Page:Elementary Text-book of Physics (Anthony, 1897).djvu/353

§ 288] positive pole of a voltaic cell, and that in the tube with the negative pole, the meniscus in the capillary tube moves upward, in the sense in which it would move if its surface tension were increased.

This movement may be explained as follows: An electrical double-sheet will be formed on the curved surface of contact of the mercury and acid in the capillary tube, and the interaction of the parts of this double-sheet will give rise to an electrical pressure (§ 256), that diminishes the apparent surface tension in that surface. If a weak current be sent through the solution, the difference of potential between the liquid and mercury will be diminished or increased by the ionic charges transferred by the current, according as the current flows in one direction or the other. The apparent surface tension will be altered and the end of the mercury column will be displaced; the true surface tension of the surface will be efficient only when the mercury and solution are at the same potential, and this surface tension will be a maximum. The experiments of Helmholtz and A. König have shown that such a maximum exists in a way consistent with this view.

The arrangement described can manifestly be used to produce the effects just discussed only when the electromotive force introduced into the circuit is less than that required to cause active decomposition of the electrolyte.

Lippmann constructed an apparatus similar to the one described, with the addition of an arrangement by which pressure can be applied to force the end of the mercury column in the capillary tube back to the fixed position which it occupies when no electromotive force is introduced into the circuit. He found that when small electromotive forces were introduced, the pressures required to bring the end of the column back to the fixed position were proportional to the electromotive forces. He hence called this apparatus a capillary electrometer.

Lippmann also found that if the area of the surface of separation between the mercury and the liquid in the capillary tube were altered by increasing the pressure and driving the mercury down the tube, a current was set up in a galvanometer inserted in the