Abstract :
It is shown that a complete verification of the well-known theory of eddy-current shielding in plane sheets can be obtained by measuring the variation of the impedance with frequency of a coil wound on a laminated core consisting of non-magnetic material. The verification of the theory is straightforward when the screening effect is small, because laminations thin enough to simulate the conditions of the plane sheet can be used, whereas much thicker laminations are necessary when the screening effect is large, and the simple theory must be extended to include the ratio of lamination thickness to lamination width. This extension is derived from an analysis of the eddy currents in an elliptical core, which, as the measurements show, applies to a rectangular core when the screening effect is large. When the screening effect is small, however, the results of the analysis of the elliptical core no longer apply to the rectangular one, and the difficulties of obtaining a mathematical solution in this case are indicated. With magnetic laminations the simple theory no longer holds in practice, and Peterson and Wrathall have shown that it is necessary to modify this theory to take into account the presence of a surface layer. The modifications are examined in an Appendix, and the results obtained indicate that they apply when the screening effect is negligible but not when it is large. Nevertheless, the equations obtained by Peterson and Wrathall are shown to provide a means of evaluating the thickness of the surface layer.
