A thermocouple is a temperature sensing element and an instrument. It directly measures temperature and converts the temperature signal into a thermoelectromotive force signal, which is then converted into the temperature of the measured medium through an electrical instrument (secondary instrument). The basic principle of thermocouple temperature measurement is that two conductors of different materials form a closed loop. When there is a temperature gradient between the two ends, a current will flow through the loop, and an electromotive force—thermoelectromotive force—will exist between the two ends. This is the so-called Seebeck effect. The two homogeneous conductors of different materials are the thermoelectrodes. The end with higher temperature is the working end, and the end with lower temperature is the free end, which is usually kept at a constant temperature. Based on the functional relationship between thermoelectromotive force and temperature, a thermocouple graduation table is made. The graduation table is obtained under the condition that the free end temperature is 0°C, and different thermocouples have different graduation tables.
When a third metal material is inserted into the thermocouple circuit, as long as the temperatures at the two junctions of this material are the same, the thermoelectromotive force generated by the thermocouple will remain unchanged, meaning it is not affected by the insertion of the third metal into the circuit.
