A thermometer works by tracking a physical property of a substance that varies in a known, repeatable way with temperature — this is the practical expression of the Zeroth Law of Thermodynamics, which lets us assign a temperature scale through thermal equilibrium. In a constant volume gas thermometer the property that is read is pressure.

The instrument traps a fixed mass of gas in a bulb of fixed volume. Because the volume (and amount) of gas is held constant, the ideal-gas law reduces to Gay-Lussac's Law:

• P ∝ T at constant V, i.e. P/T = constant.
• As the bulb is heated, the gas molecules strike the walls harder and more often, so the pressure rises in direct proportion to the absolute temperature.

A mercury manometer (with a movable limb) is used to keep the gas volume constant and to read the resulting pressure, from which temperature is deduced. This design is a highly accurate primary standard used to calibrate other thermometers.

The other choices are not the sensed quantity here: volume is the property held deliberately constant, so it cannot also be the indicator (that would describe a constant-pressure thermometer instead); entropy and enthalpy are derived thermodynamic quantities that cannot be read off directly by any simple thermometer. Hence the measured property is pressure.