Barometer

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Barometer, instrument for measuring atmospheric pressure, that is, the force exerted on a surface of unit area by the weight of the atmosphere. Because this force is transmitted equally in all directions through any fluid, it is most easily measured by observing the height of a column of liquid that, by its weight, exactly balances the weight of the atmosphere. A water barometer is far too large to be used conveniently. Liquid mercury, however, is 13.6 times as heavy as water, and the column of mercury sustained by normal atmospheric pressure is only about 760 mm (about 30 in) high.

Normal, or standard, atmospheric pressure is usually defined at 1013.25 millibars, which is equivalent to 760 mm (29.9213 in) of mercury or 1.03323 kg/sq cm (14.6960 lb/sq in).

An ordinary mercury barometer consists of a glass tube about 840 mm (about 33 in) high, closed at the upper end and open at the lower. When the tube is filled with mercury and the open end placed in a cup full of the same liquid, the level in the tube falls to a height of about 760 mm (about 30 in) above the level in the cup, leaving an almost perfect vacuum at the top of the tube. Variations in atmospheric pressure cause the liquid in the tube to rise or fall by small amounts, rarely below 737 mm (29 in) or above 775 mm (30.5 in) at sea level. When the mercury level is read with a form of gradated scale, known as a vernier attachment, and suitable corrections are made for altitude and latitude (because of the change of gravity), for temperature (because of the expansion or contraction of the mercury), and for the diameter of the tube (because of capillarity), the reading of a mercury barometer is reliable to within 0.1 mm (0.004 in).

A more convenient form of barometer (and one that is almost as accurate) is the aneroid, in which atmospheric pressure bends the elastic top of a partially evacuated drum, actuating a pointer. A suitable aneroid barometer is often used as an altimeter (instrument measuring altitude), because pressure decreases rapidly with increasing altitude (about 25 mm/1 in. of mercury per 305 m/1000 ft at low altitudes).

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