In Situ Method for Measuring the Permeability and Resistivity of Metal Sheets

The electromagnetic shielding properties of enclosures, constructed of well-jointed metal sheets, depends primarily upon the permeability, resistivity, and thickness of these sheets. A nondestructive method of measuring the permeability and resistivity of these sheets is described. This method requires access to only a single surface of the metal to be tested and does not require preparation of samples or other cutting operations. The portable test equipment can be applied to the exterior surfaces of small enclosures or to the interior surfaces of large enclosures even if these latter are buried in the earth. Sets of electrical contacts are used to inject a unit-step pulse of current into the exposed surface of the sheet and the resulting surface voltage gradient between the contacts is measured. The permeability and resistivity of the metal determine the rate of diffusion of current into the surface and, therefore, the rate of decrease of this surface voltage gradient. At late times the current density will be uniform throughout the thickness of the sheet resulting in the surface voltage gradient stabilizing at a value depending on the resistivity and thickness of the metal.