Page:Text-book of Electrochemistry.djvu/146

 Kohlrausch's method (4) Diore closely resembles that described for the determination of the resistance of metals. If a current be passed in the direction AB through a column of liquid lying between the electrodes A and B (Fig. 31), polarisation takes place, and the current is thereby weakened. If the direction of the current be now altered, i.e. passes in the direction from B to A, after it has produced its greatest polarisation effect, this polarisation intensifies the new current, which becomes stronger than it would be without the conjoint action of the polarisation. The new current, however, weakens the original polarisation, which depends on the separation of a small quantity of substance on the elec- trodes, and if it acts for a sufiGicient time, polarisation in the opposite sense takes place. By making the quantity of electricity which passes through the liquid small in com- parison with the surface of the electrodes, whereby, according to Faraday's law, the quantity of substance separated per square centimetre, and consequently the polarisation, is inconsiderable, and at the same time applying an alternating current so that the polarisation of the principal current is intensified as often as it is weakened, the resistance of electrolytes can be measured according to the same principle as that used in the determination of the resistance of metals. This is the basis of the i^^ohlrausch method. The source of the electric energy E (Tig. 31) consists of a small induction- coil actuated by a galvanic element, and the galvanometer G, which is not suitable for alternating currents, is replaced by a telephone. The movable contact Z> is slid along AC until a tone minimum is established in the telephone, and then there exists the following relationship between the resistances : —

AB :BC = AD: DC.

The solutions are contained in " resistance " or " conduc- tivity vessels," the form of which varies according to the magnitude of the resistance to be measured (Fig. 33, a, J, c, d).

The vessel is filled to such an extent that the electrode is

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