A second type of barometer, a manometer has two arms, one opened to the atmosphere and one closed or connected to a container filled with gas. The pressure exerted by the atmosphere or by gas in a container is proportional to the difference in the mercury levels (h).
R, the proportionality constant is called the gas constant.
Ideal gas – a hypothetical gas whose pressure- volume-temp. behavior can be completely accounted for by the ideal gas equation. The molecules of an ideal gas do not attract or repel one another, and their vol. is negligible compared with the volume of the container.