“Metals, however, probably hold it for a moment, as other things do for a longer time; an end coming at last to all.”
This, it will be observed, is nothing more or less than Clerk Maxwell’s theory of conduction as being the breaking down of an electrostatic strain.
In January, 1836, followed the famous experiment of building a twelve-foot cube, which when electrified exteriorly to the utmost extent, showed inside no trace of electric forces. The account in the unpublished MS. of the laboratory book is, as is the case with so many of these middle-period researches, much fuller than the published résumé of them in the “Experimental Researches.” All through 1836 he was still at work. Even when on a holiday in the Isle of Wight, in August, he took his notebook with him, and writes:—
“After much consideration (here at Ryde) of the manner in which the electric forces are arranged in the various phenomena generally, I have come to certain conclusions which I will endeavour to note down without committing myself to any opinion as to the cause of electricity, i.e. as to the nature of the power. If electricity exist independently of matter, then I think that the hypothesis of one fluid will not stand against that of two fluids. There are, I think, evidently, what I may call two elements of power of equal force and acting towards each other. These may conventionally be represented by oxygen and hydrogen, which represent them in the voltaic battery. But these powers may be distinguished only by direction, and may be no more separate than the north and south forces in the elements of a magnetic needle. They may be the polar points of the forces originally placed in the particles of matter; and the description of the current as an axis of power which I have formerly given suggests some similar general impression for the forces of quiescent electricity. Law of electric tension might do, and though I shall use the terms positive and negative, by them I merely mean the termini of such lines.”
Right on until November 30th, 1837, this research was continued. The summary of this and the succeeding researches of 1838 on the same subject, drawn up by Professor Tyndall,45 is at once so masterly and so impartial that it cannot be bettered. It is therefore here transcribed without alteration.
His first great paper on frictional electricity was sent to the Royal Society on November 30, 1837. We here find him face to face with an idea which beset his mind throughout his whole subsequent life—the idea of action at a distance. It perplexed and bewildered him. In his attempts to get rid of this perplexity he was often unconsciously rebelling against the limitations of the intellect itself. He loved to quote Newton upon this point: over and over again he introduces his memorable words, “That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum and without the mediation of anything else, by and through which this action and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial I have left to the consideration of my readers.”46
Faraday does not see the same difficulty in his contiguous particles. And yet by transferring the conception from masses to particles we simply lessen size and distance, but we do not alter the quality of the conception. Whatever difficulty the mind experiences in conceiving of action at sensible distances, besets it also when it attempts to conceive of action at insensible distances. Still the investigation of the point whether electric and magnetic effects were wrought out through the intervention of contiguous particles or not, had a physical interest altogether apart from the metaphysical difficulty. Faraday grapples with the subject experimentally. By simple intuition he sees that action at a distance must be exerted in straight lines. Gravity, he knows, will not turn a corner, but exerts its pull along a right line; hence his aim and effort to ascertain whether electric action ever takes place in curved lines. This once proved, it would follow that the action is carried on by means of a medium surrounding the electrified bodies. His experiments in 1837 reduced, in his opinion, this point to demonstration. He then found that he could electrify by induction an insulated sphere placed completely in the shadow of a body which screened it from direct action. He pictured the lines of electric force bending round the edges of the screen, and reuniting on the other side of it; and he proved that in many cases the augmentation of the distance between his insulated sphere and the inducing body, instead of lessening, increased the charge of the sphere. This he ascribed to the coalescence of the lines of electric force at some distance behind the screen.
Faraday’s theoretic views on this subject have not received general acceptance, but they drove him to experiment, and experiment with him was always prolific of results. By suitable arrangements he places a metallic sphere in the middle of a large hollow sphere, leaving a space of something more than half an inch between them. The interior sphere was insulated, the external one uninsulated. To the former he communicated a definite charge of electricity. It acted by induction upon the concave surface of the latter, and he examined how this act of induction was affected by placing insulators of various kinds between the two spheres. He tried gases, liquids, and solids, but the solids alone gave him positive results. He constructed two instruments of the foregoing description, equal in size and similar in form. The interior sphere of each communicated with the external air by a brass stem ending in a knob. The apparatus was virtually a Leyden jar, the two coatings of which were the two spheres, with a thick and variable insulator between them. The amount of charge in each jar was determined by bringing a proof-plane into contact with its knob, and measuring by a torsion balance the charge taken away. He first charged one of his instruments, and then dividing the charge with the other, found that when air intervened in both cases, the charge was equally divided. But when shell-lac, sulphur, or spermaceti was interposed between the two spheres of one jar, while air occupied this interval in the other, then he found that the instrument occupied by the “solid dielectric” took more than half the original charge. A portion of the charge was absorbed in the dielectric itself. The electricity took time to penetrate the dielectric. Immediately after the discharge of the apparatus no trace of electricity was found upon its knob. But after a time electricity was found there, the charge having gradually returned from the dielectric in which it had been lodged. Different insulators possess this power of permitting the charge to enter them in different degrees. Faraday figured their particles as polarised, and he concluded that the force of induction is propagated from particle to particle of the dielectric from the inner sphere to the outer one. This power of propagation possessed by insulators he calls their “Specific Inductive Capacity.”
Faraday visualises with the utmost clearness the state of his contiguous particles; one after another they become charged, each succeeding particle depending for its charge upon its predecessor. And now he seeks to break down the wall of partition between conductors and insulators. “Can we not,” he says, “by a gradual chain of association carry up discharge from its occurrence in air through spermaceti and water to solutions, and then on to chlorides, oxides, and metals, without any essential change in its character?” Even copper, he urges, offers a resistance to the transmission of electricity. The action of its particles differs from those of an insulator only in degree. They are charged like the particles of the insulator, but they discharge with greater ease and rapidity; and this rapidity of molecular discharge is what we call conduction. Conduction, then, is always preceded by atomic induction; and when through some quality of the body, which Faraday does not define, the atomic discharge is rendered slow and difficult, conduction passes into insulation.
Though they are often obscure, a fine vein of philosophic thought runs through these investigations. The mind of the philosopher dwells amid those agencies which underlie the visible phenomena of induction and conduction; and he tries by the strong light of his imagination to see the very molecules of his dielectrics. It would, however, be easy to criticise these researches, easy to show the looseness, and sometimes the inaccuracy, of the phraseology employed; but this critical spirit will get little good out of Faraday. Rather let those who ponder his works seek to realise the object he set before him, not permitting his occasional vagueness to interfere with their appreciation of his speculations. We may see the ripples, and eddies, and vortices of a flowing stream, without being able to resolve all these motions into their constituent elements; and so it sometimes strikes me that Faraday clearly saw the play of fluids and ethers and atoms, though his previous training did not enable him to resolve what he saw into its constituents, or describe it in a manner satisfactory to a mind versed in mechanics. And then again occur, I confess, dark sayings, difficult to be understood, which disturb my confidence in this conclusion. It must, however, always be remembered that he works at the very boundaries of our knowledge, and that his mind habitually dwells in the “boundless contiguity of shade” by which that knowledge is surrounded.
In the researches now under review the ratio of speculation and reasoning to experiment is far higher than in any of Faraday’s previous works. Amid much that is entangled and dark we have flashes of wondrous insight and utterances which seem less the product of reasoning than of revelation. I will confine myself here to one example of this divining power:—By his most ingenious device of a rapidly rotating mirror, Wheatstone had proved that electricity required time to pass through a wire, the current reaching the middle of the wire later than its two ends. “If,” says Faraday, “the two ends of the wire in Professor Wheatstone’s experiments were immediately connected with two large insulated metallic surfaces exposed to the air, so that the primary act of induction, after making the contact for discharge, might be in part removed from the internal portion of the wire at the first instance, and disposed for the moment on its surface jointly with the air and surrounding conductors, then I venture to anticipate that the middle spark would be more retarded than before. And if those two plates were the inner and outer coatings of a large jar or Leyden battery, then the retardation of the spark would be much greater.” This was only a prediction, for the experiment was not made. Sixteen years subsequently, however, the proper conditions came into play, and Faraday was able to show that the observations of Werner Siemens and Latimer Clark on subterraneous and submarine wires were illustrations, on a grand scale, of the principle which he had enunciated in 1838. The wires and the surrounding water act as a Leyden jar, and the retardation of the current predicted by Faraday manifests itself in every message sent by such cables.
The meaning of Faraday in these memoirs on induction and conduction is, as I have said, by no means always clear; and the difficulty will be most felt by those who are best trained in ordinary theoretic conceptions. He does not know the reader’s needs, and he therefore does not meet them. For instance, he speaks over and over again of the impossibility of charging a body with one electricity, though the impossibility is by no means evident. The key to the difficulty is this. He looks upon every insulated conductor as the inner coating of a Leyden jar. An insulated sphere in the middle of a room is to his mind such a coating; the walls are the outer coating, while the air between both is the insulator, across which the charge acts by induction. Without this reaction of the walls upon the sphere, you could no more, according to Faraday, charge it with electricity than you could charge a Leyden jar, if its outer coating were removed. Distance with him is immaterial. His strength as a generaliser enables him to dissolve the idea of magnitude; and if you abolish the walls of the room—even the earth itself—he would make the sun and planets the outer coating of his jar. I dare not contend that Faraday in these memoirs made all these theoretic positions good. But a pure vein of philosophy runs through these writings; while his experiments and reasonings on the forms and phenomena of electrical discharge are of imperishable importance.
In another part of the twelfth memoir, not included in the above summary, Faraday deals with the disruptive discharge, and with the nature of the spark under varying conditions. This is continued on into the thirteenth memoir, read February, 1838, and is extended to the cases of “brush” and “glow” discharges. He discovered the existence of the very remarkable phenomenon of the “dark” discharge near the cathode in rarefied air. He sought to correlate all the various forms of discharge, as showing the essential nature of an electric current. “If a ball be electrified positively,” he says, “in the middle of a room, and be then moved in any direction, effects will be produced, as if a current in the same direction (to use the conventional mode of expression) had existed.” This is the theory of convection currents later adopted by Maxwell, and verified by experiment by Rowland in 1876.
In the course of this research on induction, Faraday had, as we have seen, been compelled to adopt new ideas, and therefore to adopt new names to denote them. The term dielectric for the medium in or across which the electric forces operate was one of these. As in previous cases, he consulted with his friends as to suitable terms. In this instance the following letter from Whewell explains itself. The letter to which it is a reply has not been preserved, but the reference to Faraday’s objection to the word current may be elucidated by a comparison with what Faraday wrote in criticism of that word on pages 146 and 212.
[Rev. W. Whewell to M. Faraday.]
Trin. Coll., Cambridge, Oct. 14, 1837.
My dear Sir,—I am always glad to hear of the progress of your researches, and never the less so because they require the fabrication of a new word or two. Such a coinage has always taken place at the great epochs of discovery; like the medals that are struck at the beginning of a new reign:—or rather like the change of currency produced by the accession of a new sovereign; for their value and influence consists in their coming into common circulation. I am not sure that I understand the views which you are at present bringing into shape sufficiently well to suggest any such terms as you think you want. I think that if I could have a quarter of an hour’s talk with you I should probably be able to construct terms that would record your new notions, so far as I could be made to understand them better than I can by means of letters: for it is difficult without question and discussion to catch the precise kind of relation which you want to express. However, by way of beginning such a discussion, I would ask you whether you want abstract terms to denote the different and related conditions of the body which exercises and the body which suffers induction? For though both are active and both passive it may still be convenient to suppose a certain ascendancy on one side. If so would two such words as inductricity and inducteity answer your purpose? They are not very monstrous in their form; and are sufficiently distinct. And if you want the corresponding adjectives you may call the one the inductric, and the other the inducteous body. This last word is rather a startling one; but if such relations are to be expressed, terminations are a good artifice, as we see in chemistry: and I have no doubt if you give the world facts and laws which are better expressed with than without such solecisms, they will soon accommodate to the phrases, as they have often done to worse ones. But I am rather in the dark as to whether this is the kind of relation which you want to indicate. If not, the attempt may perhaps serve to shew you where my dulness lies. I do not see my way any better as to the other terms, for I do not catch your objection to current, which appears to me to be capable of jogging on very well from cathode to anode, or vice versa. As for positive and negative, I do not see why cathodic and anodic should not be used, if they will do the service you want of them.
I expect to be in London at the end of the month, and could probably see you for half an hour on the 1st of November, say at 10, 11, or 12. But in the mean time I shall be glad to hear from you whether you can make anything of such conundrums as I have mentioned, and am always yours very truly,
W. Whewell.
M. Faraday Esqre.
Royal Institution.
The concluding part of the thirteenth memoir, in which these new terms are used, is an exceedingly striking speculation on the lateral or transverse effects of the current. In calling special attention to them, he says: “I refer of course to the magnetic action and its relations; but though this is the only recognised lateral action of the current, there is great reason for believing that others exist and would by their discovery reward a close search for them.” He seems to have had an instinctive perception of something that eluded his grasp. Not until after Maxwell had given mathematical form to Faraday’s own suggestions was this vision to be realised. He is dimly aware that there appears to be a lateral tension or repulsion possessed by the lines of electric inductive action; and onward runs his thought in free speculation:—
When current or discharge occurs between two bodies, previously under inductrical relations to each other, the lines of inductive force will weaken and fade away, and, as their lateral repulsive tension diminishes, will contract and ultimately disappear in the line of discharge. May not this be an effect identical with the attractions of similar currents? i.e. may not the passage of static electricity into current electricity, and that of the lateral tension of the lines of the inductive force into the lateral attraction of lines of similar discharge, have the same relation and dependences, and run parallel to each other?
Series fourteen of the memoirs is on the nature of the electric force and on the relation of the electric and magnetic forces, and comprises an inconclusive inquiry as to a possible relation between specific inductive capacity and axes of crystallisation in crystalline dielectrics—a relation later assumed as true by Maxwell even before it was demonstrated by Von Boltzmann. In this memoir, too, occurs a description of a simple but effective induction balance. Then he asks what happens to insulating substances, such as air or sulphur, when they are put in a place where the magnetic forces are varying; they ought, he thinks, to undergo some state or condition corresponding to the state that causes currents in metals and conductors, and, further, that state ought to be one of tension. “I have,” he says, “by rotating non-conducting bodies near magnetic poles, and poles near them, and also by causing powerful electric currents to be suddenly formed and to cease around and about insulators in various directions, endeavoured to make some such state sensible, but have not succeeded.” In short, he was looking for direct evidence of the existence of what Maxwell called “displacement currents”—evidence which was later found independently by the author and by Röntgen. And, again, there rises in his mind a perception of that electrotonic state which had haunted his earlier researches as a something imposed upon the surrounding medium during the growth or dying of an electric current.
In these years (1835–1838) Faraday was still indefatigable in his lecture duties. In 1835 he gave four Friday discourses, and in May and June eight afternoon lectures at the Royal Institution on the metals; also a course of fourteen lectures on electricity to the medical students at St. George’s Hospital. In 1836 he published in the Philosophical Magazine a paper on the magnetism of the metals—notable as containing the still unverified speculation that all metals would become magnetic in the same way as iron if only cooled to a sufficiently low temperature—and three other papers, including one on the “passive” state of iron. He gave four Friday discourses and six afternoon lectures on heat. In 1837 also four Friday night discourses and six afternoon lectures were delivered. In 1838 three Friday discourses and eight afternoon lectures on electricity, ending in June with a distinct enunciation of the doctrine of the transformations of “force” (i.e. energy) and its indestructibility, afforded evidence of his industry in this respect. At the same time he was giving scientific advice to the authorities of Trinity House as to their lighthouses.
The laboratory notebook for March to August, 1838, shows a long research, occupying nearly 100 folio pages, on the relation of specific inductive capacity to crystalline structure. This is followed by some experiments upon an electric eel, at the Royal Adelaide Gallery, with some unpublished sketches of the distribution in the water of the currents it emits. He proved, with great satisfaction, that the currents it gave were capable of producing magnetic effects, sparks, and chemical decomposition. These observations were embodied in the fifteenth series of memoirs.
One entry in the laboratory book, of date April 5th, 1838, is of great interest, as showing how his mind ever recurred to the possibility of finding a connection between optical and electric phenomena: “Must try polarized light across a crystalline dielectric under charge. Good reasons perhaps now evident why a non-crystalline dielectric should have no effect.”
Faraday was now feeling greatly the strain of all these years of work, and in 1839 did little research until the autumn. Then he returned to the question of the origin of the electromotive force of the voltaic cell, and by the end of the year completed two long papers on this vexed question; they formed the sixteenth and seventeenth series, and conclude the memoirs of this second period.
In the eighth series, completed in April, 1834, on the “Electricity of the Voltaic Pile,” Faraday had dealt with the question—at that time a topic of excited controversy—of the origin of the electromotive force in a cell, Volta, who knew nothing of the chemical actions, ascribed it to the contact of dissimilar metals, whilst Wollaston, Becquerel, and De la Rive considered it the result of chemical actions. The controversy has long ceased to interest the scientific world; for, with the recognition of the principle of the conservation of energy, it became evident that mere contact cannot provide a continuing supply of energy. It would now be altogether dead but for the survival of a belief in the contact theory on the part of one of the most honoured veterans in science. But in the years 1834 to 1840 it was of absorbing interest. Faraday’s work quietly removed the props which supported the older theory, and it crumbled away. He found that the chemical and electrical effects in the cell were proportional one to the other, and inseparable. He discovered a way of making a cell without any metallic contacts. He showed that without chemical action there was no current produced. But his results were ignored for the time. After six years Faraday reopened the question. Again the admirable summary of Professor Tyndall is drawn upon for the following account:—
The memoir on the “Electricity of the Voltaic Pile,” published in 1834, appears to have produced but little impression upon the supporters of the contact theory. These indeed were men of too great intellectual weight and insight lightly to take up, or lightly to abandon, a theory. Faraday therefore resumed the attack in two papers communicated to the Royal Society on February 6 and March 19, 1840. In these papers he hampered his antagonists by a crowd of adverse experiments. He hung difficulty after difficulty about the neck of the contact theory, until in its efforts to escape from his assaults it so changed its character as to become a thing totally different from the theory proposed by Volta. The more persistently it was defended, however, the more clearly did it show itself to be a congeries of devices, bearing the stamp of dialectic skill rather than that of natural truth.
In conclusion, Faraday brought to bear upon it an argument which, had its full weight and purport been understood at the time, would have instantly decided the controversy. “The contact theory,” he urged, “assumes that a force which is able to overcome powerful resistance, as for instance that of the conductors, good or bad, through which the current passes, and that again of the electrolytic action where bodies are decomposed by it, can arise out of nothing; that without any change in the acting matter, or the consumption of any generating force, a current shall be produced which shall go on for ever against a constant resistance, or only be stopped, as in the voltaic trough, by the ruins which its exertion has heaped up in its own course. This would indeed be a creation of power, and is like no other force in nature. We have many processes by which the form of the power may be so changed, that an apparent conversion of one into the other takes place. So we can change chemical force into the electric current, or the current into chemical force. The beautiful experiments of Seebeck and Peltier show the convertibility of heat and electricity; and others by Oersted and myself show the convertibility of electricity and magnetism. But in no case, not even in those of the gymnotus and torpedo, is there a pure creation or a production of power without a corresponding exhaustion of something to supply it.”
In 1839 Faraday gave five Friday discourses and a course of eight afternoon lectures on the non-metallic elements. In 1840 he gave three Friday discourses and seven lectures on chemical affinity. But in the summer came the serious breakdown alluded to on page 75. He did no experimental work after September 14th, nor indeed for nearly two years. Even then it was only a temporary return to research to investigate the source of the electrification produced by steam in the remarkable experiments of Mr. (afterwards Lord) Armstrong. He proved it to be due to friction. This done, he continued to rest from research until the middle of 1844, though he lectured a little for the Royal Institution. In 1841 he gave the juvenile lectures. In 1842 he gave two Friday discourses, one of them being on the lateral discharge in lightning-rods. He also gave the Christmas lectures on electricity.
In 1843 he gave three Friday discourses, one of which was on the electricity generated by a jet of steam; and repeated the eight afternoon lectures he had given in 1838. In 1844 he gave eight lectures on heat and two Friday discourses. He also resumed research on the condensation of gases, and vainly tried to liquefy oxygen and hydrogen, though he succeeded with ammonia and nitrous oxide.
During these years of rest he also did a little work for Trinity House, chiefly concerning lighthouses and their ventilation.