A method is described which is used for measuring the thickness of a metal plate of which only one side is readilyaccessible. No knowledge of the conductivity or permeability of the material is presumed. The method isparticularly applied to the determination of the extent of corrosion. The theory of the method when applied to an extensive plate given. Only a brief indication of the theory underlying the application to plates of limited area is here published, but curves are given enabling the method to be applied in practice.