Chapter 3b

The Discovery by Volta

by Edmund Taylor Whittaker

Scarcely more than a year after the death of Galvani, the new science suddenly regained the eager attention of philosophers.

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[10] 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 moistened pasteboard (so that the order was copper, zinc, pasteboard, copper, zinc, pasteboard, &c.), the effect of the pile this formed was much greater than that of any galvanic apparatus previously introduced. When the highest and lowest disks were simultaneously 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 automatically 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 delicate 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.[11] 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 - 1 2 , and that of the zinc by the number + 1 2 , 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 distinguished 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 - 2 3 for the lower copper, + 1 3 for the zinc, and + 1 3 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 zine 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 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 that of conductors.[12] 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.

Volta 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. Volta inferred that the electric current persists during the whole time that communication by conductors 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.”

Volta announced his discovery in a letter to Sir Joseph Banks, dated from Como, March 20th, 1800. Sir Joseph, who was then President of the Royal 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[13] 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 decomposition of water into its constituent gases, was obtained on May 2nd, 1800.[14]

Although it had long been known that frictional electricity is capable of inducing chemical action,[15] the discovery of Nicholson and Carlisle was of tho first magnitude. It was at once extended by William Cruickshank, of Woolwich (b. 1745, d. 1800), who[16] showed that solutions of metallic salts are also decomposed by the current; and William Hyde Wollaston (b. 1766, d. 1828) seized on it as a test[17] of the identity of the electric currents of Volta with those obtained by the discharge of frictional electricity. He found that water could be decomposed 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 Marum (b, 1750, d. 1837) and Christian Heinrich Pfaff (b. 1773, d. 1852) arrived at the same conclusion by carrying out on a large scale[18] Volta’s plan of using the pile to charge batteries of Leyden jars.

The discovery of Nicholson and Carlisle made a great impression on the mind of Humphry Davy (b. 1778, d. 1829), a young Cornishman who about this time was appointed Professor of Chemistry at the Royal Institution in London. Davy at once began to experiment with Voltaic piles, and in November, 1800,[19] showed that they give no current when the water between the 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 Fabroni’s idea that galvanic effects are always accompanied by chemical action. After a series of experiments he definitely 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 to conclude, though with our present quantity of facts we are unable to explain the exact mode of operation, that the oxydation 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[20] and by Nicholson,[21] 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[22] 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 discharge 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 electromotion continues. Thus the contact of metals is the cause which disturbs the equilibrium, while the chemical changes of continually restore the conditions under which the contact energy can be exerted.

In this and other memoirs Davy asserted that chemical affinity is essentially of an electrical nature. “Chemical and electrical attractions,” he declared,[23] “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 Grothusst[24] (b.1785, d. 1822) and by Davy [25] 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 attracting 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.

The hypothesis of Grouhuss and Davy was attacked in 1825 by Anguste De La Rive[26] (b. 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 zine 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 & 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 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 Rive, little real progress was made with the special problems of the cell; but meanwhile 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, Jöns Jacob Berzelius (b. 1779, d. 1848).


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