CHAPTER IV.

SCIENTIFIC RESEARCHES: SECOND PERIOD.

With the year 1831 begins the period of the celebrated “Experimental Researches in Electricity and Magnetism.” During the years which had elapsed since his discovery of the electromagnetic rotations in 1823, Faraday, though occupied, as we have seen, with other matters, had not ceased to ponder the relation between the magnet and the electric current. The great discoveries of Oersted, Ampère, and Arago had culminated in England in two results: in Faraday’s discovery that the wire which carries an electric current tends to revolve around the pole of a neighbouring magnet; and in Sturgeon’s invention of the soft-iron electromagnet, a core of iron surrounded by a coil of copper wire, capable of acting as a magnet at will when the electric current is transmitted to the coil and so caused to circulate around the iron core.

[Sidenote: FORESHADOWINGS.]

This production of magnetism from electricity, at will, and at a distance, by the simple device of sending the electricity to circulate as a current around the central core of iron was then, as now, a cause of much speculation. The iron core which is to be made temporarily into a magnet stands alone, isolated. Though surrounded outwardly by the magnetising coil of copper wire, it does not touch it; nay, must be screened from contact with it by appropriate insulation. The electric current entering the copper coil at one end is confined from leaving the copper wire by any lateral path: it must circulate around each and every convolution, nor be permitted to flow back by the return-wire until it has performed the required amount of circulation. That the mere external circulation of electric current around a totally disconnected interior core of iron should magnetise that core; that the magnetisation should be maintained so long as the circulation of electricity is maintained; and that the magnetising forces should cease so soon as the current is stopped, are facts, familiar enough to every beginner in the science, but mysterious enough from the abstract point of view. Faraday was firmly persuaded that, great as had been these discoveries of the production of magnetism and magnetic motions from electricity, there remained other relations of no less importance to be discovered. Again and again his mind recurred to the subject. If it were possible to use electricity to produce magnetism, why should not the converse be true? In 1822 his notebook suggestion was, as we have seen, “Convert magnetism into electricity.” Yes, but how?

He possessed an intuitive bent of mind to inquire about the relations of facts to one another. Convinced by sheer converse with nature in the laboratory, of the correlation of forces and of the conservation of energy long before either of those doctrines had received distinct enunciation as principles of natural philosophy, he seems never to have viewed an action without thinking of the necessary and appropriate reaction; never to have deemed any physical relation complete in which discovery had not been made of the converse relations for which instinctively he sought. So in December, 1824, we find him experimenting on the passage of a bar magnet through a helix of copper wire (see _Quarterly Journal_ for July, 1825), but without result. In November, 1825, he sought for evidence that might prove an electric current in a wire to exercise an influence upon a neighbouring wire connected to a galvanometer. But again, and yet again in December of the same year, the entry stands “No result.” A third failure did not convince him that the search was hopeless: it showed him that he had not yet found the right method of experimenting. It is narrated of him how at this period he used to carry in his waistcoat pocket a small model of an electromagnetic circuit--a straight iron core about an inch long, surrounded by a few spiral turns of copper wire--which model he at spare moments would take out and contemplate, using it thus objectively to concentrate his thoughts upon the problem to be solved. A copper coil, an iron core. Given that electricity was flowing through the one, it evoked magnetism in the other. What was the converse? At first sight it might seem simple enough. Put magnetism from some external source into the iron core, and then try whether on connecting the copper coil to a galvanometer there was any indication of an electric current. But this was exactly what was found not to result.

[Sidenote: OTHER MEN’S FAILURES.]

And not Faraday alone, but others, too, were foiled in the hope of observing the expected converse. Not all who tried were as wise or as frank as Faraday in confessing failure. Fresnel, in the height of the fever of Oersted’s discovery, had announced to the Academy of Sciences at Paris, on the 6th of November, 1820, that he had decomposed water by means of a magnet which was laid motionless within a spiral of wire. Emboldened by this announcement, Ampère remarked that he too had noticed something in the way of production of currents from a magnet. But before the end of the year both these statements were withdrawn by their authors. Again, in the year 1822, Ampère, being at Geneva, showed to Professor A. de la Rive in his laboratory a number of electromagnetic experiments from his classical researches; and amongst them one[20] which has been almost forgotten, but which, had it been followed up, would assuredly have led Ampère to the discovery of the induction of currents. In the experiment in question a thin copper ring, made of a narrow strip folded into a circle, was hung inside a circular coil of wire, traversed by a current. To this apparatus a powerful horse-shoe magnet was presented; and De la Rive states that, when the magnet was brought up, the suspended ring was observed sometimes to move between the two limbs of the magnet, and sometimes to be repelled from between them according to the sense of the current in the surrounding coil. He and Ampère both attributed the effect to temporary magnetism conferred upon the copper ring. Ampère himself was at the time disposed to attribute it to the possible presence of a little iron as an impurity in the copper. There are, however, some discrepancies in the three published versions of the story. According to Becquerel, Ampère had by 1825 satisfied himself of the non-existence of induction currents.

[Sidenote: A PUZZLING EXPERIMENT.]

Quite independently, the question of the possibility of creating currents by magnets was raised by another discovery, that of the so-called “magnetism of rotation.” In 1824 Arago had observed that a fine magnetic compass constructed for him by Gambey, having the needle suspended in a cell, the base of which was a plate of pure copper, was thereby damped in its oscillations, and instead of making two or three hundred vibrations before it came to rest, as would be the case in the open air, executed only three or four of rapidly decreasing amplitude.[21] In vain did Dumas at the request of Arago analyse the copper, in the supposition that iron might be present. Inquiry compelled the conclusion that some other explanation must be sought. And, reasoning from the apparent action of stationary copper in bringing a moving magnetic needle to rest, he conjectured that a moving mass of copper might produce motion in a stationary magnetic needle. Accordingly he set into revolution, beneath a compass needle, a flat disc of copper, and found that, even when a sheet of card or glass was interposed to cut off all air-currents, the needle tended to follow the moving copper disc, turning as if dragged by some invisible influence. To the suggestion that mere rotation conferred upon copper a sort of temporary magnetism Arago listened with some impatience. All theories proposed to account for the phenomenon he discredited, even though emanating from the great mathematician Poisson. He held his judgment in absolute suspense. Babbage and Herschel measured the amount of retarding force exerted on the needle by different materials, and found the most effective to be silver and copper (which are the two best conductors of electricity), after them gold and zinc, whilst lead, mercury, and bismuth were inferior in power. The next year the same experimenters announced the successful inversion of Arago’s experiment; for by spinning the magnet underneath a pivoted copper disc they caused the latter to rotate briskly. They also made the notable observation that if slits are cut radially in the copper disc they diminish its tendency to be dragged by the spinning magnet. Sturgeon showed that the damping effect of a moving copper disc was diminished by the presence of a second magnet pole of contrary kind placed beside the first. All these things were most suggestive of the real explanation. It clearly had something to do with the electric conductivity of the metal disc, and therefore with electric currents. Sturgeon five years later came very near to the explanation: after repeating the experiments he concluded that the effect was an electric disturbance in the copper disc, “a kind of reaction to that which takes place in electromagnetism.”

Faraday knew of all the discussions which had arisen respecting Arago’s rotations. They may have been the cause of his unsuccessful attempts of 1824 and 1825. In April, 1828, for the fourth time he tried to discover the currents which he was convinced must be producible by the magnet, and for the fourth time without result. The cause of failure was that both magnet and coil were at rest.

The summer of 1831 witnessed him for the fifth time making the attack on the problem thus persistently before him. In his laboratory note-book he heads the research “Experiments on the production of electricity from magnetism.” The following excellent summary of the laboratory notes is taken from Bence Jones’s “Life and Letters”:--

I have had an iron ring made (soft iron), iron round and ⅞ths of an inch thick, and ring six inches in external diameter. Wound many coils of copper round, one half of the coils being separated by twine and calico; there were three lengths of wire, each about twenty-four feet long, and they could be connected as one length, or used as separate lengths. By trials with a trough each was insulated from the other. Will call this side of the ring A. On the other side, but separated by an interval, was wound wire in two pieces, together amounting to about sixty feet in length, the direction being as with the former coils. This side call B.[22]

Charged a battery of ten pairs of plates four inches square. Made the coil on B side one coil, and connected its extremities by a copper wire passing to a distance, and just over a magnetic needle (three feet from wire ring), then connected the ends of one of the pieces on A side with battery: immediately a sensible effect on needle. It oscillated and settled at last in original position. On breaking connection of A side with battery, again a disturbance of the needle.

[Sidenote: SUCCESS IN SIGHT.]

In the seventeenth paragraph, written on the 30th of August, he says, “May not these transient effects be connected with causes of difference between power of metals at rest and in motion in Arago’s experiments?” After this he prepared fresh apparatus.

As was his manner, he wrote off to one of his friends a letter telling what he was at work upon. On this occasion the recipient of his confidences was his friend Phillips:--

[_Michael Faraday to Richard Phillips._]

Royal Institution. Sept. 23, 1831.

MY DEAR PHILLIPS,

I write now, though it may be some time before I send my letter, but that is of no great consequence. I received your letter to Dr. Reid and read it on the coach going to Hastings, where I have been passing a few weeks, and I fancy my fellow passengers thought I had got something very droll in hand; they sometimes started at my sudden bursts, especially when I had the moment before been very grave and serious amongst the proportions. As you say in the letter there are some new facts and they are always of value; otherwise I should have thought you had taken more trouble than the matter deserved. Your quotation from Boyle has nevertheless great force in it.

I shall send with this a little thing in your own way “On the Alleged decline of science in England.” It is written by Dr. Moll of Utrecht, whose name may be mentioned in conversation though it is not printed in the pamphlet. I understand the view taken by Moll is not at all agreeable to some. “I do not know what business Moll had to interfere with our scientific disputes” is however the strongest observation I have heard of in reply.

I do not think I thanked you for your last Pharmacopœia. I do so now very heartily. I shall detain this letter a few days that I may send a couple of my papers (_i.e._ a paper and appendix) with it, for though not chemical I think you will like to have them. I am busy just now again on Electro-Magnetism, and think I have got hold of a good thing, but can’t say; it may be a weed instead of a fish that after all my labour I may at last pull up. I think I know why metals are magnetic when in motion though not (generally) when at rest.

We think about you all very much at times, and talk over affairs of Nelson Square, but I think we dwell more upon the illnesses and nursings and upon the sudden calls and chats rather than the regular parties. Pray remember us both to Mrs. Phillips and the damsils--I hope the word is not too familiar.

I am Dear Phillips, Most Truly Yours, M. FARADAY.

R. Phillips, Esq., &c., &c., &c.

[Sidenote: TEN DAYS OF SPLENDID WORK.]

September 24 was the third day of his experiments. He began (paragraph 21) by trying to find the effect of one helix of wire, carrying the voltaic current of ten pairs of plates, upon another wire connected with a galvanometer. “No induction sensible.” Longer and different metallic helices (paragraph 22) showed no effect; so he gave up those experiments for that day, and tried the effects of bar magnets instead of the ring magnet he had used on the first day.

In paragraph 33 he says:--

An iron cylinder had a helix wound on it. The ends of the wires of the helix were connected with the indicating helix at a distance by copper wire. Then the iron placed between the poles of bar magnets as in accompanying figure (Fig. 5). Every time the magnetic contact at N or S was made or broken, there was magnetic motion at the indicating helix--the effect being, as in former cases, not permanent, but a mere momentary push or pull. But if the electric communication (_i.e_. by the copper wire) was broken, then the disjunction and contacts produced no effect whatever. Hence here distinct conversion of magnetism into electricity.

The fourth day of work was October 1. Paragraphs 36, 37, and 38 describe the discovery of induced voltaic currents:--

36. A battery of ten troughs, each of ten pairs of plates four inches square, charged with good mixture of sulphuric and nitric acid, and the following experiments made with it in the following order.

37. One of the coils (of a helix of copper wire 203 feet long) was connected with the flat helix, and the other (coil of same length round same block of wood) with the poles of the battery (it having been found that there was no metallic contact between the two); the magnetic needle at the indicating flat helix was affected, but so little as to be hardly sensible.

38. In place of the indicating helix, our galvanometer was used, and then a sudden jerk was perceived when the battery communication was _made_ and _broken_, but it was so slight as to be scarcely visible. It was one way when made, the other when broken, and the needle took up its natural position at intermediate times.

Hence there is an inducing effect without the presence of iron, but it is either very weak or else so sudden as not to have time to move the needle. I rather suspect it is the latter.

The fifth day of experiment was October 17. Paragraph 57 describes the discovery of the production of electricity by the approximation of a magnet to a wire:--

A cylindrical bar magnet three-quarters of an inch in diameter, and eight inches and a half in length, had one end just inserted into the end of the helix cylinder (220 feet long); then it was quickly thrust in the whole length, and the _galvanometer_ needle moved; then pulled out, and again the _needle moved_, but in the opposite direction. This effect was repeated every time the magnet was put in or out, and therefore a wave of electricity was so produced from _mere approximation of a magnet_, and not from its formation _in situ_.

The cause of all the earlier failures was, then, that both magnet and coil were at rest. The magnet might lie in or near the coil for a century and cause no effect. But while moving towards the coil, or from it, or by spinning near it, electric currents were at once induced.

The ninth day of his experiments was October 28, and this day he “made a copper disc turn round between the poles of the great horse-shoe magnet of the Royal Society. The axis and edge of the disc were connected with a galvanometer. The needle moved as the disc turned.” The next day that he made experiments, November 4, he found “that a copper wire one-eighth of an inch drawn between the poles and conductors produced the effect.” In his paper, when describing the experiment, he speaks of the metal “cutting” the magnetic curves, and in a note to his paper he says, “By magnetic curves I mean lines of magnetic forces which would be depicted by iron filings.”

[Sidenote: SUCCESS AND ITS SECRET.]

We here come upon those “lines of force” which played so important a part in these and many of Faraday’s later investigations. They were known before Faraday’s time--had, in fact, been known for two hundred years. Descartes had seen in them evidence for his hypothetical vortices. Musschenbroek had mapped them. But it was reserved to Faraday to point out their true significance. To the very end of his life he continued to speculate and experiment upon them.

All this splendid work had occupied but a brief ten days. Then he rearranged the facts which he had thus harvested, and wrote them out in corrected form as the first series of his “Experimental Researches in Electricity.” The memoir was read to the Royal Society on November 24, 1831, though it did not appear in printed form until January, 1832--a delay which gave rise to serious misunderstandings. The paper having been read, he went away to Brighton to take a holiday, and in the exuberance of his heart penned the following letter[23] to Phillips:--

[_M. Faraday to R. Phillips._]

Brighton: November 29, 1831.

DEAR PHILLIPS,--For once in my life I am able to sit down and write to you without feeling that my time is so little that my letter must of necessity be a short one and accordingly I have taken an extra large sheet of paper intending to fill it with news and yet as to news I have none for I withdraw more and more from Society, and all I have to say is about myself.

But how are you getting on? are you comfortable? and how does Mrs. Phillips do; and the girls? Bad correspondant as I am, I think you owe me a letter and as in the course of half an hour you will be doubly in my debt pray write us, and let us know all about you. Mrs. Faraday wishes me not to forget to put her kind remembrances to you and Mrs. Phillips in my letter.

To-morrow is St. Andrew’s day,[24] but we shall be here until Thursday. I have made arrangements to be _out_ of the Council and care little for the rest although I should as a matter of curiosity have liked to see the Duke in the chair on such an occasion.

We are here to refresh. I have been working and writing a paper and that always knocks me up in health, but now I feel well again and able to pursue my subject and now I will tell you what it is about. The title will be, I think, EXPERIMENTAL RESEARCHES IN ELECTRICITY: §I. _On the induction of electric currents._ § II. _On the evolution of Electricity from magnetism._ § III. _On a New electrical condition of matter._ § IV. _On Arago’s magnetic phenomena._ There is a bill of fare for you; and what is more I hope it will not disappoint you. Now the pith of all this I must give you very briefly; the demonstrations you shall have in the paper when printed--

[Sidenote: THE PITH OF THE DISCOVERY.]

§ I. When an electric current is passed through one of two parallel wires it causes at first a current in the same direction[25] through the other, but this induced current does not last a moment, notwithstanding the inducing current (from the Voltaic battery) is continued all seems unchanged except that the principal current continues its course, but when the current is stopped then a return current occurs in the wire under induction of about the same intensity and momentary duration but in the opposite direction to that first found. Electricity in currents therefore exerts an inductive action like ordinary electricity but subject to peculiar laws: the effects are a current in the same direction when the induction is established: a reverse current when the induction ceases and a _peculiar state_ in the interim. Common electricity probably does the same thing but as it is at present impossible to separate the beginning and the end of a spark or discharge from each other, all the effects are simultaneous and neutralise each other--

§ II. Then I found that magnets would induce just like voltaic currents and by bringing helices and wires and jackets up to the poles of magnets, electrical currents were produced in them these currents being able to deflect the galvanometer, or to make, by means of the helix, magnetic needles, or in one case even to give a spark. Hence the evolution of _electricity from magnetism_. The currents were not permanent, they ceased the moment the wires ceased to approach the magnet because the new and apparently quiescent state was assumed just as in the case of the induction of currents. But when the magnet was removed, and its induction therefore ceased, the return currents appeared as before. These two kinds of induction I have distinguished by the terms _Volta-electric_ and _Magneto-electric_ induction. Their identity of action and results is, I think, a very powerful proof of the truth of M. Ampère’s theory of magnetism.

[Sidenote: A JUBILANT EPISTLE.]

§ III. The new electrical condition which intervenes by induction between the beginning and end of the inducing current gives rise to some very curious results. It explains why chemical action or other results of electricity have never been as yet obtained in trials with the magnet. In fact, the currents have no sensible duration. I believe it will explain perfectly the _transference of elements_ between the poles of the pile in decomposition but this part of the subject I have reserved until the present experiments are completed and it is so analogous, in some of its effects to those of Ritter’s secondary piles, De la Rive and Van Beck’s peculiar properties of the poles of a voltaic pile, that I should not wonder if they all proved ultimately to depend on this state. The condition of matter I have dignified by the term _Electrotonic_, THE ELECTROTONIC STATE. What do you think of that? Am I not a bold man, ignorant as I am, to coin words but I have consulted the scholars,[26] and now for § IV. The new state has enabled me to make out and explain all Arago’s phenomena of the rotating magnet or copper plate, I believe, perfectly; but as great names are concerned Arago, Babbage, Herschel, &c., and as I have to differ from them, I have spoken with that modesty which you so well know you and I and John Frost[27] have in common, and for which the world so justly commends us. I am even half afraid to tell you what it is. You will think I am hoaxing you, or else in your compassion you may conclude I am deceiving myself. However, you need do neither, but had better laugh, as I did most heartily when I found that it was neither attraction nor repulsion, but just one of _my old rotations_ in a new form. I cannot explain to you all the actions, which are very curious; but in consequence of the electrotonic state being assumed and lost as the parts of the plate whirl under the pole, and in consequence of magneto-electric induction, currents of electricity are formed in the direction of the radii; continuing, for simple reasons, as long as the motion continues, but ceasing when that ceases. Hence the wonder is explained that the metal has powers on the magnet when moving, but not when at rest. Hence is also explained the effect which Arago observed, and which made him contradict Babbage and Herschel, and say the power was repulsive; but, as a whole, it is really tangential. It is quite comfortable to me to find that experiment need not quail before mathematics, but is quite competent to rival it in discovery; and I am amused to find that what the high mathematicians have announced as the _essential condition_ to the rotation--namely, that _time is required_--has so little foundation, that if the time could by possibility be anticipated instead of being required--_i.e._ if the currents could be formed _before_ the magnet came over the place instead of _after_--the effect would equally ensue. Adieu, dear Phillips.

Excuse this egotistical letter from yours very faithfully,

M. FARADAY.

The second section shows that Faraday had discovered the cause of all the previous failures to evoke electric currents in wires by means of a magnet: it required _relative motion_. What the magnet at rest fails to do, the magnet in motion accomplishes. This crucial point is admirably commemorated in the following impromptu given by Mr. Herbert Mayo to Sir Charles Wheatstone:--

Around the magnet Faraday Was sure that Volta’s lightnings play: But how to draw them from the wire? He took a lesson from the heart: ’Tis when we meet, ’tis when we part, Breaks forth the electric fire.

Faraday’s holiday was brief; by December 5 he was again at work on his researches. He re-observed the directions of the induced currents about which, as the slip in his letter to Phillips shows, his mind was in some doubt. Then on December 14th comes the entry:--“Tried the effects of terrestrial magnetism in evolving electricity. Obtained beautiful results.”

“The helix had the soft iron cylinder (freed from magnetism by a full red heat and cooling slowly) put into it, and it was then connected with the galvanometer by wires eight foot long; then inverted the bar and helix, and immediately the needle moved; inverted it again, the needle moved back; and, by repeating the motion with the oscillations of the needle, made the latter vibrate 180°, or more.”

The same day he “made Arago’s experiment with the earth magnet, only no magnet used, but the plate put horizontal and rotated. The effect at the needle was slight but very distinct.... Hence Arago’s plate a new electrical machine.”

[Sidenote: POINTS IN THE DISCOVERY.]

When we compare these manuscript notes, recording the experiments in the order in which they were made with the published account of them in the “Experimental Researches,” we find many of them transcribed almost verbatim. But there is a difference in the order of their arrangement. In point of time the experiments on the evolution of electricity from magnetism, beginning with the ring (p. 108), preceded those on the induction of a current by another current. In the printed “Researches” the experiments on the induction of currents are put first, with an introductory paragraph on the general phenomenon of induction.[28] Faraday’s habit of working up an experiment--whether successful or unsuccessful--by increasing the power to the maximum available is illustrated in the course of the experiments on the iron ring. At first he used a battery of ten pairs of plates four inches square. Then, having been eminently successful in producing deflexions of his galvanometer, he increased the battery to one hundred pairs of plates, with the result that when contact was completed or broken in the primary circuit the impulse on the galvanometer in the secondary circuit was so great as to make the needle spin round rapidly four or five times before its motion was reduced to a mere oscillation. Then he removed the galvanometer and fixed small pencils of charcoal to the ends of the secondary helix; and to his great joy perceived a minute _spark_ between the lightly touching charcoal points whenever the contact of the battery to the primary helix was completed. This was the first transformer, for the first time set--on a small scale--to produce a tiny electric light. The spark he regarded as a precious indication that what he was producing really was an electric current. Using the great compound steel magnet of the Royal Society (constructed by Dr. Gowin Knight) at Christie’s house at Woolwich he had, as narrated above, also obtained a spark from the induced current. For some time he failed to obtain either physiological or chemical effects. But upon repeating the experiments more at leisure at the Royal Institution, with Daniell’s armed loadstone capable of lifting thirty pounds, a frog was found to be convulsed very strongly each time magnetic contact between the magnet and the iron core of the experimental coil was made or broken.

The absence of evidence as to chemical action seemed still to disquiet him. He wanted to be sure that his induced currents would do everything that ordinary voltaic currents would do. Failing the final proof from chemical action, he rested the case on the other identical properties. “But an agent,” he says, “which is conducted along metallic wires in the manner described; which, whilst so passing, possesses the peculiar magnetic actions and force of a current of electricity; which can agitate and convulse the limbs of a frog; and which, finally, can produce a spark by its discharge through charcoal, can only be electricity. As all the effects can be produced by ferruginous electro-magnets, there is no doubt that arrangements like the magnets of Professors Moll, Henry, Ten Eyke, and others, in which as many as two thousand pounds have been lifted, may be used for these experiments; in which case not only a brighter spark may be obtained, but wires also ignited, and as the currents can pass liquids, chemical action be produced. These effects are still more likely to be obtained when the magneto-electric arrangements, to be explained in the fourth section, are excited by the powers of such apparatus.” The apparatus described in the fourth section comprised several forms of magneto-electric machines, that is to say, primitive kinds of dynamos. Having in his mind the phenomenon discovered by Arago, and the experiments of Babbage and Herschel on the so-called magnetism of rotation, he followed up the idea that these effects might be due to induced currents eddying round in the copper disc. No sooner had he obtained electricity from magnets than he attempted to make Arago’s experiment a new source of electricity, and, as he himself says, “did not despair” “of being able to construct a new electrical machine.”

[Sidenote: A NEW ELECTRICAL MACHINE.]

The “new electrical machine” was an exceedingly simple contrivance. A disc of copper, twelve inches in diameter (Fig. 6), and about one-fifth of an inch in thickness, fixed upon a brass axle, was mounted in frames, so as to allow of revolution, its edge being at the same time introduced between the magnetic poles of a large compound permanent magnet, the poles being about half an inch apart.[29] The magnet first used was the historical magnet of Gowin Knight. The edge of the plate was well amalgamated, for the purpose of obtaining a good but movable contact, and a part round the axle was also prepared in a similar manner. Conducting strips of copper and lead, to serve as electric collectors, were prepared, so as to be placed in contact with the edge of the copper disc; one of these was held by hand to touch the edge of the disc between the magnet poles. The wires from a galvanometer were connected, the one to the collecting-strip, the other to the brass axle; then on revolving the disc a deflexion of the galvanometer was obtained, which was reversed in direction when the direction of the rotation was reversed. “Here, therefore, was demonstrated the production of a permanent current of electricity by ordinary magnets.” These effects were also obtained from the poles of electro-magnets, and from copper helices without iron cores. Several other forms of magneto-electric machines were tried by Faraday.

[Sidenote: NEW FORMS OF APPARATUS.]

In one,[30] a flat ring of twelve inches’ external diameter, and one inch broad, was cut from a thick copper plate, and mounted to revolve between the poles of the magnet, two conductors being applied to make rubbing contact at the inner and outer edge at the part which passed between the magnetic poles. In another,[31] a disc of copper, one-fifth of an inch thick and only 1½ inch in diameter (Fig. 7), was amalgamated at the edge, and mounted on a copper axle. A square piece of sheet metal had a circular hole cut in it, into which the disc fitted loosely; a little mercury completed communication between the disc and its surrounding ring. The latter was connected by wire to a galvanometer; the other wire being connected from the instrument to the end of the axle. Upon rotating the disc in a horizontal plane, currents were obtained, though the earth was the only magnet employed.

Faraday also proposed a multiple machine[32] having several discs, metallically connected alternately at the edges and centres by means of mercury, which were then to be revolved alternately in opposite directions, In another apparatus,[33] a copper cylinder (Fig. 8), closed at one extremity, was put over a magnet, one half of which it enclosed like a cap, and to which it was attached without making metallic contact. The arrangement was then floated upright in a narrow jar of mercury, so that the lower edge of the copper cap touched the fluid. On rotating the magnet and its attached cap, a current was sent through wires from the mercury to the top of the copper cap. In another apparatus,[34] still preserved at the Royal Institution, a cylindrical bar magnet, half immersed in mercury, was made to rotate, and generated a current, its own metal serving as a conductor. In another form,[35] the cylindrical magnet was rotated horizontally about its own axis, and was found to generate currents which flowed from the middle to the ends, or _vice versâ_, according to the rotation. The description of these new electrical machines is concluded with the following pregnant words:--

[Sidenote: AN EARTH-INDUCTOR.]

I have rather, however, been desirous of discovering new facts and relations dependent on magneto-electric induction, than of exalting the force of those already obtained; being assured that the latter would find their full development hereafter.

In yet another machine (Fig. 9), constructed by Faraday some time later,[36] a simple rectangle of copper wire _w_, attached to a frame, was rotated about a horizontal axis placed east and west, and generated alternate currents, which could be collected by a simple commutator _c_.

Within a few months machines on the principle of magneto-induction had been devised by Dal Negro, and by Pixii. In the latter’s apparatus a steel horseshoe magnet, with its poles upwards, was caused to rotate about a vertical shaft, inducing alternating currents in a pair of bobbins fixed above it, and provided with a horseshoe core of soft iron. Later, in 1832, Pixii produced, at the suggestion of Ampère,[37] a second machine, provided with mercury cup connections to rectify the alternations of the current. One of these machines was shown at the British Association meeting at Oxford in the same year (p. 64).

The idea developed in the third part of this research was intensely original and suggestive. Faraday’s own statement is as follows:--

[Sidenote: THE ELECTROTONIC STATE.]

Whilst the wire is subject to either volta-electric or magneto-electric induction, it appears to be in a peculiar state; for it resists the formation of an electrical current in it, whereas, if left in its common condition, such a current would be produced; and when left uninfluenced it has the power of originating a current, a power which the wire does not possess under common circumstances. This electrical condition of matter has not hitherto been recognised, but it probably exerts a very important influence in many, if not most, of the phenomena produced by currents of electricity. For reasons which will immediately appear, I have, after advising with several learned friends, ventured to designate it as the _electrotonic_ state.

This peculiar condition shows no known electrical effects whilst it continues; nor have I yet been able to discover any peculiar powers exerted or properties possessed by matter whilst retained in this state.

* * * * *

This state is altogether the effect of the induction exerted, and ceases as soon as the inductive force is removed.... The state appears to be instantly assumed, requiring hardly a sensible portion of time for that purpose.... In all those cases where the helices or wires are advanced towards or taken from the magnet, the direct or inverted current of induced electricity continues for the time occupied in the advance or recession; for the electro-tonic state is rising to a higher or falling to a lower degree during that time, and the change is accompanied by its corresponding evolution of electricity; but these form no objections to the opinion that the electro-tonic state is instantly assumed.

This peculiar state appears to be a state of tension, and may be considered as _equivalent_ to a current of electricity, at least equal to that produced either when the condition is induced or destroyed.

Faraday further supposed that the formation of this state in the neighbourhood of a coil would exert a reaction upon the original current, giving rise to a retardation of it; but he was unable at the time to ascertain experimentally whether this was so. He even looked--though also unsuccessfully--for a self-induced return current from a conductor of copper through which a strong current was led and then suddenly interrupted, the expected current of reaction being “due to the discharge of its supposed electrotonic state.”

If we would understand the rather obscure language in which this idea of an electrotonic state is couched, we must try to put ourselves back to the epoch when it was written. At that date the only ideas which had been formulated to explain magnetic and electric attractions and repulsions were founded upon the notion of action at a distance. Michell had propounded the view that the electric and magnetic forces vary, like gravity, according to a law of the inverse squares of the distances. Coulomb, in a series of experiments requiring extraordinary patience as well as delicacy of manipulation, had shown--by an application of Michell’s torsion balance--that in particular cases where the electric charges are concentrated on small spheres, or where the magnetic poles are small, so as to act as mere points, this law--which is essentially a geometric law of point-action--is approximately fulfilled. The mathematicians, Laplace and Poisson at their head, had seized on this demonstration and had elaborated their mathematical theories. Before them, though the research lay for a century unpublished, Cavendish had shown that the only law of force as between one element of an electric charge and another compatible with a charge being in equilibrium was the law of inverse squares. But in all these mathematical reasonings one thing had been quite left out of sight--namely, the possible properties of the intervening medium. Faraday, to whom the idea of mere action at a distance was abhorrent, if not unthinkable, conceived of all these forces of attraction and repulsion as effects taking place by something going on _in the intervening medium_, as effects propagated from point to point continuously through space. In his earlier work on the electromagnetic rotations he had grown to regard the space around the conducting wire as being affected by the so-called current; and the space about the poles of a magnet he knew to be traversed by curved magnetic lines, invisible indeed, but real, needing only the simplest of expedients--the sprinkling of iron filings--to reveal their existence and trend. When therefore he found that these new effects of the induction of one electric current by another could likewise cross an intervening space, whether empty or filled with material bodies, he instinctively sought to ascribe this propagation of the effect to a property or state of the medium. And finding that state to be different from any state previously known, different from the state existing between two magnets at rest or between two stationary electric charges, he followed the entirely philosophical course of exploring its properties and of denoting it by a name which he deemed appropriate. As we shall see, this idea of an electrotonic state recurred in his later researches with new and important connotations.

He was soon at work again, as we have seen.

He experimented, in January, 1832, on the currents produced by the earth’s rotation--on the 10th at the round pond in Kensington Gardens, and on the 12th and 13th at Waterloo Bridge.

[Sidenote: A SPARK FROM A MAGNET.]

“This evening,” he writes in his notebook under date February 8, “at Woolwich, experimenting with magnet,[38] and for the first time got the magnetic spark myself. Connected ends of a helix into two general ends, and then crossed the wires in such a way that a blow at _a b_ would open them a little [Fig. 10]. Then bringing _a b_ against the poles of a magnet, the ends were disjoined, and bright sparks resulted.”

From succeeding with a steel magnet it was but a short step to succeed when a natural loadstone was used. The next day we find this entry:--“At home succeeded beautifully with Mr. Daniell’s magnet. Amalgamation of wires very needful. This is a natural loadstone, and perhaps the first used for the spark.”

He sent to the Royal Society an account of these and the earlier experiments; his paper on terrestrial magneto-electric induction, and on the force and direction of magneto-electric induction, received the distinction of being read as the Bakerian lecture of the year.

[Sidenote: TYNDALL’S SUMMARY.]

The following summary of this second paper is from the pen of Professor Tyndall:--

He placed a bar of iron in a coil of wire, and lifting the bar into the direction of the dipping needle, he excited by this action a current in the coil. On reversing the bar, a current in the opposite direction rushed through the wire. The same effect was produced, when, on holding the helix in the line of dip, a bar of iron was thrust into it. Here, however, the earth acted on the coil through the intermediation of the bar of iron. He abandoned the bar, and simply set a copper plate spinning in a horizontal plane; he knew that the earth’s lines of magnetic force then crossed the plate at an angle of about 70°. When the plate spun round, the lines of force were intersected and induced currents generated, which produced their proper effect when carried from the plate to the galvanometer. “When the plate was in the magnetic meridian, or in any other plane coinciding with the magnetic dip, then its rotation produced no effect upon the galvanometer.”

At the suggestion of a mind fruitful in suggestions of a profound and philosophic character--I mean that of Sir John Herschel--Mr. Barlow, of Woolwich, had experimented with a rotating iron shell. Mr. Christie had also performed an elaborate series of experiments on a rotating iron disc. Both of them had found that when in rotation the body exercised a peculiar action upon the magnetic needle, deflecting it in a manner which was not observed during quiescence; but neither of them was aware at the time of the agent which produced this extraordinary deflection. They ascribed it to some change in the magnetism of the iron shell and disc.

But Faraday at once saw that his induced currents must come into play here, and he immediately obtained them from an iron disc. With a hollow brass ball, moreover, he produced the effects obtained by Mr. Barlow. Iron was in no way necessary; the only condition of success was that the rotating body should be of a character to admit of the formation of currents in its substance; it must, in other words, be a conductor of electricity. The higher the conducting power, the more copious were the currents. He now passes from his little brass globe to the globe of the earth. He plays like a magician with the earth’s magnetism. He sees the invisible lines along which its magnetic action is exerted, and, sweeping his wand across these lines, he evokes this new power. Placing a simple loop of wire round a magnetic needle, he bends its upper portion to the west; the north pole of the needle immediately swerves to the east; he bends his loop to the east, and the north pole moves to the west. Suspending a common bar magnet in a vertical position, he causes it to spin round its own axis. Its pole being connected with one end of a galvanometer wire, and its equator with the other end, electricity rushes round the galvanometer from the rotating magnet. He remarks upon the “_singular independence_” of the magnetism and the body of the magnet which carries it. The steel behaves as if it were isolated from its own magnetism.

And then his thoughts suddenly widen, and he asks himself whether the rotating earth does not generate induced currents as it turns round its axis from west to east. In his experiment with the twirling magnet the galvanometer wire remained at rest; one portion of the circuit was in motion _relatively_ to _another portion_. But in the case of the twirling planet the galvanometer wire would necessarily be carried along with the earth; there would be no relative motion. What must be the consequence? Take the case of a telegraph wire with its two terminal plates dipped into the earth, and suppose the wire to lie in the magnetic meridian. The ground underneath the wire is influenced, like the wire itself, by the earth’s rotation; if a current from south to north be generated in the wire, a similar current from south to north would be generated in the earth under the wire; these currents would run against the same terminal plate, and thus neutralise each other.

This inference appears inevitable, but his profound vision perceived its possible invalidity. He saw that it was at least possible that the difference of conducting power between the earth and the wire might give one an advantage over the other, and that thus a residual or differential current might be obtained. He combined wires of different materials, and caused them to act in opposition to each other, but found the combination ineffectual. The more copious flow in the better conductor was exactly counterbalanced by the resistance of the worst. Still, though experiment was thus emphatic, he would clear his mind of all discomfort by operating on the earth itself. He went to the round lake near Kensington Palace, and stretched 480 feet of copper wire, north and south, over the lake, causing plates soldered to the wire at its ends to dip into the water. The copper wire was severed at the middle, and the severed ends connected with a galvanometer. No effect whatever was observed. But though quiescent water gave no effect, moving water might. He therefore worked at Waterloo Bridge for three days, during the ebb and flow of the tide, but without any satisfactory result. Still he urges, “Theoretically it seems a necessary consequence, that where water is flowing there electric currents should be formed. If a line be imagined passing from Dover to Calais through the sea and returning through the land, beneath the water, to Dover, it traces out a circuit of conducting matter, one part of which, when the water moves up or down the Channel, is cutting the magnetic curves of the earth, whilst the other is relatively at rest.... There is every reason to believe that currents do run in the general direction of the circuit described, either one way or the other, according as the passage of the waters is up or down the Channel.” This was written before the submarine cable was thought of, and he once informed me that actual observation upon that cable had been found to be in accordance with his theoretic deduction.

It may here be apposite to discuss a fundamental question raised in these researches. In Faraday’s mind there arose the conviction of a connection between the induction of currents by magnets and the magnetic lines which invisibly fill all the space in the neighbourhood of the magnet. That relation he discovered and announced in the following terms:--

[Sidenote: THE LAW OF INDUCTION.]

“The relation which holds between the magnetic pole, the moving wire or metal, and the direction of the current evolved--_i.e._ _the law_ which governs the evolution of electricity by magneto-electric induction, is very simple, though rather difficult to express. If in Fig. 11, P N represent a horizontal wire passing by a marked [_i.e._ ‘north-seeking’] magnetic pole, so that the direction of its motion shall coincide with the curved line proceeding from below upwards; or if its motion parallel to itself be in a line tangential to the curved line, but in the general direction of the arrows; or if it pass the pole in other directions, but so as to cut the magnetic curves[39] in the same general direction, or on the same side as they would be cut by the wire if moving along the dotted curved line; then the current of electricity in the wire is from P to N. If it be carried in the reverse direction, the electric current will be from N to P. Or if the wire be in the vertical position, figured P´ N´, and it be carried in similar directions, coinciding with the dotted horizontal curve so far as to cut the magnetic curves on the same side with it, the current will be from P´ to N´.”

[Sidenote: CUTTING THE MAGNETIC LINES.]

When resuming the research in December, Faraday investigated the point whether it was essential or not that the moving wire should, in “cutting” the magnetic curves, pass into positions of greater or lesser magnetic force; or whether, always intersecting curves of equal magnetic intensity, the mere motion sufficed for the production of the current. He found the latter to be true. This notion of _cutting_ the invisible magnetic lines as the essential act necessary and sufficient for induction was entirely original with Faraday. For long it proved a stumbling-block to the abstract mathematicians, since there was, in most cases, no direct or easy way in which to express the number of magnetic lines that were cut. Neither had any convention been adopted up to that time as to how to reckon numerically the number of magnetic lines in any given space near a magnet. Later, in 1851, Faraday himself gave greater precision to these ideas. He found that the current was proportional to the velocity, when the conductor was moving in a uniform magnetic field with a uniform motion. Also, that the quantity of electricity thrown by induction into the circuit was directly proportional to the “amount of curves intersected.” The following passage, from Clerk Maxwell’s article on Faraday in the “Encyclopædia Britannica,” admirably sums up the matter:--

The magnitude and originality of Faraday’s achievement may be estimated by tracing the subsequent history of his discovery. As might be expected, it was at once made the subject of investigation by the whole scientific world, but some of the most experienced physicists were unable to avoid mistakes in stating, in what they conceived to be more scientific language than Faraday’s, the phenomena before them. Up to the present time the mathematicians who have rejected Faraday’s method of stating his law as unworthy of the precision of their science, have never succeeded in devising any essentially different formula which shall fully express the phenomena without introducing hypotheses about the mutual action of things which have no physical existence, such as elements of currents which flow out of nothing, then along a wire, and finally sink into nothing again.

After nearly half a century of labour of this kind, we may say that, though the practical applications of Faraday’s discovery have increased and are increasing in number and value every year, no exception to the statement of these laws as given by Faraday has been discovered, no new law has been added to them, and Faraday’s original statement remains to this day the only one which asserts no more than can be verified by experiment, and the only one by which the theory of the phenomena can be expressed in a manner which is exactly and numerically accurate, and at the same time within the range of elementary methods of exposition.

In the year 1831, which witnessed this masterpiece of scientific research, Faraday was busy in many other ways. He was still undertaking chemical analyses and expert work for fees, as witness his letter to Phillips on p. 62. He was also, until November, on the Council of the Royal Society. To the “Philosophical Transactions” he contributed a paper “On Vibrating Surfaces,” in which he solved a problem in acoustics which had previously gone without explanation. It had long been known that in the experiments of obtaining the patterns called “Chladni’s figures,” by strewing powders upon vibrating plates, while the heavier powders, such as sand, moved into the nodal lines, lighter substances, such as lycopodium dust, collected in little circular heaps over the parts where the vibration was most energetic. Faraday’s explanation was that these lighter powders were caught and whirled about in little vortices which formed themselves at spots where the motions were of greatest amplitude.

He also wrote a paper “On a Peculiar Class of Optical Deceptions,” dealing with the illusions that result from the eye being shown in successive glimpses, as between the teeth of a revolving wheel, different views of a moving body. This research was, in effect, the starting point of a whole line of optical toys, beginning with the phenakistiscope or stroboscope, which developed through the zoetrope and praxino-scope into the kinematograph and animatograph of recent date.

[Sidenote: LECTURES ON PHYSICAL SUBJECTS.]

He gave four afternoon lectures at the Royal Institution and five Friday evening discourses. These were on optical deceptions, on light and phosphorescence, being an account of experiments recently made by Mr. Pearsall, chemical assistant in the Institution; on oxalamide, then recently discovered by M. Dumas; on Trevelyan’s experiments about the production of sound by heated bodies; and on the arrangements assumed by particles upon vibrating surfaces.

In 1832 he gave five Friday evening discourses, four of which related to his own researches. In August he entered upon the third series of “Experimental Researches in Electricity,” which was devoted to the identity of electricities derived from different sources, and on the relation by measure of common [_i.e._ frictional] and voltaic electricity. He did not like any doubt to hang about as to whether the electricity obtained from magnets by induction was really the same as that obtainable from other sources. Possibly he had in his mind the difficulties which had arisen thirty years before over the discoveries of Galvani and Volta, when it was so far doubted whether the electricity in currents from piles and batteries of cells was the same as the electricity evoked by friction, that the distinctive and misleading name of “galvanism” was assigned to the former. He commented on the circumstance that many philosophers--and he included Davy by name in an explicit reference--were vainly drawing distinctions[40] between electricities from different sources, or at least doubting whether their identity were proven. His first point was to consider whether “common electricity,” “animal electricity,” and “magneto-electric currents” could, like “voltaic electricity,” produce chemical decompositions. He began by demonstrating that an ordinary electric discharge from a friction machine can affect a suitably disposed galvanometer. One of his instruments of sufficient sensitiveness was surrounded by an enclosing cage of double metal foil and wire-work, duly connected to “earth,” so as to render it independent of all disturbances by external electric charges in its neighbourhood. His “earth” for this purpose consisted of a stout metal wire connected through the pipes in the house to the metallic gas-pipes belonging to the public gas works of London, and also with the metallic water-pipes of London--an effectual “discharging train.” He used a friction electric machine with a glass plate 50 inches in diameter, and a Leyden-jar battery of fifteen jars, each having about 84 square inches of coated glass. This battery of jars was first charged from the machine and then discharged through a wet thread four feet long, and through the galvanometer to earth _viâ_ the “discharging train.” Having by this means satisfied himself that these electric discharges could deflect a galvanometer, whether through the wet thread, a copper wire, or through water, or rarefied air, or by connection through points in air, he went on to the question of chemical decomposition. Dipping two silver wires into a drop of solution of sulphate of copper, he found that one of them became copper-plated by the electricity that was evolved by 100 or 200 turns of the disc machine. He bleached indigo, turned starch purple with iodine liberated from iodide of potassium, exactly as might have been done by a “volta-electric current” from a battery of cells. He also decomposed water, giving due recognition to the antecedent experiments of Van Troostwyk, Pearson, and Wollaston.

[Sidenote: IDENTITY OF ELECTRICITIES.]

In the paper which he drew up he compares these results with others made with electric discharges from an electric kite and with those of the torpedo and other electric fishes. He recapitulates the properties of magneto-electricity and the proofs now accumulating that it can decompose water. He drew up a schedule of the different effects which electricity can produce, and of the different sources of electricity, showing in tabular form how far each so-called kind of electricity had been found to produce each effect. The conclusion was that there is no philosophical difference between the different cases; since the phenomena produced by the different kinds of electricity differ not in their character but only in degree. “_Electricity, whatever may be its source, is identical in its nature._” On comparing the effects produced by different discharges, he concludes that “if the same absolute quantity[41] of electricity pass through the galvanometer, whatever may be its intensity, the deflecting force upon the magnetic needle is the same.” He was then able to go on to a quantitative comparison between the “quantity” of electricity from different sources, and came to the conclusion that both in magnetic deflection and in chemical force the current of electricity given by his standard battery for eight beats of his watch was equal to that of the friction machine evolved by thirty revolutions; further, that “the chemical power, like the magnetic force, is in direct proportion to the absolute quantity of electricity which passes.”

[Sidenote: ELECTRO-CHEMICAL WORK.]

This series of researches was published in January, 1833. In April of the same year he sent to the Royal Society another paper--the fourth series--on electric conduction. It arose from the surprising observation that, though water conducts, ice acts as a complete non-conductor. This led to an examination of the conducting power of fusible solids in general. He found that as a rule--excepting on the one hand the metals, which conduct whether solid or liquid, and on the other hand fatty bodies, which are always non-conductors--they assume conducting power when liquefied, and lose it when congealed. Chloride of lead, of silver, of potassium, and of sodium, and many chlorates, nitrates, sulphates, and many other salts and fusible substances were found to follow this rule. All the substances so found to act were compound bodies, and capable of decomposition by the current. When conduction ceased, decomposition ceased also. An apparent exception was found in sulphide of silver, which, when heated, acquired conducting powers even before it assumed the liquid state, yet decomposed in the solid state. This led him on to study electro-chemical decompositions more closely. Here he was following directly in the footsteps of his master Davy, whose discovery of the decomposition of potash and soda by the electric current had been one of the most prominent scientific advances resulting from the invention of the voltaic cell. The fifth series of researches, published in June, 1833, embodies the work. He first combats the prevailing opinion that the presence of water is necessary for electro-chemical decomposition; then analyses the views of various philosophers--Grotthuss, Davy, De la Rive, and others--who had discussed the question whether the decompositions are due to attractions exercised by the two poles of the electric circuit. This he contests in the most direct manner. Already he has reason to believe that for a given quantity of electricity passed through the liquid the amount of electro-chemical action is a constant quantity, and depends in no way on the distance of the particles of the decomposable substance from the poles. He regards the elements as progressing in two streams in opposite directions parallel to the current, while the poles “are merely the surfaces or doors by which the electricity enters into or passes out of the substance suffering decomposition.”

Amongst the laboratory notes of this time are many which were never published in the “Experimental Researches,” or of which only brief abstracts appeared. Some of these are of great interest.

Here is one literally transcribed:--

26 Feb. 1833.

_Chloride Magnesium._--When solid and wire fuzed in non-conductor--When fuzed conducted very well and was decomposed A and P Pole much action and gas--chlorine? At N Pole Magnesium separated and no gas. Sometimes Magnesium burnt flying off in globules burning brilliantly. When wire at that pole put in water or white M A [muriatic acid] matter round it acted powerfully evolving hydrogen and forming Magnesia; and when wire and surrounding matter heated in spirit lamp _Magnesium_ burnt with intense light into _Magnesia_. VERY GOOD EXPT.

This recalls the “capital experiment” entry which Sir Humphry Davy wrote after the account of his decomposition of caustic potash. On the 7th of April we come to a marvellous page of speculations. He has seen that liquids, both solutions and fused salts, can be decomposed by the current, and that at least one solid is capable of electrolysis. But he finds that alloys and metals are not decomposed. He finds that electrolysis is easiest for those compounds that consist of the most diverse elements, and is led on to speculate as to the possible constitution of those conductors that the current does not decompose. This may involve a recasting of accepted ideas; but from such a step he does not shrink, as the following extracts show:--

Metals _may_ not be compounds of elements most frequently combined, but rather of such as are so similar to each other as to pass out of the limit of voltaic decomposition.

13th April (same page).

If voltaic decomposition of the kind I believe then review all substances upon the new view to see if they may not be decomposable, &c. &c. &c.

[Sidenote: ATTRACTION BY POLES DOUBTED.]

He has now found that the facts observed do not admit of being explained on the supposition that the motion of the ions is due to the attraction of the poles, and accordingly there follows the entry:--

(Ap. 13, 1833.)

A single element is never attracted by a pole, _i.e._ without attraction of other element at other pole. Hence doubt Mr. Brande’s Expts on attraction of gases and vapours. Doubt attraction by poles altogether.

To this subject he returned in 1834; an intervening memoir--the sixth--being taken up with the power of metals and solids to bring about the combination of gaseous bodies. In the seventh series, published in January, 1834, his first work is to explain the new terms which he has adopted, on the advice of Whewell, to express the facts. The so-called poles, being in his view merely doors or ways by which the current passes, he now terms _electrodes_, distinguishing the entrance and exit respectively as _anode_ and _cathode_,[42] while the decomposable liquid is termed an _electrolyte_, and the decomposing process _electrolysis_. “Finally,” he says, in a passage (here italicised) worthy to be engraved in gold for the essential truth it enunciates on a question of terminology, “I require a term to express those bodies which can pass to the _electrodes_, or, as they are usually called, the poles. Substances are frequently spoken of as being _electronegative_, or _electropositive_, according as they go under the supposed influence of a direct attraction to the positive or negative pole. But these terms are much too significant for the use to which I should have to put them; _for though the meanings are perhaps right, they are only hypothetical, and may be wrong; and then, through a very imperceptible but still very dangerous, because continual, influence, they do great injury to science, by contracting and limiting the habitual views of those engaged in pursuing it_. I propose to distinguish such bodies by calling those _anions_ which go to the anode of the decomposing body; and those passing to the _cathode_, _cations_; and when I shall have occasion to speak of these together, I shall call them _ions_.[43] Thus, the chloride of lead is an _electrolyte_, and when _electrolyzed_ evolves the two _ions_, chlorine and lead, the former being an _anion_ and the latter a _cation_.” In Faraday’s own bound volume of the “Experimental Researches” he has illustrated these terms by the sketch here reproduced. (Fig. 12.)

Faraday’s letter to Whewell when he consulted him as to the new words has not been preserved. He discarded, when the paper was printed, the terms he had first used. Whewell’s replies of April 25th and May 5th, 1834, have been preserved and are printed in Todhunter’s biography of Whewell. From the later of the two the following passage is extracted:--

[Sidenote: NEW NOMENCLATURE.]

[_Whewell to Faraday_], May 5, 1834.

If you take _anode_ and _cathode_, I would propose for the two elements resulting from _electrolysis_ the terms _anion_ and _cation_, which are neuter participles signifying _that which goes up_, and _that which goes down_; and for the two together you might use the term _ions_.... The word is not a substantive in Greek, but it may easily be so taken, and I am persuaded that the brevity and simplicity of the terms you will thus have will in a fortnight procure their universal acceptation. The _anion_ is that which goes to the _anode_, the _cation_ is that which goes to the _cathode_. The _th_ in the latter word arises from the aspirate in _hodos_ (way), and therefore is not to be introduced in cases where the second term has not an aspirate, as _ion_ has not.

On May 15th Faraday replied as follows:--

[_Faraday to Whewell._]

I have taken your advice and the names, and use _anode_, _cathode_, _anions_, _cations_ and _ions_; the last I shall have but little occasion for. I had some hot objections made to them here, and found myself very much in the condition of the man with his Son and Ass, who tried to please everybody; but when I held up the shield of your authority it was wonderful to observe how the tone of objection melted away. I am quite delighted with the facility of expression which the new terms give me, and shall ever be your debtor for the kind assistance you have given me.

As though to prepare the way for a still further cutting of himself adrift from the slavery of using terms that might be found misleading, he added the following note:--

It will be well understood that I am giving no opinion respecting the nature of the electric current now, beyond what I have done on former occasions; and that though I speak of the current as proceeding from the parts which are positive to those which are negative, it is merely in accordance with the conventional, though in some degree tacit, agreement entered into by scientific men, that they may have a constant, certain, and definite means of referring to the direction of the forces of that current.

The “former occasions” is a reference to an earlier suggestion that a _current_ might mean anything progressive, whether a flow in one direction or two fluids moving in opposite directions, or merely vibrations, or, still more generally, progressive forces. He had expressly said that what we call the electric current “may perhaps best be conceived of as _an axis of power having contrary forces, exactly equal in amount, in contrary directions_.”

[Sidenote: ELECTRO-CHEMICAL LAWS.]

He then suggests as a measurer of current the standard form of electrolytic cell ever since known as the _voltameter_. He preferred that kind in which water is decomposed, the quantity of electricity which had flowed through it being measured by the quantity of the gas or gases evolved during the operation. Before adopting this he undertook careful experiments in which his fine manipulative skill, no less than his chemical experience, was called into service to verify the fact that the quantity of water decomposed was really proportionate to the quantity of electricity which has been passed through the instrument. Having this standard, he investigated numerous other cases of decomposition by the current, and so arrived at a substantial basis for the doctrine of _definite electro-chemical_ action. Speaking of the substances into which electrolytes are divided by the current, and which he had called ions, he says: “They are combining bodies; are directly associated with the fundamental parts of the doctrine of chemical affinity; and have each a definite proportion, in which they are always evolved during electrolytic action.... I have proposed to call the numbers representing the proportions in which they are evolved _electro-chemical equivalents_. Thus hydrogen, oxygen, chlorine, iodine, lead, tin are _ions_; the three former are _anions_, the two metals _cations_, and 1, 8, 36, 125, 104, 58, are their _electro-chemical equivalents_ nearly.”

This fundamental law being set upon an impregnable basis of facts, he goes on to speculate upon the _absolute quantity_ of electricity or electric power belonging to different bodies; a notion which only within the last few years has found general acceptance.

In developing this theory he uses the following language:--

According to it [_i.e._ this theory], the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric powers; it being the ELECTRICITY which _determines_ the equivalent number, _because_ it determines the combining force. Or, if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalents to each other in their ordinary chemical action, have equal quantities of electricity naturally associated with them. But I must confess I am jealous of the term _atom_....

Here we find the modern doctrine of _electrons_ or unitary atomic charges, clearly formulated in 1834. In the course of this speculation he remarks that “if the electrical power which holds the elements of a grain of water in combination, or which makes a grain of oxygen or hydrogen in the right proportions unite into water when they are made to combine, could be thrown into the condition of _a current_, it would exactly equal the current required for the separation of that grain of water into its elements again.” And all this years before there was any doctrine of the conservation of energy to guide the mind of the philosopher! The passage just cited contains the germs of the thermodynamic theory of electromotive forces worked out a dozen years later by Sir William Thomson (now Lord Kelvin), by which theory we can predict the electromotive forces of any given chemical combination from a knowledge of the heat evolved by a given mass of the product in the act of combining.

[Sidenote: ANOTHER UNSUCCESSFUL QUEST.]

The eighth series of the researches, which was read in June, 1834, deals chiefly with voltaic cells and batteries of cells. He is now applying to the operations inside the primary cell the electrochemical principles learned by the study of electrolysis in secondary cells. His thoughts have been incessantly playing around the problem of electrolytic conduction. He was convinced that the forces which shear the anions from combination with the cations and transfer them in opposite directions must be inherent before the circuit is completed, and therefore before any actual transfer or movement takes place. “It seems to me impossible,” he says, “to resist the idea that it [the “transfer,” or “what is called the voltaic current”] must be preceded by a _state of tension_ in the fluid. I have sought carefully for indications of a state of tension in the electrolytic conductor; and conceiving that it might produce something like structure, either before or during its discharge, I endeavoured to make this evident by polarised light.” He used a solution of sulphate of soda, but without the slightest trace of optical action in any direction of the ray. He repeated the experiment, using a solid electrolyte, borate of lead, in its non-conducting state, but equally without result.

During the time of these electrochemical researches in 1833 and 1834, Faraday’s activities for the Royal Institution were undiminished. In 1833 he gave seven Friday discourses, three of them on the researches in hand, one on Wheatstone’s investigation of the velocity of the electric spark, and one on the practical prevention of dry rot in timber, which was afterwards republished as a pamphlet, and ran to two editions. In 1834 he gave four Friday discourses; two on his electrochemical researches, one on Ericsson’s heat-engine, and the other on caoutchouc.

The ninth series of electrical researches occupied the autumn of 1834. In it he returns to the study of the magnetic and inductive actions of the current, investigating the self-induced spark at the break of the circuit, to which his attention had been directed by Mr. W. Jenkin. Several points in this research are little known even now to electricians, the laboratory notes being much more detailed than the published paper. He describes an exceedingly neat high-speed break for producing rapid interruptions, using for that purpose stationary ripples on the surface of a pool of mercury. In a wonderful day’s work on 13th November, filling thirty-four pages of the laboratory book, illustrated with numerous unpublished sketches, he tracks out the properties of self-induction. He proves that the spark (on breaking circuit) from a wire coiled up in a helix is far brighter than that from an identical wire laid out straight. He finds that a non-inductive and, therefore, sparkless coil can be made by winding the wire in two opposite helices. “Thus the whole [inductive] effect of the length of wire was neutralised by the reciprocal and contrary action of the two halves which constituted the helices in contrary directions.” The next day he writes: “These effects show that every part of an electric circuit is acting by induction on the neighbouring parts of the same current, even in the _same wire_ and the _same part_ of the wire.”

[Sidenote: EFFECTS OF SELF-INDUCTION.]

On 22nd November he is trying another set of experiments, also never fully published. They relate to the diminution of self-induction of a straight conductor by dividing it into several parallel strands at a small distance apart from one another. The note in the laboratory book runs thus:--

Copper wire 1/23 of inch in diameter. Six lengths of five feet each, soldered at ends to piece of copper plate so as form terminations, and these amalgamated. When this bundle was used to connect the electro-motor it gave but very feeble spark on breaking contact, but the spark was sensibly better when the wires are held together so as to act laterally than when they were opened out from each other, thus showing lateral action.

Made a larger bundle of the same fine copper wire. There were 20 lengths of 18 feet 2 inches each and the thick terminal pieces of copper wire 6 inches long and ⅓ of inch thick.

This bundle he compared with a length of 19 feet 6 inches of a single copper wire ⅕ inch in diameter, having about equal sectional area. The latter gave decidedly the largest sparks on breaking circuit.

Faraday did not see fit at this time to accept the idea, suggested indeed by himself in 1831, that these effects of self-induction were the analogue of momentum or inertia. That explanation he set aside on finding that the same wire when coiled had greater self-inductive action than when straight. Had he at that time grasped this analogy, he would have seen that the very property which gives rise to the spark at break of circuit also retards the rapid growth of a current; and then the experiment described above would have shown him that Sir W. Snow Harris was right in preferring flat copper ribbon to a round wire of equivalent section as a material for lightning conductors. He was, however, disappointed to find so small a difference between round wires and parallel strands. The memoir as published contains an exceedingly interesting conclusion:--

Notwithstanding that the effects appear only at the making and breaking of contact (the current, remaining unaffected, seemingly, in the interval,) I cannot resist the impression that there is some connected and correspondent effect produced by this lateral action of the elements of the electric stream during the time of its continuance. An action of this kind, in fact, is evident in the magnetic relations of the parts of the current. But admitting (as we may do for the moment) the magnetic forces to constitute the power which produces such striking and different results at the commencement and termination of a current, still there appears to be a link in the chain of effects--a wheel in the physical mechanism of the action, as yet unrecognised.

The tenth series of researches, on the voltaic battery, though completed in October, 1834, was not published till June, 1835.

[Sidenote: ACTION IN A MEDIUM.]

The next research, begun in the autumn of 1835, after a lull of about eight months, lasted over two years. It was not completed till December, 1837. This investigation took Faraday away from magnetic and electrochemical matters to the old subject of statical electric charges, a subject hitherto untouched in his researches. But he had long brooded over the question as to the nature of an electric charge. Over and over again, as he had watched the inductive effect of electric currents acting from wire to wire, his mind turned to the old problem of the inductive influence--discovered eighty years before, by John Canton--exerted, apparently at a distance, by electric charges. He had learned to distrust action at a distance, and now the time was ripe for a searching inquiry as to whether electric _influence_, or induction[44] as it was then called, was also an action propagated by contiguous actions in the intervening medium.

Faraday had done no special electric work during the first nine months of 1835. He had worked at a chemical investigation of fluorine through the spring, and in July took a hurried tour in Switzerland, and returned to work at fluorine. Not till November 3rd does he turn to the subject over which he had been brooding. On that date, intercalated between notes of his chemical studies, filling a dozen pages of the laboratory book, are a magnificent series of speculations as to the nature of charges, and on the part played by the electric--or, as we should now say, the dielectric--medium. They begin thus:--

“Have been thinking much lately of the relation of common and voltaic electricity, of induction by the former and decomposition by the latter, and am quite convinced that there must be the closest connection. Will be first needful to make out the true character”--note the phrase--“of ordinary electrical phenomena.” The following notes are for experiment and observation.

“Does common electricity reside upon the surface of a conductor or upon the surface of the [di-]electric in contact with it?”

He goes on to consider the state of a dielectric substance, such as glass, when situated between a positively charged and a negatively charged surface, as in a charged Leyden jar, and argues from analogy thus:--

“Hence the state of the plate [of glass] under induction is the same as the state of a magnet, and if split or broken would present new P[ositive] and N[egative] surfaces before not at all evident.” This speculation was later verified by Matteucci.

“Probable that phenomena of induction prove more decidedly than anything else that the electricity is in the [di-]electric not in the conductor.”

He still worked for a week or two on fluorine, interposing some experiments on the temperature-limit of magnetisation, but on December 4th decides not to go on with fluorine at present. Then, beginning on December 5th, there follow twenty-nine pages of the laboratory diary, illustrated with sketches. He had borrowed from a Mr. Kipp a large deep copper pan thirty-five inches in diameter, and he set to work electrifying it and exploring the distribution of the charges, inside and out, and the inductive effect on objects placed within. Everywhere he is mentally comparing the distribution of the effects with that of the flow of currents in an electrolyte. Before many days he writes:--

[Sidenote: PREGNANT SUGGESTIONS.]

“It appears to me at present that _ordinary_ and _electrolytic_ induction are identical in their first nature, but that the latter is followed by an effect which cannot but from the nature and state of the substances take place with the former.” Then comes this pregnant suggestion:--

“Try induction through a solid crystalline body as to the consequent action on polarized light.”

By the end of a week he had begun to suspect that his magnet analogy went farther than he was at first prepared to hold. The action of a magnet was along curved lines of force. So he asks:--

“Can induction through air take place in curves or round a corner--can probably be found experimentally--if so not a radiating effect.”

After ten days more he has made another step.

“Electricity appears to exist only in _polarity_ as in air, glass, electrolytes, etc. Now metals, being conductors, cannot take up that polar state of their own power, or rather retain it, and hence probably cannot retain developed electric forces.

* * * * *

“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.

[Sidenote: ACTION AT A DISTANCE UNTHINKABLE.]

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.

[Sidenote: SPECIFIC INDUCTIVE CAPACITY.]

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.

[Sidenote: CABLE RETARDATION PREDICTED.]

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.

[Sidenote: COINAGE OF NEW WORDS.]

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 Esq^{re.} Royal Institution.

[Sidenote: LATERAL ACTIONS OF CURRENT.]

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.

[Sidenote: INCESSANT ACTIVITIES.]

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.

[Sidenote: THE CONTACT THEORY OF ELECTRICITY.]

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.

[Sidenote: END OF SECOND ACTIVE PERIOD.]

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.