Abstract A number of measurements of the contact potential difference between pairs of dissimilar metals has been made during the last few years. The interest and importance of such measurements arise chiefly in connexion with the theoretical relation which exists between the contact potential difference and the work functions of the surfaces considered. This relation, due to Richardson, may be written Vc = Φ1 - Φ2 + P, (1) where P is a correction for the Peltier effect which, in practice, is negligibly small. If Φ2 > Φ1, the sign of Vc will be such that surface 1 is positive with respect to surface 2. It has been pointed out by Compton and Langmuir that equation (1) cannot hold for surfaces which are not homogeneous, since contact potential measurements would yield average values for the whole surfaces, while the magnitude of the work function, measured either by the photoelectric or the thermionic method, would be determined principally by the most electropositive portions of the surface. Farnsworth and Rose have shown that these considerations may apply even for clean metal surfaces, since recent measurements by Rose§ indicate that comparatively large contact potential differences may exist between different faces of crystals of the same metal, so that polycrystalline surfaces cannot necessarily be considered as homogeneous.