Fahrenheit

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 Today, the temperature scale has been replaced by the Celsius scale in most countries,^[3] but it remains the official scale of the United States and Belize^[4] and is retained as a secondary scale in Canada.^[5][6]

Contents 1 Definition and conversions 2 History 3 Usage 4 Unicode representation of symbol 5 Temperatures and intervals 6 See also 7 External links 8 References

Definition and conversions

Fahrenheit temperature conversion formulae

	from Fahrenheit	to Fahrenheit
Celsius	[°C] = ([°F] − 32) × 5⁄9	[°F] = [°C] × 9⁄5 + 32
Kelvin	[K] = ([°F] + 459.67) × 5⁄9	[°F] = [K] × 9⁄5 − 459.67
Rankine	[°R] = [°F] + 459.67	[°F] = [°R] − 459.67
For temperature intervals rather than specific temperatures, 1 °F = 1 °R = 5⁄9 °C = 5⁄9 K Comparisons among various temperature scales

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.^[7] A degree on the Fahrenheit scale is ¹⁄₁₈₀ 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 degree Fahrenheit is equal to an interval of ⁵⁄₉ degrees Celsius. The Fahrenheit and Celsius scales intersect at −40 °F (−40 °F and −40 °C represent the same temperature).^[7]

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.

History

According to an article Fahrenheit wrote in 1724, he based his scale on three reference points of temperature.^[8] In his initial scale (which is not the final Fahrenheit scale), 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 stabilizes its temperature automatically: that stable temperature was defined as 0 °F (-17.78 °C). A mixture of ice and water also stabilizes, either freezing or melting at 32 °F.^[8] The second point, 100 degrees, was the horse body temperature, said at the time to be more stable than that of a human.^[9] The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat."^[10]

According to a letter Fahrenheit wrote to his friend Herman Boerhaave,^[11] his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer’s scale, brine freezes at 0 degrees, ice melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale. 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 six times (since 64 is 2 to the sixth power).^[10][12]

Fahrenheit observed, somewhat incorrectly, that water boils at about 212 degrees using this scale. Later, other scientists decided to redefine the degree slightly to make the freezing point exactly 32°F, and the boiling point exactly 212°F or 180 degrees higher.^[8][10] It is for this reason that normal human body temperature is approximately 98° (oral temperature) on the revised scale (whereas it was 90° on Fahrenheit's multiplication of Rømer, and 96° on his original scale).^[13][14]

Usage

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in some English-speaking countries until the 1960s. In the late 1960s and 1970s, the Fahrenheit scale was replaced by the Celsius (known until 1948 as centigrade) scale in most of these countries as part of a standardization called metrication.

The Fahrenheit scale is still used in the United States and Belize^[4] for everyday applications. For example, U.S. weather forecasts, food cooking, and freezing temperatures are typically given in degrees Fahrenheit. It may be noted, however, that scientists in these countries commonly use Celsius or Kelvin. In the Cayman Islands, the Fahrenheit scale is used at least for weather reporting.^[15]

Various reasons are given for the resistance to the Celsius system in the U.S., including the larger size of each degree Celsius (resulting in the need for decimals where integer Fahrenheit degrees were adequate for much technical work). For example, 68 °F, 69 °F, and 70 °F correspond to 20 °C, 20.6 °C, and 21.1 °C, respectively. Another reason is the lower zero point in the Fahrenheit system which reduces the number of negative signs when measurements such as weather data were averaged.^[16] Nonetheless, most countries which formerly used the Fahrenheit system (of which New Zealand and Australia are examples) have switched entirely to the Celsius system.^[17]

In some countries, both systems are used. In the United Kingdom and Canada,^[6] Celsius is mainly used in the news, weather forecasts, books, magazines and daily conversations, but many outdoor thermometers display temperatures in both Fahrenheit and Celsius. Likewise, indoor thermometers, including both digital and analogue, may be in Fahrenheit, Celsius or both.

Unicode representation of symbol

The Fahrenheit symbol has its own Unicode character: "℉"(U+2109). This is a compatibility character encoded for roundtrip compatibility with legacy CJK encodings (which included it to conform to layout in square ideographic character cells) and vertical layout. Use of compatibility characters is discouraged by the Unicode Consortium. The ordinary degree sign (U+00B0) followed by the Latin letter F ("°F") is thus the preferred way of recording the symbol for degree Fahrenheit.

Temperatures and intervals

The Fahrenheit scale uses (in the same manner as the later Celsius scale) the symbol ° to denote a point on the temperature scale and the letter F to indicate the Fahrenheit scale is being used (e.g. "Gallium melts at 85.5763 °F"), as well as to denote a difference between temperatures or an uncertainty in temperature (e.g. "The output of the heat exchanger experiences an increase of 72 °F" and "Our standard uncertainty is ±5 °F").

See also

• Comparison of temperature scales

External links

Look up fahrenheit in Wiktionary, the free dictionary.

• NOAA web site: Fahrenheit to Celsius converter

References

v · d · eScales of temperature Celsius · Delisle · Fahrenheit · Gas Mark · Kelvin · Leiden · Newton · Planck · Rankine · Réaumur · Rømer · Wedgwood Conversion formulas