Page:Text-book of Electrochemistry.djvu/247

 ��POTENTIAL DIFFERENCE.

��CHAP.

���Fig. 45.

��tends to increase the surface. The latter increases with the magnitude of the charge.

Capillary Electrometer. — In order to observe the change of surface tension, the mercury is put into a tube which is

drawn out to a capillary, slightly conical point, R (Fig. 45), so that the pressure of the mercury column acts on the con- tents of the capillary. The lower meniscus of the mercury is in contact with a saturated solution of mercurous sulphate in sulphuric acid contained in the vessel K, in the bottom of which is placed a layer of mercury. If the charge at the contact surface be altered by introducing a potential diflference (electromotive force) at P, there is a simultaneous change of surface tension. If this tension is decreased, the mercury meniscus in B falls ; if it increases, the mercury rises in R, and the movement can be observed with a microscope, M. An instrument of this kind, called

a capillary electrometer, was first constructed by Lippmann ?). It can be used to determine when the potential difference reaches the value 0, and is therefore useful for comparing potential differences. It is most commonly employed as a null instrument in the form shown in Fig. 46.

At the beginning of the experi- ment let the potential difference be zero, i.e, P = 0. If now P be so altered that the mercury in the tube becomes nega- tively charged, the mercury rises in the tube, i.e. the surface tension increases. The cause of this is that the original chai'ge of the mercury is diminished, which proves that this was positive. Mercury, therefore, in contact with sulphuric acid becomes positively charged, and the acid negatively. If the contact surface be now charged with increasing

���Fig. 46.

�� �