Abstract: A thermocouple temperature sensor employs a protecting tube made of metal-boride-based ceramics, and a platinum -rhodium-based thermocouple. To prevent a deterioration of the thermocouple due to a reducing gas generated in the protecting when used at a high service temperature, the thermocouple is placed in an insulation tube, which is placed inside the protecting tube. In addition, a front end portion of the insulation tube is covered with a noble-metal cap mainly composed of platinum group metal, to prevent the reducing gas generated in the protecting tube from reaching a connection point of the thermocouple. The top end of the protecting tube is open, and a terminal box with a ventilation means to smoothly discharge the reducing gas in provided. This arrangement remarkably prolongs the service life of the thermocouple, and thus it is able to continuously measure the temperature of molten pig iron and molten steel for a long interval.

Abstract: The service life of a bottom electrode for a direct current arc furnace for producing steel is prolonged by using ZrB.sub.2 type sintered bodies for a contacting pins which constitutes a major part of the bottom electrode wherein the ZrB.sub.2 type sintered bodies have a corrosion-resistance to molten steel and slag. Further, the number of contacting pins to be used is reduced by forming each of the pins into a form of pillar with a through hole at the axis, or by assembling a plurality of longitudinally divided pin portions to form a contacting pin, whereby the diameter of the contacting pin can be made large. Then the space between contacting pins is made broad, and brick having good corrosion-resistance are filled in the space so that the service life of the bottom electrode is brought closer to that of the refractory lining of the direct current arc furnace.