A thermocouple is formed by welding two conductors of different compositions to form a circuit. The direct temperature measuring end is called the measuring end, and the wiring terminal is called the reference end. When there is a temperature difference between the measurement and reference ends, a thermal current will be generated in the circuit. When connected to a display instrument, the instrument will indicate the temperature value corresponding to the thermoelectric potential generated by the thermocouple. Thermoelectric properties are a universal characteristic of matter, but only a pair of metal conductors with good linearity, stability, repeatability, high thermoelectric potential, easy standardization, abundant material resources, easy purification, and good corrosion resistance in the relationship curve between thermoelectric potential and temperature can become materials for making thermocouples. Thermocouple is the most widely used on-site temperature measuring instrument.