The test object materials were a cold-reduced carbon steel
sheet and a carbon tool steel with and without annealing with
different magnetic properties. The test object was 400 mm
long  400 mm wide  1 mm thick. Thin plates whose thickness
was less than the skin depth were used as the test objects so that
skin effect could be neglected in the experiments. The artificial
grooved surface of the type A standard test shim was placed on
the surface of the test object. The type A standard test shim was
placed at the center of the test object, which was also the center
of the magnetizing yoke.