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Today, the scale has been replaced by the
Celsius scale in most countries; it is still
in use for non-scientific purposes in the United States and a few other nations, such as Belize.
Definition and conversions
On the Fahrenheit scale, the
freezing
point of
water is 32 degrees Fahrenheit
(°F) and the
boiling point (at
standard atmospheric
pressure), placing the boiling and freezing points of water
exactly 180 degrees apart. A degree on the Fahrenheit scale is 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, hence the
unit
of this scale. A temperature interval of 1 degree Fahrenheit is
equal to an interval of degrees Celsius. The Fahrenheit and Celsius
scales converge at −40 degrees (i.e. and represent the same
temperature).
Absolute zero is . 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 Rankine is equal
to a difference of , except that absolute zero is – the same way
that the
Kelvin temperature scale
matches the Celsius scale, except that absolute zero is .
History
According to a journal article Fahrenheit wrote in 1724, 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 type of
frigorific mixture. The mixture
automatically stabilizes its temperature at . He then put a
thermometer into the mixture and let the
liquid in the thermometer descend to its lowest
point. The second point is the 32 degree found by putting the
thermometer in still water as ice is just forming on the surface.
The third point, the 96 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.
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. It is for
this reason that
normal body
temperature is 98.6 on the revised scale (whereas it was 96 on
Fahrenheit's original scale).
According to a letter Fahrenheit wrote to his friend
Herman Boerhaave, his scale 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).
Usage
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 1960's and 1970's, the
Celsius (known until 1948 as
centigrade) scale was adopted by most of these countries
as part of the standardizing process called
metrication.
Only in
the United
States and a few other countries (such as Belize) does the
Fahrenheit system continue to be used, and only for non-scientific
use. Most other countries have adopted Celsius as the
primary scale in all use, although Fahrenheit continues to be the
scale of preference for a minority of people in the United Kingdom,
particularly when referring to summer temperatures.
In the U.S., weather forecasts, food cooking and freezing
temperatures are common in Fahrenheit.
Resistance to the Celsius system was partly due to the larger size
of each degree Celsius, resulting in the need for fractions, where
integral Fahrenheit degrees were adequate for much technical work.
The lower zero point in the Fahrenheit system reduced the number of
negative signs when measurements such as weather data were
averaged.
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 "), and to denote a difference
between temperatures or an uncertainty of temperature (e.g. "The
output of the heat exchanger is hotter by " and "Our standard
uncertainty is ").
See also
References
- Ernst Cohen and W. A. T. Cohen-De Meester. Chemisch
Weekblad, volume 33 (1936), pages 374–393, cited and
translated in
http://www.sizes.com:80/units/temperature_Fahrenheit.htm
- [1]Halsey, Frederick A., Dale, Sanuel S., "The
metric fallacy," The American institute of weights and measures,
Second Edition, 1919. Pages 165-166, 176-177. Retrieved May 19,
2009