
 The thing observed in the Volta effect is not a differ- 
 ence of potential between zinc and copper, but a 
 
 
 
 -4 L= 
 
 
 - 1 1 
 
 
 
 I i I 
 
 
 I Cu - l 
 
 ir 
 
 \- 1 
 
 
 Y 
 
 XV 
 
 / / 
 
 FIG. ISA. Diagrammatic representation of the Volta effect on the plan of the cord 
 models (Figs. 5, 6, 7, &c.). 
 
 I. shows a piece of zinc and copper before contact, with a cord representing 
 
 negative electricity passing through both, and beads representing oxygen 
 .atoms. The arrows indicate that the oxygen is being pulled by the 
 zinc on all sides of it ; and that it is also pulled by the copper but with 
 less force. 
 
 II. shows the effect of sweeping away the oxygen atoms between the two 
 
 metals and establishing metallic contact, so that the greater atom-attract- 
 ing force of zinc over copper can now produce an effect until it is balanced 
 by the elastic stress called out by an electric displacement. The surface of 
 the zinc has now less than its normal share of negative^ cord it is 
 positively charged. The copper is negatively charged. This is the Volta 
 effect. 
 
 difference of charge ; the two metals being charged 
 so as to make their potential uniform. 
 
 What is observed in the Sir William Thomson 
 form of the experiment is again not a difference 
 of potential between zinc and copper, but a slope of 
 
 I 
 
ii4 MODERN VIEWS OF ELECTRICITY. [PART IT. 
 
 potential in the air near them, from the zinc towards 
 the copper. The metals when in contact are both 
 at a common potential, 1*3 volts below the 
 atmosphere, the mean of their original potentials, 
 but the original difference of potential between each 
 and the air in contact with it remains unaltered ; 
 hence there is a gradual slope of potential of I 
 volt from the layer of air in contact with zinc 
 to the layer in contact with copper ; and this slope 
 of potential is what the electrometer needle feels. 
 The diagram Fig. I5A may possibly help in making 
 the thing clear. 
 
 True Contact Force. 
 
 57. So far we have assumed that there is actually 
 no force at the contact of zinc with copper. There 
 is indeed none of any appreciable magnitude, but 
 the force is not absolutely zero. Between some 
 metals, bismuth and antimony for example, the force 
 is much larger, but it is still only a few hundredths 
 of a volt. It is an important thing that there can be 
 a true contact force at the junction of two metals, 
 only it has nothing to do with the chemically 
 produced Volta effect. If the Volta effect be called 
 a contact force at all, it is a contact force between 
 metal and air ; any true contact force between 
 metals acts as a slight and insignificant disturber of 
 
CHAP, vi.] PRODUCTION OF A CURRENT. 115 
 
 the simple Volta effect, and what is really observed 
 in electroscopic experiments is the sum of the two. 
 
 58. That there is a true though weak contact 
 force at the junction of metals is proved by the 
 reversible heat effects which are found there when 
 a current is passed across the junction : a current 
 one way produces more heat than a current the 
 other way. In a simple homogeneous piece of 
 metal the heat produced by a current is utterly 
 independent of direction : it is called irreversible 
 heat ; it is proportional to the square of the cur- 
 rent strength, as Joule showed. But at a junction 
 of different substances, or even at a junction of 
 the same substance in two different states two dif- 
 ferent temperatures, for example, in addition to the 
 irreversible heat produced by mere resistance there 
 is a reversible heat production, one which changes 
 sign with the direction of the current, so that the 
 current one way actually tends to cool the junction 
 instead of heating it. With care this may be got 
 to overpower and mask the irreversible heat, and a 
 junction may be cooled and water frozen by steadily 
 passing a moderate current in the right direction across 
 it. This curious effect was discovered by Peltier. 
 
 It may be considered as the fundamental fact of 
 thermo-electricity. Its meaning is that something in 
 the metals at the junction is helping to propel the 
 current along ; doing work in fact, and consuming its 
 
 I 2 
 
ii6 MODERN VIEWS OF ELECTRICITY. [PART u. 
 
 own heat in the process. The vibratory motion 
 of the molecules is getting used up in propelling elec- 
 tricity. The contact force is acting in the direction 
 of the current. 
 
 If the current be reversed, it will be driven 
 against the force of the molecules, and an extra 
 amount of heat will be added to the irreversible or 
 frictional generation of heat. 
 
 59. This thermal evidence of contact force, though 
 the most direct, was not the earliest discovered. The 
 earliest known fact in thermo-electricity was that in a 
 complete circuit of different metals a current could be 
 excited by having the parts at different temperatures ; 
 manifestly because these contact forces we have 
 been speaking of change with temperature some 
 increasing, others decreasing. They are accurately 
 balanced in a circuit of uniform temperature, but 
 they have a resultant whenever the temperature is 
 not uniform, and this resultant propels the current 
 discovered by Seebeck. 
 
 Thermo-electric Pile. 
 
 60. A thermopile may be thought of in the 
 following way, but in trying to understand the nature 
 of these actions at present one must admit that some 
 speculation and vagueness exist. 
 
 We have seen that when electricity is propelled 
 
CHAP, vi.] PRODUCTION OF A CURRENT. 117 
 
 through or among the molecules of a metal it experi- 
 ences a certain resistance or opposition force which 
 is exactly proportional to the speed of its motion 
 ( 28). In other words, there is a connection between 
 matter and electricity in many respects analogous to 
 fluid friction but varying accurately as the first powei 
 of the relative velocity. Hence, if an atom of matter 
 be vibrating about a fixed point, it will tend to drive 
 electricity to and fro with it ; but if it be only one of a 
 multitude, all quivering in different phases, they will 
 none of them achieve any propulsion. This may be 
 considered the state of an ordinary warm solid. But 
 if from any cause a set of atoms could be made to 
 move faster in one direction than in the reverse 
 direction to move forwards quickly and backwards 
 slowly then such an unsymmetrically-moving set 
 will exert a propulsive tendency and tend to drive a 
 current of electricity forwards, simply because the 
 force exerted is proportional to the velocity, and so 
 is greater on the forward journey than on the return. 
 Referring back to the cord model, Fig. 5, Ohm's law 
 requires that the friction between cord and beads should 
 be directly as the velocity, hence if a bead begins to 
 oscillate unsymmetrically, travelling forward quickly 
 and back slowly, it will propel the cord along in the 
 direction in which it moves most quickly, somewhat 
 as a child can propel its chair by jogging it upon a 
 rough floor. 
 
ii8 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 Wherever conduction of heat is going on along a 
 substance the atoms are in this condition. They are 
 driven forward infinitesimally quicker by the more 
 rapidly moving atoms at the hot end, than they are 
 driven back by the less rapidly moving atoms in front. 
 And hence such a slope of temperature exerts a 
 propulsive tendency : there is an electromotive force 
 in a substance unequally heated. 
 
 This fact was discovered theoretically and verified 
 experimentally by Sir William Thomson. 
 
 61. But not only is there such a force at a junction 
 of a hot and cold substance ; there is also a force at 
 the junction of two substances of different kinds, 
 even though the temperature be uniform. It is not 
 quite so easy to explain how it now comes about that 
 the atoms at this kind of junction are moving faster 
 one way than the other ; nevertheless, such a thing is 
 not unlikely, considering the state of constraint and 
 accommodation which must necessarily exist at the 
 boundary surface of two different media. However it 
 be caused, there is certainly an E.M.F. at such a 
 junction. 
 
 Thus, then, in a simple circuit of two metals, with 
 their junctions at different temperatures, there are 
 altogether four electromotive forces one in each 
 metal, from hot to cold or vice versa, and one at each 
 junction ; and the current which flows round such a 
 circuit is propelled by the resultant of these four. 
 
CHAP, vi.] PRODUCTION OF A CURRENT. 119 
 
 These four forces, two Thomson forces in the metals, 
 and two Peltier forces at their junctions, may some 
 of them help and some hinder the current. Whenever 
 they help, the locality is to that extent cooled ; when- 
 ever they hinder, it is to that extent warmed. 
 
 Frictional Electricity. 
 
 62. But the contact force at a junction is by no 
 means confined to metals. It occurs between in- 
 sulators also, and it is to it that the striking effects 
 produced by all frictional electric machines are due. 
 The essential thing in the production of "frictional 
 electricity " is the contact of dissimilar substances. 
 It is by their contact force that electricity gets 
 transferred from one to the other so that one becomes 
 positive and the other negative. The violence of 
 friction is sometimes necessary to aid the transfer ; 
 the substances being so badly conducting. 
 
 By thus noticing that the connection between 
 matter and electricity, known as resistance and defined 
 by Ohm's law ( 28), is competent to produce contact 
 electromotive forces, we may perceive how it comes 
 to pass that in good conductors such forces are so 
 weak, while in insulators they are so strong. Elec- 
 tricity slips through the fingers of a metal as it were, 
 and the driving force it can exert is very feeble ; 
 while an insulator gets a good grip and thrusts it 
 along with violence. 
 
120 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 Specific Relation between Matter and Electricity ; 
 sometimes called " Specific Heat of Electricity" 
 
 63. The metals differ in their gripping power, 
 and, roughly speaking, the best conductor makes 
 the worst thermo-electric substance. A bad conduc- 
 tor, like antimony, or, still better, galena, or selenium, 
 or tellurium, makes a far more effective therm6- 
 electric element than a well-conducting metal. Not 
 that specific resistance is all that has to be con- 
 sidered in the matter ; there is also a specific relation 
 between each metal and the two kinds of electricity. 
 Thus, iron is a metal whose atoms have a better 
 grip of positive than of negative electricity, and so 
 a positive current gets propelled in iron from hot to 
 cold. Copper, on the other hand, acts similarly on 
 negative electricity, and it is a negative current 
 which is driven from hot to cold in copper. And all 
 the metals can be classed with one or other of these 
 two, except perhaps lead, which appears to grip 
 both equally, and so to exert no differential effect 
 upon either. 
 
 How this relation can be likened to a "specific 
 heat," may be thought out by attending to the last 
 paragraph of 61, and by regarding electricity as a 
 material fluid (see also 182). 
 
CHAP. VI.] PRODUCTION OF A CURRENT. 121 
 
 Pyro-electricity. 
 
 Certain crystals, called by mineralogists hemihedral, 
 having different forms at the two ends of their axis, 
 which may be called the A end and the B end respec- 
 tively, exhibit some properties not quite the same 
 in the direction A B as in the direction B A. They are 
 more easily scratched, for instance, in one sense than 
 in the other. Such crystals, of which the class of 
 tourmalines may be taken as the type, have other 
 very singular properties. Such of them as are fairly 
 clear are opaque to light in a singular fashion not 
 opaque to light polarized in all planes, but selectively 
 opaque. Vibrations occurring perpendicular to the 
 axis are rapidly quenched, so that one cannot see at 
 all through a slice taken perpendicular to the axis, 
 while vibrations occurring along the axis are trans- 
 mitted with but moderate absorption. This opacity 
 seems quite different from the conducting opacity of 
 metals, about which we shall speak later, for, in 
 the first place, the light stopped is not reflected, 
 but absorbed ; and, in the second place, a crystal 
 of tourmaline is not a conductor, but a very fair 
 insulator. 
 
 And yet there are some peculiarities about such 
 conducting power as it has which are very note- 
 
122 MODERN VIEWS OF ELECTRICITY. [PARTI I. 
 
 worthy, and which may be intimately connected with 
 the selective opacity which fits a slice of crystal cut 
 parallel to the axis for use as a "polarizer" in optics. 
 One of these peculiarities was found by Dr. S. P. 
 Thompson in conjunction with the present writer, viz. 
 that while, like all other uniaxial crystals, the con- 
 ductivities for heat along and across the axis are not 
 the same (being, in the case of tourmaline, less good 
 along the axis than across), yet, in addition to this, 
 a warming crystal conducts heat better in the sense 
 B A than in the sense A B, while a cooling crystal 
 does the opposite. While the temperature is rising 
 their heat gets conveyed more easily towards A than 
 towards B. 
 
 Whether on account of inequalities of temperature 
 thus set up, or for some more direct reason, the same 
 is true of electricity. And accordingly, while a crystal 
 is rising in temperature, positive electricity accumu- 
 lates at the A end, and negative electricity at the B 
 end. So long as the temperature remains constant 
 nothing further happens, except ordinary leakage, 
 principally no doubt over the surface, which may in 
 time completely mask the effect produced. On now 
 cooling the crystal, an inverse electrification will 
 be set up ; or, if no leakage had been permitted, 
 the effect of cooling will be to simply replace the 
 electricity displaced by the warming. 
 
 While the crystal is steady in temperature no per- 
 
CHAP, vi.] GASEOUS CONDUCTION. 123 
 
 ceptible difference in electric conductivity has been 
 detected by the writer between the sense A B and the 
 sense B A. Neither is there any di fference in the thermal 
 conductivity when the temperature is steady. Both 
 effects depend on a varying temperature. 
 
 Passage of Rlectricity through a Gas. 
 
 64. There remains to be said something about 
 the way in which electricity can be conveyed by 
 gases. 
 
 The first thing to notice is that there is no true 
 conduction through either gases or vapours ; in other 
 words, a substance in this condition seems to be- 
 have as a perfect insulator perhaps the only perfect 
 insulator there is. Not water vapour, not even 
 mercury vapour, is found to conduct in the least. 
 This shows that mere bombardment of molecules, 
 such as is known to go on in gases, is not sufficient 
 either to remove or to impart any electric charge. 
 
 The commonest way in which electricity makes its 
 way through a gas, setting aside the mere mechani- 
 cal conveyance by solid carrier, is that of disruptive 
 discharge. Let us try and look into the manner of 
 this a little more closely, if possible. 
 
 First of all, since locomotion is possible to the 
 molecules of a gas the same as of any other fluid, 
 
124 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 it is natural to ask why electrolysis does not go on 
 as in a liquid. Now, for electrolysis in a liquid two 
 conditions seemed necessary : first, that the atoms or 
 radicles in a molecule should be oppositely charged 
 with electricity ; second, that they should be in such 
 a condition (whether by dissociation or otherwise) 
 that interchanges of atoms from molecule to mole- 
 cule, or, in some other way, a procession of atoms, 
 could be directed in a given direction by a very 
 feeble or infinitesimal force. 
 
 Since a gas does not act as an electrolyte, one of 
 these conditions, or perhaps both, must fail. Either 
 the atoms of a gas-molecule are not charged, which is 
 a plausible hypothesis for elementary gases, or else 
 the atoms belonging to a gas-molecule remain indivi- 
 dually belonging to it, and are not readily passed on 
 from one to another. 
 
 When one says that a gas does not act as a com- 
 mon electrolyte, the experimental grounds of the state- 
 ment are that a finite electrostatic stress certainly is 
 possible in its interior a stress of very considerable 
 amount ; and when this stress does overstep the mark 
 and cause the material to yield, the yielding is evi- 
 dently not a quiet and steady glide or procession, but a 
 violent breaking down and collapse, due to insufficient 
 tenacity of something. One may therefore picture the 
 molecules of a gas, between two opposite electrodes 
 or discharge terminals maintained at some great 
 
CHAP, vi.] GASEOUS CONDUCTION. 125 
 
 difference of potential, as arranged in a set of paral- 
 lel chains from one to the other, and strained nearly 
 up to the verge of being torn asunder. In making 
 this picture one need not suppose any fixture of 
 individual molecules : there may be a strong wind 
 blowing between the plates ; but all molecules as 
 they come into the field must experience the stress, 
 and be relieved as they pass out. 
 
 65. If the applied slope of potential overstep a 
 certain limit, fixed by observation at something like 
 33,000 volts per linear centimetre for common air, the 
 molecules give way, the atoms with their charges rush 
 across to the plates, and discharge has occurred. The 
 number of atoms thus torn free and made able to 
 convey a charge by locomotion is so great that there 
 has never been found any difficulty in conveying any 
 amount of electricity by their means. In other words, 
 during discharge the gas becomes a 'conductor, and, 
 being a conductor by reason of locomotion of atoms, 
 it may be called an electrolytic conductor. 
 
 But whether the charge then possessed by each 
 carrier atom intrinsically belonged to it all the time, 
 or whether it was conferred upon the components of 
 the molecules during the strain and the disruption, 
 is a point not yet decided. 
 
 What is called " the dielectric strength " of a gas- 
 that is, the strain it can bear without suffering dis- 
 ruption and becoming for the instant a conductor 
 
126 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 depends partly on the nature of the gas, and very 
 largely on its pressure. Roughly, one may say 
 that a gas at high pressure is very strong, a gas 
 at low pressure very weak. An ordinary electrolyte 
 might be called a dielectric of zero strength. 
 
 One reason why pressure affects the dielectric 
 tenacity of a gas readily occurs to one : it is 
 certainly not the only one, but it can hardly help 
 being at least partially a vera causa ; and that is, 
 the fact that in a rare gas there are fewer molecules 
 between the plates to share the strain between them. 
 
 Thus if 40,000 volts per centimetre break down 
 ordinary air, 40 volts per centimetre ought to be 
 enough to effect discharge through air at a pressure 
 of about f millimetre of mercury ; and at a pressure 
 of 50 atmospheres 2,000,000 volts per centimetre 
 should be needed. 1 
 
 A Current regarded as a Moving Charge. 
 
 66. To review the ground we have covered so far. 
 We first tried to get some conception of the nature 
 of electrostatic charge, and the function of a dielectric 
 medium in static electricity. We next proceeded to 
 
 1 It is true that tension per unit area, or energy per unit volume, is 
 proportional to the sqttare of the potential-slope, and I attach no 
 special importance to the simple proportion assumed in the text. There 
 is a great deal more to be said on these subjects, but this is scarcely 
 the proper lace to say it. 
 
CHAP, vi.] CONVECTION CURRENT. 127 
 
 see how far the phenomena of current electricity could 
 be explained by reference to electrostatics. For a 
 current, being merely electricity in locomotion, need 
 consist of nothing but a charged body borne rapidly 
 along. 
 
 Charge a sphere with either positive or negative 
 electricity, and throw it in some direction ; this con- 
 stitutes a positive or a negative current in that 
 direction. There is nothing necessarily more occult 
 than that. And a continuous current between two 
 bodies may be kept up by having a lot of pith 
 balls, or dust particles, oscillating from one to the 
 other, and so carrying positive electricity one way, 
 and negative the other way. But such carriers, as 
 they pass each other with their opposite charges, 
 would be very apt to cling together and combine. 
 They might be torn asunder again electrically, or 
 they might be knocked asunder by collision with 
 others. Unless they were one or other, the current 
 would shortly have to cease, and nothing but a 
 polarized medium would result. 
 
 Instead of pith balls, picture charged atoms as so 
 acting, and we have a rough image of what is going 
 on in an electrolyte on the one hand, and a di- 
 electric on the other. The behaviour of metals and 
 solid conductors is more obscure. Locomotive car- 
 riage is not to be thought of in them ; but, inas- 
 much as no new phenomenon appears in their case, 
 
128 MODERN VIEWS OF ELECTRICITY. [PART n. 
 
 it is natural to try and picture the process as one 
 not wholly dissimilar ; and this is what in 27 
 we tried to do ; with, however, but poor success. 
 
 67. I have said that an electric current need be 
 nothing more occult than is a charged sphere moving 
 rapidly ; and a good deal has been made out concern- 
 ing currents by minutely discussing all that happens 
 in such a case. But, even so, the problem is far from 
 being a simple one. One has to consider not only the 
 obviously moving charge, but also the opposite induced 
 charge tied to it by lines of force (or tubes of induc- 
 tion, as they are sometimes called), and we have this 
 whole complicated system in motion. And the effect 
 of this motion is to set up a new phenomenon in the 
 medium altogether a spinning kind of motion that 
 would not naturally have been expected ; whereby 
 two similarly charged spheres in motion repel one 
 another less than when stationary, and may even 
 begin to attract, if moving fast enough ; whereby also 
 a relation arises between electricity and magnetism, 
 and the moving charged body deflects a compass needle 
 ( 113 and 184). Of which more in the next Part. 
 
PART III. 
 MAGNETISM. 
 
 K 
 
CHAPTER VII. 
 
 RELATION OF MAGNETISM TO ELECTRICITY. 
 
 68. WE next proceed to consider electricity in a 
 state of rotation. What happens if we make a whirl- 
 pool of electricity ? Coil up a wire conveying a 
 current, and try. The result is it behaves like a 
 magnet : compass-needles near it are affected, steel 
 put near it gets magnetized, and iron nails or filings 
 get attracted by it sucked up into it if the current 
 be strong enough. In short, it is a magnet. Not of 
 course a permanent one, but a temporary one, lasting 
 as long as the current flows. It is thus suggested that 
 magnetism may perhaps be simply electricity in 
 rotation. Let us work out this idea more fully. 
 
 First of all, one may notice that everything that 
 can be done with a permanent magnet can be imitated 
 by a coiled wire conveying a current. (It would not 
 do altogether to make the converse statement.) Float 
 a coil attached to a battery vertically on water, and 
 
 K 2 
 
132 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 you have a compass-needle : it sets itself with its axis 
 north and south. Suspend two coils, and they will 
 attract or repel or turn each other round just like two 
 magnets. 
 
 69. As long as one only considers the action of a 
 coil at some distance from itself, there is no need to 
 trouble about the shape of the particular magnet 
 which it most closely simulates ; but as soon as one 
 begins to consider the action of a coil on things close 
 
 FIG. 16. Floating battery and helix acting as a compass-needle. 
 
 to it, it is necessary to specify the shape of the 
 corresponding magnet. 
 
 If the coil be a long cylindrical helix like a close- 
 spired corkscrew, as in Fig. 16, it behaves like a 
 cylindrical magnet filling the same space. If the coil 
 be a short wide hank, like a curtain-ring, it behaves 
 again like a cylindrical magnet, but one so short that 
 it is more easily thought of as a disk. A disk or 
 plate of steel magnetized with one face all north and 
 
CHAP. VIL] ELECTRO-MAGNETISM. 133 
 
 the other face all south can be cut to imitate any thin 
 hank of wire conveying a current. It will be round 
 if the coil be round, square if it be square, and 
 irregular in outline if the coil be irregular. 
 
 There is no need for the coil to have a great num- 
 ber of turns of wire except to increase its power : 
 one is sufficient, and it may be of any shape or 
 size. So when we come to remember that every 
 current of electricity must necessarily flow in a 
 closed circuit, one perceives that every current of 
 electricity is virtually a coil of more or less fantastic 
 sliape, and accordingly imitates some magnet or other 
 which can be specified. Thus we learn that every 
 current of electricity must exhibit magnetic phe- 
 nomena : the two are inseparable a very important 
 truth. See Appendix (a). 
 
 There is one detail in which the magnetized disk 
 and the coil are not equivalent, and thfe advantage lies 
 on the side of the coil : it has a property beyond that 
 possessed by any ordinary magnet. It has a penetrable 
 interior, which the magnet has not. For space outside 
 both, they simulate each other exactly ; for space 
 inside either, they behave differently. The coil can 
 be made to do all that the magnet can do ; but the 
 magnet cannot in every respect imitate and replace 
 the coil : else would perpetual motion be an every-day 
 occurrence. 
 
 70. Now I want to illustrate and bring home 
 
134 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 forcibly the fact that there is something rotatory about 
 magnetism something in its nature which makes 
 rotation an easy and natural effect to obtain if one 
 goes about it properly. One will not observe this by 
 taking two magnets : one will see it better by taking 
 a current and a magnet, and studying their mutual 
 action. 
 
 A magnet involves, as you know, two poles a 
 north and a south pole of precisely opposite pro- 
 perties : it may be considered as composed of these 
 two poles for many purposes ; and the action of a 
 current on a magnet may be discussed as compounded 
 of its action on each pole separately. Now how does 
 a current act on a magnetic pole ? Two currents at- 
 tract or repel each other ; two poles attract or repel 
 each other ; but a current and a pole exert a mutual 
 force which is neither attraction nor repulsion : it is a 
 rotatory force. They tend neither to approach nor to 
 recede ; they tend to revolve round each other. A 
 singular action this, and at first sight unique. All 
 ordinary actions and reactions between two bodies 
 take place in the line joining them : the force 
 between a current and a pole acts exactly at right 
 angles to the line joining them. 
 
 Helmholtz long ago (in 1847) showed that the con- 
 servation of energy could only be true if forces 
 between bodies varied in some way with distance and 
 acted in the line joining them. Now here is a case 
 
CHAP, vii.] ELECTRO-MAGNETISM. 135 
 
 where the force is not in the line joining the bodies, 
 and accordingly the conservation of energy is defied : 
 the two things will revolve round each other for ever. 
 This affords, and has afforded, a fine field for the 
 perpetual motionist ; and if only the current would 
 maintain itself without a sustaining power, perpetual 
 motion would in fact be attained. But this after all 
 is scarcely remarkable, for the same may be said of a 
 sewing-machine or any other piece of mechanism : if 
 only it would continue to go without sustaining power 
 it would be a perpetual motion. Attend to pole and 
 current only, and energy is not conserved, it is per- 
 petually being wasted ; but include the battery as an 
 essential part of the complete system, and the mystery 
 disappears : everything is perfectly regular. 
 
 71. The easiest way perhaps of showing the rotation 
 of a conductor conveying a current round a magnetic 
 pole is to take an 8-feet -long piece of gold thread, such 
 as is stitched upon the garments of military officers, 
 and hanging it vertically supply it with as strong a 
 current as it will stand. Then bring near it a vertical 
 bar-magnet, and instantly you will see the thread coil 
 itself into a spiral, half of it twisting round the north 
 end of the bar, and half twisting as part of the same 
 spiral round the south end (Fig. 17). 
 
 If the magnet were flexible and the conductor rigid, 
 instead of vice versa, the magnet would in like manner 
 coil itself in a spiral round the current : the force is 
 
136 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 strictly mutual. A rigid magnet, put near a stiff con- 
 ductor, shows only the last remnants of this action : 
 it sets itself at right angles to the wire, and ap- 
 proaches its middle to touch it, but that is all it 
 can do. 
 
 FIG 1 7. A long flexible conductor twisting itself into a spiral round a powerful 
 bar-magnet raised to meet it. 
 
 The experiment with the flexible gold thread is 
 simple, satisfactory, and striking, but the rotatory- 
 properties connected with a magnet may be illustrated 
 in numbers of other ways. Thus, pivot a disk at its 
 
CHAP. VII.] 
 
 ELECTRO-MAGNETISM. 
 
 137 
 
 centre, and arrange some light contact to touch its 
 edge, either at one point or all round, it matters not ; 
 then supply a current to disk from centre to circum- 
 ference, and bring a bar-magnet near it along its axis, 
 or, better, two bar-magnets, with opposite poles one 
 
 FIG. 18. Pivoted disk with radial current, revolving in a magnetic field and winding 
 up a weight. The current is supplied to the axle by screw A, and leaves the rim 
 by mercury trough M. The same apparatus obviously serves to demonstrate 
 currents induced by motion ; both directly and by the damping effect. 
 
 on each side, near the contact place of the rim ; the 
 disk at once begins to rotate (Figs. 18 and 19). 
 
 Instead of a disk one may use a single radius of it, 
 viz. a pivoted arm (Fig. 20) dipping into a circular 
 trough of mercury ; or we may use a light sphere 
 rolling on two concentric circular lines of railway 
 
138 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 ",. ig. Another pivoted disk with flange to dip into liquid so as to make contact 
 all round its rim. It rotates when a magnet is brought above or below ; or even 
 in the field of the earth. 
 
 FIG. 20. A couple of radii.of the above disk provided with points to dip into mercury, 
 and rotating constantly under the influence of the steel magnet A, 
 
CHAP. VIL] ELECTRO-MAGNETISM. 139 
 
 (Gore's arrangement, Fig. 2I). 1 In every case rotation 
 begins as soon as a magnet is brought near. 
 
 72. Nor is the revolving action confined to metallic 
 conductors and to true conduction. Liquids and 
 
 FIG. 21. Gore's circular railway. The light spherical metal ball revolves round the 
 two concentric metal hoops or rails whenever it is made to convey a current 
 between them in a vertical magnetic field. 
 
 gases, although they convey electricity by something 
 of the nature of convection, are susceptible to rotation 
 in a precisely similar manner. 
 
 1 This is not what Gore's railway is commonly used to illustrate, nor 
 is it the cause of the motion as observed by the inventor, or as described 
 in Tyndall's Heat. Ordinarily the ball moves by reason of an irregular 
 disturbance due to heat at its point of contact with the rails, and it is 
 mere accident which way it goes. But, in so far as the earth's vertical 
 magnetic field is strong enough, it should exhibit a preference for one 
 direction over the other ; and if the field is strengthened by bringing the 
 south pole of a bar-magnet below the apparatus, true magnetic rotation 
 is bound to occur. It may, however, be convenient to state that the 
 current's own lines of force are entirely powerless to cause motion in 
 this case. An external field is essential. 
 
HO MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 To show the rotation of liquid conductors under 
 the influence of a magnet, take a circular shallow 
 trough of liquid, supply it with stout sheet copper 
 electrodes at centre and circumference, and put the 
 pole of a magnet below it. The liquid at once begins 
 to rotate, and by using a magnet and current of fair 
 strength can easily be made to whirl so fast as to fly 
 
 FIG. 22. Rotation of a liquid disk conveying a radial current in a vertical 
 magnetic field. 
 
 over the edge of the trough (Fig. 22). 1 The experi- 
 ment is plainly the same as Fig. 19, except that a 
 liquid disk is used in place of a solid one. Or, again, 
 
 1 In practice it is most convenient to split a battery current between 
 magnet and liquid : i.e. to connect them in parallel instead of in series. 
 It is also well to make the smaller surface of copper the cathode ; 
 because with intense currents (say 3 amperes per square centimetre) 
 a crust of oxide forms on the anode which almost entirely stops the 
 current by its resistance. 
 
CHAP. VI!.] 
 
 ELECTRO-MAGN ETISM. 
 
 141 
 
 it may be considered the same as Fig. 21. Reverse 
 the magnet, and the rotation is rapidly reversed. 
 Another method is to send a current along a jet of 
 
 u \1 u w 
 
 A 
 
 
 1 
 
 FIG. 23. A falling stream of liquid conveying a current between two magnetic poles, 
 and being thereby twisted into a spiral. (Copied from a paper in Phil. Mag. by 
 Dr. Silvanus Thompson.) 
 
 mercury near a magnet and note the behaviour of the 
 jet. It twists itself into a flat spiral as shown in 
 
 Fig- 23- 
 
 The rotation of a gas discharge is most commonly 
 
142 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 illustrated by an arrangement like Fig. 24, where the 
 terminals of the induction coil are connected to the 
 rarefied gas respectively above one pole and round the 
 middle of a magnetized bar. If the discharge can be 
 got to concentrate itself principally down one side, 
 
 FIG. 24. Induction coil discharge from a to b through rarefied gas, rotating round 
 a glass-protected magnetized iron rod. 
 
 which it is not easy always to do (it seems to depend 
 on the presence in the vacuum of some traces of 
 foreign vapour, e.g. CS 2 vapour), the line of light so 
 formed is seen to revolve. 
 
CHAP, vii.] ELECTRO-MAGNETISM. 143 
 
 Action between a Magnet and an Electric Charge in 
 Relative Motion. 
 
 73. From all this it is not to be doubted that a 
 charged pith ball moving in the neighbourhood of a 
 magnet is subject to the same action. There is no 
 known action between a magnet and a stationary 
 charged body, but directly either begins to move there 
 is an action between them tending to cause one to 
 rotate round the other. It is true that for ordinary 
 speeds of motion this force is extremely small ; but 
 still it is not to be doubted that if a shower of charged 
 pith balls or Lycopodium granules are dropped on to 
 a magnet pole, they will fall, not perfectly straight, 
 but slightly corkscrew fashion. And again, if a set 
 of charged particles were projected horizontally and 
 radially from the top of a magnet, the^r paths would 
 revolve like the beams of a lighthouse. And if by 
 any means their paths were kept straight, or deflected 
 the other way, they would exert on the magnet an 
 infinitesimal " couple " tending to make it spin on its 
 own axis. 
 
 Conversely, if a magnet were spun on its axis 
 rapidly by mechanical means, there is very little doubt 
 but that it would act on charged bodies in its neigh- 
 bourhood, tending to make them move radially either 
 to or from it. This, however, is an experiment that 
 
144 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 ought to be tried ; and the easiest way of trying it 
 would be to suspend a sort of electrometer needle 
 electrified positive at one end and negative at the other, 
 near the spinning magnet, and to look for a trace of 
 deflection to be reversed when the spin is reversed. 
 A magnet of varying strength might be easier to try 
 than a spinning one. (See 114 116.) 
 
 Rotation of a Magnet by a Current. 
 
 74. The easiest way to show the actual rotation of 
 a magnet is to send a current half-way along it and 
 back outside. Thus, take a small, round, polished 
 steel bar-magnet with pointed ends, pivot it vertically, 
 and touch it steadily with two flakes or light pads of 
 tin-foil, one near either end and one near the middle ; 
 supply a current by these contact pieces, and the 
 magnet spins with great rapidity. Reverse the current, 
 and it rotates the other way. Conversely, by pro- 
 ducing the rotation mechanically a current will be 
 excited in a wire joining the two pieces of tin-foil 
 (Figs. 25, 26, and 27). 
 
 The two contacts may be made anywhere on the 
 magnet except symmetrically : if the two are equi- 
 distant from the middle, no effect will be produced. 
 The nearer one is to the middle and the other to 
 the end the stronger the effect ; stronger still if 
 
CHAP. VII.] 
 
 ELECTRO-MAGNETISM. 
 
 FIG. 25. Round bright steel bar-magnet pivoted at its ends, spinning rapidly on its 
 axis under the influence of a current supplied to either the bottom or top pivot, 
 or both, and removed near the middle by a scrap of tin-foil lightly touching it. 
 
 FIG. 26. Another mode of exhibiting the same thing as Fig. 23. 
 loaded so as to float upright in mercury. 
 
 The magnet is 
 
146 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 one of the contacts is either at or beyond the 
 end. 
 
 75. The customary or Faraday plan of exhibiting the 
 effect depicted in Fig. 25, with a mercury ring trough 
 round the magnet into which a projecting wire carried 
 by the magnet dips, is not quite so simple and obvious 
 a method as Fig. 25 ; neither is it so effective unless 
 
 FIG. 27. The converse of Fig. 25. Spinning th'e magnet mechanically gives a current 
 between two springs, one touching it near or beyond either end, the other 
 touching it near the middle. 
 
 the ring trough fits the magnet pretty closely. The 
 arrangement in Fig. 25, where the contact is made 
 actually on the surface of the magnet, gives the 
 theoretically greatest force. 
 
 Many more variations of the experiment could be 
 shown, but these are typical ones, and will suffice. 
 They all call attention to the fact that a magnet, 
 considered electrically, is a rotatory phenomenon. 
 
CHAPTER VIII. 
 
 NATURE OF MAGNETISM. 
 
 AmpMs Theory. 
 
 76. THE idea that magnetism is nothing more nor 
 less than a whirl of electricity is no new one it is 
 as old as Ampere. Perceiving that a magnet could 
 be imitated by an electric whirl, he made the hypo- 
 thesis that an electric whirl existed in every magnet 
 and was the cause of its properties. Not of course 
 that a steel magnet contains an electric current cir- 
 culating round and round it, as an electro-magnet 
 has ; nothing is more certain than the fact that a 
 magnet is not magnetized as a whole, but that each 
 particle of it is magnetized, and that the actual mag- 
 net is merely an assemblage of polarized particles. The 
 old and familar experiment of breaking a magnet 
 into pieces proves this. Each particle or molecule 
 of the bar must have its circulating electric current, 
 and then the properties of the whole are explained. 
 
 L 2 
 
148 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 There is only one little difficulty which suggests 
 itself in Ampere's theory How are these molecular 
 currents maintained ? Long ago a similar difficulty 
 was felt in astronomy What maintains the motions 
 of the planets ? Spirits, vortices, and other contrivances 
 were invented to keep them going. 
 
 But in the light of Galileo's mechanics the difficulty 
 vanishes. Things continue in motion of themselves 
 until they are stopped. Postulate no resistance, and 
 motion is essentially perpetual. 
 
 What stops an ordinary current ? Resistance. Start 
 a current in a curtain-ring, by any means, and leave 
 it alone. It will run its energy down into heat in the 
 space of half a second or so. But if the metal con- 
 ducted infinitely well there would be no such dissipa- 
 tion of energy, and the current would be permanent. 
 
 In a metal rod, electricity has to pass from atom to 
 atom, and it meets with resistance in so doing ; but 
 who is to say that the atoms themselves do not 
 conduct perfectly ? They are known to have various 
 infinite properties already ; they are infinitely elastic, 
 for instance. Pack up a box of gas in cotton-wool 
 for a century, and see whether it has got any cooler. 
 The experiment, if practicable, should be tried ; but 
 our present experience warrants us in assuming no 
 loss of motion among colliding atoms until the 
 contrary has been definitely proved by experiment. 
 To all intents and purposes certainly atoms are 
 
CHAP, viii.] NATURE OF MAGNETISM. 149 
 
 infinitely elastic ; why should they not also be 
 infinitely conducting ? Why should dissipation of 
 energy occur in respect of an electric current cir- 
 culating wholly inside an atom ? There is no known 
 reason why it should. There are many analogies 
 against it. 
 
 How did these currents originate ? We may as 
 well ask, How did any of their properties originate ? 
 How did their motion originate ? These questions are 
 unanswerable. Suffice it for us, there they are. The 
 atoms of a particular substance iron for instance, 
 or zinc have an electric whirl of certain strength 
 circulating in them as one of their specific physical 
 properties. 
 
 This much is certain, that the Amperian currents - 
 are not producible by magnetic experiments. When 
 a piece of steel or iron is magnetized, the act of 
 magnetization is not an excitation , of Amperian 
 current in each molecule is not in any sense a 
 magnetization of each molecule. The molecules were - 
 all fully magnetized to begin with : the act of magnet- 
 ization consists merely in facing them round so as 
 to look mainly one way in polarizing them, in fact. 
 This was proved by Beetz long ago : I will not stojx 
 to explain it further, but will refer students to^ 
 Maxwell (vol. ii. chap, vi.) 
 
ISO MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 Ampere's Theory extended by Weber to explain 
 Diamagnetism also. 
 
 77. Let us see how far we have got. We have made 
 the following assertions : 
 
 (1) That a magnet consists of an assemblage of 
 polarized molecules. 
 
 (2) That these molecules are each of them perman- 
 ent magnets, whether the substance be in its ordinary 
 or in its magnetized condition, and that the act of 
 magnetization consists in turning them round so as to 
 face more or less one way. 
 
 (3) That when all the molecules are faced in the 
 same direction the substance is magnetically com- 
 pletely saturated. 
 
 (4) That if each molecule of a definite substance 
 contains an electric current of definite strength cir- 
 culating in a channel of infinite conductivity the 
 magnetic behaviour of the substance is completely 
 explained. 
 
 But now, supposing all this granted, how comes it 
 that the molecular currents are not capable of being 
 generated by magnetic induction ? And if we cannot 
 excite them, are we able to vary their strength ? 
 
 78. The answer to these questions is included in 
 the following propositions, which I will now for con- 
 venience state, and then proceed to explain and justify. 
 
CHAP. VIIL] NATURE OF MAGNETISM. 151 
 
 (5) If a substance possessing these molecular 
 currents be immersed in a magnetic field, all those 
 molecules which are able to turn and look along the 
 lines of force in the right direction will have their 
 currents weakened ; but on withdrawal from the field 
 they will regain their normal strength. 
 
 (6) If the currents naturally flowing in conducting 
 channels be feeble or nil, the act of immersion of the 
 substance in a magnetic field will reverse them or 
 excite opposite currents, which will last so long as 
 the body remains in the field, but will be destroyed 
 by its removal. 
 
 (7) The same thing will happen whatever the 
 strength of the natural molecular currents, provided 
 the molecules are completely fixed and unable to 
 face round under the influence of the field. 
 
 (8) The molecular currents so magnetically in- 
 duced are sufficient to explain the phenomena of 
 diamagnetism. 
 
 79. Let us first just recall to mind the well-known 
 elementary facts of current induction. A conducting 
 circuit, such as a ring or a coil of wire, suddenly 
 brought near a current-conveying coil or a magnet, has 
 a momentary current induced in it in the opposite 
 direction to the inducing current in other words, 
 such as to cause momentary repulsion between the 
 two. So long as it remains steady, nothing further 
 happens ; but on withdrawing it another momentary 
 
152 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 current is induced in it in the contrary direction to 
 that first excited. The shortest way of expressing 
 the facts quite generally is to say, that, while from 
 any cause the magnetic field through a conductor 
 is increasing in strength, a momentary current is 
 excited in it tending to drive it out of the field ; 
 and that, whenever the magnetic field decreases again 
 to its old value, a reverse flow of precisely the 
 same quantity of electricity occurs. Fig. 28 shows 
 a mode of illustrating these facts. A copper disk 
 is supported at the end of a torsion arm and brought 
 close to the face of an unexcited bar electro-magnet. 
 On exciting the magnet the disk is driven violently 
 away : to be sucked back again, however, whenever 
 the magnetism ceases. 
 
 80. Now, why are all these effects so momentary? 
 What makes the induced current cease so soon after 
 excitation ? Nothing but dissipation of energy : only 
 the friction of imperfect conductivity. There is 
 nothing to maintain the current, it meets with resist- 
 ance in its flow through the metal, and so it soon 
 stops. 
 
 But in a perfect conductor like a molecule no such 
 dissipation would occur. Electricity in such a body 
 will obey the first law of motion, and continue to 
 flow till stopped. Destroying the magnetic field will 
 stop an induced molecular current, but nothing else 
 will stop it. Hence it follows that the repulsion 
 
CHAP, viii.] NATURE OF MAGNETISM. 
 
 153 
 
 experienced by a molecule is no transitory effect like 
 that in Fig. 28, but is as permanent as the magnetic 
 field which excites and exhibits it. 
 
 Thus, then, a body whose molecules are perfectly 
 conducting, but without specific current circulating in 
 them, will behave diamagnetically, i.e. will move away 
 
 FIG. 28. Stout disk of copper supported on a horizontal arm near one pole of a bar 
 electro-magnet. The disk is repelled while the magnet is being excited, and is 
 attracted while the magnetism is being destroyed. 
 
 from strong parts of the field towards weak ones, and 
 thereby set its length equatorially, just as bismuth is 
 known to do. 
 
 Whether this be the true explanation of diamagnet- 
 ism or not, it is at least a possible one. It seems to me 
 extremely probable. It is known as Weber's theory. 
 
154 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 It does not necessarily follow that the specific mole- 
 cular currents of a diamagnetic substance are really 
 nil ; all that is needful is that they shall be weaker 
 than those induced by an ordinary magnetic field. By 
 using an extremely weak field, however, the specific 
 currents need not be quite neutralized, and in such 
 a field the body ought to behave as a very feebly mag- 
 netic substance. Such an effect has been looked for. 1 
 
 8 1. One loop-hole there is, however, viz. that every 
 molecule may be so jammed as to be unable to turn 
 round, and such a substance could hardly exhibit any 
 noticeable magnetic properties. The molecules would 
 have got themselves into a state of minimum potential 
 energy, and if jammed therein nothing could be got 
 out of them. The induced currents of diamagnetism 
 would be superposed upon them just as if no initial 
 molecular currents existed. By varying the tem- 
 perature of such a substance, however, one might 
 expect to alter its molecular arrangement, and so 
 develop magnetic properties in it, just as electrical 
 properties are developed in crystals like tourmaline 
 by heat or by cold. 
 
 We are now able clearly to appreciate this much 
 that the molecular currents needful to explain mag- 
 netism are not conceivably excited by the act of 
 magnetization, for they are in the wrong direction. 
 Induced molecular currents will be such as to cause 
 
 1 See Nature, vol. xxxiii. p. 484. 
 
CHAP. VIIL] NATURE OF MAGNETISM. 155 
 
 repulsion : those which cause attraction must have 
 existed there before, and be merely rotated into fresh 
 positions by the magnetizing force. An intense mag- 
 netic field will weaken them, and thus tend to render 
 a magnetic substance less magnetic. 
 
 Function of the Iron in a Magnet. Two Modes of 
 expressing it. 
 
 82. We can now explain the function of iron, or 
 other magnetic substance, in strengthening a magnetic 
 field. Take a circular coil of wire, Fig. 29, and send 
 a current round it : there is a certain field a certain 
 number of lines of force between its faces. Fill the 
 coil with iron, so as to make it a common electro- 
 magnet, and the strength of the field is greatly in- 
 creased. Why ? The common mode of statement 
 likens the magnetic circuit to a voltaic circuit ; there 
 is a certain magneto-motive force, anfl a certain re- 
 sistance, or, as Mr. Heaviside preferably calls it, " re- 
 luctance " : the quotient gives the resulting magnetic 
 induction, or total number of lines of force. Iron is 
 more permeable than air say, 3000 times more 
 permeable and accordingly the resistance of the iron 
 part of the circuit is almost negligible in comparison 
 with that of the air-gap between the poles. Thus a 
 good approximation to the total intensity of field is 
 obtained by dividing the magneto-motive force by the 
 width of the air-gap ; or more completely and generally 
 
156 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 by treating the varying material and section of a 
 magnetic circuit just as the varying material and 
 section of a voltaic circuit is treated, and so obtaining 
 its total resistance. Iron is thus to be regarded as a 
 magnetic conductor between 100 and 10,000 times 
 better than air. Its specific magnetic conductivity or 
 inductivity, or, as it is more usually called (after 
 Thomson), permeability, is measured by the ratio of 
 
 FIG. 29. 
 
 the magnetization produced to the magnetizing force 
 applied, and is generally denoted by the symbol /&. 1 
 
 83. This mode of regarding the case is undoubtedly 
 simple and convenient, but it is not the fundamental 
 mode. If we look at it less with a view to practical 
 simplicity than with the aim of seeing what is really 
 going on, we shall express it thus : 
 
 Before the iron was inserted in the coil there were 
 a certain number of circular lines of force inside it 
 
 1 See Appendix (8). 
 
CHAP. VIIL] NATURE OF MAGNETISM. 157 
 
 due to the current alone. A piece of common iron, 
 although full of polarized molecules, has no external 
 or serviceable lines of force : they are all shut up, as 
 it were, into little closed circuits inside the iron. But 
 directly the iron finds itself in a magnetic field some 
 of these open out, a chain of polarized molecules is 
 formed, and the lines due to its molecular currents 
 add themselves to those belonging to the current of 
 the magnetizing helix. 
 
 Thus our ring electro-magnet has now not only its 
 own old lines of force, but a great many of those 
 belonging to the iron which have sympathetically laid 
 themselves alongside the first. 
 
 Parenthetically we may make the following remark. 
 So long as the iron adds some 3000 lines of its own 
 (more or less according to the quality of the iron) for 
 every one otherwise excited in the field, so long it 
 has its maximum permeability : it is infinitely far from 
 saturation. But after a certain call upon it, it begins to 
 show signs of poverty, and ultimately may refuse to 
 add any lines of its own at all ; it is then said to be 
 completely saturated : its permeability is then just 
 as if it were air. The permeability of iron is an 
 extremely indefinite quantity. Not only does it vary 
 with the same piece as it nears saturation, but it is 
 exceedingly different for different specimens. Thus 
 some manganese-steel exists with a permeability one 
 and a half times that of air, or only about as much as 
 
158 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 zinc, while Ewing finds some iron with a permeability 
 as high as 20,000 under shaking. 
 
 The end result of either mode of regarding the 
 matter is of course the same the lines of force 
 between the poles are increased in number by the 
 presence of iron ; but whereas, in the first-mentioned 
 mode of treatment, the fact of permeability had to be 
 accepted unexplained, in the second nothing is unex- 
 plained except the fundamental facts of the subject, 
 such as the reason why currents tend to set them- 
 selves with their axes parallel, and other matters of 
 that sort. 
 
 Permanent Magnetism. 
 
 84. There is one curious effect of introducing iron 
 or other solid magnetic medium into a magnetic field 
 which must not be overlooked, This effect depends 
 on the solidity of the substance, i.e. the fixedness or 
 stiffness of its molecules. In a fluid the molecules 
 are free to take up any fresh arrangement with ease : 
 there is no set arrangement in the internal structure 
 of a fluid, any more than there is a definiteness in 
 its external shape. But with a solid it is different : 
 its molecules once set into any position tend to remain 
 more or less in that position ; the substance may be 
 elastic to a certain extent, but after large disturbances 
 there will always be a certain amount of permanent 
 
CHAP. VIIL] NATURE OF MAGNETISM. 159 
 
 set. Hence it is that solid bodies have a definite 
 shape, which it requires force to change ; hence it is 
 also that their molecules are able to crystallize into 
 geometrical patterns. 
 
 Now, since the act of magnetization consists in 
 making a number of already polarized molecules face 
 round more or less in one direction, it follows that 
 solid magnetic substances will behave differently from 
 fluid ones. In fluid media the magnetized arrange- 
 ment can only be maintained by a continuous 
 exertion of magnetizing force ; and directly this is 
 withdrawn the molecules will quickly take up their 
 old higgledy-piggledy arrangement of minimum 
 energy, and all trace of magnetization will cease. 
 They will be perfectly easy to magnetize, and they 
 will automatically demagnetize themselves. But with 
 solids it is otherwise. The molecules if set in their 
 magnetized position by only a feeble force will spring 
 back almost completely when the magnetizing force 
 is removed ; but if they have been arranged by a 
 force of some violence, the spring back will be only 
 partial, and a permanent set will remain. The spring- 
 back portion of the whole arrangement is called 
 temporary magnetism, the set portion is called 
 permanent magnetism. The difference can be illus- 
 trated by bending a bit of tin plate or paper nearly 
 double, and then letting it go. 
 
 Substances differ greatly in their power of thus 
 
160 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 retaining magnetization, and, as is well known, steel 
 has the property well developed ; but all substances 
 exhibit it more or less. 1 Moreover, many substances can 
 retain a little of the set if they are left carefully un- 
 disturbed, but they lose it if shaken or heated : 
 sometimes even if gently touched. A long thin bar 
 of soft iron is most instructive in this respect. It can 
 be easily magnetized by the earth's magnetism if held 
 vertically and struck with a finger. If then inverted 
 slowly and cautiously, it will retain nearly the whole 
 of the induced magnetism ; but if struck again, or 
 even if the fingers are shuffled on it (so sensitive some 
 bars are), the whole is immediately reversed. Soft 
 iron can, in fact, retain enormously more magnetism 
 than steel can, but it retains it in a very feeble and 
 loose manner. Its magnetism can only be styled 
 sub-permanent. 
 
 85. A short thick bar can retain much less magnet- 
 ism than a long thin one ; in fact, if a stout bar is 
 made of the softest iron, it can retain hardly any. 
 A piece of iron shaped like Fig. 29 would have a 
 much better chance of retaining its magnetism, and if 
 the gap were closed by another piece of iron or 
 keeper, it would retain it very well ; while if the last 
 trace of air-gap, the air-films between keeper and mag- 
 net, be abolished by making the whole one welded 
 ring, then its magnetism is retained almost perfectly. 
 
 1 See a letter in Nature, vol. xxxiii. p. 484. 
 
CHAP. VIIL] NATURE OF MAGNETISM. 161 
 
 There is some- demagnetizing force even in this case, 
 for a fluid magnetized as a ring would not remain 
 magnetized, and I find that tapping or beating such a 
 ring does appreciably weaken its magnetization, but 
 the demagnetizing force is very small compared with 
 what it is when there is an air-gap. 
 
 Hence we learn that the specially demagnetizing 
 portion of a magnetic current is the fluid portion 
 the air portion ; and the greater the proportion of the 
 fluid portion to the whole, the more easily is de- 
 magnetization accomplished. Fluids having no power 
 of their own for retaining magnetism, if lines of force 
 are forcibly maintained in air or other fluid in the 
 neighbourhood of a solicf magnet, all the strain of 
 upholding not only its own magnetism, but all the 
 rest of the magnetism in the field the strain of 
 keeping the molecules faced round in opposition to 
 their mutual restoring forces has to he thrown upon 
 the viscosity and retentivity of the solid. 
 
 86. All the known facts of magnetism have had new 
 life and interest put into them lately by the researches 
 of Ewing. The long-known fact that solid substances 
 store up in their structure any previous arrangement 
 of their molecules, so that traces of the effect are re- 
 cognizable long after the cause of the effect has been 
 withdrawn, is not indeed by any means confined to 
 magnetism ; it is a general property of solids, and 
 constitutes a considerable difficulty in dealing with 
 
 M 
 
162 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 them theoretically. The properties of all fluids, 
 whether liquids or gases, depend upon their state at 
 the moment, and upon nothing else : not at all upon 
 how they reached that state, or upon what has 
 happened to them in past times. Hydrogen at o C. 
 and 76 centimetres pressure is a perfectly definite sub- 
 stance. Water at 50 C. and one atmosphere pressure, 
 is again a quite complete specification. And the same 
 is very nearly true of some crystalline solids. Quartz 
 or ice at given temperature and pressure is generally 
 considered quite a definite statement, though perhaps 
 it is not so exactly definite as we imagine. But glass, 
 or steel, or copper, at a specified temperature, is by no 
 means a definite substance. If it has been cooled 
 down to that temperature it will not be the same as if 
 it had been warmed up to it. We must be told 
 whether it has been hardened, or tempered, or an- 
 nealed, and so on. The properties of a solid body 
 depend on its past history as well as on its present state. 
 
 All this is pre-eminently true with magnetization. 
 To understand completely the behaviour of a magnet 
 we must not only know its present state, but we must 
 know how it got to that state. A piece of steel once 
 magnetized and then demagnetized is not in the same 
 condition as if it had never been in a magnetic field : 
 unless, indeed, it has been melted and made afresh. 
 
 This much, however, must be granted, that if every- 
 thing were known about the instantaneous state of a 
 
CHAP. VIIL] NATURE OF MAGNETISM. 163 
 
 body there would be no need to go back upon its past 
 history : it might even be possible to deduce some of 
 its past history from its present state. But it is pre- 
 cisely because a knowledge of the position and relation 
 of every individual molecule is impossible, and because 
 we have to put up with a few salient features of 
 information, that an inquiry into past history is 
 necessary. A few salient features are sufficient in the 
 case of fluids : they are, in general, not sufficient in 
 the case of solids. 
 
 I have laid stress upon this matter because it is an 
 important general distinction between states which are 
 self-contained, so to speak, and states which are led up to. 
 
 87. A further detail of the distinction is that, in 
 solids, a direct and return series of changes are not 
 usually the same ; a precisely inverse cause does not 
 precisely invert the effect. Take a body from one 
 self-contained state to another. Then, whenever you 
 reverse the series of operations which brought it there, 
 it will return by the same path to its previous condi- 
 tion, and everything will be as it had been, and no 
 work need have been done on the whole. Not so with 
 the led-up-to states. Magnetize a piece of steel by 
 one series of operations, and then perform the same 
 operations in reverse order, it will not return by the 
 same path at all, nor will it return to its original con- 
 dition. Continue the process of magnetization and 
 reverse magnetization several times, and you may at 
 
 M 2 
 
1 64 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 length succeed in getting the body to go through a 
 cycle of changes, at least approximately. But it will 
 go by one path and return by another. 
 
 Now, when a body of any kind is taken from a state 
 A to a state B by one path, and back by some other 
 path, as steam is for instance in a steam-engine, the 
 result is always that some work is either clone by the 
 substance, or has to be done upon it, in performing the 
 cycle. The return by a different path is optional in 
 the case of steam, and accordingly work may or may 
 not be done on it ; but in the case of a magnetized 
 solid it is not optional. The ascending curve of in- 
 creasing magnetism, and the descending curve of 
 decreasing magnetism, do not coincide, and cannot 
 be made to coincide. Consequently, whenever a 
 piece of iron is taken round a cycle of magnetic 
 changes, some work is necessarily done. 
 
 This work, in general, results in a production of 
 heat, and accordingly a piece of iron magnetized and 
 demagnetized successively in rapid succession gets 
 slightly warm. This direct heating effect is, however, 
 very small, and is usually inappreciable in practice. 1 
 
 All this behaviour of iron and other substances 
 with regard to magnetism is called by Ewing 
 hysteresis. 
 
 1 The large indirect heating by induced currents the so-called 
 Foucault effect is too familiarly known to need any other statement 
 than that it is quite distinct from what we are here discussing. 
 
CHAP, viii.] NATURE OF MAGNETISM. 165 
 
 Electrical Momentum once more. 
 
 88. There is just one point which I must stop here 
 to call attention to. The theories of magnetism and 
 diamagnetism, which I have given according to 
 Ampere, Weber, and Maxwell, require as their found- 
 ation that in a perfect conductor electricity shall obey 
 the first law of motion shall continue to flow until 
 stopped by force. But the property of matter which 
 enables it to do this is called inertia; the law is called 
 the law of inertia ; and anything which behaves in 
 this way must be granted to possess inertia. 
 
 It would not do to deduce so important a fact from 
 a yet unverified theory ; but at least one must notice 
 that momentum is essentially involved in Ampere's 
 theory of magnetism. It is the only theory of mag- 
 netism yet formulated, and it breaks down unless 
 electricity possesses inertia. 
 
 Nevertheless it is a fact that an electro-magnet does 
 not behave in the least like a fly-wheel or spinning- 
 top : there is no momentum mechanically discover- 
 able ( 39). Supposing this should turn out to be 
 strictly and finally true, we must admit that a molecular 
 electric current consists of two equal opposite streams 
 of the two kinds of electricity : one must begin to 
 regard negative electricity not as merely the negation 
 or defect of positive, but as a separate entity. Its 
 
166 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 relation to positive may turn out to be something 
 more like that of sodium to chlorine than that of cold 
 to heat. 
 
 89. I said that no effect due to electric inertia was 
 mechanically discoverable ; and on the hypothesis that 
 an electric current consists of a pair of equal opposite 
 currents of positive and negative electricity respec- 
 tively this is very natural. Think of a couple of 
 india-rubber pipes tied together so as to form a double 
 tube, and through each propel a current of water, one 
 in an opposite direction to the other. The double 
 current has no gyrostatic properties, and the only way 
 the water can exhibit momentum is by its resistance 
 to change of velocity, like the " extra-current " effects 
 in electricity. 1 
 
 So long as one considered the flow of electricity in 
 ordinary conductors, we could partially avoid the 
 question of inertia by considering it urged forward at 
 every point with a force sufficient to overcome the 
 resistance there and no more ; but though this ex- 
 
 1 When these articles originally appeared in Nature I made a serious 
 hydraulic mistake about this place, saying that, just as a loop of very 
 light flexible thinly-covered stranded wire or gold-thread, thrown at 
 random on a glass slab and a strong current passed through it, tended 
 to round off its sharp corners, open out its tangled loops, and do its best 
 to become a perfect circle, so would a stream of water behave, as 
 regards kinks and bends and curves in its pipe. Prof. Riicker was kind 
 enough to call my attention to this mistake. It is, indeed, well known 
 that not the slightest effect on the shape of a flexible pipe is produced 
 by a stream of water in it, so long as its ends are fixed : the stream 
 rather confers a kind of rigidity on the pipe. 
 
CHAP, viii.] NATURE OF MAGNETISM. 167 
 
 plained the shape of the stream -lines (p. 103) yet it 
 did not suffice to render clear the phenomena of self- 
 induction the lag of the interior electricity in a wire 
 behind the outside until definitely pushed ( 43-48) ; 
 still less does it explain its temporary persistence in 
 motion after the pushing force has ceased. 
 
 But, now that we are dealing with perfect conduc- 
 tors with no pushing force at all, the persistence of 
 molecular currents without inertia, or an equivalent 
 property so like it as to be rightly called by the same 
 name at present, becomes inexplicable. True, the 
 molecular currents are as yet an hypothesis ; and that 
 is the only loop-hole out of a definite conclusion 
 ( 98 and 185). 
 
CHAPTER IX. 
 
 STRUCTURE OF A MAGNETIC FIELD. 
 
 90. LET us now pass in review the various facts and 
 experiences which have led us to a dual view of 
 electricity ; a kind of two-fluid theory, but in a very 
 modified form. 
 
 First, there are the old experiments which vaguely 
 suggest the separate existence of negative electricity, 
 such as : 
 
 (1) The wind from a point whether positive or 
 negative ; so that a candle gets blown always away 
 from it, whether the point be on the prime conductor 
 and the candle held in the hand, or whether the point 
 be held in the hand and presented to the candle or 
 prime conductor ; so, also, that a point whirligig 
 turns the same way, whether supported on the prime 
 conductor, or whether attached to the earth and 
 placed near it. 
 
 (2) Phenomena connected with the spark discharge, 
 
CHAP, ix.] STRUCTURE OF A MAGNETIC FIELD. 169 
 
 such as Wheatstone's old experiment on what he 
 called the velocity of electricity, with the three pair 
 of knobs ; and the double burr produced in cardboard 
 when pierced with a spark, suggesting that something 
 has pierced it both ways at once. 
 
 Then there are the more recently observed facts ; as, 
 for instance : 
 
 (3) The fact that an electrostatic strain scarcely 
 affects the volume of a dielectric ; thereby at once 
 suggesting something of the nature of a shearing or 
 distorting stress, which alters shape but not size ; a 
 displacement of positive outwards and simultaneous 
 negative inwards ( 13). 
 
 (4) The facts of electrolysis, and the double 
 procession of atoms past each other in opposite 
 directions. 
 
 (5) The phenomena of self-induction, and the 
 behaviour of a thick wire to an alternating current. 
 The delay also in magnetizing iron, and especially 
 the possibility of permanent magnetism ; combined 
 with 
 
 (6) The absence of momentum in an electric 
 current, or moment of momentum in an electro- 
 magnet, as tested by all mechanical means yet tried. 
 
 I admit at once that many of these are mere super- 
 ficial suggestions which may hardly bear examination 
 nd criticism. Only (3), (4), (5), and (6) can be at all 
 seriously appealed to ; but (5) and (6), in conjunction, 
 
i;o MODERN VIEWS OF ELECTRICITY. [PART HI. 
 
 seem to me to afford a sort of provisional and 
 hypothetical proof, which (3) greatly strengthens. 
 
 At this point we must for the present again leave 
 the question. We return to it in 118 and 155. 
 
 Representation of a Magnetic Field. 
 
 91. The disturbance called magnetism, which we 
 have shown in Chap. VII. to be something of the 
 nature of a spin a rotation about an axis is con- 
 spicuously not limited to the steel or iron of the 
 magnet : it spreads out through all adjacent space, 
 and constitutes what is called the magnetic field. A 
 map of the field is afforded by the use of iron filings, 
 which cling end to end and point out the direction of 
 the force at every point (Fig. 33). 
 
 These lines of force so mapped are to be regarded 
 as the axes of molecular whirls (Fig. 30). They are 
 continuous with similar lines in the substance of the 
 steel, and every line really forms a closed curve, of 
 which a portion is in the steel and a portion in the air. 
 In a wire helix, such as Figs. 16 or 29, the lines are 
 wholly in the air, but in one part of their course they 
 thread the helix, and in another part they spread out 
 more or less between its faces. 
 
 But according to Ampere's theory of molecular cur- 
 rents there is no essential difference between such a 
 
CHAP, ix.j STRUCTURE OF A MAGNETIC FIELD. 171 
 
 helix and a steel magnet ; directly the currents in the 
 molecules of the magnet are considered, everything 
 resolves itself into chains of molecular currents, thread- 
 ing themselves along a common closed curve or axis. 
 Each atom, whether in the steel or in the air, is the 
 seat of a whirl of electricity, more or less faced round 
 
 FIG. 30. A, an element of a magnetic line of force with the electric whirl round it ; 
 B, a bit of an electric circuit with one of its magnetic lines offeree shown round 
 it, and the electric whirl round this : each magnetic line of force round a current 
 being an electric vortex ring. Compare Fig. 39. 
 
 so as on the average to have its plane at right angles 
 to the lines of force. The simplest plan of avoiding 
 having to consider those only partially faced round, is 
 to imagine the whole number divided into a set which 
 face accurately in the right direction, and a set which 
 
172 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 look any way at perfect random ; and to neglect this 
 latter set. 
 
 92. Well now try and picture a chain of whirls 
 like beads spinning on a wire threading them all, and 
 think of the effect of a material fluid thus rotating. 
 Obviously it would tend to whirl itself fatter, and to 
 shorten its length. An assemblage of such parallel 
 straight whirls would thus squeeze each other later- 
 ally, or cause a lateral pressure, and would tend to 
 drag their free ends together, causing a longitudinal 
 tension. 
 
 Such whirls cannot in truth have free ends except 
 at the boundary of a medium as at the free surface of 
 a liquid. Magnetic whirls are in reality all closed 
 curves ; but inasmuch as part of them may be in a 
 mobile fluid like air, and part of them in a solid like 
 iron or steel, it is convenient to distinguish between 
 their two portions ; and one may think of the air whirls 
 alone, as reaching from one piece of iron to another 
 and by their shortening tendency or centrifugal force 
 pulling the two pieces together. 
 
 The arrangement shown in Fig. 31 illustrates the 
 kind of force exerted by a spinning elastic framework, - 
 along and perpendicular to its axis of rotation. 
 
 One can easily see this effect of a whirl in a tea-cup 
 or inverted bell-jar full of liquid. Stir it vigorously, 
 and leave it. It presses against the walls harder than 
 before, so that if they were elastic they would bulge 
 
CHAP, ix.] STRUCTURE OF A MAGNETIC FIELD. 173 
 
 out with the lateral pressure ; and it sucks down the 
 top or free end of its axis of rotation, producing quite 
 a depression or hollow against the force of gravity 
 Or, as a more striking illustration, make the apparatus 
 sketched in Fig. 32. 
 
 FIG. 31. A " shape of the earth" model which, when whirled, exerts a tension along 
 its axis, pulling up the weight attached to it, and a pressure at right angles, by 
 reason of its bulging out. 
 
 Two circular boards joined by a short wide elastic 
 tube or drum : a weight hung to the lower board, the 
 top board hung from a horizontal whirling table, the 
 drum filled with water, and the whole spun round. 
 
174 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 The weight is raised by the longitudinal tension ; the 
 sides bulge out with the lateral pressure. 
 
 There is no need for the whole vessel to rotate. 
 If the liquid inside rotates, the same effect is 
 produced. 
 
 93. Imagine now a medium composed of a mul- 
 titude of such cells with rotating liquid inside : let 
 the cells be either very long, or else be joined end to 
 
 FIG 
 
 . 32. An elastic-walled cylindrical vessel full of liquid hanging from a whirling 
 table, and, by reason of centrifugal force, raising a weight and bulging out 
 laterally when spun, thereby illustrating a tension along the axis of rotation and 
 a pressure in every perpendicular direction. 
 
 end so as to make a chain a series of chains side by 
 side and you have a picture of a magnetic medium 
 traversed by a field of force. End-boundaries of the 
 field will be dragged together, thus representing mag- 
 netic attraction ; while, sideways, the lines of force 
 (axes of whirl) squeeze each other apart, thus illus- 
 trating repulsion. This is Clerk-Maxwell's view of 
 
CHAP, ix.] STRUCTURE OF A MAGNETIC FIELD. 175 
 
 Attraction. 
 
 Repulsion. 
 
 FIG. 33. Attraction and Repulsion. The tension along the lines of force or axes of 
 rotation drags the one pair of poles together ; and the pressure in directions per- 
 pendicular to the axis of rotation, due to the centrifugal force of the whirls, drives 
 the other pair apart. 
 
176 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 an electro-magnetic medium, and of the mode in 
 which magnetic stress, and magnetic attractions and 
 repulsions between bodies, arise. 
 
 Wherever lines of force reach across from one body 
 to another, those bodies are dragged together as if 
 pulled by so many elastics (Fig. 33) ; but wherever 
 lines of force from one body present their sides to 
 those proceeding from another body, then those bodies 
 are driven apart. 
 
CHAPTER X. 
 
 MECHANICAL MODELS OF A MAGNETIC FIELD. 
 
 First Representation of the Field due to a Current. 
 
 94. RETURN now to the consideration of a simp 
 circuit, or, say, a linear conductor, and start a current 
 through it ; how are we to picture the rise of the lines 
 of force in the medium ? how shall we represent the 
 spread of magnetic induction ? First ( think of the 
 current as exciting the field (instead of the field as 
 exciting the current, which may be the truer plan 
 ultimately). 
 
 If we can think of electricity in the several mole- 
 cules of the insulating medium connected like so 
 many cog-wheels gearing into one another and also 
 into those of the metal, it is easy to picture a side- 
 ways spread of rotation brought about by the current, 
 just as a moving rack will rotate a set of pinions 
 gearing into it and into each other (Fig. 34). But 
 
 N 
 
178 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 then half the wheels will be rotating one way and 
 half the other way, which is not exactly right. 
 
 FIG. 34. 
 
 How is it possible for a set of parallel whirls to be 
 all rotating in the same direction ? 
 
 ooo 
 ooo 
 
 FIG. 35. 
 
 If there is any sort of connection between them they 
 will stop each other, because they are moving in 
 opposite directions at their nearest points ; and yet, 
 if there is no connection, how can the whirl spread 
 through the field ? 
 
 Well, return to the old models by which we endea- 
 voured to explain electrostatics, and think whether 
 they will help us if we proceed to superpose upon 
 them a magnetic whirl in addition to the properties 
 they already possess. Looking at Figs. 5, 6, and /A, 
 we remember we were led to picture atoms and elec- 
 
CHAP, x.] MODELS OF MAGNETIC FIELD. 179 
 
 tricity like beads threaded on a cord. And these 
 cords had to represent, alternately, positive and 
 negative electricity, which always got displaced in 
 different directions (see 90). 
 
 We are forced to a similar sort of notion in respect 
 of the wheels at present under discussion ; in order 
 that they may co-operate properly, they must repre- 
 sent positive and negative electricity alternately. If 
 
 FIG. 36. Rows of cells alternately positive and negative, Beared together 
 free to turn about fixed axle;. 
 
 they then rotate alternately in opposite directions, all 
 is well, and the electrical circulation or rotation in the 
 field is all in one direction. Each wheel gears into 
 and turns the next, and so the spin gets propagated 
 right away through the medium, at a speed depend- 
 ing on the elasticity and density concerned in such 
 disturbances. 
 
 It is not convenient at the present stage to ask the 
 
 N 2 
 
i8o MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 question whether the wheels represent atoms of matter 
 or merely electricity. It may be that each atom is 
 electrostatically charged and itself rotates, in which 
 case it would carry its charge round with it, and 
 thereby constitute the desired molecular current. 
 The apparent inertia of electricity would thus be 
 explained simply enough, as really the inertia of the 
 spinning atoms themselves ; and the absence of any 
 
 FIG. 37. Section of a magnetic field perpendicular to the lines of force Another 
 mode of drawing Fig. 36. 
 
 moment of momentum in an electro-magnet as tested 
 mechanically would be equally explained by the 
 simultaneous opposite rotation of adjacent atoms. 
 A question may arise as to why the opposite 
 molecules should have exactly equal opposite in- 
 ertiae, as they have, else a fluid magnetized medium 
 would bodily rotate ; and there may be other 
 difficulties connected with a bodily rotation of 
 electrostatically charged molecules : it is merely a 
 
CHAP.X.] MODELS OF MAGNETIC FIELD. 181 
 
 possibility upon which stress must not be laid till it 
 has been proved able to bear it. For our present 
 purpose a spin of the electricity inside each atom, or 
 even independently of any atoms, is quite sufficient. 
 Besides, since magnetic induction can spread through 
 a vacuum quite easily, the wheel-work has to be largely 
 independent of material atoms. 
 
 If any difficulty is felt concerning the void spaces 
 in Fig. 36, it is only necessary to draw it like Fig. 37^ 
 which does every bit as well, and reduces the difficulty 
 to any desired minimum. 
 
 Representation of an Electric Current. 
 
 95. Now notice that in a medium so constituted 
 and magnetized that is, with all the wheel-work 
 revolving properly there is nothing of the nature of 
 an electric current proceeding in any direction what- 
 ever. For, at every point of contact of two wheels, 
 the positive and negative electricities are going at 
 the same rate in the same direction ; and this is no 
 current at all. Only when positive is going one way 
 and negative going the opposite way, or standing 
 still, or at least going at a different rate, can there be 
 any advance of electricity, or anything of the nature 
 of a current. 
 
 A current is nevertheless easily able to be 
 
1 82 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 represented : for it only needs the wheels to gear 
 imperfectly and to work with slip. At any such 
 slipping-place the positive is going faster than the 
 negative, or vice versa, and so there is a current there. 
 A line of slip among the wheels corresponds therefore 
 to a linear current ; and, if one thinks of it, it is quite 
 plain that such a line of slip must always have a 
 closed contour. For, if only one wheel slip, then the 
 circuit is limited to its circumference ; if a row slip, 
 then the direct and return circuit are on opposite sides 
 of the row. But if a large area of any shape with no 
 slip inside it is inclosed by a line of slip, then this 
 gives us a circuit of any shape, but always closed. 
 Understand : one is not here thinking of a current as 
 analogous to a locomotion of the wheels their axes 
 may be quite stationary, the slip contemplated is 
 that of one rim on another. 
 
 Imagine all the wheels inside the empty contour of 
 Fig. 38 to be rotating, the positive clockwise, the 
 negative counter clockwise, and let all those outside 
 the contour be either stationary or rotating at a 
 different rate or in an opposite direction ; then the 
 boundary of the inside region is a line of slip, along 
 which the positive rims are all travelling clockwise 
 and the negative rims the other way, and hence it 
 represents a clockwise positive current round the 
 inside of the empty contour. 
 
 But it may be said that the spin inside the contour, 
 
CHAP, x.] MODELS OF MAGNETIC FIELD. 183 
 
 if maintained, must sooner or later rotate the wheels 
 outside as fast as themselves, and then all slip will 
 cease. Yes, that is so, unless there is a complete 
 breach of connection at the contour, as in Fig. 38 
 
 FIG. 38. Diagram of a peripheral current partitioned off from surrounding medium 
 by a perfect conductor, which transmits no motion, and therefore acts as a perfect 
 magnetic screen. See also 101, and Fig. 41. Shaded wheels stand for positive. 
 
 there is. If the outer region has any sort of connec- 
 tion with the inner one, the slip at its boundary 
 can only be temporary, lasting during the era of 
 acceleration. 
 
 Distinction between a Dielectric and a Metal, as affected 
 by a spreading Magnetic Field. 
 
 96. In a dielectric the connection between the 
 atoms is definite and perfect. If one rotates, the next 
 must rotate too ; there is no slip between the geared 
 
1 84 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 surfaces ; it is a case of cogged wheels like Fig. 36. 
 A conduction-current is impossible. 
 
 But in a metallic conductor the gearing is im- 
 perfect ; it is a case of friction-gearing with more or 
 less lubrication and slip, so that turning one wheel 
 only starts the next gradually it may be very quickly, 
 but not instantaneously and there is a motion of a 
 positive rim incompletely compensated by an equal 
 similar motion of a negative rim while getting up 
 speed ; in other words, there is a momentary electric 
 current, lasting till the wheels have fairly started. 
 
 In a perfect conductor the gearing is absent ; the 
 lubrication is so perfect that all the atoms are quite 
 free of one another, and accordingly a spin ceases to 
 be transmitted into such a medium at all. The only 
 possible current in a perfect conductor is a skin-deep 
 phenomenon. (See also Chap. V. and 104.) 
 
 A magnetized medium of whatever sort is thus to 
 be regarded as full of spinning wheels, the positive 
 rotating .one way and the negative the other way. 
 If the medium is not magnetized, but only magnetic 
 i.e. capable of being magnetized it may be thought 
 of either as having its wheels stationary, or as having 
 them facing all ways at random ; the latter being 
 probably the truer, the former the easier, representa- 
 tion, at least to begin with. 
 
 Whether the medium be conducting or insulating 
 makes no difference to the general fact of spinning 
 
CHAP, x.] MODELS OF MAGNETIC FIELD. 185 
 
 wheels inside it wherever lines of force penetrate it. 
 But the wheels of a conductor are imperfectly cogged 
 together ; and accordingly, in the variable stages of a 
 magnetic field, while its spin is either increasing or 
 decreasing, there is a very important distinction to 
 be drawn between insulating and conducting matter. 
 During the accelerating era conducting matter is full 
 of slip, and a certain time elapses before a steady state 
 is reached. A certain time may be necessary for the 
 propagation of spin in a dielectric, but it is excessively 
 short, and the process is unaccompanied by slip, only 
 by slight distortion and recovery. (See 103 and 1 59.) 
 
 97. As for strongly magnetic substances like iron, 
 nickel, and cobalt, one must regard them as con- 
 stituted in the same sort of way, but with wheels 
 greatly more massive, or very much more numerous, 
 or both. The quantity which we have called perme- 
 ability in 82, 83, and denoted by the symbol //,, may 
 now be thought of as physically equivalent to a 
 density of the magnetic medium ; so that substances 
 with a large /^, like iron, have their magnetic 
 mechanism or wheel-work exceedingly massive. 
 
 Phenomena connected with a varying Current. Nature 
 of Self-induction. 
 
 98. Proceed now to think what happens in the 
 region round a conductor in which a current is rising. 
 
1 86 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 Without attempting a complete and satisfactory 
 representation of what is going on, we can think of 
 some mechanical arrangements which have some 
 close analogy with electrical processes Chap. V. 
 
 Take first a system of wheel-work connected to- 
 gether and moved at some point by a rack. Attend 
 to alternate wheels more especially, as representing 
 
 FIG. 39. A provisional representation of a current surrounded by dielectric medium, 
 either propelling or being propelled. Section through the wire. 
 
 positive electricity. The intermediate negative wheels 
 are necessary for the transmission of the motion, and 
 they also serve to neutralize all systematic advance of 
 positive electricity in any one direction, except where 
 slip occurs, but they need not otherwise be specially 
 attended to. 
 
 Remember that every wheel is endowed with inertia, 
 like a fly-wheel ( 88). 
 
CHAP, x.] MODELS OF MAGNETIC FIELD. 187 
 
 Directly the rack begins to move, the wheels begin 
 to rotate, and in a short time they will all be going 
 full speed. Until they are so moving, the motion of 
 the rack is opposed, not by friction or ordinary 
 resistance, but by the inertia of the wheel-work. 
 
 This inertia represents what is called self-induction, 
 and the result of it is what has been called the " extra- 
 current at make," or, more satisfactorily, the opposing 
 E.M.F. of electro-magnetic inertia or self-induction. 
 
 Having once started the rack, so long as it moves 
 steadily forward, the wheel-work has no further effect 
 upon it ; but, directly it tries to stop, it finds itself 
 unable to stop dead without great violence : its motion 
 is prolonged for a short time by the inertia of the 
 wheel-work, and we have what is known as the " extra- 
 current at break." 
 
 99. If the rack is for a moment taken to represent 
 the advancing electricity in a copper wire, then the 
 diagram may be regarded as a section of the complete 
 field : the complete field being obtained from it by 
 rotating the diagram round the axis of the rack. 
 Imagining this done, we see that the axis of each 
 wheel becomes prolonged into a circular core, and 
 each wheel into a circular vortex ring surrounding 
 the rack and rolling down it as it moves forward, 
 as when a stick is pushed through a tight-fitting 
 umbrella-ring held stationary (see Fig. 30, B). 
 
 As one goes further and further from the rack the 
 
i88 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 lengths of the vortex cores increase, but there is only 
 a given amount of rotation to be shared among more 
 and more stuff, hence it is not difficult to imagine the 
 rate of spin diminishing as the distance increases, so 
 that at a reasonable distance from the conductor the 
 medium is scarcely disturbed. 
 
 100. To perceive how much rotation of the medium 
 is associated with a given circuit, one must consider 
 the shape of its contour the position of the return 
 current. 
 
 Take first a long narrow loop and send a current 
 up one side and down the other. The rotations 
 belonging to each are superposed, and though they 
 agree in direction for the space inclosed by the loop, 
 they oppose each other outside, and so there is barely 
 any disturbance of the medium outside such a looped 
 conductor ; very little dielectric is disturbed at all, and 
 accordingly the inertia or self-induction is very small 
 (see Fig. 40). 
 
 If the loop opens out so as to inclose an area, as the 
 centrifugal force of the wheels will tend to make it do, 
 then there is a greater amount of rotation, a greater 
 moment of momentum inside it, and accordingly its 
 self-induction is increased. The axis of every wheel 
 is, however, continuous, and must return outside the 
 loop : so the outside region is somewhat affected 
 by rotation, but of a kind opposite to that inside. 
 
 101. Figs. 38 and 41 show the state of things for a 
 
CHAR x.] MODELS OF MAGNETIC FIELD. 189 
 
 closed circuit conveying a current. The free space in 
 Fig. 38 represents a perfect conductor, or perfect breach 
 of connection. Along the inner boundary of this 
 space positive electricity is seen streaming in the 
 direction of the arrows, and it may be streaming 
 quite independently on the outer boundary also, but 
 
 FIG. 40. Diagram of a direct and return current close together, showing distribution 
 of rotation and of slip in the thickness of the conduct6r, and in the dielectric 
 between. The dielectric outside is very little disturbed. The smooth wheels 
 represent a metallic conductor, the cog wheels represent a thin layer of dielectric 
 between the direct and return part of the metallic circuit. 
 For two currents going in the same direction, see Fig. 44. 
 
 nothing happens in its interior which is therefore 
 not represented. 
 
 The corresponding portion in Fig. 41 is intended for 
 an ordinary conductor, full of wheels capable of slip. 
 And slip in this case is a continuous necessity, for the 
 rotation on either side of the conductor is in opposite 
 directions, so the atoms of the conductor have to 
 
IQO MODERN VIEWS OF ELECTRICITY. [PART m. 
 
 accommodate themselves as best they can to the 
 conditions ; some of them rotating one way, some the 
 other, and some along a certain neutral line of the 
 
 oooooooo 
 
 oooooooo 
 
 FIG. 41. Diagram of simple conducting circuit like a galvanometer ring, with the 
 alternate connecting-wheels omitted. The same number of dielectric wheels are 
 drawn outside as inside, to indicate the fact that the total spin is equal inside 
 and out, though the outside is so spread out as to be much less intense. The 
 diagram shows a clockwise positive current flowing steadily round the ring ; with 
 the accompanying distribution of magnetism. Section in the plane of the ring. 
 
 conductor being stationary. If a conductor is straight 
 and infinitely long, the neutral line of no rotation is 
 in the middle. If it be a loop, the neutral line is 
 
CHAP, x.] MODELS OF MAGNETIC FIELD. 191 
 
 nearer the outside than the inside, because the rotation 
 of the medium inside is the strongest. If the loop be 
 shut up to nothing, the neutral line is its outer boundary 
 or nearly so (Fig. 40). If, again, the circuit is wound 
 round and round a ring, as string might be lapped 
 upon a common curtain-ring to cover it, then the axes 
 of whirl are wholly inclosed by the wire, and there is 
 no rotation outside at all. 
 
 FIG. 42. Section of a closed magnetic circuit, or electric vortex-ring, or hollow bent 
 solenoid like Fig. 29, inclosing an anchor-ring air space ; the axis of the ringbeine 
 A B, the sections of its core being E and F. The arrows indicate the intensity of- 
 the spin, i.e. of the magnetic field, which is a maximum at the middle of each 
 section and nothing at all outside. If the core contains iron instead of air, its 
 wheels have to be from 100 to 10,000 times as massive : slipping wheels if solid 
 iron, cogged wheels if a bundle of fine varnished iron wires. Cf. Fig. 47. 
 
 Fig. 42 shows a section of this last-mentioned con- 
 dition, and here the wheels of the dielectric outside 
 are not rotating at all. The inside is revolving, it may 
 be furiously, and so between the inner and outer layers 
 of the conductor we have a great amount of slip and 
 dissipation of energy. 
 
192 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 102. The process of slip which we have depicted 
 goes on in all conductors conveying a current, whether 
 steady or variable, and in fact is the current. The 
 slip is necessarily accompanied by dissipation of 
 energy and production of heat : only in a perfect con- 
 ductor can it occur without friction. In a steady cur- 
 rent the slip is uniformly distributed throughout the 
 section of the conductor ; in the variable stages it is 
 unequally distributed, being then more concentrated 
 near the periphery of the wire ( 43). 
 
 When a current is started in a wire, the outer 
 layers start first, and it gradually though very quickly 
 penetrates to the axis. Hence the lag or self-induc- 
 tion of a wire upon itself is greater as the wire is 
 thicker, and also as it is made of better conducting 
 substance. If it is of iron, the mass or number of the 
 wheels is so great that the lag is much increased, and 
 the spin of its outer layers is great enough to produce 
 the experimental effects discovered by Prof. Hughes. 
 
 One must never confuse the slip with the spin. Slip 
 is current, spin is magnetism. There is no spin at the 
 axis of a straight infinite wire conveying a current, 
 and it occurs in opposite directions as you recede 
 from the axis either way ; arranging itself in circular 
 vortex cores round the axis (Fig. 30, B). But the slip 
 is uniformly distributed all through the wire as soon 
 as the current has reached a steady state. The slip is 
 wholly in the direction of the wire. The axes of spin 
 are all at right angles to that direction. 
 
CHAPTER XL 
 
 MECHANICAL MODELS OF CURRENT INDUCTION. 
 
 Rise of Induced' Current in a Secondary Circuit. 
 
 103. To study the way in which a magnetic field ex- 
 cited in any manner spreads itself into and through a 
 conducting medium, look at Fig. 43, and suppose the 
 region inside the contour A B c D to be an ordinary 
 conducting region that is, full of wheels imperfectly 
 geared together, and capable of slip. 
 
 Directly the rack begins to move, all the wheels 
 outside A B C D begin to rotate, and quickly get up full 
 speed. The outer layer of wheels inside the contour 
 likewise begins to rotate, but not at once ; there is a 
 slight delay in getting them into full motion. For the 
 next inner layer the delay is rather greater, and so on. 
 But ultimately the motion penetrates everywhere 
 equally, and everything is in a steady state. 
 
 But while the process of starting the wheels was 
 
 Q 
 
194 MODERN VIEWS OF ELECTRICITY. [PART m. 
 
 going on, a slip took place round the contour A B C D, 
 and round every concentric contour inside it ; the 
 periphery of the positive wheels moving in a direction 
 opposite to that of the wheels in contact with the 
 rack, and so suggesting the opposite induced current 
 excited at u make " in the substance of a conductor 
 near a growing current, or generally in an increasing 
 magnetic field. 
 
 A E 
 
 FIG. 43. Diagram illustrating the way in which an induced current arises in a mass 
 of metal immersed in an increasing magnetic field ; also how it decays. The dotted 
 lines ABCD, EFGH, IJKL, are successive lines of slip. 
 
 The penetration of the motion deeper and deeper 
 and the gradual dying away of all slip, illustrate also 
 the mode in which this induced current arises and 
 gradually dies away ; becoming nil as soon as the 
 magnetic field (i.e. the rotation) has penetrated to the 
 interior of all conductors and become permanently 
 established there as elsewhere. 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 195 
 
 Suppose the motion of the rack now stopped : all 
 the cogged wheels stop too, though it may be with a 
 jerk and some violence and oscillation due to their 
 momentum ; but those inside the contour A B C D will 
 continue moving for a little longer. The outside layer 
 of this region will slip in such direction as to illustrate 
 the direct induced current at " break " and will begin 
 to stop first ; the slip and the stop gradually pene- 
 trating inwards just as happened during the inverse 
 process, until all trace of rotation ceases. This inverse 
 slipping process is the direct induced current at 
 " break." 
 
 104. Through a perfect conductor the disturbance 
 could never pass, for the slip of the dielectric wheels 
 on its outer skin would be perfect, and would never 
 penetrate any deeper. A superficial current lasting 
 for ever, or rather as long as the magnetic field (the 
 rotation of the dielectric wheels) lasts, is all that would 
 be excited, and it would be a perfect magnetic screen 
 to any dielectric beyond and inclosed by it. Such a 
 perfect conductor is represented by the empty space 
 in Fig. 38. A magnetic field or spin excited outside 
 that space would never reach the set of wheels inclosed 
 and protected by it ( 153). 
 
 105. It will now be perceived that a fly-wheel in 
 rotation is the mechanical analogue of magnetism, 
 or more definitely of a section of a line (or tube) of 
 magnetic force ; and that a brake applied to such a 
 
 O 2 
 
196 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 fly-wheel, with consequent slip, dissipation of energy, 
 and production of heat, is in some sort a mechanical 
 analogue of an electric current. 
 
 The field is regarded as full of geared elastic 
 vortices or whirls, some of which are cogged together, 
 so to speak, while others are merely pressed together 
 by smooth rims. It is among these latter that slip is 
 possible, and in the regions occupied by them that 
 currents exist ; the energy dissipated here being trans- 
 mitted through the non-slippery or dielectric regions 
 from the source of power, just as energy is transmitted 
 from a steam-engine through mill-work or shafting to 
 the various places where it is dissipated by friction. 
 
 Transfer of Energy to a Distance. 
 
 106. Let us now try to understand the use of a 
 telegraph wire from this point of view. Given the 
 means of exciting a magnetic field at one place, how 
 is one to transmit it to another place so as to move 
 magnetic needles or make other signals there ? The 
 first idea of a method might be that inasmuch as no 
 perfect conductor, or absolute magnetic screen, in- 
 tervenes, the field of any magnet is infinite in extent, 
 and consequently already reaches the distant place. 
 Have here a long iron bar capable of being magnetized 
 and demagnetized at pleasure, and have tJiere a very 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 197 
 
 sensitive magnetometer, and the thing is done. I 
 see no reason why, under certain circumstances, this 
 mode of signalling without wire over short distances 
 should not be attempted. But an obvious objection 
 to it is that the effect produced by a given magnet 
 varies inversely with the cube of its distance, so that 
 a few miles away the force of a magnet, even several 
 yards long, is terribly weak. 
 
 The next idea might be to carry some of the 
 magnetic lines of force to the distant place by means 
 of an iron rod or wire. A soft iron wire transmits 
 them so much better than air, that an arrangement 
 of a very elongated loop of iron, with a magnetizing 
 coil upon it at one end, and a receiving coil upon 
 it at the other, might serve to establish some con- 
 nection between the two places, and enable primary 
 currents at this end to produce secondary induced 
 currents at that. This would be a magnetic telegraph 
 in which a magnetic whirl only is propagated along 
 the wire, and a current excited by it at the distant 
 place. 
 
 The current loop and the magnetic loop are, 
 however, reciprocal : and the next idea might be 
 that instead of a long magnetic loop with little 
 current loops threading it at either end, it might 
 be better to have a long current loop with little 
 magnetic loops threading it at either end ; and that 
 is exactly what we have in the electric telegraph. 
 
198 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 It is a better plan for this reason. Iron conducts 
 magnetism, say, a thousand times better than air, 
 but by no means infinitely better ; hence from a long 
 loop of iron a great many lines of force would leak, 
 and would take a shorter circuit through air. 
 
 But copper wire conducts electricity almost in- 
 finitely better than gutta-percha or porcelain. That 
 is why an electric telegraph is better than a magnetic. 
 Lead or German silver conducts a million times better 
 than dilute sulphuric acid, and yet it would be very 
 unsatisfactory to have to signal through an ocean 
 of dilute acid with an uncovered lead or German 
 silver wire. The percentage of loss in the case 
 of a corresponding magnetic circuit of iron would be 
 far greater still. 
 
 107. But now what is it precisely that the wire 
 of the electric telegraph does ? A magnetic field 
 at this end is made to excite a magnetic field at 
 the other end with very little loss ; it is nearly all 
 concentrated upon the other end by means of the 
 wire. Somehow the wire enables us to transmit the 
 magnetic effect exactly in the direction we wish, 
 and to reproduce it where we please. It is all 
 very well to talk of a current going by the wire ; 
 but now that we are regarding a current as a mere 
 slipping of the gearing of the magnetic medium, we 
 see that nothing really travels along the wire at all. 
 
 Suppose, for simplicity, the wire to be a perfect 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 199 
 
 conductor : the magnetic gearing, which penetrates 
 everywhere else and communicates the magnetic 
 spin, breaks down at its surface and nothing is trans- 
 mitted into the wire. The wire, in fact, is electrically 
 nothing but a line of slip of arbitrary shape pene- 
 trating and modifying the magnetic field. Up to its 
 surface magnetic propagation goes on : at its surface 
 it stops. 
 
 Well, how does that help us ? How can this fact 
 enable the wire to transmit the signals ? That is what 
 we have got to see. 
 
 Refer back to Fig. 30, B, and look at it in the light 
 of Fig. 39, taking the latter as a section only, so that 
 each of the little arrows in Fig. 30, B, represents the 
 same fact as the cog-wheels do in Fig. 39. Further, 
 imagine the rack in Fig. 39 to be removed, or replaced 
 by a perfectly smooth rod, and let the wheels rotate 
 just as they would have done had the rack been 
 pushed along ; then on the surface of the stationary or 
 non-existent rod we have the state of slip which we 
 have now learned to recognize as current. What now 
 is the function of the rod or of the space it occupies ? 
 It permits free rotation of the wheel-work on either 
 side of it in opposite directions ; whereas if the rod or 
 space were removed, and the cogs allowed to gear 
 across, they would at once jamb, and the action would 
 be stopped. 
 
 Take away the long conductor, and the only 
 
200 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 magnetic field you can have is the ordinary state 
 of spin round lines of force, rapidly becoming fainter 
 and fainter with distance ; but with the long con- 
 ductor, acting as core to the whirls, you have a 
 different state of things altogether. Away from the 
 wire the field is weak, but close to it it is strong : all 
 round the wire is an intense magnetic field as in 
 Fig. 30, B, and this state of things extends the whole 
 length of the wire undiminished by distance. 
 
 In order that a wire may act in this manner, it 
 must form a closed circuit, and it must have a pro- 
 pelling arrangement at some part of it able to excite 
 the vortex cores upon it just there. Given these 
 conditions, nothing can stop the vortex cores from 
 travelling right along the wire, however long it may 
 be, and producing their effect at the distant station. 
 
 It is not very easy to draw a diagram of the 
 arrangement, because it entails such a number of 
 wheels, and because the diminution of spin with 
 distance is not well represented by them. But the 
 diagram may be imagined thus : 
 
 Let the rack in Fig. 39 be regarded as an in- 
 finitesimal portion of a long circuit extending to 
 New York and back. It may be considered smooth 
 or cogged as one pleases : to avoid the idea of any 
 material transfer it is better to think of it as smooth. 
 At some one point, by means of a battery, or a 
 dynamo, or any other electromotive arrangement, 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 201 
 
 excite in a few of the wheels the motion which in 
 Fig. 39 would occur if the rack were pushed. Out 
 and away through the dielectric the motion trans- 
 mitted by the cog-wheels spreads, at a pace which for 
 the present we may consider infinite, but which we 
 shall learn later is the velocity of light. At a distance 
 from the wire the spin is small it diminishes as 
 the inverse distance ; but close to the wire, all the way 
 along, the opposing cogs are kept apart, and there 
 the spin is most intense. Right along the wire flashes 
 the vortex-like motion ; all the way along it is sur- 
 rounded with rings of whirl, as in Fig. 30, B, and by 
 concentrating some quantity of this whirl into small 
 compass at the distant station, a visible motion or 
 signal is produced. 
 
 This is the function of the wire, it guides the effect 
 transmitted through the dielectric. The wire transmits 
 nothing it is the insulating sheath that transmits 
 all the energy : the wire directs it on its way by 
 holding asunder the mutually opposing gearing of 
 the dielectric. 
 
 So much if the wire be perfectly conducting. If 
 it be an ordinary wire it acts in just the same way, 
 only that the slip on its surface is not perfect it is 
 accompanied, as it were, by friction ; and so its own 
 wheel-work is more or less set in motion in concentric 
 cylinders, except just the axis of the wire, which is 
 undisturbed. The process of transmission is just the 
 
202 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 same as in the case of a perfect conductor, except for 
 two things : First, the having to start the wheel-work 
 inside the metal may delay the process a little, espe- 
 cially if the wheels be very massive as they are in 
 the case of iron. That is the first distinction the 
 delay caused by the conductor being either very 
 thick or very magnetic, or both. The second dis- 
 tinction is that the friction and slip on the imperfect 
 conductor dissipates some of the original supply of 
 energy into heat, and that which is transmitted is 
 accordingly less than with a perfect conductor. But 
 observe still that although the wire now dissipates 
 energy, it transmits none ; all that enters it is lost : 
 the dielectric alone, with its cogged gearing, transmits 
 energy to the distant station ( 42 45). 
 
 Later on we shall learn that the gearing in a 
 dielectric is not rigid, but elastic, and that this is 
 why a certain time is required for transmission why 
 a definite velocity of transmission exists. We may 
 be said to have learnt it already, indeed ; and we have 
 also learned that some dielectrics are less rigid than 
 others gutta-percha less than air, for instance, (see 
 1 6 and 23), and accordingly transmit the disturb- 
 ances more slowly ; but always, as we shall find, 
 with the appropriate velocity of light, so far as the 
 dielectric alone is concerned ( 133 et seq.}. 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 203 
 
 Mechanical Force acting on a Conductor conveying a 
 Current. 
 
 108. In Fig. 41 the conducting portion of a circuit 
 is shown with its appropriate opposite rotations on 
 either side of it. Now superpose a uniform rotation 
 all in one direction upon this, so as to increase the 
 spin on one side of the conductor and diminish it on 
 the other ; in other words, immerse the circuit in a 
 
 
 FIG. 44. Two parallel conductors conveying equal currents in one direction and 
 getting pushed together by the centrifugal force of the outside whirls, no whirl 
 existing between them. The length of the arrows again suggests the distribution 
 of magnetism in the conductors. Fig. 40 showed the correlative repulsion of 
 opposite currents. 
 
 magnetic field. Immediately the extra centrifugal 
 force on one side will urge any movable part of the 
 conductor from the stronger to the weaker portion of 
 the field. And whether there be any movable portion 
 or not, the whole circuit will tend to expand if the 
 
204 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 superposed magnetic whirl agree in direction with 
 the whirl already inside ; while it will tend to 
 contract if the superposed whirl agree with that 
 outside. 
 
 The field for a direct and return circuit may be 
 similarly drawn by superposition of their separate 
 whirls (see Fig. 40). In this case there is strong 
 centrifugal force of the whirl between the wires, while 
 outside there is next to no whirl at all. Hence the 
 wires tend to get driven apart ; and so it becomes 
 evident why a circuit tends to expand so as to inclose 
 the largest possible area, even if no other magnetic 
 field than its own be acting on it. The circuit shown 
 in Fig. 41, for instance, tends to expand even without 
 any superposed magnetic field, simply because the 
 whirl inside is more concentrated, and therefore more 
 intense, than the whirl outside. 
 
 Also if two circuits are arranged near each other 
 
 & t 
 
 in a plane, with their currents in opposite directionsj 
 they will more or less neutralize each other's effect on 
 the space between them, causing (if equal) a region of 
 no spin there. The two conductors will thus get 
 urged together by the unbalanced pressure of the 
 centrifugal force due to the whirl on the other side ; 
 or, currents in the same direction attract. 
 
 109. As for the effect of iron introduced into a 
 circuit, it brings into the region of space it occupies 
 some hundred or thousand times as many lines of 
 
CHAP, xi.] MODELS OF CURRENT INDUCTION. 205 
 
 whirl as were there before, and these naturally con- 
 tribute mightily to the effects, both those exhibiting 
 mechanical force and those exhibiting inertia. 
 
 When one says, as roughly one may do, that iron 
 
 brings 1000 fresh lines into the field, one means that, for 
 
 every whirl otherwise excited, icco more are faced 
 
 round in the iron. And this process goes on while 
 
 the field is increasing in strength until the total 
 
 number of whirls in the iron begins to be called upon ; 
 
 when this point is reached the rate of addition is not 
 
 maintained, and the iron is said to show signs of 
 
 saturation. Ultimately, if ever all its whirls were 
 
 faced round, the iron would be quite saturated ; but 
 
 long before this point is reached another cause is 
 
 likely to make itself felt, viz. the falling off in the 
 
 strength of the whirls already faced round, by the 
 
 action of the strong magnetic induction, which is all 
 
 the time acting so as to weaken thp iron currents 
 
 so far as it is able. And thus at a certain point 
 
 hitherto, unreached by experiment the iron may hot 
 
 only fail to increase the strength of the field any more, 
 
 but may actually begin to diminish it. That is to 
 
 say, its .permeability may conceivably become less 
 
 than I, as if it were a diamagnetic substance (cf. 81). 
 
 The easiest way to picture the effect of iron is to 
 
 think of its wheels as some hundred or thousand 
 
 times as massive as those of air, so that their energy 
 
 and momentum are very great (cf. 97). 
 
206 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 That which is commonly called magnetic permeabi- 
 lity, and denoted by //, ( 82), may in fact be thought 
 of as a kind of inertia, an inertia per unit volume ; in 
 other words, a density an ethereal density ; though 
 how it comes to pass that the ether inside iron is en- 
 dowed with so great inertia one cannot say. Perhaps 
 it is that the iron atoms themselves revolve with the 
 electricity ( 94), perhaps it is something quite different. 
 Whatever the peculiar behaviour of iron, nickel, &c., 
 be due to, it must be something profoundly interesting 
 and important as soon as our knowledge of their 
 molecular structure enables us to perceive its nature. 
 
 Induction in Conductors not originally carrying Currents 
 but moving in a Magnetic Field, 
 
 no. To explain the currents induced in a con- 
 ductor moving through a uniform magnetic field is 
 not quite easy, because none of the diagrams lend 
 themselves naturally and simply to the idea of circuits 
 changing in form or size. 
 
 If we take a rigid circuit in a magnetic field, like 
 Fig. 45, and revolve it out of its plane 180, it is 
 obvious that a current will be excited in it, for the 
 process is essentially the same as if the conductor 
 were kept still and the field reversed. 
 
 But to understand the current excited in a closed 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 207 
 
 circuit when a portion of it moves across the lines so 
 as to embrace a greater number of them, one has to 
 take into account the fact that the inside whirls are 
 expanding and doing work in forcing the conductor 
 away, while the outer whirls are resisting the motion, 
 and being thereby compressed and rendered more ener- 
 getic. Thus the wheels inside revolve slightly slower 
 
 FIG. 45. Section of a uniform magnetic field with two rails and a slider in it. If 
 the slider be moved to or fro, the wheels inside get initially compressed or 
 extended, and thereby gain or lose energy respectively, thus exciting the state of 
 slip known as induced current. 
 
 as the circuit expands, and those on the other side 
 the slider revolve slightly quicker. Both these pro- 
 cesses cause a slipping of the gearing, first all round 
 the inside, and then all through the substance of the 
 wire, whereby positive electricity moves forward in 
 one direction round the circuit, the negative moving 
 oppositely ; and so a current is accounted for. It is 
 
208 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 not to be supposed, however, that any finite expansion 
 of the wheels really occurs : the motion is rapidly 
 equalized by diffusion through the wire, and fresh 
 wheels come in round it from outside ; hence directly 
 after the conductor has stopped moving the field is 
 again steady, but with many more wheels inside the 
 contour than it possessed at first. 
 
 Representation of an Electrostatic Field again, and 
 superposition of it on a perpendicular Magnetic Field. 
 
 in. An electrostatic strain is, we know, caused by 
 a displacement of positive electricity one way along 
 the lines of force, and by an equal displacement of 
 negative the other way. The process was indicated 
 crudely in Fig. /A ; we may now represent it rather 
 more fully with the help of our elastic cells by 
 Fig. 46. 
 
 Here the positive cells have been pulled one way, the 
 negative the other way ; and when the distorting force 
 is removed, the medium tends to spring back to its 
 normal condition, exerting an obvious tension on 
 bodies attached to it in the direction of its lines offeree, 
 its elongated direction, and an obvious pressure in all 
 perpendicular directions, its compressed directions. 
 
 Now, if all the cells are full of parallel whirls, as in 
 the preceding magnetic diagrams, it is not improbable 
 
CHAP, xi.] MODELS OF CURRENT INDUCTION. 209 
 
 that this electrostatic distortion or "shear" of the 
 medium may affect its magnetic properties slightly, 
 and that, if the direction of electrostatic strain were 
 
 FIG. 46. A portion of an electrostatic field between two oppositely charged bodies, 
 with its lines of force going from right to left, and showing a tension along and a 
 pressure at right angles to them, due to the elasticity of the cells (which elasticity 
 may be due to their containing fluid in a state of whirl, see 156). Magnetic lines 
 of force perpendicular to the paper are also shown in section. While this magnetic 
 field was being excited and propagated from below upwards, a slight strain would 
 be produced in the elastic cells, like but immensely less than that shown ; as con- 
 trasted with its normal condition (Fig. 37). Conversely, while this electrostatic 
 strain was being produced, the positive whirls would be infinitesimally quickened 
 and the negative ones retarded during the displacement, thus producing a minute 
 magnetic effect. If the medium is not magnetized, the whirls are not necessarily 
 absent, only faced all ways. 
 
 rapidly reversed, a small magnetic oscillation would 
 also ensue ; but the exact details of these mutual 
 actions are difficult to specify at present. 
 
 Disruptive Discharge. 
 
 112. Disruptive discharge may be thought of as a 
 pulling of the shaded cells violently along past the 
 others ; the process being accompanied by a true 
 disruption a sort of electrolysis of the medium, 
 and a passage of the two electricities in opposite 
 directions along the line of discharge. 
 
 p 
 
2io MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 Consider the locomotion of any one horizontal row 
 of shaded cells in Fig. 46 during the occurrence of such 
 a disruption of the medium. The cells slide on towards 
 the right, and, as they slide, the spin of the negative 
 cells above them is retarded while that of those below 
 them is accelerated ; consequently a true magnetic 
 effect is produced, just like that accompanying a 
 current, and a disruptive discharge has therefore all the 
 magnetic properties of a current. 
 
 Effects of a Moving Charge. 
 
 113. This locomotion of a set of positive cells, or 
 of negative cells the other way, as just considered, is 
 very near akin to the motion of a charge through a 
 dielectric medium. 
 
 When a charged body moves along with extreme 
 rapidity, it can be thought of as exciting a rotation in 
 the cells most closely in contact with it, greater than 
 that which it excites in the opposite kind of cells, and 
 thus produces the whirl proper to a magnetic field. 
 Thus does a moving charge behave just like a current 
 of a certain strength. 
 
 It may be, indeed, that this is the customary way of 
 exciting a voltaic current ; for the chemical forces in 
 a cell cause a locomotion of charged atoms, and thus 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 211 
 
 set up a field, which, spreading out in the way Prof. 
 Poynting has sketched, ( 42) reaches every part of the 
 metallic circuit and excites the current there. 
 
 Electrostatic Effects of a Moving or Varying Magnetic 
 Field. 
 
 1 14, Just as we have seen that a moving or varying 
 electrostatic field may produce slight magnetic effects, 
 so one can perceive that a moving or varying magnetic 
 field brings about something of the nature of an 
 electrostatic strain. 
 
 For a spreading out field is continually propagating 
 the rotation on from one layer of wheels to the next. It 
 there is any slip, we thus get induced currents (Fig. 43), 
 and the rate of propagation is comparatively slow, being 
 a kind of diffusion ; but even if there is not any slip, yet, 
 unless the wheel-work is absolutely rigid, the rate of 
 propagation will not be infinite. The actual rate ot 
 propagation is very great, which shows that the 
 rigidity or elasticity of the wheels is very high in 
 proportion to their inertia, but it is not infinite ; and 
 accordingly the propagation of rotation is accompanied 
 by a temporary strain. One part of the field is in full 
 spin, another more distant part is as yet unreached by 
 the spin ; between the two we have the region of strain , 
 the wheel-work being distorted a little while taking up 
 
 P 2 
 
212 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 the motion. Thus does a spreading out magnetic 
 field cause a slight and temporary electrostatic strain, 
 at right angles both to the direction of the lines offeree 
 and to the direction of their advance. 
 
 Generation of a Magnetic Field. Induction in closed 
 Circuits. 
 
 115. Picture to one's self an unmagnetized piece of 
 iron ; its whirls are all existent, but they are shut up 
 into little closed circuits, and so produce no external 
 effect ; magnetize it slightly, and some of the closed 
 circuits open out and expand, with one portion of 
 them in the air. Magnetize it strongly, and we have 
 a whole set of them opened out into vertex cores, still 
 with the whirl round them, and constituting the 
 common magnetic lines of force. There is no need to 
 think of iron and steel in this connection. In air or 
 any substance the whirls are still present, though much 
 fewer or feebler, and their axes ordinarily form little 
 closed circuits it may be inside the atoms themselves. 
 But wrap a current-conveying wire round them, and 
 at once they open out into the lines of force proper to 
 a circular current. 
 
 Again, think of an iron ring, or a hank of wire 
 as bought at an ironmonger's : wrap a copper wire 
 several times round it, as a segment of a Gramme ring 
 is wound (Fig. 47) and pass a current. The closed 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 213 
 
 vortices in the iron at once expand : a portion of each 
 flashes out and across the air-space inclosed by the 
 ring (not by any means confining itself to a plane, of 
 course), and enters the ring on the opposite side ; so 
 that directly the current is steady the lines all lie inside 
 the iron again, but now inclosing an area the area of 
 the ring instead of being shut up into infinitesimal 
 links. In a sense the iron is still unmagnetized, for 
 its lines of force still form closed contours within it, 
 and none protrude any part of themselves into the air, 
 
 FIG. 47. Closed magnetic circuit like Fig. 42, with a single-ring secondary circuit, 
 and another open secondary loop ; also with a short conducting-rod standing up 
 in it. 
 
 except for irregularities. But in another sense it is 
 highly and permanently magnetized round and round 
 in itself, the magnetism being not easy to get out of it 
 again, except by judiciously arranged reverse currents. 
 
 It is now like one great electric vortex ring instead 
 of like a confused jumble of microscopic ones. Its 
 section was shown in Fig. 42. (See also Appendix 
 (d) and (;;).) 
 
 During the variable period, while the current is in- 
 creasing in strength, or while it is being reversed, the 
 
214 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 region inclosed by the ring and all around it is full of 
 myriads of expanding lines of force flashing across, 
 broadside on, from one side of the iron to the other, 
 and there stopping. It is the presence of these 
 moving lines, changing rapidly from a " simply- 
 connected " into a " multiply-connected " state, or 
 vice versa, which causes the powerful induced currents 
 of " secondary generators." 
 
 In every case of varying magnetic field, in fact, we 
 have lines moving broadside on, propagating their 
 whirl, and more or less disturbing the medium through 
 which they move. 
 
 Next consider a moving or spinning magnet. Its 
 lines travel with it, and, being closed curves, they also 
 must move broadside through the field, so that in this 
 case we may expect just the same effect as can be 
 obtained from a varying magnetic field. 
 
 If a broadside-moving line of force cut across a 
 conductor, its motion is delayed, for its wheels slip 
 and only gradually get up a whirl inside the ill-geared 
 substance ; thus, as we know, causing an induced 
 current (see 103). 
 
 If a conducting ring is looped with the iron ring 
 previously mentioned, as a snap-hook is looped with 
 an eye, then every expanding vortex, while the ring is 
 being magnetized, has necessarily to cut through the 
 conducting ring once and no more, no matter what its 
 shape or size. The electromotive force of induction 
 
CHAP. XL] MODELS OF CURRENT INDUCTION. 215 
 
 is in this case therefore perfectly definite, and simply 
 proportional to the number of turns made by the 
 secondary round the core of the ring (Fig. 47). 
 
 Instead of supposing a closed conducting secondary 
 circuit, imagine an open one : there is still an E.M.F. 
 in it, though rather less than before because a few of 
 the expanding lines flash through the gap and produce 
 no effect ; so electricity must surge to and fro in the 
 conductor, while the ring is being magnetized and de- 
 magnetized, as water surges up and down in a tilted 
 trough, and a small condenser attached to the free 
 ends will be alternately charged and discharged. The 
 gap might become so large that nothing is left but a 
 short rod (Fig. 47) : in this also similar oscillations 
 would occur. 
 
 But now suppose no secondary conductor at all ; 
 nothing but dielectric inclosed by the ring. In it 
 there must be an electric displacement excited every 
 time the magnetism of the ring is reversed. It may 
 be an oscillatory displacement, but still on the whole 
 in one direction during rise of magnetism, and in an 
 opposite direction during reversal of magnetism. A 
 charged body delicately suspended within the ring 
 may feel the effect of the minute electrostatic strain 
 so magnetically produced. 1 
 
 1 1 6. To see the mode in which an electrostatic 
 displacement arises in the space embraced by the ring 
 1 See Phil. Mag. June, 1889. 
 
216 MODERN VIEWS OF ELECTRICITY. [PART in. 
 
 we have only to turn to Fig. 42, and look at the set 
 of wheels along the line A B separating one half the 
 section from the other. They cannot steadily rotate 
 either way, for they are urged in opposite directions by 
 the two halves ; in other words, there is no magnetic 
 field anywhere near such a ring, as is well known ; 
 but, nevertheless, during a change of magnetism, 
 while the whirls inside are changing in speed, the rub 
 on the dielectric necessary for checking the outer 
 wheels of the conductor is either increased or 
 diminished ; and if the wheels have any elastic 
 "give" in them, as we know they have, ( in) the 
 electrostatic strain in the field is thereby altered during 
 the varying stage of the magnetism. 
 
PART IV. 
 
 RADIA TION. 
 
CHAPTER XII. 
 
 RELATION OF ETHER TO ELECTRICITY. 
 
 117. So far as we have been able to understand and 
 explain electrical phenomena, it has been by assuming 
 the existence of a medium endowed with certain 
 mechanical or quasi-mechanical properties, such as 
 mobility ( 12), incompressibility or infinite elasticity 
 of volume ( 5), combined with a certain amount of 
 plasticity or finite elasticity of shape ( 9). We also 
 imagined the medium as composed of two opposite 
 constituents, which we called positive and negative 
 electricity respectively ( 90), and which were con- 
 nected in such a way that whatever one did the other 
 tended to do the precise opposite. Further, we were 
 led to endow each of these constituents with a certain 
 amount of inertia ( 38 and 88), and we recognized 
 something of the nature of friction between each 
 constituent and ordinary matter ( 28 and 63). 
 
 Broadly speaking we may say 
 
 (i) That friction makes itself conspicuous in the 
 
220 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 discussion of current-electricity or the properties of 
 conductors, and that the laws of it are summarized in 
 the statement known by the name of Ohm, viz. that 
 the current through a given conductor is proportional 
 to the force that drives it, or that the opposition force 
 exerted by a conductor upon a current is simply 
 proportional to the strength of that current. 
 
 (2) That elasticity is recognized as necessary when 
 studying the facts of electrostatics or the properties of 
 insulators electric displacement and recoil, or charge 
 and discharge : the laws having been studied by 
 Faraday, and the relative pliability (or shearability if 
 there were such a word) of the medium in different 
 substances being measured and stated in terms of that 
 of air as their specific inductive capacity, K. 
 
 (3) That inertia is brought into prominence by the 
 facts of magnetism, studied chiefly perhaps by 
 Thomson, who has called the relative density of the 
 medium in different substances their magnetic perme- 
 ability or magnetic inductive capacity ; the ratio of its 
 value for any substance to its value for common air 
 being called &. 
 
 (4) That the doubleness of constitution of the 
 medium its being composed of two precisely op- 
 posite entities is suggested by the facts of electro- 
 lysis, by the absence of mechanical momentum in 
 currents and magnets, and by the difficulty of other- 
 wise conceiving a medium endowed with rigidity 
 
CHAP. XIL] ETHER AND ELECTRICITY. 221 
 
 which yet is perfectly fluid to masses of matter 
 moving through it. 
 
 1 1 8. With the hypothesis of doubleness of con- 
 stitution this last-mentioned difficulty disappears. 
 The ether as a whole may be perfectly fluid and allow 
 bodies to pass through it without resistance, while its 
 two components may be elastically attached together 
 and may resist any forces tending to separate them 
 with any required rigidity. It is like the difference 
 between passing one's hand through water, and 
 chemically decomposing it ; it is like the difference 
 between waving a piece of canvas about, and tearing 
 it into its constituent threads. 
 
 To put the matter boldly and baldly : we are 
 familiar with the conceptions of matter and of ether, 
 and it is known that the two things react on each 
 other in some way, so that although a free portion of 
 the ether appears to move freely through matter, yet 
 another portion appears to move with matter as if 
 bound to it. This mode of regarding the facts is as 
 old as Fresnel ( 184). We now proceed a step further, 
 and analyze the ether into two constituents two equal 
 opposite constituents each endowed with inertia, and 
 each connected to the other by elastic ties : ties 
 which the presence of gross matter in general weakens 
 and in some cases dissolves. The two constituents 
 are called positive and negative electricity respectively ; 
 and of these two electricities we imagine the ether to be 
 
222 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 composed. The tie between them is dissolved in metals, 
 it is relaxed or made less rigid in ordinary insulators. 
 The specific inductive capacity of a substance means 
 the reciprocal of the rigidity of its doubly constituted 
 
 ether. Let us call this rigidity /, so that k = ^ 
 
 iv 
 
 The neighbourhood of gross matter seems also to 
 render ether more dense. It is difficult to suppose that it 
 can really condense an incompressible fluid, but it may 
 load it or otherwise modify it so as to produce the effect 
 of increased density. In iron this density reaches its 
 highest known value, and in all substances the density or 
 inertia per unit volume of their ether may be denoted 
 by p, and called their magnetic permeability. 1 
 
 119. Let it be understood what we are doing. 
 In Part I. we discussed effects very analogous to 
 those which would be produced by an elastic incom- 
 pressible medium (roughly like india-rubber or jelly); 
 that is, we were led to postulate a medium possessing 
 elasticity or something very analogous to elasticity. 
 In Parts II. and III. we discussed effects suggesting, 
 and more or less necessitating, the idea of a property 
 of the medium very analogous to inertia ; and we 
 were also led to postulate a doubleness of constitution 
 for the medium, so that shearing strains may go on in 
 it and yet it be perfectly fluid as a whole. We are 
 
 1 Strictly speaking, the density is more likely to be 4717*, and the 
 rigidity ^ir/k ; but the 4?r is omitted for simplicity. 
 
CHAP. XII.] ETHER AND ELECTRICITY. 223 
 
 now pushing these analogies and ideas into greater 
 definiteness and baldness of statement. We already 
 know of a continuous incompressible fluid filling all 
 space, and we call it the ether. Let us suppose that 
 it is composed of, and by electromotive force ana- 
 lyzable into, two constituents ; let these constituents 
 cling together with a certain tenacity, so that the me- 
 dium shall have an "electromotive elasticity, though 
 mechanically quite fluid ; and let each constituent pos- 
 sess inertia, or something so like inertia as to produce 
 similar effects. Making this hypothesis, electrical 
 effects are to a certain extent explained. Not 
 ultimately, indeed few things can be explained 
 ultimately : not even as ultimately as could be 
 wished ; for the nature of the connection between the 
 two constituents of the ether and between the ether 
 and gross matter the nature of the force, that is, and 
 the nature of the inertia remains untouched. This is 
 a limitation to be clearly admitted ; but if that were 
 the only one if all else in the hypothesis were true 
 we should do well, and a distinct step would have been 
 gained. It is hardly to be hoped that this is so 
 hardly to be expected that the bald statement above 
 is more than a kind of parody of the truth ; neverthe- 
 less, supposing it only a parody, supposing what we 
 call electromotive elasticity and inertia are things 
 capable of clearer conception and more adequate state- 
 ment, ( 156) yet, inasmuch as they correspond to and 
 
224 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 represent a real analogy, and inasmuch as we find that 
 a medium so constructed would behave in a very elec- 
 trical manner, and might in conjunction with matter 
 be capable of giving rise to all known electrical 
 phenomena, we are bound to follow out the conception 
 into other regions, and see whether any other abstruse 
 phenomena, not commonly recognized as electrical, 
 will not also fall into the dominion of this hypothetical 
 substance and be equally explained by it ( 8). This 
 is what we shall now proceed to do. 
 
 1 20. Before beginning, however, let me just say 
 what I mean by "electromotive elasticity." It 
 might be called chemical elasticity, or molecular 
 elasticity. There is a well-known distinction be- 
 tween electro-motive force and ordinary matter- 
 moving force. The one acts upon electricity, 
 straining or moving or, in general, " displacing " it ; 
 the other acts upon matter, displacing it. The 
 nature of neither force can be considered known, but 
 crudely we may say that as electricity is to matter 
 so is electromotive force to common mechanical 
 force ; so also is electromotive elasticity to the 
 common shape-elasticity or rigidity of ordinary 
 matter : so perhaps, once more, may electrical 
 inertia be to ordinary inertia. 
 
 Inertia is defined as the ratio of force to ac- 
 celeration ; similarly electric inertia is the ratio of 
 electromotive force to the acceleration of electric 
 
CHAP, xii.] ETHER AND ELECTRICITY. 225 
 
 displacement. It is quite possible that electric iner- 
 tia and ordinary inertia are the same thing, just as 
 electric energy is the same with mechanical energy. If 
 this were known to be so, it would be a step upward 
 towards a mechanical explanation ; but it is by no 
 means necessarily or certainly so ; and whether it be 
 so or not, the analogy undoubtedly holds, and may be 
 fruitfully pursued. 
 
 And as to " electromotive elasticity," one may say 
 that pure water or gas is electromotively elastic, though 
 mechanically limpid ; each resists electric forces up to 
 a certain limit of tenacity, beyond which it is broken ; 
 and it recoils when they are withdrawn. Glass acts 
 in the same way, but that happens to be mechanically 
 elastic too. Its mechanical elasticity and tenacity 
 have, however, nothing to do with its electric elasticity 
 and tenacity. 
 
 One perceives in a general way why fluids can be 
 electrically, or chemically, or molecularly elastic : it is 
 because their molecules are doubly or multiply com- 
 posed, and the constituent atoms cling together, while 
 the several molecules are free of one another. Mechan- 
 ical forces deal with the molecule as a whole, and to 
 them the substance is fluid ; electrical or chemical 
 forces deal with the constituents of the molecule, 
 setting up between them a shearing strain and en- 
 deavouring to tear them asunder. To such forces, 
 therefore, the fluid is elastic and tenacious up to a 
 
 Q 
 
226 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 certain limit. Extend this view of things to the 
 constitution of the ether, and one has at least a 
 definite position whence to further proceed. 
 
 121. It may be convenient and not impertinent here 
 to say that a student might find it a help to re-read 
 Parts I. and II. in the light of what has just been 
 said : remembering that, for the sake of simplicity, 
 only the simple fact of an elastic medium was at first 
 contemplated and insisted on ; no attempt being 
 made to devise a mechanism for its elasticity by 
 considering it as composed of two constituents. 
 Hence the manifest artificiality of such figures as 
 Fig. 6, where fixed beams are introduced to serve as 
 the support of the elastic connections. But it is 
 pretty obvious now, and it has been already indicated 
 in Fig. 7 A, that a closer analogy will be obtained 
 by considering two sets of beads arranged in alter- 
 nate parallel rows connected by elastic threads, and 
 displaced simultaneously in opposite directions. A still 
 further progressive analogy is attempted in Fig. 46. 
 We have gradually passed, therefore, from a sort of one- 
 fluid theory to a modified two-fluid theory ; believing 
 it to be in some sense or other nearer the truth. 
 
 Recovery of the Medium from Strain. 
 
 122. We have now to consider the behaviour of a 
 medium endowed with an elastic rigidity, k, and a 
 density, /*, subject to displacements or strains. One 
 
CHAP, xii.] ETHER AND ELECTRICITY. 227 
 
 obvious fact is that when the distorting force is 
 removed the medium will spring back to its old 
 position, overshoot it on the other side, spring back 
 again, and thus continue oscillating till the original 
 energy is rubbed away by viscosity or internal friction. 
 If the viscosity is very considerable, it will not be 
 able so to oscillate ; it will then merely slide back in 
 a dead-beat manner towards its unstrained state, 
 taking a theoretically infinite time to get completely 
 back, and practically restoring itself to something 
 very near its original state in what may be quite a 
 short time. The recovery may in fact be either a 
 brisk recoil or a leak of any degree of slowness, 
 according to the amount of viscosity as compared 
 with the inertia and elasticity ( 19). 
 
 The matter is one of simple mechanics. It is a 
 case of simple harmonic motion modified by a friction 
 proportional to the speed. The electrical case is 
 simpler than any mechanical one, for two reasons : 
 first, because so long as capacity is constant (and 
 no variation has yet been discovered) Hooke's law 
 will be accurately obeyed restoring force will be 
 accurately proportional to displacement ; secondly, 
 because for all conductors which obey Ohm's law 
 (and no true conductor is known to disobey it) the 
 friction force is accurately proportional to the first 
 power of velocity. 
 
 123. There are two, or perhaps one may say three, 
 
 Q 2 
 
228 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 main cases. First, where the friction is great In 
 that case the recovery is of the nature of a slow leak, 
 according to a decreasing geometrical progression 
 or a logarithmic curve ; the logarithmic decrement 
 being independent of the inertia, and being equal 
 to the quotient of the elasticity and the resistance 
 coefficients. 
 
 As the resistance is made less, the recovery be- 
 comes quicker and quicker until inertia begins to 
 prominently assert its effect and to once more 
 lengthen out the time of final recovery by carrying 
 the recoiling matter beyond its natural position, and 
 so prolonging the disturbance by oscillations. The 
 quickest recovery possible is obtained just before 
 these oscillations begin ; and it can be shown that 
 this is when the resistance coefficient is equal to twice 
 the geometric mean of the elasticity and the inertia. 
 One may consider this to be the second main case. 
 The recoil is then exactly dead-beat, and occurs in the 
 minimum of time. 
 
 The third principal case is when the resistance is 
 quite small, and when the recovery is therefore 
 distinctly oscillatory. If the viscosity were really 
 zero, the motion would be simply harmonic for ever, 
 unless some other mode of dissipating energy were 
 provided ; but if some such mode were provided, or 
 if the viscosity had a finite value, then the vibra- 
 tions would be simply harmonic with a dying out 
 
CHAP, xii.] ETHER AND ELECTRICITY. 229 
 
 amplitude, the extremities of all the swings lying on 
 a logarithmic curve. In such a case as this, the rate 
 of swing is practically independent of friction ; it 
 depends only on elasticity and inertia ; and, as is well 
 known for simple harmonic motion, the time of a 
 complete swing is 2?r times the square root of the ratio 
 of inertia and elasticity coefficients. 
 
 124. Making the statement more electrically con- 
 crete, we may consider a circuit with a certain amount 
 of stored-up potential energy or electrical strain in it : 
 for instance, a charged Leyden jar provided with a 
 nearly complete discharge circuit. The main elastic 
 coefficient here is the reciprocal of the capacity of the 
 jar : the more capacious the jar the more "pliable" it 
 is the less force of recoil for a given displacement, 
 so that capacity is the inverse of rigidity. The main 
 inertia coefficient is that which is known electrically 
 as the " self-induction " of the circuit : it involves the 
 inertia of all the displaced matter and ether, of every- 
 thing which will be moved or disturbed when the jar 
 is discharged. It is not a very simple thing to calcu- 
 late its value in any given case ; still it can be done, 
 and the general idea is plain enough without under- 
 standing the exact function and importance of every 
 portion of the surrounding space. (See Appendix.) 
 
 Corresponding, then, to the well-known simple 
 
 harmonic T = 2?r ./ -, we have, writing L for the 
 
230 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 self-induction or inertia of the circuit, and S for its 
 capacity or inverse elasticity constant, 
 
 T = 27TX/LS. 
 
 This, therefore, is the time of a complete swing. 
 Directly the jar is discharged, these oscillations begin, 
 and they continue like the vibration of a tuning-fork 
 until they are damped out of existence by viscosity 
 and other modes of dissipation of energy. 
 
 125. But now just consider a tuning-fork. Suppose 
 its substance were absolutely unviscous, would it go 
 on vibrating for ever ? In a vacuum it might : in air 
 it certainly would not. And why not ? Because it is 
 surrounded by a medium capable of taking up vibra- 
 tions and of propagating them outwards without limit. 
 The existence of a vibrating body in a suitable 
 medium means the carving of that medium into a 
 succession of waves and the transmission of these 
 waves away into space or into absorbing obstacles. 
 It means, therefore, the conveyance away of the 
 energy of the vibrating body, and its subsequent 
 appearance in some other form wherever the radiating 
 waves are quenched ( 141). 
 
 The laws of this kind of wave-propagation are well 
 known ; the rate at which waves travel through the 
 medium depends not at all on any properties of the 
 original vibrating body, the source of the disturbance ; 
 it depends solely on the properties of the medium. 
 
CHAP, xii.] ETHER AND ELECTRICITY. 231 
 
 They travel at a rate precisely equal to the square 
 root of the ratio of its elasticity to its density. 
 
 Although the speed of travel is thus fixed inde- 
 pendently of the source, the length of the indivi- 
 dual waves is not so independent. The length of 
 the waves depends both on the rate at which they 
 travel and on the rate at which the source vibrates. 
 It is well known and immediately obvious that the 
 length of each wave is simply equal to the product of 
 the speed of travel into the time of one vibration. 
 
 126. But not every medium is able to convey every 
 kind of vibration. It may be that the mode of 
 vibration of a body is entirely other than that which 
 the medium surrounding it can convey : in that case 
 no dissipation of energy by wave- propagation can 
 result, no radiation will be excited. The only kind 
 of radiation which common fluids are mechanically 
 able to transmit is well known : it is that which 
 appeals to our ears as sound. The elasticity con- 
 cerned in such disturbance as this is mere volume 
 elasticity or incompressibility. But electrical ex- 
 periments (the Cavendish experiment, 4, and 
 Faraday's ice-pail experiment) prove the ether to 
 be enormously perhaps absolutely incompressible ; 
 and if so, such vibrations as these would travel with 
 infinite speed and not carve proper waves at all. 
 
 Conceivably (I should like to say probably) 
 gravitation is transmitted by such longitudinal 
 
232 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 impulses or thrusts, and in that case it is nearly or 
 quite instantaneous ; and the rate at which it travels, 
 if finite, can be determined by a still more accurate 
 repetition of the Cavendish experiment than has yet 
 been made ; but true radiation transmitted by the 
 ether cannot be of this longitudinal character. The 
 elasticity possessed by the ether is of the nature 
 of rigidity : it has to do with shears and distortions ; 
 not mechanical stresses, indeed to them it is quite 
 limpid and resistless but electromotive stresses : it 
 has an electrical rigidity, and it is this which must 
 be used in the transmission of wave-motion. 
 
 But the oscillatory discharge of a Leyden jar is 
 precisely competent to apply to the ether these 
 electromotive vibrations : it will shake it in the 
 mode suitable for it to transmit ; and accordingly, 
 from a discharging circuit, waves of electrical dis- 
 tortion, or transverse waves, will spread in all 
 directions at a pace depending on the properties 
 of the medium. 
 
 Thus, then, even with a circuit of perfect conduc- 
 tivity the continuance of the discharge would be 
 limited, the energy would be dissipated ; not by 
 friction, indeed there would in such a circuit be 
 no direct production of heat it would be dissipated 
 by radiation, dissipated in the same way as a hot 
 body cooling, in somewhat the same way as a vibrating 
 tuning-fork mounted on its resonant box. The energy 
 
CHAP, xii.] ETHER AND ELECTRICITY. 233 
 
 of the vibrating body would be transferred gradually 
 to the medium, and would by this be conveyed out 
 and away ; its final destination being a separate ques- 
 tion, and depending on the nature and position of 
 the material obstacles it meets with ( 160 ; see also a 
 lecture on Discharge of Leyden Jar, p. 367). 
 
CHAPTER XIII. 
 
 CONSTANTS OF THE ETHER. 
 
 Velocity of Electrical Radiation. 
 
 127. THE pace at which the radiation-waves travel 
 depends, as we have said, solely on the properties of 
 the medium, solely on the relation between its elasticity 
 and its density. The elasticity considered must be of 
 the kind concerned in the vibrations ; but the vibra- 
 tions are in this case electrical, and so electrical 
 elasticity is the pertinent kind. This kind of elasticity 
 is the only one the ether possesses of finite value, and 
 its value can be measured by electrostatic experiments. 
 Not absolutely, unfortunately : only the relative elas- 
 ticity of the ether as modified by the proximity of 
 gross substances has yet been measured : its reciprocal 
 being called their specific inductive capacity, or di- 
 electric constant, K. The absolute value of the quan- 
 tity K is at present unknown, and so a convention has 
 
CHAP. XIIL] CONSTANTS OF THE ETHER. 235 
 
 arisen whereby in air it is called I. This convention 
 is the basis of the artificial electrostatic system of 
 units. No one supposes, or at least no one has a right 
 to suppose, that its value is really I. The only rational 
 guess at its value is one by Sir William Thomson, 1 viz. 
 
 - . Whether known or not, the absolute value of 
 0420 
 
 the dielectric constant is manifestly a legitimate 
 problem which may any year be solved. 
 
 The other thing on which the speed of radiation 
 waves depends is the medium's density its electric 
 density, if so it must be distinguished. Here, again, 
 we do not know its absolute value. Its relative or 
 apparent amount inside different substances is mea- 
 sured by magnetic experiments, and called their 
 specific magnetic capacity, or permeability, and is 
 denoted by //-. 
 
 Being unknown, another convention has arisen, quite 
 incompatible with the other convention just mentioned, 
 that its value in air shall be called I. This convention 
 is the basis of the artificial electro-magnetic system of 
 units volts, ohms, amperes, farads, and the like. 
 Both of these conventions cannot be true : no one 
 has the least right to suppose either true. The only 
 rational guess at ethereal free density is one by Sir 
 William Thomson, viz. 9-36 x IO" 19 . 
 
 1 Trans. K. S. Edin., xxi. 60 ; see also article "Ether, "Jin the 
 Encyc. Brit. ; and page 341 of this book. 
 
236 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 128. Very well, then ; it being clearly understood 
 
 that these two great ethereal constants, k or ~' and p, 
 
 are neither of them at present known, but are both of 
 them quite knowable, and may at any time become 
 known, it remains to express the speed of wave 
 transmission in terms of them. But it is well known 
 that this speed is simply the square root of the ratio 
 of elasticity to density, or 
 
 v = 4 / ~ or 
 
 This then is the speed with which waves leave the 
 discharging Leyden jar circuit, or any other circuit 
 conveying alternating or varying currents, and travel 
 out into space. 
 
 Not knowing either k or ytt, we cannot calculate 
 this speed directly, but we can try to observe it 
 experimentally. 
 
 129. The first and crudest way of making the 
 attempt would be to arrange a secondary circuit near 
 our oscillating primary circuit, and see how soon the 
 disturbance reached it. For instance, we might take 
 a nearly closed loop, make it face a Leyden jar circuit 
 across a measured distance, and then look for any 
 interval of time between the spark of the primary 
 discharge and the induced spark of the secondary 
 circuit ; using a revolving mirror or what we please. 
 
CHAP, xiii.] CONSTANTS OF THE ETHER. 237 
 
 But in this way we should hardly be able to detect 
 any time at all : the propagation is too quick. 
 
 130. Since this was first written, Dr. Hertz, of 
 Karlsruhe, has succeeded in making a measure of 
 velocity on this very plan. He did not indeed 
 actually measure the time which elapsed between 
 the closing of the primary circuit and the start of the 
 induced current in the secondary, neither did he use 
 a Leyden jar, but he converted the advancing waves 
 from an electrically oscillating arrangement, excited 
 by means of an induction coil, into stationary waves, 
 by means of reflection at a plane metallic wall. Just 
 as waves travelling along a rope or stretched cord 
 are converted into stationary waves, or nodes and 
 loops, by the interference of direct and reflected 
 pulses : reflection taking place from the fixed end of 
 the cord ; so waves advancing from an electrostatic 
 oscillator, or charged body connected with the ter- 
 minals of an induction coil, were reflected at the wall 
 of the room (lined with sheet zinc on purpose to 
 make it a conductor, and therefore a good reflector, 
 see 164), and by interference with the direct waves 
 converted them into stationary nodes and loops : the 
 interval between two nodes being half a wave-length. 
 
 By now moving the secondary circuit about, be- 
 tween the primary and the wall, places of maxi- 
 mum and minimum disturbance could be found, and 
 thus the wave-length measured. By calculating the 
 
238 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 oscillation period of the primary circuit (or part-cir- 
 cuit, for it was unclosed) an indirect measure of the 
 velocity of propagation was arrived at. So far as 
 could be told it agreed with measurements made 
 by other means, such as those now to be described. 
 
 131. We might next make use of the principle of 
 the electric telegraph, viz. the propagation of a dis- 
 turbance round a single circuit from any one point of 
 origin. Consider a large closed circuit, either convey- 
 ing or not conveying a current : introduce at any one 
 point a sudden change a sudden E.M.F., for instance, 
 or a sudden resistance if there be a current already. 
 Out from that point a disturbance will spread into the 
 ether, just as happens in air when a blow is struck or 
 gun-cotton fired. A regular succession of disturbances 
 would carve the ether into waves : a single disturb- 
 ance will merely cause a pulse or shock ; but the rate 
 of transmission is the same in either case, and we may 
 watch for the reception of the pulse at a distant station. 
 If the station has to be very distant in order to give 
 an appreciable lapse of time, a speaking-tube is de- 
 sirable to prevent spreading out in all directions to 
 concentrate the disturbance at the desired spot. What 
 a speaking-tube is to sound, that is the wire of the 
 circuit the telegraph wire to ethereal pulses. 
 
 It is a curious function, this of the telegraph wire : 
 it does not convey the pulses, it directs them. They 
 are conveyed wholly by the ether, at a pace deter- 
 
CHAP, xiii.] CONSTANTS OF THE ETHER. 239 
 
 mined by the properties of the ether, modified as it 
 may be by the neighbourhood of gross matter. Any 
 disturbance which enters the wires is rapidly dissipated 
 into heat, and gets no further : it is the insulating 
 medium round it which transmits the pulses to the 
 distant station. 
 
 All this was mentioned in Part III., and an at- 
 tempt was made to explain the mechanism of the 
 process, and to illustrate in an analogical way what 
 is going on (Chap. XL). 
 
 The point of the matter is that currents are not 
 propelled by end-thrusts, like water in a pipe or air in 
 a speaking-tube, but by lateral propulsion, as by a 
 series of rotating wheels with their axes all at right 
 angles to the wire surrounding it as a central core, 
 and slipping with more or less friction at its surface. 
 This is characteristic of ether modes in general : it 
 does not convey longitudinal waves or end-thrust 
 pulses, like sound, but it conveys transverse vibrations 
 or lateral pulses, like light ( 42). 
 
 132. Without recapitulating further, we can perceive, 
 then, that the transmission of the pulse round the 
 circuit to its most distant parts depends mainly on 
 the medium surrounding it. The process is somewhat 
 as follows : Consider two long straight parallel wires, 
 freely suspended, and at some great distance joined 
 together. At the near end of each, start equal opposite 
 electromotive impulses, as by suddenly applying to 
 
240 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 them the poles of a battery ; or apply a succession of 
 such pulses by means of an alternating machine. Out 
 spread the pulses into space, starting in opposite 
 phases from the two wires, so that at a distance from 
 the wires the opposite pulses interfere with each other, 
 and are practically non-existent, just as but little sound 
 is audible at a distance from the two prongs of a freely 
 suspended tuning-fork. But near the wires, and 
 especially between them, the disturbance may be 
 considerable. The energy emitted by the source, as 
 it reaches each wire is dissipated, and so a fresh supply 
 of energy goes on continually arriving at the wires, 
 always flowing in from outside, to make up the de- 
 ficiency. If the wires are long enough, hardly any 
 energy may remain by the time their distant ends are 
 reached ; but whatever there is will still be crowding 
 in upon the wires and getting dissipated, unless by 
 some mechanism it be diverted and utilized to effect 
 some visible or audible or chemical change, and so to 
 give the desired signal ( 107). 
 
 133. Now the pace at which this transmission of 
 energy goes on in the direction of the wires is pretty 
 much the same as in free space. 1 There are various cir- 
 cumstances which can retard it ; there are none which 
 can accelerate it. The circumstances which can retard 
 it are, first, constriction of the medium by too great 
 proximity of the two conducting wires, as, for instance, 
 
 1 Appendix (d). 
 
CHAP. XIIL] CONSTANTS OF THE ETHER. 241 
 
 if they consisted of two flat ribbons close together 
 with a mere film of dielectric between, or if one were 
 a small-bore tube and the other its central axis or 
 core. In such cases as this the general body of ether 
 takes no part in the process, the energy has all to be 
 transmitted by the constricted portion of dielectric, 
 and the free propagation of ethereal pulses is interfered 
 with : the propagation is no longer a simple true wave- 
 propagation, it approximates more or less closely to 
 a mere diffusion creep : rapid it may be, and yet 
 without definite velocity, like the conduction of heat 
 or the diffusion of a salt into water. One well-known 
 effect of this is to merge successive disturbances 
 into one another, so that their individuality, and 
 consequently the distinctness of signalling, is lost. 
 
 1 34. Another circumstance which can modify rate 
 of transmission of the pulses is ethereal inertia in the 
 substance of the conducting wires : especially extra 
 great inertia, as, for instance, if they are made of iron. 
 For the dissipation of energy does not go on accu- 
 rately at their outer surface ; it has usually to penetrate 
 to a certain depth, and until it is dissipated the fresh 
 influx of energy from behind does not fully occur. 
 Now, so long as the value of fi for the substance of 
 the wires is the same as that of air or free space, no 
 important retardation is thus caused, unless the wires 
 are very thick ; but directly the inertia in the sub- 
 stance of the wires is some hundred or thousand times 
 
 R 
 
242 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 as big as that outside, it stands to reason that more 
 time is required to get up the needful magnetic spin 
 in its outer layer ; and so the propagation of pulses 
 is more or less retarded. At the same time this cir- 
 cumstance does not alter the character of the pro- 
 pagation, it does not change it from true wave 
 velocity to a diffusion, it leaves its character unaltered ; 
 and so the signals, though longer in coming, may 
 arrive quite clear, independent, and distinct. It is 
 much the same, indeed, as if the density of the 
 surrounding medium had been slightly increased. 
 
 I have several times mentioned the name of Prof. 
 Poynting as one who has developed Maxwell's equa- 
 tions, and thrown great light upon the mode in which 
 electro-magnetic energy is transmitted : in the same 
 connection, and also still more prominently in con- 
 nection with the general theory of telegraphy and 
 of electro-magnetic waves, I must mention with due 
 emphasis the name of Mr. Oliver Heaviside. It is not 
 for me to attempt to apportion credit, but the wide 
 scope of his mathematical investigation into these 
 difficult fields of research is remarkable. 
 
 135. These, then, are the main circumstances which 
 affect the rate of transmission of a pulse from one 
 part of a closed circuit to another : extra inertia or 
 so-called magnetic susceptibility in the conducting 
 substance, especially in its outer layers ; and undue 
 constriction or throttling of the medium through 
 
CHAP, xiii.] CONSTANTS OF THE ETHER. 243 
 
 which the disturbance really has to go. Both these 
 circumstances diminish rate of transmission ; and one 
 (the last mentioned) modifies the law, and tends 
 to obliterate individual features and to destroy 
 distinctness. 
 
 Of course, besides these, the nature of the insulating 
 medium will have an effect on the rate of propagation 
 but that is obvious all along ; it is precisely the rate 
 at which any given medium transmits pulses that we 
 want to know, and on which we are thinking of making 
 experiments. If we use gutta-percha (more accu- 
 rately the ether inside gutta-percha) as our transmit- 
 ting medium in an experiment, we are not to go and 
 pretend that we have obtained a result for air. 
 
 136. The circumstances we have considered as 
 modifying the rate of transmission are both of them 
 adventitious circumstances, independent of the nature 
 of the medium, and they are entirely at our own 
 disposal. If we like to throttle our medium, or to use 
 thick iron wires, we can do so, but there is no com- 
 pulsion : and if we wish to make the experiment in 
 the simplest manner, we shall do no such thing. We 
 shall use thin copper wires (the thinner the better), ar- 
 ranged parallel to one another a fair distance apart, and 
 we shall then observe the time which an electromotive 
 impulse communicated at one end takes to travel to 
 the other. Instead of using two wires, we may if we 
 like use what comes to much the same thing, viz. a 
 
 R 2 
 
244 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 single wire suspended at a reasonable height above 
 the ground, as in a common land telegraph. Such a 
 case as this is much the same as if two wires were 
 used at a distance apart equal to about twice the 
 height above the ground. 
 
 The experiment, if it could be accurately made, 
 would result in the observation of a speed of pro- 
 pagation equal to 3 x io 10 centimetres (300,000 kilo- 
 metres, or about 185,000 miles) per second. The 
 actual speed in practice may be less than this, by 
 reason of the various circumstances mentioned, but 
 it can never be greater. This, then, is the rate of 
 transmission of transverse impulses, and therefore of 
 transverse waves, through ether as free as it can be 
 easily obtained. 
 
 137. The writer has succeeded in making a rough 
 preliminary determination on this very plan, but 
 avoiding the necessity for excessive lengths of wire by 
 using the principle of reflection and interference to 
 obtain stationary waves in a pair of parallel wires of 
 known length attached as lateral appendages to a 
 Leyden jar circuit at every discharge. Alternating 
 pulses travel along these wires, and are reflected at 
 their far ends, just as pulses travel along a string 
 attached to a tuning-fork in Melde's experiment. 
 Reflection of the pulses at the free ends of the wires 
 is not accomplished without a considerable recoil or 
 kick, which can be observed by the brightness of the 
 
CHAP, xiii.] CONSTANTS OF THE ETHER. 245 
 
 brush or the length of spark it gives. The length of 
 the wires or the size of the circuit is adjusted until 
 this recoil kick is a maximum, and the length of each 
 wire is then taken as half a wave-length. Knowing 
 the rate of oscillation proper to the particular Leyden 
 jar circuit employed, a determination of the velocity 
 of the pulses can at once be made. It agrees with 
 what is said above. 1 
 
 138. There are many methods known to physicists 
 by which an indirect experimental determination of 
 this velocity can be made. These methods have been 
 more largely practised than the one described, but 
 they do not determine directly the speed with 
 which electrical pulses or waves travel : they directly 
 determine the ratio ///A, or, what is the same thing 
 the product K//,, and it is left to theory to say that 
 this is really the velocity of electrical pulses in free 
 ether. It is unnecessary to say more about them 
 here. They are generally referred to as methods 
 of determining the ratio " v? or the number of 
 electrostatic units of quantity in an electro-magnetic 
 unit ; which is a roundabout and forced mode of 
 expression, but it serves. 
 
 1 Phil: Mag. August, 1888. See also Appendix (o). 
 
CHAPTER XIV. 
 
 ELECTRICAL RADIATION, OR LIGHT. 
 
 139. HAVING now described one or two possible 
 methods of measuring the velocity of electric wave 
 propagation, and therefore of at least the ratio of the 
 two ethereal constants k and //, (or, what is the same 
 thing, the product of the two constants K and yu) ; 
 return to the consideration of the ordinary small 
 discharging Ley den jar or other alternating current 
 circuit of a moderate size, it may be a few yards or 
 a foot or an inch in diameter. 
 
 If the alternating currents are produced artificially 
 by some form of alternating machine, their frequency 
 is, of course, arbitrary ; but if they be automatically 
 caused by the recoil of a given Leyden jar in a given 
 circuit, their frequency is, as we have already said 
 ( 124), 
 
 per second ; 
 
 27Tv/(LS) 
 
 where L is the electrical inertia or self-induction of 
 
CHAP. XIV.] ELECTRICAL RADIATION, OR LIGHT. 247 
 
 the circuit, and where S is the capacity or reciprocal 
 of the elasticity-constant of the jar. 
 
 140. It is not convenient here to go into the deter- 
 mination of the quantity L, but roughly one may 
 say that for an ordinary open single-loop circuit 
 it is a quantity somewhat comparable with ten or 
 twelve times its circumference multiplied by the 
 constant /u. 1 
 
 The value of S has to do with the area and 
 
 thickness of the condenser, being, as is well known, 
 j\^ 
 multiplied by the constant K. 
 
 47T 
 
 The product LS in the above expression contains 
 therefore two factors, each of linear dimensions, ex- 
 pressing the sizes of circuit and jar ; and likewise 
 contains a factor /-tK expressing the properties of the 
 surrounding medium. Hence, so far as the ether is 
 concerned, the above expression for frequency of 
 vibration demands only a knowledge of the product 
 of its two constants K and /u ; and since this is known 
 by the previous velocity experiments, it is easy to 
 calculate the rate of oscillation of any given con- 
 denser-discharge. It is also easy to calculate the 
 wave-length ; for if there are n waves produced per 
 second, and each travels with the velocity v, the length 
 
 r 1 V 
 
 of each wave is -. 
 
 See Appendix (e). 
 
248 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 Hence the wave-length is 2-rr 
 
 141. Now, if we go through these numerical calcu- 
 lations for an ordinary Leyden jar and discharger, we 
 shall find waves something like, say, 50 or 100 yards 
 long. They may plainly be of any length, according 
 to the size of the jar and the size of the circuit. The 
 bigger both these are the longer will be the waves. 
 
 A condenser of i microfarad capacity, discharging 
 through a coil of self-induction I secohm, will give rise 
 to ether waves 1900 kilometres or 1200 miles long; 
 and the rate of its oscillation is 157 complete swings 
 per second. 
 
 A common pint Leyden jar discharging through a 
 pair of tongs may start a system ofetherwaves each not 
 longer than about 15 or 20 metres ; and its rate of oscil- 
 lation will be something like ten million per second. 
 
 A tiny thimble-sized jar overflowing its edge may 
 propagate waves only about 2 or 3 feet long. (See 
 also 157 and Appendix (/).) 
 
 142. The oscillations of current thus recognized as 
 setting up waves have only a small duration, unless 
 there is some means of maintaining them. How long 
 they will last depends partly upon the conductivity of 
 the circuit ; but even in a circuit of infinite conductivity 
 they must die out if left to themselves, from the mere 
 fact that they dissipate their energy by radiation. 
 One may get 10 or 100, or perhaps even 1,000, 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 249 
 
 perceptible oscillations of gradually decreasing ampli- 
 tude, but the rate of oscillation is so great that their 
 whole duration may still be an extremely small 
 fraction of a second. For instance, to produce 
 ether waves a metre in length requires 300,000,000 
 oscillations per second. 
 
 To keep up continuous radiation naturally requires 
 a supply of energy, and unless it is so supplied the 
 radiation rapidly ceases. Commercial alternating 
 machines are artificial and cumbrous contrivances for 
 maintaining electrical vibrations in circuits of finite 
 resistance, and in despite of loss by radiation. 
 
 In most commercial circuits the loss by radiation 
 is probably so small a fraction of the whole dissipation 
 of energy as to be practically negligible ; but one is, 
 of course, not limited to the consideration of commer- 
 cial circuits or to alternating machines as at present 
 invented and used. It may be possible to devise 
 some less direct method some chemical method, 
 perhaps for supplying energy to an oscillating circuit, 
 and so converting what would be a mere discharge or 
 flash into a continuous source of radiation. 
 
 143. So far we have only considered ordinary prac- 
 ticable electrical circuits, and have found their waves in 
 all cases pretty long, but getting distinctly shorter the 
 smaller we take the circuit. Continue the process of 
 reduction in size further, and ask what sized circuit 
 will give waves 6000 tenth-metres (three-fifths of a 
 
250 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 micron, or 25 millionths of an inch) long. We have 
 only to put 2?r /(__) = O'ooooo, and we find 
 
 V VyU, K/ 
 
 that the necessary circuit must have a self-induction 
 in electro-magnetic units, and a capacity in electro- 
 static units, such that their geometric mean is io- 5 
 centimetre (one-tenth of a micron). This gives us 
 at once something near atomic dimensions for the 
 circuit, and suggests immediately that those short 
 ethereal waves which are able to affect the retina, 
 and which we are accustomed to call " light," may 
 be really excited by electrical oscillations or surgings 
 in circuits of atomic dimensions ( 157-9). 
 
 If after the vibrations are once excited there is no 
 source of energy competent to maintain them, the 
 light production will soon cease, and we shall have 
 merely the temporary phenomenon of phosphores- 
 cence ; but if there is an available supply of suitable 
 energy, the electrical vibrations may continue, and 
 the radiation may become no longer an evanescent 
 brightness, but a steady and permanent glow. 
 
 Velocity of Electrical Radiation compared with Velocity 
 of Light in Free Space and in Material Substances. 
 
 144. We have thus imagined the now well-known 
 Maxwellian theory of light, viz. that it is produced by 
 electrical vibrations, and that its waves are electrical 
 waves. 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 251 
 
 But what justification is there for such an hypothesis 
 beyond the mere fact which we have here insisted 
 on, viz. that waves in all respects like light-waves 
 except size, i.e. transverse vibrations travelling at a 
 certain pace through ether, can certainly be produced 
 temporarily in practicable circuits by familiar and 
 very simple means, and could be produced of exactly 
 the length proper to any given kind of light if only 
 it were feasible to deal with circuits ultra-micro- 
 scopic in size ? The simplest point to consider 
 is : Does light travel at the same speed as the 
 electrical disturbances we have been considering ? 
 We described one method of measuring how fast 
 electrical radiation travels in free space, and there 
 are many other methods : the result was 300,000 
 kilometres per second. Does light travel at the same 
 pace ? 
 
 Methods of measuring the velocity of light have 
 long been known, and the result of those measure- 
 ments in free space or air is likewise 300,000 kilo- 
 metres a second. The two velocities agree in free 
 space. Hence surely light and electrical radiation 
 are identical. 
 
 145. But there is a further test The speed of elec- 
 trical radiation was not the same in all media : it 
 depended on the electrical elasticity and the ethereal 
 density of the transparent substance ; in other words, 
 it was equal to the reciprocal of the geometric mean 
 
252 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 of its specific inductive capacity and its magnetic 
 permeability 
 
 I 
 
 Now, although the absolute value of neither K nor 
 fM is known, yet their values relatively to air are often 
 measured, and are known for most substances. 
 
 Also, it is easy to compare the pace at which light 
 goes through any substance with its velocity in free 
 space : the operation is called finding the refractive 
 index of a substance. The refractive index means, in 
 fact, simply the ratio of the velocity of light in space 
 to its velocity in the given substance. The reciprocal 
 of the index of refraction is therefore the relative 
 velocity of light. Calling the index of refraction n , 
 therefore, we ought, if the electrical theory of light 
 be true, to find that ;z 2 =K//.; or that the index of 
 refraction of any substance is the geometric mean of 
 its electrostatic and magnetic specific capacities. 
 
 146. That this is precisely true for all substances 
 cannot at present be asserted. There are some sub- 
 stances for which it is very satisfactorily true : there are 
 others which are apparent exceptions. It remains to 
 examine whether they are not only apparent but 
 real exceptions, and, if so, to what their exceptional 
 behaviour is due, 
 
 It must be understood what the essential point is- 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 253 
 
 It has been proved by various methods, and with 
 greater approach to exactness as the accuracy of the 
 methods is improved, that electrical disturbances 
 such as the long waves emitted by any alternating 
 machine travel through air or free space with exactly 
 the same velocity as light ; in other words, that there 
 is no recognizable difference in speed between waves 
 several hundred miles long and waves so small that a 
 hundred thousand of them can lie in an inch. This is 
 true in free ether, and it is a remarkable fact. If it 
 proves anything concerning the structure of the ether, 
 it proves that it is continuous, homogeneous, and 
 simple beyond any other substance ; or at least that if 
 it does possess any structural heterogeneity, the parts 
 of which it is composed are so nearly infinitesimal that 
 a hundred miles and the hundred-thousandth of an 
 inch are quantities of practically the same order of 
 magnitude so far as they are concerned : its parts are 
 able to treat all this variety of wave-length in the 
 same manner. 
 
 But directly one gets to deal with ordinary gross 
 matter we know that this is certainly not the case. 
 Ordinary matter is composed of molecules which, 
 though small, are far from being infinitesimal. Atoms 
 are much smaller than light-waves, indeed, but not 
 incomparably smaller. Hence it is natural to suppose 
 that the ether as modified by matter will be modified 
 in a similarly heterogeneous manner ; and will accord- 
 
254 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 ingly not be able to treat waves of all sizes in the 
 same way. 
 
 The speed of all waves is retarded by entering gross 
 matter, but we should expect the smallest waves to be 
 retarded most. The phenomenon is well marked even 
 within the range of such light-waves as can affect the 
 retina : the smaller waves those which produce the 
 sensation of blue are more retarded, and travel a 
 little slower, through, say, glass or water, than the 
 somewhat larger ones which produce the sensation of 
 red. This phenomenon has long been known, and is 
 called dispersion. One result of it is that it is not easy 
 to say at what rate waves a few inches or a few yards 
 or miles long ought to travel, by merely knowing at 
 what rate the ultra-microscopic light-waves travel. 
 
 147. But there is even more to be said than this. 
 There is not only dispersion, there is selective absorp- 
 tion possessed by matter : not only does it transmit 
 different-sized waves at different rates, but it absorbs 
 and quenches some much faster than others. Few 
 substances, perhaps none, are equally transparent to 
 all sizes of waves. Glass, for instance, which transmits 
 readily the assortment of waves able to affect the 
 retina, is practically quite opaque to waves two or 
 three times longer or shorter. And whenever this 
 selective absorption occurs, the laws of dispersion are 
 extraordinary so extraordinary that the dispersion 
 is often spoken of as " anomalous " ; which of course 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 255 
 
 means, not that it is lawless, but that its laws are 
 unknown. Dispersion in any case is an obscure and 
 little understood subject, but dispersion modified by 
 selective absorption is still worse. Until the theory of 
 dispersion is better understood, no one is able to say 
 at what speed waves of any given length ought to 
 travel. One can only examine experimentally at 
 what rate they do travel. This has been done for long 
 electrical waves, and it has been done for short light- 
 waves : in the case of some substances the speed is the 
 same, in the case of others it is different. But that 
 the speed should be different is as I have now ex- 
 plained very natural, and can by no means be twisted 
 into an admission that light-waves and electrical waves 
 are not essentially identical. That the speed of both 
 should agree at all is noteworthy ; the agreement 
 appears to be exact in air, and practically exact in 
 such simple substances as sulphur, and in the class of 
 hydrocarbons known as paraffins ; whereas in arti- 
 ficial substances like glass, and in organic substances 
 like fats and oils, the agreement is less perfect. 
 
 148. So much for the vital question of the speed at 
 which electrical and optical disturbances travel. In 
 some cases the speeds are accurately the same, in 
 no case are they entirely different ; and in those cases 
 where the agreement is only rough, an efficient and 
 satisfactory explanation of the difference is to hand 
 in the very different lengths of wave which have at 
 
256 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 present been submitted to experiment. To compare 
 the speeds properly, we must either learn to shorten 
 electrical waves, or to lengthen light- waves, or both, 
 and then compare the two things together when of 
 the same si^e. 
 
 It cannot be seriously doubted that they will turn 
 out identical. 
 
 Manufacture of Light. 
 
 149. The conclusions at which we have arrived, that 
 light is an electrical disturbance, and that light-waves 
 are excited by electric oscillations, must ultimately, 
 and may shortly, have a practical import. 
 
 Our present systems of making light artificially are 
 wasteful and ineffective. We want a certain range of 
 oscillation, between 7000 and 4000 billion vibrations per 
 second : no other is useful to us, because no other has 
 any effect on our retina ; but we do not know how to 
 produce vibrations of this rate. We can produce a de- 
 finite vibration of one or two hundred or thousand per 
 second ; in other words, we can excite a pure tone of 
 definite pitch ; and we can command any desired range 
 of such tones continuously by means of bellows and a 
 keyboard. We can also (though the fact is less well 
 known) excite momentarily definite ethereal vibrations 
 of some million per second, as I have explained at 
 length; but we do not at present seem to know how to 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 257 
 
 maintain this rate quite continuously. To get much 
 faster rates of vibration than this we have to fall back 
 upon atoms. We know how to make atoms vibrate : it 
 is done by what we call " heating " the substance ; and 
 if we could deal with individual atoms unhampered by 
 others, it is possible that we might get a pure and 
 simple mode of vibration from them. It is possible, 
 but unlikely ; for atoms, even when isolated, have a mul- 
 titude of modes of vibration special to themselves, of 
 which only a few are of practical use to us, and we do 
 not know how to excite some without also the others. 
 However we do not at present even deal with individual 
 atoms ; we treat them crowded together in a compact 
 mass, so that their modes of vibration are really infinite. 
 We take a lump of matter, say a carbon filament or 
 a piece of quick-lime, and by raising its temperature 
 we impress upon its atoms higher and higher modes 
 of vibration, not transmuting the lower into the higher 
 but superposing the higher upon the 'lower, until at 
 length we get such rates of vibration as our retina is 
 constructed for, and we are satisfied. But how waste- 
 ful and indirect and empirical is the process. We 
 want a small range of rapid vibrations, and we know 
 no better than to. make the whole series leading up to 
 them. It is as though, in order to sound some little 
 shrill octave of pipes in an organ, we were obliged to 
 depress every key and every pedal, and to blow a 
 young hurricane. 
 
258 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 150. I have purposely selected as examples the 
 more perfect methods of obtaining artificial light, 
 wherein the waste radiation is only useless, and not 
 noxious. But the old-fashioned plan was cruder even 
 than this, it consisted simply in setting something 
 burning : whereby not only the fuel but the air was 
 consumed, whereby also a most powerful radiation 
 was produced, in the waste waves of which we were 
 content to sit stewing, for the sake of the minute, almost 
 infinitesimal, fraction of it which enabled us to see. 
 
 Everyone knows now, however, that combustion is 
 not a pleasant or healthy mode of obtaining light ; but 
 everybody does not realize that neither is incandes- 
 cence a satisfactory and unwasteful method which is 
 likely to be practised for more than a few decades, or 
 perhaps a century. 
 
 Look at the furnaces and boilers of a great steam- 
 engine driving a group of dynamos, and estimate the 
 energy expended ; and then look at the incandescent 
 filaments of the lamps excited by them, and estimate 
 how much of their radiated energy is of real service to 
 the eye. It will be as the energy of a pitch-pipe to 
 an entire orchestra. 
 
 It is not too much to say that a boy turning a handle 
 could, if his energy were properly directed, produce 
 quite as much real light as is produced by all this 
 mass of mechanism and consumption of material. 
 
 151. There might, perhaps, be something contrary to 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 259 
 
 the laws of Nature in thus hoping to get and utilize 
 some specific kind of radiation without the rest, but 
 Lord Rayleigh has shown in a short communication 
 to the British Association at York 1 that it is not so, 
 and that therefore we have a right to try to do it. 
 
 We do not yet know how, it is true,* but it is one of 
 the things we have got to learn. 
 
 Anyone looking at a common glow-worm must be 
 struck with the fact that not by ordinary combustion, 
 nor yet on the steam-engine and dynamo principle, 
 is that easy light produced. Very little waste radia- 
 tion is there from phosphorescent things in general. 
 Light of the kind able to affect the retina is directly 
 emitted, and for this, for even a large supply of 
 this, a modicum of energy suffices. 
 
 Solar radiation consists of waves of all sizes, it 
 is true ; but then solar radiation has innumerable 
 things to do besides making things visible. The 
 whole of its energy is useful. In artificial lighting 
 nothing but light is desired ; when heat is wanted 
 it is best obtained separately, by combustion. And 
 so soon as we clearly recognize that light is an 
 electrical vibration, so soon shall we begin to beat 
 about for some mode of exciting and maintaining 
 an electrical vibration of any required degree of 
 rapidity. When this has been accomplished, the 
 problem of artificial lighting will have been solved. 
 
 1 &. A. Report, 1881, p. 526. 
 
 S 2 
 
260 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 Mechanism of Electrical Radiation. 
 
 152. In forming a mental image of an electrical 
 wave, we have to note that three distinct direc- 
 tions are involved. There is (i) the direction of 
 propagation the line of advance of the waves ; 
 (2) the direction of the electric displacements, at right 
 angles to this ; and (3) the direction of the magnetic 
 axis, at right angles to each of the other two. 
 
 One may get a rough mechanical idea of the 
 process of electrical radiation (at any rate in a plane) 
 by means of the cog-wheel system already used in 
 Part III. Imagine a series of elastic wheels, in one 
 plane, all geared together, and let one of them be 
 made to twist to and fro on its axis ; from it, as 
 centre, the disturbance will spread out in all direc- 
 tions, each wheel being made to oscillate similarly 
 and to transmit its oscillation to the next. Looking 
 at what is happening at a distance from the source, 
 we shall see the pulses travelling from left to right ; 
 the electrical displacement, such as it is, being up 
 and down ; and the oscillating axes of the wheels 
 being to and fro, or at right angles to the plane 
 containing the wheels. The electric displacement 
 is small, because the positive and negative wheels 
 gearing into one another move almost equally, and 
 accordingly there is the merest temporary balance 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 261 
 
 of one above the other, due to the elastic " give " 
 of the wheels. The magnetic oscillations, on the 
 other hand, are all in one sense, the positive wheels 
 rotating one way and the negative the other : all 
 act together, and so the magnetic oscillation is a 
 more conspicuous fact than the electric oscillation. 
 Hence it is often spoken of as electro-magnetic radia- 
 tion rather than as electric radiation. But the energy 
 of the electrostatic strain is just as great as that 
 of the electro-magnetic motion ; in fact the energy 
 alternates from the potential to the kinetic form, 
 or vice versa, at every quarter swing, just like every 
 other case of vibration. 
 
 153. As a matter of fact the magnetic oscillations 
 are very small too. For just consider that the wheel-work 
 extends right away to infinity in all directions : how 
 is any moderate force going to make one of these wheels 
 oscillate ? If they were rigid it would be impossible, 
 but as they are elastic it is possible, though only with 
 a very small amplitude of vibration ; and it sets up a 
 strain all round which rapidly spreads out as we have 
 said in all directions from the source. If the source 
 were inclosed in a perfect conductor of moderate 
 dimensions if, for instance, one tried to oscillate one 
 of the wheels inside the empty contour of Fig. 38 it 
 would be easy enough : the wheels are limited in 
 number, and can be easily got to oscillate considerably 
 by a feeble source of disturbance, 
 
262 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 This is commonly spoken of as the concentration of 
 light by reflection ; the conductor is said to act as a 
 perfect mirror ; and, since none of the light escapes, 
 any amount of illumination can be produced inside a 
 closed spherical mirror of perfect conductivity. Such 
 illumination would not be much use, however ; for, 
 directly a bit of matter is introduced to receive the 
 benefit of it, dissipation goes on at its surface, and 
 the violence of the ethereal disturbance is brought 
 down to something more moderate. Nevertheless, 
 even when dissipation is allowed, and when the 
 reflecting surface is by no means perfectly conduct- 
 ing, but is bright silver, which is the best conductor 
 we know, a considerable increase in illumination is 
 caused by reflection, if we choose to say so by 
 limitation, in at least some directions, of the extent of 
 ethereal medium to be affected by a given source, as 
 we might now prefer to express it ( 164). 
 
 154. Prof. Fitzgerald, of Dublin, has devised a 
 model of the ether, which by help of a little arti- 
 ficiality represents the two kinds of displacement 
 the electric and magnetic very simply and clearly. 
 
 His wheels are separated from one another by a 
 certain space, and are geared together by elastic 
 bands. They thus turn all in one direction, and 
 no mention need be made of positive and negative 
 electricity as separate entities. 
 
 But, the wheels being massivej a rotatory disturb- 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 263 
 
 ance given to one takes time to spread through 
 the series, at a pace depending on the elasticity of 
 the bands and the inertia of the wheels ; and during 
 the period of acceleration one side of every elastic 
 is stretched, while the other side is relaxed and there- 
 fore thickened. This thickening of the elastics goes 
 on in one direction, and corresponds to an electric 
 displacement in that direction ; the direction being 
 
 FIG. 48. Fitzgerald's Ether Model. A set of massive-rimmed brass wheels on 
 fixed pivots with their axles connected by common elastic bands. If the bands 
 are taken off any region, it becomes a perfect conductor, into which disturbances 
 cannot penetrate. (Cf. Fig. 38.) 
 
 perpendicular both to the direction of advance of 
 the disturbance and to the axes of the wheels. A 
 row of wheels corresponds to a section of a wave- 
 front ; the displacements of india-rubber and the 
 rotating axes, i.e. the electric and the magnetic 
 disturbances, both lie in the wave-front. 
 
 155. Clerk Maxwell's originally suggested repre- 
 sentation was not unlike this. 1 It consisted of a 
 series of massive wheels, connected together not by a 
 series of elastic bands but by a row of elastic particles 
 or " idle wheels." These particles represented " elec- 
 tricity" ; their displacement during the period of ac- 
 
 1 Phil Mag., April 1861, 
 
264 ' MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 celeration corresponding to the one-sided thickening 
 of the elastic bands in Fitzgerald's model. 
 
 I have proposed to contemplate a double series of 
 wheels geared directly into one another, and repre- 
 senting positive and negative electricity respectively, 
 because it seems to me that so many facts point to 
 the existence of these two entities, and because then no 
 distinction has to be drawn between one part of the 
 medium which is ether, and another part which is 
 electricity, but the whole is ether and the whole is also 
 electricity, while, nevertheless, a much-needed distinc- 
 tion can be drawn between a motion of the ether as a 
 whole, and a relative motion of its component parts 
 a distinction between forces able to move ether, i.e. to 
 displace the centre of gravity of some finite portion of 
 it, and forces which shear it and make its components 
 slide past each other in opposite senses : these latter 
 forces being truly electromotive ( 1 20). 
 
 156. If it be asked how the elasticity of the ether is 
 to be explained, we must turn to the vortex sponge 
 theory, suggested by Mr. Hicks l (Principal of Firth 
 College, Sheffield), and recently elaborated by Sir 
 William Thomson. 2 But this is too complicated a 
 matter to be suited for popular exposition just at 
 present. It must suffice to indicate that the points 
 here left unexplained are not necessarily at the present 
 
 1 Brit. Assoc. Report, 1885, Aberdeen, p. 930. 
 
 8 Ibid. 1887, Manchester, p. 486. Also Phil. Mag., October 1887. 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 265 
 
 time unexplainable, but that the explanations have not 
 yet been so completely worked out that an easy grasp 
 can be obtained of them by simple mechanical illus- 
 trations and conceptions. At the same time, the 
 general way in which motion is able to simulate the 
 effects of elasticity will be found popularly illustrated 
 by help of gyrostats in Sir William Thomson's article 
 "Elasticity" in the Encyclopedia Britannica ; l and 
 the fact that elastic rigidity of a solid can be produced 
 by impressing motion on a homogeneous and other- 
 wise structureless fluid must be regarded as one of the 
 most striking among his many vital discoveries. 
 
 We have found it necessary all through Part III. 
 to imagine the ether as composed of cells containing 
 electricity in rotation, and that the act of magnetiza- 
 tion consisted in facing these whirls round. Sir 
 William Thomson has taught us that a medium 
 containing whirls like this will simulate the behaviour 
 of an elastic solid, and in fact that whirling motion 
 is all that is required to explain elasticity (Fig. 46). 
 With this hint, which might be developed at greater 
 length, I must leave this part of the subject. 
 
 157. We have seen that to generate radiation an 
 electrical oscillation is necessary and sufficient, and we 
 have attended mainly to one kind of electric oscilla- 
 tion, viz. that which occurs in a condenser circuit when 
 
 1 Also in a recent volume of the Nature Series Popular Lectures and 
 Addresses. 
 
266 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 the distribution of its electricity is suddenly altered 
 as, for instance, by a discharge ( 124, 141). But the 
 condenser circuit need not be thrown into an obviously 
 Leyden jar form ; one may have a charged cylinder 
 with a static charge accumulated mainly at one end, 
 and then suddenly released, The recoil of the charge 
 is a true current, though a weak one ; a certain amount 
 of inertia is associated with it, and accordingly oscilla- 
 tions will go on, the charge surging from end to end of 
 the cylinder like the water in a tilted bath suddenly 
 levelled. 
 
 In a spherical or any other conductor, the like 
 electric oscillations may go on : and the theory of 
 these oscillations has been treated with great mathe- 
 matical power both by Mr. Niven and by Prof. Lamb. 1 
 
 Essentially, however, the phenomenon is not distinct 
 from a Leyden jar or condenser circuit, for the ends of 
 the cylinder have a certain capacity, and the cylinder 
 has a certain self-induction ; the difficulty of the 
 problem may be said to consist in finding the values 
 of these things for the given case. The period of 
 an oscillation may still be written 27r v /(LS) ; only, 
 since L and S are both very small, the "frequency" 
 of vibration is likely to be excessive. And when we 
 come to the oscillation of an atomic charge the 
 frequency may easily surpass the rate of vibration 
 
 1 Phil. Trans., 1881 and 1883. Also by Prof. J. J. Thomson, 
 Math. Soc. Proc., April 1884, 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 267 
 
 which can affect the eye. The damping out of such 
 vibrations, if left to themselves, will be also a very 
 rapid process, because the initial energy is but small. 
 It can be calculated that the oscillation of an atomic 
 charge would give rise to only ultra-violet rays. It is 
 probably because these ultra-violet rays synchronize 
 with the period of vibration of atomic charges that 
 they have such extraordinarily powerful chemical 
 effects ( 187). 
 
 158. But whether the charge oscillates in a station- 
 ary conductor, or whether a charged body vibrates as 
 a whole, it equally constitutes an alternating current, 
 and can equally well be treated as a source of radiation. 
 Now, when we were considering the subject of elec- 
 trolysis, we were led to think of molecules as com- 
 posed of two atoms or groups of atoms, each charged 
 with equal quantities of opposite kinds of electricity. 
 Under the influence of heat the components of the 
 molecules arc set in vibration like the prongs of a 
 tuning-fork, the rate of vibration depending on and 
 being characteristic of the constants of the particular 
 molecule. The atoms being charged, however, their 
 mechanical oscillation is necessarily accompanied by 
 an electric oscillation, and so an electric radiation is 
 excited and propagated outwards. These vibrations 
 would appear to be often of the frequency suited to 
 our retina, hence these vibrating atoms indirectly 
 constitute our usual source of light. The " frequency " 
 
268 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 of the visible radiation can be examined and deter- 
 mined by optical means (some form of interference 
 experiment, usually a diffraction grating), and hence 
 many of the rates of vibration possible to the atoms 
 of a given molecule under given circumstances become 
 known, and this is the foundation of the science of 
 spectroscopy. 
 
 It is possible that the long duration of some kinds 
 of phosphorescence may be due to the atoms receiving 
 indirectly some of the ethereal disturbance, and so 
 prolonging it by their inertia, instead of leaving it to 
 the far less inertia of the ether alone. It is possible 
 also that the definite emissivity of some fluorescent 
 substances is due to periods of vibration proper to 
 their atoms, which, being disturbed in an indirect way 
 by receipt of radiation, re-emit the same radiation in 
 a modified, and, as it were, laden manner. 
 
 159. To get some further idea concerning the way in 
 which an oscillating charge or an oscillating charged 
 body can propagate radiation, refer back to Fig. 39, 
 Part III., and imagine the rack oscillating to and fro. 
 It will produce rotatory oscillation in the wheels 
 gearing into it, these again in the next, and soon. If 
 the wheel-work \vere rigid, the propagation would go 
 on at an infinite speed to the most distant wheels, but 
 if it be elastic then the pace of propagation depends 
 on the elasticity and the density in a way we have 
 already said enough about. The line of rack is the 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 269 
 
 direction of electric oscillation, the axes of the wheels 
 the direction of magnetic rotatory oscillation, and at 
 right angles to both these is the direction of advance 
 of the waves. True, the diagram is not a space 
 representation, it is a mere section, and a very crude 
 suggestion of a mechanical analogy to what may be 
 taking place. 
 
 The wheels being perfectly geared together and into 
 the rack represent an insulator or dielectric : there is 
 no slip or frictional dissipation of energy in other 
 words, there are no true electric currents. The electric 
 oscillation is a mere displacement oscillation due to 
 elasticity and temporary give of the elastic wheels, 
 whereby during each era of acceleration they are 
 thrown slightly into the state represented in Fig. 46 
 as contrasted with Fig. 37. 
 
 Effects of encountering a New Medium. 
 
 1 60. Now contemplate an advancing system of 
 waves, and picture their encounter with an obstacle ; 
 say, a medium of greater density, or less elasticity, or 
 both. If the new medium is a perfect insulator, it 
 must be considered as having its wheels thoroughly 
 geared up both with themselves and with those of 
 the initial medium, so that there is no slip or dissipa- 
 tion of energy at the surface. In this case none of 
 the radiation will be lost : some will be reflected 
 
2/o MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 and some transmitted according to ordinary and 
 well-known mechanical laws. The part transmitted 
 will suddenly begin to travel at a slower pace, and 
 hence if the incidence were oblique would pursue 
 a somewhat different path. Also, at the edges of 
 the obstacle, or at the boundary of any artificially 
 limited portion of the wave, there will be certain 
 effects due to spreading out and encroaching on 
 parts of the medium not lying in the direct path. 
 These refraction and diffraction effects are common to 
 all possible kiuds of wave propagation, and there is 
 nothing specially necessary to be said concerning 
 electrical radiation on these heads which is not to be 
 found in any work on the corresponding parts of 
 optics. 
 
 161. Concerning the amount and direction of the 
 reflected vibrations there is something to be said 
 however, and that something very important ; but it 
 is no easy subject to tackle, and I fear must be 
 left, so far as I am concerned, as a distinct, but 
 perhaps subsequently-to-be-filled-up, gap. 
 
 If the gearing between the new medium and the 
 old is imperfect, if, for instance, there were a layer of 
 slippery wheels between them, representing a more or 
 less conducting film, then some of the radiation would 
 be dissipated at the surface, not all would be reflected 
 and transmitted, and the film would get to a certain 
 extent heated. By such a film the precise laws of re- 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 271 
 
 flection might be profoundly modified, as they would 
 be also if the transition from one medium to another 
 were gradual instead of abrupt, But all these things 
 must remain for the present part of the unfilled gap. 
 
 Electric Radiation encountering a Conductor. 
 
 162. We will proceed now to the case of a conducting 
 obstacle that is, of waves encountering a medium 
 whose electrical parts are connected, not by elasticity, 
 but by friction. It is plain here that not only at the 
 outer layer of such a medium, but at every subse- 
 quent layer, a certain amount of slip will occur during 
 every era of acceleration, and hence that in penetrating 
 a sufficient thickness of a medium endowed with any 
 metallic conductivity the whole of the incident radia- 
 tion must be either reflected or destroyed : none can 
 be transmitted ( 104). 
 
 Refer back to Fig. 43, and think of the rack in that 
 figure as oscillating. Through the cog-wheels the dis- 
 turbance spreads without loss, but at the outer layer 
 of the conducting region A B C D a finite slip occurs, 
 and a less amount of radiation penetrates to the next 
 layer, E F G H, and so on. Some thickness or other, 
 therefore, of a conducting substance must necessarily 
 be impervious to electric radiation : that is, it must be 
 opaque. And since it dissipates very little energy, it 
 must act as a reflector. (See 153 and 164.) 
 
272 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 Conductivity is not the sole cause of opacity. It 
 would not do to say that all opaque bodies must be 
 conductors. But conductivity is a very efficient cause 
 of opacity, and it is true to say that all conductors of 
 electricity are necessarily opaque to light ; under- 
 standing, of course, that the particular thickness of 
 any homogeneous substance which can be considered 
 as perfectly opaque must depend on its conductivity. 
 It is a question of dissipation, and a minute but 
 specifiable fraction of an original disturbance may be 
 said to get through any obstacle. Practically, however, 
 it is well known that a thin, though not the thinnest, 
 film of metal is quite impervious to light. 
 
 163. When one says that conductivity is not the sole 
 cause of opacity, one is thinking of opacity caused by 
 heterogeneity. A confused mass of perfectly trans- 
 parent substance may be quite opaque ; witness foam, 
 powdered glass, chalk, &c. 
 
 Hence, though a transparent body must indeed be 
 an insulator, the converse is not necessarily true. An 
 insulator need not necessarily be transparent. A 
 homogeneous flawless insulator must, however, be 
 transparent to some, though not necessarily to all 
 wave-lengths. A homogeneous and flawless opaque 
 body, if really opaque to all wave-lengths, must be a 
 conductor. 
 
 These, then, are the simple connections between 
 two such apparently distinct things as conducting 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 273 
 
 power for electricity and opacity to light which 
 Maxwell's theory points out ; and it is possible 
 to calculate the theoretical opacity of any given 
 simply-constructed substance by knowing its specific 
 electric conductivity. 
 
 Fate of the Radiation 
 
 164. To understand what happens to radiation im- 
 pinging on a conducting body it is most simple to 
 proceed to the limiting case at once and consider a 
 perfect conductor. In the case of a perfect conductor 
 the wheels are connected not even by friction ; they 
 are not connected at all. Consequently the slip at 
 the boundary of such a conductor is perfect, and there 
 is no dissipation of energy accompanying it. The 
 blank space in Fig. 38 represented a perfectly con- 
 ducting layer. Ethereal vibrations impinging on a 
 perfect conductor practically arrive at an outer con- 
 fine of their medium : beyond, there is nothing capable 
 of transmitting them ; the outer wheels receive an im- 
 petus which they cannot get rid of in front, and which 
 they therefore return back the way it came to those 
 behind them with a reversal of phase : the radiation is 
 totally reflected. It is like what happens when a 
 sound-pulse reaches the open end of an organ-pipe ; 
 like what happens when sound tries to go from water 
 to air ; like the last of a row of connected balls along 
 
 T 
 
274 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 which a knock has been transmitted ; and our massive 
 elastic wheels, especially the wheels of Fig. 48, are able 
 to represent the reversal of phase and reflection quite 
 properly. 
 
 165. The reflected pulses will be superposed upon 
 and interfere with the direct pulses, and accordingly 
 if the distances are properly adjusted we can have 
 the familiar formation of fixed nodes and stationary 
 waves ( 130). 
 
 1 66. The point of main interest, however, is to 
 notice that a perfect conductor of electricity, if there 
 were such a thing, would be utterly impervious to 
 light : no light could penetrate its outer skin, it 
 would all be reflected back : the substance would be 
 a perfect reflector for ethereal waves of every size. 
 
 Thus with a perfect conductor, as with a perfect 
 non-conductor, there is no dissipation. Radiation 
 impinging on them is either all reflected or some 
 reflected and some transmitted. It is the cases of 
 intermediate conductivity which destroy some of the 
 radiation and convert its ethereal vibrations into 
 atomic vibrations, i.e. which convert it into heat. 
 
 167. The mode in which radiation or any other 
 electrical disturbance diffuses with continual loss 
 through an imperfect conductor can easily be appre- 
 ciated by referring to 103 again. The successive 
 lines of slip, A B c D, E F G H, &c., are successive layers 
 of induced currents. An electromotive impulse loses 
 
CHAP, xiv.] ELECTRICAL RADIATION, OR LIGHT. 275 
 
 itself in the production of these currents, which are 
 successively formed deeper and deeper in the material 
 according to laws of diffusion. 
 
 If the waves had impinged on one face of a slab, a 
 certain fraction of them would emerge from the other 
 face a fraction depending on the thickness of the slab 
 according to a logarithmic or geometrical-progression 
 law of decrease. 
 
CHAPTER XV, 
 
 ELECTRO-MAGNETIC AND ELECTROSTATIC EFFECTS 
 ON LIGHT. 
 
 1 68. WE must now mention one or two phenomena 
 which depend entirely upon a modification of ether 
 by the neighbourhood of matter, and which we have 
 reason to believe would not occur in free ether at all. 
 These are the optical phenomena of Faraday and Kerr, 
 and the electric phenomenon of Hall. 
 
 Faraday discovered, long before there was any other 
 connection known between electricity and light, that 
 the plane in which light-vibrations occur could be 
 rotated by transmitting light through certain magnet- 
 ized substances along the lines of magnetic force. To 
 make this effect easily manifest, one uses plane-polar- 
 ized light, and transmits it through a fair length of 
 magnetized substance, analyzing it after emergence 
 and showing that, though it remains plane-polarized, 
 the plane has been rotated, possibly through a right 
 angle or more. 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 277 
 
 Now, in a general way it is easy to imagine that, 
 inasmuch as something of the nature of a rotation is 
 going on in a magnetic field round the lines of force, 
 vibrations travelling into such a field along these lines 
 should be twisted round, corkscrew fashion, and emerge 
 vibrating in a different plane. But when one tries to 
 follow out this process into detail, one finds it not 
 quite so simple a matter. It has, however, no business 
 to be a very simple and obvious consequence of the 
 existence of a magnetic rotation round the rays of 
 light, else would it occur in free space, and in the 
 same direction in all media. But the facts are that 
 in free space that is, in free ether it does not occur 
 at all, and the direction of rotation is not the same 
 for all media : substances can, in fact, be divided into 
 two groups, according to the way in which given 
 magnetization shall rotate the plane of polarized 
 light passing through them. 
 
 169. Similar statements can be made concerning the 
 electrostatic optical effect discovered by Dr. Kerr, who 
 showed that plane-polarized light transmitted across 
 the lines of force in an electrostatic field could, in cer- 
 tain media, come out elliptically polarized. Now, in- 
 asmuch as an electric field is a region of strain, and 
 strain in transparent bodies is well known to make 
 them slightly doubly refracting and able to turn plane- 
 polarized into elliptically-polarized light, it is very 
 easy to imagine such a result in an electric field to be 
 
2;8 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 natural and probable. But the explanation is not so 
 simple as that, else it ought to be a large effect, occur- 
 ring in all sorts of media in the same direction, and 
 likewise in free space. But the facts are that it does 
 not occur at all in free space, and it occurs in different 
 senses in different substances ; so that again they can 
 be grouped into two classes according to the sign of 
 the Kerr effect. 
 
 Thus, then, the rotatory effect of a magnetic field 
 upon light, discovered by Faraday, and the doubly 
 refracting effect of an electrostatic field upon light, 
 discovered by Kerr, agree in this : that they are both 
 small or residual effects, depending on the existence 01 
 a dense medium, and both varying in sign according 
 to the nature of the medium. 
 
 170. The only substance in which the Faraday effect 
 is large, is iron, including with iron the other highly 
 magnetic substances. The discovery of the effect in 
 these bodies was likewise made by Kerr. The diffi- 
 culty of dealing with them is that they are very 
 opaque, and hence that the merest film of them can 
 be used. The film can be used either by way of 
 transmission or by way of reflection, it matters 
 not which, but reflection is the way in which it was 
 first done. Light reflected from the polished face 
 of a magnet has indeed barely penetrated at all 
 into the substance of the iron before being sent 
 back ; still, it has penetrated deep enough to be 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 279 
 
 distinctly rotated by the tremendous magnetic whirl 
 which it finds there. 
 
 171. All these highly magnetic substances are 
 metallic conductors, and are therefore very opaque. 
 Whether there is any real connection between high 
 magnetic susceptibility and conductivity is more than 
 I can say. But it is quite natural, and indeed 
 necessary, that the greatest portion of light should 
 be reflected on entering a highly magnetic medium, 
 because in such a medium the ethereal density, /u,, is 
 so great, and hence the velocity of wave transmission 
 must undergo a sudden and immense decrease a 
 circumstance always causing a great amount of 
 reflection, just as when sound tries to pass from any 
 one medium to a much denser one. 
 
 But the opacity of iron and other magnetic sub- 
 stances may be explained by the mere fact of their 
 conducting power, just like other metals, and no 
 noteworthy effect of their large value of /JL need be 
 detectable optically. 
 
 If a non-conducting highly magnetic substance 
 could be found, it would probably reflect a great deal 
 of light at its surface, though it would not dissipate 
 that which entered it. Such a substance would be 
 most interesting to submit to experiment, but perhaps 
 its existence presupposes a combination of impossible 
 properties. Certainly it has not yet been discovered. 
 
 As to the phenomenon detected by Hall, it appears 
 
280 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 intimately associated with that of Faraday, and it will 
 be most simple to omit all reference to it for the 
 present. 
 
 172. A general idea of what is happening in the 
 Faraday and Kerr phenomena can be given thus. 
 
 A simple vibration, like a pendulum-swing, or any 
 other oscillation in one plane, can be resolved into 
 two others in an infinite variety of ways ; just as one 
 force can be resolved into any number of pairs of 
 equivalent forces. The two most useful modes of 
 
 D 
 
 P C 
 
 FIG. 49. 
 
 analyzing a simple vibration into a pair of constituents 
 are these : (l) two equal components, likewise plane 
 vibrations, each inclined at 45 to the original one, as 
 when P Q is resolved into A B and C D (Fig. 49) ; and 
 (2) two equal circular or rotatory oscillations in oppo- 
 site directions, as when P Q is resolved into P M Q and 
 P N Q (Fig. 50). The first method of resolution is 
 useful in explaining Kerr's effect, the second 
 explaining Faraday's. 
 
 in 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 281 
 
 Of the two component vibrations, A B and C D, into 
 which P Q can be supposed analyzed, let some cause, 
 no matter what, make one gain upon the other, so 
 that in travelling along a line perpendicular to the 
 paper one goes a little the quicker : the effect at once 
 is to change the character of the vibration into which 
 they will recompound. After the gain, they no longer 
 reproduce the original simple vibration P Q : they give 
 rise to elliptic, or it may be to circular, vibrations ; this 
 last, if the retardation is equal to a quarter period. 
 
 These are matters fully treated in any elementary 
 treatise on polarized light, and they are quite easily 
 illustrated by means of a simple pendulum. One 
 may assume them known. 
 
 Similarly with the second system of analyzing the 
 vibration into two opposing circular ones. If the 
 components travel through any interposed medium 
 at the same rate, they will, on emergence, reproduce 
 the original vibration in its original position ; but if 
 one travels quicker than the other they recombine 
 into a vibration of the same character as at first, but 
 turned through a certain angle. Thus anything which 
 retards one of the rectangular components behind the 
 other, changes the character of the vibration from 
 plane into elliptical ; while anything which retards one 
 of the circular components behind the other, leaves 
 the character of the vibration unaltered, but rotates it 
 through a certain angle. 
 
282 MODERN VIEWS OF ELECTRICITY. [PART Iv. 
 
 173. So far one has said nothing but the simplest 
 mechanics. ^The next point to consider is what deter- 
 mines the rate at which light travels through any sub- 
 stance ? This we have discussed at length ( 1 28), and 
 
 shown to be Anything which increases either 
 
 the electric or the magnetic permeability of the 
 medium decreases the velocity of light. Now, when 
 a medium is already subject to a violent strain in any 
 one direction it is possibly less susceptible to further 
 strain in that direction and responds less readily. Not 
 necessarily so at all : such an effect would only be 
 produced when the strain was excessive, when the 
 medium was beginning to be overdone, and when its 
 properties began thereby to be slightly modified. 
 There are reasons for believing the specific inductive 
 capacity of most media to be very constant ; of some 
 media, perhaps, precisely constant ; but if there were 
 any limit beyond which the strain could not pass it is 
 probable that on nearing the limit the specific induc- 
 tive capacity would be altered possibly increased, 
 possibly diminished one could hardly say which. 
 Quincke has investigated this matter, and has shown 
 that the value of K is affected by great electric strain. 
 Suppose now that a dielectric is subject to a violent 
 electrical stress, so that its properties along the lines 
 of force become slightly different from its properties 
 at right angles to those lines. The value of K will 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 283 
 
 not be quite the same along the lines of strain as across 
 them, and accordingly the rectangular component of 
 a vibration resolved along the lines of force will travel 
 rather quicker or rather slower than the component at 
 right angles, because the velocity of transmission 
 depends upon K, as already explained : such a me- 
 dium at once acquires the necessary doubly-refractive 
 character, and will show Kerr's effect. 
 
 174. Similarly with magnetization. It is well 
 known that for many media //, is not constant. Take 
 iron, for instance. For very small magnetizing forces 
 the susceptibility is moderate, and increases as they 
 increase ; at a certain magnetization it reaches a maxi- 
 mum, and then steadily decreases. But not only is 
 it thus very inconstant, its ascending and descending 
 values are not the same. To forces tending to mag- 
 netize it more, the susceptibility has one value ; to 
 forces tending to demagnetize it, it has another and 
 in general smaller value. This property has been 
 specially studied by Ewing, and has been called by 
 him "hysteresis." Slightly susceptible substances 
 cannot be magnetized to anything like the same 
 extent, and hence the property in them has been less 
 noticed, perhaps not noticed at all. Nevertheless it 
 must exist in every substance which exhibits a trace 
 of permanent magnetism, and every substance I have 
 tried appears to show some such trace. 1 
 
 1 See Nature, vol. xxxiii. p. 484. 
 
284 MODERN VIEWS OF ELECTRICITY, [PART iv. 
 
 An already strongly magnetized medium will be 
 rather differently susceptible to additional magnetizing 
 forces in the same direction from what it is to those 
 in a contrary direction. Nothing more is wanted to 
 explain Faraday's effect. The vibration being resolved 
 into two opposite circular components, one of them 
 must agree in direction with the magnetism already in 
 the medium and try to magnetize it for the instant 
 infinitesimally more ; the other component will for 
 the instant infinitesimally tend to demagnetize it. 
 The value of fj, offering itself to the two components 
 will be different, hence they will go at different rates, 
 and the plane of vibration will be rotated. 1 
 
 175. The direction of rotation will depend on 
 whether the value of yu, is greater for small relaxa- 
 tions, or for small intensifications, of magnetizing 
 force ; and diamagnetic substances may be expected 
 
 1 The connection which I here trace between hysteresis and the 
 magnetic rotation of plane of polarization, is not one which I at all 
 press. Prof. Fitzgerald has intimated to me that if I take a whole 
 wave-front into consideration, the theory will hardly work, and that it 
 would have been better if the real electro-magnetic disturbance were 
 the thing acted on instead of having to fall back upon a secondary 
 magnetic effect of the electrostatic displacement. And Prof. Ewing, 
 though he adduced at first some facts which appeared to strengthen my 
 view, now doubts whether the kinematic resolution of a displacement 
 into two circular components is under the circumstances legitimate. I 
 have doubts too. If I were quite sure that there were no vestige of 
 truth in the suggestion I have made in the text, I should of course 
 suppress it ; but as I am not quite sure, I let it stand for the present, 
 taking any possible harm out of it by this note. See also 180 et seq. 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 285 
 
 to be opposite in this respect to paramagnetic ones. 
 Any substance for which //, is absolutely constant, 
 whatever the strength of magnetic polarization to 
 which it is submitted, can hardly be expected to ex- 
 hibit any hysteresis ; the ascending and descending 
 curves of magnetization will coincide, being both 
 straight lines, and such a substance will show no 
 Faraday effect. Similarly, any substance for which K 
 is absolutely constant, whatever the electric polariza- 
 tion to which it is submitted, can show no Kerr's effect. 
 Free space appears to be of this nature ; and gases 
 approach it very nearly, but not quite. 
 
 In iron, //, is greater for an increasing than for a 
 decreasing force, as is shown by the loops in Ewing's 
 curves ; hence the circular component agreeing in 
 direction with the magnetizing current will travel 
 slower than the other component, and hence the 
 rotation in iron will be against the direction of the 
 magnetizing current. The same appears to hold in 
 most paramagnetic substances, and the opposite in 
 most diamagnetic ; but the mere fact of paramagnetism 
 or diamagnetism is not sufficient to tell us the sign of 
 the effect in any given substance. We must know 
 the mode in which its magnetic permeability is affected 
 by waxing and by waning magnetization respectively. 
 
286 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 Possible Electrical Method of detecting the Faraday 
 Effect. 
 
 176. Thus far we have considered the rotation o< 
 electric displacement by a magnetic field as being 
 examined optically, the displacements being those 
 concerned in light, and the rotation being detected 
 by a polarizing analyzer suitable for determining the 
 direction in which the vibrations occur before and 
 after the passage of light through a magnetized sub- 
 stance. This is the only way in which the effect has 
 at present been observed in transparent bodies. But 
 one ought not to be limited to an optical method of 
 detection. 
 
 Electrical displacements are easily produced in any 
 insulator, and if it be immersed in a strong magnetic 
 field, so that the electric and magnetic lines of force 
 are at right angles to each other, every electric dis- 
 turbance ought to experience a small rotation. A 
 steady strain will not be affected ; it is the variable 
 state only which will experience an effect, but every 
 fresh electric displacement should experience a slight 
 rotatory tendency just like the displacements which 
 occur in light. 
 
 Now to rotate a displacement A B into the position 
 A C requires the combination with it of a perpen- 
 dicular displacement B C (Fig. 51). Hence the effect 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 
 
 287 
 
 of the magnetic field upon an electric displacement, 
 A B, may be said to be the generation of a small 
 perpendicular E.M.F., B c, which, compounded with the 
 original one, has the resultant effect A C. It will be 
 
 FIG. 51. 
 
 only a temporary effect, lasting while the displace- 
 ment is being produced, and ceasing directly a steady 
 state of strain is set up. 
 
 An inverse E.M.F., A D, will be excited by the same 
 magnetic field directly the displacement is reversed. 
 
 JL 
 
 FIG. 52. 
 
 And so, if a continual electric oscillation is kept up 
 between A and B in a magnetic field,' an accompanying 
 very minute transverse oscillation may be expected, 
 and may be looked for electrically. 
 
288 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 Some such arrangement as that here shown (Fig. 52) 
 may be employed : a square of heavy glass, per- 
 forated with four holes towards the centre, supplied 
 with electrodes : one pair of electrodes, A, B, to be 
 connected with the poles of some alternating machine, 
 and the other pair, C, D, connected to a telephone or 
 other detector of minute oscillatory disturbance. So 
 soon as a strong steady magnetic field is applied, by 
 placing the glass slab between the poles of a strong 
 magnet, the telephone ought to be slightly affected 
 by the transverse oscillations. This effect has not yet 
 been experimentally observed, but it seems to me a 
 certain consequence of the Faraday rotation of the 
 plane of polarization of light. 
 
 Hall Effect. 
 
 177. Although the existence of this transverse 
 E.M.F., excited by a magnetic field in substances 
 undergoing varying electric displacement, has a t 
 present only been detected optically in transparent 
 bodies, i.e. in insulators, yet in conductors the corre- 
 sponding effect with a steady current has been dis- 
 tinctly observed electrically. By many persons it had 
 been looked for (by Prof. Carey Foster and the writer, 
 among others, though unfortunately they were not suffi- 
 ciently prepared for its extreme smallness) ; by Mr. 
 Hall, at Baltimore, was it first successfully observed. 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 289 
 
 In conductors it is natural to use a conduction- 
 current instead of a displacement-current. A steady 
 current can be maintained in a square or cross of 
 gold-leaf or other thin sheet of metal between the 
 electrodes A, B ; and a minute transverse E.M.F. can be 
 detected, causing a very weak steady current through 
 a galvanometer connected to the terminals c, D, so soon 
 as a strong magnetic field is applied perpendicularly 
 
 FIG. 53. The direction of the transverse E.M.F. excited by the earth's vertical 
 magnetic field in this conductor, conveying a current 4 as shown, is c D if it 
 represents gold, D c if it represents iron. 
 
 to the plate. Fig. 53 will sufficiently indicate the 
 arrangement. The poles of the magnet are one above 
 and one below the paper. 
 
 In iron it is easy to see which way the transverse 
 E.M.F. ought to be found. It has been shown ( 175) 
 that a displacement will be rotated in iron against the 
 magnetizing current ; hence to rotate the displace- 
 ment A l* to AC (Fig. 51), requires in iron a clockwise 
 
 U 
 
290 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 magnetizing current. Such a current, or, what is the 
 same thing, a south pole below the paper, a north pole 
 above, excites, in the cross of Fig. 53, an E.M.F. in 
 the direction n C ; and this by Ampere's rule is just the 
 direction in which the conductor itself is urged by the 
 magnetic forces acting on the current-conveying sub- 
 stance. Most diamagnetic substances should exhibit a 
 transverse E.M.F. in the opposite sense. This trans- 
 verse E. M. F. excited in conductors conveying a 
 current in a magnetic field is the effect known by 
 the name of Hall. It is, as Prof. Rowland and 
 others have pointed out, intimately connected with 
 the Faraday rotation of light. 
 
 178. Unfortunately a pure and simple Hall effect is a 
 difficult thing to observe. Magnetism affects the con- 
 ductivity of metals in a rather complicated manner ; 
 and strain affects their thermo-electric properties. Now 
 a metal conveying a current in a magnetic field is 
 certainly more or less strained by mechanical forces ; 
 and hence heat will be developed unequally in different 
 parts, by a sort of Peltier effect ; and the result of this 
 will be to modify the resistance in patches, and so to 
 produce a disturbance of the flow which may easily 
 result partly in a transverse E.M.F. This has been 
 pointed out by Mr. Shelford Bidwell. 
 
 The more direct effect of magnetism on conductivity 
 may be negligibly small in many metals, but in bis- 
 muth it is certainly large. Both of these spurious 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 291 
 
 effects seem to be large in bismuth, and probably 
 quite mask any true Hall effect there may be in that 
 metal. In all cases the existence of these spurious 
 effects makes it difficult to be sure of the magnitude 
 and sign oT the real rotational effect. 
 
 179. But, it may be asked, what right have we to 
 distinguish between a real and a spurious Hall effect ? 
 If a transverse E.M.F. can be predicted by reason of 
 known strains and thermo-electric properties, as well 
 as by known rotation of light effects, why should the 
 two things be considered different ? Why should they 
 not be different modes of regarding one and the same 
 phenomenon ? 
 
 In other words, may not the Faraday rotation of 
 light vibration be due to infinitesimal temporary 
 strains and heatings in the medium, caused by the 
 fact that minute electric displacements are occurring in 
 a violent magnetic field ? This is a question capable 
 of being answered by a quantitative determination of 
 the amounts and direction of the effects to be expected, 
 and a comparison with those actually observed. I 
 do not know of data, at present obtained, sufficient 
 to enable us to answer it. If its answer should turn 
 out in the affirmative, several apparently distinct 
 phenomena will be linked together. 
 
 U 2 
 
292 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 Possible Accounts of the Faraday and Hall Effects. 
 
 i So. The account I have given of the magnetic rota- 
 tion of the plane of polarization has made it depend 
 on the phenomenon of hysteresis, in a way which may 
 be thus summarized. The value of //, for increasing 
 magnetization is different from that for decreasing 
 magnetization ; an electric displacement such as 
 occurs in every half-swing of a light-vibration is 
 resolvable into two opposite circular components, one 
 of which increases, while the other decreases, any 
 magnetization already existing in the direction of the 
 ray : the value of //, affects the speed of transmission 
 of light ; hence the two circular components will not 
 proceed at the same pace, and the direction of vibra- 
 tion will infinitesimally rotate. The same thing is 
 repeated at every half-swing, the elemental rotations 
 being all in the same sense, and so the ultimate 
 rotation of the plane of polarization in transparent 
 bodies is accounted for. 
 
 The Hall effect observed in conductors follows at 
 once ; for the rotation of a displacement is equivalent 
 to combining it with a small perpendicular displace- 
 ment ; and it is this perpendicular or transverse E.M.F. 
 exerted by a magnetic field which Hall discovered. 
 At the same time, there are one or two facts which 
 militate against this view of the Hall effect, chief 
 among which is the singular behaviour of nickel, which 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 293 
 
 rotates light one way and electric displacement the 
 other way. For some time it was possible to hope 
 for a way out of this through the usual convenient 
 avenue of " impurity " ; but now that both experiments 
 have been performed on the same identical piece of 
 metal, still with opposite results, this exit is closed. In 
 this unsettled state, so far as I know, the connection 
 between the rotation of light and the Hall effect at 
 present stands. 
 
 1 8 1. It may be well here to repeat the caution 
 appended as a footnote to 174, not to assume that 
 this account of the magnetic rotation of light and the 
 Hall effect is true. If true, however, it is convenient as 
 linking the phenomena on to hysteresis, and the direc- 
 tion of the effect in iron is correctly given namely, 
 a rotation against the magnetizing current ( 177). 
 
 Prof. Ewing has since pointed out, in a letter to me, 
 that, attending more precisely to the instruction of 
 his curves, we find the difference in jjt> for positive 
 and negative magnetizing forces only lasts through 
 a number of cycles for the time during which the 
 final state has been approached, and does not persist 
 after a steady state has been reached. This would 
 make the magnetic rotation of light a function of 
 time ; and certain experiments by Villari on spinning 
 a glass disk between the poles of a magnet, so that 
 fresh and fresh portions of glass were continually 
 exposed to the magnetic field, showed a marked 
 
294 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 falling off in the amount of rotation as soon as 
 high speeds were obtained ; thus proving, apparently, 
 that a certain short time was necessary to set up 
 the effect. This experiment, and other modifications 
 of it, want repeating, however. 1 
 
 Prof. Ewing has subsequently expressed a doubt 
 as to whether the kinematic resolution of a displace- 
 ment into two equal opposite circular components 
 is, under the circumstances, legitimate. 
 
 Prof. Fitzgerald has further pointed out that, 
 although when attending to one element only the 
 theory might possibly work, yet, as soon as one 
 takes into account the whole wave-front, it breaks 
 down ; for all the main magnetic disturbance lies 
 in the wave-front, as is well known, and the extra 
 magnetic disturbance which I have postulated as a 
 consequence of electrostatic displacement is annulled 
 by interference of adjacent elements. 
 
 If I were quite sure that there were no vestige 
 of truth in the suggestion I have made, I should, 
 of course, withdraw it ; but, as I do not feel perfectly 
 sure either way, I leave it in a dilapidated condition 
 for the present. 
 
 182. Another and apparently distinct account of 
 the magnetic rotation can also be hinted at, which 
 links the phenomenon on to the facts of thermo- 
 
 1 See in this connexion a paper by the present writer in the Philo- 
 sophical Magazine for April, 1889. Also a paper by Mr. A. W. Ward, 
 to be read before the Royal Society shortly. 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 295 
 
 electricity. It labours under worse disadvantages 
 than the preceding, being more hazy. 
 
 Referring back to 63, we find that, to explain 
 what is called the " Thomson effect " in metals, we 
 were led to suppose a connection between one kind 
 of electricity and some kinds of matter more 
 intimate than between the other kind of elec- 
 tricity and the same matter. Thus, the atoms 
 of iron were said to have a better grip of positive 
 electricity than of negative ; while copper, on 
 the other hand, had a better grip of negative 
 than of positive. All metals could be arranged in 
 one or other of the two classes, with the exception 
 of lead, which appears to grip both equally. It is 
 the same phenomenon as was originally named by 
 Sir W. Thomson " the specific heat of electricity in a 
 substance." Certain it is that vibrating atoms of iron 
 push positive electricity from places of more rapid, to 
 places of less rapid, vibration that is, from hot to cold 
 and a whole class of the metals do the same ; while 
 another class, like copper, push it from cold to hot. 
 
 Permitting ourselves to picture this effect as a direct 
 consequence of the Ohm's law relation between elec- 
 tricity and matter ( 60), combined with a special 
 relationship between certain kinds of matter and one or 
 other kind of electricity, a relationship which can exhibit 
 itself in other ways also, we get a possible though rather 
 hazy notion of a Faraday rotation in a magnetic field 
 
296 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 by supposing that the Amperian molecular currents 
 in these substances consist not of precisely equal 
 positive and negative currents, but of opposite currents 
 slightly unequal ; say, for instance, that the density 
 of the positive constituent of the bound ether of a 
 substance is slightly different from that of the negative 
 constituent ; so that on the whole the bound ether in a 
 magnetized molecule is slowly rotating one way or 
 the other, at a pace equal to the resultant rotation 
 of its constituents. Suppose that in iron the positive 
 Amperian electric current is the weaker of the two ; 
 then the ether, as a whole, will be rotating with the 
 negative current, and accordingly an ethereal vibration 
 entering such a medium will begin to screw itself 
 round in a direction opposite to that of the mag- 
 netizing current ; whereas in copper or other such 
 substance it would be rotated the other way. 
 
 183. According to this (admittedly indistinct) view, 
 lead ought to show no rotatory effect at all ; and of 
 course, therefore, no Hall effect either. - And the 
 classes into which metals are divided by the sign of 
 their Hall effect should coincide with the classes into 
 which the sign of their Thomson effect throws them. 
 
 Hall finds that, of the metals he examined, iron, 
 cobalt, and zinc fall into one class, while gold, silver, 
 tin, copper, brass, platinum, nickel, aluminium, and 
 magnesium fall into the other. Now, referring to the 
 thermo-electric results of Prof. Tait, we find iron, 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 297 
 
 cobalt, platinum, and magnesium with a negative sign 
 to their Thomson-effect-coefficient, or with lines in the 
 thermo-electric diagram sloping downwards ; while 
 gold, silver, tin, copper, aluminium, and zinc slope 
 upwards, or have a positive sign to their " specific heat 
 of electricity." 
 
 According to this, therefore, the discordant metals 
 are zinc, platinum, and magnesium. The proper 
 thing to say under these circumstances is that the 
 metals used in the very different experiments were 
 not pure. They certainly were not ; but I do not feel 
 able to conscientiously bolster up so inadequate a 
 theory by help of this convenient fact. 
 
 In the Philosophical Magazine for May 1885, Mr. 
 Hall gives some more measurements, showing that in 
 bismuth the effect is enormous, and in the same direc- 
 tion as in copper, whereas in antimony it is also great, 
 and in the same direction as in iron. All these things 
 seem to point to some thermo-electric connection 
 whether it -be of the sort I have vaguely tried to 
 indicate, or some other. 
 
 Other Outstanding Problems. 
 
 1 84. Outstanding problems bristle all over the sub- 
 ject, and if I pick out any for special mention it will 
 only be because I happen to have made some experi- 
 ments in their direction myself, or otherwise have had 
 my thoughts directed to them, and because they have 
 
298 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 not been so directly called attention to in the body of 
 the book. 
 
 Referring back to 66 at the end of Part II., " a cur- 
 rent regarded as a moving charge," it is natural to ask, 
 Is this motion to be absolute, or relative to the ether 
 only, or must it be relative to the indicating magneto- 
 meter ? In other words, if a charged body and a mag- 
 netic needle are flying through space together, as, for^ 
 instance, by reason of the orbital motion of the earth, 
 will the needle experience any deflecting couple ? 
 
 It is one of many problems connected with the 
 ether and its motion near gross matter problems 
 which the experiments of Fizeau (showing that a 
 variable part of ether was bound with matter and 
 transmitted with it, while another constant portion 
 was free and blew through it) began to throw light 
 upon ; aberration problems such as have been partially 
 solved by the genius of Stokes ; problems connected 
 with the motion of ether near great masses of matter, 
 like those which Michelson is so skilfully attacking 
 experimentally : it is among these that we must 
 probably relegate the question whether absolute or 
 relative motion of electric charges is concerned in the 
 production of magnetic field, and what absolute 
 motion through the ether precisely means. It is 
 doubtless a question capable of being attacked ex- 
 perimentally, but the experiments will be very difficult. 
 I believe that Prof. Ayrton has attempted them. 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 299 
 
 185. Referring back to Parts I., II., and III., 
 ;, 39, 41, 48, 88, 89, 97, 98, 109, 122, 134, we 
 find a number of questions regarding momentum left 
 unsettled. Has an electric current any true momen- 
 tum mechanically discoverable ? Now, this question 
 before it can be answered in the negative, will have 
 to be attacked under a great number of subdivisions. 
 One may classify them thus. Two main heads : (i) 
 When steady, Does a magnet behave in the least like 
 a gyrostat ? (2) When variable, Is there a slight 
 mechanical kick on starting or stopping a current ? 
 With four or more subsidiary heads under each, viz. 
 (a) in metallic conductors ; (b) in electrolytes ; (c) in 
 gases ; (d) in dielectrics. 
 
 Suppose the answer turns out negative in metals, 
 it by no means follows that it should be negative in 
 electrolytes too. In fact, as matter travels with the 
 current in the case of electrolytic conduction ( 36), it is 
 hardly possible that there is not some momentum, 
 though it may be too small to observe either a kick 
 of the vessel as a whole at starting and stopping, or 
 a continuous impact on an electrode receiving a de- 
 posit. The present writer has looked for these things, 
 but after gradually eliminating a number of spurious 
 effects the result has been so far negative. In a light 
 quill vessel fixed to the end of a torsion arm, the 
 main disturbance was due to variations of temperature 
 which gradually introduced a minute air-bubble, and 
 
300 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 by kicking this backwards and forwards simulated the 
 effects sought. In the case of the suspended elec- 
 trode, convection currents in the electrolyte, caused by 
 extra concentration or the reverse, seem determined 
 to mask any possible effect. 
 
 One obvious though very troublesome source of 
 disturbance in all cases is the direct effect of terres- 
 trial magnetism on the circuit. To get over this, 
 the writer not only made his circuits as nearly as 
 possible of zero area, but also inclosed them in the 
 iron case of a Thomson marine galvanometer, lent 
 for the purpose by Dr. Muirhead. 
 
 In gases, the experiment of Mr. Crookes, where an 
 electrical stream inside a vacuum-tube propels a mill 
 along rails perhaps even the ancient experiment of 
 the blast from a point shows that momentum is by 
 no means absent from an electric current through 
 a gas ( 64). 
 
 To see if there are any momentum effects ac- 
 companying variation of electric displacement in 
 dielectrics, the writer has suspended a mica-disk 
 condenser at the end of a torsion arm, and arranged 
 it so that it could be charged and discharged in situ. 
 Many spurious effects, but no really trustworthy ones, 
 were observed. 
 
 In the writer's opinion the subject is by no means 
 thoroughly explored, and he only mentions his old 
 attempts as a possible guide to future experimenters. 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 301 
 
 1 86. Then, again, there is the influence of light 
 on conductivity. Annealed selenium, and perhaps a 
 few other things, improve in conductivity enormously 
 when illuminated. The cause of this is unknown at 
 present, and whether it is a general property of 
 matter, possessed by metals and other bodies to a 
 slight degree, is uncertain ; for the experiments of 
 Bornstein, with an affirmative result for the case 
 of metals, have been seriously criticized. 
 
 Even though metals show no effect, yet electro- 
 lytes might possibly do so, but the effect, if any, 
 is small ; and it is particularly difficult in their case 
 to distinguish any direct radiation effect from the 
 similar effect of mere absorbed radiation or heat. 
 
 The writer has found that a glass test-tube kept 
 immersed in boiling water conducts distinctly better 
 when the blinds of a room are raised than when 
 they are lowered, though nothing but diffuse daylight 
 falls upon it. But as the effect could have been 
 produced by a rise in temperature of about the 
 tenth of a degree, and as the absorption of diffuse 
 daylight is competent to produce a rise of tempera- 
 ture as great as this in the glass of a thermometer- 
 bulb even though immersed in boiling water, he feels 
 constrained to regard the result, though very clear 
 and distinct, as after all a negative one, and has 
 accordingly not published* it. 
 
 187. The fact that ultra-violet waves have a period 
 
302 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 of vibration synchronous with probable electric vibra- 
 tion in molecules ( 157) seems to cause a multitude of 
 consequences now being discovered. Hertz noticed 
 that the light of one spark influenced another at 
 a distance, so that a sparking interval was virtually 
 shortened when illuminated. Wiedemann and Ebert 
 have further investigated this, and obtained several 
 interesting results, distinctly proving that it is ultra-- 
 violet light which is effective. Hallwachs has dis- 
 covered that a clean metallic plate becomes electrified 
 when light falls upon it. And there are a number 
 of other similar facts, some long known, some recent, 
 which all illustrate the molecular effects of light. It 
 appears probable that they all depend on some 
 synchronized disturbance set up in the air or other 
 film in contact with the substance, a disturbance re- 
 sulting in some kind of chemical action, and hence 
 that these physical effects are of the same order 
 as those other familiar but vaguely grasped facts 
 summed up under the category of the chemical 
 or actinic power of light. For that light affects 
 silver salts, ebonite, hydrogen and chlorine, &c., is an 
 old story. Some progress is now likely to be made 
 in ascertaining the precise mode in which these 
 changes occur ( 33). 
 
 1 88. A few months ago I should have put in a 
 prominent position among outstanding problems the 
 production of electric radiation of moderate wave- 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 303 
 
 length, and the performance with this radiation of all 
 the ordinary optical experiments reflection, refrac- 
 tion, interference, diffraction, polarization, magnetic 
 rotation, and the like ( i). But a great part of this 
 has now been done, and so these things come to 
 be mentioned under a different heading : 
 
 Conclusion. 
 
 " Conclusion " is an absurd word to write at the 
 present time, when the whole subject is astir with 
 life, and when every month seems to bring out some 
 fresh aspect, to develop more clearly some already 
 glimpsed truth. The only proper conclusion to a 
 book dealing with electricity at the present time is 
 to herald the advent of the very latest discoveries, 
 and to prepare the minds of readers for more. 
 
 189. Referring back to Chap. XIV., to I and 8, 
 and all Part IV., we spoke confidently of a radiation 
 being excited by electric oscillations, a radiation which 
 travelled at the same rate as light, which is reflected 
 and refracted according to the same laws, and which, 
 in fact, is identical with the radiation able to affect 
 our retina, except in the one matter of wave-length. 
 Such a radiation has now been definitely obtained 
 and examined by Dr. Hertz, of Karlsruhe ; and in 
 the last, month of last year, Prof, von Helmholtz 
 communicated to the Physical Society of Berlin an 
 account of Dr. Hertz's latest researches. 
 
304 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 The step in advance which has enabled Dr. Hertz to 
 do easily that which others have long wished to do, has 
 been the invention of a suitable receiver. Light when 
 it falls on a conductor excites first electric currents 
 and then heat. The secondary minute effect was what 
 we had thought of looking for ; but Dr. Hertz has 
 boldly taken the bull by the horns, looked for the 
 direct electric effect, and found it manifesting itself 
 in the beautifully simple form of microscopic sparks 
 across a gap between two conductors, or between the 
 ends of a looped conductor. 
 
 He takes a brass cylinder, some inch or two in 
 diameter, and a foot or so long, divided into two 
 halves with a small sparking interval between, and 
 connects the halves to the terminals of a small induc- 
 tion-coil ; every spark of the coil causes the charge in 
 the cylinder to surge to and fro about five hundred 
 million times a second, and to disturb the ether in a 
 manner precisely equivalent to a diverging beam of 
 plane-polarized light, with waves about thrice the 
 length of the cylinder. 
 
 The radiation, so emitted, can be reflected by plane 
 conducting surfaces, and it can be concentrated by 
 metallic parabolic mirrors ; the mirror ordinarily used 
 being a large parabolic cylinder of sheet zinc, with the 
 electric oscillator situate along its focal line. By this 
 means the effect of the wave could be felt at a fail- 
 distance, the receiver consisting of a synchronized 
 
CHAP, xv.] ELECTRO-OPTIC EFFECTS. 305 
 
 pair of straight conductors with a microscopic spark- 
 gap between them, across which the secondary induced 
 sparks were watched for. By using a second mirror like 
 the first to catch the parallel rays and reconverge 
 them to a focus, the effect could be appreciated at a 
 distance of 20 yards. If the receiving mirror were 
 rotated through a right angle, it lost its converging 
 power on this particular light. 
 
 Apertures in a series of interposed screens proved 
 that the radiations travelled in straight lines (roughly 
 speaking, of course). 
 
 A gridiron of metallic wires is transparent to the 
 waves when arranged with the length perpendicular 
 to the electric oscillations, but it reflects them when 
 rotated through a right angle, so that the oscillations 
 take place along the conducting wires ; thus repre- 
 senting a kind of analyzer proving the existence 
 of polarized light. The receiver itself also acted 
 as analyzer, for if rotated much it failed to feel 
 the disturbance. 
 
 Conducting sheets, even thin ones, were very 
 opaque to the electrical radiation ; but non-con- 
 ducting obstacles, even such as wood, interrupt it 
 very little, and Dr. Hertz remarks, " not without 
 wonder," that the door separating the room contain- 
 ing the source of radiation from that containing 
 the detecting receiver might be shut without inter- 
 
 x 
 
306 MODERN VIEWS OF ELECTRICITY. [PART iv. 
 
 cepting the communication. The secondary sparks 
 were still observed. 
 
 But the most crucial test yet applied is that of 
 refraction. A great prism of pitch was made, its 
 faces more than a yard square, and its refracting 
 angle about 30. This being interposed in the path 
 of the electric rays, they were lost to the receiver 
 until it was shifted considerably Adjusting it till 
 its sparks were again at a maximum, it was found 
 that the rays had been bent by the pitch prism, 
 when set symmetrically, some 22 out of their original 
 course, and hence that the pitch had an index of 
 refraction for these 2-foot waves about 17. 
 
 190. These are great experiments. As I write, the 
 latest of them are but a month or two old, and they 
 are manifestly only a beginning. Most of the earlier 
 ones are very simple, and have already been repeated. 1 
 They seem likely to settle many doubtful points. 
 There has been a long-standing controversy in optics, 
 nearly as old as the century, as to whether the direction 
 of the vibrations was in, or was perpendicular to, the 
 plane of polarization ; in other words, whether it was 
 the elasticity or the density of the ether which varied 
 in dense media ; or, in Maxwell's theory, whether 
 
 1 See Fitzgerald and Trouton, A : atnrc, Vol. 39, p 391 ; also 
 Dr. Dragoumis, Nature, Vol. 39, p. 548. Also Lodge and Howard, 
 'Phil. Mag. July 1889. 
 
CHAP, xv | ELECTO-OPTIC EFFECTS. 507 
 
 it was the electro-magnetic or the electrostatic dis- 
 turbance that coincided with that plane. This point 
 has indeed by the exertion of extraordinary power 
 been almost settled already, through the considera- 
 tion of common optical experiments ; but now that 
 we are able electrically to produce radiation with 
 a full knowledge of what we are doing, of its direc- 
 tions of vibration and all about it, the complete 
 solution of this and of many another recondite 
 optical problem may be expected during the next 
 decade to drop simply and easily into our hands. 
 
 We have now a real undulatory theory of light, 
 no longer based on analogy with sound, and its 
 inception and early development are among the 
 most tremendous of the many achievements of the 
 latter half of the nineteenth century. 
 
 In 1865, Maxwell stated his theory of light. 
 Before the close of 1888 it is utterly and completely 
 verified. Its full development is only a question of 
 time, and labour, and skill. The whole domain of 
 Optics is now annexed to Electricity, which has 
 thus become an imperial science. 
 
 X 2 
 
APPENDED LECTURES. 
 
 (The following lectures bearing on the subject of this book 
 are here conveniently appended. In one or two places the 
 dale of their delivery must be taken into account?) 
 
LECTURE I. 
 
 THE RELATION BETWEEN ELECTRICITY AND 
 LIGHT. 1 
 
 EVER since the subject on which I have the honour 
 to speak to you to-night was arranged, I have been 
 astonished at my own audacity in proposing to deal in 
 the course of sixty minutes with a subject so gigantic 
 and so profound that a course of sixty lectures would 
 be inadequate for its thorough and exhaustive 
 treatment. 
 
 I must indeed confine myself carefully to some few 
 of the typical and most salient points in the relation 
 between electricity and light, and I must economize 
 time by plunging at once into the middle of the matter 
 without further preliminaries. 
 
 Now when a person is setting off to discuss the 
 relation between electricity and light it is very natu- 
 ral and very proper to pull him up short with the two 
 
 1 Delivered at the London Institution on December 16, 1880. 
 
312 MODERN VIEWS OF ELECTRICITY. [LKCT. i. 
 
 questions : What do you mean by electricity ? and 
 What do you mean by light ? These two questions 
 I intend to try briefly to answer. And here let me 
 observe that in answering these fundamental questions 
 I do not necessarily assume a fundamental ignorance 
 on your part of these two agents, but rather the 
 contrary ; and must beg you to remember that if 
 I repeat well-known and simple experiments before 
 you, it is for the purpose of directing attention to 
 their real meaning and significance, not to their obvious 
 and superficial characteristics : in the same way that 
 I might repeat the exceedingly familiar experiment 
 of dropping a stone to the earth if we were going to 
 define what we meant by gravitation. 
 
 Now then we will ask first, What is Electricity ? 
 and the simple answer must be, We don't know. Well, 
 but this need not necessarily be depressing. If the 
 same question were asked about Matter, or about 
 Energy, we should have likewise to reply, No one 
 knows. 
 
 But then the term Matter is a very general one, 
 and so is the term Energy. They are heads, in fact, 
 under which we classify more special phenomena. 
 
 Thus if we were asked \Vhat is sulphur ? or What 
 is selenium ? we should at least be able to reply, A 
 form of matter ; and then proceed to describe its pro- 
 perties, i.e. how it afifectecl our bodies and other bodies. 
 
 Again, to the question, What is heat ? we can reply, 
 
i-ECT. I.] ELECTRICITY AND LIGHT.. 313 
 
 A form of energy ; and proceed to describe the 
 peculiarities which distinguish it from other forms 
 of energy. 
 
 But to the question, What is electricity ? we have 
 no answer pat like this. We cannot assert that it is 
 a form of matter, neither can we deny it ; on the 
 other hand, we certainly cannot assert that it is a form 
 of energy, and I should be disposed to deny it. It 
 may be that electricity is an entity per se, just as 
 matter is an entity per se. 
 
 Nevertheless I can tell you what I mean by electricity 
 by appealing to its known behaviour. 
 
 Here is a battery that is, an electricity pump : it 
 will drive electricity along. Prof. Ayrton is going, 
 I am afraid, to tell you, on the 2Oth of January next, 
 that it produces electricity ; but if he does, I hope you 
 will remember that that is exactly what neither it nor 
 anything else can do. It is as impossible to generate 
 electricity in the sense I am trying to give the word, 
 as it is to produce matter. Of course I need hardly 
 say that Prof. Ayrton knows this perfectly well ; it is 
 merely a question of words, i.e., of what you understand 
 by the word electricity. 1 
 
 1 Or rather of what one understands by the word "produces." The 
 title of Prof. Ayrton's lecture was "The Production of Electricity'' ; 
 and it was to guard persons from supposing that it is right to speak 
 of the generation or creation of electricity in the same way as it is 
 possible to speak of the generation or creation (or, as it is often called, 
 "production") of heat, that I gave this caution. 
 
3H MODERN VIEWS OF ELECTRICITY. [LECT. I. 
 
 I want you then to regard this battery and all 
 electrical machines and batteries as kinds of electricity 
 pumps, which drive the electricity along through the 
 wire very much as a water-pump can drive water along 
 pipes, and that no electric machine can manufacture 
 electricity any more than a pump can manufacture 
 water. While the flow of electricity is going on, the 
 wire manifests a whole series of properties, which are 
 called the properties of the current. 
 
 [Here were shown an ignited platinum wire, the 
 electric arc between two carbons, an electric machine 
 spark, an induction-coil spark, and a vacuum tube 
 glow. Also a large nail was magnetized by being 
 wrapped in the current, and two helices were sus- 
 pended and seen to direct and attract each other.] 
 
 To make a magnet, then, we only need a current of 
 electricity flowing round and round in a whirl. A 
 vortex or whirlpool of electricity is in fact a magnet ; 
 and vice versa. And these whirls have the power of 
 directing and attracting other previously existing- 
 whirls according to certain laws, called the laws of 
 magnetism. And, moreover, they have the power of- 
 exciting fresh whirls in neighbouring conductors, and- 
 of repelling them according to the laws of dia- 
 magnetisrn. The theory of the actions is known ; 
 though the nature of the whirls, as of the simple 
 stream of electricity, is at present unknown. 
 
 [Here was shown a large electro-magnet and an 
 
J.ECT. i.] ELECTRICITY AND LIGHT. 315 
 
 induction-coil vacuum discharge spinning round and 
 round when placed in its field (Fig. 24).] 
 
 So much for what happens when electricity is made 
 to travel along conductors, i.e. when it travels along 
 like a stream of water in a pipe, or spins round and 
 round like a whirlpool. 
 
 But there is another set of phenomena, usually re- 
 garded as distinct and of another order, but which 
 are not so distinct as they appear, which manifest 
 themselves when you join the pump to a piece of glass 
 or any non-conductor and try to force the electricity 
 through that. You succeed in driving some through, 
 but the flow is no longer like that of water in an open 
 pipe ; it is as if the pipe were completely obstructed 
 by a number of elastic partitions, or diaphragms. 
 The water cannot move without straining and bending 
 these diaphragms, and if you allow it, these strained 
 partitions will recover themselves and drive the water- 
 back again. [Here was explained the process of 
 charging a Leyden jar, and the model (Fig. n)was 
 shown.] The essential thing to remember is that we 
 may have electrical energy in two forms, the static 
 and the kinetic ; and it is therefore also possible to 
 have the rapid alternation from one of these forms to 
 the other, called vibration. 
 
 Now we will pass to the second question : What- 
 do you mean by light? And the first and obvious 
 answer is, Everybody knows. And everybody that is- 
 
316 MODERN VIEWS OF ELECTRICITY. [LECT. T. 
 
 not blind does know to a certain extent. We have a 
 special sense-organ for appreciating light, whereas we 
 have none for electricity. Nevertheless, we must ad- 
 mit that we really know very little about the intimate 
 nature of light very little more than about electricity. 
 But we do know this, that light is a form of energy ; 
 and, moreover, that it is energy rapidly alternating 
 between the static and the kinetic forms that it is, in 
 fact, a special kind of energy of vibration. We arc ab- 
 solutely certain that light is a periodic disturbance in 
 some medium, periodic both in space and time ; that is 
 to say, the same appearances regularly recur at certain 
 equal intervals of distance at the same time, and also 
 present themselves at equal intervals of time at the same 
 place ; that in fact it belongs to the class of motions 
 called by mathematicians undulatory or wave motions. 
 The wave motion in this model (Powell's wave ap- 
 paratus) results from the simple up-and-down motion 
 popularly associated with the term wave. But when 
 a mathematician calls a thing a wave he means that 
 the disturbance is represented by a certain general 
 type of formula, not that it is an up-and-down motion, 
 or that it looks at all like those things on the top of 
 the sea. The motion of the surface of the sea falls 
 within that formula, and hence is a special variety of 
 wave motion, and the term wave has acquired in 
 popular use this signification and nothing else. So 
 that when one speaks ordinarily of a wave or undula- 
 
LECT. I.] ELECTRICITY AND LIGHT. 317 
 
 lory motion one immediately thinks of something- 
 heaving up and down, or even perhaps of something 
 breaking on the shore. But when we assert that the 
 form of energy called light is nndulatory, we by no 
 means intend to assert that anything whatever is 
 moving up and down, or that the motion, if we could 
 see it, would be anything at all like what we are 
 accustomed to in the ocean. The kind of motion is 
 unknown ; we are not even sure that there is anything 
 like motion in the ordinary sense of the word at all. 
 
 Now how much connection between electricity and 
 light have we perceived in this glance into their 
 natures ? Not much truly. It amounts to about this : 
 That on the one hand electrical energy may exist in 
 either of two forms the static form, when insulators 
 are electrically strained by having had electricity 
 driven partially through them (as in the Leyden jar), 
 which strain is a form of energy because of the 
 tendency to discharge and do work ; and the kinetic 
 form, where electricity is moving bodily along through 
 conductors or whirling round and round inside them, 
 which motion of electricity is a form of energy, 
 because the conductors and whirls can attract or repel 
 each other and thereby do work. 
 
 And, on the other hand, that light is the rapid alter- 
 nation of energy from one form to another from the 
 static form where the medium is strained, to the 
 kinetic form when it moves. It is just conceivable 
 
31 8 MODERN VIEWS OF ELECTRICITY. [I.KCT. I. 
 
 then that the static form of the energy of light is 
 etectro-static that is, that the medium is electrically 
 strained and that the kinetic form of the energy of 
 light is e/ectro-k'metic that is, that the motion is not 
 ordinary motion, but electrical motion ; in fact that 
 light is an electrical vibration, not a material one. 
 
 On November 5 last year there died at Cambridge 
 a man in the full vigour of his faculties such faculties 
 as do not appear many times in a century whose 
 chief work has been the establishment of this very 
 fact, the discovery of the link connecting light and 
 electricity ; and the proof for I believe it amounts to 
 a proof that they are different manifestations of one 
 and the same class of phenomena : that light is, in 
 fact, an electro-magnetic disturbance. The premature 
 death of James Clerk Maxwell is a loss to science 
 which appears at present utterly irreparable, for he 
 was engaged in researches that no other man can hope 
 as yet adequately to grasp and follow out ; but for- 
 tunately it did not occur till he had published his 
 book on Electricity and Magnetism, one of those 
 immortal productions which exalt one's idea of the 
 mind of man, and which has been mentioned 
 by competent critics in the same breath as the 
 Principia itself. 
 
 But it is not perfect like the Principia; much 
 of it is rough-hewn, and requires to be thoroughly 
 worked out. It contains numerous misprints and 
 
LECT. i.] ELECTRICITY AND LIGHT. 319 
 
 errata, and part of the second volume is so difficult 
 as to be almost unintelligible. Some, in fact, con- 
 sists of notes written for private use, and not 
 prepared for publication. It seems next to im- 
 possible now to mature a work silently for twenty 
 or thirty years, as was done by Newton two and a 
 half centuries ago. But a second edition was pre- 
 paring, and much might have been improved in form 
 if life had been spared to the illustrious author. 
 
 The main proof of the electro-magnetic theory of 
 light is this. The rate at which light travels has been 
 measured many times, and is pretty well known. 
 The rate at which an electro-magnetic wave disturb- 
 ance would travel, if such could be generated, can be 
 also determined by calculation from electrical measure- 
 ments. The two velocities agree exactly. This is 
 the great physical constant known as the ratio " 7'," 
 which so many physicists have been measuring, and are 
 likely to be measuring for some time to come ( 138). 
 
 Many and brilliant as were Maxwell's discoveries, 
 not only in electricity, but also in the theory of the 
 nature of gases, and in molecular science generally, I 
 cannot help thinking that if one of them is more 
 striking and more full of future significance than the 
 rest, it is the one I have just mentioned the theory 
 that light is an electrical phenomenon. 
 
 The first glimpse of this splendid generalization 
 
320 MODERN VIEWS OF ELECTRICITY. [LECT I. 
 
 was caught in 1845, fi ye an d thirty years ago, by that 
 prince of pure experimentalists, Michael Faraday. 
 His reasons for suspecting some connection between 
 electricity and light are not clear to us in fact they 
 could not have been clear to him ; but he seems to 
 have felt a conviction that if he only tried long enough, 
 and sent all kinds of rays of light in all possible 
 directions across electric and magnetic fields in all 
 sorts of media, he must ultimately hit upon some- 
 thing. Well, this is very nearly what he did. With 
 a sublime patience and perseverance which remind 
 one of the way Kepler hunted down guess after 
 guess in a different field of research, Faraday com- 
 bined electricity, or magnetism, and light in all manner 
 of ways, and at last he was rewarded with a result. 
 And a most out-of-the-way result it seemed. First 
 you have to get a most powerful magnet and very 
 strongly excite it ; then you have to pierce its two 
 poles with holes, in order that a beam of light may 
 travel from one to the other along the lines of force ; 
 then, as ordinary light is no good, you must get a 
 beam of plane-polarized light and send it between the 
 poles. But still no result is obtained until, finally, you 
 interpose a piece of a rare and out-of-the-way material 
 which Faraday had himself discovered and made, a 
 kind of glass which contains borate of lead, and which 
 is very heavy, or dense, and which must be perfectly 
 annealed. 
 
LKCT. 1.] ELECTRICITY AND LIGHT. 321 
 
 And now, when all these arrangements are 
 completed, what is seen is simply this, that if an 
 analyzer is arranged to stop the light and make the 
 field quite dark before the magnet is excited, then 
 directly the battery is connected and the magnet 
 called into action a faint and barely perceptible 
 brightening of the field occurs ; which will disappear 
 if the analyzer be slightly rotated. [The experiment 
 was then shown.] Now no wonder that no one under- 
 stood this result. Faraday himself did not under- 
 stand it at all : he seems to have thought that the 
 magnetic lines of force were rendered luminous, or 
 that the light was magnetized ; in fact he was in a fog, 
 and had no idea of its real significance. Nor had 
 anyone. Continental philosophers experienced some 
 difficulty and several failures before they were able to 
 repeat the experiment. It was in fact discovered too 
 soon, before the scientific world was ready to receive it, 
 and it was reserved for Sir William Thomson briefly, 
 but very clearly, to point out, and for Clerk Maxwell 
 more fully to develop, its most important consequences. 
 
 [The principle of the experiment was then illus- 
 trated by the aid of a mechanical model. The 
 model was a Wheatstone photometer consisting of 
 one cogged circle rolling inside a fixed outer circle 
 of twice the diameter, so that a bead attached to the 
 inner one described some ellipse. An extra adjust- 
 ment was provided whereby the bead could be set 
 
 Y 
 
322 MODERN VIEWS OF ELECTRICITY. [LECT. j. 
 
 exactly over the circumference of the smaller wheel : 
 it then describes a straight line, a diameter of the 
 large circle, with a simple harmonic motion ; and this 
 simple harmonic motion is actually compounded of 
 two equal opposite circular motions, viz. the revolu- 
 tion of the centre of the smaller wheel, and the revolu- 
 tion of the bead about this moving centre in an 
 opposite direction and at the same speed. 
 
 The whole instrument was mounted in such a way 
 that it could be slowly rotated one way or other 
 by a second handle and endless screw ; by this means 
 one of these circular motions was accelerated and the 
 other retarded, and as a consequence the path of the 
 oscillating bead slowly rotated, describing a more 
 complicated hypocycloid, and representing the rota- 
 tion of the direction of vibration of light ( 172).] 
 
 This is the fundamental experiment which probably 
 suggested Clerk Maxwell's theory of light ; but of late 
 years many fresh facts and relations between electricity 
 and light have been discovered, and at the present 
 time they are tumbling in in great numbers. 
 
 It was found by Faraday that many other trans- 
 parent media besides heavy glass would show the 
 phenomenon if placed between the poles : only in a 
 less degree ; and the very important observation that 
 air itself exhibits the same phenomenon, though to an 
 exceedingly small extent, has just been made by Ku'ndt 
 and Rontgen in Germany. 
 
LF.CT. T.] ET.I-:CTRICITY AND LIGHT. 323 
 
 Dr. Kerr, of Glasgow, has extended the result to 
 opaque bodies, and has shown that if light be passed 
 through magnetized iron its plane is rotated. The film 
 of iron must be exceedingly thin, because of its opacity, 
 and hence, though the intrinsic rotating power of iron 
 is undoubtedly very great, the observed rotation is 
 exceedingly small and difficult to observe ; and it is 
 only by very remarkable patience and care and inge- 
 nuity that Dr. Kerr has obtained his result. Mr. 
 Fitzgerald, of Dublin, has examined the question 
 mathematically, and has shown that Maxwell's theory 
 would have enabled Dr. Kerr's result to be predicted. 
 
 Another requirement of the theory is that bodies 
 which are transparent to light must be insulators or 
 non-conductors of electricity, and that conductors of 
 electricity are necessarily opaque to light. Simple 
 observation amply confirms this ; metals are the best 
 conductors, and are the most opaque bodies known. 
 Insulators such as glass and crystals are transparent 
 whenever they are sufficiently homogeneous ; and the 
 very remarkable researches of Prof. Graham Bell in 
 the last few months have shown that even ebonite, one 
 of the most opaque insulators to ordinary vision, is 
 certainly transparent to some kinds of radiation, and 
 transparent to no small degree. 
 
 [The reason why transparent bodies must insulate, 
 and why conductors must be opaque, was here 
 illustrated by mechanical models. 
 
 Y 2 
 
324 MODERN VIEWS OE ELECTRICITY. [LECT. I. 
 
 The model which represented a dielectric has already 
 been depicted in Fig. 8 ; and when the cord thread- 
 ing alt the elastically supported balls is vibrated, waves 
 travel readily through it. 
 
 The model which represented a metallic conductor 
 is shown here in Fig. 54. It has its wooden balls 
 sliding on smooth brass rods so that they have no 
 tendency to recoil to a settled position but remain 
 where placed. On shaking the cord connecting these 
 
 FIG. 54 Rude model 10 pair with Fig. 8 (p. 43) and to call attention to some of 
 the differences between a metal and an insulator. 
 
 balls the waves penetrate a certain small depth into 
 the medium but fail to get through it. 
 The two models were connected in series, and waves 
 which had been transmitted along the cord by one 
 were partly quenched, partly reflected, by the other.] 
 A further consequence of the theory is that the 
 velocity of light in a transparent medium will be 
 affected by its electrical strain constant ; in other 
 words, that its refractive index will bear some close 
 but not yet quite ascertained relation to its specific 
 
LECT. 1.] ELECTRICITY AND LIGHT. 325 
 
 inductive capacity. Experiment has partially con- 
 firmed this, but the confirmation is as yet very 
 incomplete. 
 
 But there are a number of results not predicted by 
 theory, and whose connection with theory is not 
 clearly made out. We have the fact that light falling 
 on the platinum electrode of a voltameter generates a 
 current ; first observed, I think, by Sir W. R. Grove 
 at any rate it is mentioned in his Correlation of 
 Forces extended by Becquerel and Robert Sabine 
 to other substances, and now being extended to 
 fluorescent and other bodies by Prof. Minchin. And 
 finally for I must be brief we have the remarkable 
 action of light on selenium. This fact was discovered 
 accidentally by an assistant in the laboratory of Mr. 
 Willoughby Smith, who noticed that a piece of 
 selenium conducted electricity very much better when 
 light was falling upon it than when it was in the dark. 
 The light of a candle is sufficient, and instantaneously 
 brings down the resistance to something like one-fifth 
 of its original value. 
 
 I could show you these effects, but there is not much 
 to see ; it is an intensely interesting phenomenon, but 
 its external manifestation is not striking any more 
 than Faraday's heavy glass experiment was. 
 
 This is the phenomenon which, as you know, has 
 been utilized by Prof. Graham Bell in that most 
 ingenious and striking invention, the photophone. 
 
326 MODERN VIEWS OF ELECTRICITY. [LECT. 1. 
 
 By the kindness of Prof. Silvanus Thompson I have 
 a few slides to show the principle of the invention, 
 and Mr. Shelford Bidwell has been good enough to 
 lend me his home-made photophone, which answers 
 exceedingly well for short distances. 
 
 I have now trespassed long enough upon your 
 patience, but I must just allude to what may very 
 likely be the next striking popular discovery, and that 
 is the transmission of light by electricity ; I mean the 
 transmission of such things as views and pictures by 
 means of the electric wire. It has not yet been done, 
 but it seems already theoretically possible, and it may 
 very soon be practically accomplished. 
 
LECTURE II. 
 
 THE ETHER AND ITS FUNCTIONS. 1 
 
 I HOPE that no one has been misled by an error in 
 the printing of the title of this lecture, viz. the 
 omission of the definite article before the word ether, 
 into supposing that I am going to discourse on 
 chemistry and the latest anaesthetic ; you will have 
 understood, I hope, that " ether " means the ether, and 
 that the ether is the hypothetical medium which is 
 supposed to fill otherwise empty space. 
 
 The idea of an ether is by no means a new one. As 
 soon as a notion of the enormous extent of space had 
 been grasped, by means of astronomical discoveries, 
 the question presented itself to men's minds, what 
 was in this space ? was it full, or was it empty ? and 
 the question was differently answered by different 
 metaphysicians. Some felt that a vacuum was so 
 
 1 Delivered at the London Institution on December 28, 1882. 
 
323 MODERN VIEWS OF ELECTRICITY. [LECT. n. 
 
 abhorrent a thing that it could not by any possibility 
 exist anywhere, but that Nature would not be 
 satisfied unless space were perfectly full. Others, 
 again, felt that empty space could hardly exist, that it 
 would shrink up to nothing like a pricked bladder 
 unless it were kept distended by something material. 
 In other words, they made matter the condition of 
 extension. On the other hand, it was contended 
 that, however objectionable the idea of empty space 
 might be, yet emptiness was a necessity in order that 
 bodies might have room to move ; that, in fact, if all 
 space were perfectly full of matter everything would 
 be jammed together, and nothing like free attraction 
 or free motion of bodies round one another could 
 go on. 
 
 And indeed there are not wanting philosophers at 
 the present day who still believe something of this 
 same kind, who are satisfied to think of matter as 
 consisting of detached small particles acting on one 
 another with forces varying as some inverse power ol 
 the distance, and who, if they can account for a 
 phenomenon by an action exerted across empty space, 
 are content to go no further, nor seek the cause and 
 nature of the action more closely. 1 
 
 Now metaphysical arguments, in so far as they 
 
 1 In illustration of this statement an article has since appeared in 
 the January number of the Philosophical Magazine for 1883, by 
 Mr. Walter Browne. 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 329 
 
 have any weight or validity whatever, are unconscious 
 appeals to experience ; a person endeavours to find 
 out whether a certain condition of things is by him 
 conceivable, and if it is not conceivable he has some 
 prinid facie ground for asserting that it probably does 
 not exist. I say he has some ground, but whether it 
 be much or little depends partly on the nature of the 
 thing thought of, whether it be fairly simple or highly 
 complex, and partly on the range of the man's own 
 mental development, whether his experience be wide 
 or narrow. 
 
 If a highly-developed mind, or set of minds, find 
 a doctrine about some comparatively simple and 
 fundamental matter absolutely unthinkable, it is an 
 evidence, and is accepted as good evidence, that the 
 unthinkable state of things is one that has no 
 existence; the argument being that if it did exist, 
 either it or something not wholly unlike it would 
 have come within the range of experience. We have 
 no further evidence than this for the statement that 
 two straight lines cannot inclose a space, or that the 
 three angles of a triangle are equal to two right 
 angles. 
 
 Nevertheless there is nothing final about such an 
 argument ; all that the inconceivability of a thing 
 really proves, or can prove, is that nothing like it 
 has ever come within the thinker's experience ; and 
 this proves nothing as to the reality or non-reality of 
 
330 MODERN VIEWS OF ELECTRICITY. [I,KCT. ri. 
 
 the thing, unless his experience of the same kind of 
 things has been so extensive as to make it reasonably 
 probable that if such a thing had existed it would 
 not have been so completely overlooked. 
 
 The experience of a child or a dog, on ordinary 
 scientific phenomena, therefore, is worth next to 
 nothing ; and as the experience of a dog is to 
 ordinary science, so is the experience of the human 
 race to some higher phenomena, of which they at 
 present know nothing, and against the existence of 
 which it is perfectly futile and presumptuous to bring 
 forward arguments about their being inconceivable, as 
 if they were likely to be anything else. 
 
 Now if there is one thing with which the human race 
 has been more conversant from time immemorial than 
 another, and concerning which more experience has 
 been unconsciously accumulated than about almost 
 anything else that can be mentioned, it is the action of 
 one body on another ; the exertion of force by one 
 body upon another, the transfer of motion and 
 energy from one body to another ; any kind of effect, 
 no matter what, which can be produced in one body 
 by means of another, whether the bodies be animate 
 or inanimate. The action of a man in felling a tree, in 
 thrusting a spear, in drawing a bow ; the action of the 
 bow again on the arrow, of po\vder on a bullet, of a 
 horse on a cart ; and again, the action of the earth on 
 the moon, or of a magnet on iron. Every activity of 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 331 
 
 every kind that we are conscious of may be taken 
 as an illustration of the action of one body on 
 another. 
 
 Now I wish to appeal to this mass of experience, 
 and to ask, is not the direct action of one body on 
 another across empty space, and with no means 
 of communication whatever, is not this absolutely 
 unthinkable ? We must not answer the question 
 off-hand, but must give it due consideration, and we 
 shall find, I think, that wherever one body acts on 
 another by obvious contact, we are satisfied and have 
 a feeling that the phenomenon is simple and intelli- 
 gible ; but that whenever one body apparently acts on 
 another at a distance, we are irresistibly impelled to 
 look for the connecting medium. 
 
 If a marionette dances in obedience to a prompting 
 hand above it, any intelligent child would feel for the 
 wire, and if no wire or anything corresponding to it 
 was discovered, would feel that there was something 
 uncanny and magical about the whole thing. An- 
 cient attempts at magic were indeed attempts to 
 obtain results without the trouble of properly causing 
 them, to build palaces by rubbing rings or lanterns, to 
 remove mountains by a wish instead of with the 
 spade and pickaxe, and generally to act on bodies 
 without any real means of communication ; and 
 modern disbelief in magic is simply a statement of 
 the conviction of mankind that all attempts in this 
 
332 MODERN VIEWS OF ELECTRICITY. [LECT. 11. 
 
 direction have turned out failures, and that action at a 
 distance is impossible. 
 
 If a man explained the action of a horse or a cart 
 by saying that there was an attraction between them 
 varying as some high direct power of the distance, he 
 would not be saying other than the truth the facts 
 may be so expressed but he would be felt to be 
 giving a wretchedly lame explanation, and anyone 
 who simply pointed out the traces would be going 
 much more to the root of the matter. Similarly with 
 the attraction of a magnet for a distant magnetic 
 pole. To say that there is an attraction as the 
 inverse cube of the distance between them is true, but 
 it is not the whole truth ; and we should be obliged to 
 anyone who will point out the traces, for traces we 
 feel sure there are. 
 
 If anyone tries to picture clearly to himself the 
 action of one body on another without any medium of 
 communication whatever, he must fail. A medium is 
 instinctively looked for in most cases ; and if not in all, 
 as in falling weights or magnetic attraction, it is only 
 because custom has made us stupidly callous to the 
 real nature of these forces. 
 
 When we see a vehicle bowling down-hill without 
 any visible propelling force, we ought to regard it with 
 the same mixture of curiosity and wonder as the 
 Chinaman felt when he saw for the first time in 
 the streets of Chicago a tram-car driven by a rope 
 
LECT. II.] THE ETHER AND ITS FUNCTIONS. 333 
 
 buried in a pipe underground. The attachment to 
 these cars comes through a narrow slit in the pipe, 
 and is quite unobtrusive. After regarding the car 
 with open-mouthed astonishment for some time, the 
 Chinaman made use of the following memorable 
 exclamation, " No pushee No pullee Go like mad ! " 
 He was a philosophic Chinaman. 
 
 Remember, then, that whenever we see a thing 
 being moved we must look for the rope ; it may be 
 visible or it may be invisible, but unless there is 
 either " pushee " or " pullee " there can be no action. 
 And if you further consider a pull it resolves itself 
 into a push ; to pull a thing towards you, you have to 
 put your finger behind it and push ; a horse is said to 
 pull a cart, but he is really pushing at the collar ; an 
 engine pushes a truck by means of a hook and eye ; 
 and so on. 
 
 There is still the further very important and 
 difficult question as to why the parts hang together, 
 and why when you push one part the rest follows. 
 Cohesion is a very striking fact, and an explanation of 
 it is much to be desired ; I shall have a little more to 
 say about it later, but at present we have nothing more 
 than an indication of the direction in which an ex- 
 planation seems possible. We cannot speak distinctly 
 about those actions which are as yet mysterious to us ; 
 but concerning those which are comparatively simple 
 and intelligible we may make this general state- 
 
334 MODERN VIEWS OF ELECTRICITY [I.KCT. 11. 
 
 mcnt : The only way of acting on a body directly 
 is to push it behind. 
 
 There must be contact between bodies before they 
 can directly act on each other ; and if they are not in 
 contact with each other and yet act, they must both 
 be in contact with some third body which is the 
 medium of communication, the rope. 
 
 Consider now for an instant the most complex case, 
 the action of one animate body on another not touch- 
 ing it. To call the attention of a dog, for instance, 
 there are several methods : one plan is to prod him 
 with a stick, another is to heave a stone at him, a 
 third is to whistle or call, while a fourth is to beckon 
 him by gesture, or, what is essentially the same pro- 
 cess, to flash sunlight into his eye with a mirror. In 
 the first two of these methods the media of com- 
 munication are perfectly obvious the stick and the 
 stone ; in the third, the whistle, the medium is not so 
 obvious, and this case might easily seem to a savage 
 like action at a distance, but we know of course that 
 it is the air, and that if the air between be taken away, 
 all communication by sound is interrupted. But the 
 fourth or optical method is not so interrupted ; the 
 dog can see through a vacuum perfectly well, though 
 he cannot hear through it ; but what the medium now 
 is which conveys the impression is not so well known. 
 The sun's light is conveyed to the earth by such a 
 medium as this across the emptiness of planetary space. 
 
I.F.IT, ii.] THK ETHER ANT) ITS FUNCTIONS. 335 
 
 The only remaining typical plans of acting on 
 the dog would be either by electric or magnetic 
 attractions, or by mesmerism, and I would have you 
 seek for the medium which conveys these impressions 
 with just as great a certainty that there is one as you 
 feel in any of the other cases. 
 
 Leaving these more mysterious and subtle modes 
 of communication, let us return to the two most 
 simple ones, viz. the stick and the stone. These two 
 arc. representative of the only possible fundamental 
 modes of communication between distant bodies, 
 for one is compelled to believe that every more 
 occult mode of action will ultimately resolve itself 
 into one or other of these two. The stick repre- 
 sents the method of communication by continuous 
 substance ; the stone represents the communi- 
 cation by actual transfer of matter, or, as I shall 
 call it, the projectile method. There are no other 
 known methods for one body to act on another 
 than by these two by continuous medium, and by 
 projectile. 
 
 We know one clear and well-established example 
 of the projectile method, viz. the transmission of 
 pressure by gases. A gas consists of particles per- 
 fectly independent of each other, and the only way in 
 which they can act on each other is by blows. The 
 pressure of the air is a bombardment of particles, and 
 actions are transmitted through gases as through a 
 
336 MODERN VIEWS OF ELECTRICITY. [LECT. n. 
 
 row of ivory balls. Sound is propagated by each 
 particle receiving a knock and passing it on to the 
 next, the final effect being much the same as if the 
 first struck particles had been shot off through the 
 whole distance. 
 
 The explanation of the whole behaviour of gases 
 in this manner is so simple and satisfactory, and 
 moreover is so certainly the true account of the 
 matter, that we are naturally tempted to ask whether 
 this projectile theory is not the key to the universe, 
 and whether every kind of action whatever cannot be 
 worked out on this hypothesis of atoms blindly driving 
 about in all directions at perfect random, and with 
 complete -independence of each other except when 
 they collide. 1 And accordingly we have the cor- 
 puscular theories of light and of gravitation : both 
 account for their respective phenomena by a battering 
 of particles. The corpuscular theory of gravitation 
 is, however, full of difficulties, for it is not obvious 
 according to it why the weight of a plate is the same 
 when held edgeways as when held broadside on, in 
 the stream of corpuscles ; while it is surprising (as 
 indeed it perhaps is on any hypothesis) that the 
 weight of a body is the same in the solid, liquid, and 
 gaseous states. It has been attempted to explain 
 cohesion also on the same hypothesis, but the diffi- 
 
 1 To this hypothesis Mr, Tolver Preston has addressed himself with 
 much ingenuity, 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 337 
 
 culties, which were great enough before, are now 
 enormous ; and to me at any rate it seems that it is 
 only by violent straining and by improbable hypotheses 
 that we can explain all the actions of the universe by 
 a mere battery of particles. 
 
 Moreover, it is difficult to understand what the 
 atoms themselves can be like, or how they can 
 strike and bound off one another without yielding 
 to compression and then springing out .again like 
 two elastic balls ; it is difficult to understand the 
 elasticity of really ultimate hard particles. And if 
 the atoms are not such hard particles, but are elastic 
 and yielding, and rebound from one another according 
 to the same sort of law that ivory balls do : of what 
 are they composed ? We shall have to begin all over 
 again, and explain the cohesion and elasticity of the 
 parts of the atom. 
 
 The more we think over the matter, the more are 
 we compelled to abandon mere impact as a complete 
 explanation of action in general. But if this be so 
 we are driven back upon the other hypothesis, the 
 only other, viz. communication by continuous medium. 
 
 We must begin to imagine a continuous connecting 
 medium between the particles a substance in which 
 they are embedded, which penetrates into all their 
 interstices, and extends without break to the remotest 
 limits of space. Once grant this, and difficulties 
 begin rapidly to disappear. There is now continuous 
 
 Z 
 
338 MODERN VIEWS OF ELECTRICITY. [LKCT. n. 
 
 contact between the particles of bodies, and if one is 
 pushed the others naturally receive the motion. The 
 atoms of gas are impinging as before, but we have 
 now a different idea of what impact means. 
 
 Gravitation is explainable by differences of pressure 
 in the medium, caused by some action between it and 
 matter not yet understood. (See page 352.) Cohesion 
 is explainable also probably in the same way. 
 
 Light consists of undulation or waves in the 
 medium ; while electricity is turning out quite 
 possibly to be an aspect of a part of the very 
 medium itself. 
 
 The medium is now accepted as a necessity by all 
 modern physicists, for without it we are groping in 
 the dark, with it we feel we have a clue which, if 
 followed up, may lead us into the innermost secrets of 
 Nature. It has as yet been followed up very partially, 
 but I will try and indicate the directions in which 
 modern science is tending. 
 
 The name you choose to give to the medium is a 
 matter of very small importance, but " the ether" is 
 as good a name for it as another. 
 
 As far as we know it appears to be a perfectly 
 homogeneous incompressible continuous body, in- 
 capable of being resolved into simpler elements or 
 atoms ; it is, in fact, continuous, not molecular. 
 There is no other body of which we can say this, 
 and hence the properties of ether must be somewhat 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 339 
 
 different from those of ordinary matter. But there 
 is little difficulty in picturing a continuous substance 
 to ourselves, inasmuch as the molecular and porous 
 nature of ordinary matter is by no means evident to 
 the senses, but is an inference of some difficulty. 
 
 Ether is often called a fluid, or a liquid, and again 
 it has been called a solid and has been likened to 
 a jelly because of its rigidity; but none of these names 
 are very much good ; all these are molecular group- 
 ings and therefore not like ether ; let us think simply 
 and solely of a continuous frictionless medium pos- 
 sessing inertia, and the vagueness of the notion will 
 be nothing more than is proper in the present state 
 of our knowledge. 
 
 We have now to try and realize the idea of 
 a perfectly continuous, subtle, incompressible sub- 
 stance pervading all space and penetrating between 
 the molecules of all ordinary matter, which are em- 
 bedded in it, and connected with one another by its 
 means. And we must regard it as the one universal 
 medium by which all actions between bodies are 
 carried on. This, then, is its function to act as the 
 transmitter of motion and of energy. 
 First consider the propagation of light. 
 Sound is propagated by direct excursion and im- 
 pact of the atoms of ordinary matter. Light is not 
 so propagated. How do we know this ? 
 
 (l) Because of its speed, 3 X io 10 centimetres per 
 
 z 2 
 
340 MODERN VIEWS OF ELECTRICITY. [LECT. n. 
 
 second, which is greater than anything transmissible 
 by ordinary matter. 
 
 (2) Because of the kind of vibration, as revealed by 
 the phenomena of polarization. 
 
 The vibrations of light are not such as can be 
 transmitted by a set of disconnected molecules ; if 
 by molecules at all, it must be by molecules con- 
 nected into a solid, i.e. by a body with rigidity. 
 Rigidity means active resistance to shearing stress, 
 i.e. to alteration in shape ; it is also called elasticity 
 of figure ; it is by the possession of rigidity that 
 a solid differs from a fluid. For a body to transmit 
 vibrations at all it must possess inertia ; transverse 
 vibrations can only be transmitted by a body with 
 rigidity. All matter possesses inertia, but fluids 
 only possess volume elasticity, and accordingly can 
 only transmit longitudinal vibrations. Light consists 
 of transverse vibrations ; air and water have no 
 rigidity,' yet they are transparent, i.e. transmit trans- 
 verse vibrations ; hence it must be the ether inside 
 them which really conveys the motion, and the ether 
 must have properties which, if it were ordinary matter, 
 we should style inertia and rigidity. No highly 
 rarefied air will serve the purpose ; the ether must be 
 a distinct body. Air may exist indeed in planetary 
 space, even to infinity, but if so it is of almost in- 
 finitesimal density compared with the ether there. 
 It is easy to calculate the density of the atmosphere 
 
LECT. II.] THE ETHER AND ITS FUNCTIONS. 341 
 
 at any height above the earth's surface, supposing 
 
 other bodies absent and supposing the temperature 
 
 constant. (All numbers following are in C.G.S. units.) 
 
 The density of the air at a distance of ;/ earth 
 
 radii from the centre of the earth is equal to a 
 
 i 
 quarter the density here divided by IO 3 ^~"~. So at a 
 
 height of only 4000 miles above the surface, the 
 atmospheric density is a number with 127 ciphers 
 after the decimal point before the significant figures 
 begin. 1 The density of ether, on the other hand, 
 has been calculated by Sir William Thomson from 
 data furnished by Pouillet's experiments on the 
 energy of sunlight, and from a justifiable guess as 
 to the amplitude of a vibration ; and it comes out 
 about 10" l8 , a number with only 17 ciphers before 
 the significant figures. In inter-planetary space, 
 therefore, all the air that exists is utterly negligible ; 
 the density of the ether there, though small, is 
 enormous by comparison. [See also page 235.] 
 
 Once given the density of the ether, its rigidity 
 follows at once, because the ratio of the rigidity 
 to the density is the square of the velocity of trans- 
 
 1 I have left this statement in, because it is a view which has been 
 apparently held by high authority that the atmosphere has no limit. 
 To me I confess it appears much more reasonable to suppose that at a 
 certain height, which on the hypothesis of thorough stirring or convec- 
 tion equilibrum is only 16 or 17 miles, but is pro I- ably a good deal 
 more in reality (because rare air is very viscous), a free surface exists 
 although of very small density. 
 
342 MODERN VIEWS OE ELECTRICITY. [I.F.CT. n 
 
 verse wave propagation, viz. in the case of ether, 
 9 x io 20 . The rigidity of ether comes out, therefore, 
 to be about 900. The most rigid solid we know 
 is steel, and compared with its rigidity, viz. Sxio 11 , 
 that of ether is insignificant. Neither steel nor glass. 
 however, could transmit vibrations with anything like 
 the speed of light, because of their great density. The 
 rate at which transverse vibrations are propagated by 
 crown glass is half a million centimetres per second 
 a considerable speed, no doubt, but the ether inside 
 the glass transmits them 40,000 times as quick, viz. 
 at twenty thousand million centimetres per second. 
 
 The ether outside the glass can do still better than 
 this, it comes up to thirty thousand million, and the 
 question arises what is the matter with the ether 
 inside the glass that it can only transmit undulations 
 at two-thirds the normal speed. Is it denser than free 
 ether, or is it less rigid ? Well, it is not easy to say ; 
 but the fact is certain that ether is somehow affected 
 by the immediate neighbourhood of gross matter, 
 and it appears to be concentrated inside it to an ex- 
 tent depending on the density of the matter. Frcsncl's 
 hypothesis is that the ether is really denser inside 
 gross matter, that there is a sort of attraction be- 
 tween ether and the molecules of matter which results 
 in an agglomeration or binding of some ether round 
 each atom, and that, this additional or bound ether 
 belongs to the matter, and travels about with it. The 
 
I.ECT. II.] THE ETHER AND ITS FUNCTIONS. 343 
 
 rigidity of the bound ether Fresnel supposes to be the 
 same as that of the free, except in some crystals. 
 
 If anything like this can be imagined, a measure 
 of the relative density of the bound ether is easily 
 given. For the inverse velocity-ratio of light is 
 n (the index of refraction), and the density is inversely 
 as the square of the velocity ; hence the density- 
 measure is ;/ 2 . The density of ether in free space 
 being called T, that inside matter has a density ;/ 2 , 
 and the density of the bound portion of this is ;/ 2 I. 
 
 This may all sound very fanciful, but something 
 like it is sober truth ; not as it is here stated very 
 
 likely, but the fact that (i 1 )th of the whole 
 
 //- / 
 
 ether inside matter is bound to it and travels with it, 
 
 while the remaining th is free and blows freely 
 n 2 
 
 through the pores, is fairly well established and 
 confirmed by direct experiment ( 1 1*8). 
 
 Consider the effect of wind on sound. Sound is 
 travelling through the air at a certain definite rate 
 depending simply on the average speed of the atoms 
 in their excursions, and at the rate at which they 
 therefore pass the knocks on ; if there is a wind 
 carrying all the atoms bodily in one direction, natur- 
 ally the sound will travel quicker in that direction 
 than in the opposite. Sound travels quicker with the 
 wind than against it. Now is it the same with light ? 
 
344 MODERN VIEWS OF ELECTRICITY. [LECT. n. 
 
 does it too travel quicker with the wind ? Well that 
 altogether depends on whether the ether is blowing 
 along as well as the air ; if it is, then its motion must 
 help the light on a little ; but if the ether is at rest, no 
 motion of air or matter of any kind can make any 
 difference. But according to Fresnel's hypothesis it 
 is not wholly at rest nor wholly in motion ; the free is 
 at rest, the bound is in motion ; and therefore the 
 speed of light with the wind should be increased by an 
 
 addition of (i . 2 -)th of the velocity of the wind. 
 
 Utterly infinitesimal, of course, in the case of air, 
 whose n is but a trifle greater than I ; but for water 
 the fraction is 7-i6ths, and Fizeau thought this not 
 quite hopeless to look for. He accordingly devised 
 a beautiful experiment, executed it successfully, and 
 proved that when light travels with a stream of 
 water, 7-i6ths of the velocity of the water must 
 be added to the velocity of the light ; and when it 
 travels against the stream the same quantity must 
 be subtracted, to get the true resultant velocity 
 with which the light is travelling through space. 
 
 Arago suggested another experiment. When light 
 passes through a prism, it is bent out of its course 
 by reason of its diminished velocity inside the glass, 
 and the refraction is strictly dependent on the re- 
 tardation ; now suppose a prism carried rapidly 
 forward through space, say at the rate of nineteen 
 
LECT. II.] THE ETHER AND ITS FUNCTIONS. 345 
 
 miles a second by the earth in its orbit, which is 
 the quickest accessible carriage ; if the ether is all 
 streaming freely through the glass, light passing 
 through the prism will be less retarded when going 
 with the ether than when going against it, and hence 
 the bending will be different. 
 
 Maxwell tried the experiment in a very perfect 
 form, but found no difference. If all the ether were 
 free there would have been a difference ; if all the 
 ether were bound to the glass there would have been 
 a difference the other way ; but according to Fresnel's 
 hypothesis there should be no difference, because 
 according to it, the free ether, which is the portion in 
 relative motion, has nothing to do with the refraction, 
 it is the addition of the bound ether which causes the 
 refraction, and this part is stationary relatively to the 
 glass, and is not streaming through it at all. Hence 
 the refraction is the same whether the pr.ism be at rest 
 or in motion through space. 1 
 
 An atom embedded in ether is vibrating and send- 
 ing out waves in all directions ; the length of the 
 wave depends on the period of the vibration, and 
 
 1 Several of this class of experiments have been recently performed 
 with consummate skill and with refined appliances by Mr. Michelson 
 in America. The result of his repetition of the Fizeau experiment is 
 entirely confirmatory of Fizeau's result and of Fresnel's theory. The 
 results of some of the other experiments, having reference to the theory 
 of aberration and the motion of the ether near the earth, are more puzzling, 
 and seem discordant with ordinarily received notions at present. 
 
346 MODERN VIEWS OF ELECTRICITY. [i.ECT. n. 
 
 different lengths of wave produce the different colour 
 sensations. Now through free ether all kinds of waves 
 appear to travel at the same rate ; not so through 
 bound ether ; inside matter the short waves arc more 
 retarded than the long, and hence the different sizes 
 of waves .can be sorted out by a prism. Now a free 
 atom has its own definite period of vibration, like a 
 tuning-fork has, and accordingly sends out light of a 
 certain definite colour or of a few definite colours, just 
 as a tuning-fork emits sound of a certain definite pitch 
 or of a few definite pitches called harmonics. By the 
 pitch of the sound it is easy to calculate the rate of 
 vibration of the fork ; by the colour of the light one 
 can determine the rate of vibration of the atom. 
 
 When we speak of the atoms vibrating, we do not 
 mean that they are wagging to and fro as a whole ; it 
 is more likely that they are crimping themselves, that 
 they are vibrating as a tuning-fork or a bell vibrates ; 
 we know this because it is easy to make the free 
 atoms of a gas vibrate. It is only in the gaseous 
 state, indeed, that we can study the rate of vibration 
 of an atom ; when they are packed closely together in 
 a solid or liquid, they arc cramped, and all manner of 
 secondary vibrations are induced. They then, no 
 doubt, wag to and fro also ; and in fact these con- 
 strained vibrations are executed in every variety, but 
 the simple periodicity of the free atom is lost. 
 
 To study the free atoms we take a gas the rarer 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 347 
 
 the better : heat it, and then sort out the waves it 
 produces in the ether by putting a triangular prism 
 of bound ether in their path. 
 
 Why the bound ether retards different waves 
 differently, or "disperses" the light, is quite unknown, 
 beyond the fact that it has something to do with the 
 size of the atoms of matter being comparable to the 
 size of waves : being most nearly comparable to the 
 smallest waves, and therefore affecting them most. 
 It is not easy accurately to explain refraction, but 
 it is extremely difficult to explain dispersion. How- 
 ever, the fact is undoubted, and more light will 
 doubtless soon fall upon its theory. 
 
 The result of the prismatic analysis is to prove 
 that every atom of matter has its own definite rate 
 of vibration, as a bell has ; it may emit several 
 colours or only one, and the number it emits may 
 depend upon how much it is struck (err heated) ; but 
 those it can emit arc a perfectly definite selection, 
 and depend in no way on the previous history of 
 the atom. Every free atom of sodium, for instance, 
 vibrates in the same way, and has always vibrated 
 in the same way, whatever other clement it may 
 have been at intervals combined with, and whether 
 it exists in the sun or in the earth, or in the most 
 distant star. The same is true of every other 
 kind of matter, each has its own mode of vibration 
 which nothing but bondage changes ; and hence 
 
348 MODERN VIEWS OF ELECTRICITY. [LECT. IT. 
 
 has arisen a new chemical analysis, wherein sub- 
 stances are detected simply by observing the rate 
 of vibration of their free atoms, a branch of physical 
 chemistry called spectrum analysis. 
 
 The atoms are small bodies, and accordingly 
 vibrate with inconceivable rapidity. 
 
 An atom of sodium vibrates 5 x io 14 times in a 
 second ; that is, it executes five hundred million 
 complete vibrations in the millionth part of a second. 
 
 This is about a medium pace, and the waves it emits 
 produce in the eye the sensation of a deep yellow. 
 
 4 x io 14 corresponds to red light, 7 x io 14 to blue. 
 
 An atom of hydrogen has three different periods, 
 viz. 4*577, 6*179, and 6-973, each multiplied by the 
 inevitable io 14 . 
 
 Atoms may, indeed, vibrate more slowly than 
 this, but the retina is not constructed so as to be 
 sensible of slower vibrations ; however, thanks to 
 Capt. Abney, there are ways now of photographing 
 the effect of much slower vibrations, and thus of 
 making them indirectly visible ; so we can now 
 hope to observe the motion of atoms over a much 
 greater range than the purely optical ones, and so 
 learn much more about them. 1 
 
 1 Still more perhaps may we now hope from the modified line 
 thermopile or Siemens pyrometer, which Prof. Langley has so ably 
 developed and used in a series of fine researches : the instrument 
 which he calls the "bolometer." Or from Mr. Boys's still more recent 
 *' Radio-micrometer." 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 349 
 
 The distinction between free and bound ether is 
 forced on our notice by other phenomena than 
 those of light. When we come to electricity, we 
 find that some kind of matter has more electricity 
 associated with it than others, so that for a given 
 electromotive force we get a greater electric dis- 
 placement ; that the electricity is, as it were, denser 
 in some kinds of matter than in others. The density 
 of electricity in space being called I, that inside 
 matter is called K, its specific inductive capacity. In 
 optics the relative density of the ether inside matter 
 was n 2 , the square of the index of refraction (p. 343). 
 These numbers appear to be the same. 
 
 Is the ether electricity then ? I do not say so, 
 neither do I think that in that coarse statement 
 lies the truth ; but that they are connected there 
 can be no doubt. 
 
 What I have to suggest is that positive and 
 negative electricity together may make up the ether, 
 or that the ether may be sheared by electromo- 
 tive forces into positive and negative electricity. 
 Transverse vibrations are carried on by shearing 
 forces acting in matter which resists them, or which 
 possesses rigidity. The bound ether inside a con- 
 ductor has no rigidity ; it cannot resist shear ; such 
 a body is opaque. Transparent bodies are those 
 whose bound ether, when sheared, resists and 
 springs back again ; such bodies are dielectrics. 
 
350 MODERN VIEWS OF ELECTRICITY. [LKCT. ii. 
 
 We have no direct way of exerting force upon 
 ether at all ; we can, however, act on it in a very 
 indirect manner, for we have learnt how to arrange 
 matter so as to cause it to exert the required 
 shearing (or electromotive) force upon the ether 
 associated with it. Continuous shearing force applied 
 to the ether in metals produces a continuous and 
 barely resisted stream of the two electricities in 
 opposite directions : or a conduction current. 
 
 Continuous shearing force applied to the ether 
 in transparent bodies produces an electric displace- 
 ment accompanied by elastic resilience, and thus 
 all the phenomena of electric induction (Chap. III.). 
 
 Some chemical compounds, consisting of binary 
 molecules, distribute the bound ether of the molecule, 
 at any rate as soon as it is split up by dissociation ; 
 and, instead of each nascent radicle or atom taking 
 with it neutral ether, one takes a certain definite 
 quantity of positive, the other the same amount 
 of negative, electricity. In the liquid state the 
 atoms are capable of locomotion ; and a continuous 
 shearing force applied to the ether in such liquids 
 causes a continual procession of the matter and 
 associated electricity, the positive one way, and 
 the negative the other, and thus all the phenomena 
 -of electrolysis (Chap. IV.). 
 
 What I say about electricity, however, is not to be 
 taken without salt ; you will not regard it as recognized 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 351 
 
 truth, but as a tentative belief of your lecturer's which 
 may be found to be more or less, and possibly more 
 rather than less, out of accordance with facts. I can 
 only say that it hangs phenomena together, and that 
 it has been forced upon my belief in various ways. 
 
 Now what about the free ether of space, is it a con- 
 ductor of electricity ? There are certain facts which 
 suggest that it is, and Edlund has suggested that it is 
 an almost perfect conductor. When a sun-spot or 
 other disturbance breaks out on the sun, accompanied 
 as it is, no doubt, by violent electric storms, the electric 
 condition of the earth is affected, and we have auroras 
 and magnetic disturbances. Is this by induction 
 through space ? or can it be due to conduction and 
 the arrival of some microscopic portion of a derived 
 current travelling our way ? 
 
 For my part I cannot think the ether a conductor. 
 Maxwell has shown that conductors must be opaque, 
 and ether is nothing, if not transparent ; one is driven, 
 then, to conclude that what we call conduction does 
 not go on except in the presence of ordinary matter 
 in other words, perhaps, that it is a phenomenon more 
 connected with bound ether than with free. 
 
 But now, looking back to Fresnel's hypothesis of 
 the extra density of ether inside gross matter, and 
 also to the fact that it must be regarded as incom- 
 pressible, the question naturally arises, How can it be 
 clensified by matter or anything else ? Perhaps it is 
 
352 MODERN VIEWS OF ELECTRICITY. [LECT. n. 
 
 not ; perhaps matter only strains the ether towards 
 itself, thus slackening its tension, as it were, inside 
 bodies, not producing any real increase of density ; 
 and fchis is roughly McCullagh's form of the undulatory 
 theory. In this form gravitation may be held to be 
 partially explained ; for two bodies straining at the 
 ether in this way will tend to pull themselves together. 
 Newton himself dimly suggested, in one of the queries 
 appended to the later editions of his " Opticks," that 
 gravitation would be produced if only matter exerted 
 a kind of pressure on an all pervading ether, the pres- 
 sure varying as the inverse distance. (See Appendix.) 
 He did not follow the idea up, however, because 
 he had then no other facts to confirm him in his 
 impression of the existence of such an ether, or to 
 inform him concerning its properties. We now not 
 only feel sure that an ether exists, but we know some- 
 thing of its properties ; and we also have learnt 
 from light and from electricity, that some such action 
 between matter and ether actually occurs, though how 
 or why it occurs we do not yet know. I am therefore 
 compelled to believe that this is certainly the direction 
 in which an ultimate explanation of gravitation and 
 of cohesion is to be looked for. 
 
 In thinking over the Fresnel and McCullagh forms 
 of the undulatory theory, with a view to the recon- 
 ciliation between them which appears necessary and 
 
 
LECT. n.j THE ETHER AND ITS FUNCTIONS. 353 
 
 imminent, one naturally asks, is there any such clear 
 distinction to be drawn between ether and matter as 
 we have hitherto tacitly assumed ? may they not be 
 different modifications, or even manifestations, of the 
 same thing ? 
 
 Again, when we speak of atoms vibrating, how can 
 they vibrate ? of what are their parts composed ? 
 
 And now we come to one of the most remarkable 
 and suggestive speculations of modern times a 
 speculation based on this experimental fact, that the 
 elasticity of a solid may be accounted for by the 
 motion of a fluid ; that a fluid in motion may possess 
 rigidity. 
 
 I said that rigidity was precisely what no fluid 
 possessed : at rest this is true ; in motion it is not 
 true ( 156). 
 
 Consider a perfectly flexible india-rubber O-shaped 
 tube full of water ; nothing is more flaccid and limp. 
 But set the water rapidly circulating, and it becomes 
 at once stiff ; it will stand on end for a time without 
 support ; kinks in it take force to make, and are more or 
 less permanent. A practicable form of this experiment 
 is the well-known one of a flexible chain over a pulley, 
 which becomes stiff as soon as it is set in rapid 
 motion (page 1 66, footnote). 
 
 This is called a vortex stream-line, and a vortex is a 
 thing built up of a number of such stream-lines. If they 
 are arranged parallel to one another about a straight 
 
 A A 
 
354 MODERN VIEWS OF ELECTRICITY. [LECT. II. 
 
 axis or core, we have a vortex cylinder, such as is 
 easily produced by stirring a vessel of water or by 
 pulling the plug out of a wash-hand basin ; or such 
 as are made in the air on a large scale in America, 
 and telegraphed over here, when they are called 
 " cyclones," or " depressions." The depression is visible 
 enough in the middle of revolving water. These 
 vortices are wonderfully permanent things, and last a 
 long time, though they sometimes break up unex- 
 pectedly. 
 
 Vortices need not have straight cores : they 
 may have cores of various ring forms, the simplest 
 being a circle. To make a vortex ring, we must take 
 a plane disk of the fluid, and at a certain instant give 
 to every atom in the disk a certain velocity forward, 
 graduating the velocity according to its distance from 
 the edge of the disk. We have as yet no means of 
 doing this in a frictionless fluid, but with a fluid such 
 as air and water it happens to be easy ; we have only 
 to knock a little of the fluid suddenly out of a box 
 through a sharp-edged hole, and the friction of the 
 edges of the hole does what we want. The central 
 portion travels rapidly forward, and returns round 
 outside the core, rolling back towards the hole. But 
 the impetus sends the whole forward, and none really 
 returns ; it rolls on its outer circumference as a wheel 
 rolls along a road. In a perfect fluid under conceivable 
 circumstances it need not so roll forward, as there would 
 
LECT. ii.] THE ETHER AND ITS FUNCTIONS. 355 
 
 be no friction, but in air or water a vortex ring has 
 always a definite forward velocity, just as a locomotive 
 driving-wheel has when it does not slip on the rails. 
 
 We have in these rings a real mass of air moving 
 bodily forward, and it .impinges on a face or a gas 
 flame with some force. One is thus easily able to 
 blow out a distant gas flame ten or twelve yards away 
 by an invisible projectile of air. It is differentiated 
 from the rest of the atmosphere by reason of its 
 peculiar rotational motion. The ring may be rendered 
 visible by means of smoke, but it is in no way 
 improved by the addition except in the matter of 
 visibility. 
 
 The cores of these rings are elastic they possess 
 rigidity ; the circular is their stable form, and if this 
 is altered, they oscillate about it. Thus when two 
 vortex rings impinge or even approach fairly near one 
 another, they visibly deflect each other, and also cause 
 each other to vibrate. 
 
 The theory of the impact or interference of vortex 
 rings whose paths cross but which do not come very 
 near together, has been quite recently worked out by 
 Mr. J. J. Thomson. It is quite possible to make the 
 rings vibrate without any impact, by serrating the 
 opening out of which they are knocked. The simplest 
 serration of a circle turns it into an ellipse, and here 
 you have an elliptic ring oscillating from a tall to a 
 squat ellipse and back again. Here is a four-waved 
 
 A A 2 
 
356 MODERN VIEWS OF ELECTRICITY. [LECT. n. 
 
 opening, and the vibrations are by this very well 
 shown. A six-waved opening makes the vibrations 
 almost too small to be perceived at a distance, but 
 still they are sometimes distinct. 
 
 The rings vibrate very much like a bell vibrates : 
 perhaps very much like an atom vibrates. They have 
 rigidity, although composed of fluid : they are com- 
 posed of fluid in motion. These vortices are imper- 
 fect, they increase in size, and decrease in energy ; in 
 a perfect fluid they would not do this, they would 
 then be permanent and indestructible, but then also 
 you would not be able to make them. 
 
 Now does not the idea strike you that atoms of 
 matter may be vortices like these vortices in a perfect 
 fluid, vortices in the ether. This is Sir William 
 Thomson's theory of matter. It is not yet proved to 
 be true, but is it not highly beautiful ? a theory about 
 which one may almost dare to say that it deserves to 
 be true ? The atoms of matter, according to it, are 
 not so much foreign particles imbedded in the all- 
 pervading ether, as portions of it differentiated off 
 from the rest by reason of their vortex motion, thus 
 becoming virtually solid particles, yet with no transi- 
 tion of substance ; atoms indestructible and not able 
 to be manufactured, not mere hard rigid specks, but 
 each composed of whirling ether ; elastic, capable of 
 definite vibration, of free movement, of collision. 
 The crispations or crimpings of these rings illustrate 
 
LfiCT.II.JTHE ETHER AND ITS FUNCTIONS. 357 
 
 the kind of way in which we may suppose an atom to 
 vibrate. They appear to have all the properties of 
 atoms except one, viz. gravitation ; and before the 
 theory can be accepted, I think it must account for 
 gravitation. This fundamental property of matter 
 cannot be left over to be explained by an artificial 
 battery of ultra-mundane corpuscles. We cannot go 
 back to mere impact of hard bodies after having 
 allowed ourselves a continuous medium. Vortex 
 atoms must be shown to gravitate. 
 
 But then remember how small a force gravitation 
 is. Ask any educated man whether two pound- 
 masses of lead attract each other, and he will reply no. 
 He is wrong, of course, but the force is exceedingly 
 small. Yet it is the aggregate attraction of trillions 
 upon trillions of atoms ; the slightest effect of each 
 upon the ether would be sufficient to account for 
 gravitation ; and no one can say that vortices do not 
 exert some such residual, but uniform, effect on the 
 fluid in which they exist, till second, third, and every 
 other order of small quantities have been taken into 
 account, and the theory of vortices in a perfect fluid 
 worked out with the most final accuracy. 
 
 At present, however, the Thomsonian theory of 
 matter is not a verified one ; it is, perhaps, little more 
 than a speculation, but it is one that it is well worth 
 knowing about, working at, and inquiring into. It 
 may stand or it may fall ; but if it is the case, as I 
 
358 MODERN VIEWS OF ELECTRICITY. [LF.CT. 11. 
 
 believe it is, that our notions of natural phenomena, 
 though they often fall short, yet never exceed in 
 grandeur the real truth of things, how splendid must 
 be the real nature of matter if the Thomsonian 
 hypothesis turns out to be inadequate and untrue. 
 
 I have now endeavoured to introduce you to the 
 simplest conception of the material universe which 
 has yet occurred to man the conception, that is, of 
 one universal substance, perfectly homogeneous and 
 continuous and simple in structure, extending to the 
 furthest limits of space of which we have any know- 
 ledge, existing equally everywhere ; some portions 
 either at rest or in simple irrotational motion trans- 
 mitting the undulations which we call light ; other 
 portions in rotational motion, in vortices that is, 
 and differentiated permanently from the rest of the 
 medium by reason of this motion. 
 
 These whirling portions constitute what we call 
 matter ; their motion gives them rigidity, and of 
 them our bodies and all other material bodies with 
 which we are acquainted are built up. 
 
 One continuous substance filling all space : which 
 can vibrate as light ; which can be sheared into 
 positive and negative electricity ; which in whirls 
 constitutes matter ; and which transmits by continuity, 
 and not by impact, every action and reaction of which 
 matter is capable. This is the modern view of the 
 Ether and its functions. 
 
LECTURE III. 
 
 THE DISCHARGE OF A LEYDEN JAR. 1 
 
 IT is one of the great generalizations established by 
 Faraday, that all electrical charge and discharge is 
 essentially the charge and discharge of a Leyden jar. 
 It is impossible to charge one body alone. Whenever 
 a body is charged positively, some other body is ipso 
 facto charged negatively, and the two equal opposite 
 charges are connected by lines of induction. The 
 charges are, in fact, simply the ends of these lines, and 
 it is as impossible to have one charge without its cor- 
 relative as it is to have one end of a piece of string 
 without there being somewhere, hidden it may be, 
 split up into strands it may be, but somewhere existent, 
 the other end of that string. 
 
 This I suppose familiar fact that all charge is 
 virtually that of a Leyden jar being premised, our 
 subject for this evening is at once seen to be a very 
 
 1 Delivered at the Royal Institution of Great Britain, on Friday 
 evening, March 8, 1889. 
 
360 MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 wide one, ranging in fact over the whole domain of 
 electricity. For the charge of a Leyden jar includes 
 virtually the domain of electrostatics ; while the dis- 
 charge of a jar, since it constitutes a current, covers 
 the ground of current electricity all except that por- 
 tion which deals with phenomena peculiar to steady 
 currents. And since a current of electricity necessarily 
 magnetizes the space around it, whether it flow in a 
 straight or in a curved path, whether it flow through 
 wire or burst through air, the territory of magnetism 
 is likewise invaded ; and inasmuch as a Leyden jar 
 discharge is oscillatory, and we now know the vibra- 
 tory motion called light to be really an oscillating 
 electric current, the domain of optics is seriously 
 encroached upon. 
 
 But though the subject I have chosen would permit 
 this wide range, and though it is highly desirable to 
 keep before our minds the wide-reaching import of the 
 most simple-seeming fact in connection with such a 
 subject, yet to-night I do not intend to avail myself of 
 any such latitude, but to keep as closely and distinctly 
 as possible to the Leyden jar in its homely and well- 
 known form, as constructed out of a glass bottle, two 
 sheets of tinfoil, and some stickphast. 
 
 The act of charging such a jar I have permitted 
 myself now for some time to illustrate by the 
 mechanical analogy of an inextensible endless cord 
 able to circulate over pulleys, and threading in some 
 
LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 361 
 
 portion of its length a row of tightly-gripping 
 beads which are connected to fixed beams by elastic 
 threads. 
 
 The cord is to represent electricity ; the beads 
 represent successive strata in the thickness of the 
 glass of the jar, or, if you like, atoms of dielectric or 
 
 Fig- 55- Mechanical analogy of a circuit partly dielectric ; for instance, of a 
 charged condenser. A is its positive coat, v> its negative. 
 
 insulating matter. Extra tension in the cord repre- 
 sents negative potential, while a less tension (the 
 nearest analogue to pressure adapted to the circum- 
 stances) represents positive potential. Forces applied 
 to move the cord, such as winches or weights, are 
 electromotive forceps; a clamp- or fixed obstruction 
 represents a rheostat or contact-breaker ; and an 
 
362 MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 excess or defect of cord between two strata of matter 
 represents a positive or a negative charge. 
 
 The act of charging a jar is now quite easily 
 depicted as shown in the diagram. 
 
 To discharge the jar one must remove the charging 
 E.M.F. and unclamp the screw, i.e. close the circuit. 
 The stress in the elastic threads will then rapidly drive 
 the cord back, the inertia of the beads will cause it to 
 overshoot the mark, and for an instant the jar will 
 possess an inverse charge. Back again the cord 
 swings, however, and a charge of same sign as at first, 
 but of rather less magnitude, would be found in the 
 jar if the operation were now suspended. If it be 
 allowed to go on, the oscillations gradually subside, 
 and in a short time everything is quiescent, and the 
 jar is completed discharged. 
 
 All this occurs in the Leyden jar, and the whole 
 series of oscillations, accompanied by periodic reversal 
 and re-reversal of the charges of the jar, is all accom- 
 plished in the incredibly short space of time occupied 
 by a spark. 
 
 Consider now what the rate of oscillation depends 
 on. Manifestly on the elasticity of the threads and 
 on the inertia of the matter which is moved. Take 
 the simplest mechanical analogy, that of the vibration 
 of a loaded spring, like the reeds in a musical box. 
 The stifTer the spring and the less the load, the faster 
 it vibrates. Give a mathematician these data, and he 
 
I.ECT. in.] THE DISCHARGE OF A LEYDEN JAR. 363 
 
 will calculate for you the time the spring takes to 
 execute one complete vibration, the " period " of its 
 swing. [Loaded lath in vice.] 
 
 The electrical problem and the electrical solution are 
 precisely the same. That which corresponds to the 
 flexibility of the spring is in electrical language called 
 static capacity, or, by Mr. Heaviside, permittance. 
 That which corresponds to the inertia of ordinary 
 matter is electro-magnetic inertia, or self-induction, or 
 by Mr. Heaviside, inductance. 
 
 Increase either of these, and the rate of oscillation 
 is diminished. Increasing the static capacity corre- 
 sponds to lengthening the spring ; increasing the 
 self-induction corresponds to loading it. 
 
 Now the static capacity is increased simply by 
 using a larger jar, or by combining a number of jars 
 into a battery in the very old-established way. Increase 
 in the self-induction is attained by giving the discharge 
 more space to magnetize, or by making it magnetize 
 a given space more strongly. For electro-magnetic 
 inertia is wholly due to the magnetization of the space 
 surrounding a current, and this space may be increased, 
 or its magnetization intensified, as much as we please. 
 
 To increase the space we have only to make the 
 discharge take a long circuit instead of a short one. 
 Thus we may send it by a wire all round the room, or 
 by a telegraph wire all round a town, and all the space 
 inside it and some of that outside will be more or less 
 
364 MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 magnetized. More or less, I say, and it is a case of 
 less rather than more. Practically very little effect is 
 felt except close to the conductor, and accordingly the 
 self-induction increases very nearly proportionally to 
 the length of the wire, and not in proportion to the 
 area inclosed : provided also the going and return wires 
 are kept a reasonable distance apart, so as not to 
 encroach upon each other's appreciably magnetized 
 regions. See Appendix (e). 
 
 But it is just as effective, and more compact, to 
 intensify the magnetization of a given space by send- 
 ing the current hundreds of times round it instead of 
 only once ; and this is done by inserting a coil of wire 
 into the discharge circuit. 
 
 Yet a third way there is of increasing the 
 magnetization of a given space, and that is to fill it 
 with some very magnetizable subtance such as iron. 
 This, indeed, is a most powerful method under many 
 circumstances, it being possible to increase the 
 magnetization and therefore the self-induction or 
 inertia of the current some 5000 times by the use 
 of iron. 
 
 But in the case of the discharge of a Leyden jar 
 iron is of no advantage. The current oscillates so 
 quickly that any iron introduced into its circuit, 
 however subdivided into thin wires it may be, is 
 protected from magnetism by inverse currents induced 
 in its outer skin, as your Professor of Natural 
 
-LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 365 
 
 Philosophy 1 has shown, and accordingly it does not 
 get magnetized ; and so far from increasing the 
 inductance of the discharge circuit it positively 
 diminishes it by the reaction effect of these induced 
 currents : it acts, in fact, much as a mass of copper 
 might be expected to do. 
 
 The conditions determining rate of oscillation 
 being understood, we have next to consider what 
 regulates the damping out of the vibrations, i.e. the 
 total duration of the discharge. 
 
 Resistance is one thing. To check the oscillations 
 of a vibrating spring you apply to it friction, or 
 make it move in a viscous medium, and its vibrations 
 are speedily damped out. The friction may be made 
 so great that oscillations are entirely prevented, the 
 motion being a mere dead-beat return to the position 
 of equilibrium ; or, again, it may be greater still, and 
 the motion may correspond to a mere leak or slow 
 sliding back, taking hours or days for its accomplish- 
 ment. With very large condensers, such as are used 
 in telegraphy, this kind of discharge is frequent, but 
 in the case of a Leyden jar discharge it is entirely 
 exceptional. It can be caused by including in the 
 circuit a wet string, or a capillary tube full of distilled 
 water, or a slab of wood, or other atrociously bad 
 conductor of that sort ; but the conditions ordinarily 
 associated with the discharge of a Leyden jar, 
 
 1 Lord Rayleigh. 
 
366 MODERN VIEWS OF ELECTRICITY. [LECT. m. 
 
 whether it discharge through a long or a short wire, 
 or simply through its tongs, or whether it overflow its 
 edge or puncture its glass, are such as correspond 
 to oscillations, and not to leak. [Discharge jar first 
 through wire and next through wood.] 
 
 When the jar is made to leak through wood or 
 water the discharge is found to be still not steady : it 
 is not oscillatory indeed, but it is intermittent. It 
 occurs in a series of little jerks, as when a thing is 
 made to slide over a resined surface. The reason of 
 this is that the terminals discharge faster than the 
 circuit can supply the electricity, and so the flow is 
 continually stopped and begun again. 
 
 Such a discharge as this, consisting really of a 
 succession of small sparks, may readily appeal to the 
 eye as a single flash, but it lacks the noise and 
 violence of the ordinary discharge ; and any kind 
 of moving mirror will easily analyze it into its 
 constituents and show it to be intermittent. [Shake 
 a mirror, or waggle head or opera-glass.] 
 
 It is pretty safe to say, then, that whenever a jar 
 discharge is not oscillatory it is intermittent, and 
 when not intermittent is oscillatory. There is an 
 intermediate case when it is really dead-beat, but it 
 could only be hit upon with special care, while its 
 occurrence by accident must be rare. 
 
 So far I have only mentioned resistance or friction 
 as the cause of the dying out of the vibrations ; but 
 
LECT. Hi.] THE DISCHARGE OF A LEYDEN JAR. 367 
 
 there is another cause, and that a most exciting 
 one. 
 
 The vibrations of a reed are damped partly indeed 
 by friction and imperfect elasticity, but partly also by 
 the energy transferred to the surrounding medium 
 and consumed in the production of sound. It is the 
 formation and propagation of sound-waves which 
 largely damp out the vibrations of any musical 
 instrument. So it is also in electricity. The 
 oscillatory discharge of a Leyden jar disturbs the 
 medium surrounding it, carves it into waves which 
 travel away from it into space : travel with a velocity 
 of 185,000 miles a second : travel precisely with the 
 velocity of light. [Tuning-fork.] 
 
 The second cause, then, which damps out the 
 oscillations in a discharge circuit is radiation : 
 electrical radiation if you like so to distinguish it, but 
 it differs in no respect from ordinary radiation (or 
 "radiant heat" as it has so often been called in 
 this place) ; it differs in no respect from Light except 
 in the physiological fact that the retinal mechanism, 
 whatever it may be, responds only to waves of a 
 particular, and that a very small, size, while radiation 
 in general may have waves which range from 10,000 
 miles to a millionth of an inch in length. 
 
 The seeds of this great discovery of the nature of 
 light were sown in this place : it is all the outcome of 
 Faraday's magneto-electric and electrostatic indue- 
 
368 MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 tion : the development of them into a rich and full- 
 blown theory was the greatest part of the life-work of 
 Clerk Maxwell : the harvest of experimental verifica- 
 tion is now being reaped by a German. But by no 
 ordinary German. Dr. Hertz, now Professor in 
 the University of Bonn, is a young investigator 
 of the highest type. Trained in the school of 
 Helmholtz, and endowed with both mathematical 
 knowledge and great experimental skill, he has 
 immortalized himself by a brilliant series of investiga- 
 tions which have cut right into the ripe corn of scien- 
 tific opinion in these islands, and by the same strokes 
 as have harvested the grain have opened up wide and 
 many branching avenues to other investigators. 
 
 At one time I had thought of addressing you this 
 evening on the subject of these researches of Hertz, 
 but the experiments are not yet reproducible on a 
 scale suited to a large audience, and I have been so 
 closely occupied with some not wholly dissimilar, but 
 independently conducted, researches of my own- 
 researches led up to through the unlikely avenue of 
 lightning-conductors that I have had as yet no 
 time to do more than verify some of them for my 
 own edification ( 189). 
 
 In this work of repetition and verification Prof. 
 Fitzgerald has, as related in a recent number of 
 NATURE (vol. xxxix. p. 391), probably gone further ; 
 and if I may venture a suggestion to your Honorary 
 
LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 369 
 
 Secretary, I feel sure that a discourse on Hertz's 
 researches from Prof. Fitzgerald next year would be 
 not only acceptable to you, but would be highly 
 conducive to the progress of science. 
 
 I have wandered a little from my Leyden jar, and 
 I must return to it and its oscillations. Let me very 
 briefly run over the history of our knowledge of the 
 oscillatory character of a Leyden jar discharge. It 
 was first clearly realized and distinctly stated by that 
 excellent experimentalist, Joseph Henry, of Wash- 
 ington, a man not wholly unlike Faraday in his mode 
 of work, though doubtless possessing to a less degree 
 that astonishing insight into intricate and obscure 
 phenomena ; wanting also in Faraday's circumstantial 
 advantages. 
 
 This great man arrived at a conviction that the 
 Leyden jar discharge was oscillatory, by studying the 
 singular phenomena attending the magnetization of 
 steel needles by a Leyden jar discharge, first observed 
 in 1824 by Savary. Fine needles, when taken out of 
 the magnetizing helices, were found to be not always 
 magnetized in the right direction, and the subject is re- 
 ferred to in German books as " anomalous magnetiza- 
 tion." It is not the magnetization which is anomalous, 
 but the currents which have no simple direction ; and 
 we find in a memoir published by Henry in 1842, the 
 following words : 
 
 B B 
 
370 MODERN VIEWS OF ELECTRICITY. [LRCT. in. 
 
 " This anomaly, which has remained so long unex- 
 plained, and which, at first sight, appears at variance 
 with all our theoretical ideas of the connection of 
 electricity and magnetism, was, after considerable 
 study, satisfactorily referred by the author to an 
 action of the discharge of the Leyden jar which had 
 never before been recognized. The discharge, what- 
 ever may be its nature, is not correctly represented 
 (employing for simplicity the theory of Franklin) by 
 the single transfer of an imponderable fluid from one 
 side of the jar to the other ; the phenomenon requires 
 us to admit the existence of a principal discharge in one 
 direction and then several reflex actions backivard and 
 forward, each more feeble tJian the preceding, until the 
 equilibrium is obtained. All the facts are shown to 
 be in accordance with this hypothesis, and a ready 
 explanation is afforded by it of a number of pheno- 
 mena, which are to be found in the older works 
 on electricity, but which have until this time 
 remained unexplained." l 
 
 The italics are Henry's. Now if this were an 
 isolated passage it might be nothing more than a 
 lucky guess. But it is not. The conclusion is one 
 at which he arrives after a laborious repetition and 
 serious study of the facts, and he keeps the idea con- 
 stantly before him when once grasped, and uses it in 
 
 1 Scientific Writings of Joseph Henry , vol. i. p. 201. Published 
 by the Smithsonian Institution, Washington, 1886. 
 
LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 371 
 
 all the rest of his researches on the subject. The facts 
 studied by Henry do in my opinion support his 
 conclusion, and if I am right in this it follows that he 
 is the original discoverer of the oscillatory character of 
 a spark, although he does not attempt to state its 
 theory. That was first done, and completely done, in 
 1853, by Sir William Thomson ; and the progress of 
 experiment by Feddersen, Helmholtz, Schiller, and 
 others has done nothing but substantiate it. 
 
 The writings of Henry have been only quite 
 recently collected and published by the Smithsonian 
 Institution of Washington in accessible form, and 
 accordingly they have been far too much ignored. 
 The two volumes contain a wealth of beautiful ex- 
 periments clearly recorded, and well repay perusal. 
 
 The discovery of the oscillatory character of a 
 Leyden jar discharge may seem a small matter, but it 
 is not. One has only to recall the fact that the 
 oscillators of Hertz are essentially Leyden jars one 
 has only to use the phrase " electro-magnetic theory 
 of light " to have some of the momentous issues of 
 this discovery flash before one. 
 
 One more extract I must make from that same 
 memoir by Henry, 1 and it is a most interesting one : 
 it shows how near he was, or might have been, to 
 obtaining some of the results of Hertz ; though, if he 
 had obtained them, neither he nor any other experi- 
 
 1 Loc. tit., p. 204. 
 
 B B 2 
 
372 MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 mentalist could possibly have divined their real 
 significance. 
 
 It is, after all, the genius of Maxwell and of a few 
 other great theoretical physicists whose names are on 
 everyone's lips l which endows the simple induction 
 experiments of Hertz and others with such stupendous 
 importance. 
 
 Here is the quotation : 
 
 " In extending the researches relative to this part 
 of the investigations, a remarkable result was obtained 
 in regard to the distance at which induction effects are 
 produced by a very small quantity of electricity ; a 
 single spark from the prime conductor of a machine, 
 of about an inch long, thrown on to the end of a cir- 
 cuit of wire in an upper room, produced an induction 
 sufficiently powerful to magnetize needles in a parallel 
 circuit of iron placed in the cellar beneath, at a per- 
 pendicular distance of 30 feet, with two floors and 
 ceilings, each 14 inches thick, intervening. The author 
 is disposed to adopt the hypothesis of an electrical 
 plenum " [in other words, of an ether], " and from the 
 foregoing experiment it would appear that a single 
 spark is sufficient to disturb perceptibly the electricity 
 
 1 And of one whose name is not yet on everybody's lips, but whose 
 profound researches into electro-magnetic waves have penetrated further 
 than anybody yet understands into the depths of the subject, and whose 
 papers have very likely contributed partly to the theoretical inspiration 
 of Hertz I mean that powerful mathematical physicist, Mr. Oliver 
 Ileaviside, 
 
LECT. in] THE DISCHARGE OF A LEYDEN JAR. 373 
 
 of space throughout at least a cube of 400,000 feet of 
 capacity; and when it is considered that the magnetism 
 of the needle is the result of the difference of two 
 actions, it may be further inferred that the diffusion of 
 motion in this case is almost comparable with that of 
 a spark from a flint and steel in the case of light." 
 
 Comparable it is, indeed, for we now know it to be 
 the self-same process. 
 
 One immediate consequence and easy proof of the 
 oscillatory character of a Leyden jar discharge is the 
 occurrence of phenomena of sympathetic resonance. 
 
 Everyone knows that one tuning-fork can excite 
 another at a reasonable distance if both are tuned to 
 the same note. Everyone knows, also, that a fork can 
 throw a stretched string attached to it into sympathetic 
 vibration if the two are tuned to unison or to some 
 simple harmonic. Both these facts have their electrical 
 analogue. I have not time to go fully into the matter 
 to-night, but I may just mention the two cases which 
 I have myself specially noticed. 
 
 A Leyden jar discharge can so excite a similarly- 
 timed neighbouring Leyden jar circuit as to cause the 
 latter to burst its dielectric if thin and weak enough. 
 The well-timed impulses accumulate in the neighbour- 
 ing circuit till they break through a quite perceptible 
 thickness of air. 
 
 Put the circuits out of unison, by varying the capacity 
 or by including a longer wire in one of them ; then, 
 
374 MODERN VIEWS OF ELECTRICITY. [LECT. Hi. 
 
 although the added wire be a coil of several turns, well 
 adapted to assist mutual induction as ordinarily under- 
 stood, the effect will no longer occur. It can be 
 obtained again by diminishing the static capacity. 
 
 That is one case, and it is the electrical analogue of 
 one tuning-fork exciting another. It is too small at 
 present to show here satisfactorily, for I only recently 
 observed it, but it is exhibited in the library at the back. 
 
 The other case, analogous to the excitation of a 
 stretched string of proper length by a tuning-fork, I 
 published last year under the name of the experiment 
 of the recoil kick ; where a Leyden jar circuit sends 
 waves along a wire connected by one end with it, 
 which waves splash off at the far end with an electric 
 brush or long spark. 
 
 I will show merely one phase of it to-night, and 
 that is the reaction of the impulse accumulated in the 
 wire upon the jar itself, causing it to either overflow 
 or burst. [Sparks of gallon or pint jar made to over- 
 flow by wire round room. 1 ] 
 
 1 During the course of this experiment, the gilt paper on the wall was 
 observed by the audience to be sparkling, every gilt patch over a certain 
 area discharging into the next, after the manner of a spangled jar. It 
 was probably due to some kind of sympathetic resonance. Electricity 
 splashes about in conductors in a surprising way everywhere in the 
 neighbourhood of a discharge. For instance, a telescope in the hand 
 of one of the audience was reported afterwards to be giving off little 
 sparks at every discharge of the jar. Everything which happens to have 
 a period of electric oscillation corresponding to some harmonic of the 
 main oscillation of a discharge is liable to behave in this way. When 
 light falls on an opaque surface it is quenched ; producing minute 
 electric currents, which subside into heat. What the audience saw was 
 
LKCT. in.] THE DISCHARGE OF A LEYDEN JAR. 375 
 
 The early observations by Franklin on the bursting 
 of Leyden jars, and the extraordinary complexity 
 or multiplicity of the fracture that often results, are 
 most interesting. (See Electrician for March 29 and 
 April 5, 1889.) 
 
 His electric experiments as well as Henry's well 
 repay perusal, though of course they belong to the 
 infancy of the subject. 
 
 He notes the striking fact that the bursting of a jar 
 is an extra occurrence it does not replace the ordinary 
 discharge in the proper place, it accompanies it ; and 
 we now know that it is precipitated by it, that the 
 spark occurring properly between the knobs sets up 
 such violent surgings that the jar is far more violently 
 strained than by the static charge or mere difference 
 of potentials between its coatings ; and if the surgings 
 are at all even roughly properly timed, the jar is 
 bound to either overflow or burst. 
 
 Hence a jar should always be made without a lid, 
 and with a lip protruding a carefully considered dis- 
 tance above its coatings : not so far as to fail to act 
 as a safety valve, but far enough to prevent overflow 
 under ordinary and easy circumstances. 
 
 probably the result of waves of electrical radiation being quenched or 
 reflected by the walls of the room, and generating electrical currents 
 in the act ( 166). It is these electric surgings which render such 
 severe caution necessary in the erection of lightning-conductors. 
 
 This explanation has since been entirely confirmed by similar 
 occurrences in other places. 
 
376 MODERN VIEWS OF ELECTRICITY [LECT. in. 
 
 And now we come to what is after all the main 
 subject of my discourse this evening, viz. the optical 
 and audible demonstration of the oscillations occur- 
 ring in the Leyden jar spark. Such a demonstration 
 has, so far as I know, never before been attempted, but 
 if nothing goes wrong we shall easily accomplish it. 
 
 And first I will do it audibly. To this end the 
 oscillations must be brought down from their extra- 
 ordinary frequency of a million or hundred thousand 
 a second to a rate within the limits of human audition. 
 One does it exactly as in the case of the spring one 
 first increases the flexibility and then one loads it. 
 [Spark from battery of jars and varying sound of 
 same.] 
 
 Using the largest battery of jars at our disposal, I 
 take the spark between these two knobs not a long 
 spark, J inch will be quite sufficient. Notwithstanding 
 the great capacity, the rate of vibration is still far 
 above the limit of audibility, and 'nothing but the 
 customary crack is heard. I next add inertia to the 
 circuit by including a great coil of wire, and at once 
 the spark changes character, becoming a very shrill 
 but an unmistakable whistle, of a quality approxi- 
 mating to the cry of a bat. Add another coil, and 
 down comes the pace once more, to something like 
 5000 per second, or about the highest note of a piano. 
 Again and again I load the circuit with magnetiza- 
 bility, and at last the spark has only 500 vibrations 
 
LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 377 
 
 a second, giving the octave, or perhaps the double 
 octave, above the middle C. 
 
 One sees clearly why one gets a musical note : the 
 noise of the spark is due to a sudden heating of the 
 air ; now if the heat is oscillatory, the sound will be 
 oscillatory too, but both will be an octave above the 
 electric oscillation, if I may so express it, because 
 two heat-pulses will accompany every complete electric 
 vibration, the heat production being independent of 
 direction of current. 
 
 Having thus got the frequency of oscillation down 
 to so manageable a value, the optical analysis of it 
 presents no difficulty : a simple looking-glass waggled 
 in the hand will suffice to spread out the spark into 
 a serrated band, just as can be done with a singing 
 or a sensitive flame : a band - too of very much the 
 same appearance. 
 
 Using an ordinary four-square rotating mirror driven 
 electro-magnetically at the rate of some two or three 
 revolutions per second, the band is at the lowest pitch 
 seen to be quite coarsely serrated ; and fine serrations 
 can be seen, with four revolutions per second, in even 
 the shrill whistling sparks. 
 
 The only difficulty in seeing these effects is to catch 
 them at the right moment. They are only visible for 
 a minute fraction of a revolution, though the band 
 may appear drawn out to some length. The further 
 away a spark is from the mirror, the more drawn 
 
378 MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 out it is, but also the less chance there is of 
 catching it. 
 
 With a single observer it is easy to arrange a contact 
 maker on the axle of the mirror which shall bring on 
 the discharge at the right place in the revolution, and 
 the observer may then conveniently watch for the 
 image in a telescope or opera-glass ; though at the 
 lower pitches nothing of the kind is necessary. 
 
 But to show it to a large audience various plans can 
 be adopted. One is to arrange for several sparks 
 instead of one ; another is to multiply images of a 
 single spark by suitably adjusted reflectors, which if 
 they are concave will give magnified images ; another 
 is to use several rotating mirrors ; and indeed I do use 
 two, one adjusted so as to suit the spectators in the 
 gallery. 
 
 But the best plan that has struck me is to combine 
 an intermittent and an oscillatory discharge. Have 
 the circuit in two branches, one of high resistance so 
 as to give intermittences, the other of ordinary resist- 
 ance so as to be oscillatory, and let the mirror analyze 
 every constituent of the intermittent discharge into a 
 serrated band. There will thus be not one spark, but 
 a multitude of successive sparks, close enough together 
 to sound almost like one, separate enough in the 
 rotating mirror to be visible on all sides at once. 
 
 But to achieve it one must have great exciting 
 power. In spite of the power of this magnificent 
 
LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 379 
 
 Wimshurst machine, it takes some time to charge 
 up our great Leyden battery, and it is tedious waiting 
 for each spark. A Wimshurst does admirably for 
 a single observer, but for a multitude one wants an 
 instrument which shall charge the battery not once 
 only but many times over, with overflows between, 
 and all in the twinkling of an eye. 
 
 To get this I must abandon my friend Mr. 
 Wimshurst, and return to Michael Faraday. In 
 front of the table is a great induction coil ; its 
 secondary has the resistance needed to give an inter- 
 mittent discharge. The quantity it supplies at a 
 single spark will fill our jars to overflowing several 
 times over. The discharge circuit and all its circum- 
 stances shall remain unchanged. [Excite jars by 
 coil.] 
 
 Running over the gamut with this coil now used 
 as our exciter instead of the Wimshurct machine 
 everything else remaining exactly as it was you 
 hear the sparks give the same notes as before, but 
 with a slight rattle in. addition, indicating inter- 
 mittence as well as alternation. Rotate the mirror, 
 and everyone should see one or other of the serrated 
 bands of light at nearly every break of the primary 
 current of the coil. [Rotating mirror to analyze 
 sparks.] 
 
 The musical sparks which I have now shown you 
 
38o MODERN VIEWS OF ELECTRICITY. [LECT. in. 
 
 were obtained by me during a special digression l 
 which I made while examining the effect of dis- 
 charging a Leyden jar round heavy glass or bi- 
 sulphide of carbon. The rotation of the plane of 
 polarization of light by a steady current, or by a 
 magnetic field of any kind properly disposed with 
 respect to the rays of light, is a very familiar one in 
 this place. Perhaps it is known also that it can be 
 done by a Leyden jar current. But I do not think it 
 is ; and the fact seems to me very interesting. It is 
 not exactly new in fact, as things go now it may be 
 almost called old, for it was investigated six or seven 
 years ago by two most highly skilled French experi- 
 menters, Messrs. Bichat and Blondlot. 
 
 But it is exceedingly interesting as showing how 
 short a time, how absolutely no time, is needed by 
 heavy glass to throw itself into the suitable rotatory 
 condition. Some observers have thought they had 
 proved that heavy glass requires time to develop the 
 effect, by spinning it between the poles of a magnet 
 and seeing the effect decrease ; but their conclusions 
 cannot be right, for the polarized light follows every os- 
 cillation in a discharge, the plane of polarization being 
 waved to and fro as often as 70,000 times a second 
 in my own observation. (See Phil. Mag. April 1889.) 
 
 1 Most likely it was a conversation which I had with Sir Wm. 
 Thomson, at Christmas, which caused me to see the interest of getting 
 slow oscillations. My attention has mainly been directed to getting 
 them quick. 
 
LECT. in.] THE DISCHARGE OF A LEYDEN JAR. 381 
 
 Very few persons in the world have seen the effect. 
 In fact, I doubt if anyone had seen it a month ago 
 except Messrs. Bichat and Blondlot. But I hope to 
 make it visible to most persons here, though I hardly 
 hope to make it visible to all. 
 
 Returning to the Wimshurst machine as exciter, I 
 pass a discharge round the spiral of wire inclosing this 
 long tube of CS 2 , and the analyzing Nicol being 
 turned to darkness, there may be seen a faint by 
 those close to not so faint, but a very momentary 
 restoration of light on the screen at every spark. 
 [CS 2 tube experiment on screen.] 
 
 Now I say that this light restoration is also 
 oscillatory. One way of proving this fact is to insert 
 a biquartz between the Nicols. With a steady current 
 it constitutes a sensitive detector of rotation, its sen- 
 sitive tint turning green on one side and red on the 
 other. But with this oscillatory current a biquartz 
 does absolutely nothing. [Biquartz.] 
 
 That is one proof. Another is that rotating 
 the analyzer either way weakens the extra bright- 
 ening of the field, and weakens it equally either 
 way. 
 
 But the most convincing proof is to reflect the light 
 coming through the tube upon our rotating mirror, 
 and to look now not at the spark, or not only at the 
 spark, but at the faint band into which the last residue 
 of light coming through polarizer and tube and 
 
382 MODERN VIEWS OF ELECTRICITY [LECT. in. 
 
 analyzer is drawn out. [Analyze the light in rotating 
 mirror.] 
 
 At every discharge this faint streak brightens in 
 places into a beaded band : these are the oscilla- 
 tions of the polarized light ; and when examined 
 side by side they are as absolutely synchronous 
 with the oscillations of the spark itself as can be 
 perceived. 
 
 Out of a multitude of phenomena connected with 
 the Leyden jar discharge I have selected a few only 
 to present to you here this evening. Many more 
 might have been shown, and great numbers more are 
 not at present adapted for presentation to an audience, 
 being only visible with difficulty and close to. 
 
 An old and trite subject is seen to have in the 
 light of theory an unexpected charm and brilliancy. 
 So it is with a great number of other old familiar 
 facts at the present time. 
 
 The present is an epoch of astounding activity in 
 physical science. Progress is a thing of months and 
 weeks, almost of days. The long line of isolated 
 ripples of past discovery seem blending into a mighty 
 wave, on the crest of which one begins to discern 
 some oncoming magnificent generalization. The sus- 
 pense is becoming feverish, at times almost painful. 
 One feels like a boy who has been long strumming on 
 the silent keyboard of a deserted organ, into the chest 
 
LKCT. ill.] THE DISCHARGE OF A LEYDEN JAR. 383 
 
 of which an unseen power begins to blow a vivifying 
 breath. Astonished, he now finds that the touch of a 
 finger elicits a responsive note, and he hesitates, half 
 delighted, half affrighted, lest he be deafened by the 
 chords which it would seem he can now summon 
 forth almost at will. 
 
APPENDIX. 
 
 c c 
 
APPENDIX. 
 
 CERTAIN portions of electrical science have recently come 
 into considerable prominence, and, as they are hardly satis- 
 factorily treated in text-books yet, it may be a help to students 
 to say something about them here in less popular language 
 than in the body of the book. 
 
 Electro-magnetism . 
 
 (a] The fundamental fact of electro-magnetism, ascertained 
 by direct experiment, is that a circuit conveying a current 
 exactly imitates a magnet of definite moment, the equivalent 
 moment being 
 
 ml = /zAC, 
 
 where A is the mean area of the coil, n the number of turns of 
 wire, C the current, and /z a constant characteristic of the 
 medium inside the coil, whose absolute value we have as yet 
 no means of ascertaining ( 68, 69, 127). 
 
 Magnetic Induction, Reluctance, and Permeability. 
 
 (b) The intensity of magnetic field at a distance r from a pole 
 of strength m is ^, and this may be called the number of lines 
 
 of force (or tubes if the idea be preferred) per unit area. The 
 total number of lines offeree through a spherical surface of this 
 
 radius is X 4 nr", or fcrm. 
 
 C C 2 
 
388 APPENDIX. 
 
 This number must likewise thread any closed surface what- 
 ever inclosing the pole ; and in fact it is the number the pole 
 possesses. It may be called the total magnetic flux or displace- 
 ment, or the total magnetic induction, due to the pole ; the 
 name " induction," first used vaguely in the sense of influence by 
 Faraday, having been given this definite connotation by Maxwell. 
 The same expression likewise gives the number of lines of force 
 due to a complete magnet ; for the superposition of lines due to an 
 equal opposite pole curves the original lines but alters not their 
 number. With two detached poles the lines simply go from 
 one to the other. With a complete magnet the lines all form 
 closed loops extending from north to south through air, and 
 back through steel. In the case of a coil they likewise are 
 closed loops, all threading the coil and then spreading out 
 through the surrounding medium. In all real cases, therefore, 
 the lines of force form closed curves. Magnetic circuits are 
 always closed, just as electric circuits are. 
 
 Take the simplest case of an anchor-ring coil, a helix bent 
 into a closed circuit (like Fig. 47 or 29) : all its lines are then 
 
 inside it, and their total number, being 477;;?, is 4^M ; where 
 
 / is the mean circumference of the anchor-ring, or length of the 
 magnetic circuit. This is called the total flux of magnetic 
 induction, or briefly the total induction, and we will denote it 
 by I. 
 
 Now, in the analogous case of a voltaic circuit, the current 
 is ratio of electromotive force to resistance, and the resistance 
 
 may be written ; K being specific conductivity, and A sectional 
 
 area of conductor of length /. 
 
 To bring out the analogy, we shall write the magnetic flux 
 
 T 47T/2C 
 1 r~> 
 
 where the numerator is sometimes called magneto-motive force, 
 
APPENDIX. 389 
 
 and the denominator magnetic resistance, or preferably, as 
 suggested by Mr. Heaviside, magnetic reluctance. Obviously /* 
 takes the place of electric conductivity, and is a sort of mag- 
 netic conductivity : it was from this point of view that Sir W. 
 Thomson long ago christened it " permeability " (see 82). 
 
 If the magnetic circuit is not so simply constituted, but is 
 composed of portions of different areas, length, and material in 
 series as the magnetic circuit of a dynamo is, for instance 
 the magnetic reluctance can be written (still pursuing the 
 analogy) 
 
 R = -4- + -4- + . 
 
 and I = --? 
 K. 
 
 Mutual Induction. 
 
 (c] If a single turn of secondary wire surround this closed 
 magnetic circuit, as in Fig. 47, the total induction through it, 
 whatever its shape or size, is just I ; and if it surround the ring 
 n' times, the effective total induction is n'l. This is the induc- 
 tion of the primary through the secondary, which, written out 
 in full, is 
 
 The relation is a mutual one ; and if the same current 
 were to flow in secondary, the same number of lines would 
 thread effectively the primary. Hence we call it mutual in- 
 duction, and write it MC : where M, the coefficient of mutual 
 induction between the two coils, is 
 
 M = iirptrn A . 
 
 the A and the / referring most easily to the simply and obviously 
 closed magnetic circuit. Two detached coils situated anyhow 
 
390 APPENDIX. 
 
 with respect to each other, will have a specifiable value of M, 
 but it is not so easy to write down. 
 
 Self-induction. 
 
 (d ) Instead of using a secondary coil to surround the induction 
 caused by the primary, we may consider the primary as sur- 
 rounding the induction itself has produced, and so speak of its 
 " self-induction " as 
 
 / 
 which, written LC, gives us the coefficient of self-induction 
 
 L = 
 or, 
 
 where ;/ t = number of turns per unit length. ( 115 and 98) 
 
 Here, again, every coil has a specifiable self-induction, but in 
 most cases it is not so easy to write down. It always means, 
 however, the ratio of the self-produced magnetic induction to 
 the current which has produced it 
 
 Value of Coefficient of Self- Induction in a few other Simple 
 
 Cases. 
 
 (e) The magnetic field produced by a straight wire varies in- 
 versely with the distance ; being, at a distance r from a straight 
 wire of sectional radius a 3 conveying a current, C 
 
 and this therefore specifies the number of lines through unit 
 area. 
 
APPENDIX. 391 
 
 So the whole number of lines of force included between the 
 wire and any distance b, in a drum of thickness /, is 
 
 Now, if at the distance b there is a parallel wire, conveying the 
 return current, it, too, will have the same number of lines of 
 force, and the whole number lying between a length, /, of each 
 of the two parallel wires is 
 
 ^ X C; 
 
 and as all the lines of force that exist pass between the 
 wires, this expression sums up the whole magnetic flux pro- 
 duced by the going and return parallel currents ; and the co- 
 efficient of C in the last expression is therefore the coefficient of 
 self-induction for the case of two thin parallel wires at a 
 distance b. 
 
 For a circular loop of radius r, radius of section of wire being 
 , this modifies itself to 
 
 Sr 
 L = 47r/zr log 
 
 (see 140). In every case /* refers to the space near the wire, 
 not to the substance of the wire itself. 
 
 In both these cases, the magnetization of the substance of the 
 wires themselves is supposed nil. In the case of extremely 
 rapidly alternating currents, this is correct ( 47). In the case 
 of copper wires not too close together, it is never very incorrect. 
 
 Energy of a Current. 
 
 (/) A magnet of moment ml, in a magnetic field of intensity 
 H, experiences a couple //H sin 6 ; and therefore a simple 
 
392 APPENDIX. 
 
 stiff coil of wire conveying a current experiences a couple 
 /xACH sin 6. If it turns a small angle, d6, the work done, or 
 the change of potential energy, is /n;;ACH sin 6 dB ; and there- 
 fore the potential energy of the circuit in any position is 
 -fifzACH cos 6 ; which may be written 1C, because nA. cos 6 
 is the effective area of the coil resolved perpendicularly to the 
 lines of force which thread it to the number /uH per unit area. 
 
 This result may be generalized ; a current in a magnetic field 
 always possesses energy 1C. If the field is due to external 
 causes, i.e. having an existence independent of the current, the 
 energy is potential energy of strain, and tends to cause the cir- 
 cuit to rotate. This is the principle of electric motors. But if 
 the field is due to nothing but the current itself if it is a self- 
 produced and self-maintained field the value of I is LC, and 
 the energy is now more conveniently called kinetic energy. 
 To obtain its value, we must remember that the induction and 
 the current die out together : it is not as if they had an 
 independent existence, and so the energy is 
 
 f. 
 
 UC 
 
 This is the work which must be done at starting and at 
 stopping the current (Chap. V.). 
 
 Pole near a Circuit. 
 
 (g) If a single pole find itself on the axis of a circle, the 
 number of its lines of force which penetrate the circle is 
 
 ^- 27rr 2 (i cos 0), the latter factor being the area of the portion 
 
 of a sphere with centre at m, cut off by the said circle. The 
 expression 277(1 cos 6), since it measures the ratio of the area 
 subtended by a conical angle to the square of the radius, is, in 
 analogy with the circular measure of a plane angle, called a solid 
 
APPENDIX. 393 
 
 angle : the solid angle of the cone with vertex m and base the 
 circle, or the angle subtended by the circle to an eye placed at 
 m. Call this angle o> ; then the number of lines of force, or 
 the magnetic induction through the circle is ma>. 
 
 If the circle becomes now a circuit conveying a current C, 
 the system has energy ?;zcoC, and accordingly there will be a 
 tendency to relative motion, the force in any direction being 
 equal to the rate of change of mwC per unit distance in that 
 direction. 
 
 The potential of the pole on the circuit is m<o ; the potential 
 of the circuit on the pole is Ceo. If the pole is situated any- 
 where, and the coil is of any shape, o> can still be specified, 
 but not so easily. If there is a collection of magnets, their 
 potential on a circuit, or induction through it, can be written 
 
 Magneto-electricity. 
 
 (h] The fundamental fact of magneto-ejectricity is that if the 
 induction through a circuit change from any cause whatever, an 
 E.M.F. is set up in the circuit equal to the rate of change of the 
 magnetic induction 
 
 e- dl 
 ~ dt' 
 
 This is not strictly a relation independent of the fundamental 
 fact of electro-magnetism : the two are connected by the law of 
 the conservation of energy.' I may indicate this important 
 
394 APPENDIX. 
 
 fact sufficiently for our present purpose by quoting the conser- 
 vation of energy, in a form applicable to the case of a circuit 
 conveying a steady current, as 
 
 ECdf = RCV/ + O/I ; 
 whence 
 
 RC E dl 
 RC = E ~ dt> 
 
 or the resultant E.M.F. consists not only of the E.M.F. applied, 
 but contains also an intrinsic or indirect E.M.F. magnetically 
 excited in the circuit ; this being what Faraday discovered as 
 magneto-electricity. 
 
 Various Modes of exciting Induction Currents. 
 
 (t) Now /may be made up in a multitude of ways. It may 
 be a component of terrestrial magnetic field, say, nAH cos 0. It 
 may be caused by magnets in the neighbourhood 2(;;zco). It 
 may be due to induction from some other coil, MC'. It may 
 be due to the current passing in the coil itself, say LC. The 
 
 total induced E.M.F. is the rate of change of the sum of all 
 these, or 
 
 e = { ;/AH cos 9 + 2(;o>) -f- MC' + LC } ; 
 
 and accordingly it may be excited in many ways : by changes 
 in size or shape of coil ; by changing its aspect to the 
 field (as in a dynamo) ; by moving magnets in its neigh- 
 bourhood (as in an alternating-current machine) ; by varying 
 the current in or shifting the position of other circuits 
 (as in a Ruhmkorff coil) ; or, lastly, by changing its own 
 current, or its own coefficient of self-induction. Changes 
 
 in the last term, -^ (LC), are specially called E.M.F. of self- 
 dt 
 
 induction, and used to be called extra- currents. 
 
APPENDIX. 395 
 
 Primary Current alone : and Coil with Revolving 
 Commutator. 
 
 (f) The equation to a current of varying strength in the 
 simplest case of a lone circuit is 
 
 E - RC = |(LC), 
 
 where E is the applied E.M.F. ; and this may be written out 
 more fully 
 
 -|- C - E 
 
 + dt) ~ ' 
 
 which shows that in the case of circuits of variable self-induction 
 the resistance has not its most simple value, but has an extra 
 
 term in it, a spurious or imitative resistance, r- . 
 
 An example of a circuit of variable self-induction is one 
 which is continually having wire withdrawn from or added to 
 it, so that a current has to be stopped in portions where it was 
 already established^ and started in hitherto stagnant portions : 
 a case quite analogous to the viscosity of gases, and commonly 
 illustrated by passengers of appreciable inertia getting in and out 
 of a moving train. An instance of the case occurs in every 
 Gramme ring, or indeed every dynamo armature, when 
 spinning with a commutator, quite independently of the 
 magnetic field in which it may happen to be spinning. In 
 all such cases the effective resistance is rather greater than 
 
 R, being R + ~k or R + n L ; where the self-induction 
 virtually added to the circuit n times a second is L. 
 
 Ley den Jar. 
 
 (k] In the case of a discharging condenser of capacity S, the 
 quantity stored in it at any instant is such that C = - - , 
 
396 APPENDIX. 
 
 rt 
 or that Q = Q ~ / Cdt ; and the difference of potential be- 
 
 tween its terminals is S, which is the E.M.F. applied to the 
 
 o 
 
 circuit. So the equation to the discharge current is 
 
 . 
 
 The solution of the equation in this case is 
 C= JL*- 
 
 PL 
 
 -p 
 
 where m = , and regulates the total duration of the discharge, 
 
 and where p = * approximately 
 
 v(LS) 
 
 | more accurately ^(~ ' 2 ) } , 
 and regulates the rapidity of alternation, which is *-. The 
 
 27T 
 
 wave-length of the emitted radiation (Chapter XIV.) is 
 
 ^ = // t ,S\ 
 
 p V U K/ 
 
 With these quick oscillations, R is nothing at all like its 
 ordinary value for steady currents ; because the outside of the 
 wire only is used ( 45 and 102) ; but, calling the ordinary value 
 R , R is very approximately, for high rates of alternation, l 
 
 / being the length of the wire, and /* the magnetic permeability 
 of its substance ( 46). 
 
 1 See Rayleigh, Phil Mag., May 1886. 
 
APPENDIX. 397 
 
 The emission of radiation by such a circuit goes to increase 
 R still more ( 142 and p. 367). See also in. 
 
 Alternating Current. 
 
 (/) In case of any coil or armature spinning in a magnetic 
 field, the equation to the current is 
 
 - RC= 4 
 
 and the E. M.F. is therefore alternating according to a sine 
 function. Writing this equation 
 
 the solution is 
 
 Epcosd/- e) 
 
 where tan * = = The R' differs from simple R, as already 
 R * 
 
 explained in (/), only when a commutator is employed : which 
 it often is not. The denominator of the above expression 
 
 may be called impedance, and denoted by P (see next section), 
 the quantities being related as in this little diagram. The 
 quantity c is the lag of the current behind the applied E.M F. 
 
398 APPENDIX. 
 
 Two Definitions of Electric Resistance, and Distinction between 
 the Two. 
 
 (w) The oldest definition of the term " resistance of a con- 
 ductor " is that given by Ohm, viz. the ratio 
 E.M.F. applied to the conductor 
 Current excited in it 
 
 But another is contained in the law of Joule, viz. the ratio 
 
 Energy dissipated per second by the conductor 
 Current squared which it transmits 
 
 In cases of no reversible obstruction the two definitions agree, 
 but in cases of chemical action, of reversible heat effects, and of 
 varying magnetic induction, some of the energy may be stored, 
 all is not dissipated, and under these circumstances the two 
 definitions do not agree. A distinction must be drawn between 
 them : the term resistance cannot properly be applied to both 
 quantities. 
 
 Now it is found convenient to retain the name resistance for 
 the second definition the dissipation of energy coefficient ; 
 and to realize that in the total obstruction specified by the first 
 definition there is included "back E.M F.," "polarization," or 
 other reversible obstruction, in addition to resistance proper ; 
 while in the very important case of the total obstruction met 
 with by an alternating current, it has become convenient to 
 call the quantity defined by the first of the two equations, 
 " impedance." 
 
 The two definitions of resistance may indeed be always made 
 to agree, if, in the Ohm's law definition, instead of applied 
 E.M.F., we reckon resultant E.M.F. And this is the neatest 
 and simplest mode of taking into account such things as che- 
 mical or thermal polarization, and also a magnetic back E.M.F., 
 so long as it is steady and external, as in the case of electric 
 motors. But, when dealing with alternating generators, some 
 understanding has to be come to as to how the value of their 
 
APPENDIX. 399 
 
 E.M.F. is to be reckoned, and no simple subtraction of a back 
 E.M.F. is convenient. Referring to last section, we see that 
 the expression for current contains as numerator a lessened or 
 lagging E.M.F., and as denominator an obstruction or impedance 
 containing a term in addition to what is usually called resist- 
 ance. It is from this point of view that the idea and term 
 " impedance " become so useful. 
 
 The value of this quantity is, in general, as has been shown, 
 
 and its two portions may be styled respectively the inertia, or 
 conservative portion, and the frictional or dissipative portion 
 
 ( 38). 
 
 Part of the energy dissipated appears as heat in the con- 
 ductor, and this is the only portion on which Joule experi- 
 mented, but another portion we now know is propagated out as 
 radiation into space ( 142) : both portions together are included 
 in the numerator proper to the second definition of R. 
 
 Induced Current in Secondary Circuit. Transformers. 
 
 (ri) The E.M.F. induced in a secondary circuit surrounding 
 a ring like Fig. 47, whose primary coil has an alternating or 
 intermittent current, C, sent round it, is, referring back to (h] 
 and (c}~ 
 
 and depends, therefore, directly on the number of turns of 
 wire in the secondary coil, and on the rate of variation of the 
 primary current. This is the principle of induction-coils, and of 
 "secondary-generators" or transformers ( 115). The E.M.F. 
 thus obtained is completely under control by choosing a suit- 
 able value for ', according as high E.M.F. (in Ruhmkorff coils) 
 or a powerful current (for electric welding) is required. They 
 
400 APPENDIX. 
 
 are called transformers, because, of the two electrical factors in 
 mechanical " power," EC, they can change their ratio, leaving 
 the product nearly constant ; just as ordinary machines do with 
 the force and velocity factors of the same product " power." 
 So, in precise analogy with gaining in force what you lose in 
 speed, you gain in E.M.F. what you lose in current ; or vice 
 versa. 
 
 The equations to primary and secondary currents, C and C 
 are 
 
 E-RC =4-(LC + M 
 
 o-R'C = 
 
 and from the solution of these, the effective or apparent self- 
 induction of primary, when its secondary is short-circuited 
 and when all resistances are kept small, comes out equal to 
 
 M 2 
 L . Now since, for a simply closed magnetic circuit, 
 
 L : L' : M = n 2 : n' 2 : nn', 
 
 the effective self-induction (and therefore the impedance) of the 
 primary is approximately zero when its secondary is short- 
 circuited a fact which is the Magna Charta of commercial 
 transformers. 
 
 Rate of Transmission of Telegraph Signals, in the Simplest 
 Case. 
 
 (d) Consider a unit length of a pair of parallel thin copper 
 wires not very close together, a going and return wire, at a 
 distance b apart, the sectional radius of each wire being a. 
 The self-induction of this portion, see (e), is 
 
 L! = 4/1 log -, 
 
APPENDIX. 401 
 
 and the static capacity of the same portion is (by somewhat 
 similar reasoning) 
 
 S 1 = 
 
 Hence 
 
 LA = /xK. 
 
 The resistance of the same unit length may be called R^ 
 Now consider an element of the pair of wires of length dx, 
 and write down the slope of potential between its ends when a 
 current, C, flows along it, and also their rise of potential with 
 time ; we get 
 
 L >f + R > c + f = > 
 
 and 
 
 St g+-o, 
 
 dt dx 
 
 Now, a " wave " being any disturbance periodic both in space 
 and time, its general fundamental equation is 
 
 y a sin (pt nx), 
 
 where y is the extent of the disturbance at any place distant x 
 from the origin, and at any time, /, from the era of reckoning. 
 
 The coefficient a is the amplitude of the vibration ; n is the 
 
 space-period-constant, or 27r ; p is the time-period-constant, or 
 
 A 
 
 ; the velocity of advance of the waves is one space-period in y 
 
 one time- period, viz. or *, 
 
 The solution of these equations for the case of an applied 
 rapidly alternating E.M.F., V sin //, at the origin, may be 
 written 
 
 sin j 
 
 where m l = r *- and ^ = 77^-5-^. 
 
 D D 
 
APPENDIX. 
 
 Hence the above bracketed pair of equations give waves 
 travelling along the wires with the speed -^-- -, which we 
 
 have seen equals , and with an amplitude dying out along 
 
 v(/*K) 
 
 the length of the wires according to a logarithmic decrement 
 
 The speed of propagation of pulses along wires is therefore 
 precisely the same, in this simple case, as the propagation of 
 
 waves out through free space, viz. the velocity . ( 128, 
 
 I 3 2 ) r 37)- All complications go to decrease, not to increase, the 
 speed ( 135). 
 
 Dimensions of Electrical Quantities. 
 
 (p] Writing L, M, T, F, v, for units of length, mass, time, 
 orce, velocity, as usual, and A for area ; the fundamental and 
 certain experimental relations, independent of all considerations 
 about units and systems of measurement, are 
 Of electrostatics, Q = LV(KF) ...... (i) 
 
 Of magnetism, m = L VO^F) ...... (2) 
 
 Of electro-magnetism, ;;zL = /iAC ....... (3) 
 
 The last may also be written 
 
 m - pvQ ....... (3') 
 
 in which form it suggests the magnetic action of a moving 
 charge, which Rowland's experiment has established. 
 Combining the three equations, we deduce 
 
 m 
 
 T ^ I density 
 
 whence uK =_.. = _- r-r t 
 
 v 2 elasticity 
 
 the well-known relation connecting the two etherial constants. 
 
 Comparing many electrical equations with correspond- 
 ing mechanical ones, we find that the product LC takes the 
 
APPENDIX. 403 
 
 place of momentum (mv\ and that LC 2 takes the place of 
 kinetic energy (%,mv), and indeed is the energy of a current, 
 see (_/). Hence it is natural to think of L as involving inertia, 
 and of p. or 4717* as a kind of density of the medium concerned. 
 
 Assuming this, ^ at once becomes an elasticity coefficient 
 K 
 
 (as indeed electrostatics itself suggests), because /uK^EEi ; and 
 the dimensions of all electrical units can be specified as fol- 
 lows, without any arbitrary convention or distinction between 
 electrostatic and electro-magnetic units : 
 
 Sp. ind. cap., K = = = = shearability. 
 
 stress force M 
 
 Permeability, /. = = = density. 
 
 volume L 3 
 
 T-I i. a. /~v T 9 volume 
 
 Electric charge, Q = L 2 
 
 displacement 
 Magnetic pole, m = = momentum per unit length. 
 
 I 2 
 Electric current, C = = displacement X velocity. 
 
 Magnetic moment, ml = - - = momentum. 
 
 E.M.F., E - wor /k = a = pressure X displacement, or 
 
 work per unit area. 
 
 Intensity of magnetic field, H = = - = velocity. 
 
 Intensity of electrostatic field, = ^ = energy per unit 
 
 volume. 
 
 Surface density, a = ^ = a pure number. 
 A 
 
 Electric tension,^ = = a pressure or tension. 
 
 i\. L, L 
 
 D D 2 
 
404 APPENDIX 
 
 Capacity, S = - = - = displacement per unit pressure. 
 
 Coefficient of resistance, = j-^-p impulse or momentum 
 
 per unit volume. 
 Magneto-motive force, ^nnC = = current. 
 
 Reluctance, - = = r . 
 p.A. M inertia 
 
 Magnetic induction, I = = moment of momentum per 
 
 unit area. 
 Coefficient of induction (self or mutual), = 2 = inertia per 
 
 unit area. 
 
 This is, or may be, an improvement on the rough practical 
 system which assumes as of no dimensions sometimes K, and 
 sometimes /n, according as one is dealing with electrostatics or 
 with magnetism ; but very likely it is only a stepping-stone. 
 Prof. Fitzgerald has recently suggested that, regarding every- 
 thing from the strictly kinematic and etherial point of view, 
 both K and p, may be a slowness of the vorticity ; and by that 
 assumption also everything becomes simple and of unique 
 dimensions. Whatever of this turns out true, it is not to be 
 supposed that we can long go on with two distinct systems 
 of units, the electrostatic and the electromagnetic, and two 
 distinct sets of dimensions for the same quantities ; knowing 
 as we do that neither set can by any reasonable chance turn 
 out to be the right one. 
 
 NEWTON'S GUESSES CONCERNING THE ETHER. 
 
 (q) Newton's queries at the end of his " Opticks" finish in 
 the early editions with. Query 16, and I have found it difficult to 
 
APPENDIX. 405 
 
 come across the later queries except in Latin. I therefore here 
 copy such portions of these queries as have an obvious bearing 
 on our present subject, in order to make them more easy of 
 reference. 
 
 " Qte. 17. If a Stone be thrown into stagnating Water, the 
 Waves excited thereby continue some time to arise in the place 
 where the Stone fell into the Water, and are propagated from 
 thence in concentrick Circles upon the Surface of the Water 
 to great distances. And the Vibrations or Tremors excited in 
 the Air by percussion, continue a little time to move from the 
 place of percussion in concentrick Spheres to great distances. 
 And in like manner, when a Ray of Light falls upon the Surface 
 of any pellucid Body, and is there refracted or reflected, may 
 not Waves of Vibrations or Tremors be thereby excited in the 
 refracting or reflecting Medium at the point of Incidence ...?'' 
 
 " Qu. 1 8. If in two large tall cylindrical Vessels of Glass 
 inverted, two little Thermometers be suspended so as not to 
 touch the Vessels, and the Air be drawn out of one of these 
 Vessels, and these Vessels thus prepared be carried out of a 
 cold place into a warm one ; the Thermometer in vacua will 
 grow warm as much and almost as soon as the Thermometer 
 which is not in vacua. And when the Vessels are carried back 
 into the cold place, the Thermometer in vacua will grow cold 
 almost as soon as the other Thermometer. Is not the Heat of 
 the warm Room conveyed through the Vacuum by the Vibra- 
 tions of a much subtiler Medium than Air, which after the Air 
 was drawn out remained in the Vacuum ? And is not this 
 Medium the same with that Medium by which Light is re- 
 fracted and reflected, and by whose Vibrations Light communi- 
 cates Heat to Bodies, 1 and is put into Fits of easy Reflexion 
 
 1 Note the precision and propriety of this phrase : far superior to 
 most of the writing on the subject of absorption of radiation during 
 the present century. It could only be improved by substituting 
 generates in for " communicates to," in accordance with the modern 
 kinetic theory of heat. 
 
4o6 APPENDIX. 
 
 and easy Transmission? And do not the Vibrations of this 
 Medium in hot Bodies contribute to the intenseness and 
 duration of their Heat ? And do not hot Bodies communicate 
 their Heat to contiguous cold ones, by the Vibrations of this 
 Medium propagated from them into the cold ones ? And is not 
 this Medium exceedingly more rare and subtile than the Air, 
 and exceedingly more elastick and active ? And doth it not 
 readily pervade all bodies ? And is it not (by its elastick force) 
 expanded through all the Heavens ? " 
 
 " O,u. 19. Doth not the Refraction of Light proceed from 
 the different density of this yEtherial Medium in different 
 places, the Light receding always from the denser parts of the 
 Medium ? And is not the density thereof greater in free and 
 open Space void of Air and other grosser Bodies, than within 
 the Pores of Water, Glass, Crystal, Gems, and other compact 
 Bodies?" 1 . . . 
 
 " Qu. 21. Is not this medium much rarer in the denser Bodies 
 of the Sun, Stars, Planets, and Comets, than in the empty 
 celestial Spaces between them ? And in passing from them to 
 great distances, doth it not grow denser and denser perpetually, 
 and thereby cause the gravity of those great Bodies towards 
 one another, and of their parts towards the Bodies ; every body 
 endeavouring to go from the denser parts of the Medium 
 towards the rarer ? For if this Medium be rarer within the 
 Sun's Body than at its surface, and rarer there than at the 
 hundredth part of an Inch from its Body, and rarer there than 
 at the fiftieth of an Inch from its Body, 2 and rarer there than at 
 
 1 In Newton's opinion light travelled quicker in gross matter than 
 in space, and hence it is that he inverts our Fresnel-derived views. He 
 continues the same inversion in his query concerning gravitation, here 
 next following. 
 
 2 It was his experiments in diffraction which made him think of this 
 gradual change in the properties of ether as one recedes from a body. 
 A few years ago such gradual changes would have seemed to us quite 
 unlikely ; but the most recent experiments of Michelson shake all 
 preconceived opinions. 
 
APPENDIX. 407 
 
 the Orb of Saturn ; I see no reason why the Increase of density 
 should stop anywhere, and not rather be continued through all 
 distances from the Sun to Saturn, and beyond. And though 
 this Increase of density may at great distances be exceeding 
 slow, yet if the elastick force l of the medium be exceeding 
 great, it may suffice to impel Bodies from the denser parts of the 
 Medium towards the rarer, with all that power which we call 
 Gravity. And that the elastick force of the Medium is 
 exceeding great, may be gathered from the swiftness of its 
 Vibrations. Sounds move about 1140 English Feet in a second 
 Minute of Time, and in seven or eight Minutes of Time they 
 move about one hundred English Miles. Light moves from 
 the Sun to us in about seven or eight Minutes of Time, which 
 distance is about 70,000,000 English Miles, supposing the 
 horizontal Parallax of the Sun to be about 12". And the 
 Vibrations or Pulses of this Medium, that they may cause the 
 alternate Fits of easy Transmission and easy Reflexion, must 
 be swifter than Light, and by consequence above 700,000 times 
 swifter than Sounds. And therefore the elastick force of this 
 Medium, in proportion to its density, must be above 700,000 X 
 700,000 (that is, above 490,000,000,000) times greater than the 
 elastick force of Air is in proportion to its density. For the 
 Velocities of the Pulses of Elastick Mediums are in a sub- 
 duplicate Ratio of the Elasticities and the Rarities of the 
 Mediums taken together." . . . 
 
 " Qu. 22. May not Planets and Comets, and all gross Bodies, 
 perform their motions more freely, and with less resistance in 
 this ^therial Medium than in any Fluid, which fills all Space 
 adequately without leaving any Pores, and by consequence is 
 much denser than Quick-silver and Gold ? And may not its 
 resistance be so small as to be inconsiderable ? For instance ; 
 
 1 Meaning what we call the pressure. This is, of course, pursuing 
 the analogy of sound-waves, and does not accord with our present 
 knowledge. 
 
408 APPENDIX. 
 
 if this ALiher (for so I will call it ^ should be supposed 700,000 
 times more elastick than our Air, and above 700,000 times more 
 rare ; its resistance would be above 600,000,000 times less than 
 that of Water. And so small a resistance would scarce make 
 a sensible alteration in the Motions of the Planets in ten 
 thousand Years. If any one would ask me how a Medium can 
 be so rare, let him tell me how the Air in the upper parts of the 
 Atmosphere can be above an hundred thousand times rarer 
 than Gold. Let him also tell me how an electrick Body can by 
 Friction emit an Exhalation so rare and subtile, and yet so 
 potent, as by its Emission to cause no sensible Diminution of 
 the weight of the electrick Body, and to be expanded through 
 a Sphere whose Diameter is. above two Feet, and yet to be 
 able to agitate and carry up Leaf Copper, or Leaf Gold, at the 
 distance of above a Foot from the electrick Body ? And how 
 the Effluvia of a Magnet can be so rare and subtile, as to pass 
 through a Plate of Glass without any Resistance or Diminution 
 of their Force, and yet so potent as to turn a magnetick Needle 
 beyond the Glass?" 
 
 1 The interest of these extracts lies largely in their belonging to the 
 very early days of the conception of an ether, and in their remarkable 
 insight into many things, though in detail they often do not completely 
 accord with present knowledge. 
 
INDEX. 
 
 E E 
 
INDEX. 
 
 A. 
 
 ABERRATION, 298, 345 
 
 Abney, 348 
 
 Absolute minimum of electricity, 77 
 
 Absolute motion, 298 
 
 Absorption, model of, 274 
 
 selective, 254 
 
 Action at a distance, 328 333 
 Actinic rays, 267 
 Affinity, chemical, 77, 78 
 Air-battery, 112 
 
 Air-gap, effect of, in magnets, 161 
 Alternate contact, discharge by, 56, 
 
 57 
 Alternating current, equation to, 397 
 
 resistance to, 100, 396 
 Ampere, 92 
 Ampere's theory of magnetism, 147 
 
 150, 170 
 
 Amperian currents, 149 
 Analogies, see Models 
 Analysis of oscillatory discharge, 
 
 377 
 
 Anion, 84 
 Anomalous dispersion, 254 
 
 magnetization, 369 
 Arago experiment, 344 
 Argument from experience, 329, 
 
 330 
 
 Artificial lighting, 257 259 
 
 Atmosphere, density of, 341 
 Atomic charge, 73, 76, 77 
 
 potential, 77 
 
 vibration, 250 
 
 vibration, frequency of, 266 
 
 268 
 
 Atomicity, 75 
 Atom, of electricity, 77 
 
 current in, 149 
 
 elasticity of, 337 
 
 infinite properties of, 148 
 
 locomotion of, 82 87 
 
 intensity of attraction, 78 
 
 surgings in, 250 
 
 vibration of, 346 348 
 Attraction and repulsion, caused by 
 
 strain in medium, 28 
 Attraction of atoms, intensity of, 78 
 Ayrton, 298, 313 
 
 B. 
 
 BEETZ, 149 
 
 Bell, Graham, 323, 325 
 Bichat and Blondlot, 380 
 Bidwell, Shelford, 290, 326 
 Biot, ii 
 
 Bird-cage experiment, 10 
 Bismuth, effect of magnetism 011,291 
 Bernstein, 301 
 Boys, C. V., 12, 348 
 
 E E 
 
412 
 
 INDEX. 
 
 Browne, Walter, 328 
 Burr in pierced card, 169 
 Bursting of jar, 375 
 
 CAMBRIDGK physicists, 5 
 Capacity, 30 
 
 effect of, on signals, 241 
 Carey Foster, 288 
 Cation, 83 
 Cavendish, 5, 6 
 
 experiment, n, 27 
 
 experiment, moral of, 231 
 Cavities in medium, 20, 27, 28, 65 
 Centrifugal force analogue of 
 
 magnetic repulsion, 172 176 
 Charge, 26, 28, 32 
 
 atomic, 73, 76, 77 
 
 and varying magnetic field, 144 
 
 by alternate contact, 56 
 
 by induction, 42 53 
 
 internal, 38 
 
 moving, 126128, 143, 210, 
 298 
 
 residual, 39 
 
 spurious, 53 
 
 surface, 53 
 
 Charged air penetrating wire gauze, 
 II 
 
 sphere, motion of, 128 
 Chemical affinity, 77, 78 
 
 elasticity, 224 
 
 equivalent, 75 
 Chinaman, 332 
 Clausius, 79 
 
 Clerk Maxwell, see Maxwell 
 Cohesion, 333, 338 
 
 and gravitation, 352 
 Coil imitating magnet, 132 
 Combustion, an indirect source of 
 
 light, 258 
 Communication, modes of, 334 
 
 337 
 
 Commutated circuit, resistance of, 
 
 395 
 Concentration by reflexion, 262 
 
 Condenser, stratified, 36 
 
 discharge of, 228 233 
 
 momentum in, 300 
 Condensers, 30 
 Conduction, gaseous, 123 126 
 
 in liquid, 72 87 
 
 in metals, 67 72 
 
 surface, 101 
 Conductivity, magnetic, 156 
 
 effect of light on, 301 
 Conductor,moving in magnetic field, 
 206 
 
 perfect, 165, 167, 183, 184, 195, 
 261, 274 
 
 radiation encountering, 271 
 Conductors, like cavities, 20, 65 
 
 opacity of, 271275, 323, 324 
 Connexion of Faraday and Hall 
 
 effects, 288, 290 
 Contact-force, 109 119 
 Convection of heat, water, and elec- 
 tricity, 66 
 
 Copper-disc experiment, 152 
 Cord-models, 32 53 
 Corpuscular theories, 336 
 Crookes, 300 
 
 Currents, action between two, 91 
 Current, action of, on pole, 134 
 
 alternating, 100 
 
 Amperian, 149 
 
 as moving charge, 298 
 
 brake analogue of, 196 
 
 condition of medium near, 91 
 
 density of, 69 
 
 distribution of, 192 
 
 disturbance not confined to 
 conductor, 91 97 
 
 energy of, 94 97, 39 1 
 
 excited by light, 304 
 
 exerting mechanical force, 203 
 
 extra, 89, 92 94, 187, 394 
 
 heat of, 70 
 
 imitating magnet, 387 
 
 in atom, 149 
 
 induced, 394 
 
 in perfect conductor, 184 
 
 intensity of, 69 
 
 induction, 92 
 
INDEX. 
 
 413 
 
 Current, magnetic properties of, 91 
 
 94 
 
 model of, 181 192 
 molecular, 149 155, 167 
 produced by rotating magnet, 
 
 146 
 
 production of, 210 
 propelled by side-thrust, 94, 
 
 98, 239 
 rise in secondary circuit, 193 
 
 196 
 
 rotating magnet, 144 
 starts at surface of wire, 98, 239 
 stopped by resistance, 148 
 time taken to start, 99 
 varying, 185 192 
 viscosity, analogue of, 98, 99 
 Cycle, magnetic, 164 
 Cyclone, 354 
 
 D. 
 
 DECOMPOSITION, 78 84 
 Density, of current, 69 
 
 of ether, 185, 206, 222 
 
 of ether and atmosphere, 341 
 Diamagnetism, experiment illustrat- 
 ing, 153 
 
 Weber's theory of, 150 155 
 Dielectric, breaking down of, 31 
 
 constant, see Specific inductive 
 capacity 
 
 cord- model of, 33, 43 
 
 stratified, 35 40 
 
 strength, 125, 126 
 Differences between electricity and 
 
 fluid, 15 
 Diffraction, 270 
 
 Diffusion velocity in cables, 241 
 Dimensions of electric quantities, 
 
 402 
 
 Direction of vibration, 306 
 Discharge, by alternate contact, 57 
 
 disruptive 31, 35, 41, 209, 210 
 
 dissipation of energy from, 232 
 
 intermittent, 366, 378 
 
 of condenser, three main cases, 
 228, 365 
 
 Discharge of condenser, dying out 
 
 of vibrations, 229, 367 
 of condenser, theory of, 396 
 oscillatory, 41, 42, 94, 227 - 
 
 233. 361382, 395 
 tuning-fork analogue, 230, 362, 
 
 367 
 
 wave-length, 248, 396 
 Dispersion, 253255 
 
 anomalous, 254 
 Displacement, 33, 34, 40 
 Disruptive discharge, 31, 3-5, 41, 
 
 209, 210 
 
 Dissipation of energy from dis- 
 charge, 232 
 
 Dissociation, 7880, 106, 350 
 Dog, modes of calling, 334 
 Double decomposition, 79, 80 
 Doubleness of constitution of ether, 
 
 220, see also Dual view of electricit y 
 Dual view of electricity, 26, 28, 48, 
 
 81, 85, 86, 90, 165, 168170, 
 
 221, 350, preface 
 
 E. 
 
 EDLUND, 351 
 
 Elastic bags, 14, 18, 27, 31 
 
 cells, 209 
 Elasticity, 227 
 
 chemical, 224 
 
 electromotive, 223 225 
 
 of ether, 220 
 
 of ether accounted for, 264, 265 
 
 of ether, probable real value, 
 235 
 
 of moving fluid, 166, 209, 264, 
 265, 353, 356 
 
 Thomson theory of, 209, 265, 
 
 Electricity, absolute minimum of, 77 
 always flows in closed circuit, 9 
 and ether, 1 7, 338, 349, preface 
 and light, 307, 317 
 atom of, 77 
 conduction of, 67 87 
 convection of, 85, 86 
 
INDEX. 
 
 Electricity, displacement of, 33, 34, 
 
 40 
 dual view of, 26, 28, 41, 81, 
 
 85, 86, 90, 165, 168170, 
 
 221 
 
 fluid theories of, 7, 20, 26, 28 
 four ways of recognizing, 14 
 frictional, 119 
 inertia of, 15, 1 6, 42, 88 90, 
 
 94, 102 105, 165 167, 180, 
 
 187, 299, 300 
 liquid theory of, 12 
 like incompressible fluid, 12, 18, 
 
 27, 231, differences, 15 
 locomotion of, 66, 127 
 modes of transfer, 67, 72 
 momentum of, 16, 88 90, 102 
 
 105, 165167, 1 80, 299, 
 
 300 
 
 "-.natural unit of, 77 
 not a form of energy, 7 
 not created nor destroyed, 7, 9, 
 
 3.13 
 
 positive and negative, see Dual 
 view of 
 
 possibly a form of matter, 7, 9 
 
 pyro, 1 20 
 
 rotation of, 90, 131 
 
 specific heat of, 120, 295 
 
 stream line of, 103 
 
 subdivisions of, I 
 
 weight unascertainable, 14 
 
 what is, 313 
 
 Wheatstone's experiment on 
 velocity of, 169 
 
 whirl of, 90, 131, 171, 314 
 Electric analyzer (Hertz), 305 
 
 eye, 304 
 
 light, a temporary phase, 258 
 
 oscillation, model of, 269 
 
 radiation refracted, 306 
 
 radiation, speed of, 234 256 
 
 resonance, 304, 374 
 
 tenacity, 225 
 
 vortex ring, 171, 187, 213 
 
 wave length, 247, 248 
 
 wind, 1 68, 300 
 Electrode, 71, 74, 82 
 
 Electrolysis, 350 
 
 laws of, 74 80, 84 
 Electrolyte, 73 
 
 differing from metal and di- 
 electrics, 8 1, 84 
 
 model of, 80 83 
 Electrolytes and gases, 124, 125 
 Electrolytic conduction, 72 87 
 
 momentum, 299 
 Electromagnetic system of units. 
 
 235 
 Electromagnetism, fundamental fact 
 
 of, 132, 387 
 
 Electromotive elasticity, 223 225 
 force, 34, 39, 393 
 force, thermal, 117 
 Electro-optic effects,.^ Faraday and 
 
 Kerr, also Hall 
 Electrophorus, cord model of, 47 
 
 50 
 Electrostatic and magnetic fields 
 
 superposed, 208 
 displacement inside ring mag- 
 net, 215 
 
 effect of moving or varying 
 magnetic fields, 144, 211, 
 215 
 
 system of units, 235 
 Empiricism of present modes of 
 
 getting light, 257 
 Energy, of current, 9497, 391 
 paths of, 95 97 
 transfer of, to distance, 196 
 
 202 
 
 transmission of, 96, 97 
 two forms of, 317 
 Ether, and electricity, 17, 338, 349 
 and gravitation, 9 
 and matter, 352 358 
 constants of, 234, 403 
 constitution of, 223, 338, 339, 
 
 349, preface 
 
 density of, 185, 206, 222, 349 
 density of, probable real value, 
 
 235, 341 
 
 doubleness of constitution, 220 
 dual view of, 221, 349, 350 
 effect of matter on, 342 
 
INDEX. 
 
 415 
 
 Ether, elasticity of, 220, 235, 340 
 elasticity accounted for, 264, 
 
 265 
 
 Fitzgerald's model of, 262 264 
 fluidity and rigidity of, 221, 
 
 222 
 
 free and bound, 343, 349 
 free, simple structure of, 253 
 functions of, 358 
 incompressibility of, 231 
 inertia of, 220, 340 
 jelly theory of, 17, 19, 339 
 Maxwell's model, 263 
 motion through, 298 
 rigidity, probable value, 342 
 sheared, 350 
 
 Ewing, 161, 164, 283 293, 294 
 Extra current, 89, 92, 93, 187 
 Eye, electric, 304 
 
 F. 
 
 FARADAY, 5, 6, n, 21, 22, 92, 277, 
 
 320, 367 
 electroptic effect, 276288, 
 
 320322 
 electroptic effect, connected 
 
 with Hall's, 290 
 electroptic effect, model of, 
 
 321 
 electroptic effect, time of, 293, 
 
 38o 
 
 laws of electrolysis, 74 77, 84 
 Feddersen, 42 
 
 Film affecting reflexion, 270 
 Fitzgerald, 294, 306, 323, 368, 
 
 preface 
 
 Fitzgerald's ether model, 262 264 
 Fizeau experiment. 298, 343, 344 
 Fluids, magnetization of, 161 
 Fluid theories of electricity, 7 
 Fluidity and rigidity of ether, 221, 
 
 222 
 
 Fluorescence, 268 
 Fly-wheel, analogue of magnetism, 
 
 195 
 
 Forces acting on conductors, 203 
 
 Foster, 288 
 
 Franklin, 5, 6, 26 
 
 Frequency of atomic vibration, 266 
 
 268 
 
 Fresnel, 221 
 Fresnel's ether theory, 342 345, 
 
 351, 406 
 Friction, between matter and ether, 
 
 219 
 
 resistance, 32, 69, 70, 219 
 Frictional electricity, 119 
 
 G. 
 
 GALILEO, 148 
 Galvanometer analogy, 15 
 Gas, momentum in, 300 
 Gaseous conduction, 123 126 
 Gases and electrolytes, 124, 125 
 
 kinetic theory of, 336 
 Gore's railway, 139 
 Generation of magnetic field, 212 
 Glass conductivity, affected by light, 
 
 301 
 
 Glow-worm, 259 
 
 Gold thread experiment, 135, 136 
 Gravitation and the ether, 9 
 
 Newton's views on, 352, 407 
 
 propagation of, 231 
 
 smallness of, 357 
 
 theories of, 336, 338, 352 
 Gravity compared with chemical 
 
 affinity, 78 
 Green, 6 
 Grotthus, 80 
 Grove, Sir W. R., 325 
 Gyrostat explaining elasticity, 265 
 Gyrostatic inaction of magnet, 89, 
 90, 1 66, 299 
 
 H. 
 
 HALL, 279, 288, 297 
 effect, 276, 288 
 effect in insulators, 287 
 effect connected with Thom- 
 son effect, 295 297 
 
4i6 
 
 INDEX. 
 
 Hall effect, thermo-electric view of, 
 291, 292297 
 
 Hallwachs, 302 
 
 Heat and radiation, 66, 
 
 modes of transfer, 66, 72 
 produced by current, 70 
 produced by magnetization, 
 164 
 
 Heaviside, 155, 242, 363, 372 
 
 Heavy glass, 320 
 
 Helmholtz, 79, 108, 134, 371 
 
 Henry, Joseph, 369372 
 
 Hertz, 237, 302, 303307, 368 
 experiments, 302 307 
 
 Hicks, W. M., 264 
 
 Hollow vessel, experiments in, 8, 
 11, 231 
 
 Holtz machine, 86 
 
 Hooke's law in electrical case, 227 
 
 Horse and cart, 332, 333 
 
 Hot glass conducting, 301 
 
 Hughes, 97, 192 
 
 Hydraulic analogies, breakdown of, 
 
 95 
 
 models, 5360 
 Hydrogen, speed of, 87 
 Hysteresis, 164, 283, 285 
 
 J. 
 
 ICE pail experiment, 10, 23 
 
 moral of, 231 
 
 Illustrations, 2225, 33, 34, 38, 40, 
 43, 45, 48, 52, 55, 58, 59, 83, 
 
 102, 103, 113, 132, 136142, 
 145, 146, 153, 156, 171, 173 
 
 175, 178180, 183, 1 86, 189 
 
 191, 194, 203, 207, 209, 213, 263, 
 
 280, 287, 289, 324, 361 
 Impact theories, 335337 
 Impedance, 397, 398 
 
 and resistance, 398 
 Impetus, see Momentum 
 Incompressibility of ether, 231 
 Index of refraction and sp. ind. cap. 
 
 250 256 
 
 Induced charge, 42 53 
 
 current, 193196, 394 
 Induction coil theory, 399 
 Induclion, cord model of, 43 
 
 illustrated by strained medium 
 28 
 
 in conductors moving in mag- 
 netic field, 206 
 
 magnetic, 155, 388 
 
 mutual, 389 
 
 self, see Self-induction 
 Inductive retardation, 29 
 Inductivity, 34 
 
 Inertia, of electricity, 15,16,42,88 
 90, 94, 102 105, 165 167, 
 180, 187, 299, 300, 363 
 
 electromagnetic, 363, 364 
 
 of ether, 220 
 
 of iron, 241 
 Insulating medium conveys signals, 
 
 239 
 Insulators, Hall effect in, 287 
 
 transparency of, 269 272, 3 2 3 
 Intensity of current, 69 
 Internal charge, 38 
 Ions, speed of, 87 
 Iron, function of, in retarding 
 
 signals, 192, 241 
 ether density in, 205 
 magnetic effect of, 155, 192, 
 
 205, 364 
 
 permeability of, 157, 185, 206 
 saturation of, 157 
 wire, properties of, 98, 101 
 
 J- 
 
 JELLY, analogue of ether, 17, 19, 
 
 60, 65 
 Joule, 70, 115 
 
 KEEPER, use of, 160 
 Kepler, 320 
 Kerr, 278, 323 
 
 electroptic effect, 276285 
 
INDEX. 
 
 417 
 
 Kinds of wave propagation, 16, 231 
 Kinetic theories, 334 337 
 Kohlrausch, 87 
 Kundt and Rontgen, 322 
 
 Liquid rotated by magnet, 140 
 spiral jet of, 141 
 theory of electricity, 12 
 
 Locomotion of electricity, 66, 127 
 
 LAMB, Prof. Horace, 266 
 
 Langley, 348 
 
 Lateral propulsion of currents, 94, 
 
 98, 239 
 
 Leyden jar, 3032, 41, 94, 101, 
 358-382 
 
 bursting, 375 
 
 discharge, rotating plane of 
 polarization, 380 382 
 
 emitting waves, 232 
 
 frequency, 246 
 
 hydraulic model of, 53 60 
 
 insulated, 57 
 
 method of determining speed 
 of pulses in wires, 244 
 
 theory, 395 
 
 wave-length, 248 
 Light, affecting conductivity, 301 
 
 and electricity, 317 
 
 common indirect mode of 
 obtaining, 258 
 
 exciting currents, 304 
 
 manufacture of, 256 259, 302 
 
 modern theory of, 307 
 
 present modes of obtaining, 257 
 
 reflected at surface, amount of, 
 270 
 
 velocity of, 246 256, 340, 344 
 
 and electric radiation velocities 
 compared in matter, 250 
 
 violet, on spark-length, 302 
 
 waves, length of, 249 
 
 what is, 312, 315 
 Lighting, artificial, 257 259 
 Lightning conductor, IOI 
 Lines of force, 21 25 
 
 between two discs, 23 
 
 like elastic threads, 22 
 
 like rays of light, 25 
 
 magnetic, 171 176 
 
 M. 
 
 McCuLLAGH's ether theory, 352 
 Magic, 331 
 
 Magnet, and gold-thread, 136 
 and moving charge, 143 
 gyrostatic inaction of, 89, 90, 
 
 1 66, 299 
 
 imitated by coil, 131 133, 387 
 in form of ring, 157, 161, 213 
 
 216 
 
 rotated by current, 144 
 rotating, producing current, 146 
 rotating liquid, 140 
 Magnetic and electrostatic fields 
 
 superposed, 208 
 attraction and repulsion, 172 
 
 176 
 
 circuit, 155 
 conductivity, 156 
 cycle, 164 
 
 field, generation of, 212 
 field, map of, 170 
 field, models of, 177 192 
 field, producing electrostatic 
 
 effect, 144, 211, 215 
 field, spreading, 183 
 function of iron, 155, 192, 205, 
 
 241, 364 
 
 induction, 155, 387 
 line of force, 171 
 medium, stress in, 172 176 
 moment of circuit, 387 
 permeability, 156, 157, 185, 
 
 206, 389 
 permeability, probable real 
 
 value, 235 
 
 properties of disruptive dis- 
 charge, 210 
 reluctance, 155, 389 
 rotation experiments, 135 146 
 screen, 195 
 
4i8 
 
 INDEX. 
 
 Magnetic substances, opacity of, 279 
 
 whirls, 172 176 
 
 Magnetism, Ampere's theory of, 146 
 150, 170 
 
 fly-wheel, analogue of, 165, 186 
 
 permanent, 158 
 
 permanent, universal, 283 
 
 sub-permanent, 160 
 Magnetization, 149, 150, 159, 212 
 
 anomalous, 369 
 
 heat produced by, 164 
 
 of fluids, 161 
 
 of steel, 163 
 
 Magneto-motive force, 155, 388 
 Maintenance of radiation, 249, 250 
 Manganese steel, 157 
 Manufacture of light, 256 259, 302 
 Map of magnetic field, 170 
 Marionette, 331 
 Matter and ether, 352 358 
 Matter, vortex theory of, 356358 
 Maxwell, 5, 6, 165, 307, 318, 368, 
 
 372 
 Maxwell's ether model, 263 
 
 experiment on Fresnel's theory, 
 
 345 
 
 momentum experiment, 90 
 
 theory of light, 307 
 Mechanical analogies, see Models 
 Mechanism of radiation, 260 275 
 Media of communication, 331 337 
 Medium, strain of, 226 
 Melde's experiment analogue, 244 
 Mental imagery, advantage of, 61 
 Metallic conduction, 67 72 
 
 cord-model, 33 
 
 reflexion, 262 
 Metaphysical arguments, validity of, 
 
 3 2 9 
 Methods of communication, 334 
 
 337 
 Michelson, 298 
 
 recent experiments of, 345 
 Minchin, 325 
 
 Model of rotation of plane of polari- 
 zation, 321 
 cord, 3253 
 ethers. 262 264 
 
 Mode), hydraulic, 53 6c 
 of absorption, 274, 324 
 of electric currents, 181192 
 of electrolyte, 8083 
 of magnetic field, 177 192 
 of radiation, 260 
 of reflexion, 271 275, 324 
 of self-induction, 185 192 
 
 Molecular currents, 149 155, 167, 
 170 
 
 Momentum in condenser looked for, 
 
 300 
 
 electric, in gas, 300 
 of electricity, 16, 88 90, 102 
 105, 165 167, 180, 299, 
 300 
 
 Motion absolute, 298 
 
 Moving charge, 126128, 210, 298 
 charge and magnet, 143 
 
 Muirhead, 300 
 
 Musical sparks, 376 380 
 
 Mutual induction, 389 
 
 N. 
 
 NEGATIVE electricity, existence of, 
 
 see Dual view of electricity 
 Newton, 319 
 Newton's guesses on the ether, 406 
 
 queries, 352, 404 
 Niven, 266 
 
 O. 
 
 OCEAN of incompressible fluid, 18 
 Ohm's law, 69, 220 
 Opacity, model of, 324 
 
 of conductors, 271 275, 305, 
 
 3 2 3, 324 
 
 of magnetic substances, 279 
 Optical controversies, 306 
 Optics and electricity, 307 
 Organ analogue, 257 
 Oscillation in conductor, 266 269 
 
 period, 230 
 Oscillators of Hertz, 304 
 
INDEX. 
 
 419 
 
 Oscillatory discharge, 41, 42, 94, 
 
 227233, 360382, 396 
 analysed by mirror, 377 
 Outstanding problems, 297 303 
 Overflow of jar, 374 
 
 P. 
 
 PATHS of energy, 95 97 
 Peltier effect, 115 
 Penetrability of ether, 19 
 Perfect conductor, 165, 167, 183, 184, 
 
 195, 261, 274 
 
 Period of oscillation, 230, 362370 
 Permanent magnetism, 158 
 
 magnetism, universal, 283 
 Permeability, 156, 157, 185, 206, 
 
 387 
 
 not constant, 283 
 
 real value of, 235 
 Perpetual motion, 135 
 Phosphorescence, 250, 259, 268 
 Photophone, 325 
 Pitch, index of refraction of, 306 
 
 prism, 306 
 Plane of polarization rotated, model 
 
 of, 321 
 Point whirligig, 168 
 
 wind, 300 
 Polarization, electrolytic, 109 
 
 of electric radiation, 304 
 Pole, acted on by current, 134 
 Potential, 28, 61 
 
 of atoms, 77 
 
 of isolated metals, 112 
 
 of pole on circuit, 393 
 
 uniform in conductors, 20 
 Poynting, 16, 95, 96, 98, 105, 242 
 Pressure and dielectric-strength, 126 
 Preston, Tolver, 336 
 Principia, 318 
 Problems outstanding, 297 
 Production of electricity, 313 
 Projectile method of communication, 
 
 335337 
 Pyro-electricity, 120 
 
 Q- 
 
 )UANTI VALENCE, 75 
 
 )uincke, 282 
 
 R. 
 
 RADIATION and heat, 66 
 
 electric, speed of, 234 256 
 encountering conductor, 271 
 
 275 
 
 exciting currents, 304 
 
 loss of energy, 249 
 
 maintenance of, 249, 250 
 
 mechanism of, 260 275 
 
 polarized, 305 
 
 production of, 266, 303 
 
 reflected, 304 
 
 refracted, 306 
 
 speed, modes of observing, 
 236256 
 
 waste, 258, 259 
 Rails and slider, 207 
 Range of light waves, 367 
 Rate of oscillation, 42 
 Ratio of units, 245 
 Rayleigh, Lord, 259, 365, 396 
 Reflected light, amount of, 270 
 Reflexion, 269 275 
 
 by magnetic medium, 279 
 
 concentration of light by, 262 
 
 metallic, 262 
 
 model of, 271 274 
 Refraction of electric waves, 270, 
 306 
 
 index and spec. ind. cap., 250 
 
 256 
 
 Reluctance, 155, 389 
 Residual charge, 39 
 Resistance, 32, 69, 70 
 
 and impedance, 398 
 
 of commutated circuit, 395 
 
 to alternating currents, 397 
 
 magnetic, see Reluctance 
 Resonance, electric, 304, 374 
 Retentivity, 159, 161 
 Return circuit, 28 
 
420 
 
 INDEX. 
 
 Reversible heat, 115 
 
 Rigidity and fluidity of ether, 221, 
 
 222 
 
 of moving fluid, 166, 209, 265, 
 
 Ring magnet, 157, 161, 213 216 
 Rise of induced current in second- 
 ary circuit, 193 196 
 Rotating discharge in vacuum-tube, 
 
 142 
 
 magnet producing current, 146 
 Rotation experiments, 135 146 
 of magnet by current, 144 
 of plane of polarization, 276 
 of polarization by Ley den jar 
 
 discharge, 380 382 
 of plane of polarization, model 
 
 illustrating, 321 
 of viscous liquid, 98, 99 
 Rowland, 290, 402 
 Royal Institution, 367 
 
 wall-paper, 374 
 Riicker, 166 
 
 S. 
 
 SATURATION of iron, 157 
 Savary, 369 
 Schiller, 371 
 
 Secondary circuit, rise of current in, 
 193196 
 
 generators, 214, 399 
 Seebeck, 116 
 Selective absorption, 254 
 Selenium, 301, 325 
 Self-induction, 89, 62, 93, 94, 229, 
 390 
 
 coefficient of, 390 
 
 effect of, on signals, 242 
 
 explained, 363, 364 
 
 model of, 185 192 
 Shearing of ether, 350, preface 
 
 stress, 41 
 Signalling by wire, 199, 238 
 
 speed, 29, 202, 238244, 400 
 Signals affected by capacity and 
 self-induction, 241, 242 
 
 Simple harmonic motion analyzed, 
 280 
 
 harmonic motion period, 229 
 Slider on rails, 207 
 Slip and spin, 192 
 Smith, Willoughby, 325 
 Soaking in, 35, 37 
 Solid angle, 393 
 Solids, properties dependent on 
 
 past history, 162 
 Sound, propagation of, 72 
 Space not a conductor, 17, 351 
 Spark-length affected by light, 302 
 Spark, light of, one starting another, 
 
 302 
 Sparks, in wall paper, 374 
 
 musical, 376 380 
 Specific heat of electricity 120, 
 
 295 
 Specific inductive capacity, 34, 53> 
 
 22O, 222 
 
 compared with index of refrac- 
 tion, 250 256 
 
 not constant, 282 
 
 probable real value, 235 
 Specific resistance, 70 
 Speed, see Velocity 
 Spin and slip, 192 
 Spiral liquid jet, 141 
 Spring analogue to Leyden jar, 
 
 362 
 
 Spurious charge, 53 
 Stationary waves, Hertz's, 237, 274 
 Steel, magnetization of, 163 
 
 state of, 162 
 
 Stokes, Sir G. G., 6, 298 
 Strain in dielectric, 20 
 
 in medium, causing attraction 
 and repulsion, 28 
 
 of medium, recovery, 226 
 Stratified condenser, 35 40 
 Stream lines, 102 105 
 Stress in magnetic medium, 172 
 
 176 
 
 Sub-permanent magnetism, 1 60 
 Surface charge, 53 
 
 conduction, 101 
 Surgings in circuit, 374, 375 
 
INDEX. 
 
 T. 
 
 TAIT, 296 
 
 Telegraph, function of wire in, 
 
 198 202, 238 
 
 Telegraphic return circuit, 28 
 Telegraphing, methods of, 196 
 
 202 
 
 speed of, 202, 400 
 affected by capacity and self- 
 induction, 241, 242, 402 
 Telephone currents, 101 
 Tenacity, electric, 225 
 Tension along magnetic lines of 
 
 force, 172 176 
 
 Temperature raised by current, 71 
 Thermal E.M.F., 117 
 Thermo-electric pile, 116 
 
 view of Hall effect, 291, 292 
 
 297 
 
 Thompson, S. P., 121, 141, 326 
 Thomson, J. J., 266, 355 
 Thomson, Sir W., 5, 6, 42, in, 
 235, 265, 321, 371, 380, 
 preface 
 
 effect, 1 1 8, 295 
 effect, connected with Hall's, 
 
 295 2 97 
 
 form of Volta effect, 113 
 theory of elasticity, 265 
 theory of matter, 356358 
 Time necessary to start current, 99 
 
 of Faraday effect, 293, 380 
 Tourmaline, 121, 122 
 Traces, 332, 333 
 Tram-car, electric, 97 
 
 rope, 96, 333 
 Transfer of energy to distance, 96, 
 
 97, 196202 
 of heat, water, and electricity, 
 
 66 
 
 Transformers, 214, 399 
 Transmission of energy, 96, 97 
 
 of pictures by electricity, 326 
 Transparency of insulators, 269 
 
 272, 305, 323 
 
 Transverse vibrations, transmissible 
 by ether, 231, 239 
 
 Tuning-fork analogue to discharge, 
 
 230232 
 Two-fluid theory, 26, 28, see Dual 
 
 view of electricity 
 
 U. 
 
 ULTRA-VIOLET light, effect on 
 sparks, 302 
 
 rays, 267 
 Undulations, 15 
 
 Undulatory, meaning of word, 317 
 Unit, natural, of electricity, 77 
 Units, artificial systems of, 235 
 
 ratio of, 245 
 
 system of, 404 
 
 V. 
 
 "V," modes of determining, 245, 
 
 319 
 
 Vacuum-tube, rotating discharge in, 
 
 142 
 
 Vacuum versus Plenum, 328 
 Valency, 75 
 Varying current, 185 195 
 
 magnetic field and electrostatic 
 
 charge, 144, 211, 215 
 Velocity of electricity, Wheatstone's 
 
 experiment on, 169 
 of electric pulses along wires, 
 method of determining, 244 
 of electric pulses along wire, 
 
 of electric radiation, 234, 236 
 
 -256 
 
 of gravitation, 232 
 of ions, 87 
 
 of light, 246 256, 340, 407 
 of signalling, 29,202, 238 244, 
 
 400 
 
 of wave propagation, 230 
 Velocities of light and electric waves 
 
 compared in substances, 250 
 Vibration, direction of, 306 
 Villari, 293 
 
422 
 
 . INDEX. 
 
 Viscosity analogue of starting 
 current, 98, 99 
 
 resistance, 227 
 Volta-effect, cord model of, 113 
 
 Thomson form of, 113 
 Voltaic battery, 106 109 
 
 E. M, F., 210 
 Voltameter, 75, 108 
 Volta's contact force, 109 114 
 Vortex, elasticity of, 355 
 
 ring, electric, 171, 187, 213 
 
 theory of matter, 356358 
 
 vibration of, 355 
 Vortices, 15, 354 35 8 
 
 W. 
 
 WALL paper, sparks in, 374 
 Ward, A. W., 294 
 
 Waste radiation, 258, 259 
 
 Water-ram, 88 
 
 Wave, definition of, 316 
 
 length, electric, 247, 248, 396 
 
 propagation, 16, 230 233, 367 
 Waves, Hertz's, 237, 304 
 
 length of Leyden jar, 248, 396 
 
 stationary, 274 
 Weber's theory of diamagnetism, 
 
 Wheatstone photometer, 321 
 
 velocity of electricity, 169 
 Whirl of electricity, 90, 131, 171 
 
 176, 3H 
 
 Wiedemann and Ebert, 302 
 Wimshurst machine, 379, 381 
 Wind, electric, 168, 300 
 Wire, function of, 96, 238 
 Work done in magnetic cycle, 164 
 
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