apparatus using leakage current through a sample to determine resistivity . an optically active material of the type which changes the polarization of a light beam passing therethrough , responsive to an electric field , is placed between a first pair of conducting plates . the sample is placed between a second pair of conducting plates which are in parallel connection with the first pair of plates . a light source transmits light through the material and a light detector detects changes in polarization of the light which emerges from the material . each pair of plates is charged and the rate of discharge of the plates is monitored by the light detector .