The temperature is measured with a bimetal system inside the thermometer stem. The bimetal system consists of two metal strips bonded together that have different expansion coefficients. Therefore, one strip will  expand faster than the other causing the bimetal strip to curl in proportion to its temperature. The bimetal  system is helically wound and heat treated for long term stability. Temperature variations cause the bimetal strip to unwind  or wind tighter, which in turn rotates the pointer.