An explicit thermodynamic analysis of the Volta effect for a system including adsorbed films or spread monolayers is presented in terms of surface variables. From an appropriate fundamental equation, Maxwell relations are obtained describing the effect of an applied electric field on some of the parameters characterizing surface films. It is shown that the assumptions of Bridgman, Kelvin & Lorentz (B.K.L.) concerning the properties of the Volta and compensation potentials are strictly incorrect, but that in many cases of practical interest the errors introduced by these assumptions are negligible, and lie well within current experimental accuracy. Measurable effects associated with the breakdown of the B.K.L. assumptions are predicted at high fields corresponding to phenomena such as field desorption. New experiments are suggested by the demonstration that in the transition region of a two dimensional first-order phase change, the B.K.L. assumptions can break down significantly when the chemical potentials of the adsorbed components are among the variables to be kept constant, but much less so at constant average surface densities of all surface active components. Finally, a possible method is suggested for obtaining the slopes of the isotherms for spread monolayers in regions in which they are too small to measure by conventional means.