Page:Elementary Text-book of Physics (Anthony, 1897).djvu/349

§ 286] conventionally said to flow through the liquid from the anode to the cathode, from the electrode above which oxygen is collected to the electrode above which hydrogen is collected. The current existing during the recombination of the gases flows through the liquid from the hydrogen electrode to the oxygen electrode, or outside the liquid from the positive to the negative pole. Such an arrangement as is here described was devised by Grove, and is called the Grove's gas battery.

A combination known as Smee's cell consists of a plate of zinc and one of platinum, immersed in dilute sulphuric acid. It is such a cell as would be formed by the complete electrolysis of a solution of zinc sulphate, if the zinc plate were made the cathode. When the zinc and platinum plates are joined by a wire, a current is set up from the platinum to the zinc outside the liquid, and the zinc combines with the acid to form zinc sulphate. The hydrogen thus liberated appears at the platinum plate, where, since the oxygen which was the electro-negative ion of the hypothetical electrolysis by which the cell was formed does not exist there ready to combine with it, it collects in bubbles and passes up through the liquid. The presence of this hydrogen at once lowers the current from the cell, for it sets up a counter electromotive force, and also diminishes the surface of the platinum plate in contact with the liquid, and thus increases the resistance of the cell. It may be partially removed by mechanical movements of the plate or by roughening its surface. The counter electromotive force is called the electromotive force of polarization. It occurs soon after the circuit is joined up in all cells in which only a single liquid is used, and very much diminishes the currents which are at first produced.

Advantage is taken of secondary chemical reactions to avoid this electromotive force of polarization. The best example, and a cell which is of great practical value for its cheapness, durability, and constancy, is the Daniell's cell. Two liquids are used—solutions of cupric sulphate and zinc sulphate. They are best separated from one another by a porous porcelain diaphragm. A plate