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Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German[1][2] physicist Daniel Gabriel Fahrenheit (1686–1736). Today, the temperature scale has been replaced by the Celsius scale in most countries,[3] but it is still used in a handful of nations such as the United States and Belize.[4]


Definition and conversions

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F (at standard atmospheric pressure), placing the boiling and freezing points of water exactly 180 degrees apart.[5] A degree on the Fahrenheit scale is 1180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 Fahrenheit degree is equal to an interval of 59 degrees Celsius. The Fahrenheit and Celsius scales converge at −40 °F (i.e. −40 °F and −40 °C represent the same temperature).[5]

Absolute zero is defined as −273.15 °C. The Rankine temperature scale was created to use degree intervals the same size as those of the Fahrenheit scale, such that a temperature difference of one degree Rankine (1 °R) is equal to a difference of 1 °F, 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.


According to an article Fahrenheit wrote in 1724,[6] he based his scale on two reference points of temperature.[7] The zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt. This is a frigorific mixture which automatically stabilizes its temperature at 0 °F. (A mixture of ice and water also stabilizes, either freezing or melting at 32 °F though Fahrenheit did not use this point in defining his temperature scale). The second point, 100 degrees, was the level of the liquid in the thermometer when held in the mouth or under the armpit of his wife — subsequent refinements. Fahrenheit noted that, using this scale, water boils at about 212 degrees.[7]

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