The principle behind a bimetallic strip is that different metals expand to different extents with temperature changes. By combining two different metals one on top of another into a strip, a bimetallic strip is formed. As the two metals expand or contract differently under the same temperature change, the strip bends. It can then be used to switch on or off a circuit at certain temperatures.
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper , or in some cases steel and brass. The strips are joined together throughout their length by riveting , brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.
Bulb thermometers are good for measuring temperature accurately, but they are harder to use when the goal is to control the temperature. The bimetallic strip thermometer, because it is made of metal, is good at controlling things. The principle behind a bimetallic strip thermometer relies on the fact that different metals expand at different rates as they warm up.