PURPOSE: To reduce the variation in operating current values by lapping the attracting surface of an iron core with coarse sand, then lapping or polishing with fine sand.

CONSTITUTION: The attracting surface of the iron core is lapped with coarse sand of #1,000 and less, then, said surface is lapped or polished with fine sand of #3,000 and more. By this method, the roughness of the surface is made to become about 1W1.4μm. Since convex portions of the attracting surface are removed by this machining, the initial operating current is decreased, and the generation of powder due to the wear of the surface is decreased. The operating current is not increased even through the number of usage is increased.