In any method for quantitatively determining the Kerr constant of nitrobenzene, the question of the purity of the material cannot be ignored. In the method described, account is explicitly taken of the effect of impurity ions in the dielectric on the optical performance of the Kerr cell. Thus, although nitrobenzene of less than the highest attainable purity is employed, results comparable in accuracy to expert determinations of the Kerr constant are obtained. The method, therefore, offers possibilities for a student experiment. Details of cell construction, nitrobenzene purification, optical procedures, and data reduction are given. Also included is a general discussion of electrical effects and the electro‐optical effects in Kerr cells having dielectrics that are less‐than‐perfect electrical insulators.