A bimetal is a strip of metal obtained by cladding two metals which have different temperature coefficients of expansion. The bimetal, bends in different directions, which results in deflection of the bimetallic strip. If the bimetallic strip is coiled, the strip will uncoil when heat is applied. Bimetallic coils can also be helically wound. 
In a bimetallic Thermometer, the bimetallic strip is helically wound and placed inside them stem. A long shaft is connected to the bimetallic helix and the dial which moves across the scale.
When the temperature rises, the helix unwinds and the shaft rotates moving the dial across the pointer. The Bimetallic Thermometer is a simple device. It is also accurate. The downside of the bimetallic thermometer is that they are not accurate at low temperatures as there is not much of a difference in the expansion of the two metals at low temperatures.