INSTRUMENTATION A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site barometer The aneroid barometer comprises a partially evacuated corrugated metal box (a), prevented from collapsing by a spring (b). The strain in this spring, proportional to the difference in pressures between the air inside and that outside the box, is amplified by a train of levers (c) that operator a pointer (d) that moves over a calibrated scale. Aneroid barometers are convenient to use but require regular calibration against an accurate mercury barometer. An instrument for measuring air pressure used in weather forecasting and for determining altitude. Most commonly encountered is the aneroid barometer in which the effect of the air in compressing an evacuated thin cylindrical corrugated metal box is amplified mechanically and read off on a scale or, in the barograph, used to draw a trace on a slowly rotating drum, thus giving a continuous record of the barometric pressure. The aneroid instrument is that used for aircraft altimeter. The earliest barometers, as invented by Torricelli in 1643, consisted simply of a glass tube about 800mm long closed at one end and filled with mercury before being inverted over a pool of mercury. Air pressure acting on the surface of pool held up a column of mercury about 760mm tall in the tube, a "Torricellian" vacuum appearing in the closed end of the tube. The height of the column was read as a measure of the pressure. In the Fortin barometer, devised by Jean Fortin (1750–1831) and still used for accurate scientific work, the lower mercury level can be finely adjusted and the column height is read off with the aid of a vernier scale. Related category    • INSTRUMENTATION Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP