Air is sucked out of the straw, and the atmospheric air pressure, which acts upon the surface of the liquid in the glass, will force the liquid up through the straw. If a long glass tube, which is sealed at one end and open at the other, is filled with mercury and is then stood upright, with the open end downwards, in a dish containing mercury, then so much mercury will flow out of the tube until a column of mercury not more than 760 mm in height above the mercury surface in the dish remains. The air pressure which acts upon the surface of the mercury in the dish is therefore able to hold up a 760 mm high mercury column. If the same experiment were carried out with water instead of mercury, the tube would have to be at least 33.9 ft. A mercury-filled tube over 760 mm in length as described above, represent the simplest form of the mercury barometer.
Modern barometers are mostly of the aneroid type.
An aneroid barometer consists of a hermetically sealed metal box, exhausted of air. The top and bottom of this box are thin corrugated plates. In the interior of the box is a spring which strives to force the top and bottom plates apart, against the external air pressure. The movement of the top plate is transmitted through a train of levers to a pointer which indicates the atmospheric pressure on an appropriately graduated scale. The unit of atmospheric pressure usually employed in meteorology is the millibar.