Faraday as a Discoverer
John Tyndall
447745 fc k "» ^ . '■ « « » ^ • . " * •> • •■ V « « « t «• < • « « • • •» • • • • ^ . . . _ / /-/^ <■ I - *■ w <- ' .-? .' t " '' ■ •/•,/* / « V ^{l n T( ! . .^ ^' > ' * - * / 1 / • • / . t *" i < ^ , U,U- '< ' <''s ^ / /' / / -</ •:.__ -'-■''■ J, f. "^'S /.i- NOTE. <, A. /-27 Some tears ago I accompanied Mr, Faraday to a little Photographic Studio in Lambeth, with the view of exchanging portraits. The Frontispiece is engraved from one of the negatives taken on that occasion, and which is now in the possession, of Dr. Bence Jones. The portrait facing p. 79 is from a Daguen^eotype by Claudet, the property of Mrs. Faraday, taken when her husband was about fifty years old. Its position in the book has been chosen with reference to his age. The present memoir embraces two discourses on Fara- day, delivered at the Royal Institution of Great Britain on January 17th and 24th, 1868. John Tyndall. RoYAii iNSTiTunoN, ^st February ^ 1868, CI" r- ■ 1 / /. - * ■ ^ ^^^t'*^ X . .1 '. y . } CONTENTS. PAGB PABENTAGE — ^ESTTRODTTCTION TO THE BOTAL INSTITUTION —  OP GASES — HEAVY GLASS — ClIABLES AITOEBSON— OONTBIBTT- TlOlSrS TO PHYSICS . . . . • .12 DISOOVEBY OP MAGNET0-ELECTBIC3ITY —r EXPLANATION OP AB^GO'S MAGNETISK OP BOTATION — TEBBESTBIAL MAG- NETO-ELECTBIC INDUCTION — THE EXTRA CUBBENT . • 19 POINTS OP CHABACTER . . . . .36 IDENTITY  ORIGIN OP POWER IN THE YOLTAIC PILE . . .59 RESEARCHES ON PRICTIONAL ELECTRICITY — ^INDUCTION — CON- DUCTION — SPECIFIC INDUCTIYB CAPAaiX — THEORY OP CONTIGUOUS PARTICLES . • • • .66 • • • Vm CONTENTS. PAOB REST NEEDED — ^VISIT TO SWITZERLAIJD . • * 75 MAGNETIZATION OF liaHT . . • • .79 DISCOYEBT  AND OASES — ^ATMOSFHEBIG MAGNETISM . 108 8PECXTLAT10NS — NATTJBE OF MATTEB — KLINES OF POECE . 119 UNITY AND CONYEBTIBILITY OF NATUBAL FOBCES — THEOBY OF THE ELECTBIO CUBBENT .... 133 SUMMABY ••••»•• 145 ILLT7STBATI0NS OF CHABACTEB • • • • 147 FARADAY AS A DISCOVERER. parentage : inteoduotiolir to the botal institu- tion: eabliest expeeiments: first boyal society papeb: habbiage. It has been thought desirable to give you and the world some image of Michael Fabaday^ as a scien- tific investigator and discoverer. The attempt to respoiid to this desire has been to me a labour of difficulty, if also a labour of love. For however well acquainted I may be with the researches and dis- coveries of that great master — ^however numerous the illustrations which occur to me of the loftiness of Faraday's character and the beauty of his life — still to grasp him and his researches as a whole ; to seize upon the ideas which guided him, and connected them ; to gain entrance into that strong and active brain, and read from it the riddle of the world — this is a work not easy of performance, and all but impos- sible amid the distraction of duties of another kind. That I should at one period or another speak to you 2 FARADAY AS A DISCOVERER. regarding Faraday and liis work, is natural, if not inevitable ; but I did not expect to be called upon to speak so soon. Still the bare suggestion that this is the fit and proper time for speech sent me imme- diately to my task : from it I have returned with such results as I could gather, and also with the wish that those results were more worthy than they are of the greatness of my theme. It is not my intention to lay before you a life of Faraday in the ordinary acceptation of the term. The duty I have to perform is to give you some notion of whajb he has done in the world ; dwelling incidentally on the spirit in which his work was executed, and introducing such personal traits as may be necessary to the completion of your picture of the philosopher^ though by no means adequate to give you a complete idea of the man. The newspapers have already informed you that Michael Faraday was bom at Newington Butts, on September 22, 1791,* and that he fell finally asleep at Hampton Court, on August 25, 1867. Believing, as I do, in the general truth of the doctrine of here- ditary transmission — sharing the opinion of Mr. Carlyle, that * a really able man never proceeded from entirely stupid parents * — I once used the privilege of my intimacy with Mr. Faraday to ask "hiTn whether his parents showed any signs of un* FARADAY AS A DISCOVERER. 3 usual ability. He could remember none. His father^ I believe, was a great sufferer during the latter years of bis life, and this might have masked whatever inteUectual power he possessed. When thirteen years old, that is to say in 1804, Faraday was apprenticed to a bookseller and bookbinder in Blandford-street, Manchester-square: here he spent eight years of his life, after which he worked as a journeyman elsewhere. You have also heard the account of Faraday's first contact with the Eoyal Institution; that he was introduced by one of the members to Sir Humphry Davy's last lectures; that he took notes of those lectures, wrote them fairly out, and sent them to Davy, entreating him at the same time to enable him to quit trade, which he detested, and to pursue science, which he loved. Davy was helpful to the young man, and this should never be forgotten : he at once wrote to Faraday, and, afterwards when an opportunity occurred, made him his assistant.* Mr. * Here is Davy's recommendation of Faraday, presented to the managers of the Royal Institution, at a meeting on the 18th of March, 1813, Charles Hatchett, Esq., in the chair :^ ' Sir Humphry Dayy has the honour to inform the managers that he has found a person who is desirous to occupy the situation in the Insti- tution lately filled by William Payne. His name is Michael Faraday. He is a youth of twenty-two years of age. As far as Sir H. Davy has been able to observe or ascertain, h^ appears well fitted for the situa- tion. His habits seem good ; his disposition active and cheerful, and 4 FARADAY AS A DISCOVERER. Gassiot has lately favoured me with the following Feminiscence of this time : — * Clapham Common, Surrey, * November 28, 1867. * Mt dear Ttndall, — Sir H. Davy was accustomed to call on the late Mr. Pepys, in the Poultry, on his way to the London Institution, of which Pepys was one of the original managers ; the latter told me that on one occasion Sir H. Davy, showing him a letter, said, " Pepys, what am I to do, here is a letter from a yoimg man named Faraday ; he has been attending my lectures, and wants me to give him employment at the Eoyal Institution — what can I do ? " " Do ? " replied Pepys, " put him to wash bottles ; if he is good for anything he will do it directly, if he refuses he is good for nothing/' " No, no,'' replied Davy; " we must try him with something better than that." The result was, that Davy engaged him to assist in the Laboratory at weekly wages. * Davy held the joint of&ce of Professor of Chemis- try and Director of the Laboratory; he ultimately gave up the former to the late Professor Brande, but he insisted that Faraday should be appointed Direc- tor of the Laboratory, and, as Faraday told me, this his manner intelligent. He is willing to engage himself on the same terms as giyen to Mr. Payne at tl^ time of quitting the Institution. * Sesolved, — That Michael Faraday be engaged to fill the situation lately occupied by Mr. Payne, on the same terms.*. PAKADAY AS A DISCOVERER. 5 enabled him on saljaequent occaeiona to hold a defi- nite position in the Institution, in which He was always supported bj Davy. I believe he held that office to the last. ' Believe me, my dear Tyndall, yours traly, ' J. P. Uassiot. 'Dr. Tyndall.' From a letter written by Faraday himself soon after his appointment as Davy's assistant, I extract the following acconnt of his introduction to the Boyal Institation : — 'London, Sept, 18, 1813. ' As for myself, I am absent (£rom home) nearly day and night, except occasional calls, and it is likely shall shortly be absent entirely, bnt this (having nothing more to say, and at the request of my mother) I will explain to you. I was formerly a bookseller and binder, but am now turned philoso- pher,* irfiich happened thus : — ^Whilst an apprentice, I, for amusement, learnt a little chemistry and other parts of philosophy, and felt an eager desire to pro- ceed in that way farther. After being a joomeyman for six months, under a disagreeable master, I gave up my business, and through the interest of a Six H. Davy, filled the situation of chemical assistant to thi? * Faraday loved ibis irord and employed it to the last; he liad Ha intense dislike to the modem term ph^^citt. 6 FARADAY AS A DISCOVERER. Eoyal Institution of Great Britain, in which, of&ce I now remain ; and where I am constantly employed in observing the works of nature, and tracing the man- ner in which she directs the order and arrangement of the world. I have lately had proposals made to me by Sir Humphry Davy to accompany him in his travels through Europe and Asia, as philosophical assistant. If I go at all I expect it will be in Ootober next — about the end ; and my absence from home will perhaps be as long as three years. But as yet all is imcertain.' This account is supplemented by the following letter, written by Faraday to his friend De la Eive,^ on the occasion of the death of Mrs. Marcet. The letter is dated Sept. 2, 1858 : — ^ My dear Friend, — Tour subject interested me deeply every way; for Mrs. Marcet was a good friend to me, as she must have been to many of the human race. I entered the shop of a bookseller and bookbinder at the age of 13, in the year 1804, remained there eight years, and during the chief part of the time bound books. Now it was in those books, in the hours after work, that I found the be- ginning of my philosophy. There were two that especially helped me, the "Encyclopaedia Britannica,^' * To whom I am indebted for a copy of the original letter. FARADAY AS A DISCOVERER. 7 from which I gained my first notions of electricity, and Mrs. Marcet's "Conversations on Chemistry/' which gave me my foundation in that science. ^ Do not suppose that I was a very deep thinker, or was marked as a precocious person. I was a very lively imaginative person, and could believe in the "Arabian Nights'' as easily as in the "Encyclo- paedia." But facts were important to me, and saved me. I could trust a fact, and always cross-examined an assertion. So when I questioned Mrs. Marcet's book by such little experiments as I could find means to perform, and found it true to the facts as I could understand them, I felt that I had got hold of an anchor in chemical knowledge, and clung fast to it. Thence my deep veneration for Mrs. Marcet — first as one who had conferred great personal good and pleasure on me ; and then as one able to convey the truth and principle of those boundless fields of know- ledge which concern natural things, to the young, untaught, and inquiring mind. *Tou may imagine my delight when I came to know Mrs. Marcet personally ; how often I cast my thoughts backward, delighting to connect the past and the present ; how often, when sending a paper to her as a thank-offering, I thought of my first instructress, and such like thoughts will remain with me. O FARADAY AS A DISCOVERER. * I have some such thoughts even as regards your own father j who was, I may say, the first who per- sonally at Geneva, and afterwards by correspondence, encouraged, and by that sustained, me/ Twelve or thirteen years ago Mr, Faraday and myself quitted the Institution one evening together, to pay a visit in Baker-street. He took my arm at the door, and, pressing it to his side in his warm genial way, said, * Come, Tyndall, I will now show you something that will interest you/ We walked northwards, passed the house of Mr. Babbage, which drew forth a reference to the famous evening parties once assembled there. We reached Blandford-street, and after a little looking about, he paused before a stationer's shop, and then went in. On entering the shop, his usual animation seemed doubled; he looked rapidly at everything it contained. To the left on entering was a door, through which he looked down into a little room, with a window in front facing Blandford-street. Drawing me to- wards him, he said eagerly, 'Look there, Tyndall, that was my working-place. I bound books in that little nook.' A respectable-looking woman stood behind the counter : his conversation with me was too low to be heard by her, and he now turned to the counter to buy some cards as an excuse for our being there. He askied the woman her name — ^her prede- FAKADAT AS A DISCOVEREB. 9 cesser's name — ^his predecessor's name. * That won't do,' lie said, with good-humoured impatience ; * who was his predecessor?' 'Mr. Eiebau,' she replied, and immediately added, as if suddenly recollecting herself, *He, sir, was the master of Sir Charles Faraday.' * Nonsense ! ' he responded, * there is no such person.* Great was her delight when I told her the name of her visitor ; but she assured me that as soon as she saw him running about the shop, she felt — ^though she did not know why — ^that it must be * Sir Charles Faraday.* Faraday did, as you know, accompany Davy to Rome ; he was re-engaged by the managers of the Royal Institution on May 15, 1815. Here he made rapid progress in chemistry, and after a time was entrusted with easy analyses by Davy. had applied to them an explanation which com- pletely accounted for a class of sounds discovered by De la Rive himself. By a few simple and conclusive experiments, Faraday proved that the explanation was insufficient. It is an epoch in the life of a young man, when he finds himself correcting a person of eminence, and in Faraday's case, .where its effect was to develop a modest self-trust, such an event could not fail to act profitably. From time to time between 1818 and 1820 Faraday published scientific notes and notices of minor weight. At this time he w^s acquiring, not producing ; work- ing hard for his master and storing and strengthen- ing his own mind. He assisted Mr. Brande in his lectures, and so quietly, * skilfully, and modestly was his work done, that Mr. Brande's vocation at the time was pronounced ^ lecturing on velvet.* In 1820 Faraday published a chemical paper * on two new compounds of chlorine and carbon, and on a new compound of iodine, carbon, and hydrogen. This 4 paper was read before the Eoyal Society on December 21, 1820, and it was the first of his that was honoured with a place in the * Philosophical Transactions.' On June 12, 1821, he married, and obtained leave to bring his young wife into his rooms at the Eoyal Institution. There for forty-six years they lived to- gether, occupying the suite of apartments which had FARADAY AS A DISCOVEHEK. 11 been previously in the succesaire occupancy of Young, Davy, and Brande. At the time of Her marria^ Mrs. Faraday was twenty-one years of age, te being nearly thirty. Begarding this marriage I will at present limit myself to quoting an entry written in Faraday's own hand in his book of diplomas, which caught my eye while in his company some years ago. It ran thus; — ' 25th Jannarj, 1847- * Amongst these records and events, I here insert the date of one which, as a source of honour and happiness, far exceeds all the rest. We were married on June 12, 1821. ' M. Faeadat.' Then follows the copy of the minutes, dated May 21, 1821, vfhich gave him additional rooms, and thus enabled bin) to bring his wife to the Boyal Institu- tion. A feature of Faraday's character which I have often noticed makes itself apparent in this entry. In his relations to his wife he added chivalry to affection. / 12 FARADAY AS A DISCOVEKER. EARLY RESEARCHES : MAGNETIC ROTATIONS : LIQUE- FACTION ' OP GASES : HEAVY GLASS : CHARLES AN- DERSON : CONTRIBUTIONS TO PHYSICS. Oersted, in 1820, discovered the action of a voltaic current on a magnetic needle; and immediately afterwards the splendid intellect of Ampere suc- ceeded in showing that every magnetic phenomenon then known might be reduced to the mutual action of electric currents. ^ The subject occupied aU men's thoughts ; and in this country Dr. Wollaston sought to convert the deflection of the needle by the current into a permanent rotation of the needle round the current. He also hoped to produce the reciprocal eflFect of causing a current to rotate round a magnet. In the early part of 1821, Wollaston attempted to realise this idea in the presence of Sir Humphry Davy in the laboratory of the Eoyal Institution. This was well calculated to attract Faraday's atten- tion to the subject. He read much about it ; and in the months of July, August, and September, he wrote *a history of the progress of electro-magnetism,' which he published in Thomson's ^Annals of Phi- losophy/ Soon afterwards he took up the subject of * Magnetic Rotations,' and on the morning of Christ- mas-day, 1821, he called his wife to witness for the FARADAY AS A DISCOVERER. 13 first time, the revolution of a magnetic needle round an electric current. Incidental to tlie * historic sketch,' he repeated almost all the experiments there referred to ; and these, added to his own subsequent work, made him practical master of all that was then known regarding the voltaic current. In 1821, he also touched upon a subject which subsequently received his closer attention — the vaporization of mercury at common temperatures ; and immediately afterwards conducted, in company with Mr. Stodart, experiments on the alloys of steel. He was accus- tomed in after years to present to his friends razors formed from one of the alloys then discovered. During Faraday's hours of liberty from other duties, he took up subjects of inquiry for himself; and in the spring of 1823, thus self-prompted, he began the examination of a substance which had long been regarded as the chemical element chlorine, in a solid form, but which Sir Humphry Davy, in 1810, had proved to be a hydrate of chlorine, that is, a compound of chlorine and water. Faraday first analysed this hydrate, and wrote out an account of its composition. This account was looked over by Davy, who suggested the heating of the hydrate under pressure in a sealed glass tube. This was done. The hydrate fused at a blood-heat, the tube became filled with a yellow atmosphere, and was 14 FABADAY AS A DISCOVEREK. found to contain two liquid substances. Dr. Paris happened to enter the laboratory while Faraday was at work. Seeing the oily liquid in his tube, he rallied the young chemist for his carelessness in employing soiled vessels. On filing off the end of the tube, its contents exploded and the oily matter vanished. Early next morning, Dr. Paris received the following note : — ^ Deab Sie, — ^The oil you noticed yesterday turns out to be liquid chlorine. ^ Tours faithfully, ' M. Paeaday.' * The gas had been liquefied by its own pressure. Fa- raday then tried compression with a syringe, and succeeded thus in liquefying the gas. To the published account of this experiment Davy added the following note : — ^ In desiring Mr. Faraday to expose the hydrate of chlorine in a closed glass tube, it occurred to me that one of three things would happen : that it would become fluid as a hydrate ; that decomposition of water would occur; ... or that the chlorine would separate in a fluid state.' Davy, moreover, immediately applied the method of self-compressing atmospheres to the liquefaction of muriatic gas. Faraday continued the experiments, * Paris : Z(/b of Davy, p. S91. FARADAY AS A DISCOVERER. 15 and succeeded in reducing a number of gases till then deemed permanent to tlie liquid condition. In 1844 lie returned to the subject, and considerably expanded its limits. These important investiga- tions established the fact that gases are but the vapours of liquids possessing a very low boiling-point, and gave a sure basis to our views of molecular ag- gregation. The account of the first investigation was read before the Royal Society on April 10, 1823, and was published, in Faraday's name, in the ^ Phi- losophical Transactions.' The second memoir was sent to the Eoyal Society on December 19, 1844. I may add that whfle lie was conducting his first ecc- peiiments on the liquefaction of gases, thirteen pieces of glass were on one occasion driven by an explosion into Faraday's eye. Some small notices and papers, including the observation that glass readily changes colour in sunlight, follow here. In 1825 and 1826 Faraday published papers in the * Philosophical Transactions ' on ^new compounds of carbon and hydrogen,' and on * sulphonaphthalic acid.' In the former of these papers he announced the discovery of Benzol, which, in the hands of modem chemists, has become the foundation of our splendid aniline dyes. But he swerved incessantly from chemistry into physics; and in 1826 we find him engaged in investigating 16 FABADAT AS A DISCOVERER. the limits of yaporizatioii, and showing, by exceed- ingly strong and apparently condnsiye argoments, that even in the case of mercury sach a limit exists ; much more he conceived it to be certain that onr atmosphere does not contain the vaponr of the fixed constituents of the earth's crust. This question, I may say, is likely to remain an open one. Dr. Bankine, for example, has lately drawn attention to the odour of certain metals; whence comes this odour, if it be not from the vapour of the metal 9 In 1825 Faraday became a member of a com- mittee, to which Sir John Herschel and Mr. Dollond also belonged, appointed by the ^loyal Society to examine, and if possible improve, the manufacture of glass for optical purposes. Their experiments continued till 1829, when the account of them con- stituted -the subject of a ^Bakerian Lectfare.' This lectureship, founded in 1774 by Henry Baker, Esq., of the Strand, London, provides that every year a lecture shall be given before the Eoyal Socieiy, the sum of four pounds being paid to the lecturer. The Bakerian Lecture, however, has long since passed from the region of pay to that of honour, papers of mark only being chosen for it by the council of the Society. Faraday's first Bakerian Lecture, * On the Manufacture of Glass for Optical Purposes,' was de- livered at the close of 1829. It is a most elaborate FARADAY AS A DIStJOVERER, 17 and conscientious description of processes, pre- cautions, and results : the details were so exact and so minute, and the paper consequently so long, thai three successive sittings of the Eoyal Society were taken up by the delivery of the lecture.* This glass did not turn out to be of important practical use, but it happened afterwards to be the foundation of two of Faraday's greatest discoveries.f The experiments here referred to, were commenced at the Falcon Glass Works, on the premises of Messrs. Green and Pellatt, but Faraday could not conveniently attend to them there. In 1827, therefore, a furnace was erected in the yard of the Eoyal Institution ; and it was at this time, and with a view of assisting him at the furnace, that Faraday engaged Sergeant An- derson, of the Eoyal Artillery, the respectable, truth- ful, and altogether trustworthy man whose appearance * Fi?. November 19, December 3 and 10. t I make the following extract from a letter from Sir John Herschel, written to me from Collingwood, on the 3rd of November, 1867 : — ' I will take this opportunity to mention that I believe myself to have originated the suggestion of the employment of borate of lead for optical purposes: It was somewhere in the year 1822, as woU as I can re- collect, that I mentioned it to Sir James (then Mr.) South ; and, in con- sequence, the trial was made in his laboratory in Blackman Street, by precipitating and working a large quantity of borate of lead, and fusing it under a muffle in a porcelain evaporating dish. A very limpid (though slightly yellow) glass resulted, the refractive index 1*8661 (which you will find set down in my table of refractive indices in my article " Light," Encyclopadia Meiropolitana). It was, however, too soft for optical use as an object-glass. This Faraday overcame, at least to a considerable degree, by the introduction of silica,* 18 FARADAY, AS A DISCOVERER. here is so firesh in our memories. Anderson con- tinued to be the reverential helper of Faraday and the faithful servant of this Institution for nearly forty years.* In 1831 Faraday published a paper * On a peculiar class of Optical Deceptions/ to which I believe the beautiful optical toy called the Chromatrope owes its origin. In the same year he published a paper on Vibrating Surfaces, in which he solved an acoustical problem which, though of extreme simplicity when solved^ appears to have baffled many eminent men. The problem was to account for the fact that light bodies, such as the seed of lycopodium, collected at the vibrating parts of sounding plates, while sand ran to the nodal lines. Faraday showed that the light bodies were entangled in the little whirlwinds formed in the air over the places of vibration, and through which the heavier sand was readily projected. Faraday's resources as an experimentalist were so wonderful, and his delight in experiment was so great, that he sometimes almost ran into excess in * Regarding Anderson, Faraday writes thus in 1845 : — * I cannot resist the occasion that is thus offered to me of mentioning the name of Mr. Anderson, who came to me as an assistant in the glass experiments, and has remained ever since in the laboratory of the Royal Institution. He assisted me in all the researches into which I have entered since that time ; and to his care, steadiness, exactitude, and faithfulness in the performance of all that has been committed to his charge, I am much indebted. — ^M.P.* {Exp. Researches, toI. iii. p. 3, footnote.) FAEADAY AS A DISCOVERER. 19 this direction. I have heard him say that this paper on vibrating surfaces was too heavily laden with experiments. DISCOVERT OP magneto-electricity: explanation OP ARAGO'S MAGNETISM OP ROTATION : TERRES- TRIAL MAGNETO-ELECTRIC INDUCTION: THE EXTRA CURRENT. The work thus far referred to, though sufficient of itself to secure no mean scientific reputation, forms but the vestibule of Faraday's achievements. He had been engaged within these walls for eighteen years."*^ During part of the time he had drunk in knowledge from Davy, and during the remainder he continually exercised his capacity for independent inquiry. In 1831 we have him at the climax of his intellectual strength, forty years of age, stored with knowledge and full of original power. Through reading, lec- turing, and experimenting, he had become thoroughly familiar with electrical science : he saw where light was needed and expansion possible. The phenomena of ordinary electric induction belonged, as it were, to the alphabet of his knowledge : he knew that under ordinary circumstances the presence of an electrified body was sufficient to excite, by induction, an une- * He used to say that it required twenty years of work to make a man in Physical Science ; the previous period being one of infancy. 20 FARADAY AS A DISCOVERER. lectrified body. He knew tliat tli6 wire whicli carried an electric current was an electrified bodj, and stiU tliat aU attempts liad failed to make it excite in other wires a state similar to its own. What was the reason of this failure ? Faraday never could work from the experiments of others, however clearly described. He knew well that from every experiment issues a kind of radiation, lumi- nous in different degrees to different minds, and he hardly trusted himself to reason upon an ex- periment that he had not seen. In the autumn of 1831 he began to repeat the experiments with electric currents, which, up to that time, had pro- duced no positive result. And here, for the sake of younger inquirers, if not for the sake of us all, it is worth while to dwell for a moment on a power which Faraday possessed in an extraordinary degree. He imited vast strength with perfect flexibility. His momentum was that of a river, which combines weight and directness with the ability to yield to the flexures of its bed. The intentness of his vision in any direction did not apparently diminish his power of perception in other directions; and when he attacked a subject, expecting results, he had the faculty of keeping his mind alert, so that results different from those which he expected should not escape him through pre-occupation. FARADAY AS A DISCOVEBEB. 21 He began his experiments ^on the induction of electric currents ' bj composing a helix of two insu- lated wires, which were wound side by side round the same wooden cylinder. One of these wires lie connected with a voltaic battery of ten cells, and the other with a sensitive galvanometer. When con- nection with the battery was made, and while the current flowed, no effect whatever was observed at the galvanometer. But he never accepted an experi- mental result, until he had applied to it the utmost power at his command. He raised his battery from 10 cells to 120 cells, but without avail. The current flowed calmly through the battery wire without pro- ducing, during its flow, any sensible result upon the galvanometer. ' During its flow,' and this was the time when an effect was expected — but here Fara'day's power of lateral vision, separating, as it were, from the line of expectation, came into play — ^he noticed that a feeble movement of the needle always occurred at the mo- ment when he made contact with the battery ; that the needle would afterwards return to its former posi- tion and remain quietly there unaffected by the flowing current. At the moment, however, when the circuit was interrupted the needle again moved, and in a direction opposed to that observed on the com- pletion of the circuit. 22 FAHADAX AS A D15COVEKEB. ThiB result, and others of a similar kind, led him to the conclusion * that the battery current through the one irire did in reality induce a similar current through the other; but that it continued for an in- stant only, and partook more of the nature of the electric wave from a common Leyden jar than of the current from a voltaic battery/ The momentary currents thus generated were called induced cwrrenis, while the current which generated them was called the inducing current. It was immediately proved that the current generated at making the circuit was always opposed in direction to its generator, while that developed on the rupture of the circuit coin- cided in direction with the inducing current. It appeared as if the current on its first rush through the primary wire sought a purchase in the secondai*y one, and, by a kind of kick, impelled backward through the latter an electric wave, which subsided as soon as the primary current was fully established. Faraday, for a time, believed that the secondary- wire, though quiescent when the primary current had been once established, was not in its natural condition, its return to that condition being declared by the current observed at breaking the circuit. He called this hypothetical state of the wire the electro- tonic state : he afterwards abandoned this hypothesis, but seemed to return to it in later life. The term FARADAY AS A DISCOVERER. 23 electro-tonic is also preserved by Professor Du Bois Eeymotfd to express a certain electric condition of the nerves, and Professor Clerk Maxwell has ably defined and illustrated the hypothesis in the Tenth Volume of the ^Transactions of the Cambridge Philosophical Society,' The mere approach of a wire forming a closed curve to a second wire through which a voltaic cur- rent flowed was then shown by Faraday to be suf- ficient to arouse in the neutral wire an induced current, opposed in direction to the inducing cur- rent; the withdrawal of the wire also generated a current having the same direction as the inducing current ; those currents existed only during the time of approach or withdrawal, and when neither the primary nor the secondary wire was in motion, no matter how close their proximity might be, no in- duced current was generated. Faraday has been called a purely inductive philo- sopher. A. great deal of nonsense is, 1 fear, uttered in this land of England about induction and deduc- tion. Some profess to befriend the one, some the other, while the real vocation of an investigator, like Faraday, consists in the incessant marriage of both. He was at this time full of the theory of Ampere, and it cannot be doubted that numbers of his ex- periments were executed merely to test his deductions 24 FARADAY AS A DISCOVERER. % from that theory. Starting from the discovery of Oersted, the celebrated Trench philosopher Lad shown that all the phenomena of magnetism then known might be reduced to the mutual attractions and repulsions of electric currents. Magnetism had been produced jfrom electricity, and Faraday, who all his life long entertained a strong belief in such re- ciprocal actions, now attempted to effect the evolu- tion of electricity jfrom magnetism. Round a welded iron ring he placed two distinct coils of covered wire, causing the coils to occupy opposite halves of the ring. Connecting the ends of one of the coils with a galvanometer, he found that the moment the ring was magnetized, by sending a current through the other coily the galvanometer needle whirled round four or five times in succession. The action, as before, was that of a pulse, which vanished imme- diately. On interrupting the circuit, a whirl of the needle in the opposite direction occurred. It was only during the time of magnetization or demagne- tization that these effects were produced. The in- duced currents declared a change of condition only, and they vanished the moment the act of magnetiza- tion or demagnetization was complete. The effects obtained with the welded ring were also obtained with straight bars of iron. Whether the bars were magnetized by the electric current, or FARADAY AS A DISCOVEBER. 25 were excited by the contact of permanent steel mag- nets, induced currents were always generated during the rise, and during the subsidence of the magnetism. The use of iron was then abandoned, and the same effects were obtained by merely thrusting a perma- nent steel magnet into a coil of wire, A rush of electricity through the coil accompanied the inser- tion of the magnet ; an equal rush in the opposite direction accompanied its withdrawal. The precision with which Faraday describes these results, and the completeness with which he defines the boundaries of his facts, are wonderful. The magnet, for example, must not be passed quite through the coil, but only half through, for if passed wholly through, the needle is stopped as by a blow, and then he shows how this blow results from a reversal of the electric wave in the helix. He next operated with the power- ful permanent magnet of the Eoyal Society, and ob- tained with it, in an exalted degree, all the foregoing phenomena. And now he turned the light of these discoveries upon the darkest physical phenomenon of that day. Arago had discovered in 1824, that a disk of non- magnetic metal had the power of bringing a vibrating magnetic needle suspended over it rapidly to rest; and that on causing the disk to rotate the magnetic needle rotated along with it. When both were 28 FARADAY AS A DISCOVERER. On January 12, 1832, he comintmicated to the Eoyal Society a second paper on Terrestrial Magneto- electric Induction, which was chosen as the Bakerian Lecture for the year. He placed a bar of iron in a coil of wire, and lifting the bar into the direction of the dipping needle, he excited by this action a current in the coil. On reversing the bar, a current in the opposite direction rushed through the wire. The same effect was produced, when, on holding the helix in the line of dip, a bar of iron was thrust into * it. Here, however, the earth acted on the coil through the intermediation of the bar of iron. He abandoned the bar and simply set a copper-plate spinning in a horizontal plane ; he knew that the earth's lines of magnetic force then crossed the plate at an angle of about 70°. When the plate spun roimd, the lines of force were intersected and induced currents generated, which produced their proper effect when carried from the plate to the galvano- meter. 'When the plate was in the magnetic meridian, or in any other plane coinciding with the magnetic dip, then its rotation produced no effect upon the galvanometer.' At the suggestion of a mind fruitful in suggestions of a profound and philosophic charactei: — I mean that of Sir John Herschel — ^Mr. Barlow, of Woolwich, had experimented with a rotating iron shell. Mr. FARADAY AS A DISCOVERER. 29 Christie had also performed an elaborate series of experiments on a rotating iron disk. Both of them had found that when in rotation the body exercised a peculiar action upon the magnetic needle, deflect- ing it in a manner which was not observed during quiescence ; but neither of them was aware at the time of the agent which produced this extraordinary- deflection. They ascribed it to some change in the magnetism of the iron shell and disk. But Faraday at once saw that his induced curren{s must come into play here, and he immediately ob- tained them from an iron disk. With a hollow brass ball, moreover, he produced the effects obtained by Mr. Barlow. Iron was in no way necessary: the only condition of success was that the rotating body should be of a character to admit of the formation of currents in its substance : it must, in other words, be a conductor of electricity. The higher the con- ducting power the more copious were the currents. He now passes from his little brass globe to the globe of the earth. He plays like a magician with the earth's magnetism. He sees the invisible lines along which its magnetic action is exerted, and sweeping his wand across these lines evokes this new power. Placing a simple loop of wire round a magnetic needle he bends its upper portion to the west : the north pole of the needle immediately swerves to the 30 FARADAY AS A DISCOVERER. east: lie bends his loop to the east, and the north pole moves to the west. Suspending a common bar magnet in a vertical position, he causes it to spin round its own axis. Its pole being connected with one end of a galvanometer wire, and its equator with the other end, electricity rushes round the galvano- meter from the rotating magnet. He remarks upon the ^ singular independence ' of the magnetism and the body of the magnet which carries it. The steel be- haves as if it were isolated from its own magnetism. And then his thoughts suddenly widen, and he asks himself whether the rotating earth does not generate induced currents as it turns round its axis from west to east. In his experiment with the twirl- ing magnet the galvanometer wire remained at rest ; one portion of the circuit was in motion relatively to another portion. But in the case of the twirling planet the galvanometer wire would necessarily be carried along with the earth ; there would be no rela- tive motion. What must be the consequence ? Take the case of a telegraph wire with its two terminal plates dipped into the earth, and suppose the wire to lie in the magnetic meridian. The ground under- neath the wire is influenced like the wire itself by the earth's rotation ; if a current from south to north be generated in the wire, a similar current from south to north woidd be generated in the earth under the FARADAY AS A DISCOVERER. 31 vrire; these currents would run against the same terminal plate, and thus neutralize each other. This inference appears inevitable, but his profound vision perceived its possible invalidity. He saw that it was at least possible that the difference- of con- ducting power between the earth and the wire might give one an advantage over the other, and that thus a residual or differential current might be obtained. He combined wires of different materials, and caused them to act in opposition to each other : but found the combination ineffectual. The more copious flow in the better conductor was exactly counterbalanced by the resistance of the worst. Still, though ex- periment was thus emphatic, he would clear his mind of aU discomfort by operating on the earth itself. He went to the round lake near Kensington Palace, and stretched 480 feet of copper wire, north and south, over the lake, causing plates soldered to the wire at its ends to dip into the water. The copper wire was severed at the middle, and the severed ends connected with a galvanometer. No effect whatever was observed. But though quiescent water gave no effect, moving water might. He therefore worked at London Bridge for three days during the ebb and flow of the tide, but without any satisfactory result. Still he urges, ^Theoretically it seems a necessary consequence, that where water is flowing there elec- 32 FARADAY AS A DISCOVERER. trie currents should be formed. If a line be imagined passing from Dover to Calais through the sea, and returning through the land, beneath the water, to Dover, it traces out a circuit of conducting matter one part of which, when the water moves up or down the channel, is cutting the magnetic cuitcs of the earth, whilst the other is relatively at rest. 