The working principle of bimetallic thermometers

The sensing element of a bimetallic thermometer is made by welding together two layers of metal sheets with different coefficients of linear expansion. The metal sheet with the higher coefficient of linear expansion is called the active layer, while the other sheet is known as the passive layer. One end of the element is fixed, while the other end is free. When the temperature being measured changes, due to the difference in coefficients of linear expansion between the two metal sheets, the free end bends (or deforms) as a result of a combined torque. The degree of bending depends on the physical properties of the materials making up the bimetallic sheets, the thickness of each layer, and the temperature; it does not depend on the width. Once the temperature range is determined, along with the materials and geometric dimensions of the bimetallic sheets, the degree of bending depends solely on temperature. If a transmission mechanism, along with a pointer and a scale marked with temperature values, is installed at the free end, it becomes possible to directly display the temperature reading.