Two types of the superconducting magnets, whose insulating layers inserted between every successive superconducting winding layer consist of the usual insulator named Mylar sheets in the one and of the metallic copper foils in the other, were examined. The former gives a comparatively definite magnetic field with regard to the exciting current but is liable to be damaged. The latter shows a large magnetic hysteresis owing to the intra- and inter-layer persistent eddy currents between successive layers of windings, but is not damaged by the super- to normal-phase transition
