Barometer
From Encyclopædia
The barometer, the single most important meteorological instrument, measures atmospheric
pressure.Mercury BarometerThe invention of the mercury barometer (1643) by Evangelista TORRICELLI depended on his realization that air has weight. He noted that if the open end of a glass tube filled with mercury is inverted in a bowl of mercury, the atmospheric
pressure on the bowl of mercury will affect the height of the
column of mercury in the glass tube. The greater the air
pressure, the longer is the mercury
column. The atmospheric
pressure may be calculated by multiplying the height of the mercury
column by the mercury
density and the acceleration due to gravity. At sea level, atmospheric
pressure is equal to about 15 lb per sq in, or 29.9 in. of mercury. This is equivalent to 101.3 kilopascals, the
pressure unit meteorologists now use, besides millibars.Mercury is ideal for a liquid barometer, since its high
density permits a short
column, whereas a water barometer would be 10 m (33 ft) tall at one
atmosphere. Another advantage of mercury is its negligible vapor
pressure. This is important because the few mercury vapor
molecules in the empty space above the mercury
column will add only slightly to the
pressure exerted by the mercury
column itself. In a water barometer the vapor would cause an error of 2 percent at 15 deg C (59 deg F).Aneroid BarometerMost barometers are of the aneroid type and function without liquid. The aneroid barometer, dating from 1843, consists of a small
metal box, almost totally evacuated of air. One side is immovable, and the opposite side is connected to a strong spring to keep the box from collapsing. The movable side will expand if the air
pressure decreases and will compress if the air
pressure increases. The position of the movable side is indicated by a pointer. An aneroid barometer is checked regularly against a mercury barometer for calibration.The aneroid barometer can be easily converted into a barograph, or recording barometer, by adding a pen to the pointer. The ink in the pen describes a trace (barogram) on the paper wrapped around a cylinder. The cylinder usually rotates once a day or once a week.The mercury barometer is used in research laboratories and in the most important weather stations. Aneroid barometers, used in the home, on board
ships, and in all weather stations, are also a prominent part of RADIOSONDE instruments.Willem Van Der BijlBibliography: Bolle, Bert, Barometers (1984); Middleton, W. E. K., Invention of the Meteorological Instruments (1969); Wang, J.-Y., Instruments for Physical Environmental Measurements, 2d ed. (1983).Picture Caption[s]A mercury barometer (left) consists of an upright mercury-filled glass tube with its lower end submerged in an adjustable mercury reservoir and a vernier-equipped scale for reading the mercury
column height. A
cross-section of an aneroid barometer (right) reveals a small evacuated
metal box, with an immobilized end and a movable end that expands and
contracts with changes in air
pressure. Any motion of the movable end is transmitted by means of a spring, a system of levers, and a chain to a rotating needle, which indicates the air
pressure on a graduated scale.
