The Attenuation of a Pulsed Field by a Conducting Sheet

Instruments that produce pulsed eddy currents in metals have been used to measure the thickness of one metal on another and to determine the properties of metals. The currents are produced by a pulsed dipole, which is surrounded by a conducting mask with a small aperture near the metal to concentrate the field and thus provide better resolution. A pickup coil near the metal is used to detect the currents. One configuration had the pickup coil on one side of a thin metal sheet with the aperture on the other side. This was analyzed assuming a plane electromagnetic wave with the magnetic field parallel to the surface of the metal. Experimental measurements showed that the attenuation as measured was about one-tenth that predicted by the analysis. An analysis was then made assuming that the magnetic field entered the metal surface perpendicularly to the surface. The results of this analysis were compared with the experimentally obtained results with good agreement. Possibly the main reason for the failure of the earlier plane-wave analysis was the reflection at the metal surfaces.