SOLUTION

The surface resistivity is the resistance between opposite edges of a square area of the surface of the material. It depends largely on the condition of the surface and upon humidity and therefore it is not constant under all conditions of the weather. In order to measure the surface resistivity, the sample is kept in a pool of mercury as shown in Figure.

To measure the surface resistivity the galvanometer is placed as shown in Fig. The galvanometer measures the leakage current. The current flowing between the upper and lower electrodes will be eliminated from the measurement.

The negative terminal of the battery is connected to the pool of mercury. This mercury pool is contained in an annular groove to which the positive terminal of the battery is connected. A pool of mercury is also provided inside the specimen and a wire from this is connected to the positive terminal of the battery. This wire acts as a guard wire and prevents the leakage current which passes through the body of the specimen, from passing through the galvanometer.