The bimetallic thermometers are made up of bimetallic strips formed by joining two different metals having different thermal expansion coefficients. Basically, bimetallic strip is a mechanical element which can sense temperature and transform it into a mechanical displacement.
The bimetallic thermometer is basically an expansion thermometer. It consists of two strips of metals which are welded together. The bimetallic strips are active transducers. Both metals have different coefficient of thermal expansion. The welded structure is fixed at one end and other end is free to move. A pointer is fixed on free end which moves across calibrated scale.
The bimetallic strip is installed at the place to which temperature is measured. As the temperature increases it, the metals structure demonstrates deflection. Both metals show different expansion with increase in temperature. The deflection depends upon the coefficient of thermal expansion; therefore the metal strip bends towards the side of low coefficient of thermal expansion as shown in figure. The pointer attached will deflect with the strip and the temperature can be measured by using this pointer and calibrated scale.