Manometer

Manometer is an instrument used for measuring the pressure of a gas enclosed in a definite space. A simple form of the instrument consists of a box containing mercury; through the air-tight cover of the box passes a tube into the mercury, the upper end of the tube being open to the air. The vapour whose pressure is to be measured enters the box by another inlet, and if its pressure is greater than that of the air, mercury is forced up the tube, the height to which it rises indicating the pressure of the vapour. The change of form of a metallic tube, when filled with gases of different pressures, is the principle upon which metallic manometers work.