Faraday's Theory

The previous theories were demolished and Faraday was free to construct his own theory.

He retained one of the ideas of Grothuss’ and Davy’s doctrine – a chain of decompositions and recombinations takes place in the liquid.

But these molecular processes he attributed not to any action of the terminals, but to a power possessed by the electric current itself, at all places in its course through the solution.

If as an example we consider neighbouring molecules A, B, C, D, … of the compound—say water, which was at that time believed to be directly decomposed by the current—Faraday supposed that before the passage of the current the hydrogen of A would be in close union with the oxygen of A, and also in a less close relation with the oxygen atoms of B, C, D, …: these latter relations being conjectured to be the cause of the attraction of aggregation in solids and fluids.[29]

When an electric current is sent through the liquid, the affinity of the hydrogen of A for the oxygen of B is strengthened, if A and B lie along the direction of the current; while the hydrogen of A withdraws some of its bonds from the oxygen of A, with which it is at the moment combined.

So long as the hydrogen and oxygen of A remain in association, the state thus induced is merely one of polarization; but the compound molecule is unable to stand the strain thus imposed on it, and the hydrogen and oxygen of A part company from each other, Thus decompositions take place, followed by recombinations: with the result that after each exchange an oxygen atom associates itself with a partner nearer to the positive terminal, while a hydrogen atom associates with a partner nearer to the negative terminal.

This theory explains why, in all ordinary cases, the evolved substances appear only at the terminals; for the terminals are the limiting surfaces of the decomposing substance; and, except. at them, every particle finds other particles having a contrary tendency with which it can combine. It also explains why, in numerous cases, the atoms of the evolved substances are not retained by the terminals (an obvious difficulty in the way of all theories which suppose the terminals to attract the atoms): for the evolved substances are expelled from the liquid, not drawn out by an attraction.

Many of the perplexities which had harassed the older theories were at once removed when the phenomena were regarded from Faraday’s point of view. Thus, mere mixtures (as opposed to chemical compounds) are not separated into their constituents by the electric current; although there would seem to be no reason why the Grothuss-Davy polar attraction should not operate as well on elements contained in mixtures as on elements contained in compounds.

In the latter part of the same year (1833) Faraday took up the subject again.[30] It was at this time that he introduced the terms which have ever since been generally used to describe the phenomena of electro-chemical decomposition. To the terminals by which the electric current passes into or out of the decomposing body he gave the name electrodes. The electrode of high potential, at which oxygen, chlorine, acids, &c., are evolved, he called the anode, and the electrode of low potential, at which metals, alkalis, and bases are evolved, the cathode. Those bodies which are decomposed directly by the current he named electrolytes; the parts into which they are decomposed, ions; the acid ions, which travel to the anode, he named anions; and the metallic ions, which pass to the cathode, cations,

Faraday now proceeded to test the truth of a supposition which he had published rather more than a year previously,[31] and which indeed had apparently been suspected by Gay-Lussac and Thénard[32] so early as 1811; namely, that the rate at which an electrolyte is decomposed depends solely on the intensity of the electric current passing through it, and not at all on the size of the electrodes or the strength of the solution. Having established the accuracy of this law,[33] he found by a comparison of different electrolytes that the mass of any ion liberated by a given quantity of electricity is proportional to its chemical equivalent, i.e. to the amount of it required to combine with some standard mass of some standard element. If an element is n-valent, so that one of its atoms can hold in combination n atoms of hydrogen, the chemical equivalent of this element may be taken to be 1/n of its atomic weight; and therefore Faraday’s result may be expressed by saying that an electric current will liberate exactly one atom of the element in question in the time which it would take to liberate n atoms of hydrogen.[34]

The quantitative law seemed to Faraday[35] to indicate that “the atoms of matter are in some way endowed or associated with electrical powers, to which they owe their most striking qualities, and amongst them their mutual chemical affinity.”

Looking at the facts of electrolytic decomposition from this point of view, he showed how natural it is to suppose that the electricity which passes through the electrolyte is the exact equivalent of that which is possessed by the atoms separated at the electrodes; which implies that there is a certain absolute quantity of the electric power associated with each atom of matter.

The claims of this splendid speculation he advocated with conviction. “The harmony,” he wrote,[36] “which it introduces into the associated theories of definite proportions and electro-chemical affinity is very great. According to it, the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric powers; it being the electricity which determines tho equivalent number, because it determines the combining force. Or, if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalent to each other in their ordinary chemical action, have equal quantities of electricity naturally associated with them. “But,” he added, “I must confess I am jealous of the term atom: for though it is very easy to talk of atoms, it is very difficult to form a clear idea of their nature, especially when compound bodies are under consideration.”

These discoveries and ideas tended to confirm Faraday in preferring, among the rival theories of the voltaic cell, that one to which all his antecedents and connexions predisposed him. The controversy between the supporters of Volta’s contact hypothesis on the one hand, and the chemical hypothesis of Davy and Wollaston on the other, had now been carried on for a generation without any very decisive result. In Germany and Italy the contact explanation was generally accepted, under the influence of Christian Heinrich Pfaff, of Kiel (b. 1773, d. 1852), and of Ohm, and, among the younger men, of Gustav Theodor Fechner (b. 1801, d. 1887), of Leipzig,[37] and Stefano Marianini (b. 1790, d. 1866), of Modena. Among French writers De La Rive, of Geneva, was, as we have seen, active in support of the chemical hypothesis; and this side in the dispute had always been favoured by the English philosophers.

There is no doubt that when two different metals are put in contact, a difference of potential is set up between them without any apparent chemical action; but while the contact party regarded this as a direct manifestation of a “contact-force” distinct in kind from all other known forces of nature, the chemical party explained it as a consequence of chemical affinity or incipient chemical action between the metals and the surrounding air or moisture. There is also no doubt that the continued activity of a voltaic cell is always accompanied by chemical unions or decompositions; but while the chemical party asserted that these constitute the efficient source of the current, the contact party regarded them as secondary actions, and attributed the continual circulation of electricity to the perpetual tendency of the electromotive force of contact to transfer charge from one substance to another.