A bimetallic strip thermometer uses a helical metal strip that unwinds when heated and rotates a pointer over a calibrated scale. When warmed, one metal (usually copper) expands more than the other (usually Invar), so causing the bimetallic strip to unwind. Invar is an alloy of iron and nickel, which has a low coefficient of thermal expansion.
A bimetallic strip is a device used in thermostats and mechanical thermometers. It consists of bonded strips of two metals with dissimilar coefficients of thermal expansion. When heated, the bimetallic strip bends because one metal expands more than the other. This reversible distortion is used to move a dial pointer or to open or close a switch in an electric circuit.