nal, Oersted took no immediate measures either to complete or to publish his discovery. Some months appear to have elapsed whilst waiting for the convenience of a larger battery before he repeated the experiment with the aid of Professor Esmark and other friends. The battery then employed contained 20 twelve-inch elements, charged with water andth of mixed nitric and sulphuric acids. The conducting wire was heated red hot, which must have rather diminished the effect than otherwise. The nature of the wire was found to be unimportant. If positive electricity passed from north to south through a conducting wire placed horizontally in the magnetic meridian, then a compass needle suspended over it had its north end deviated to the west; if under it, to the east; if the needle was placed on the east side of the conductor, its north end was raised; if on the west side, it was depressed. Oersted further found that needles of non-magnetic substances, such as brass and gumlac, were not affected, and that the electrical efficiency depended on the quantity, not the intensity, of the current. These experiments seem to have been made in July 1820; and Oersted and his friends being now fully alive to the novelty and importance of the discovery, he circulated extensively copies of a Latin tract, dated the 21st July, in which the effects of the "electric conflict," as he terms the presumed combination of the opposite electricities in the "conjunctive wire" upon a magnet, were described.1 In this tract we find the following expressions: The electric conflict acts in a revolving manner." "It resembles a helix." "The electric conflict is not confined to the conducting wire, but it has around it a sphere of activity of considerable extent."

:

(792.) The effect of this pamphlet, consisting of a few Speedily pages only, was instantaneous and wonderful. The taken up author probably counted on the opportunity of develby Ampère, oping his discovery at leisure, but it was seized on Arago, and with such avidity, and pursued with such signal sucDavy. cess, particularly in France, that he probably gave up the race of invention in despair. Ampère had already communicated experiments to the Institute on the 18th and 26th September. Arago and Davy separately, and but little later, discovered the magnetizing power which the voltaic conductor exerted on iron filings, and the latter tried in vain the magnetizing power of common or machine electricity, which, however, was soon after shown by Arago, who enclosed steel wires in helices of copper wire, through which the discharges were passed. When soft iron was placed in such a helix, it was found to become a temporary magnet of great power whilst the voltaic current continued. Thus magnets of enormously greater power than any previously known were constructed; one of the first large ones was made by Professor Henry of the United States.

(793.)

But the progress of electro-magnetismas a science

was far more indebted to Ampère, a professor at
Paris, than to any other philosopher.
I shall,
therefore, introduce here some account of his dis-
coveries before closing what I have to say of Oer-
sted.

ANDRE MARIE AMPERE was born in 1775 at Lyon. (794.) He was an able mathematician, and wrote several Electromemoirs on Chances, and on the Integration of Par- theory of dynamic tial Differential Equations. But with this he com- Ampère. bined a taste for, and a practical acquaintance with, the experimental sciences. He was a very good chemist, and showed himself particularly attentive to Davy on his first visit to Paris. He was also much attached to metaphysical speculation. His skill in devising apparatus and in performing experiments was eminently shown in his electro-magnetic researches ; whilst he judiciously rendered his mathematical knowledge subservient to them. In this respect he had greatly the advantage of Oersted, who appears to have been little acquainted with mathematics, and, perhaps, in common with his metaphysical friends of the German school, misapprehended their utility in physical discoveries. Three different hypotheses Various were speedily broached to represent mechanically opinions the singular kind of force mutually exerted between nature a conductor and a magnet. The first and most ob- of the elecvious was, that this action was not a push-and-pull tro-magforce, but a force producing rotation without direct netic force. attraction and repulsion, or of the nature of a couple exerted between any part of an electric current, and a small magnet or magnetic element. The second opinion was, that an electric current may be esteemed equivalent to a magnetizing force at right angles to it. The third, that a magnet is composed of elements which act as if a closed electric circuit existed independently within each of them; that is, each magnetic molecule may be replaced by a small conducting wire bent upon itself, in which some unfailing source of electricity, like a galvanic pair, keeps up, in the same direction, a constant current.

on the

This last hypothesis, arbitrary and improbable as (795.) it may sound, was that defended by Ampère. Whilst Theory or few will be disposed to regard it as a true and com- hypothesis adopted by plete physical picture of the condition of magnetized Ampère. bodies, it seems impossible not to award to it the same sort of credit which we do to Newton's "fits of easy reflection and transmission" of light, when we find that it not only serves to represent the more obvious phenomena, but has suggested experiments absolutely new, and which turned out in accordance with the anticipation; and that, finally, by the sagacity and industry of its author it was made to include, by merely mathematical deductions, and without any complication of the hypothesis, certain experiments of a very singular kind, which at first seemed inexplicable by it. I proceed to develope a little farther this consideration.

(796.) Mutual action of electric

The theory of Ampère rejects all but push-andpull forces, such as are commonly recognized in mechanical physics. These forces are mutual, and beconductors. long to electric currents. A permanent magnet is a congeries of minute parallel and circular currents, all acting in the same direction, which is at right angles to the magnetic axis or line of force. Granting this for a moment, Oersted's experiment shows that the current in the conductor acts on the currents in the magnet; and as a magnet places itself transversely to a conductor, the currents in the magnet tend to place themselves parallel to that in the conductor. Do we then find such properties in moveable electric conductors alone? Have they any mutual action? Does that mutual action tend to produce parallelism? And if so, may it be farther analysed into direct attractions or repulsions of the several parts of the electric currents upon one another? All these questions were answered by Ampère affirmatively after due appeal to experiment. Two copper wires connected with voltaic circuits, and suspended with the requisite degree of freedom, approach when the currents have a similar direction, but are repelled when the direction is opposite in the two. When two moveable conductors are placed at right angles, or indeed at any angle, they tend to parallelism. All the usual phenomena of a magnet may be imitated by a long helix of copper wire through which electricity is made by some artifice continually to circulate. The position of the poles is the same as in a real magnet, and the name of pole is determined by the direction (right or left handed) in which the helix is wound. Such an instrument, not containing one particle of iron, is attracted and repelled by a steel magnet, obeys the directive influence of the earth,-gives transverse motion to an electric conductor near it,-in short, does whatever magnetized iron does.

the dis

tance.

(797.) Thus, in the mutual action of electric currents Elementary law of (for the phenomena of static electricity are wholly the inverse unlike) we recognize the great discovery of Ampère. square of A new science was formed, which he called electrodynamics, which he proceeded to develope with great skill and success. MM. Biot and Savary found that the electro-magnetic force exerted by an indefinite straight conductor and needle, varies inversely as the simple distance from the conductor; but looking to the elementary actions of each portion of the current, it will be found that this corresponds to the usual physical law of the inverse square of the distance between the magnetic and the electric ele

lar kind of motion, that in which a magnet floating in mercury is made to revolve continuously around a central conducting wire, and in like manner a conductor may be made to revolve round a fixed magnet; nay, stranger still, a magnet acting at once as conductor and magnet, revolves with great velocity on its own axis when an electric stream is made to traverse one half of its length. These astonishing experiments, which, in an earlier age, might have founded a new sect of astronomers and replaced the theory of Vortices, offered also considerable difficulties in the application of Ampère's theory. They were, however, ulti-Accounted for by mately removed by Ampère himself, who analysed Ampere. with great skill the mechanical conditions of each case, and interpreting them into the language of his theory, showed how continuous rotations might be produced, according to the laws which he had established, by electric currents alone suitably arranged; and he effected by most ingenious experimental combinations purely electro-dynamic rotations. Some other experiments, in which magnets seemed to produce a different effect from electro-dynamic cylinders, presented a more serious obstacle, which, however, was removed by a rigorous demonstration of the effects which must ensue, if we regard the elementary molecules of a magnet as very small, and consequently the entire magnet as a collection of indefinitely small and correspondingly numerous electro-dynamic cylinders. By means of four critical experiments, Ampère determined completely the elementary laws of the mutual action of currents, including that previously established by Biot and Savary in the case of a magnet and a conductor. This investigation was one of great intricacy, and was carried out with remarkable skill. Ampère had the field almost to himself, Savary making some contributions; and, what is remarkable, little or nothing has been added either to the theory, or to the deductions from it, since his death. The progress of the science of electro-magnetism has been so astonishingly rapid since the year 1820, that one set of phenomena after another has for the time attracted almost exclusive notice. The discovery of diamagnetism will probably lead to a reconsideration of Ampère's theory as applicable to all matter in a more general form.

This rapid succession of interesting topics has pre- (799.) vented attention from being perhaps sufficiently di- Great rected to the importance of Ampère's labours. He merit of is at least as well entitled as any other philosopher Ampère. who has yet appeared, to be called "the Newton of Electricity."

His death.

Ampère was of an amiable, though rather eccentric (800.) character. His absence of mind was proverbial, and his style is somewhat cumbrous and obscure. But he was devoted to science, the promotion of which was ever his first consideration, and he evidently himself possessed great clearness in his conceptions. He died on the 17th May 1836.

(801.)

Whilst Ampère, Arago, Davy, the two De la Seebeck. Rives, and Mr Faraday, were throwing light on Discovery of thermo- the causes, and developing the consequences of electricity. Oersted's experiment, SEEBECK of Berlin discovered in 1822 a new source of electric excitement, which has since become indirectly of very great importance. This was Thermo-Electricity. He found that when heterogeneous metals are united, either by soldering or pressure, and the junction heated, a current of electricity is established. The order of metals which produces the most energetic combinations, is wholly unlike the arrangement of the voltaic series, and has no apparent reference to any other known property of those substances. Bismuth and antimony stand at the opposite extremities of the scale, and a pair formed of them is consequently the most powerful which can be made. When heated at the junction, positive electricity passes from bismuth to antimony. In 1823, Oersted, then on a visit to Paris, united with Fourier in making experiments on this subject, and was probably the first who constructed thermo-electrical piles. Unquestionably, the most important application of these was to the construction of an instrument for measuring the effects of radiant heat, by Nobili and Melloni, of which an account has already been given, Art. (709).

(802.) Invention

of the galvan

ometer

Schweigger and Nobili.

(803.)

An application of electro-magnetism of extreme importance, was the Multiplier or Galvanometer, contrived by Schweigger of Halle. In it the idea was first realized of measuring the power of an electric current by its effect in deviating a magnetic needle. Schweigger perceived that he could multiply the action of one and the same current, by causing it to traverse successive parallel coils of the conducting wire carried round the needle. Its sensibility was still farther, and almost indefinitely increased by Nobili's invention of rendering the needle astatic, or diminishing its natural directive power in any required degree. This he did by connecting it firmly with a second needle parallel to the first, of nearly equal strength, with its poles placed in an inverted position relatively to the other, and moving freely in a plane altogether exterior to the coil, so that whilst the directive effect of the earth's polarity is almost neutralized, the electro-magnetic effect of the coil tends to produce a similar deviation in both needles. This is one of the most precious philosophical instruments ever invented. It has been employed for thirty years in almost every electrical research or application. One of its best forms for many purposes (though hitherto little used) is the Torsion Galvanometer of Ritchie.

Oersted, of course, interested himself in this new Oersted's application of his own great discovery. Indeed, havcontinued. ing the good fortune to survive that discovery for

history

more than thirty years, with a full enjoyment of his intellectual vigour, he had the gratification of contemplating a body of science entirely

new as its results, and a variety of useful applications scarcely less astonishing, which might, in one sense, be called his own creation. The discoveries of Ampère, Seebeck, and Mr Faraday, were all based upon his; and during those thirty years, this elegant and interesting branch of experimental physics underwent an almost uninterrupted extension, such as hardly any other affords an example of. The Electric Telegraph is one of its most direct and practical results; nor should we omit that Oersted himself proposed, as far back as 1818, the application of electricity to blasting rocks by the very same process in which it has of late years been so usefully applied, namely, that of heating a fine wire to incandescence.

the com

pression of

Though Oersted was the author of numerous (804.) papers connected with science down nearly to His experithe close of his life, they do not contain any impor- ments on tant discovery, and with reference to electro-magnetism, he appears to have contented himself princi- water. pally with repeating and expounding the observations of his contemporaries. But some of his experiments on other subjects deserve mention, especially those on the compressibility of water. This fact, which the Florentine Academicians had vainly sought to establish in the 17th century, had been clearly demonstrated by Canton in the middle of the 18th, but Oersted first devised a compendious and effective apparatus for producing and measuring it more effectually. His result, that the compression amounts to 46-millionths of the bulk, for a pressure equal to one atmosphere, agrees almost precisely with Canton's. In 1845, he considered that he had established that the heat developed by the same amount of compression is .0203 of a centigrade degree. He also made some experiments on the Law of the Compressibility of Air and upon other subjects.

Iceives the

tute of

France.

The desideratum of a clear expression of the mani- (805.) fest alliance between Electricity and Magnetism had Oersted rebeen so long and so universally felt, that the discovery prize of placed its author in the first rank of scientific men, the InstiThere was not even, so far as I am aware, a suspicion that he had been, however remotely or dimly, anticipated. The prize of the French Institute which had been awarded to Davy for his galvanic discoveries, was bestowed upon Oersted, and so far as I am informed, has not been since adjudicated. He was elected first Correspondent, and finally Associate of the Academy of Sciences. He was personally known to many of the philosophers of Europe, having made repeated journeys in France, Germany, and England. His His scienagreeable manners and general information rendered tific charachim popular. Sir H. Davy, who visited him at Co- ter. penhagen, describes him as a man of simple manners, of no pretensions, and not of extensive resources." Niebuhr, however, who viewed his character in a different light, says, "I scarcely know another natural philosopher with so much intellect, and freedom from prejudice and esprit de corps." His writings were indeed too discursive. Professor Forch

hammer has enumerated above 200 of his publications or articles, on a vast variety of subjects; but of all these, only a single tract of a few pages will perhaps be ultimately remembered. As I before remarked, his mind, though capable of continued application, appears to have wanted the sort of concentration which prolonged physical researches require, and the school of philosophy in which he was considered by his own countrymen as a proficient, has

never been fruitful in researches based on Induction.

His death.

In November 1850, the fiftieth anniversary of his (806.) connection with the University of Copenhagen was celebrated by a jubilee. Though in his 74th year, his activity was unimpaired, and he continued his lectures and other employments until within a few days of his death, which occurred on the 9th of March 1851, closing a life full of years and honour.

of Dr Faraday.

and the

Royal In

stitution.

(808.) This eminent philosopher was born, I believe, in His early 1791. He was originally "a bookseller's apprentice, history, and fond of experiment and very averse to trade." connection very with Davy In 1812 he sent to Sir H. Davy, then at the height of his reputation, a copy of a set of notes taken at his lectures, desiring his assistance "to escape from trade, and enter into the service of science." To the credit of the popular and distinguished chemist, he gave Mr Faraday a courteous answer, and appointed him as chemical assistant in the Laboratory of the Royal Institution in March 1813. Leaving England to travel in the autumn of the same year, Davy engaged Mr Faraday to accompany him as secretary and scientific assistant; they returned in April 1815, and from that time to the present Mr Faraday has been constantly engaged in the scientific business of the Royal Institution, which is as completely associated with his numerous and splendid discoveries as Cambridge is with those of Newton, and Slough with those of the elder Herschel. By a rare, perhaps unexampled good fortune, that establishment, founded principally for the promotion of original research and the promulgation of discoveries, has been indebted during the first fifty years of its existence to the talents of two men only, for a succession of new scientific truths which might have done credit to a whole academy; indeed, if to the names of Davy and Mr Faraday we add that of Young, who here first promulgated the doctrines of the Interference of Light, there is scarcely an academy in Europe which has within the same period added so extensively to our choicest stock of original science.

(809.) Partly in consequence of his official duty of bringVariety of ing forward and explaining the most important cotemhis publications porary discoveries, partly also in consequence of his his Re- own matchless talent of elucidating, by original illussearches on trations, if not by new facts, whatever he undertakes Electricity.

VOL. I.

to expound, the variety of subjects on which Dr Faraday has made essential additions to our knowledge is so great that it is difficult to comprehend them under one section. In conformity, however, with our plan of suppressing minor facts, and insisting on the most important, I shall confine myself to a summary statement of his main discoveries connected with Electricity and Electro-Magnetism as contained in a continuous series of "Researches," published in the Philosophical Transactions between 1831 and the present time; which, when collected (as they have been in a distinct form), now fill three closely printed octavo volumes. It would be difficult to name in the history of any progressive experimental subject so large an amount of research prosecuted for so long a time in so methodical a manner and with such remarkable uniformity in plan, and with such unvarying success.

rotations.

I shall only farther premise that Dr Faraday's (810.) earliest essays were naturally of a chemical charac- Electroter. In 1820 he assisted Davy, in prosecuting Oer-magnetic sted's researches on the relations of Electricity and Magnetism, and the following year he himself succeeded in producing, for the first time, the continuous rotation of a magnet round an electric conductor, and the converse rotation of the conductor round the magnet (798). These experiments were the germ of others which continued to interest philosophers as well as the curious public for a long time after. But it was in 1831 (when the author had attained his 40th year) that the genius of Dr Faraday was displayed in a commanding manner by the appearance of his First and Second series of the Researches on Electricity, which have not perhaps been surpassed by even the most brilliant of their successors. The subject was the Induction of Electric Currents from other Currents and from Magnets. But we shall find it most convenient to take an order different from that of the discovery, and to present the main results of Dr Faraday's electrical labours under the following Heads of heads :

his chief and mag

I. The law of definite Electro-chemical Decompo- electrical sition, and the theory of the pile connected therewith. II. The Induction of Electric Currents from other coveries. 6 н

(811.) Electro

chemical

tion, and theory of the pile.

Currents and from Magnets, or the discovery of
Magneto-Electricity.

III. The influence of the Magnet on all bodies, and the consequent division of substances into two classes, Magnetics and Diamagnetics.

IV. Optical changes induced by Magnetism. I. With regard to electro-chemical decomposition and the theory of the pile, the great extent and intridecomposi- cacy of the subject require us to restrict our analysis to a few of the leading conclusions. The most important of these may be summed up in the following propositions:-1st, The amount of a decomposable substance (or electrolyte) analysed into its elements by a current of electricity depends solely on the amount of electricity passing through it, and is independent of the form of apparatus employed, the dimensions of the poles (or electrodes), the strength of the solution, or any other circumstance. It is thence inferred, with respect, for instance, to water, that the amount of it decomposed in a given time is an exact measure of the quantity of electricity set in motion in that time. 2d, When a substance is thus decomposed, it is a necessary, or at least a highly probable, consequence of Dalton's laws, that the elements separated are in atomic proportions to one another. But Mr Faraday also found that when several decompositions are effected at the same time by interposing different electrolytes in intervals of the same circuit, the whole of the series of elements separated bear the atomic relations to one another. Thus, to take a single case; an electric current decomposes in the same time 0.497 grain of water and 3-2 grains of protochloride of tin. Now, these are exactly the proportions of the atomic weights of those bodies. From this and numerous other cases of decom- Mr Faraday infers, that universally the amount of position electrical action required to dissolve a combination is electrical in a constant proportion to the force of chemical affinity by which its elements are united. The corollary seems therefore highly probable that it is one and the same force which is exerted in either case. But the conclusion as to their identity becomes almost irresistible when we add to these propositions the following: 3dly, That the oxidation of one atom of zinc by the acid of the battery generates precisely so much electricity as would resolve one atom of water into its elements. Thus, 8.45 grains of zinc dissolved occasioned the analysis of 2·35 grains of water; but these numbers are in the ratio of 32.5 to 9; the equivalents or atomic weights of zinc and water.

Definite character

equiva

lents.

of the science of voltaic electricity, are supported by Mr Faraday by a great variety of collateral proofs; and, on the whole, I cannot see that they admit of any reasonable doubt. The contact theory of Volta still, however, holds its ground in Germany, where the number of influential writers on electricity is considerable; and so perseveringly is it maintained, that it is difficult to perceive how it is ever to be dislodged. But on this wide and not very profitable controversy we cannot here enter.

duction

There are a great many other considerations (813.) connected with the action of the voltaic battery Dr Farawhich are independent of these primary ones, and day on Inwhich are scarcely less important. Dr Faraday and Conhas entered into a most elaborate experimental duction. argument to show that induction always precedes both conduction and decomposition, and that decomposable bodies or electrolytes must be all more or less perfect conductors. His views may be thus concisely summed up in his own words :-"The first effect" of the electrifying influence, whether of frictional electricity or of voltaic electricity, upon bodies, is "the production of a polarized state of their particles which constitutes induction; and this arises from its action upon the particles in immediate contact with" the excited body, " which again act upon those contiguous to them, and thus forces are transferred to a distance. If the induction remain undiminished, perfect insulation is the consequence; if the contiguous particles" thus polarized "have the power to communicate their forces, then conduction occurs, conduction being a distinct act of discharge between neighbouring particles." "In the inductive condition assumed by water" when about to be decomposed, "the discharge between particle and particle is not, as before, a mere interchange of their powers and forces, but an actual separation of them, the oxygen travelling in one direction and carrying with it its amount of force acquired during polarization, and the hydrogen doing the same thing in the other direction, until they each meet the next approaching particle, which is in the same electrical state with that they have left, and by association of their forces with it produce discharge. This action may be regarded as a carrying one performed by the constituent particles of the dielectric."2 Again, "the current is an indivisible thing; an axis of power, in every part of which both electric forces are present in equal amount."3

These views respecting the molecular progress of (814.) conduction and decomposition, though perhaps never so categorically stated as by Dr Faraday, have been, I imagine, substantially held by a majority of those who have considered the subject since the time of Davy, who first gave them a partial expression. And when Davy and others speak of the electric forces in decomposition as if they emanated from the

3 Ib., Art. 1642.