The measuring system in the gas thermometers consists of the stem, capillary and Bourdon tube in a case. These parts are connected to form a unit. The complete measuring system is filled with an inert gas under pressure.
A temperature change causes the internal pressure in the stem to change. The deflection of the Bourdon tube thus caused is transmitted to the pointer by a movement. A bimetallic element is fitted between the movement and Bourdon tube to compensate for fluctuations of ambient temperature at the case.