Torr

The torr (symbol: Torr) is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere (101325 Pa). Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa). Historically, one torr was intended to be the same as one 'millimeter of mercury'. However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr. The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644. The unit's name torr is written in lower case, while its symbol ('Torr') is always written with upper-case initial; including in combinations with prefixes and other unit symbols, as in 'mTorr' (millitorr) or 'Torr⋅L/s' (torr-litres per second). The symbol (uppercase) should be used with prefix symbols (thus, mTorr and millitorr are correct, but mtorr and milliTorr are not). The torr is sometimes incorrectly denoted by the symbol 'T', which is the SI symbol for the tesla, the unit measuring the strength of a magnetic field. Although frequently encountered, the alternative spelling 'Tor' is incorrect. 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.