PURPOSE: To obtain a contrate type electrode of a high durability by using a beryllium-copper alloy with a thermal expansibility larger than a composite metal to compose a contact at a magnetic field generator, and incorporating it with the contact by soldering.

CONSTITUTION: A magnetic field generator 30 is formed of an alloy of beryllium and copper, while a contact 40 is formed of a composite metal made by fusing copper to a porous sintered body of at least two sorts of metallic powders with the melting point higher than copper, and setting its thermal expansibility less than that of the magnetic field generator 30. The beryllium-copper alloy improves the mechanical strength after a high temperature soldering, and has a higher thermal expansibility compared with the composite metal to compose the contact 40. By the difference of the thermal expansibility, the thermal stress is left at a cylinder 33 in which including grooves 34 are formed after the soldering. Consequently, the mechanical strength as an electrode is made same as or higher than that before the soldering.