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    Pressure, in mechanics, the force per unit area exerted by a liquid or gas on a body or surface, with the force acting at right angles to the surface uniformly in all directions. In the British system, pressure is usually measured in pounds per square inch (PSI); in international usage, in kilograms per square centimeters, or in atmospheres; and in the international metric system (SI), in newtons per square meter. The unit atmosphere (atm) is defined as a pressure of 1.03323 kg/sq cm (14.696 lb/sq in), which, in terms of the conventional mercury barometer, corresponds to 760 mm (29.921 in) of mercury. The unit kilopascal (kPa) is defined as a pressure of 0.0102 kg/sq cm (0.145 lb/sq in).

    Pascal’s law, developed by French mathematician Blaise Pascal, states that the pressure on a fluid is equal in all directions and in all parts of the container. As liquid flows into the large container at the bottom of this illustration, pressure pushes the liquid equally up into the tubes above the container. The liquid rises to the same level in all of the tubes, reguardless of the shape or angle of the tube.

    Boyle’s law, developed by English scientist Robert Boyle, states that the pressure of a gas times its volume is equal to a constant number, for a gas at a constant temperature. This relationship means that pressure increases as volume decreases, and vice versa.

    Most gauges record the difference between the fluid pressure and local atmospheric pressure. For small pressure differences, a U-tube manometer is used. It consists of a U-shaped tube with one end connected to the container and the other open to the atmosphere. Filled with a liquid, such as water, oil, or mercury, the difference in the liquid surface levels in the two manometer legs indicates the pressure difference from local atmospheric conditions. For higher pressure differences, a Bourdon gauge, named after the French inventor Eugène Bourdon, is used. This consists of a hollow metal tube with an oval cross section, bent in the shape of a hook. One end of the tube is closed, the other open and connected to the measurement region. If pressure (above local atmospheric pressure) is applied, the oval cross section will become circular, and at the same time the tube will straighten out slightly. The resulting motion of the closed end, proportional to the pressure, can then be measured via a pointer or needle connected to the end through a suitable linkage. Gauges used for recording rapidly fluctuating pressures commonly employ piezoelectric or electrostatic sensing elements that can provide an instantaneous response.

    As most pressure gauges measure the difference between the fluid and the local atmospheric pressure, the atmospheric pressure must be added to the gauge pressure to arrive at the true absolute pressure. A negative gauge-pressure reading corresponds to a partial vacuum.

    Low gas pressure (down to about 10-6 mm mercury absolute) can be measured by the so-called McLeod gauge, in which a measured volume of gas at the unknown low pressure is compressed at constant temperature to a much smaller volume, and then the pressure is measured directly with a manometer. The unknown pressure is then calculated from Boyle's law. For still lower pressures, various gauges depending on radiation, ionization, or molecular effects are used.




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