- This article is about the temperature scale; see also Fahrenheit graphics API.
Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724.
In this scale, the freezing point of water is 32 degrees (this is written "32 °F"), and the boiling point is 212 degrees Fahrenheit, placing the boiling and melting points of water 180 degrees apart. Thus the unit of this scale, a degree Fahrenheit, is 5/9ths of a kelvin (or of a degree Celsius), and minus 40 degrees Fahrenheit is equal to minus 40 degrees Celsius.
There are several competing versions of the story of how Fahrenheit came to devise his temperature scale. One 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's blood to calibrate his scale). Initially, his scale had only contained 12 equal subdivisions, but then later he subdivided each division into 8 equal degrees ending up with 96. He then observed that plain water would freeze at 32 degrees and boil at 212 degrees.
Another 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 freezing point of water 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).
His measurements were not entirely accurate, though; by his original scale, the actual freezing 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. This resulted in the healthy human body temperature being 98.6 °F rather than 96 °F.
The Fahrenheit scale was widely used in many English-speaking countries until the Celsius (formerly centigrade) scale was adopted in the late 1960s and 1970s. In the United States and Jamaica, the Fahrenheit system continues to be used by the general population for everyday, non-scientific temperature measurement; despite its murky origins, it remains well-suited for this use as temperature in temperate latitudes tends to range from around 0 °F to 100 °F. Conversion between the Fahrenheit and Celsius scales is given by the formula TC = 5/9 · (TF − 32), where TC and TF are the Celsius and Fahrenheit temperatures, respectively.