The thermometer consists of a steel bulb, a steel tube and Bourdon tube.
The whole system is filled with a gas at a high pressure and sealed off. The gas commonly used is nitrogen which is an inert gas, with a high cubical expansion coefficient and which is readily available.
It may be assumed that the volume of the gas remains nearly constant. For an ideal gas, the gas law applies:
PV = nRT.
It is clear from this equation that if V is constant, the pressure is proportional to the temperature T. Temperature is thus converted to pressure, which is detected by the Bourdon tube.