"A pair of plug members 25 and 26 are respectively disposed in the open ends 23 and 24 of the tubular member 22 to close the open ends 23 and 24, the plug members 25 and 26 respectively being formed of electrically insulating material, such as plastic material, and being secured in the tubular member 22 in any suitable manner, such as by press-fitting, adhesive means, etc., or even by a turning ove" . . . .