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In this scale, the freezing point of water is 32 degrees Fahrenheit (written "32 °F"), the boiling point is 212 degrees, placing the boiling and freezing points of water exactly 180 degrees apart. On the Celsius scale, the freezing and boiling points of water are exactly 100 degrees apart, thus the unit of this scale, a degree Fahrenheit, is 5⁄9 of a degree Celsius. The Fahrenheit scale coincides with the Celsius scale at −40 °F, which is the same temperature as −40 °C.
There are a few competing versions of the story of how Fahrenheit came to devise his temperature scale. According to Fahrenheit himself in an article he wrote in 1724  he determined three fixed points of temperature. The zero point is determined by placing the thermometer in a mixture of ice, water, and ammonium chloride. This is a type of frigorific mixture. The mixture automatically stabilizes its temperature at zero degrees Fahrenheit (0 °F). He then put an alcohol or mercury thermometer into the mixture and let the liquid in the thermometer descend to its lowest point. The second point is the 32nd degree found by mixing ice and water without the salt. His third point, the 96th degree, was the level of the liquid in the thermometer when held in the mouth or under the armpit. Fahrenheit noted that, using this scale, mercury boils at around 600 degrees.
Other theories are similar in nature. One states that Fahrenheit established the zero (0 °F) and 96 °F points on his scale by recording the lowest outdoor temperatures he could measure, and his own body temperature. He took the lowest temperature which he measured in the harsh winter of 1708 through 1709 in his hometown of Danzig (now Gdańsk, Poland) (−17.8 °C) as his zero point. (He was later able to reach this temperature under laboratory conditions using a mixture of ice, sodium chloride and water.)
A variant of this version is that the mixture of ice, salt, and water registered the lowest temperature Fahrenheit could attain in the lab, so he used that for his zero point, using his body temperature as 96 °F.
Fahrenheit wanted to avoid the negative temperatures that the Rømer scale had produced in everyday use. He fixed his own body temperature as 96 °F. (As noted below, the scale has since been re-calibrated so that normal body temperature is closer to 98.6 °F). He then divided his scale into twelve sections, and subsequently each of these into 8 equal subdivisions, producing a scale of 96 degrees. Fahrenheit noted that his scale placed the freezing point of water at 32 °F and the boiling point at 212 °F.
Another story holds that Fahrenheit established the zero of his scale (0 °F) as the temperature at which an equal mixture of ice and salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and ninety-six degrees as the temperature of blood (he initially used horse blood to calibrate his scale, as the normal body temperature of horses is 100 °F). Initially, his scale only contained 12 equal subdivisions, but later he subdivided each division into 8 equal degrees ending up with 96.
A fourth well-known version of the story, as described in the popular physics television series The Mechanical Universe, holds that Fahrenheit simply adopted Rømer’s scale, at which water freezes at 7.5 degrees, and multiplied each value by 4 in order to eliminate the fractions and increase the granularity of the scale (giving 30 and 240 degrees). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he took to be 96 degrees); the melting point of ice was adjusted to 32 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).
A fifth version maintains that Fahrenheit based 0 degrees on an estimate of the temperature at which someone's bare skin would freeze, and 100 degrees on the temperature at which one would be unable to live, therefore making 0 to 100 the livable range for human beings. This, however, is arguable because the human body has been known to survive at temperatures above and below these thresholds.
A sixth version maintains that Fahrenheit marked the melting point of ice, normal human body temperature, and the boiling point of water. He then divided the span from melting to boiling into 180 degrees. Setting the normal human body temperature as 96 resulted in the freezing point and boiling point being 32 and 212, respectively.
A seventh version maintains that the coldest temperature he could achieve in the lab was designated with 0 degrees, and the melting point of butter was 100 degrees.
His measurements were not entirely accurate; by his original scale, the actual melting and boiling points would have been noticeably different from 32 °F and 212 °F. Some time after his death, it was decided to recalibrate the scale with 32 °F and 212 °F as the exact freezing and boiling points of plain water. That change was made to easily convert from Celsius to Fahrenheit and vice versa, with a simple formula. This change also explains why normal human body temperature, once taken as 96 °F by Fahrenheit, is today known to average 98.6 °F.
The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in most English-speaking countries until the 1960s. In the late 1960s and 1970s, the Celsius (formerly Centigrade) scale was phased in by governments as part of the standardizing process of metrication. In the United States and a few other countries (such as Belize) the Fahrenheit system continues to be the accepted standard for non-scientific use. Most other countries have adopted Celsius as the primary scale in all use. Fahrenheit is sometimes used by older generations in English speaking countries, especially for measurement of higher temperatures.
The special Unicode °F character
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 is thus the preferred way of writing the symbol for degrees Fahrenheit.
- Cecil Adams. "On the Fahrenheit scale, do 0 and 100 have any special significance?". The Straight Dope.
- O. Wallace. "What is the History of the Fahrenheit Scale?". WiseGeek.
- Belize Weather Bureau
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