A History of the Theories of Aether and Electricity (1910) — part 5 of 29

Coulomb developed this idea : " Whatever be the cause of electricity," he says,J " we can explain all the phenomena by

• Coulomb's First, Second, and Third Memoirs appear in Memoires de 1'Acad., 1785 ; the Fourth in 1786, the Fifth in 1787, the Sixth in 1788, and the Seventh in 1789.

t Phil. Trim*, li (1759), p. 371. j Sixth Memoir, p. 561.

prior to the Introduction of the Potentials. 57

supposing that there are two electric fluids, the parts of the same fluid repelling each other according to the inverse square of the distance, and attracting the parts of the other fluid according to the same inverse square law." " The supposition ^ of two fluids," he adds, " is moreover in accord with all those 7 discoveries of modern chemists and physicists, which have made known to us various pairs of gases whose elasticity is destroyed by their admixture in certain proportions — an effect which could not take place without something equivalent to a repulsion between the parts of the same gas, which is the cause of its elasticity, and an attraction between the parts of different gases, which accounts for the loss of elasticity on combination." J

According, then, to the two-fluid theory, the " natural fluid " contained in all matter can be decomposed, under the influence of an electric field, into equal quantities of vitreous and resinous electricity, which, if the matter be conducting, can then fly to the surface of the body. The abeyance of the characteristic properties of the opposite electricities when in combination was f sometimes further compared to the neutrality manifested by . the compound of an acid and an alkali.

The publication of Coulomb's views led to some controversy between the partisans of the one-fluid and two-fluid theories ; the latter was soon generally adopted in France, but was stoutly opposed in Holland by Van Marum and in Italy by Volta. The chief difference between the rival hypotheses is that, in the ^ two-fluid theory, both the electric fluids are movable within the substance of a solid conductor ; while in the one-fluid theory the actual electric fluid is mobile, but the particles of the conductor are fixed. The dispute could therefore be settled only by a deter- mination of the actual motion of electricity in discharges ; and this was beyond the reach of experiment.

In his Fourth Memoir Coulomb showed that electricity in equilibrium is confined to the surface of conductors, and does not penetrate to their interior substance ; and in the Sixth Memoir* he virtually establishes the result that the electric

• Page 677.

58 Electric and Magnetic Science

force near a conductor is proportional to the surface-density of electrification.

Since the overthrow of the doctrine of electric effluvia by Aepinus, the aim of electricians had been to establish their science upon the foundation of a law of action at a distance, resembling that which had led to such triumphs in Celestial Mechanics. When the law first stated by Priestley was at length decisively established by Coulomb, its simplicity and beauty gave rise to a general feeling of complete trust in it as the best attainable conception of electrostatic phenomena. The result was that attention was almost exclusively focused on action-at-a-distance theories, until the time, long afterwards,, when Faraday led natural philosophers back to the right' path.

Coulomb rendered great services to magnetic theory. It was he who in 1777, by simple mechanical reasoning, completed the overthrow of the hypothesis of vortices.* He also, in the second of the Memoirs already quoted,f confirmed Michell's law, according to which the particles of the magnetic fluids attract or repel each other with forces proportional to the inverse square of the distance. Coulomb, however, went beyond this, and endeavoured to account for the fact that the two magnetic fluids, unlike the two electric fluids, cannot be obtained separately; for when a magnet is broken into two pieces, one containing its north and the other its south pole, it is found that each piece is an independent magnet possessing two poles of its own, so that it is impossible to obtain a north or south pole in a state of isolation. Coulomb explained this by supposing^ that the mag- netic fluids are permanently imprisoned within the molecules of magnetic bodies, so as to be incapable of crossing from one molecule to the next ; each molecule therefore under all circumstances contains as much of the boreal as of the

• Mem. presences par divers Savans, ix (1780), p. 165.

t Mem de 1'Acad., 1785, p. 593. Gauss finally established the law by a much more refined method.

J In his Seventh Memoir, Mem, de 1'Acad., 1789, p. 488.

prior to the Introduction of the Potentials. 59

austral fluid, and magnetization consists simply in a separation of the two fluids to opposite ends of each molecule. Such a hypothesis evidently accounts for the impossibility of separating the two fluids to opposite ends of a body of finite size. The same idea, here introduced for the first time, has since been applied with success in other departments of electrical philosophy.

In spite of the advances which have been recounted, the mathematical development of electric and magnetic theory was scarcely begun at the close of the eighteenth century ; and many erroneous notions were still widely entertained. In a Eeport* which was presented to the French Academy in 1800, it was assumed that the mutual repulsion of the particles of electricity on the surface of a body is balanced by the resistance of the surrounding air; and for long afterwards the electric force outside a charged conductor was confused with a supposed additional pressure in the atmosphere.

Electrostatical theory was, however, suddenly advanced to quite a mature state of development by Simeon Denis Poisson (b. 1781, d. 1840), in a memoir which was read to the French Academy in 1812.f As the opening sentences show, he accepted the conceptions of the two-fluid theory.

" The theory of electricity which is most generally accepted," he says, " is that which attributes the phenomena to two different fluids, which are contained in all material bodies. It is supposed that molecules of the same fluid repel each other and attract the molecules of the other fluid ; these forces of attraction and repulsion obey the law of the inverse square of the distance ; and at the same distance the attractive power is equal to the repellent power; whence it follows that, when all the parts of a body contain equal quantities of the two fluids, the latter do not exert any influence on the fluids contained in neighbouring bodies, and consequently no electrical effects are discernible. This equal and uniform

• On Yolla's discoveries.

t Mem. de Plnstitut, 1811, Part i., p. 1, Part ii., p. 163.

60 Electric and Magnetic Science

distribution of the two fluids is called the natural state ; when this state is disturbed in any body, the body is said to be electrified, and the various phenomena of electricity begin to take place.

"Material bodies do not all behave in the same way with respect to the electric fluid : some, such as the metals, do not appear to exert any influence on it, but permit it to move about freely in their substance ; for this reason they are called conductors. Others, on the contrary — very dry air, for example — oppose the passage of the electric fluid in their interior, so that they can prevent the fluid accumulated in conductors from being dissipated throughout space."

When an excess of one of the electric fluids is communi- cated to a metallic body, this charge distributes itself over the surface of the body, forming a layer whose thickness at any point depends on the shape of the surface. The resultant force due to the repulsion of all the particles of this surface-layer must vanish at any point in the interior of the conductor, since otherwise the natural state existing there would be disturbed ; and Poisson showed that by aid of this principle it is possible in certain cases to determine the distribution of electricity in the surface-layer. For example, a well-known proposition of the theory of Attractions asserts that a hollow shell whose bounding surfaces are two similar and similarly situated ellipsoids exercises 110 attractive force at any point within the interior hollow; and it may thence be inferred that, if an electrified metallic conductor has the form of an ellipsoid, the charge will be distributed on it proportionally to the normal distance from the surface to an adjacent similar and similarly situated ellipsoid.

Poisson went on to show that this result was by no means all

• that might with advantage be borrowed from the theory of

I Attractions. Lagrange, in a memoir on the motion of gravitating

bodies, had shown* that the components of the attractive force

• Mem. de Berlin, 1777. The theorem was afterwards published, and ascribed to Laplace, in a memoir by Legendre on the Attractions of Spheroids, which will be found in the Mem. par divers Snvanx, published in 178o.

prior to the Introduction of the Potentials. 61

at any point can be simply expressed as the derivates of the function which is obtained by adding together the masses of all the particles of an attracting system, each divided by its distance from the point; and Laplace had shown* that this function V satisfies the equation

in space free from attracting matter. Poisson himself showed later, in 1813,f that when the point (z, y, z) is within the substance of the attracting body, this equation of Laplace must be replaced by

W VV VV

^ + w~~vr: p>

where p denotes the density of the attracting matter at the point. In the present memoir Poisson called attention to the utility of this function F in electrical investigations, remarking that its value over the surface of any conductor must be constant.

The known formulae for the attractions of spheroids show that when a charged conductor is spheroidal, the repellent force acting on a small charged body immediately outside it will be directed at right angles to the surface of the spheroid, and will be proportional to the thickness of the surface-layer of electricity at this place. Poisson suspected that this theorem might be true for conductors not having the spheroidal form — a result which, as we have seen, had been already virtually given by Coulomb ; and Laplace suggested to Poisson the following proof, applicable to the general case. The force at a point immediately outside the conductor can be divided into a part s due to the part of the charged surface immediately adjacent to the point, and a part S due to the rest of the surface. At a point close to this, but just inside the con- ductor, the force j^jpll still act; but the forces will evidently

• Mem. de 1'Acad., 1782 (published in 1785), p. 113. t Bull, de la Soc. Philomathique. iii. (1813,, p. 388.

62 Electric and Magnetic Science

be reversed in direction. Since the resultant force at the latter point vanishes, we must have S=s ; so the resultant force at the exterior point is 2s. But s is proportional to the charge per unit area of the surface, as is seen by considering the case of an infinite plate ; which establishes the theorem.

When several conductors are in presence of each other, the distribution of electricity on their surfaces may be determined by the principle, which Poisson took as the basis of his work, that at any point in the interior of any one of the conductors, the resultant force due to all the surf ace -layers must be zero. He discussed, in particular, one of the classical problems of electrostatics — namely, that of determining the surface-density on two charged conducting spheres placed at any distance from each other. The solution depends on Double Gamma Functions in the general case ; when the two spheres are in contact, it depends on ordinary Gamma Functions. Poisson gave a solution in terms of definite integrals, which is equivalent to that in terms of Gamma Functions ; and after reducing his results to numbers, compared them with Coulomb's experiments. f The rapidity with which in a single memoir Poisson passed from the barest elements of the subject to such recondite problems as those just mentioned may well excite admiration. His success is, no doubt, partly explained by the high state of development to which analysis had been advanced by the great mathematicians of the eighteenth century ; but even after allowance has been made for what is due to his predecessors, Poisson' s investigation must be accounted a splendid memorial uof his genius.

Some years later Poisson turned his attention to magnetism ; and, in a masterly paper* presented to the French Academy in 1824, gave a remarkably complete theory of the subject.

His starting-point is Coulomb's doctrine of two imponderable magnetic fluids, arising from the decomposition of a neutral fluid, and confined in their movements to the individual elements

• Mem. <le 1'Acad., v, p. 247.

prior to the Introduction of the Potentials. 63

of the magnetic body, so as to be incapable of passing from one element to the next

Suppose that an amount m of the positive magnetic fluid is located at a point (x y, z) ; the components of the magnetic intensity, or force exerted on unit magnetic pole, at a point (£, »f, £) will evidently be

-m-f-X -m~(-\ -m-(-)

where r denotes ((? - xf + (n - ?/)2 + (Z - z)2j*. Hence if we consider next a magnetic element in which equal quantities of the two magnetic fluids are displaced from each other parallel to_ the ic-axis, the components of the magnetic intensity at (g, i|, 2) will be the negative derivates, with respect to £ ij, £ respectively, of the function

where the quantity A, which does not involve (f, »j, £), may be called the magnetic moment of the element : it may be measured by the couple required to maintain the element in equilibrium at a definite angular distance from the magnetic meridian.

If the displacement of the two fluids from each other in the element is not parallel to the axis of xt it is easily seen that the expression corresponding to the last is

where the vector (A, B, C) now denotes the magnetic moment of the element.

Thus the magnetic intensity at an -external point (£, 77, £) due to any magnetic body has the components

«; - 017 where

ex oy integrated throughout the substance of the magnetic body, and

64 Electric and Magnetic Science

where the vector (A, B, C) or I represents the magnetic moment per unit- volume, or, as it is generally called, the magnetization. The function Fwas afterwards named by Green the magnetic potential.

Poisson, by integrating by parts the preceding expression for the magnetic potential, obtained it in the form

F = [[(I . dS). \ - fjp div I dx dy dz*

the first integral being taken over the surface $ of the magnetic body, and the second integral being taken throughout its volume. This formula shows that the magnetic intensity produced by the body in external space is the same as would be produced by a fictitious distribution of magnetic fluid, consisting of a layer over its surface, of surface-charge (I .- dS) per element dSy together with a volume-distribution of density - div I through- out its substance. These fictitious magnetizations are generally known as Poisson's equivalent surface- and volume-distributions of magnetism.

Poisson, moreover, perceived that at a point in a very small cavity excavated within the magnetic body, the magnetic potential has a limiting value which is independent of the shape of the cavity as the dimensions of the cavity tend to zero ; but that this is not true of the magnetic intensity, which in such a small cavity depends on the shape of the cavity. Taking the cavity to be spherical, he showed that the magnetic intensity within it is

grad F 4 ^-7rl,f where I denotes the magnetization at the place.

• If the components of a vector a are denoted by (ax, ay, az), the quantity drbjc + ayby -f- atkz is called the scalar product of two vectors a and b, and is denoted by (a . b).

The quantity ^— ' + ^ + ^ is called the divergence of the vector a, and is

fix dy 02

denoted by div a.

t The vector whose components are - — , - •?—, - -„— is denoted by grad V.

C£ dy dz J °

prior to the Introduction of the Potentials. 65

This memoir also contains a discussion of the magnetism temporarily induced in soft iron and other magnetizable metals by the approach of a permanent magnet. Poisson accounted for the properties of temporary magnets by assuming that they contain embedded in their substance a great number of small spheres, which are perfect conductors for the magnetic fluids ; so that the resultant magnetic intensity in the interior of one of these small spheres must be zero. He showed that such a sphere, when placed in a field of magnetic intensity F,* must acquire a

magnetic moment of amount -.- F x the volume of the sphere,

in order to counteract within the sphere the force F. Thus if kp denote the total volume of these spheres contained within a unit volume of the temporary magnet, the magnetization will be I, where 4-TrI = kp F,

and F denotes the magnetic intensity within a spherical cavity excavated in the body. This is Poisson s laiv of induced magnetism.

It is known that some substances acquire a greater degree of temporary magnetization than others when placed in the same circumstances : Poisson accounted for this by supposing that the quantity kp varies from one substance to another. But the experimental data show that for soft iron kp must have a value very near unity, which would obviously be impossible if kp is to mean the ratio of the volume of spheres contained within a region to the total volume of the region.f The physical inter- pretation assigned by Poisson to his formulae must therefore be rejected, although the formulae themselves retain their value.

Poisson's electrical and magiietical investigations were generalized and extended in 1828 by George Green* (b. 1793, d. 1841). Green's treatment is based on the properties of the function already used by Lagrange, Laplace, and Poisson, which

• In the present work, vectors will generally be distinguished by heavy type.

t This objection was advanced by Maxwell in § 430 of his Treatise. An attempt to overcome it was made by Betti : cf. p. 377 of his Lessons on the Potential.

J A.n essay on the application of mathematical analysis to the theories of electricity and magnetism, Nottingham, 1828 : reprinted in The Mathematical Papers of the late George Green, p. 1.

F

66 Electric and Magnetic Science.

represents the sum of all the electric or magnetic charges in the field, divided by their respective distances from some given point : to this function Green gave the name potential, by which it has always since been known.*

Near the beginning of the memoir is established the celebrated formula connecting surface and volume integrals, which is now generally called G-reeris Theorem, and of which Poisson's result on the equivalent surface- and volume-distribu- tions of magnetization is a particular application. By using this theorem to investigate the properties of the potential, Green arrived at many results of remarkable beauty and interest. We need only mention, as an example of the power of his method, the following : — Suppose that there is a hollow conducting shell, bounded by two closed surfaces, and that a number of electrified bodies are placed, some within and some without it ; and let the inner surface and interior bodies be called the interior system, and the outer surface and exterior botlies be called the exterior system. Then all the electrical phenomena of the interior system, relative to attractions, repulsions, and densities, will be the same as if there were no exterior system, and the inner surface were a perfect conductor, put in communication with the earth ; and all those of the exterior system will be the same as if the interior system did not exist, and the outer surface were a perfect conductor, containing a quantity of electricity equal to the whole of that originally contained in the shell itself and in all the interior bodies.

It will be evident that electrostatics had by this time attained a state of development in which further progress could be hoped for only in the mathematical superstructure, unless experiment should unexpectedly bring to light phenomena of an entirely new character. This will therefore be a convenient place to pause and consider the rise of another branch of electrical philosophy.

• Euler in 1744 (De melhodis inveniendi . . .) had spoken of the vis potentialis — what would now be called the potential energy — possessed by an elastic body when bent.

CHAPTEE III.

GALVANISM, FROM GALVANI TO OHM.

UNTIL the last decade of the eighteenth century, electricians were occupied solely with statical electricity. Their attention was then turned in a different direction.

In a work entitled Recherches sur Vorigine des sentiments agreables et cUsagr cables, which was published* in 1752, Johann Georg Sulzer (b. 1720, d. 1779) had mentioned that, if two pieces of metal, the one of lead and the other of silver, be joined together in such a manner that their edges touch, and if they be placed on the tongue, a taste is perceived " similar to that of vitriol of iron," although neither of these metals applied separately gives any trace of such a taste. " It is not probable," he says, " that this contact of the two metals causes a solution of either of them, liberating particles which might affect the tongue : and we must therefore conclude that the contact sets up a vibration in their particles, which, by affecting the nerves of the tongue, produces the taste in question."

This observation was not suspected to have any connexion with electrical phenomena, and it played no part in the incep- tion of the next discovery, which indeed was suggested by a mere accident.

Luigi Galvani, born at Bologna in 1737, occupied from 1775 onwards a chair of Anatomy in his native city. For many years before the event which made him famous he had been studying the susceptibility of -the nerves to irritation ; and, having been <- formerly a pupil of Beccaria, he was also interested in electrical experiments. One day in the latter part of the year 1780 he ' had, as he tells us,f " dissected and prepared a frog, and laid it on a table, on which, at some distance from the frog, was an electric machine. It happened by chance that one of my

• Mem. de 1'Acad. de Berlin, 1752, p. 356.

t Aloysii Galvani, De Viribus E 'lee trie itatis in Motu Mnsculari : Commentarii Bononiensi, vii (1791), p. 363.

F 2

68 Galvanism, from Galvani to Ohm.

assistants touched the inner crural nerve of the frog with the point of a scalpel ; whereupon at once the muscles of the limbs were violently convulsed.

" Another of those who used to help me in electrical experi- ments thought he had noticed that at this instant a spark was drawn from the conductor of the machine. I myself was at the time occupied with a totally different matter; but when he drew my attention to this, I greatly desired to try it for myself,. and discover its hidden principle. So I, too, touched one or other of the crural nerves with the point of the scalpel, at the same time that one of those present drew a spark ; and the same phenomenon was repeated exactly as before."*

After this, Galvani conceived the idea of trying whether the electricity of thunderstorms would induce muscular contractions equally well with the electricity of the machine. Having successfully experimented with lightning, he " wished," as he writes,! " to try the effect of atmospheric electricity in calm weather. My reason for this was an observation I had made,, that frogs which had been suitably prepared for these experi- ments and fastened, by brass hooks in the spinal marrow, to the iron lattice round a certain hanging-garden at my house,, exhibited convulsions not only during thunderstorms, but sometimes even when the sky was quite serene. I suspected these effects to be due to the changes which take place during the day in the electric state of the atmosphere ; and so, with some degree of confidence, I performed experiments to test the point; and at different hours for many days I watched frogs which I had disposed for the purpose ; but could not detect any motion in their muscles. At length, weary of waiting in vain, I pressed the brass hooks, which were driven into the spinal marrow, against the iron lattice, in order to see whether contractions could be excited by varying the incidental circum-

• According to a story which has often been repeated, but which rests on no sufficient evidence, the frog was one of a number which had been procured for th& Signora Galvani, who, being in poor health, had been recommended to take a soup, made of these animals as a restorative. f Loc. cit., p. 377.

Galvanism, from Galvani to Ohm. 69

stances of the experiment. I observed contractions tolerably often, but they did not seem to bear any relation to the changes in the electrical state of the atmosphere.

" However, at this time, when as yet I had not tried the experiment except in the open air, I came very near to adopt- ing a theory that the contractions are due to atmospheric electricity, which, having slowly entered the animal and accu- mulated in it, is suddenly discharged when the hook comes in contact with the iron lattice. For it is easy in experimenting to deceive ourselves, and to imagine we see the things we wish to see.

" But I took the animal into a closed room, and placed it on an iron- plate ; and when I pressed the hook which was fixed in the spinal marrow against the plate, behold ! the same spasmodic contractions as before. I tried other metals at different hours on various days, in several places, and always with the same result, except that the contractions were more violent with some metals than with others. After this I tried various bodies which are not conductors of electricity, such as glass, gums, resins, stones, and dry wood ; but nothing happened. This was somewhat surprising, and led me to suspect that electricity is inherent in the animal itself. This suspicion was strengthened by the observation that a kind of circuit of subtle nervous fluid (resembling the electric circuit which is manifested in the Leyclen jar experiment) is completed from the nerves to the muscles when the contractions are produced.

" For, while I with one hand held the prepared frog by the hook fixed in its spinal marrow, so that it stood with its feet on a silver box, and with the other hand touched the lid of the box, or its sides, with any metallic body, I was surprised to see the frog become strongly convulsed every time that I applied this artifice."*

Galvani thus ascertained that the limbs of the frog are con- vulsed whenever a connexion is made between the nerves and muscles by a metallic arc, generally formed of more than one

*This observation was made in 1786.

70 Galvanism > from Galvani to Ohm.

kind of metal ; and he advanced the hypothesis that the convul- sions are caused by the transport of a peculiar fluid from the

' nerves to the muscles, the arc acting as a conductor. To this fluid the names Galvanism and .Animal Electricity were soon generally applied. Galvani himself considered it to be the same as the ordinary electric fluid, and, indeed, regarded the entire phenomenon as similar to the discharge of a Leyden jar.

' The publication of Gralvani's views soon engaged the attention of the learned world, and gave rise to an animated controversy between those who supported Galvani's own view, those who believed galvanism to be a fluid distinct from ordinary electricity, and a third school who altogether refused to attribute the effects to a supposed fluid contained in the nervous system. The leader of the last-named party was Alessandro Volta (b. 1745, d. 1827), Professor of Natural Philosophy in the University of Pavia, who in 1792 put forward the view that the stimulus in Galvani's experiment is derived essentially from the connexion of two different metals by a moist body. "The metals used in the

• experiments, being applied to the moist bodies of animals, can by themselves, and of their proper virtue, excite and dislodge the electric fluid from its state of rest ; so that the organs of the

• animal act only passively." At first he inclined to combine this theory of metallic stimulus with a certain degree of belief in such a fluid as Galvani had supposed; but after the end of 17!. '3 he denied the existence of animal electricity altogether.

From this standpoint Volta continued his experiments and worked out his theory. The following quotation from a lettert which he wrote later to Gren, the editor of the Neucs Journal //. Physik, sets forth his view in a more developed form : —

"The contact of different conductors, particularly the metallic, including pyrites and other minerals, as well as charcoal, which I call dry conductors, or of the first class, with moist conductors, or conductors of the second class, agitates or disturbs the electric

f fluid, or gives it a certain impulse. Do not ask in what manner : it is enough that it is a principle, and a general principle. This

*Phil. Trans., 1793, pp. 10, 27. tPhil. Mag. iv (1799), pp. 59, 163, 306.

Galvanism , from Galvani to Okm. 71

impulse, whether produced by attraction or any other force, is different or unlike, both in regard to the different metals and to the different moist conductors ; so that the direction, or at least the power, with which the electric fluid is impelled or excited, is different when the conductor A is applied to the conductor B, or to another C. In a perfect circle of conductors, where either one of the second class is placed between two different from each other of the first class, or, contrariwise, one of the first class is placed between two of the second class different from each other, an electric stream is occasioned by the predominating force either to the right or to the left — a circulation of this fluid, which ceases only when the circle is broken, and which is renewed when the circle is again rendered complete."

Another philosopher who, like Volta, denied the existence of a fluid peculiar to animals, but who took a somewhat different view of the origin of the phenomenon, was Giovanni Fabroni, of Florence (b. 1752, d. 1822), who,* having placed two plates of different metals in water, observed that one of them was partially oxidized when they were put in contact ; from which he rightly concluded that some chemical action is inseparably connected with galvanic effects.

The feeble intensity of the phenomena of galvanism, which compared poorly with the striking displays obtained in electro- statics, was responsible for some falling off of interest in them towards the end of the eighteenth century ; and the last years of their illustrious discoverer were clouded by misfortune. Being attached to the old order which was overthrown by the armies of the French Ke volution, he refused in 1798 to take the oath of allegiance to the newly constituted Cisalpine Eepublic, and was deposed from his professorial chair. A profound melancholy, which had been induced by domestic bereavement, was aggra- vated by poverty and disgrace ; and, unable to survive the loss of all he held dear, he died broken-hearted before the end of the year.f

• Phil. Journal, 4to, iii. 308 ; iv. 120 ; Journal de Physique, vi. 348. t A decree of reinstatement had been granted, but had not come into operation at the time of Galvani's death.

<

72 Galvanism, Jrom Galvani to O/it/i.

Scarcely more than a year after the death of Galvani, the new science suddenly regained ' the eager attention of philo- sophers. This renewal of interest was due to the discovery by Volta, in the early spring of 1800, of a means of greatly increasing the intensity of the effects. Hitherto all attempts to magnify the action by enlarging or multiplying the apparatus had ended in failure. If a long chain of different metals was used instead of only two, the convulsions of the frog were no more violent. But Volta now showed* that if any number of couples, each consisting of a zinc disk and a copper disk in contact, were taken, and if each couple was separated from the next by a disk of moist- ened pasteboard (so that the order was copper, zinc, pasteboard, copper, zinc, pasteboard, &c.), the effect of the pile thus formed was much greater than that of any galvanic apparatus previously introduced. When the highest and lowest disks were simul- taneously touched by the fingers, a distinct shock was felt ; and this could be repeated again and again, the pile apparently possessing within itself an indefinite power of recuperation. It thus resembled a Leyden jar endowed with a power of automati- cally re-establishing its state of tension after each explosion; with, in fact, " an inexhaustible charge, a perpetual action or impulsion on the electric fluid."

Volta unhesitatingly pronounced the phenomena of the pile to be in their nature electrical. The circumstances of Galvani's original discovery had prepared the minds of philosophers for this belief, which was powerfully supported by the similarity of the physiological effects of the pile to those of the Leyden jar, and by the observation that the galvanic influence was conducted only by those bodies — e.g. the metals — which were already known to be good conductors of static electricity. But Volta now supplied a still more convincing proof. Taking a disk of copper and one of zinc, 'he held each by an insulating handle and applied them to each other for an instant. After the disks had been separated, they were brought into contact with a deli-

• I'hil. Trans., 1800, p. 403.

Galvanism, from Galvani to Ohm. 73