Page:Elementary Text-book of Physics (Anthony, 1897).djvu/351

§ 288] '''287. Theories of the Electromotive Force of the Voltaic Cell.'''—The plan followed in the preceding discussions has rendered it unnecessary for us to adopt any theory to explain the cause of the electromotive force of the voltaic cell. The different theories which have been advanced may be classed under one of two general theories, the contact theory and the chemical theory. On the contact theory, as advanced by Volta and supported by Thomson and others, the difference of potential which exists between two heterogeneous substances in contact is due to molecular interactions across the surface of contact, or, as it is commonly stated, is due merely to the contact. The chemical theory, as advocated by Faraday and Schönbein, holds that the difference of potential considered cannot arise unless chemical action or a tendency to chemical action exist at the surface of contact.

Numerous experiments have shown that the sum of all the differences of potential at the surfaces of contact of the various substances making up any voltaic cell is equal to the electromotive force of that cell. This is true even when the cell is formed solely of liquid elements. On the contact theory, this electromotive force is due merely to the several contacts, while the chemical actions of the cell begin only when the circuit is made, and supply the energy for the maintenance of the current. On the chemical theory the chemical action of the medium is concerned in the production of the difference of potential observed.

On either theory it is clear that the energy maintaining the current must have its origin in the chemical actions which go on in the voltaic cell.

288. The Electrical Double-sheet.—Suppose two plates of different materials, say one zinc and the other copper, joined by a wire and placed opposite each other like the plates of a condenser: as stated in the last section, a difference of potential then exists between them. The charge on one of them is given by $$\frac{S (V_{1} - V_{2})}{4 \pi d}$$ (§ 259, (Eq. 92)). The difference of potential will remain the same, whatever be the distance between the plates, so that the charges on