Page:Text-book of Electrochemistry.djvu/144

 reaches the value X^, which is the equivalent conductivity at infinite dilution.

The Wheatstone Bridge.— For the determination of the specific conductivity of a metal the apparatus diagram- matically represented in Fig. 31 is used. The metal wire M to be invesrtigated is introduced into the branch AB of the Wheat- stone bridge, and between B and C there is a rheostat of known re- sistance. The two other branches AD and DC consist of a metal wire, generally platinum, along which the sliding contact D can be moved. A galvanometer is

interposed between D and B, When the points A and C are connected with the poles of some source of electricity, the current distributes itself over the various parts of the bridge according to KirchhofiTs law. The galvanometer shows no deflection when the contact i> is at a certain position on AC^ and the ratio between the resistance sought (in AB) and that in BO is then the same as the ratio of the resistance AD to DC. Since -B, AD, and DC are knowri, the resistance of

M can be found from —

^ Rx AD

This method cannot, however, be used without modifica- tion for the determination of the resistance of an electrolytic solution, because the current is constantly passing in one direction, and this causes polarisation of the electrodes. This disturbing factor can be avoided in two ways : either non- polarisable electrodes are used (Fuchs and Bouty), or the direct current is replaced by an alternating current (F. Kohlrausch).

Determination of the Resistance of Electroljrtes.— The method employed by Fuchs (2) and Bouty (3) is as follows: Two vessels, K and Ki (Fig. 32), are filled with zinc sulphate solution, and two non-polarisable zinc electrodes

K

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