Peter Mark Roget

Peter Mark Roget (b. 1779, d. 1869) was one of the most active English supporters of the chemical theory immediately preceding Faraday.

In the first place, carefully distinguishing between the quantity of electricity put into circulation by a cell and the tension at which this electricity is furnished, he showed that the latter quantity depends on the “energy of the chemical action”[38]—a fact which, when taken together with Faraday’s discovery that the quantity of electricity put into circulation depends on the amount of chemicals consumed, places the origin of voltaic activity beyond all question.

Roget’s principle was afterwards verified by:

• Faraday[39] and
• De La Rive[40]

“the electricity of the voltaic pile is proportionate in its intensity to the intensity of the affinities concerned in its production,” said the former in 1834;

De La Rive wrote in 1836: “The intensity of the currents developed in combinations and in decompositions is exactly proportional to the degree of affinity which subsists between the atoms whose combination or separation has given rise to these currents.”

Roget brought up another argument of far-reaching significance:

“If [41] there could exist a power having the property ascribed to it by the [contact] hypothesis, namely, that of giving continual impulse to a fluid in one constant direction, without being exhausted by its own action, it would differ essentially from all the other known powers in nature.

All the powers and sources of motion, with the operation of which we are acquainted, when producing their peculiar effects, are expended in the same proportion as those effects are produced; and hence arises the impossibility of obtaining by their agency a perpetual effect; or, in other words, a perpetual motion.

But the electro-motive force ascribed by Volta to the metals when in contact is a force which, as long as a free course is allowed to the electricity it sets in motion, is never expended, and continues to be exerted with undiminished power, in the production of a never-ceasing effect. Against the truth of such a supposition the probabilities are all but infinite.”

This principle is little less than the doctrine of conservation of energy applied to a voltaic cell, was reasserted by Faraday.

The process imagined by the contact school would," he wrote, “indeed be a creation of power, like no other force in nature.”

In all known cases energy is not generated, but only transformed.

There is no such thing in the world as “a pure creation of force; a production of power without a corresponding exhaustion of something to supply it."[42]

As time went on, each of the rival theories of the cell became modified in the direction of the other.

The contact party admitted the importance of the surfaces at which the metals are in contact with the liquid, where of course the chief chemical action takes place.

The chemical party confessed their inability to explain the state of tension which subsists before the circuit is closed, without introducing hypotheses just as uncertain as that of contact force.

Faraday’s own view on this point[43] was that a plate of amalgamated zine, when placed in dilute sulphuric acid, “has power so far to act, by its attraction for the oxygen of the particles in contact with it, as to place the similar forces already active between these and the other particles of oxygen and the particles of hydrogen in the water, in a peculiar state of tension or polarity, and probably also at the same time to throw those of its own particles which are in contact with the water into a similar but opposed state.

Whilst this state is retained, no further change occurs. But when it is relieved by completion of the circuit, in which case the forces determined in opposite directions, with respect to the zine and the electrolyte, are found exactly competent to neutralize each other, then a series of decompositions and recompositions takes place amongst the particles of oxygen and hydrogen which constitute the water, between the place of contact with the platina and the place where the zine is active: these intervening particles being evidently in close dependence upon and relation to each other.

The zinc forms a direct compound with those particles of oxygen which were, previously, in divided relation to both it and the hydrogen: the oxide is removed by the acid, and a fresh surface of zinc is presented to the water, to renew and repeat the action.”

These ideas were developed further by the later adherents of the chemical theory, especially by Faraday’s friend Christian Friedrich Schönbein,[44] of Basle (b. 1799, d. 1868), the discoverer of ozone. Schönbein made the hypothesis more definite by assuming that when the circuit is open, the molecules of water adjacent to the zinc plate are electrically polarized, the oxygen side of each molecule being turned towards the zinc and being negatively charged, while the hydrogen side is turned away from the zinc and is positively charged.

In the third quarter of the 19th century, the general opinion was in favour of some such conception as this.

Helmholtz[45] attempted to grasp the molecular processes more intimately. He assumed that the different chemical elements have different attractive powers (exerted only at small distances) for the vitreous and resinous electricities. Thus:

• potassium and zinc have strong attractions for positive charges
• oxygen, chlorine, and bromine have strong attractions for negative electricity.

This assumes 2 electric fluids. Whereas Volta’s original hypothesis assumed only one.

The contact difference of potential between 2 metals is explained by Helmholtz’s hypothesis, as it was by Volta’s.

The activity of the voltaic cell may be referred to the same principles: for the 2 ions of which the liquid molecules are composed will also possess: different attractive powers for the electricities, and may be supposed to be united respectively with vitreous and resinous charges.

Thus, when 2 metals are immersed in the liquid, the circuit being open, the positive ions are attracted to the negative metal and the negative ions to the positive metal, thereby causing a polarized arrangement of the liquid molecules near the metals. When the circuit is closed, the positively charged surface of the positive metal is dissolved into the fluid.

As the atoms carry their charge with them, the positive: charge on the immersed surface of this metal must be perpetually renewed by a current flowing in the outer circuit.

Helmholtz did not adhere to Davy’s’s doctrine of the electrical nature of chemical affinity quite as: simply or closely as Faraday, who preferred it in its most direct and uncompromising form. He wrote:

Chemical affinity [46] can be communicated to a distance through the metals and certain forms of carbon. The electric current is only another form of the forces of chemical affinity. Its power is in proportion to the chemical affinities producing it.

When it is deficient in force it may be helped by calling in chemical aid, the want in the former being made up by an equivalent of the latter.

In other words, the forces called chemical affinity and electricity are one and the same.”