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The torr (symbol: Torr) is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimeter of mercury, i.e. a pressure of 1 Torr is approximately equal to 1 mmHg. Note that the symbol is spelled exactly the same as the unit, but the symbol is capitalized, as is customary in metric units derived from names. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.[1]



Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressure. Scientists at the time were familiar with small fluctuations in height that occurred in barometers. When these fluctuations were explained as a manifestation of changes in atmospheric pressure, the science of meteorology was born.

Over time, 760 millimeters of mercury (abbreviated mmHg) came to be regarded as the standard atmospheric pressure. In honor of Torricelli, the torr was defined as a unit of pressure equal to one mmHg.

In 1954, the definition of the atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM)[2] to the currently accepted definition: one atmosphere is equal to 101,325 pascals. The torr was then re-defined as 1760 of one atmosphere. This was necessary in place of the definition of a torr as 1 mmHg, because the height of mercury changes at different temperatures and gravities.

SI units of pressure

The SI unit of pressure is the pascal (symbol: Pa), defined as one newton per square meter. Other units of pressure are defined in terms of SI units.[3][4] These include:

  • The bar (symbol: bar), defined as 100 kPa exactly.
  • The atmosphere (symbol: atm), defined as 101.325 kPa exactly.
  • The torr (symbol: Torr), defined as 1760 atm exactly.

These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures.[5] The torr, a more convenient unit for low pressures, is used in high-vacuum physics and engineering.

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