Two conductors of different compositions (called thermocouple wires or thermoelectrodes) are joined at both ends to form a circuit. When the temperatures of the two junctions are different, an electromotive force is generated in the circuit. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric potential. Thermocouples utilize this principle for temperature measurement. The end directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensation end). The cold end is connected to a display instrument or a matching instrument, which indicates the thermoelectric potential generated by the thermocouple.
A thermocouple is essentially an energy converter that converts thermal energy into electrical energy, using the generated thermoelectric potential to measure temperature. The following points should be noted regarding the thermoelectric potential of a thermocouple:
1. The thermoelectric potential of a thermocouple is a function of the temperature difference between the two ends of the thermocouple's working end, not a function of the temperature difference between the cold end and the working end;
2. The magnitude of the thermoelectric potential generated by a thermocouple, when the thermocouple material is uniform, is independent of the length and diameter of the thermocouple, and only related to the composition of the thermocouple material and the temperature difference between the two ends;
3. Once the materials of the two thermocouple wires are determined, the magnitude of the thermocouple's thermoelectric potential is only related to the temperature difference of the thermocouple. If the temperature of the cold end of the thermocouple is kept constant, then the thermoelectric potential of the thermocouple is a single-valued function of the working end temperature. Two conductors or semiconductors A and B of different materials are welded together to form a closed circuit. When there is a temperature difference between the two junctions 1 and 2 of conductors A and B, an electromotive force is generated between them, thus forming a current in the circuit. Thermocouples work by utilizing this effect.
