We provide a quantitative analysis of the Faraday cage method for determining the leakage of electric charge across the surface of insulators, and correct some inconsistencies in earlier work. We show that it is possible to distinguish between the mechanisms of surface conduction and surface charge driven flow, since in the latter case the time constant depends on the amount of the initial charge and the logarithmic decay rate slows down at long times. ©1997 American Institute of Physics.