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barometer

(bərŏm`ətər), 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 weatherweather,
state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer), wind, humidity, cloudiness, and precipitation. The term weather
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 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 barographbarograph,
instrument used to make a continuous recording of atmospheric pressure. The pressure-sensitive element, a partially evacuated metal cylinder, is linked to a pen arm in such a way that the vertical displacement of the pen is proportional to the changes in the
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 is a self-recording aneroid barometer; an altimeteraltimeter
, device for measuring altitude. The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises.
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 is often an aneroid barometer used to calculate altitude.

Barometer

An absolute pressure gage specifically designed to measure atmospheric pressure. This instrument is a type of manometer with one leg at zero pressure absolute. See Manometer

The common meteorological barometer (see illustration) is a liquid-column gage filled with mercury. The top of the column is sealed, and the bottom is open and submerged below the surface of a reservoir of mercury. The atmospheric pressure on the reservoir keeps the mercury at a height proportional to that pressure. An adjustable scale, with a vernier scale, allows a reading of column height. Aneroid barometers using metallic diaphragm elements are usually less accurate, though often more sensitive, devices, and not only indicate pressure but may be used to record it. See Pressure measurement

Mercury barometerenlarge picture
Mercury barometer

Barometer

 

a device for measuring atmospheric pressure. The most widely used barometers are liquid barometers, which are based on the balancing of atmospheric pressure by the weight of a column of liquid; aneroid barometers, whose principle of operation is based on the elastic deformations of a membranous capsule; and hypsothermometers, which are based on the dependence of the boiling point of certain liquids—for example, water—on external pressure.

The most accurate standard devices are mercury barometers: because of the high density of mercury, a comparatively short column of liquid, convenient for measurement, is produced in the barometer. Mercury barometers consist of two mercury-filled communicating vessels, one of which is a glass tube approximately 90 cm long that is sealed at the top and evacuated. The pressure of the column of mercury, expressed in millimeters or millibars, is used to measure atmospheric pressure.

In the determination of atmospheric pressure the following corrections are made in the mercurial barometer’s readings: (1) instrumental, which includes the production error; (2) a correction for the reduction of the barometer readings to 0°C, since barometric readings depend upon temperature (the density of mercury and the linear dimensions of the barometer’s components vary with temperature); and (3) a correction for the reduction of barometric readings to the normal acceleration of free fall (gn = 9.80665 m/sec2), which is due to the fact that the readings of the mercury barometer depend on the geographical latitude and the altitude above sea level of the location where the observations are being made.

Regarding the form of the communicating vessels, mercury barometers are divided into three basic types: cistern (cup), siphon, and siphon-cup. In practice, cistern and siphon-cup barometers are used. Station cup barometers are used at meteorological stations. Such a barometer consists of a barometric glass tube lowered by an open end into a cup. The entire barometric tube is contained in a brass holder with a vertical groove in its upper section; on the edge of the groove is etched a scale for reading the position of the meniscus of the mercury column. A special sight, equipped with a vernier and a movable screw, is used for the precise sighting of the apex of the meniscus and the reading of tenths of a unit. The height of the mercury column is read by the position of the mercury in the glass tube, and the change of the position of the mercury level in the cup is accounted for by using a compensated scale, so that the reading on the scale is obtained directly in millibars. In every barometer there is a small mercury thermometer for the introduction of the temperature correction. Cistern barometers are manufactured in the measurement ranges of 810–1070 mbar and 680–1070 mbar; the accuracy of the readings is 0.1 mbar.

The siphon-cup barometer is used as a control. It consists of two tubes lowered into the barometric cup. One of the tubes is closed, and the other is open to the atmosphere. In the measurement of pressure the bottom of the cup is raised by a screw, bringing the meniscus in the open column to the scale’s zero, and then the position of the meniscus in the closed column is read. The pressure is determined by the difference of the mercury levels in the columns. The measurement range of this barometer is 880–1090 mbar, and the precision of the readings is 0.05 mbar.

AD mercurial barometers are absolute instruments, since atmospheric pressure is measured directly by their readings.

REFERENCE

Kedrolivanskii, V. N., and M. S. Sternzat. Meteorologicheskie pribory. Leningrad, 1953.

S. I. NEPOMNIASHCHII

barometer

[bə′räm·əd·ər]
(engineering)
An absolute pressure gage specifically designed to measure atmospheric pressure.

barometer

an instrument for measuring atmospheric pressure, usually to determine altitude or weather changes