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Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Røemer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.The world's average surface air temperature is 15 °C. For information on temperature changes relevant to climate change or Earth's geologic past see: Temperature record.
Technologies Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the gas thermometer.Other important devices for measuring temperature include:
Negative temperatures:See main article: Negative temperature.For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic).
History In 1701, Ole Christensen Røemer (1644-1710) made one of the first practical thermometers. As a temperature indicator it used red wine. The temperature scale used for his thermometer had 0 representing the temperature of a salt and ice mixture (at about 259 K).In 1708 Gabriel Fahrenheit (1686-1736) modified Rømer's scale and switched to mercury for more precise measurement. The Fahrenheit scale is still used in parts of the world.In 1731, René-Antoine Ferchault de Réaumur (1683-1757) made a simpler temperature scale. On this scale 0 represented the freezing point of water (273.15 K) and 80 represented the boiling point (373.15 K). In 1742, Anders Celsius (1701-1744) created an inverted centigrade or Celsius temperature scale in which 0 represented the boiling point of water (373.15 K) and 100 represented the freezing point (273.15 K).In 1744, Carl Linnaeus (1707-1778) suggested reversing the temperature scale of Anders Celsius so that 0 represented the freezing point of water (273.15 K) and 100 represented the boiling point (373.15 K).The kelvin (symbol: K) is the SI unit of temperature. It is defined by two facts: zero kelvins is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin.
Notes # The temperature of the air near the surface of the Earth, is usually determined by a thermometer in a Stevenson screen. The thermometers should be between 1.25 m (4 ft 1 in) and 2 m (6 ft 7 in) above the ground, as defined by the World Meteorological Organization (WMO). The true daily mean, obtained from a thermograph, is approximated by the mean of 24 hourly readings and may differ by 1.0 degrees C from the average based on minimum and maximum readings. [http://eobglossary.gsfc.nasa.gov/Library/glossary.php3?mode=alpha&seg=s]
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