PURPOSE: To obtain an electrode probe suitably used in aq. high pressure and high temp. high radiation field environment like the core of a nuclear reactor as an electric potential reference electrode.

CONSTITUTION: An electrode is an assembly wherein only ceramic and metal parts are soldered or welded and, concretely, a sapphire base 12 is soldered to a housing 16 made of Koval or stainless steel and a housing is welded to a signal transmission coaxial cable assembly 20. The pedestal 30 integrally formed to the base is provided and a conductor wire 36 is passed through the pedestal and the pedestal is covered with a selectively coated cylindrical seal retainer 42. This apparatus is especially proper for the use in a silver/silver chloride electrode system.