The Scientific Monthly/Volume 14/June 1922/The Ether Theories of Electrification

by Fernando Sanford

THE ETHER THEORIES OF ELECTRIFICATION

By Professor FERNANDO SANFORD

Stanford University

IN a previous paper on "The Electric Fluid Theories" it was shown that neither the one fluid nor the two fluid theory gave any physical explanation of the cause of electric attraction or repulsion, though it was this explanation which was the principal purpose of the earlier emanation theory. It was also shown that a fundamental question which was left unsettled was how the electric fluid is held to a conductor while there is no attraction between it and the particles of matter.

The only theory proposed during this time which seems to suggest any explanation of this phenomenon was one proposed by Cavendish, who regarded the electric fluid in conductors as under an external pressure and as always flowing in the direction of least pressure. Cavendish did not discuss the question of the retention of a charge upon a conductor, for he regarded the electric fluid as being attracted by the particles of material bodies; but he seems to have had a very definite notion of an external pressure exerted upon the electric fluid in a charged body and of the reaction of the electric fluid to this pressure. Thus he says:

When the electric fluid within any body is more compressed than in its natural state, I call that body positively electrified. When it is less compressed, I call the body negatively electrified.

It is plain from what has been here said that if any number of conducting bodies be joined by conductors and one of these bodies be positively electrified, that all the others must be so too.

If two bodies, both perfectly insulated, so that no electricity can escape from them, be positively electrified and then brought near each other, as they are both overcharged they will each, by the action of the other upon it, be rendered less capable of containing electricity; therefore, as no electricity can escape from them, the fluid in them will be more compressed, just as air included within a bottle will become more compressed either by heating the air or by squeezing the bottle into less compass: but it is evident that the bodies will remain just as much overcharged as before.

It will be seen from the above quotations that Cavendish regarded the compressed electric fluid in one charged body as capable of transmitting in some way a pressure to the fluid in another body brought near it. This would seem to indicate that he supposed this pressure to be exerted by some external elastic medium which was itself thrown into a state of stress by the reaction of the com pressed electric fluid.

Apparently the first physicist to definitely suggest the pressure of an elastic medium as the cause of electric attractions and repulsions was Dr. Thomas Young. In his lectures which were given before the Royal Institution and published in 1807 he says:

It must be confessed that the whole science of electricity is yet in a very imperfect state. We know little or nothing of the intimate nature of the substances and actions concerned in it: and we can never foresee, without previous experiment, where or how it will be excited. We are wholly ignorant of the constitution of bodies, by which they become possessed of different conducting powers; and we have only been able to draw some general conclusions respecting the disposition and equilibrium of the supposed electric fluid from the laws of the attractions and repulsions that it appears to exert. There seems to be some reason to suspect from the phenomena of cohesion and repulsion that the pressure of an elastic medium is concerned in the origin of these forces; and if such a medium really exists, it is perhaps nearly related to the electric fluid.

Between the time of Cavendish's writing and the publication of Young's lectures there had been great advancement in electrical knowledge and very important improvement in facilities for electrical measurement. The most important discovery in electrostatics was Bennett's discovery of contact electrical charges. Galvani's discovery of muscular contractions due to electrical stimulation of the nerves had attracted a great deal of attention, and there was much controversy over the question whether the force with which Galvani was experimenting was really electrical, or whether it was some force before unknown and which was called Galvanism. This question was finally settled by Volta, who in 1796 discovered the electrical current set up in a circuit containing two metals and an electrolytic conductor. Then in 1800 Volta announced the discovery of the voltaic pile, by means of which the electromotive force of a single metallic pair could be increased to any desired degree. This was followed the same year by the discovery by Carlisle and Nicholson of the separation of water by electrolysis and very rapidly thereafter by the experiments of Davy and others on electrolytic dissociation. As a result of these investigations, the study of electrostatic phenomena was neglected, and aside from the work of Faraday but little of consequence has since been done in this field.

Faraday's most important contributions to electrostatics were his proofs of the perfect equality of the inducing and induced charges in all cases; his proof that electric induction might sometimes act in curved lines (as he stated it) and hence could not be due to the action of forces at a distance, as such action would necessarily be rectilinear; and finally, in 1837, the discovery of specific inductive capacity.

This discovery as made by Faraday consisted in determining the division of an electric charge between two equal conductors when one was surrounded by air and the other by some other insulating medium. The apparatus used consisted of two brass balls 2.33 inches in diameter and exactly alike mounted upon insulating supports concentrically inside of two similar hollow brass spheres 3.57 inches internal diameter. The inner balls were carefully insulated from the outer hollow conductors, and the space between could be left filled with air or could be filled with some insulating solid or liquid. The outer hollow spheres were joined to earth.

Thus in the diagram in Figure 1 the inner spheres are indicated by A and B, the outer hollow spheres by C and D. C and D are joined to earth. If, now, A and B are connected and charged, then separated and the hollow spheres removed from around them, they are found to have equal charges.

Faraday found that when the space between A and C was half filled with sulphur and A and B were connected and charged as before and the outer spheres and the sulphur were removed A was found to have 2.24 times as great a charge as B. The experiment was repeated with glass, shellac and other substances instead of sulphur, and it was found that in every case A took a greater charge than B, but that the magnitude of the charge depended upon the insulating material between A and C.

Faraday believed that the so-called charges upon A and B were merely manifestations of some condition known as induction which had been produced in the medium between the inner and outer spheres, though why this condition should persist after the medium was removed he does not say. In Article 1174 of Experimental Researches he says:

The conclusion I have come to is that non-conductors, as well as conductors, have never yet had an absolute and independent charge of one electricity communicated to them, and that to all appearance such state of matter is impossible.

Again (Arts. 1177 and 1178), as far as experiment has proceeded, it appears, therefore, impossible either to evolve or make disappear one electric force without equal and corresponding change in the other. It is also equally impossible experimentally to charge a portion of matter with one electric force independently of the other. Charge always implies induction, for it can in no instance be effected without; and also the presence of the two forms of power, equally at the moment of development and afterwards. There is no absolute charge of matter with one fluid; no latency of a single electricity. This, though a negative result, is an exceedingly important one, being prob ably the consequence of a natural impossibility, which will become clear to us when we understand the true condition and theory of the electric power.

The preceding considerations already point to the following conclusions: bodies cannot be charged absolutely, but only relatively, and by a principle which is the same with that of induction. All charge is sustained by induction. All phenomena of intensity include the principle of induction. All excitation is dependent on or directly related to induction. All currents involve previous intensity and therefore previous induction. Induction appears to be the essential function both in the first development and the consequent phenomena of electricity.

From this point of view, the charges of A and B in the above experiment were merely manifestations of some condition in the medium between A and C and between B and D. If A assumed a larger proportion of the total electrification than B, it was because induction took place more freely between A and C than be tween B and D. Faraday accordingly said that sulphur had a greater capacity for electrical induction than air, and that if the inductive capacity of air were taken as 1, the inductive capacity of sulphur would be greater than 2.24.

It may be interesting at this point to consider briefly the difference between the views of Cavendish and Faraday. Cavendish believed all bodies to contain an unknown quantity of a single electric fluid, and that this fluid was always under some kind of external pressure and that in conductors it always flowed toward the region of least external pressure and could be in equilibrium only when the external pressure was everywhere the same over the surface of the conductor or system of conductors in which the fluid was confined. From his point of view, the reason that A took a greater charge than B in the Faraday experiment was that the external pressure on a given charge was less around A than around B, and hence that an excess of fluid would flow into A until this external pressure was equalized upon the fluid in the two spheres.

Faraday attributed the state known as electrification not to any changed condition inside the charged conductor, but wholly to conditions in the insulating medium surrounding the conductors concerned. A charged conductor was only one limiting boundary of an electrical field. A charge could not penetrate into a conductor, because the state of strain which was the essential condition of induction could not exist in conductors. In order for this state of strain to exist the bounding surfaces of the strained medium must be on different conductors insulated from each other, because from this theory the different surfaces must support equal and opposite stresses, and this is impossible over a conducting surface, which from its nature cannot support an electric stress at all. If the two surfaces of the region of strain, which Faraday called the dielectric, are joined by a conductor, the strain is relieved and the electrification disappears. Hence there can be no electrification upon the inner walls of a closed hollow conductor, since this would involve a condition of equal and opposite strains resting upon the same conducting surface.

It will be seen from the above that Faraday's theory does not involve the existence of any electric fluid whatever. Faraday calls attention to this in a foot-note to his Article 1298. He says:

The theory of induction which I am stating does not pretend to decide whether electricity is a fluid or fluids, or a mere power or condition of recognized matter.

Even in the production of a current Faraday does not admit the necessity for the passage of any electric fluid through the conductor. From his point of view, since charging a body consists in setting up a certain condition of strain in the dielectric between it and another body, or other bodies, so discharging an electrified body consists merely in removing this state of strain in the dielectric around it. This strain can exist permanently in an insulator; it breaks down rapidly in a conductor. While it is breaking down the current is said to be passing through the conductor.

Faraday's notion as to the nature of the condition which he called induction was not clear, as may be shown by the following quotation from Article 1298:

Induction seems to consist in a certain polarized state of the particles, into which they are thrown by the electrified body sustaining the action, the particles assuming positive or negative points or parts, which are symmetrically arranged with reference to each other and the inducting surfaces or particles. The state must be a forced one, for it is originated and sustained only by force, and sinks to the normal or quiescent state when the force is removed. It can be continued only in insulators by the same portion of electricity, because they only can retain this state of the particles. ........... When a Leyden jar is charged, the particles of the glass are forced into this polarized and constrained condition by the electricity of the charging apparatus. Discharge is the return of these particles to their natural state of tension, whenever the two electric forces are allowed to be disposed of in some other direction.

The question as to the cause of this state of polarization is left entirely unanswered by Faraday, and this question is fundamental. The fact that electrical induction could take place in the best air pump vacuum seemed to require that all space must be filled with a medium made of polarizable particles, and this assumption was not readily accepted, especially at a time when the notion of force acting at a distance had become the common heritage of physicists. For this and other reasons Faraday's electrical theory did not meet with general acceptance at the time when it was proposed.

In 1873 Maxwell published the first edition of his Electricity and Magnetism which brought the fundamental ideas of Faraday into a position of prominence in English speaking countries which they have largely maintained up to very recent times.

Maxwell undertook to show in his treatise that the quantitative laws of electricity and magnetism which had been put into mathematical form on the assumption of forces acting at a distance could also be put into mathematical form on the basis of Faraday's notion of induction.

Thus Maxwell says:

I was aware that there was supposed to be a difference between Faraday's way of conceiving phenomena and that of the mathematicians, so that neither he nor they were satisfied with each other's language. I had also the conviction that this discrepancy did not arise from either party being wrong.

I was first convinced of this by Sir William Thomson, to whose advice and assistance, as well as to his published papers, I owe most of what I have learned on the subject.

As I proceeded with the study of Faraday, I perceived that his method of conceiving the phenomena was also a mathematical one, though not exhibited in the form of mathematical symbols. I also found that these methods were capable of being expressed in the ordinary mathematical forms, and thus compared with those of the professed mathematicians.

For instance, Faraday, in his mind's eye, saw lines of force traversing all space where the mathematicians saw centers of force attracting at a distance: Faraday sought the seat of the phenomena in real actions going on in the medium; they were satisfied that they had found it in a power of action at a distance impressed on the electrical fluids.

When I had translated what I conceived to be Faraday's ideas into a mathematical form, I found that in general the two methods coincided, so that the same phenomena were accounted for, and the same laws of action deduced by both methods, but that Faraday's methods resembled those in which we begin with the whole and arrive at the parts by analysis, while the ordinary mathematical methods were founded on the principle of beginning with the parts and building up the whole by synthesis.

I also found that several of the most fertile methods of research discovered by the mathematicians could be expressed much better in terms of ideas derived from Faraday than in their original form.

Maxwell takes pains to emphasize this statement of the purpose of his treatise. Thus in Vol. II, p. 176, 3d Ed., he says:

It was perhaps for the advantage of science that Faraday, though thoroughly conscious of the fundamental forms of space, time and force, was not a professed mathematician. He was not tempted to enter into the many interesting researches in pure mathematics which his discoveries would have suggested if they had been exhibited in a mathematical form, and he did not feel called upon either to force his results into a shape acceptable to the mathematical taste of the time, or to express them in a form which mathematicians might attack. He was thus left at leisure to do his proper work, to coordinate his ideas with his facts, and to express them in natural, untechnical language.

It is mainly with the hope of making these ideas the basis of a mathematical method that I have undertaken this treatise.

Maxwell accordingly undertook to specify the conditions in a dielectric medium by means of which the induction effects discussed by Faraday could be explained from the known laws of mechanics. In doing this he used as much as possible the fundamental concepts of Faraday in so far as these could be determined.

Faraday's researches were carried on through a term of years and were presented as they were finished. Naturally, one who departed so fundamentally in his electrical concepts from all who had preceded him, and who discovered so many new phenomena in electricity and magnetism, was obliged to modify his views as he proceeded. In his Experimental Researches Faraday gives us, not his mature opinion at the conclusion of his work, but the evolution of his theory as it took shape in his mind. It is accordingly possible to get different notions of Faraday's theory from different parts of his Researches.

Thus, in the discussion of induction which has been in part quoted Faraday speaks of the phenomena as being entirely due to a condition in the dielectric medium, and he discusses the direction of the lines of force of the inductive stress in this medium. In the early stages of his work he uses the term "lines of force" in a purely mathematical sense, that is, as giving throughout their length the direction of the inductive force. Later he came to think of the dielectric medium as consisting wholly of physical lines of force. In one of his latest papers (Proc. Roy. Inst., June 11, 1852) he discusses the characteristics which must distinguish physical lines of force from abstract, or mathematical, lines of force, and decides that both electrical and magnetic phenomena are dependent upon physical lines of force; that is, the lines of force are no longer used to describe phenomena, but to explain them.

His later ideas as to the nature of these physical lines of force are perhaps most fully explained in a letter to Richard Phillips, Esq., written in May, 1846, and published in Experimental Researches, III., p. 447. Some extracts from this letter are given below.

You are aware of the speculation which I sometime since uttered respecting that view of the nature of matter which considers its ultimate atoms as centers of force, and not as so many little bodies surrounded by forces, the bodies being considered in the abstract as independent of the forces and capable of existing without them. In the latter view, these little particles have a definite form and a certain limited size; in the former view such is not the case, for that which represents size may be considered as extending to any distance to which the lines of force of the particles extend: the particle indeed is supposed to exist only by these forces, and where they are it is. The consideration of matter under this view gradually led me to look at the lines of force as being perhaps the seat of the vibrations of radiant phenomena. ............. The ether is assumed as pervading all bodies as well as space: In the view now set forth, it is the forces of the atomic centres which pervade (and make) all bodies, and also penetrate all space. As regards space, the difference is, that the ether presents successive parts or centres of action, and the present supposition only lines of action; as regards matter, the difference is, that the ether lies between the particles and so carries on the vibrations, whilst as respects the supposition, it is by the lines of force between the centres of the particles that the vibration is continued.

Again, in Experimental Researches II, p. 291, Faraday presents his theory of the nature of matter in much the same manner as above. At the conclusion of this discussion, he says:

The view now stated of the constitution of matter would seem to involve necessarily the conclusion that matter fills all space, or, at least, all space to which gravitation extends (including the sun and its system); for gravitation is a property of matter dependent on a certain force, and it is this force which constitutes the matter. In that view, matter is not merely mutually penetrable, but each atom extends, so to say, throughout the whole of the solar system, yet always retaining its own centre of force.

We see from the above that Faraday's later electrical theory was based upon a concept of the nature of matter which is no longer regarded as tenable, but which necessarily profoundly modified his views on electrical phenomena. It did away at once with all distinction between matter and the ether, unless those parts of space in which the centers of force were less numerous than in other parts could be regarded as a separate medium. Any question as to the number of electrical fluids, or whether there was any electrical fluid at all, could have little significance. The atoms of bodies were merely centers from which innumerable contractile filaments which he called lines of force radiated in all directions and throughout all space. From his reasoning, these filaments must extend to the limits of the physical universe, and every point in space must be traversed by lines from all the centers in the universe, as otherwise there would be points in which the law of gravitation would not apply. Whether these lines of force are of different kinds, so that gravitation depends upon one kind, electric phenomena upon another kind and magnetic phenomena upon a third kind, Faraday does not state, but this condition would seem to follow from the rest of his theory.

When Maxwell undertook to interpret Faraday to the mathematicians he was compelled to choose between the more or less contradictory views which Faraday had expressed at different times, and he naturally undertook to select the views which could be used as the most satisfactory basis for a mathematical theory of electricity and magnetism. In doing this, he does not adopt Faraday's extreme views of the identity of matter and force. The distinction between force and energy was much more clearly understood at the time of Maxwell's writing than it was when Faraday was carrying on his investigations. In fact, energy, as a concept distinct from force, was not known to Faraday, and Maxwell shows early in his treatise that what had been defined as electricity or electrical quantity could not be measured as energy. He does, however, adopt Faraday's concept of physical lines of force, but somewhat in the manner of Faraday's earlier views, in which the lines of force were regarded as chains of polarized material particles.

Maxwell first defines his lines of force in a purely mathematical sense. Thus he says (Elec. and Mag. I, 97):

If a line be drawn whose direction at every point of its course coincides with that of the resultant intensity at that point, the line is called a Line of Force.

In every part of the course of a line of force, it is proceeding from a place of higher potential to a place of lower potential.

Hence a line of force cannot return into itself, but must have a beginning and an end. The beginning of a line of force must, by Number 80, be in a positively charged surface, and the end of a line of force must be in a negatively charged surface.

It is easily seen that such a line of force does not pull the positively and negatively charged surfaces together. It is merely the path along which a positively or negatively electrified particle would move if set free on the line of force. It does not explain the motion of the particle, it merely describes it. When he undertakes to explain why an electrified particle would travel along a line of force, Maxwell says:

At every point of the medium there is a state of stress such that there is a tension along the lines of force and pressure in all directions at right angles to these lines, the numerical magnitude of the pressure being equal to that of the tension, and both varying as the square of the resultant force at the point.

In another place Maxwell argues that the state of stress described above is the only one consistent with the observed mechanical action of the electrified bodies and also with the observed equilibrium of the fluid dielectric which surrounds them.

Sir J. J. Thomson, who edited the third edition of Maxwell's treatise, takes exception to this claim. He says in a foot-note on page 165:

The subject of the stress in the medium will be further considered in the supplementary volume; it may however be noticed here that the problem of finding a system of stresses which will produce the same forces as those existing in the electric field is one which has an infinite number of solutions. That adopted by Maxwell is one which could not in general be produced by strains in an elastic solid.

This, in connection with the preceding quotation from Max well, indicates that Maxwell regarded his dielectric medium as necessarily a fluid; hence when the only dielectric between the positive and negative electrical condition is the ether of space, this medium must be a fluid. This seems to contradict the well-known fact that the only known forms of ether radiation are of the nature of transverse waves.

Maxwell goes no further than Faraday in explaining the condition of stress which is supposed to constitute induction. He merely attempts to describe it. He says:

It must be carefully borne in mind that we have made only one step in the theory of the action of the medium. We have supposed it to be in a state of stress, but we have not in any way accounted for this stress, or explained how it is maintained. ............. I have not been able to make the next step, namely, to account by mechanical considerations for these stresses in the dielectric. I therefore leave the theory at this point, merely stating what are the other parts of the phenomenon of induction in dielectrics.

Maxwell's claim is, accordingly, that if the dielectric medium between two charges, said charges being always necessarily upon the opposite surfaces of the dielectric, should contract in the direction of the lines of force normal to its charged boundaries and should expand in all directions at right angles to these lines of force, this contraction and expansion would enable him to account for the other phenomena of electrostatic induction.

Maxwell does make the further assumption that this stress in the dielectric is analogous to an elastic stress in material bodies. Thus he says:

The analogy between the action of electromotive intensity in producing electric displacement and of ordinary mechanical force in producing the displacement of an elastic body is so obvious that I have ventured to call the ratio of the electromotive intensity to the corresponding electric displacement the coefficient of electric elasticity of the medium. The coefficient is different in different media, and varies inversely as the specific inductive capacity of each medium.

Farther along in his treatise Maxwell argues that this "Elec tric Elasticity" is the elasticity by means of which light waves are propagated through the ether. Thus he says (Vol. II, p 431):

According to the theory of emission, the transmission of energy is effected by the actual transference of light corpuscles from the luminous to the illuminated body, carrying with them their kinetic energy, together with any other kind of energy of which they may be the receptacles.

According to the theory of undulation, there is a material medium which fills the space between the bodies, and it is by the action of contiguous parts of this medium that the energy is passed on from one portion to the next, till it reaches the illuminated body.

The luminiferous medium is therefore, during the passage of light through it, a receptacle of energy.

In the undulatory theory as developed by Huyghens, Fresnel, Young, Green, etc., this energy is supposed to be partly potential and partly kinetic. The potential energy is supposed to be due to the distortion of the elementary portions of the medium. We must therefore regard the medium as elastic. The kinetic energy is supposed to be due to the vibratory motion of the medium. We must therefore regard the medium as having a finite density.

In the theory of electricity and magnetism adopted in this treatise, two forms of energy are recognized, the electrostatic and the electrokinetic (see Arts. 630 and 636), and these are supposed to have their seat, not merely in the electrified or magnetized bodies, but in every part of the surrounding space, where electric or magnetic force is observed to act. Hence our theory agrees with the undulatory theory in assuming the existence of a medium which is capable of becoming the receptacle of two forms of energy.

Let us next determine the conditions of the propagation of an electromagnetic disturbance through a uniform medium, which we shall suppose to be at rest, that is, to have no motion except that which may be involved in electromagnetic disturbances.

Maxwell then proceeds to develop an equation for the velocity of an electromagnetic disturbance in terms of the specific inductive capacity and the magnetic permeability of the medium and which, if the specific inductive capacity be taken as the reciprocal of the elasticity and the magnetic permeability be taken as the density of the medium gives an expression for the velocity of a wave motion in an elastic medium. It also gives an expression for the ratio of the electromagnetic to the electrostatic unit of electricity, or the velocity with which a unit electrostatic charge must move in order to become electromagnetically a unit current. This ratio can be deter mined experimentally, and gives a quantity numerically equal to the velocity of light.

He then says:

In other media than air the velocity V is inversely proportional to the product of the dielectric and the magnetic inductive capacities. According to the undulatory theory, the velocity of light in different media is inversely pro portional to their indices of refraction.

There are no transparent media for which the magnetic capacity differs from that of air more than by a very small fraction. Hence the principal part of the difference between these media must depend upon their dielectric capacity. According to our theory, therefore, the dielectric capacity of a transparent medium should be equal to the square of its index of refraction.

In Maxwell's theory we accordingly find the dielectric medium of Faraday identified with the luminiferous ether. But the elasticity of the luminiferous ether which is involved in the transmission of all known forms of radiation must be of the nature of rigidity, and a fluid dielectric such as Maxwell's assumption of contracting lines of force seems to require does not possess rigidity.

It would accordingly seem that while Maxwell's method of calculating the velocity of light from purely electrical experiments seems to prove beyond question that the luminiferous ether is the medium of electric and magnetic induction, the assumption as to the contraction of this medium in the direction of the electrical lines of force and its expansion in all directions at right angles to these lines may require modification.

It is plain that this assumption that an electrical charge is merely one aspect of a stress in the ether is equivalent to a denial of an electrical substance per se. It is difficult to see, however, how from the assumption of a mere contraction of the dielectric between two conductors the surfaces of the conductors could be put in qualitatively different electrical conditions such as are known to distinguish positively and negatively electrified bodies. Both aspects of such a stress would appear to be exactly alike, just as the stresses at the opposite ends of a stretched elastic cord.

It accordingly became necessary to make some further assumptions to account for the difference in the positive and negative electrical surfaces. Here recourse was again had to Faraday's notion of a polarizable medium; that is, a medium made up of particles having opposite electrical properties at two opposite extremities. The ether accordingly came to be regarded by many of Maxwell's successors as made up of particles or "cells" holding positive charges on one side and negative charges on the opposite side, very much as the current magnetic theory regards a magnet as made up of molecules having a north magnetic pole on one face and a south magnetic pole on the opposite face. The polarization of the medium in electrical induction was supposed to consist in the orientation of these hypothetical particles so that their charges of the same kind were turned in the same direction.

Thus Lodge (Modern Views of Electricity, p. 349) says:

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 electromotive 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 conductor 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 dielectric.

A similar view to this was expressed in most text books on Electricity written in the English language between 1890 and 1900. Thus in his well-known text book on Electricity and Magnetism (Edition of 1895) S. P. Thompson attempts to define electricity as follows:

Electricity is the name given to an invisible agent known to us only by the effects which it produces and by various manifestations called electrical. These manifestations, at first obscure and even mysterious, are now well understood; though little is yet known of the precise nature of electricity itself. It is neither matter nor energy; yet it apparently can be associated or combined with matter; and energy can be spent in moving it. Indeed its great importance to mankind arises from the circumstance that by its means energy spent in generating electric forces in one part of a system can be made to appear as electric heat or light or work at some other part of the system; such transfer of energy taking place even to very great distances at an enormous speed. Electricity is apparently as indestructible as matter or energy. It can neither be created nor destroyed, but it can be transformed in its relations to matter and to energy, and it can be moved from one place to another. In many ways its behaviour resembles that of an incompressible liquid; in other ways that of a highly attenuated and weightless gas. It appears to exist distributed nearly uniformly throughout all space. Many persons (including the author) are disposed to consider it as identical with the luminiferous ether. If it be not the same thing, there is an intimate relation between the two. That this must be so is a necessary result of the great discovery of Maxwell—the greatest scientific discovery of the nineteenth century—that light itself is an electric phenomenon, and that the light waves are merely electric, or, as he puts it, electromagnetic waves.

In 1893 J. J. Thomson published his Recent Researches in Electricity and Magnetism, in which he carried the Faraday-Maxwell theory to a development almost as extreme as the later views of Faraday, to which reference has already been made. Only a short time later, he and his fellow workers succeeded in identifying the electrical fluid concerning whose existence there had been so much argument for 150 years. The development of the ether theory by Thomson should form the subject of another paper.