Barometer
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