PURPOSE: To increase the mechanical strength and the dielectric strength of the titled coil by a method wherein a solution in which a silicon organic compound and the microscopic powder of frit and natural accumulated mica or fluoric gold mica are mixed and then diluted by an organic solvent, is impregnated and hardened on the coil whereon an insulated wire was wound around.

CONSTITUTION: On the coil 1 whereon an insulated wire was wound around, a solution in which a silicon organic compound and the microscopic powder of firt and natural accumulated mica or fluoric gold mica were mixed and diluted by an organic solvent is impregnated and hardened. As a result, the dielectric strength and the mechanical strength of the coil can be improved.