A History of the Theories of Aether and Electricity (1910) — part 6 of 29

oate electroscope, which indicated by the divergence of its straws that the disks were now electrified — the zinc had, in fact, acquired a positive and the copper a negative electric charge.* Thus the mere contact of two different metals, such as those employed in / the pile, was shown to be sufficient for the production of effects ' undoubtedly electrical in character.

On the basis of this result Volta in the same year (1800) put forward a definite theory of the action of the pile. Suppose first that a disk of zinc is laid on a disk of copper, which in turn rests on an insulating support. The experiment just described shows that the electric fluid will be driven from the copper to the zinc. We may then, according to Volta, represent the state or " tension " of the copper by the number - J, and that of the zinc by the number + J, the difference being arbitrarily taken as unity, and the sum being (on account of the insulation) zero. It will be seen that Volta's idea of " tension " was a mingling of two ideas, which in modern electric theory are clearly distin- guished from each other — namely, electric charge and electric potential.

Now let a disk of moistened pasteboard be laid on the zinc, and a disk of copper on this again. Since the uppermost copper is not in contact with the zinc, the contact-action does not take place between them ; but since the moist pasteboard is a conductor, the copper will receive a charge from the zinc. Thus the states will now be represented by - f for the lower copper, + J for the zinc, and + \ for the upper copper, giving a zero sum as before.

If, now, another zinc disk is placed on the top, the states will be represented by - 1 for the lower copper, 0 for the lower zinc and upper copper, and + 1 for the upper zinc.

In this way it is evident that the difference between the numbers indicating the tensions of the uppermost and lowest

• Abraham Bennet (b. 1750, d. 1799) had previously shown (Xew Experiments in Electricity, 1789, pp. 86-102) that many bodies, when separated after contact, f are oppositely electrified ; he conceived that different bodies have different attrac- tions or capacities for electricity.

74 Galvanism , from Galvani to O/im.

disks in the pile will always be equal to the number of pairs of metallic disks contained in it. If the pile is insulated, the sum of the numbers indicating the states of all the disks must be zero; but if the lowest disk is connected to earth, the tension of this disk will be zero, and the numbers indicating the states of all the other disks will be increased by the same amount, their mutual differences remaining unchanged.

The pile as a whole is thus similar to a Leyden jar ; when the experimenter touches the uppermost and lowest disks, he receives the shock of its discharge, the intensity being proportional to the number of disks.

The moist layers played no part in Volta's theory beyond j. that of conductors.* It was soon found that when the moisture is acidified, the pile is more efficient; but this was attributed solely to the superior conducting power of acids.

Yolta fully understood and explained the impossibility of constructing a pile from disks of metal alone, without making use of moist substances. As he showed in 1801, if disks of various metals are placed in contact in any order, the extreme metals will be in the same state as if they touched each other directly without the intervention of the others ; so that the whole is equivalent merely to a single pair. When the metals are arranged in the order silver, copper, iron, tin, lead, zinc, each of them becomes positive with respect to that which precedes it, and negative with respect to that which follows it ; but the moving force from the silver to the zinc is equal to the sum of the moving forces of the metals comprehended between them in the series.

When a connexion was maintained for some time between the extreme disks of a pile by the human body, sensations were experienced which seemed to indicate a continuous activity in the entire system. Yolta inferred that the electric current persists during the whole time that communication by con-

• Volta had inclined, in his earlier experiments on galvanism, to locate the seat of power at the interfaces of the metals with the rnoist conductors. Cf. his letter to Gren, Phil. Mag. iv (1799), p. 62.

Galvanism, from Gaivani to Ohm. 75

ductors exists all round the circuit, and that the current is suspended only when this communication is interrupted. " This endless circulation or perpetual motion of the electric fluid," he says, "may seem paradoxical, and may prove inexplicable ; but it is none the less real, and we can, so to speak, touch and handle it."

Yolta announced his discovery in a letter to Sir Joseph Banks, dated from Como, March 20th, 1800. Sir Joseph, who was then President of the Eoyal Society, communicated the news to William Nicholson (b. 1753, d. .1815), founder of the Journal which is generally known by his name, and his friend Anthony Carlisle (b. 1768, d. 1840), afterwards a distinguished surgeon. On the 30th of the following month, Nicholson and Carlisle set up the first pile made in England. In repeating Volta's experiments, having made the contact more secure at the upper plate of the pile by placing a drop of water there, they noticed* a disengagement of gas round the con- ducting wire at this point ; whereupon they followed up the matter by introducing a tube of water, into which the wires from the terminals of the pile were plunged. Bubbles of an inflammable gas were liberated at one wire, while the other wire became oxidised ; when platinum wires were used, oxygen and hydrogen were evolved in a free state, one at each wire. This effect, which was nothing less than the electric decom- position of water into its constituent gases, was obtained on May 2nd, 1800.f

Although it had long been known that frictional electricity is capable of inducing chemical action,* the discovery of Nicholson and Carlisle was of the first magnitude. It was at once extended by William Cruickshank, of Woolwich (b. 1745,

i's Journal (4to), iv, 179 (1800) ; Phil. Mag. vii, 337 (1800).

t It was obtained independently four months later l>y J. "W. Hitter.

J Beccaria (Lettere deW elettricismo, Bologna, 1758, p. 282) had reduced mercury and other metals from their oxides by discharges ot fractional electricity ; and Priestley had obtained an inflammable gas from certain organic liquids in the same way. Cavendish in 1781 had established the constitution of water by electrically exploding hydrogen and oxygen.

76 Galvanism > from Galvani to Ohm.

d. 1800), who* showed that solutions of metallic salts are also decomposed by the current; and William Hyde Wollaston (ft. 1766, d. 1828) seized on it as a testf of the identity of the electric currents of Volta with those obtained by the discharge of f rictional electricity. He found that water could be decom-

vy posed by currents of either type, and inferred that all differences between them could be explained by supposing that voltaic electricity as commonly obtained is " less intense, but produced in much, larger quantity." Later in the same year (1801), Martin van Mar um (ft. 1750, d. 1837) and Christian Heinrich Pfaff (ft. 1773, d. 1852) arrived at the same conclusion by carrying out on a large scale} Volta's plan of using the pile to

V charge batteries of Leyden jars.

The discovery of Nicholson and Carlisle made a great impression on the mind of Humphry Davy (ft. 1778, d. 1829), a young Cornishman who about this time was appointed Professor of Chemistry at the E-oyal Institution in London. Davy at once began to experiment vvitli Voltaic piles, and in November, 1800,§ showed that they give no current when the water between the

y pairs of plates is pure, and that their power of action is " in great measure proportional to the power of the conducting fluid substance between the double plates to oxydate the zinc." This result, as he immediately perceived, did not harmonize well with Volta's views on the source of electricity in the pile, but was, on the other hand, in agreement with , Eabroni's idea that galvanic effects are always accompanied by chemical action. After a series of experiments he definitely

1 concluded that " the galvanic pile of Volta acts only when the conducting substance between the plates is capable of oxydating the zinc ; and that, in proportion as a greater quantity of oxygen enters into combination with the zinc in a given time, so in proportion is the power of the pile to decompose water and to give the shock greater. It seems therefore reasonable

• Nicholson's Journal (4to), iv (1800), pp. 187,245: Phil. Mag., vii (1800), p. 337.

t Phil. Mag., 1801, p. 427. J Phil. Mag., xii (1802), p. 161.

§ Nicholson's Journal (4to), iv (1800) ; Davy's Works, ii, p. 155.

Galvanism, from Galvani (o Ohm. 77

to conclude, though with our present quantity of facts we are unable to explain the exact mode of operation, that the </ oxydatioii of the zinc in the pile, and the chemical changes connected with it, are somehow the cause of the electrical effects ^ it produces." This principle of oxidation guided Davy in designing many new types of pile, with elements chosen from the whole range of the known metals.

Davy's chemical theory of the pile was supported by Wollaston* and by Nicholson,f the latter of whom urged that the existence of piles in which only one metal is used (with more than one kind of fluid) is fatal to any theory which places the seat of the activity in the contact of dissimilar metals.

Davy afterwards proposed J a theory of the voltaic pile which combines ideas drawn from both the "contact" and " chemical " explanations. Ho supposed that before the circuit is closed, the copper and zinc disks in each contiguous pair assume opposite electrostatic states, in consequence of inherent "electrical energies" possessed by the metals; and when a > communication is made between the extreme disks by a wire, the opposite electricities annihilate each other, as in the dis- charge of a Leyden jar. If the liquid (which Davy compared to the glass of a Leyden jar) were incapable of decomposition, the current would cease after this discharge. But the liquid in the pile is composed of two elements which have inherent attractions for electrified metallic surfaces : hence arises chemical action, which removes from the disks the outermost layers of molecules, whose energy is exhausted, and exposes new metallic surfaces. The electrical energies of the copper and zinc are consequently again exerted, and the process of electro- motion continues. Thus the contact of metals is the cause which disturbs the equilibrium, while the chemical changes continually restore the conditions under which the contact energy can be exerted.

In this and other memoirs Davy asserted that chemical

Phil. Trans., 1801, p. 427. t Nicholson' Journal, i (1802), p. 142.

; Phil. Trans., 1807, p. 1.

78 Galvanism, from Galvani to Ohm.

J affinity is essentially of an electrical nature. " Chemical and electrical attractions," he declared,* "are produced by the same cause, acting in one case on particles, in the other on masses, of matter; and the same property, under different modifications, is the cause of all the phenomena exhibited by different voltaic combinations."

The further elucidation of this matter came chiefly from

• researches on electro-chemical decomposition, which we must now consider.

A phenomenon which had greatly surprised Nicholson and Carlisle in their early experiments was the appearance of the products of galvanic decomposition at places remote from each other. The first attempt to account for this was made in 1806 by Theodor von Grothussf (b. 1785, d. 1822) and by Davy,} who advanced a theory that the terminals at which water is decomposed have attractive and repellent powers ; that the pole whence resinous electricity issues has the property of attracting hydrogen and the metals, and of repelling oxygen and acid substances, while the positive terminal has the power of attract- ing oxygen and repelling hydrogen ; and that these forces are sufficiently energetic to destroy or suspend the usual operation of chemical affinity in the water-molecules nearest the terminals. The force due to each terminal was supposed to diminish with the distance from the terminal. When the molecule nearest one of the terminals has been decomposed by the attractive and repellent forces of the terminal, one of its constituents is liberated there, while the other constituent, by virtue of electrical forces (the oxygen and hydrogen being in opposite electrical states), attacks the next molecule, which is then decomposed. The surplus constituent from this attacks the next molecule, and so on. Thus a chain of decompositions and recompositions was supposed to be set up among the molecules intervening between the terminals.

• Phil. Trans., 1826, p. 383. f Ann. de Cliim., Iviii (1806), p. 54.

t Bukerian lecture for 1806, Phil. Trans., 1807, p. 1. A theory similar to that of Grothuss and Davy was communicated by Peter Mark Eoget (b. 1779, d. 1869) in 1807 to the Philosophical Society of Manchester : cf. Roget's Galvanism, § 106.

Galvanism^ from ^Galvani to Ohm. 79

The hypothesis of Grothuss and Davy was attacked in 1825 by Aiiguste De La Kive* (6. 1801, d. 1873) of Geneva, on the ground of its failure to explain what happens when different liquids are placed in series in the circuit. If, for example, a solution of zinc sulphate is placed in one compartment, and water in another, and if the positive pole is placed in the solution of zinc sulphate, and the negative pole in the water, De La Rive found that oxide of zinc is developed round the latter; although decomposition and recomposition of zinc sulphate could not take place in the water, which contained none of it. Accordingly, he supposed the constituents of the decomposed liquid to be bodily transported across the liquids, in close union with the moving electricity. In the electrolysis of water, one current of electrified hydrogen was supposed to leave the positive pole, and become decomposed into hydrogen and electricity at the negative pole, the hydrogen being there liberated as a gas. Another current in the same way carried electrified oxygen from the negative to the positive pole. In this scheme the chain of successive decompositions imagined by Grothuss does not take place, the only molecules decomposed being those adjacent to the poles.

The appearance of the products of decomposition at the separate poles could be explained either in Grothuss' fashion by assuming dissociations throughout the mass of liquid, or in De La Rive's by supposing particular dissociated atoms to travel considerable distances. Perhaps a preconceived idea of economy in Nature deterred the workers of that time from accepting the two assumptions together, when either of them separately would meet the case. Yet it is to this apparent redundancy that later researches have pointed as the truth. Nature is what she is, and not what we would make her.

De La Rive was one of the most thoroughgoing opponents of Volta's contact theory of the pile ; even in the case when two metals are in contact in air only, without the intervention

• Annales de Cnimie, xxviii, 190.

80 Galvanism, from Galvani to Ohm.

of any liquid, he attributed the electric effect wholly to the chemical affinity of the air for the metals.

During the long interval between the publication of the rival hypotheses of Grothuss and De La Bive, little real progress was made with the special problems of the cell ; but mean- while electric theory was developing in other directions. One of these, to which our attention will first be turned, was the electro-chemical theory of the celebrated Swedish chemist, Jons Jacob Berzelius (b. 1779, d. 1848).

Berzelius founded his theory,* which had been in one or two of its features anticipated by Davy,f on inferences drawn from Volta's contact effects. " Two bodies," he remarked, " which have affinity for each other, and which have been brought into mutual contact, are found upon separation to be in opposite electrical states. That which has the greatest affinity for oxygen usually becomes positively electrified, and the other negatively."

This seemed to him to indicate that chemical affinity arises from the play of electric forces, which in turn spring from electric charges within the atoms of matter. To be precise, he supposed each atom to possess two poles, which are the seat of opposite electrifications, and whose electrostatic field is the cause of chemical affinity.

By aid of this conception Berzelius drew a simple and vivid picture of chemical combination. Two atoms, which are about to unite, dispose themselves so that the positive pole of one touches the negative pole of the other ; the electricities of these two poles then discharge each other, giving rise to the heat and light which are observed to accompany the act of combination.! The disappearance of these leaves the compound molecule with the two remaining poles ; and it cannot be dissociated into its constituent atoms again until some means is found of restoring to the vanished poles their charges. Such a means is afforded

• Memoirs of the Acad. of Stockholm, 1812 ; Nicholson's Journal of Nat. Phil., xxxiv (1813), 142, 153, 240, 319; xxxv, 38, 118, 159.

t Pnil. Trans., 1807. J This idea was Davy's.

Galvanism, from Gaivani to Ohm. 81

by the action of the galvanic pile in electrolysis : the opposite electricities of the current invade the molecules of the electrolyte, and restore the atoms to their original state of polarization.

If, as Berzelius taught, all chemical compounds are formed by the mutual neutralization of pairs of atoms, it is evident / that they must have a binary character. Thus he conceived a salt to be compounded of an acid and an oxide, and each of these to be compounded of two other constituents. Moreover, in any compound the electropositive member would be replace- able only by another electropositive member, and the electro- negative member only by another member also electronegative ; so that the substitution of, e.g., chlorine for hydrogen in a compound would be impossible — an inference which was overthrown by subsequent discoveries in chemistry.

Berzelius succeeded in bringing the most curiously diverse facts within the scope of his theory. Thus " the combination 1 of polarized atoms requires a motion to turn the opposite poles to each other; and to this circumstance is owing the facility with which combination takes place when one of the two bodies is in the liquid state, or when both are in that state ; and the extreme difficulty, or nearly impossibility, of effecting an union between bodies, both of which are solid. And again, since each polarized particle must have an electric atmosphere, and as this atmosphere is the predisposing cause of combination, as we have seen, it follows, that the particles cannot act but at certain distances, proportioned to the intensity of their polarity ; and hence it is that bodies, which have affinity for each other, always combine nearly on the instant when mixed in the liquid state, but less easily in the gaseous state, and the union ceases to be possible under a certain degree of dilatation of the gases ; as we know by the experiments of Grothuss, that a mixture of oxygen and hydrogen in due proportions, when rarefied to a certain degree, cannot be set on fire at any temperature whatever." j And again : " Many bodies require an elevation of temperature to

G

82 Galvanism, from Galvani to Ohm.

enable them to act upon each other. It appears, therefore, that heat possesses the property of augmenting the polarity of these bodies."

Berzelius accounted for Volta's electromotive series by assuming the electrification at one pole of an atom to be some- what more or somewhat less than what would be required to neutralize the charge at the other pole. Thus each atom would possess a certain net or residual charge, which might be of either sign ; and the order of the elements in Volta's series could be interpreted simply as the order in which they would stand when ranged according to the magnitude of this residual charge. As we shall see, this conception was afterwards overthrown by Faraday.

Berzelius permitted himself to publish some speculations on the nature of heat and electricity, which bring vividly before us the outlook of an able thinker in the first quarter of the nineteenth century. The great question, he says, is whether v the electricities and caloric are matter or merely phenomena. If the title of matter is to be granted only to such things as are ponderable, then these problematic entities are certainly not matter ; but thus to narrow the application of the term is, he believes, a mistake; and he inclines to the opinion that caloric is truly matter, possessing chemical affinities without obeying the law of gravitation, and that light and all radiations consist in modes of propagating such matter. This conclusion makes it easier to decide regarding electricity. " From the relation which exists between caloric and the electricities," he remarks, "it is clear that what may be true with regard to the materiality of one of them must also be true with regard to that of the other. There are, however, a quantity of phenomena produced by electricity which do not admit of explanation without admitting at the same time that electricity is matter. Electricity, for instance, very often detaches everything which covers the surface of those bodies which conduct it. It, indeed, passes through conductors without leaving any trace of its passage ; but it penetrates non-con-

Galvanism i from Galvani to Ohm. 83

ductors which oppose its course, and makes a perforation precisely of the same description as would have been made by something which had need of place for its passage. We often observe this when electric jars are broken by an over- charge, or when the electric shock is passed through a number of cards, etc. We may therefore, at least with some proba- bility, imagine caloric and the electricities to be matter, destitute of gravitation, but possessing affinity to gravitating bodies. When they are not confined by these affinities, they tend to place themselves in equilibrium in the universe. The ^ suns destroy at every moment this equilibrium, and they send the re-united electricities in the form of luminous rays towards the planetary bodies, upon the surface of which the rays, being arrested, manifest themselves as caloric ; and this last in its turn, during the time required to replace it in equilibrium in the universe, supports the chemical activity of organic and inorganic nature."

It was scarcely to be expected that anything so speculative as Berzelius' electric conception of chemical combination would be confirmed in all particulars by subsequent discovery ; and, as a matter of fact, it did not as a coherent theory survive the lifetime of its author. But some of its ideas have persisted, and among them the conviction which lies at its foundation, that chemical affinities are, in the last resort, of * electrical origin.

While the attention of chemists was for long directed to the theory of Berzelius, the interest of electricians was diverted from it by a discovery of the first magnitude in a different region.

That a relation of some land subsists between electricity and magnetism had been suspected by the philosophers of the eighteenth century. The suspicion was based in part on some curious effects produced by lightning, of a kind which may be illustrated by a paper published in the Philosophical Transactions in 1735.* A tradesman of Wakefield, we are told, "having put

*Phil. Trans, xxxix (1735), p. 74. G 2

84 Galvanism, from Galvani to Ohm.

up a great number of knives and forks in a large box, and having placed the box in the corner of a large room, there happen'd in July, 1731, a sudden storm of thunder, lightning, etc., by which the corner of the room was damaged, the Box split, and a good many knives and forks melted, the sheaths being untouched. The owner emptying the box upon a Counter where some Nails lay, the Persons who took up the knives, that lay upon the Nails, observed that the knives took up the Nails."

Lightning thus came to be credited with the power of magnetizing steel ; and it was doubtless this which led Franklin* in 1751 to attempt to magnetize a sewing-needle by means of the discharge of Leyden jars. The attempt was indeed success- ful ; but, as Van Marum afterwards showed, it was doubtful whether the magnetism was due directly to the current.

More experiments followed. f In 1805 Jean Nicholas Pierre Hachette (b. 1769, d. 1834) and Charles Bernard Desormes (b. 1777, d. 1862) attempted to determine whether an insulated voltaic pile, freely suspended, is oriented by terrestrial mag- netism ; bat without positive result. In 1807 Hans Christian Oersted (&. 1777, d. 1851), Professor of Natural Philosophy in Copenhagen, announced his intention of examining the action of electricity on the magnetic needle ; but it was not for some years that his hopes were realized. If one of his pupils is to be believed,* he was " a man of genius, but a very unhappy experi- menter ; he could not manipulate instruments. He must always have an assistant, or one of his auditors who had easy hands, .to arrange the experiment."

During a course of lectures which he delivered in the winter of 1819-20 on " Electricity, Galvanism, and Magnetism," the idea occurred to him that the changes observed with the compass-needle during a thunderstorm might give the clue to the effect of which he was in search ; and this led him to think that the experiment should be tried with the galvanic circuit

• Letter vi from Franklin to Collinson. f In 1774 the Electoral Academy

of Bavaria proposed the question, " Is there a real and physical analogy between electric and magnetic forces ? " as the subject of a prize.

1 Cf. a letter from Hansteen inserted inBence Jones' Life of Faraday y ii, p. 395.

Galvanism, from Galvani to Ohm. 85

closed instead of open, and to inquire whether any effect is produced on a magnetic needle when an electric current is passed through a neighbouring wire. At first he placed the wire at right angles to the needle, but observed no result. After the end of a lecture in which this negative experiment had been shown, the idea occurred to him to place the wire parallel to the needle : on trying it, a pronounced deflexion was observed, and the relation between magnetism and the electric current was discovered. After confirmatory experiments with more powerful apparatus, the public announcement was made in July, 1820 *

Oersted did not determine the quantitative laws of the ;action, but contented himself with a statement of the qualita- tive effect and some remarks on its cause, which recall the magnetic speculations of Descartes : indeed, Oersted's concep- tions may be regarded as linking those of the Cartesian school to those which were introduced subsequently by Faraday. " To the effect which takes place in the conductor and in the sur- rounding space," he wrote, " we shall give the name of the -conflict of electricity? " The electric conflict acts only on the magnetic particles of matter. All non-magnetic bodies appear penetrable by the electric conflict, while magnetic bodies, or rather their magnetic particles, resist the passage of this conflict Hence they can be moved by the impetus of the contending powers.

" It is sufficiently evident from the preceding facts that the •electric conflict is not confined to the conductor, but dispersed pretty widely in the circumjacent space.

" From the preceding facts we may likewise collect, that this conflict performs circles ; for without this condition, it seems impossible that the one part of the uniting wire, when placed below the magnetic pole, should drive it toward the east, and when placed above it toward the west; for it is the nature of a

• Schweigger's Journal fur Chemie und Physik, zxix (1820), p. 275 ; Thomson's Annals of Philosophy, xvi (1820), p. 273; Ostwald's Klattiter der ' Wi.ssenseha.ften, Nr. 63.

86 Galvanism, from Galvani to Ohm.

circle that the motions in opposite parts should have an opposite1 direction."

Oersted's discovery was described at the meeting of the French Academy on September llth, 1820, by an academician (Arago) who had just returned from abroad. Several investi- gators in France repeated and extended his experiments ; and the first precise analysis of the effect was published by two of these, Jean-Baptiste Biot (b. 1774, d. 1862) and Felix Savart (b. 1791, d. 1841), who, at a meeting of the Academy of Sciences on October 30th, 1820, announced* that the action experienced by a pole of austral or boreal magnetism, when placed at any distance from a straight wire carrying a voltaic current, may be

4 thus expressed : " Draw from the pole a perpiendicular to the wire ; the force on the pole is at right angles to this line and ta the wire, and its intensity is proportional to the reciprocal of the distance." This result was soon further analysed, the attractive force being divided into constituents, each of which was supposed to be due to some particular element of the current ; in its new form the law may be stated thus : the- magnetic force due to an element ds of a circuit, in which a current i is flowing, at a point whose vector distance from ds is r,, is (in suitable units)

i ids

— |ds,r|t or curl — .+ r3 J r

It was now recognized that a magnetic field may be produced as readily by an electric current as by a magnet ; and, as Arago soon showed,§ this, like any other magnetic field, is capable of

• Annales de Chimie, xv (1820), p. 222 ; Journal de Phys., xli, p. 51.

f If a and b denote two vectors, the vector whose components are (aybz — azby^ azb* — a*bz, axby — aybx) is called the vector product of a and b, and is denoted by [a, b]. Its direction is at right angles to those of a and b, and its magnitude is represented by twice the area of the triangle formed by them.

• If a denotes any vector, the vector whose components are ^-z - -^, -—• - ^-*r

3% 9a* • -,

z-l - -— is denoted by curl a.

fo ty

§ Annales de Chimie, xv (1820), p. 93.

Galvanism, from Galvani to Ohm. 87

inducing magnetization in iron. The question naturally sug- gested itself as to whether the similarity of properties between currents and magnets extended still further, e.g. whether conductors carrying currents would, like magnets, experience ponderomotive forces when placed in a magnetic field, and whether such conductors would consequently, like magnets, exert ponderomotive forces on each other.

The first step towards answering these inquiries was taken by Oersted* himself. " As," he said, " a body cannot put another in motion without being moved in its turn, when it possesses the requisite mobility, it is easy to foresee that the galvanic arc must be moved by the magnet " ; and this he verified experimentally.

The next step came from Andre Marie Ampere (b. 1775, d. 1836), who at the meeting of the Academy on September 18th, exactly a week after the news of Oersted's first discovery had arrived, showed that two parallel wires carrying currents attract each other if the currents are in the same direction, and repel each other if the currents are in opposite directions. During the next three years Ampere continued to prosecute the researches thus inaugurated, and in 1825 published his collected results in one of the most celebrated memoirsf in the history of natural philosophy.

Ampere introduces his work by proclaiming himself a follower of that school which explained all physical phenomena in terms of equal and oppositely directed forces between pairs of particles ; and he renounces the attempt to seek more speculative, though possibly more fundamental, explanations in terms of the motions of ultimate fluids and aethers. Never- theless, he indicates two conceptions of this latter character, on which such explanations might be founded.

In the firstj he suggests that the ponderomotive forces

• Schweigger's Journal fur Chem. u. Phys., xxix (1820), p. 364 ; Thomson's Annals of Philosophy, xvi (1820), p. 375. t Mem. de 1'Acad., vi, p. 175.

% facueil tF observations electro- dynamiques, p. 215 ; and the memoir just cited, pp. 285, 370.

88 Galvanism, from Galvani to Ohm.

between circuits carrying electric currents may be due to " the reaction of the elastic fluid which extends throughout all space, whose vibrations produce the phenomena of light," and which is " put in motion by electric currents." This fluid or aether can, he says, " be no other than that which results from the combination of the two electricities/'

In the second conception,* Ampere suggests that the interspaces between the metallic molecules of a wire which carries a current may be occupied by a fluid composed of the two electricities, not in the proportions which form the neutral fluid, but with an excess of that one of them which is opposite to the electricity peculiar to the molecules of the metal, and which consequently masks this latter electricity. In this inter- molecular fluid the opposite electricities are continually being dissociated and recombined ; a dissociation of the fluid within one inter-molecular interval having taken place, the positive electricity thus produced unites with the negative electricity of the interval next to it in the direction of the current, while the negative electricity of the first interval unites with the positive electricity of the next interval in the other direction. Such interchanges, according to this hypothesis, constitute the electric current.

Ampere's memoir is, however, but little occupied with the more speculative side of the subject. His first aim was to investigate thoroughly by experiment the ponderomotive forces on electric currents.