barometer

barometer , instrument for measuring atmospheric pressure. It was invented in 1643 by the Italian scientist Evangelista Torricelli, who used a column of water in a tube 34 ft (10.4 m) long. This inconvenient water column was soon replaced by mercury, which is denser than water and requires a tube about 3 ft (0.9 m) long. The mercurial barometer consists of a glass tube, sealed at one end and filled with pure mercury. After being heated to expel the air, it is inverted in a small cup of mercury called the cistern. The mercury in the tube sinks slightly, creating above it a vacuum (the Torricellian vacuum). Atmospheric pressure on the surface of the mercury in the cistern supports the column in the tube, which varies in height with variations in atmospheric pressure and hence with changes in elevation, generally decreasing with increases in height above sea level. Standard sea-level pressure is 14.7 lb per sq in. (1,030 grams per sq cm), which is equivalent to a column of mercury 29.92 in. (760 mm) in height; the decrease with elevation is approximately 1 in. (2.5 cm) for every 900 ft (270 m) of ascent. In weather forecasting, barometric readings are usually measured on electronically controlled instruments often tied to computers. The results are plotted on base maps so that analyses of weather-producing pressure systems can be made. At a given location a storm is generally anticipated when the barometer is falling rapidly; when the barometer is rising, fair weather may usually be expected. The aneroid barometer is a metallic box so made that when the air has been partially removed from the box the surface depresses or expands with variation of air pressure on it; this motion is transmitted by a train of levers to a pointer which shows the pressure on a graduated scale. A barograph is a self-recording aneroid barometer; an altimeter is often an aneroid barometer used to calculate altitude.

barometer, an instrument that measures atmospheric pressure either by a column of mercury or, in the case of the aneroid barometer, by a thin metal cylinder partially compressed by the atmosphere.

Mike Richey

barometer Instrument for measuring atmospheric pressure. There are two basic types. A mercury barometer has a vertical column of mercury, whose length alters with changes in atmospheric pressure. An aneroid barometer has a chamber containing a partial vacuum, and the chamber changes shape with shifts in pressure. Barometers are used in weather forecasting to predict local weather changes: a rising barometer (increasing pressure) indicates dry weather; a falling barometer indicates wet weather. In a barograph, the pointer of an aneroid barometer is replaced by a pen that traces variations in pressure on a revolving cylindrical chart. A barometer can also be used in an altimeter to measure altitude by indicating changes in atmospheric pressure. See also bar