We can measure atmospheric pressure P0 using a liquid column in a tube. This arrangement is shown in the figure and is called barometer. (There are several other modifications). Mercury is filled in a tube of length about 1m. Its mouth is covered (by thumb,say) and inverted into a vessel containing mercury. It is held in position by a stand and mouth is now uncovered.
It is observed that some mercury comes out in the vessel. A vacant space is created at the top (closed) end.
This space is historically called Torricelli’s vacuum, although same mercury vapour is present in it.
If pressure at the top of mercury column be P1, we have, for equilibrium of column,
P1 A + A ρ0 gh = P0 A
Here P1 is mercury vapour pressure which we generally ignore. Then
P0 = ρ0 gh
The atmospheric pressure is, thus, measured if h be measured.
The normal density (ρ0) of mercury is 13.6 g cm–3 and g = 981 cms–1. For a given place and temperature, these data are fixed. Hence directly h can be used to show pressure. One atmospheric pressure is a unit of pressure equal to 76 cm of Hg
