Fahrenheit

Thermometer with Fahrenheit units on the outer scale and Celsius units on the inner scale

Fahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit (1686–1736). Today, the temperature scale has been replaced by the Celsius scale in most countries.^[1] It is still in use in few nations, such as United States and Belize.^[2]

Definition and conversions

On the Fahrenheit scale , the freezing point of water is 32 Fahrenheit (F) and the boiling point 212F (at standard atmospheric pressure), placing the boiling and freezing points of water exactly 180F apart.^[3] 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 Fahrenheit is equal to an interval of ⁵⁄₉ degrees Celsius. The Fahrenheit and Celsius scales converge at −40F (i.e. −40F and −40 °C represent the same temperature).^[3]

Absolute zero is −459.67F.^[4] 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 1F, 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. In science there are no degrees in the Fahrenheit scale and degrees only exist in the Centigrade or Celsius Scale. Example, 32F (not 32°F) is 0°C.

History

According to an article Fahrenheit wrote in 1724,^[5] he based his scale on three reference points of temperature. 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,^[6] though Fahrenheit did not use this point in defining his temperature scale. The third point, 96 degrees, was the level of the liquid in the thermometer when held in the mouth or under the armpit of his wife. Fahrenheit noted that, using this scale, mercury boils at around 600 degrees.

Later, work by other scientists observed that water boils about 180 degrees higher than the freezing point and decided to redefine the degree slightly to make it exactly 180 degrees higher.^[5] It is for this reason that normal human body temperature is 98.6 on the revised scale (whereas it was 96 on Fahrenheit's original scale).^[7]

According to a letter Fahrenheit wrote to his friend Herman Boerhaave,^[8] his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer’s scale, the two fixed reference points are that brine also freezes at 0 degrees and water boils at 60 degrees. He observed that, on this scale, water freezes at 7.5 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale (resulting in 30 and 240 degrees, respectively). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he observed to be 96 degrees); he adjusted the scale so that the melting point of ice would be 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).^[9]

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 Celsius (known until 1948 as centigrade) scale was adopted by most of these countries as part of the standardizing process called metrication.

The Fahrenheit scale is used in the United States, Jamaica, Palau, Belize^[2], Burma, and Liberia^[10] for non-scientific applications. Most other countries have adopted Celsius as the primary scale in all use.

In the U.S., weather forecasts, food cooking and freezing temperatures are commonly in degrees Fahrenheit.

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.^[11] On the other hand, most countries which formerly used the Fahrenheit system, such as New Zealand and Australia, have totally switched to the Celsius system.^[12]

Even though in Canada weather is always reported in degrees Celsius, many outdoor thermometers display temperatures in both Fahrenheit and Celsius. Inside buildings, digital or analogue thermostats may be either in Fahrenheit or Celsius, and kitchen ovens are in both degrees Fahrenheit and degrees Celsius.

Representation

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

As with the Celsius scale, the same symbol, °, is used to denote both a point on the temperature scale, with a letter (C, F) indicating which scale is being used (e.g. "Gallium melts at 85.5763 °F"), and to denote a difference between temperatures or an uncertainty of temperature (e.g. "The output of the heat exchanger is hotter by 72 °F" and "Our standard uncertainty is ±5 °F").

See also

• Comparison of temperature scales

External links

• NOAA web site: Fahrenheit to Celsius converter

References