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A barometer It is an instrument that serves to measure atmospheric pressure, that is, the weight of the air column per unit area exerted by the atmosphere. The most common way is to observe the height of a column of liquid whose weight compensates for the weight of the atmosphere.

The best known is the mercury barometer, invented by Torricelli in 1643. A mercury barometer is formed by a glass tube about 850 mm high, closed at the upper end and open at the bottom. The tube is filled with mercury, inverted and the open end is placed in a bucket filled with the same liquid.

The level of mercury in the tube falls to a height of about 760 mm above the level in the bucket (barometric height) and leaves an almost perfect vacuum in the upper part of the tube (barometric chamber). Variations in atmospheric pressure cause the tube liquid to rise or fall slightly between 737 and 775 mm.

There are also metal barometers, called aneroid or Vidi barometers, which consist of a metal box in which the partial vacuum has been made. The upper cover has a spring that balances the mean atmospheric pressure, while the cover of the case is a membrane of a wavy metallic and elastic alloy that deforms easily, and rises or falls depending on the variation of the Pressure.

These movements modify the position of a needle that travels a circular scale graduated in millimeters of pressure compared to mercury barometers. The most accurate aneroid barometers have an elastic bellows mechanism.

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