The eddy current method of nondestructive evaluation involves the induction of eddy currents in a conductive test object by a time-varying field produced by a suitable distribution of impressed currents and the detection of the resultant field. The method is ordinarily used at frequencies sufficiently low to neglect effects due to displacement current; hence a theoretical analysis entails calculating the self-impedance of the coil in the presence of the test object. In practice, one often needs only the change in impedance produced by the test object or by changes in the nominal properties of the test object (e.g., changes in its geometry or position with respect to the test coil or coils, or distributed or localized changes in the resistivity of the test object).