The principle of the bimetal thermometer is to measure the temperature by the principle of thermal expansion, which is a solid expansion thermometer. Bimetallic thermometers are based on the principle of thermal expansion of solids. The measurement of temperature is usually by welding two metal sheets with relatively large expansion coefficients together to form a bimetallic temperature sensing element. When the temperature changes, due to the relatively large difference in the linear expansion coefficients of the two different materials of the bimetallic sheet, different expansion and contraction will occur, resulting in bending deformation of the bimetallic sheet. Depending on the amount of deformation, different amounts of rotation are generated. The amount of rotation drives the connected shaft, and the shaft drives the indicator needle at the other end, so that the indicator needle can point to the correct reading and indicate the temperature.