Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/396

 polarized mercury cathode in acidulated water, there must be on the electrode itself a negative charge: the surface of this electrode in the polarized state may be supposed to be either mercury, or mercury covered with a layer of hydrogen. In the solution adjacent to the electrode, there must be an excess of cations and a deficiency of anions, so as to constitute the other layer of the condenser: these cations may be either mercury cations dissolved from the electrode, or the hydrogen cations of the solution.

It was shown in 1870 by Cromwell Fleetwood Varley that a mercury cathode, thus polarized in acidulated water, shows i tendency to adopt a definite superficial form, as if the surfacetension at the interface between the mercury and the solution were in some way dependent on the electric conditions. The matter was more fully investigated in 1873 by a young French physicist, then preparing for his inaugural thesis, Gabriel Lippmann. In Lippmann's instrumental disposition, which is called a capillary electrometer, mercury electrodes are immersed in acidulated water: the anode, has a large surface, while the cathode has a variable surface  small in comparison. When the external electromotive force is applied, it is easily seen that the fall of potential at the large electrode is only slightly affected, while the fall of potential at the small electrode is altered by polarization by an amount practically equal to the external electromotive force. Lippmann found that the constant of capillarity of the interface at the small electrode was a function of the external electromotive force, and therefore of the difference of potential between the mercury and the electrolyte.

Let denote the external electromotive force: we may, without loss of generality, assume the potential of, to be zero, so that the potential of  is. The state of the system may be varied by altering either or ; we assume that these