The Search for Electromagnetic Induction: 1820-1831

The Search for Electromagnetic Induction: 1820-1831Author(s): Sydney RossSource: Notes and Records of the Royal Society of London, Vol. 20, No. 2 (Dec., 1965), pp. 184-219Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/3519873 .

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184

THE SEARCH FOR ELECTROMAGNETIC INDUCTION 1820-1831

By SYDNEY Ross

Professor of Colloid Science, Rensselaer Polytechnic Institute, Troy, New York

[Plates 19 to 21]

I. THE CLUE THAT AMPiRE NEGLECTED

O ERSTED'S discovery in 1820 of the magnetic field that surrounds a conductor during the passage of an electric current, aroused a wave of

interest among men of science in England, France, Germany, Italy, and the United States. The apparatus required to verify his results was easily put together, and anyone who cared to do so could see for himself the nature of the indissoluble connexion between electricity and magnetism, which, though long suspected and vaguely adumbrated, was now precisely defined and made a permanent portion of the corpus of science. As one subsequent discovery after another was announced from various places, the recognition became widespread that a large and unexploited field for investigations and applications had been opened up. Only one week after word of Oersted's experiment reached Paris, Ampere discovered that two parallel wires that carry parallel currents attract each other. Less than two months after Oersted's publication, J. S. C. Schweigger (1779-1857), at the University of Halle, reasoned that if the current in a single wire held above the compass needle would deflect the needle to the right, while the same wire placed beneath the needle would deflect it to the left, one turn of wire, placed around the needle in the plane of the magnetic meridian, would exert twice the deflecting force of a single wire; and a coil made of ten turns of insulated wire would exert twenty times the force. Thus were Schweigger's multiplier and Amp"re's solenoid invented. In rapid succession followed the electromagnet (Arago and Davy), the astatic galvanometer (Nobili), electromagnetic rotations (Wollaston and Faraday), and the science of electrodynamics (Ampere).

But to one question the answer was stubbornly withheld. If the presence of an electric current is always concomitant with a magnetic field, why should it not be possible to reverse Oersted's experiment and induce electric currents by the action of a magnet? The first to consider this question was Fresnel (I), who argued that since a steel bar can be magnetized by passing a current through a metallic helix surrounding the bar, it is natural to try ifa bar magnet



Plate 19

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ANDRt MARIE AMPIRE, For. Mcm. R.S. (1775-1836)

[Facing page 184



I85 would not in turn create an electric current within an enveloping helix. 'Not that such a result is a necessary consequence of the original observation,' he added, 'because the magnetic state of steel might, for example, be due only to a new arrangement of its molecules, or to a particular way in which an imponderable fluid is distributed; in which case the magnetic state would not be expected to be able to produce the movement that originally established it.' He thought, nevertheless, that it might not be useless to try the experiment. On 6 November 1820, he reported to the Academy of Sciences that he had succeeded in decomposing water by means of a current induced in a coil wound around a magnet. Emboldened by this announcement, Ampere also declared that he too had noticed something in the way of production of currents from a magnet. Before the end of the year, however, both statements were retracted. Fresnel explained that his announcement was premature and that on subsequent trials the results were not reproduced; as for Monsieur Ampere, his explanation continued, the indications he had obtained were so feeble and uncertain that he would not have published them at all, had not Fresnel's results, which he thought certain, persuaded him that his observed effects must also have originated from a current induced by a magnet (2).

With Fresnel's experiment we see for the first time the false trail that made it so difEcult to discover electromagnetic induction. Fresnel had reasoned that since a steady current produced a steady magnetic field, the converse effect would be detected by merely placing a magnet in the neighbourhood of a wire, or a coil of wire, and looking for the presence of a steady current in the wire. He argued, further, with irrefutable logic, that this converse effect, though it might be forthcoming, does not necessarily exist, so that repeated failures to obtain it would be a good reason to quit the search. We now know that this type of converse to Oersted's experiment cannot be realized; but if a fluctuating magnetic field were applied, pulses of electric current would be generated in the wire.

Six months later, Ampere sought the effect with a more carefully designed experiment, which turned out negative. He stated his conclusion with impres- sive deliberation (3): 'The proximity of an electric current does not induce another current in a metallic current made of copper, even under the most favourable conditions for its influence to be effective.' The experiment that led him to that conclusion was simple enough: a light-weight ring made from a thin strip of copper was suspended by a fine wire so that it lay inside, and almost touching, a flat coil wound parallel to the ring. Amphre's own diagram is shown in Fig. I. When a current is passed through the coil, the magnetic field generated would have the same geometric relation to the coil and to the



186

suspended ring. Ampere expected that an induced current, if one were called into existence, would then flow around the ring, and he proposed to detect it by observing the action of a magnet on the movable ring. In July 1821, when he tried the experiment, he saw no movement of the ring, despite its ease of mobility. That result persuaded him of the non-existence of induced currents.

The following year, on an eight-day visit to Switzerland, Ampere set up the experiment again, in order to demonstrate it to an ardent young physicist,

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FIGURE I. The apparatus with which Ampere and De la Rive dimly foresaw the presence of an induced current in 1822. ABCDEF-the loop of the primary circuit; M-a glass tube through which passes a fine thread, suspending the copper ring GHI that forms the secondary circuit; pk, qn-supports for an iron horse-shoe magnet (not shown in the diagram) to produce a constant magnetic field inside which the secondary circuit GHL will turn at the moment

when the induced current flows.



187

Auguste de la Rive. This time he had available a powerful horse-shoe magnet, on loan from the Geneva Museum, as a detector. By its means an effect that at first he had failed to observe became prominent. But the circumstances of the experiment are actually poorly disposed for a clear-cut demonstration of electromagnetic induction: what Ampere and De la Rive observed was that the ring would move slightly whenever a current was set up in the coil, and that it would return to its original position once the circuit was broken. Even though the momentary character of the induced current was not at all evident from this observation, they had undoubtedly witnessed a true effect of electromagnetic induction. Ampere at least immediately recognized it as such, although he erred in believing that a steady current persisted in the ring while the primary current lasted. On his return to Paris he reported the results of this and other experiments to the Academy of Sciences on i6 September 1822.

The memoir that Ampeire read on this occasion remained unpublished until long after his death: young De la Rive had sent an account (4) of the Geneva experiments to the Swiss journal Bibliotheque Universelle, and this was soon after reprinted in the Annales de Chimie et de Physique. Ampere therefore considered that the publication of his own memoir on the subject was not required. Though apparently trivial, this incident had unfortunate reper- cussions. De la Rive, who was only twenty at the time, gave an inadequate account of the experiment, which was to mislead later investigators. At a critical time in the history of the subject, the report by De la Rive was the only one readily available. The few sentences devoted to the experiment occur as the terminal paragraph of a paper largely concerned with other matters. The paragraph is quoted below in its entirety:

The second experiment has to do with the effect produced on a thin copper ring that is suspended within a band of strong electric currents, which surround it without touching it anywhere. This effect, which at first M. Ampere believed to be non-existent, has been verified by him very definitely while in Geneva. With a powerful horse-shoe magnet held near one face of the ring, it (the ring) was observed to advance or withdraw between the two branches of the magnet, depending on (suivant) the direction of the current in the surrounding conductors. This important experiment shows that bodies not otherwise able to acquire permanent magnetism by the influence of electric currents, being in this respect unlike iron and steel, can at least acquire a sort of temporary magnetism while they are under the influence of the current.



188

De la Rive's statement offered no more explanation of the effect than that it was due to a temporary magnetization acquired by the non-magnetic material. The presence of an induced current as a cause of the temporary magnetization was not suggested, which deprived the announcement of much of its potential dramatic impact: a more experienced writer might have been less timid. Ampere certainly saw no reason to hesitate about drawing a conclusion that seemed so obvious. The very first sentence of his unpublished memoir (5) made that clear: 'The objective of the experiment was to learn if an electric current could be produced by the influence of another current.' The memoir continued with a description of the apparatus, which was essentially the same as that of Fig. I. The observed results were next disposed of with the simple statement that 'the ring was alternately attracted and repelled by the magnet, when under the influence of the current flowing in the outer spiral.' So brief an account of what was observed leaves many questions unanswered, but it was intended merely as a preliminary report. It continued:

This experiment, therefore, does not permit any doubt of the production of electric currents by means of induction, assuming that the presence of a little iron in the copper forming the mobile circuit is not suspected. There had not, however, been any reaction between the ring and the magnet before the electric current passed through the spiral surrounding the ring; hence the reason that I regard this experiment as proving the production of an induced current. Nevertheless, in order to anticipate any objection, I plan to repeat it immediately with a ring-circuit made of a highly purified non-magnetic metal. The fact that electric currents can be produced by induction is extremely interesting in itself, and is besides independent of the general theory of electro-dynamic action.

Unfortunately those definitive and challenging statements were not published at the time. The comment at the end I interpret to mean that the effect was not to be predicted by Amp"re's theory of electrodynamic action, and this may well be significant in explaining his relative lack of interest in the result, as due to impatience at any diversion of his attention by irrelevant phenomena. Impatience is obvious too in his written account: in his haste he omitted to mention whether the alternate attractions and repulsions of the ring were caused by alternating the pole of the magnet presented to it, or by reversing the direction of the flow of current in the surrounding spiral-circuit. Presumably, going on the evidence of De la Rive, it was the latter effect that was obtained. And here both men cannot escape the accusation of



189

negligence, for they made no attempt to determine the direction of their postulated induced-current relative to the direction of the primary current. This determination was the obvious next step of the investigation; had they commenced seriously to undertake it, they could hardly have missed un- covering the true nature of the induced current. De la Rive was content, however, merely to put on record a general but unspecified dependence of the direction of motion of the ring on the direction of the current in the spiral- circuit. His language was not sufficiently explicit, perhaps to hide his lack of more definite information, and at least two subsequent readers, Demon- ferrand and Faraday, were to misinterpret it.

J. B. F. Demonferrand (1795-1844) described the new science of electrodynamics in his book Manuel d''lectricit6 dynamique (Paris, 1823). His account of the Amp re-De la Rive experiment has a certain precision of description that suggests he may have derived it in part directly from Amp&re; it is, at all events, a more explicit account than any published previously. Demonfer- rand wrote (6):

We are to determine if a galvanic current passing through a conductor can produce, by its influence, a current in another conducting wire that is submitted to its action without being connected to the voltaic pile. To perform that experiment a copper circle was suspended by a silk thread and surrounded with a spiral... A magnet was brought up to the mobile ring; then at the moment when the electric circuit was established in the spiral, the mobile ring turned into such an equilibrium position as it would have assumed had it been traversed by a current passing in the same direction as that in the spiral. The first attempts of M. Amp&re at this experiment were unsuccessful, because of the lack of power both in the galvanic pile and in the magnet; in subsequent trials he succeeded, and it may therefore be considered as established that an electric current tends to put the electricity of conductors, near which it is flowing, into motion in the same direction. [The italics are Demonferrand's.] Even with a knowledge of this fact, our complete ignorance about the nature of the elementary currents in iron, leaves it still uncertain whether the magnetization of metals is the result of the actual production of electric currents or simply a change of direction in currents previously existing. Note, however, that an electric current of finite magnitude has never yet been produced in a conductor by the influence of a magnet or a system of magnets. Perhaps magnetization is not uniquely due to one or the other of the causes I have indicated, but both of them at once-the



g190

action of a magnet or an electrical spiral upon iron giving a common direction to pre-existing currents, and at the same time augmenting their intensity.

Demonferrand has added significant details to De la Rive's account. Some of the more explicit statement was perhaps due to a mistaken interpretation of De la Rive's phraseology. De la Rive had written: 'En prdsentant "a un c8td de cette lame un aimant en fer " cheval, tres-fort, on I'a vue tant6t s'avancer entre les deux branches de l'aimant, tant6t au contraire en &tre repouss6, suivant le sens du courant dans les conducteurs environnans.' Did suivant imply more than a general dependence? Demonferrand took it literally, and it led him into defining a direction for the induced current just the opposite of what should have been observed: for the primary current, on starting, induces a current of the opposite direction in the secondary circuit. What had merely been non-committal in De la Rive's account was thus converted into a positive error in Demonferrand's book. Amp&re often praised the book for its accuracy; this particular point he seems never to have noticed. The book is indeed an important work in the history of electrodynamics, as it contains the earliest exposition of Amp "re's discoveries. Ampere himself advocated it and sent many copies abroad (including one to Faraday) along with his own papers; soon after, he published his Pre'cis de la theorie des phenomenes electro- dynamiques (1824), which, on its title-page, is designated as a supplement to Demonferrand's book.

Demonferrand's account is more informative in some other respects than either of the previous written versions: it mentions the position of equilibrium assumed by the movable ring. The equilibrium could only be supposed to be that caused by the torsion in the suspending thread counteracting a continuous magnetic force experienced by the ring while the current is flowing in the spiral-circuit; this force, it was clearly implied by Demon- ferrand, results from a continuous electric current in the ring. As we know there would not have been any such continuous induced current in the ring, we can explain the observation only by denying the equilibrium and supposing that the torque in the suspending thread was too slight to restore the ring to its initial position after the first pulse of electricity had passed, which would have taken place on first setting-up the current in the spiral-circuit. The notion of an equilibrium appears here for the first time, yet it coincides with a much later account given by Ampere himself (7) in I833. Furthermore, Demonferrand has indicated in the extract given above that the object of the experiment was to throw light on the question of whether electric currents



9II

already exist in iron when it is in the unmagnetized condition, or are brought into being as a result of magnetization. The result of the experiment did not settle the question, which explains Amp&re's lack of interest in pursuing the subject further. But once again we find Demonferrand has anticipated a later statement (1833) by Amp ere of his objective in performing the experiment. How could Demonferrand have learned these facts save directly from Ampere himself? And if we are to admit this, then we are left to wonder if the error about the direction of the induced current could also have originated with Ampere. Some years later, after the publication of Faraday's discovery, the history of this experiment suddenly acquired some importance. The Lycle of I January 1832, No. 36, in an article written after the bare news of Faraday's discovery had been made public, but before the published account was available, 'reasoned' that the induced current ought to move in the same direction as the primary current, and added (8): 'Amphre was so thoroughly persuaded that such ought to be the direction of the currents-by-influence, that he neglected to assure himself of it in his experiments at Geneva.' If this were so, Ampere may have had a conviction in his mind that could have obscured his recollection of the actual observation. Against this supposition, is Ampere's vehement denial, ten years later, in letters to Faraday, that he had ever put on record any conclusion about the direction of the induced current, because he had never determined it; the article in the Lycee, he asserted, was written by an enemy with malicious intentions and did not represent his thinking (9). Whatever may have happened, Ampere, prior to 1832, seems to have talked of this experiment in an offhand and careless way-a measure of its lack of importance in his eyes.

Ampere's contemporaries, however, did not allow the experiment to remain in total obscurity. Although relegated to an insignificant place in Ampere's own publications, it never failed to be included in any account of his work written by others. English readers could find it described, with a diagram of the apparatus, in a Cambridge translation of Demonferrand's book, published in 1827, edited by James Cumming, F.R.S. (io). Of more general appeal and far wider circulation was a popular account of Ampere's work that appeared in the January 1827 issue of the Quarterly Review. The Geneva experiment was singled out by the reviewer in the following terms (1I):

By a very curious experiment, Ampere has proved, that a powerful electric current has a tendency to excite similar currents in neighbouring bodies, not generally susceptible of magnetism. A copper wire of

6A



192

considerable length was rolled round a cylinder, so as to form a coil, all the turns of which were separated from each other by silk riband. Within this spiral coil, a ring of brass was freely suspended by a fine metallic thread, passing through a small glass tube, which was placed between the threads of the copper coil. The circumference of the ring, in every part, was thus brought very near to the copper wire, through which a powerful voltaic current was made to pass. Under these circumstances, the brass wire was attracted or repelled by a magnet, in the same way as it would have been, had it formed part of the same voltaic circuit. The action, indeed, was but feeble, and Ampere, in his first trials, failed in his endeavours to render it sensible; but on persevering in the attempt, his success, at last, was complete and unequivocal.

The anonymous reviewer had nothing but the warmest praise for Ampere's contributions. The readers of the Quarterly were told:

No theory of electro-magnetism hitherto devised can at all enter into competition with that of Ampire ... Every experiment that has been tried, and a great variety has been devised by the ingenuity of numerous experimentalists, has served but to confirm the correctness of Ampi"re's views of the theory of magnetism... It is impossible to deny that a great advance will have been made in the philosophy of nature, if it can be shown, or even rendered probable, that all the phenomena usually referred to the operation of magnetism, as a principle totally distinct from electricity, are mere electrical effects; that the former is, in fact, included in the latter; and that, instead of two agencies, there exists but one.

The favourable treatment thus accorded Amp'ere by this influential periodical, not usually so genial in its expressions of opinion about Frenchmen, may have been instigated by Davy, who had connexions in the higher councils of the editorial board; at all events Davy seized the propitious occasion to put forth Ampere's nomination to the Royal Society, to which he was duly elected on 8 March 1827 (12).

Geneva, no less than Paris and London, was a centre of scientific activity, with particular interest being shown in the new phenomena of electromagnetism. De la Rive was only one of a circle of eminent savans who met frequently to discuss developments in natural philosophy. One of those men, Jean Daniel Colladon (I802-I893), then, in the summer of 1825, a young man and collaborator of Pr*vost, caught the general notion that a magnet ought to be able to produce an electric current in an adjoining



Plate 20

Expdrience d'induction dlectromagndlique lenlde i Geneve en 1825,

Par H. D. COLLADON.

Je crois pouvoir dire quelques mots d'une exp6rience que j'ai faite a GenBve pendant l'6t6 de 1825, et qui, par suite de circonstances tout a fait sp6ciales, ne m'avait donn6 qu'un r6sultat negatif, dont j'avais fait part, d&s cette 6poque, A quelques personnes, notamment a MM. Aug. de la Rive, J.-L. Prevost, Ch. Sturm et, deux ou trois ans plus tard, a M. Ampire, qui ne fit aucune remarque critique a cette occasion et ne m'engagea pas a la renouveler.

J'avais admis, comme chose possible, que la presence du pole d'un fort aimant pr~sentO

' l'extr6mit6 d'une h6lice form6e d'un fil de cuivre recouvert

de soie pourrait developper dans cette helice un courant Olectrique permanent. Poss6dant un galvanombtre tris sensible, je prolongeai d'environ cinquante

m6tres une des extr6mites de son fil conducteur enveloppO de soie, que je ter- minai par une h6lice a spires serr6es, de quatre ou cinq centimetres de diametre et longue de huit ou dix. J'empruntai au cabinet de physique un tres fort aimant en fer a cheval, qui faisait partie de sa collection, pour approcher un de ses pbles de celui de l'h1lice sur le prolongement de son axe.

Pour 6viter toute influence possible de cet aimant sur le galvanometre tres sensible dont je me servais, j'avais portO ce galvanomitre dans une chambre 6loign6e de celle ofi j'op6rais, je l'avais plac' sous une cloche de verre et j'avais verifi6 avec soin la position de l'index, apres quoi je revins vers la spire et je rapprochai un des pbles du gros aimant de l'helice, puis, sans me presser, je retournai vers le galvanomietre et je constatai que son index 6tait exactement au meme point qu'auparavant.

N'ayant aucun aide avec moi et ne soupponnant pas que l'induction pfit tre un effet seulement instantanO, du^ au rapprochement ou a' l'Oloignement r4ci- proque de l'h1lice et de l'aimant, je ne pouvais mieux op6rer. - Ce fut seulement six ans apris que, les exp6riences de l'illustre Faraday 6tant connues, j'eus le regret d'apprendre que j'avais Ote bien pres de decouvrir, en 1825, un des faits les plus importants de la physique moderne et celui qui a donnO nais- sance aux applications les plus pr6cieuses au point de vue mecanique et indus- triel.

An account by J. D. Colladon of his experiments in 1825 when he almost discovered electromagnetic induction [reference 13]. Reproduced by kind permission of the Bibliothique

Nationale Suisse, Berne.

[Facing page 193



193

conductor (13). His experiment was better planned than that previously carried out by Fresnel in search of the same objective, and only very bad luck prevented his making the great discovery. As he had a very sensitive galvanometer, he feared that the proximity of a powerful magnet would affect the pointer reading; accordingly, he attached fifty metres of silk-covered copper wire to it and placed the instrument itself in a glass bell-jar in an adjoining room. He constructed a tightly-wound helix of insulated copper wire, about ten centimetres in length and four or five centimetres in diameter; the two ends of this helix were connected to the long leads from the galvanometer. He anticipated that the pole of a powerful magnet, which like Ampere he had borrowed for the purpose, brought near to one end of the helix and in line with the prolongation of its long axis, would cause a permanent current to flow through the circuit. This current he planned to detect by means of his distant galvanometer. Completing the arrangcment as planned, he then sauntered (sans me presser) across the passage to look at the needle of his distant galvanometer, which of course by the time he arrived showed no change from its original position. Had he stationed an assistant to watch the galvanometer, as Joseph Henry did some years later in conducting a similar experiment, the temporary effect of electromagnetic induction might have been discovered at that time. As it was, his lack of success probably discouraged attempts that might have been made by others.

And now a new experiment, and a second clue, makes its appearance. A French instrument maker, H. P. Gambey (1787-1847) noticed that the damping of the oscillations of a compass needle is very marked when it is placed above a sheet of copper. He drew it to the attention of Arago in 1824, who confirmed the original observation, and also found that the rotation of a copper disc beneath the magnetic needle (see Fig. 2) produced a deflection of the needle in the same direction. If the rate of rotation of the disc were

Magnet

Copper d,

c Driving wheelP FIGURE 2. The Arago rotation-experiment, disclosed 7 March 1825, by which a magnetic needle is made to drag after a revolving copper disc. (From B. Dibner, Faraday discloses

electro-magnetic induction, Burndy Library, 1949.)



194

rapid enough, the needle could be made to rotate continuously (14). Babbage andJ. F. W. Herschel, in London, performed the reverse experiment (15): by rapidly rotating the magnet, they were able to set the copper disc in motion. They also made the significant observation that, substituting other metals for copper, the better conductors of electricity responded most readily to the rotating magnet.

The effects remained mysterious, however, because nobody suspected the presence of electric currents in the copper disc. A particular form of magnetism developed by motion was postulated, to explain why a normally non-magnetic metal could be affected by a magnet. The phrase magnetism of rotation, although it seemed to be no more than a description of what was clearly observed to occur in the copper disc, actually conjured up a false hypothesis, which, in the words of E. Bauer (16), 'stood like a screen between the physicist's mind and reality'. Arago himself does not deserve this criticism: he described the observed effects without venturing any hypothesis; and remained resolutely aloof and incredulous about rotational magnetism, even when advocated by the great mathematical physicist Poisson. Faraday was later to praise the wisdom and maturity ofjudgment displayed by Arago during this time (17): 'What an education Arago's mind must have received in relation to philosophic reservation: what an antithesis he forms with the mass of table-turners; and what a fine example he has left us of that condition ofjudgment to which we should strive to attain !'

Freedom from error, however, was the most that Arago could achieve: the rotations were inherently too complex a series of phenomena from which to arrive at the discovery of electromagnetic induction. The distribution of eddy currents in the rotating disc was later elucidated by Nobili; and the experiment itself became in Faraday's hands the basis of the first electromagnetic generator of continuous current; but in the period of our interest it stood only as an intriguing puzzle, a challenge that aroused the interest of many investigators, including Faraday, but which did not contribute directly towards his great discovery. One man alone must be excepted; one man for whom the Arago rotations could well have been the final clue that solved the mystery: Amp"re. No need for him to work out the complex problem of locating the eddy currents-that could come later: the key fact was the apparent magnetism of the copper. Amp&re in 1822 had already superseded De la Rive's phrase 'a kind of temporary magnetism of the non-magnetic metal' with his own explanation of a current; usually nobody was more alert than he to transpose magnetism in his thoughts into electric current; and finally he had the grand clue that an induced current could be



195

created in an independent circuit by the action of another current-then, why not by a magnet? That chain of reasoning is so readily completed that we marvel at his not having seen it at once.

Ampere, however, seems to have had no inclination, by himself, to take up the subject of the Arago rotations. In August 1826, nearly two years after Arago had first demonstrated the effect in front of the Academy of Sciences, Ampere was approached by Arago himself who wished to make use of the voltaic pile and other equipment belonging to the College de France for a continuation of his researches. Arago had in mind substituting a solenoidal electromagnet for the magnetic needle of his original experiment. With Ampere's approval, the laboratory assistant (ripe'titeur), Ajasson de Grandsagne, assembled the equipment; but on the making the first trial, at the very moment when the pivoted electromagnet began to move in response to the rotation of the copper disc, the axle of the disc snapped across. Arago had to leave for the Pyrenees on the very next day, but he authorized Ampere to continue the experiment in his absence. Colladon, who was in Paris at the time and witnessed the first attempt, undertook to repair the equipment, at the same time making some important improvements in its strength and sensitivity. On repeating the experiment, the electromagnet was observed to move almost as soon as the copper plate was set in motion. Ampere immediately transmitted the result to Arago in the form of a Note for publication, enclosed in a letter dated I September 1826, in which he said: 'Allow me to remind you, my dear and excellent friend, that you promised me, should this experiment succeed, to adhere to my theory as the true explanation of these phenomena. In adding this effect to all the rest that I have published, I do not see how anyone can continue to find objections to it' (18).

To Amp"re, therefore, the Arago rotations were significant only as having provided an opportunity to demonstrate yet another similarity in behaviour of a solenoid and a magnet. After having demonstrated this point of prime concern to himself, he abandoned the investigation as of no further interest. His disinclination to persist was related to the fact that at that time he had relinquished the grand clue that might otherwise have stimulated his perception. Colladon had by now convinced many of the non-existence of an induced current created by the influence of a magnet, and even Ampere did not argue with his finding. In fact, many years afterwards, Colladon remarked that Amphre had neither offered any critical remarks nor encouraged him to continue the search. Such a passive reception of what was, after all, a flat contradiction of his own published statements was not typical of Ampere, but could be accounted for by some uncertainty in his own mind of



196

the validity of his original results. And, sure enough, at about this time we find that Ampere had declared to his friend A. C. Becquerel (1788-1878) that he had changed his opinion and had reverted to his former disbelief in the existence of induced currents.

The evidence that Amphre had thus changed his opinion is once again indirect: it is to be found in the course of a memoir by Becquerel on his own experiments. Becquerel was trying to find out if magnetism could be detected in various non-metallic substances: he suspended them, in the form of needles, inside a multiplier-coil and then turned on the current, to see if they would move. He found signs of magnetic action in needles made of peroxide of iron, copper, wood, and shellac. Before recounting his own experiments, he referred to the Ampere-De la Rive experiment in the following terms (19):

One would have concluded from that experiment . . . that the influence of the electric current had developed another current in the strip [i.e. the secondary circuit], such as is observed in a metal wire connected to the two poles of a voltaic pile; but Monsieur Ampere has subsequently become convinced that this is not so [my italics].

Becquerel does not disclose what were the new observations or arguments that had persuaded Ampere to alter his opinion. And what are we to make of Ampere's statement to Faraday, in his self-exculpatory and self-justifying letter of April 1833, that he had repeated the Geneva experiment a number of times between 1822 and 1828 in the presence of various people, and always with the same success? The experiment would have had no significance for him or for his audience if the possibility of an induced current in the secondary circuit were ruled out; why should he then have bothered to perform the experiment? These and other similar questions, which cannot now be answered, show some of the uncertainties in this historical reconstruction of Ampere's actual interpretation (before 1831) of the Geneva experiment. This much is clear: the subject had occupied his mind only momentarily from time to time; he had not deemed it worthy of concentrated and prolonged attention; and the very ease with which he could change his opinion about the interpretation of the experiment is itself indicative that he had not spent any significant intellectual effort on the subject.

2. A DECADE OF UNREWARDED RESEARCH

In London, in the laboratory of the Royal Institution, researches on electromagnetism were originated by Sir Humphry Davy in the first flush of interest and enthusiasm with which he had greeted the news of Oersted's



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discovery. At first, Davy was occupied with Wollaston's idea that the newly- discovered effect might be utilized to produce rotatory motion. When their initial attempts to bring this about had failed, other implications of the discovery remained to be explored. The rapid development of the subject by Amp "re gave fresh food for thought, particularly the theory that magnetism could be explained by postulating electrical currents within each atom-an idea that sounds marvellously like our modern knowledge of atomic structure. Davy too had intellectual powers amounting to genius; he was, moreover, a romantic idealist who foresaw great practical outcomes from science for the benefit of mankind. This was a new viewpoint for the man of science, and a stimulus for investigation more powerful than any other. He was always eager, therefore, to move from the realm of theory to that of practice. In the following letter (hitherto unpublished) from Davy to Ampere we can detect the writer's anxiety to bring the theories of the brilliant French mathematician to some test of experiment:

H. Davy to A.-M. Ampire (20) Dear Sir, May 26, 1821

I am very much obliged to you for the last flattering letter which I had the honour of receiving from you.

Your ingenious results and the elaborate conclusions deduced from them have excited great attention amongst our Philosophers.

I wish you may be able to furnish some direct proof of the existence of Electrical currents in the Magnet. As yet all our attempts to produce electrical from magnetic phaenomena have failed.

I have worked a good deal on this subject and I shall soon have the pleasure of sending you two memoirs containing the few facts I have been able to establish. They are at least of a novel kind though I fear of little importance for theory.

I shall seize the first favourable opportunity that offers of placing your name amongst the candidates for election on the foreign list (21), but in general it is a point of delicacy for the president rather to obey ...

[The remainder of the letter is missing.]

The possibility of producing an electric current by means of magnetism appeared to Davy to be a direct outcome of Amp"re's theory of magnetism, and it seems that Davy had himself made some unsuccessful efforts towards realizing it. Faraday, who worked so closely with him, would certainly have been aware of the objective of these experiments. We know (22) that the



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following year Faraday made out a list headed 'Chemical Notes, Hints, Suggestions, and Objects of Pursuit': it contained many of the germs of his future discoveries, and prominent amongst them was the injunction: Convert magnetism into electricity. Faraday has been praised, sometimes too fulsomely, for his intuitive perception in recognizing the relation of phenomena to one another: the influence of German Naturphilosophie has also been invoked as the underlying inspiration of his work, although unknown to himself. His conscious quest for 'electricity from a magnet', which led to the greatest of his discoveries, is most frequently cited as a striking illustration of this un- provable thesis. That Faraday, however, should simply have had the idea directly from Davy, and that it should have come to Davy as a result of reading Ampire's papers, is extremely probable, however, in the light of this letter. Nothing discreditable accrues to Faraday's memory should that have been so: the idea was not even original with Davy, having been expressed earlier by Fresnel. But when Davy was in his laboratory, on the quest for a solution to a problem, few men could have been present without being in- spired by the ardour and sharing the enthusiasm of this most vivacious re- searcher. Faraday was caught up time and again in the sweep of Davy's activities. This occasion differs only from the others in being the most illus- trious. It does not detract from Faraday's great merits to do justice to his patron and teacher, whose brilliance at grasping the wider implications of

phenomena was precisely his strongest trait. In this respect, Davy surpassed Faraday, althoughhe never equalled the younger man in power of observation, experimental skill, or doggedness of purpose.

We can detect between the lines of Davy's letter to Ampere a hint of criticism to the effect that theory was all very well, but experimental evidence was lacking to substantiate it. A more direct expression of Davy's opinion occurs in one of his published papers (23):

Is electricity a subtile elastic fluid?-or are electrical effects merely the exhibition of the attractive powers of the particles of bodies? Are heat and light elements of electricity, or merely the effects of its action? Is magnetism identical with electricity, or an independent agent, put into motion or activity by electricity? Queries of this kind might be considerably multiplied, and stated in more precise and various forms: the solution of them, it must be allowed, is of the highest importance; and though some persons have undertaken to answer themin the most positive manner, yet there are, I believe, few sagacious reasoners, who think that our present data are sufficient to enable us to decide on such abstruse and difficult parts of corpuscular philosophy.



Plate 21

? . . .

. .' ..

? 5.

.. ,

MICHAEL FARADAY, F.R.S. (1791-1867) Statue by J. H. Foley, R.A., in the possession of the Royal Institution. Reproduced by kind permission of the Royal Institution. Photograph by courtesy of Associated Electrical

Industries Limited.

[FIacinhq page 198



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Faraday merely echoed Davy's opinion, expressing it much more bluntly, in writing to Professor Gaspard de la Rive, the father of Amp&re's young collaborator (24):

September 12, 1821

You partly reproach us here with not sufficiently esteeming Ampere's experiments on electro-magnetism. Allow me to extenuate your opinion a little on this point. With regard to the experiments, I hope and trust that due weight is allowed to them; but these you know are few, and theory makes up the great part of what M. Amp&re has published, and theory in a great many points unsupported by experiments when they ought to have been adduced. At the same time, M. Ampere's experiments are excellent, and his theory ingenious; and, for myself, I had thought very little about it before your letter came, simply because, being naturally sceptical on philosophical theories, I thought there was a great want of experimental evidence....

I am by no means decided that there are currents of electricity in the common magnet. I have no doubt that electricity puts the circles of the helices [of an electromagnet] into the same state as those circles are in that may be conceived in the bar magnet, but I am not certain that this state is directly dependent on the electricity, or that it cannot be produced by other agencies. And therefore until the presence of Electrical currents is proved in the magnet by other than magnetical effects, I shall remain in doubt about Ampere's theory.

A slight coldness or reserve always seems to have mitigated the respectful courtesy with which the flood ofAmp&re's papers and pamphlets was received at the Royal Institution. The inability to appreciate the true worth of Ampere's work on electrodynamics was perhaps due to deficiencies in the mathematical training of both Davy and Faraday: they appear never to have appreciated the force of an argument that is based on the agreement between observed behaviour and the predictions of a theoretical model, when those predictions are made by mathematical deduction. Although such an argument, even when not vitiated by assumptions or approximations, can never be a conclusive proof of the validity of models, it is no worse off in that respect than the experimental 'effects' that, according to Davy and Faraday, constituted the sole criterion of hypotheses. Much of what Ampere wrote was, consequently, as a result of this attitude, discounted in advance; but when he would describe an experiment, such as his method of causing a bar magnet to rotate about its axis, Faraday was prompt to verify the observa-

6B



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tion. A just appreciation of Ampere's contribution had to wait until Maxwell, trained as was Ampere himself as a mathematical physicist, paid him the following fine tribute (25):

The experimental investigation by which Ampere established the laws of the mechanical action between electrical currents is one of the most brilliant achievements in science. The whole, theory and experiment, seems as if it had leaped, full grown and full armed from the brain of the 'Newton of electricity'. It is perfect in form, and unassailable in accuracy, and it is summed up in a formula from which all the phenomena may be deduced, and which must always remain the cardinal formula of electro-dynamics.

In September 1821, Faraday discovered how to produce electromagnetic rotations, bringing to a successful conclusion Wollaston's thought that such a motion might be possible. With the publication of this result, Faraday moved at one bound into the forefront of those actively engaged in develop- ing the new science of electromagnetism. The effect of his researches upon Ampere was described by the latter in a letter (26) to J. Bredin, 3 December 1821:

On arriving here my head was filled with metaphysics; but since Faraday's memoir has been published I dream only of electrical currents. This memoir contains some very unusual facts about electromagnetism, which perfectly confirm my theory, although the author tries to dispute it by substituting one of his own invention.

Faraday had despatched a copy of his paper to Ampere, who immediately entered into correspondence with him (27). Ampere's first letter (28) reveals his immediate reaction, which was in terms of the significance of the electromagnetic rotations to his own theory of magnetism. The obvious practical implications of the new discovery as a prime mover did not even rate a passing mention.

Paris, 23 January, 1822

The supposition of electrical currents flowing around each molecule of a magnet has always seemed to me to offer the simplest explanation, and the one most conforming to all other physical theories, of all the observed phenomena, whether they pertain to the mutual action of two magnets, or to that between a magnet and a conductor carrying current. A year ago, when I announced to the Academy of Sciences that the phenomena could be considered in this way, I looked on it as most probable. Different experiments that I have made since then have confirmed



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that opinion; but as the author of the little book that I send with this letter does not share that view, he has written it on the supposition that the currents of a magnet are concentric around its axis. I have added to that only one observation, which I think will lead to an answer to the question: the rotation of a magnet about its axis by the action of a current can decide the matter because that rotation can occur only if the electric currents of the magnet exist around each of its molecules.

Amp"re's letter also describes how he had contrived to make a magnet rotate about its axis by passing a current through it in such a way that only one of the magnetic poles was included in the electrical circuit (see Fig. 3). Faraday, in his reply (29), capped Ampere's experiment by replacing the magnet with a piece of copper similar in form to the magnet, floating it upright in mercury and passing the current through it, just as had been done with the magnet; the pole of a strong magnet was then placed beneath the cup containing the mercury; when the pole was exactly in line with the axis of the copper, the latter began to rotate slowly about its axis. Ampere too had, independently, performed a similar experiment: thus both of them had succeeded in fmding the conditions for axial rotation of a conductor carrying a current, and so finally demonstrated the very type of rotation that Wollaston and Davy had unsuccessfully sought in 1820.

N

d 7/

FIGURE 3. The rotation of a magnet on its own axis: the arrows indicate the direction of the applied electric current.



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The importance of the experiment for Ampere, however, was that it seemed to him to settle the question of the reality of the current around each molecule in a magnet. Faraday, however, saw it merely as a special case of a wire rotating around a magnetic pole: any line of particles parallel to the direction of the current, except the line that passes as an axis through the pole of the magnet, is in the same situation as such a wire, and will try to rotate round the pole; as a result of all the lines acting in the same direction round the pole, the whole magnet revolves. Nothing new, as far as Faraday could see, had been brought to light by the experiment. Ampere, however, was not shaken in his belief in his theory; for him the question was decided anyway, even without the help of this experiment, so it really was not important if the experiment after all should not be as decisive as he had hoped. He returned no answer to Faraday's observation, but he never again referred to his experimentum crucis.

Faraday was now in direct correspondence with Ampere. 'For the next ten years, there was to be a true and fertile dialogue between these two men,' says Professor Pearce Williams in his recently-published biography of Faraday. The dialogue had, however, one curious feature: although both men considered their correspondence of prime importance, so much so that Ampere treated his reply to a letter from Faraday as a major responsibility taking precedence over his other affairs of the moment, yet neither really paid close attention to the expressed ideas of the other. What they were eager to learn was the news of latest experimental discoveries. This matter they always pulled out first from each other's letters, and the absence of any such news was felt to be a proper occasion for an apology. But in response to the hypotheses that Ampere sought so earnestly to persuade Faraday to adopt, he received finally only weary and uncomprehending expressions of courteous incredulity; and Faraday's real objections, when on a rare occasion he was able to define them articulately, were blandly ignored by Ampere. Each spoke to the other seriously and intently: each listened to the other with preoccupied indifference. Highly original and creative minds are often destined to this kind of lop-sided dialogue, in which each pursues a separate course of thought, determined by his particular training and experience, and is aware of the other only so much as to feel a mild wonder or irritation at seeing him take a divergent path. The ultimate synthesis of the ideas of Ampere and Faraday was still in the womb of the future (30).

On ioJuly 1822, Amphre wrote a ten-page letter to Faraday (3 I) in which he explained a number of experimental facts in terms of his hypothesis of magnetism. Faraday in his reply (32) of 3 September 1822, did not discuss the



203 scientific reasoning at all; he stated that he understood nothing of mathematics (Amp "re's letter, however, did not employ mathematics) and so felt that he was not capable of following Ampire 'into the domain of abstractions'; in order to base a judgment about Amp"re's conclusions, he asked for facts and still more facts:

I am unfortunate in a want of mathematical knowledge, and the power of entering with facility into abstract reasoning. I am obliged to feel my way by facts closely placed together, so that it often happens I am left behind in the progress of a branch of science not merely from the want of attention but from the incapability I lay under of following it, not- withstanding all my exertions. It is just now so, I am ashamed to say, with your refined researches in electro-magnetism or electrodynamics. On reading your papers and letters, I have no difficulty in following the reasoning, but still at last I seem to want something more on which to steady the conclusions. I fancy the habit I got into of attending too closely to experiment has somewhat fettered my power of reasoning and chains me down and I cannot help now and then, comparing myself to a timid ignorant navigator who, though he might boldly and safely steer across a bay or an ocean by the aid of a compass which in its action and principles is infallible, is afraid to leave sight of the shore because he understands not the power of the instrument that is to guide him.

Later in his career Faraday was to acquire more confidence and a high degree of skill in the use of his power of reasoning, particularly with the fruitful concept of lines of force, though still without the aid of mathematics.

By this time Faraday was investigating electro-magnetism on his own initiative: Davy had turned his attention to other matters-the liquefaction of gases, the corrosion of copper sheathing by sea water, the relation of electrical to chemical changes, etc. Faraday's duties at the Royal Institution, and other assumed obligations, restricted his freedom to pursue subjects of investigation that intrigued him, but he never entirely lost sight of the grand objective, to obtain electricity from magnetism. Nearly two years passed before he took up the subject again; then, on 28 December 1824, he tried an experiment that is interesting retrospectively as an indication of the direction taken by his thinking, and how close it had brought him to the experimental arrangement that ultimately proved successful. The experiment was actually a failure, another stumble in the darkness, but so high was the interest at that time in the Arago rotations, that even a negative result was significant.



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Faraday made his experiment the subject of a note, which he published under 'Miscellaneous Intelligence' in the Royal Institution's Journal, of which he was sometimes the editor. It is given here in its entirety (33):

Electro-Magnetic Current-As the current of electricity, produced by a voltaic battery when passing through a metallic conductor, powerfully affects a magnet, tending to make its poles pass round the wire, and in this way moving considerable masses of matter, it was supposed that a reaction would be exerted upon the electric current capable of producing some visible effect; and the expectation being, for various reasons, that the approximation of a pole of a powerful magnet would diminish the current of electricity, the following experiment was made. The poles of a battery of from 2 to 30 4-inch plates were connected by a metallic wire formed in one part into a helix with numerous convolutions, whilst into the circuit, at another part, was introduced a delicate galvanometer. The magnet was then put, in various positions, and to different extents, into the helix, and the needle of the galvanometer noticed: no effect, however, upon it could be observed. The circuit was made very long, short, of wires of different metals and different diameters down to extreme fineness, but the results were always the same. Magnets more or less powerful were used, some so strong as to bend the wire in its endeavours to pass round it. Hence it appears, that however powerful the action of an electric current may be upon a magnet, the latter has no

tendency, by reaction, to diminish or increase the intensity of the former-a fact which, though of a negative kind, appears to me to be of some importance.-M.F.

The magnet would not have had any effect as long as it was not in motion; and even when moved about, the feeble currents induced were probably masked by the current already flowing in the circuit.

In performing experiments of this sort Faraday had disregarded advice coming to him directly from Ampere, who, in a letter of 27 April 1824, had tried to persuade him that the action between a current and a magnet was not likely to be a productive subject for research. 'You would thereby bring together two heterogeneous things,' wrote Amp ere, 'whereas the fundamental action should of necessity be between two entities of the same nature, such as two elements of current. This is the underlying fact on which all other phenomena of this sort depend' (34). Amp re's advice to Faraday was that he repeat for himself the celebrated experiment of the mutual action of two elements of current, from which Ampere had deduced the mathematical



205 laws for the mechanical forces between them. Starting with those formulae, Faraday was told, he would be able to deduce quantitatively the action of an electric current on a magnetic pole, and also that of one magnetic pole on another. Although Ampere could not have known it at the time, by applying the principle of the conservation of energy to his formulae, they would have successfully predicted the very results of electromagnetic induction that Faraday was ultimately to discover experimentally (35). But the principle of the conservation of energy had not yet been enunciated. Ampere's advice, therefore, while perfectly sound sub specie aeternitatis, was, as a practical course of action, rather like suggesting that Faraday read a book not yet written. In 1825, to have followed that advice would have been merely to retrace the course Ampere had already taken; it would not have led to any new discovery. Faraday, of course, ignored the suggestion anyhow, as it would have required the use of more mathematical learning than he possessed. The incident shows Ampere's predilection for mathematical deduction contrasted with Faraday's unvarying attitude to try-it-and-see: it also exempli- fies a wise general rule in research, a rule once stated by Watson as follows (36):

We have to learn physics a little at a time, and there is no good purpose served by refusing to give a hypothesis a fair trial merely because one feels that it does not fit easily into our present scheme of things; one may be right asjudged from the point of view of the distant future, but wrong in one's judgment as to the way in which the goal is to be reached.

Soon after this, Ampire sent Faraday a copy of Demonferrand's book, with the earnest recommendation that he familiarize himself with the experiments described in it. Faraday's letter of acknowledgment (hitherto unpublished) gives an interesting account of his working conditions (37):

November 17, I825 Every letter you write me states how busily you are engaged and

I cannot wish it otherwise knowing how well your time is spent. Much of mine is unfortunately occupied in very commonplace employment and this I may offer as an excuse (for want of a better) for the little I do in original research.

I am sorry to find by one of your letters that you experience an unworthy opposition to the fair and high claim you have to the approba- tion and thanks of your fellow philosophers. This however you can hardly wonder at. I do not know what it is or by whom exerted in your case but I never yet even in my short time knew a man to do



2o6

anything eminent or become worthy of distinction without becoming at the same time obnoxious to the cavils and rude encounters of envious men. Little as I have done I have experienced it and that too where I least expected it.

I think however and hope that you are somewhat mistaken in your opinion of the feeling here. It is true that some of your views were at first received here with great reserve but I think that now all your facts are admitted and are all properly attributed to you. With regard to your theory it so soon becomes mathematical that it quickly becomes beyond my reach. At the same time I know that it has received the consideration of eminent men here. I am not however competent to tell you exactly how it is accepted, for in fact being a very busy man and somewhat retired in habits I am all day long in my Laboratory, do not go much among scientific men, and am in some sort an anchorite in the Scientific world. Hence I have neither time nor opportunity for scientific conversation and am frequently surprised at information which is new to me and old to every one else.

Be assured however that whenever the opportunity occurs I do full justice to your important investigations, for as far as I can go with them I am convinced of their accuracy and great value.

Many thanks to you for M. de Monferrand's book. I had it only a day or two ago, and though I have not yet read it, have looked over the table of contents and agree with you in its accuracy.

Soon after writing this letter, Faraday returned to his experiments on electricity. He had failed to find any action of a magnet in affecting an electric current: perhaps, then, one current could produce another, as Ampere and De la Rive had reported. The experiment that he tried this time was similar to that tried originally by Fresnel in 1820, except that Faraday used a galvanometer to detect the possible current in the secondary circuit where Fresnel had tried to detect it by its electrolytic action. The entry in Faraday's laboratory notebook for 28 November 1825, reads as follows (38):

Experiments on induction by connecting wire of voltaic battery. A battery of 4 troughs, ten pair of plates each, arranged side by side. Expt. L The poles connected by a wire about 4 feet long, parallel to which was another similar wire separated from it only by 2 thicknesses of paper. The ends of the latter wire attached to a galvanometer exhibited no action. Expt. II. The battery poles connected by a silked helix-a straight



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wire passed through it and its ends connected with the galvanometer- no effect. Expt. III. The battery poles connected by a straight wire over which was a helix, its ends being connected with the galvanometer-no effect.

Could not in any way render any induction evident from the connecting wire.

The experiment was no more successful in his hands than it had been with Fresnel, and for the same reason-both researchers had been looking for a sustained effect and had not noticed the feeble indications of the induced current when the battery was connected or disconnected. Again, 2 December 1825 and 22 April 1828, Faraday made experiments on electromagnetism that gave 'no result'. These experiments were not published.

Another unpublished experiment, that of 26 February 1828, is of historical interest although not directly related to electromagnetic induction. On that occasion Faraday set out to discover if a bar of steel changes its length on being magnetized. He wrote in his notebook (39):

Conceived that when a bar of soft steel was converted into a magnet, the particles if they become each magnetic independent of the others ought to exert such power of attraction upon each other as to influence the density of the bar, for upon the received theory their attraction ought to be super added to the attraction of aggregation in the direction of the axis of the magnet. To try whether any sensible effect is really produced, a soft steel bar II or 12 inches long was put into a wedgewood pyrometer indicating the expansion or contraction by levers which multiplied the effect 400 times; but on holding powerful magnets to it, or converting the bar into a magnet, no change of length either one way or the other could be observed.

We now know that the effect Faraday was looking for is a real one; it is called magnetostriction. The change of length amounts to only a few parts per million, however, which was beyond Faraday's power to detect, though it can be measured by modern techniques with fair accuracy. The iron core of a transformer, for example, contracts and expands in unison with the alterna- tions of the electric field, which is one of the reasons why large power transformers emit an audible hum. This noise was once the cause of serious concern to the Westinghouse Company. The disagreeable noise of the transformers under the pavement on Park Avenue (New York) brought vehement com- plaints from tenants of the large apartment houses, and had to be reduced by the research efforts of the Company's scientists and engineers (40). It seems



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sometimes to the electrical engineer that whatever problem he looks at, he finds Faraday's name connected with it somewhere.

Investigating possible changes of length on magnetizing a steel bar did not divert Faraday from the main objective of his investigations on electromagnetism. As we have seen, Demonferrand's book was now in his possession and he was aware of the description therein of the Amp ere-De la Rive experiment. He repeated it for himself, but by a strange mischance he had mistrans- lated or misunderstood that Ampere had used a circle or ring of copper for the movable conductor; for this, Faraday substituted a solid copper disc. The weak effect produced would be further weakened by this substitution, and in fact Faraday did not detect any effect when he made the experiment. After that experience he wrote off the Ampere-De la Rive experiment as another erroneous report, of which, it will be remembered, there had already been one or two instances.

Were Ampere and Faraday to have had the opportunity of a personal meeting at that time they would soon have cleared up for one another their points of misunderstanding. Ampere no doubt would have heard with impatience of Faraday's efforts to produce an electric current by induction, for he believed he had settled that question long ago, and after several changes of mind on the matter had ended up with the opinion that it was not possible. Nevertheless, he was still demonstrating the Ampere-De la Rive experiment occasionally to his class in experimental general physics at the Coll ge de France. A glimpse of that demonstration would have given Faraday food for thought, because hitherto he had never witnessed even the slightest effect that might have originated from the presence of an induced current. This was the clue that he longed to obtain, and which was not to come his way until he found it for himself years later.

3. EXPLANATIONS AND DISCLOSURES EX POST FACTO

One cannot but admire Faraday's persistence in the face of all the negative results he had accumulated. But he was not ready to give up until he had checked one more possibility-perhaps the induced current he was looking for was so tiny that it could be detected only by magnifying the powers of the agents that elicited it. By leading the current around a helix of many turns and by placing a soft-iron core inside it, the magnetic lines of force could be greatly concentrated; suppose those lines of force were brought around a circle on the other side of which was wound a second helix of many turns: the lines of force would then be made to occupy the same position with respect to the inactive circuit as they already had with respect to the active circuit. Because



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of the multiplication of the lines of force produced by many turns of the helix and the concentration of them produced by the iron core, the greater power of this arrangement might call forth an observable reaction in the secondary circuit. He had a soft-iron ring made (- inch thick and 6 inches in external diameter), and wound many coils of copper wire around each half. On 29 August 1831, all was ready to begin the experiment.

One circuit was completed by connecting the ends of the coil with a copper wire that extended over a pivoted magnetic needle placed far enough away from the coil so that it would not be affected by the magnetic field emanating from the primary circuit: this needle was to be the galvanometer. The other circuit was completed by connecting the extremities of the coil to a powerful battery of io pairs of plates, each 4 inches square. As soon as this connexion was made, Faraday observed the oscillation of the needle. On breaking the connexion the needle oscillated in the opposite direction. For the moment suppressing his jubilance, Faraday wrote in his notebook (41): 'Hence effect evident but transient; but its recurrence on breaking the connection shews an equilibrium somewhere that must be capable of being rendered more distinct.' At last he had found the clue he needed to solve the mystery.

The rest of the story is too well known to need repeating here (42). True to his own adage of 'Work, Finish, Publish', Faraday read an account of his experiments to the Royal Society on 24 November 183 1, and put it into print in Part I of the 1832 volume of the Philosophical Transactions. Four months elapsed between the reading of the paper and its publication; during that time inaccurate and incomplete reports of his discovery were circulated as far as France, Italy, and the United States. Much to his subsequent regret, Faraday had sent a preliminary account of his findings to M. Hachette. His letter was translated (with some errors) and read to the Academy of Sciences at Paris on 26 December 1831. A copy of it in Le Temps of 28 Decem- ber soon reached Italy, where Nobili and Antinori immediately began to experiment on the subject, and obtained many of the results mentioned in Faraday's letter; other results they could neither obtain nor understand, because of the inadequacy of their information. Meanwhile in Paris, Ampere and Becquerel were tidying up a few loose ends in the light of Faraday's results: Ampere decided to put into print a more detailed version of the Geneva experiment (43). Of this account, Silvanus Thompson remarked (44): 'It is curious to note the change of view.' If at any time Ampere had doubted the existence of an induced current, nobody would have now suspected it from a perusal of this article:



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During my visit to Geneva in September, 1822, M. August de la Rive was eager to assist me with some experiments I wished to make bearing on the production of an electric current by the influence of another current... We presented a powerful horse-shoe magnet to the circle, so that one of its poles was within and the other outside the circle. When the extremities of the [primary] conductor were connected to the pile, the circle was attracted or repelled by the magnet, according to which pole had been placed inside the circle. This demonstrated the existence of an electric current produced there by the influence of the current in the [primary] conductor.

As was pointed out by Silvanus Thompson, this account of the experiment differs from Amp"re's former version in describing the position of the magnet, and in saying that the magnet (which in the original experiment was brought up after the current had been turned on) was placed in position before the circuit was completed.

Amphre next went on to report a new series of experiments performed by himself, by means of which he had confirmed Faraday's discovery. He claimed that the new phenomena could be foreseen (presumably by hind- sight) from his own general laws of electrodynamic action. He suggested no procedure, however, by which this could be done; but concluded with the rather patronizing remark that, although his laws actually required no further proof, yet 'physicists would not see with any less pleasure this new verification of a theory that traced all magnetic phenomena to the production of electricity in motion'.

The excited discussions that soon became rife impelled Faraday to add a footnote to his published paper to defend his priority (45):

The Lyc6e, No. 36, for January Ist, [1832] has a long and rather premature article, in which it endeavours to show anticipations by French philosophers of my researches. It however mistakes the erroneous results of MM. Fresnel and Ampere for true ones, and then imagines my true results are like those erroneous ones ...

Amp"re's knowledge of English was slight, and he read Faraday's paper, including this note, only when it appeared in a French translation in April 1833. He was puzzled by the inaccuracies of Faraday's account of the Geneva experiment and he was pained by the expression 'erroneous results of MM. Fresnel and Amptre', which seemed to apply to that experiment and not to the much earlier one already retracted in 1820. It is indeed quite clear from



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the text that Faraday considered both experiments equally erroneous. In some distress of mind, Ampire wrote first to his friend De la Rive, reminding him of the experiment they had carried out jointly in I822 and stating that he intended to write to Faraday to inform him of their claim to have been the first to obtain the induced current. Amp"re wrote (46):

It is a fact that we were the first, in 1822, to obtain an electric current by influence, or induction as M. Faraday says, at the moment when we established the current within a spiral that surrounded a circle made of a thin sheet bent in this way [see Fig. 4] and suspended by a silk thread GH from a bracket K; that the effect made itself manifest by the attraction or repulsion exerted by a strong horse-shoe magnet that we had borrowed from M. Pictet, according to which pole was in the interior of the circle at B and which was outside at D. Unfortunately neither you nor I thought to analyse this phenomenon and to explore all its circumstances. We would have seen, what M. Faraday has since discovered, that the current lasts only for an instant and that it runs in the contrary direction to the current flowing in the spiral circuit, which produced it by induction.

H K

G

D

B

A

FIGURE 4. Ampere's diagram of the apparatus used in the Geneva experiment.



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In a later letter to De la Rive, Ampere expanded further on his original interpretation of the Geneva experiment (47):

Faraday has certainly made one of the most beautiful discoveries of all the electro-magnetic phenomena; but he is not the author of the very fact of the production of a current by induction, since we obtained this current in 1822...

The thin foil bent into a circle is either drawn towards or carried away from the poles of the horse-shoe magnet, to remain almost in the same position that it first assumed, as long as the exciting current continues to flow in the spiral circuit; precisely because, the first action being only momentary, there is no other while the current continues. Then, when it is stopped, the circle of foil returns to its original position, because a current in the opposite direction has been created in it. It was this return, which I attributed to the torsional force of the wire, that made me think of the persistence of the first action (as long as the current lasted) making an equilibrium with a supposed torsional force that did not really exist. As for the direction of the currents, whether the same or contrary, I had never in fact made the necessary experiments to determine it. But it is a fact that, in the three or four places in my memoirs or books in which I had spoken of it, I always avoided declaring its direction, because I always proposed to undertake a complete work on the induced currents, which I never did.

This explanation of the experiment first performed successfully in Geneva is much more explicit than any that had been published. The discrepancies between the various details of the experiment are no more than might arise from slight alterations that could creep in during the many times the experiment was repeated. The explanation of the supposed torsional equilibrium seems, however, as if it were a rationalization conceived after the whole truth had been revealed by Faraday's experiments; though, it will be recalled, Demonferrand had vaguely mentioned an equilibrium as early as 1823. Faraday can hardly be blamed for misunderstanding an experiment so variously and so ineptly reported.

Of much greater interest than any of Ampere's belated attempts to give a more logical form to his account of this experiment, is the question of why he did not appreciate the significance of his observation at the time when it was first made. In his long letter of explanation to Faraday, he revealed the basic reasons for his failure (48):



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At that time I had but one aim in making these experiments. I was searching exclusively (as you will recognize on looking at what I have published at that period, when I described the apparatus which I used) to resolve this question: Do electric currents, which are the cause of magnetic attractions and repulsions, pre-exist, before magnetization, around molecules of iron, or steel, or the two other metals where magnetic effects are observed; but exist in such a position that they cannot exercise any action beyond. Or are the currents produced, at the moment of magnetizing, by the influence of neighbouring currents?

When, in my first experiments of July 1821, I obtained no current of this sort, I reasoned (Annales de Chimie et de Physique, vol. 18, p. 377, and Recueil d'observations ilectro-dynamiques, p. 165) that, since a current was not able to produce another one by influence, then, necessarily, magnetization takes place because the current, or the bar magnet that does the magnetizing, only acts upon pre-existing currents in the iron or steel. But, when the experiment that I made in Geneva in 1822 with M. Auguste de la Rive obliged me to retract and admit the production of currents by influence, I thought that the great question of the pre- existence of molecular currents in metals able to be magnetized, was not to be answered in this manner and that it must remain undecided until it could be resolved by other methods; and I placed no further importance on these experiments, which I erred in not having studied more deeply.

On receiving a long explanatory letter from Ampere, Faraday inserted at the end of his Third Series (49) an apology and explanation of his original remarks-an apology that mollified Amp re's feelings though it could not assuage his regret at the opportunity he had missed.

Had Ampere been impelled to follow up the phenomenon he had discovered, one cannot doubt that his genius would soon have led him to the truth of the effect. He did, indeed, plan to take it up at some indefinite future date, but the very fact that he was willing thus to postpone the investigation reveals its essential lack of importance in his eyes. He saw the effect only as it related to his theory of magnetism, and because it did not promise to throw light on that subject he dismissed it as of entirely secondary interest. Thus his intense preoccupation with his own line of thought was responsible for his neglect of a clue that could have brought him to one of the greatest scientific discoveries of the century.

Faraday, had he delayed much longer in starting his experiments, would



214

have missed the opportunity himself. Joseph Henry, working in Albany, New York, had already obtained and recognized the momentary nature of the induced current at some date prior to May 1832, before he had any knowledge of Faraday's results. His experiments had been temporarily interrupted, when he read a brief announcement in the Annals of Philosophy of Faraday's work. The notice lacked particulars; apparently Henry thought that no more were to be given, and he immediately published an account of his own experiments in the forthcoming issue of Silliman's American Journal of Science (that for July I832) (50).

It early occurred to me, that if galvanic magnets, [i.e. electromagnets] on my plan, were substituted for ordinary magnets, in researches of this kind, more success might be expected ... With this view, I commenced, last August, the construction of a much larger galvanic magnet than, to my knowledge, had before been attempted, and also made preparations for a series of experiments with it on a large scale, in reference to the production of electricity from magnetism. I was, however, at that time, accidentally interrupted in the prosecution of these experiments, and have not been able since to resume them, until within the last few weeks, and then on a much smaller scale than was at first intended. In the mean time, it has been announced in the I 7th number of the Library of Useful Knowledge, that the result so much sought after has at length been found by Mr. Faraday of the Royal Institution ...

Henry wound 30 feet of insulated copper wire around the middle of the soft-iron keeper (or armature) of his electromagnet-a magnet powerful enough to sustain a weight of more than six hundred pounds. The wire was wound upon itself so as to occupy only about one inch of the length of the keeper, which was seven inches in all. The projecting ends of the coil were connected to a galvanometer, placed at a forty-foot distance from the electromagnet. Henry stationed himself at the galvanometer and directed an assistant to activate the electromagnet. At the instant when the circuit was completed, the galvanometer needle was deflected 30' to the west, indicating that a pulse of electrical current had passed through the coil on the keeper; on de- activating the electromagnet the needle was again deflected from a state of rest, this time 200 to the east, or in a contrary direction from the first effect. So far, Henry had essentially done no more than confirm Faraday's results: his electromagnet and wound keeper were identical in principle with Faraday's iron ring with its active and secondary circuits. Before the end of his paper, however, he reported and correctly interpreted an effect of self-



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induction-the production of sparks when a long helical conductor is disconnected from a battery. 'I can account for these phaenomena', Henry concluded, 'only by supposing the long wire to become charged with electricity which by its reaction on itself projects a spark when the connection is broken.' Henry's observations on self-induction, made in the Spring of 1832, anticipated those of Faraday (13 November 1834) by more than two years: Faraday's later work, however, was considerably more detailed and penetrated farther into the nature of self-induction.

Supporters of Henry's claims are often moved by national loyalties to praise him extravagantly at the expense of Faraday. Henry's most recent biographer, for example, states that Henry's immediate recognition of the momentary nature of the induced current, when compared with Faraday's long search for a continued effect, argues a better understanding of Oersted's experiment on his hero's part (5I). If it were so, Henry would also stand intellectually ahead of Fresnel, Davy, and Ampere. But the lack of documenta- tion does not give licence to an unbridled use of the imagination. We cannot tell what difficulties Henry may have had in thinking about induced currents: we know of Amp"re's and Faraday's difficulties because they were recorded; there is no reason to believe that Henry had not met the same difficulties. Henry's recognition of the momentary character of the induced current was made easy by its unmistakable effect, which was the merit of his

powerful electromagnet rather than his superior intelligence. Faraday also, once the equipment was powerful enough to make the effect manifest, recognized its momentary character without hesitation. One hopes that this

subject will not continue to be clouded by partisanship.

NOTES

To avoid undue repetition the following abbreviations are used: Coll. for Collection de me'moires relatifs a la physique, publies par la Societe'francaise de Physique,

Paris, 1885-1887. Volumes 2 and 3 of this series refer to electromagnetism and electro-

dynamics, and include most of Ampere's papers on those subjects. Corr. for Correspondance du grand Ampere, Paris, 1936-1943. (i) A. Fresnel, Annales de chimie et de physique, [2], 15, 219-222 (1820). Reprinted in Coll., 2,

76-79 (2) Others had tried to repeat Fresnel's observation and found it to be erroneous. Gilbert, in

Gilbert's Annalen der Physik, 66, 4 10(1820), after giving a summary of Fresnel's reputed discovery, stated that he had repeated the experiment with no result. The same conclusion was reached by Pohl, Oken's Isis, pt. IV, p. 407 (1822): also see remarks signed P at the end of an article by Savary in Poggendorff's Annalen der Physik und Chemie, 8, 368 (1826).

7



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(3) A. M. Ampere, Journal de physique, 93, 447 (1821): reprinted in Coll., 2, 212. (4) A. de laRive, Bibliotheque universelle,21, 29 (I822); and Annales de chimie et de physique, [2],

21, 24-48 (1822): reprinted in Coll., 2, 328. (5) Published for the first time in Coll., 2, 329-3. (6) J.-F. Demonferrand, Manuel d'clectricite dynamique, Paris 1823, pp. 173-174- (7) Corr. No. 490, pp. 773-775. Amphre to Auguste de la Rive, 8 November 1833. (8) These statements from the Lycde are quoted by Faraday, Experimental researches in electricity,

London, 1839, I, pp. 107-109. (9) Corr. No. 485, pp. 763-770. Ampere to Faraday, 13 April 1833.

(IO) A manual of electro-dynamics, chiefly translated from the Manuel d'e'lectricit6 dynamique ofJ. F. Demonferrand, by James Cumming, Cambridge 1827.

(ii) Quarterly Review, No. LXIX,January 1827, pp. 237-269. (12) Ampere's certificate of candidature reads: We recommend M. Ampere, member of the

Royal Academy of Sciences of Paris, a distinguished Mathematician, and author of various works on the Mathematical theory of electro-Magnetism, as a proper person to be a Foreign Member of the Royal Society. H. Davy, Pres. R.S.; Everard Home; John Geo. Children; J. F. W. Herschel; J. South; Wm. H. Wollaston; Wm. Prout; George Pearson; Francis Lunn; Charles Babbage; A. B. Granville.' Ballotted for and elected March 8, 1827.

(13) J. D. Colladon, Recherches et experiences sur l'ilectricite. Eight memoirs published I825-I837, reprinted in Geneva, 1893. The fourth of those memoirs is entitled Experiment on electro-magnetic induction made in 1825.

(14) F. Arago, Annales de chimie et de physique, [2], 27, 263 (1824); ibid., [2], 28, 325 (1825). (15) C. Babbage &J. F. W. Herschel, Phil. Trans. (1825), pp. 467-496. (16) 'Toujours les mots d'aimantation passagere, de magnetisme de rotation, s'interposaient

comme un voile entre l'esprit des physiciens et la rdalitd.' E. Bauer, L'ilectromagnetisme hier et aujourd'hui, Paris, 1949, p. 93.

(17) M. Faraday, Experimental Researches in Chemistry and Physics, London, 1859, pp. 483-484. (18) F. Arago, Oeuvres de Frangois Arago, 2nd edition, Paris, 1865. vol. I, pp. 439-440. Ampere's

Note was published in the Bulletin de la Socidtd philomathique (1826), p. 134: translated into English in the Quarterly Journal of Science, Literature, and the Arts, N.S. vol. I, pp. 228-229 (1827).

(19) A. C. Becquerel, Annales de chimie et de physique, [2], 25, 269-278 (1824). (20) Now in the possession of the author. (21) For membership of the Royal Society: Amphre was elected six years later [see note (12)

above]. (22) S. P. Thompson, Michael Faraday, his life and work, London, 1898, p. 89. (23) H. Davy, Phil. Trans. (1822), p. 64; Collected Works, 6, 245 (1840). (24) Bence Jones, The life and letters of Faraday, London, 1870. Vol. I, 315-317. L. Pearce

Williams, Michael Faraday, London, 1965, p. 166. (25) J. Clerk Maxwell, A treatise on electricity and magnetism, Oxford, 1873. Vol., I, p. 162. (26) Corr., No. 359, pp. 576-577. Ampere to Bredin, 3 December 1821. (27) Faraday first met Ampire in Paris in 1813 when he accompanied Sir Humphry and Lady

Davy on a tour of the Continent. They did not correspond, however, until after Faraday's publication of his discovery of the electromagnetic rotations. In Ampere's Correspondance are printed 14 letters from Ampere to Faraday and 3 of Faraday's replies; in this paper I have added a 15th letter to the former and a 4th to the latter series.



217

(28) The original is in the Library of the Institute of Electrical Engineers, London, by whose courtesy I am permitted to publish it here. The complete text is given below.

Paris, 23 Janvier, 1822 Monsieur,

Depuis que j'ai reCu le mimoire dont vous avez eu la bontd de me faire passer un exemplaire par l'entremise de mon ami, M. Hachette, je me suis propos6 de vous ecrire pour vous remercier d'une communication qui m'a 6t6 si prdcieuse par les faits nouveaux contenus dans ce memoire, et la maniere dont vous les deduisez tous d'un d'entre eux. Sij'ai tard6 si longtems

' executer ce projet c'est qu'il m'a d'abord fallu avoir ce memoire traduit en franqais, puisqueje n'entends pas malheureusement votre langue, ensuitej'ai voulu repeter ces experiences, et y ajouter quelque chose s'il m'6tait possible. Mes tentatives m'ont amen6 " un moyen tres simple de produire le mouve- ment continu du conducteur voltalque avec un aimant, sans mercure et sans autre action voltalque que celle d'une lame de cuivre circulaire attachee a ce conducteur et plongeant dans l'eau acidulde que renfermait un vase en zinc, communiquant avec la pointe d'acier sur laquelle tournait le conducteur. J'ai obtenu le meme effet remplaqant l'aimant par une lame de cuivre plid en spirale autour duvase, maisilfaut alors employer l'electricite d'une forte pile. Quelque tems aprbsje rdussis a faire tourner un conducteur pareil mais plus grand par l'action de la terre. Enfinj'ai vu qu'un aimant tourne rapide- ment sur son axe quand flottant sur du mercure dans une situation verticale, au moyen d'un contrepoids de platine attache ' son extremit6 inf6rieure. On pratique

' son extr6mit6 superieure une petite cavit6 o0% l'on met un peu de mercure, et que l'on plonge dans ce mercure le fil communiquant a l'une des extr6mitis de la pile, tandis que l'autre est en communication avec la masse de mercure sur laquelle flotte l'aimant. Mais ce qui a le plus retard6 ma lettre c'est que je voulais y joindre un expos6 des faits connus sur le sujet dont nous nous occupons a% l'poque oui il a 6t6 redig6 par unjeune professeur de physique

' qui cette science devra probablement quelques progres, et

off j'ai ajoutd plusieurs articles, pour montrer comment ces faits rentraient dans ma maniere de les expliquer. Quoique cet expose ait 6td fait il y a six mois, il ne parait point encore parcequ'il fait partie d'un ouvrage plus considdrable qui n'est pas entiere- ment achev6, en sorte queje n'ai pu m'en procurer un exemplaire que depuis quelques jours. M. Hachette m'a depuis deux jours communiqud votre second m6moire que je fais traduire.

Il me semble toujours que la supposition des courans 6lectriques dans les aimans, autour de chacune de leurs mol6cules, offre l'explication la plus simple et la plus conforme au reste des thdories physiques, de tous les ph6nomines qu'ils pr6sentent, soit dans leur action mutuelle, soit dans celle qui a lieu entre un aimant et un fil conducteur. Depuis un an que j'annonqais a l'Academie des Sciences qu'on pouvait les considdrer de cette derniere mani*re, c'est celle que j'ai regardee comme la plus probable. Diff6rentes expdriences que j'ai faites depuis m'ont confirm6 dans cette opinion, mais comme l'auteur du petit ouvrage quejejoins

" cette lettre ne la partageait pas, il a 6td 6crit en supposantles courans des aimans concentriques autour de leurs axes. J'y ai seulement ajout6 une observation 'i ce sujet, ofi je crois avoir precis6 cette question: la rotation d'un aimant autour de son axe, par l'action d'un fil conducteur, me parait propre at la decider, cette rotation ne pouvant avoir lieu que quand on admet que les courans electriques de l'aimant existent autour de chacune de leurs molecules. Mais pour en tirer cette consequence il faudrait que je me fusse assurd que la rotation de l'aimant sur luimeme que j'ai observee dans ce cas ne venait pas de la



218

r6action du fil qui plongeait dans la cavit6 practiquie au sommet de l'aimant, contre les bords de cette cavitd. C'est ce que je me propose de vtrifier en playant dans une coupe d'un diametre sufissant, le mercure que j'y mettais pour etablir la communication avec une des extr6mitis de la pile.

Je vous prie, Monsieur, d'agreer mes excuses sur le retard des remercimens que je vous devais. Je serais trop heureux que vous voulussiez bien accueillir l'hommage de ma reconnaissance et de ma sincere admiration pour vos belles decouvertes dans les deux sciences dontj'aimerais le plus ai m'occuper si mes travaux ordinaires me permet- taient de m'y livrer exclusivement, la physique et la chimie.

J'ai l'honneur d'etre, Monsieur, votre tres humble et tres obeissant serviteur, A. Ampere

A Monsieur Faraday "i Londres

(29) Corr., No. 352 bis, pp. 908-913. Faraday to Ampere, 2 February [1822]. This letter was misdated 1821 when it was just published in the Trans. Newcomen Soc., 1922-1923, 3, 119-121 and the erroneous date has been copied in Ampere's Correspondance (supra) and in A. E. Jeffrey's Michael Faraday, a list of his lectures and published writings, London, I960, No. 78.

(30) A somewhat speculative reconstruction of the development of Faraday's ideas on electromagnetism during this decade is given by L. Pearce Williams, Michael Faraday, London, 1965, pp. 169-183. Professor Pearce Williams credits Faraday with so much logical coherence in his prior thinking that the actual discovery almost appears as the last step in a process of deduction. No doubt all the necessary elements were in existence for such a deduction, but the degree to which they were present in Faraday's mind, either consciously or subconsciously, is problematical.

(31) Corr., No. 365, pp. 586-592 Ampere to Faraday, o July 1822. (32) Corr., No. 369 bis, pp. 928-931 Faraday to Ampere, 3 September 1822.

(33) M.F., Quarterly Journal of Science, Literature, and the Arts, 19, 338 (1825). (34) Corr., No. 390, pp. 652-653. Ampere to Faraday, 27 April 1824. (35) Nobody made the prediction, however, or was likely to make it, prior to the experimental

discovery. Helmholtz and W. Thomson (Lord Kelvin) showed that Faraday's discovery of the induction of electric currents could be deduced mathematically from Amp re's laws of electrodynamics as a consequence of the principle of conservation of energy: see E. T. Whittaker, History of the theories of aether and electricity, London, 1910, pp. 243 et seq. Critical reconsideration of these proofs has thrown doubt on their generality: see E. S. Shire, Classical electricity and magnetism, Cambridge, I960, pp. 168-171.

(36) W. H. Watson, On Understanding Physics, Cambridge, 1938, p. 54. (37) Original in the Burndy Library, Norwalk, Conn., U.S.A. Published here by courtesy

of Mr Bern Dibner, Director of the Library. (38) Faraday's Diary, London, 1932. Vol. I, p. 279. (39) Faraday's Diary, London, 1932, Vol. I, pp. 309-3IO. (40) F. Bitter, Magnets: the education of a physicist. New York, 1959, pp. 93-94. (41) Faraday's Diary, London, 1932, Vol. I, p. 368. (42) For a detailed account of the history of Faraday's work on electromagnetic induction see

T. Martin, Faraday's discovery of electro-magnetic induction, London, 1949, 16o pp. (43) A. M. Ampere, 'Experiences sur les courans electriques produits par l'influence d'un

autre courant,' Annales de chimie et de physique, [2], 48, 405-412 (1831).



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(44) S. P. Thompson, 'Note on a neglected experiment of Ampere,' Phil. Mag. [5], 39, 534-541 (1895).

(45) M. Faraday, Phil. Trans. (1832), P. 146f; reprinted in Experimental Researches in Electricity, London, 1839, 579f.

(46) Corr., No. 484, pp. 760-763, Ampere to A. de la Rive, [April 1833]. (47) Corr., No. 490, pp. 773-775, Ampere to A. de la Rive, 8 November 1833. (48) Corr., No. 485, pp. 763-770, Ampere to Faraday, 13 April 1833. (49) M. Faraday, Phil. Trans. (1833), pp. 53-54; reprinted in Experimental Researches in

Electricity, London, 1839, pp. 107-1o9. (50) Joseph Henry, 'On the production of currents and sparks of electricity from magnetism,'

American Journal of Science and Arts, 22, 403-408 (1832). (51) T. Coulson, Joseph Henry: his life and work, Princeton, 1950, p. 84.



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The Search for Electromagnetic Induction: 1820-1831

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