SEARCH Articles Figures Tables A simplified diagram of a con-, stant-volume ideal gas thermometer. In this hose thermometer the product PV for a gas at various temperatures is found by measuring the pressure P at constant volume. For each measurement the mercury reservoir is raised or lowered until the mercury column at the left touches an index mark. The pressure of the gas in the bulb is then equal to the atmospheric pressure plus the pressure due to the height of the mercury column. [Publishers Imprint]