Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/395

 equivalent of ion. Thus = total current =, or. The determination of by the method of Hittorf, and of  by the method of Kohlrausch, made it possible to calculate the absolute velocities of drift of the ions from experimental data.

Meanwhile, important advances in voltaic theory were being effected in connexion with a different class of investigations.

Suppose that two mercury electrodes are placed in a solution of acidulated water, and that a difference of potential, insufficient to produce continuous decomposition of the water, is set up between the electrodes by an external agency. Initially a slight electric current—the polarizing current, as it is called—is observed; but after a short time it ceases; and after its cessation the state of the system is one of electrical equilibrium. It is evident that the polarizing current must in some way have set up in the cell an electromotive force equal and opposite to the external difference of potential; and it is also evident that the seat of this electromotive force must be at the electrodes, which are now said to be polarized.

An abrupt fall of electric potential at an interface between two media, such as the mercury and the solution in the present case, requires that there should be a field of electric force, of considerable intensity, within a thin stratum at the interface: and this must owe its existence to the presence of electric charges. Since there is no electric field outside the thin stratum, there must be as much vitreous as resinous electricity present; but the vitreous charges must preponderate on one side of the stratum, and the resinous charges on the other side; so that the system as a whole resembles the two coatings of a condenser with the intervening dielectric. In the case of the