The bimetallic thermometer uses the bimetallic strip which converts the temperature into the mechanical displacement. The working of the bimetallic strip depends on the thermal expansion property of the metal. The thermal expansion is the tendency of metal in which the volume of metal changes with the variation in temperature.

Every metal has a different temperature coefficient. The temperature coefficient shows the relation between the change in the physical dimension of metal and the temperature that causes it. The expansion or contraction of metal depends on the temperature coefficient, i.e., at the same temperature the metals have different changes in the physical dimension.

The working principle of bimetallic thermometer depends on the two fundamental properties of the metal. One, the metal has the property of thermal expansion, i.e., the metal expand and contract concerning the temperature. Two, the temperature coefficient of all the metal is not same. The expansion or contraction of metals is different at the same temperature.