The manometer is one of the
simplest tools for measuring gas pressure differences. A manometer is a u-tube.
One side of the "U" is open to atmosphere and the other side is connected to a closed container. The "U" is filled with a fluid. If both sides of the "U" have the same liquid levels then the pressure inside and the pressure outside are the same. The difference between the liquid levels equals the pressure difference between inside and outside.
The mercury level will be lower on the side with greater pressure. The higher pressure "pushes" the mercury down
. The Manometer measures the Gauge pressure. Mercury is particularly convenient for use in manometers because at room temperature it has low vapor pressure, does not wet glass, and has a high density.