Abstract: A device for continuously monitoring the internal surface temperature of a refractory lining in a metallurgical furnace. The device comprises an outer sheath, a pair of dissimilar, metallic wires within the outer sheath and a powdered oxide material closely packed within the sheath and surrounding both wires. Each wire is separately insulated by a sleeving of a refractory material having a melting point higher than the normal working temperature of the furnace at the surface of the lining. The oxide material used as a packing between the outer sheath and the wires is electrically conductive at elevated temperatures and has also a melting point higher than the temperature normally encountered at the surface of the lining.