Fahrenheit

Fahrenheit (symbol °F) is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736), after whom the scale is named. Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees—on Fahrenheit's original scale the freezing point of brine was zero degrees.

By the end of the 20th century, most countries used the Celsius scale rather than the Fahrenheit scale, though Canada retains it as a supplementary scale that can be used alongside Celsius. Fahrenheit remains the official scale in several countries: United States, Cayman Islands, Palau, Bahamas and Belize. The Rankine temperature scale was based upon the Fahrenheit temperature scale, with its zero representing absolute zero instead.

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). This puts the boiling and freezing points of water exactly 180 degrees apart. Therefore, 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 °F is equal to an interval of $5/9$ degrees Celsius. The Fahrenheit and Celsius scales intersect at −40° (−40 °F and −40 °C represent the same temperature).

Absolute zero is −273.15 °C or −459.67 °F. The Rankine temperature 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.

Like Celsius, the Fahrenheit scale uses the symbol ° to denote a point on the temperature scale and the letter F to indicate the use of the Fahrenheit scale (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").

History
Fahrenheit proposed his temperature scale in 1724, basing it on three reference points of temperature. 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, at a 1:1:1 ratio. This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat".

According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer's 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 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).

Fahrenheit observed that water boils at about 212 degrees using this scale. Later, other scientists decided to redefine the scale slightly to make the freezing point exactly 32 °F, and the boiling point exactly 212 °F or 180 degrees higher. 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).

Usage
The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in English-speaking countries until the 1960s. In the late 1960s and 1970s, the Celsius scale (known until 1948 as centigrade) replaced Fahrenheit in almost all of those countries, typically during their metrication process.

Fahrenheit is used in the United States, Belize, Palau and the United States territories of Puerto Rico, Guam and the U.S. Virgin Islands for everyday applications. For example, U.S. weather forecasts, food cooking, and freezing temperatures are typically given in degrees Fahrenheit. Scientists, such as meteorologists, use Celsius or Kelvin in all countries. In some nations, both measures are quoted.

Early in the twentieth century Halsey and Dale suggested that the resistance to the use of centigrade (now Celsius) system in the U.S. included the larger size of each degree Celsius and the lower zero point in the Fahrenheit system.

Canada has passed legislation favouring the International System of Units, while also maintaining legal definitions for traditional Canadian imperial units. Canadian weather reports are conveyed using degrees Celsius with occasional reference to Fahrenheit especially for trans border broadcasts. Virtually all Canadian ovens make legal use of the Fahrenheit scale. Thermometers, both digital and analogue, sold in Canada usually employ both the Celsius and Fahrenheit scales. Also, in some instances, (swimming pool temperature or cooking temperatures in Québec for example) temperatures are still expressed in Fahrenheit.

Fahrenheit also remains in occasional use in the United Kingdom, with some newspaper and online weather reports quoting temperatures in both Celsius and Fahrenheit (or having an option to enable this), especially during record-breaking weather. However, TV weather reports are primarily conveyed using degrees Celsius. Cooking instructions on some (but not all) packaged food retain dual temperature scales for the benefit of consumers using older ovens, and dual-unit thermometers are readily available.

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.