An open-tube manometer is a device for the measurement of the
the pressure of a fluid, such as that contained in the tank shown on the left.
The tube contains mercury, or water, or oil. One side of the tube is in
contact with the fluid in the tank; the other is in contact with the air.
The fluid in the tank, therefore, presses down on one end of the mercury
column and the air presses down on the other end. The difference h in
the heights of the levels of mercury at the two ends gives the difference
in the pressure at the two ends,

P - Po = Pgh

Hence, this kind of manometer indicates the amount of pressure in the
tank in excess of the atmospheric pressure. This excess is called the
overpressure, or gauge pressure. It is well to keep in mind that
much of the pressure is over pressure. For instance, the pressure
gauges used for automobile tires read overpressure.
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