A bimetallic strip consists of two different materials with different expansion coefficients that are bonded together. For example, for brass and steel, the coefficients of linear expansion are:
Brass: 19 x 10-6 /°C Steel: 11 x 10-6 /°C
When this bimetallic strip is heated, the brass expands more than the steel and the strip curves with the brass on the outside. If the strip is cooled, it curves with the steel on the outside.
Bimetallic strips are used as switches in thermostats.