The basic working part of both thermostats and circuit breakers. What it is is a strip with two different metals fused together. These metals expand at different rates when heated. This difference in the coefficient of expansion will cause the strip to bend or curl, thus causing switch contacts to be made, or to be broken depending on how the strip is physically configured. In thermostats, the curled bimetallic strip often rotates a glass cylinder filled with mercury which turns the heating or cooling device on or off depending on the [temperature.