the portion under the resulting condition is illuminated by laser beam the nickel is alloyed for bringing the portion into an austenite property around the yoke 2 into which the yoke 3 is fitted in a snug manner a coil 22 formed of a material of good electrical conductivity such as nichrome wire is wound between the flanges 2 a and 3 a both ends of the coil 22 are connected electrically to respective l shaped terminals 6 and 6 only one is illustrated by welding or soldering at this stage the resultant condition of the device or an integral configuration of the yokes 2 and 3 the coil 22 and the terminals 6 and 6 etc is placed in a molding device not shown and a resin molding is made such that an amount of synthetic resin in liquid phase is supplied into the molding device in order to obtain a sub assembly 13 it is desirable to employ a 6 6 nylon as the synthetic resin for improving heat resistance and an inclusion of glass is recommended for reinforcing the 6 6 nylon thus the resin formed sub assembly 13 is in the form of an integrated or one piece configuration of the yokes 2 and 3 the coil 22 and the connector 8 a plunger 4 which is formed of a magnetic material such as sum is provided in a sub assembly 13 such that the plunger 4 is slidable along the inner surface 2 b of the yoke 2 the plunger 4 is provided at its left side or one.