The thermocouple is an electric circuit consisting of two metal conductors made of different materials welded at one end. Due to the Seedbeck effect, when there is a difference in temperature between the hot joint (welding point) and the cold joint at the other end of the conductors, an electromotive force is generated, which depends on the type of the two metals. To measure temperature with a thermocouple, the cold joint needs to be at a known temperature, so that the electomotive force generated depends solely on the hot joint.
The type J thermocouple consists of a positive conductor made of iron and a negative conductor made of constantan (copper-nickel). Suitable for measuring at medium temperatures in non-oxidising atmospheres due to the iron components.