6.7 Measurement of Atmospheric Pressure

6.7 Measurement of Atmospheric Pressure

Atmospheric pressure is usually measured by the height of the mercury column which it can support. Instruments which measure atmospheric pressure are called barometers.

A simple mercury barometer consists of a glass tube about one metre long that is closed at one end. It is completely filled with mercury, then inverted vertically in a dish of mercury. A metre scale is placed by the side of the tube to measure the height of the mercury column. The space in the glass tube over the top of the mercury is almost completely empty, so the pressure there is almost zero.

The pressure at point A in the mercury column is the same as at point B at the surface of mercury in the dish because both points are at the same level. This is equal to the atmospheric pressure P = ρgh acting at the surface of mercury in the dish.

If we put P = 1.013 × 10⁵ Pa at sea level and ρ = 13.6 × 10³ kg m⁻³ for mercury, the height of the mercury column comes out to be 760 mm. By using this instrument, atmospheric pressure at any altitude can be measured in terms of the height of the mercury column.

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For Your Information! Air pressure gauges are used to measure the pressure in motor car tyres.

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For Your Information! A Fortin's Barometer is used in laboratories to measure atmospheric pressure.