6.8 Measurement of Pressure by Manometer
A simple manometer consists of a U-shaped glass tube which contains mercury. In the beginning, the atmospheric pressure at the two open ends of the tube is the same, and hence, the mercury level in the two arms remains the same.
If, on connecting a gas cylinder with the short arm while keeping the longer arm open, the mercury level in the short arm is lower than that in the long arm, then the unknown pressure is more than the atmospheric pressure.
If the mercury level in the short arm is higher than the long arm, then the unknown pressure is less than the atmospheric pressure.