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The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined his scale exist, but the original paper suggests the lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from a mixture of water, ice, and ammonium chloride (a salt). The other limit established was his best estimate of the average human body temperature, originally set at 90 °F, then 96 °F (about 2.6 °F less than the modern value due to a later redefinition of the scale).
For much of the 20th century, the Fahrenheit scale was defined by two fixed points with a 180 °F separation: the temperature at which pure water freezes was defined as 32 °F and the boiling point of water was defined to be 212 °F, both at sea level and under standard atmospheric pressure. It is now formally defined using the Kelvin scale.
It continues to be used in the United States (including its unincorporated territories), its freely associated states in the Western Pacific (Palau, the Federated States of Micronesia and the Marshall Islands), the Cayman Islands, and Liberia.
Fahrenheit is commonly still used alongside the Celsius scale in other countries that use the U.S. metrological service, such as Antigua and Barbuda, Saint Kitts and Nevis, the The Bahamas, and Belize. A handful of British Overseas Territories, including the Virgin Islands, Montserrat, Anguilla, and Bermuda, also still use both scales. All other countries now use Celsius ("centigrade" until 1948), which was invented 18 years after the Fahrenheit scale.Celsius, Anders (1742) "Observationer om twänne beständiga grader på en thermometer" (Observations about two stable degrees on a thermometer), Kungliga Svenska Vetenskapsakademiens Handlingar (Proceedings of the Royal Swedish Academy of Sciences), 3: 171–180 and Fig. 1.
Absolute zero is 0 K, −273.15 °C, or −459.67 °F. The Rankine Scale uses degree intervals of the same size as those of the Fahrenheit scale, except that absolute zero is 0 °R the same way that the Kelvin temperature scale matches the Celsius scale, except that absolute zero is 0 K.
The combination of degree symbol (°) followed by an uppercase letter F is the conventional symbol for the Fahrenheit temperature scale. A number followed by this symbol (and separated from it with a space) denotes a specific temperature point (e.g., "Gallium melts at 85.5763 °F"). A difference between temperatures or an uncertainty in temperature is also conventionally written the same way as well, e.g., "The output of the heat exchanger experiences an increase of 72 °F" or "Our standard uncertainty is ±5 °F". However, some authors instead use the notation "An increase of " (reversing the symbol order) to indicate temperature differences. Similar conventions exist for the Celsius scale, see .
There is also an exact conversion between Celsius and Fahrenheit scales making use of the correspondence −40 °F ≘ −40 °C. Again, is the numeric value in degrees Fahrenheit, and the numeric value in degrees Celsius:
According to a German story, Fahrenheit actually chose the lowest air temperature measured in his hometown Danzig (Gdańsk, Poland) in winter 1708–09 as 0 °F, and only later had the need to be able to make this value reproducible using brine.
According to a letter Fahrenheit wrote to his friend Herman Boerhaave,Ernst Cohen and W. A. T. Cohen-De Meester. Chemisch Weekblad, volume 33 (1936), pages 374–393, cited and translated in http://www.sizes.com:80/units/temperature_Fahrenheit.htm. his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by 4 in order to eliminate fractions and make the scale more granularity. He then re-calibrated his scale using the melting point of ice and normal human body temperature (which were at 30 and 90 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees, and body temperature 96 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval 6 times (since 64 = 26). (2008). 9780521715904, Cambridge University Press. ISBN 9780521715904
Fahrenheit soon after observed that water boils at about 212 degrees using this scale.Fahrenheit, Daniele Gabr. (1724) "Experimenta circa gradum caloris liquorum nonnullorum ebullientium instituta". (Experiments performed concerning the degree of heat of some boiling liquids), Philosophical Transactions of the Royal Society of London, 33 : 1–3. For an English translation, see: Le Moyne College (Syracuse, New York). The use of the freezing and boiling points of water as thermometer fixed reference points became popular following the work of Anders Celsius, and these fixed points were adopted by a committee of the Royal Society led by Henry Cavendish in 1776–77.Hasok Chang, Inventing Temperature: Measurement and Scientific Progress, pp. 8–11, Oxford University Press, 2004, . Under this system, the Fahrenheit scale is redefined slightly so that the freezing point of water was exactly 32 °F, and the boiling point was exactly 212 °F, or 180 degrees higher. It is for this reason that normal human body temperature is approximately 98.6 °F (oral temperature) on the revised scale (whereas it was 90° on Fahrenheit's multiplication of Rømer, and 96° on his original scale).
In the present-day Fahrenheit scale, 0 °F no longer corresponds to the eutectic temperature of ammonium chloride brine as described above. Instead, that eutectic is at approximately 4 °F on the final Fahrenheit scale.
The Rankine scale was based upon the Fahrenheit temperature scale, with its zero representing absolute zero instead.
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The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in Anglophone countries until the 1960s. In the late 1960s and 1970s, the Celsius scale replaced Fahrenheit in almost all of those countries—with the notable exception of the United States.
Fahrenheit is used in the United States, its territories and associated states (all serviced by the U.S. National Weather Service), as well as the (British) Cayman Islands and Liberia for everyday applications. The Fahrenheit scale is in use in U.S. for all temperature measurements including weather forecasts, cooking, and food freezing temperatures, however for scientific research the scale is Celsius and Kelvin.
For example, The Times has an all-metric daily weather page but includes a Celsius-to-Fahrenheit conversion table. Some UK tabloids have adopted a tendency of using Fahrenheit for mid to high temperatures. It has been suggested that the rationale to keep using Fahrenheit was one of emphasis for high temperatures: "−6 °C" sounds colder than "21 °F", and "94 °F" sounds more sensational than "34 °C".
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