bimetal meter

Like the moving-coil instrument, the moving-iron instrument or the moving-solenoid movement, the bimetallic movement is one of the movements used in analog indicating pointer instruments. In this movement, the torque is generated by a bimetal spiral which heats up and unwinds when current flows through it. This unwinding movement forms the pointer deflection.

The counter-torque is generated by a spiral spring. The bimetallic spring consists of two different metals with different coefficients of thermal expansion, for example zinc and steel, which are rolled onto each other and inseparably bonded. When heated by current flow, the two metals expand differently, causing the coil to lengthen or shorten. Since the meter pointer is directly connected to the coil spring, the pointer deflection changes as the current flows or the coil spring heats up. The pointer rotation angle is proportional to the current flow squared and therefore not dependent on direction. Accordingly, the scale display also has a quadratic progression.

Bimetallic movements are mainly used for measuring high currents and have a relatively high thermal inertia.