The kelvin (symbol: K) is the SI unit of temperature and is one of the seven SI base units. The kelvin is a unit measure of temperature on the thermodynamic (absolute) temperature scale where absolute zero—the coldest possible temperature where there is no heat energy—is defined as zero kelvin (0 K). A degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701-1744), who first proposed a similar system in 1742. ... Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... The electronvolt (symbol eV, or, rarely and incorrectly, ev) is a unit of energy. ... // Comparison of temperature scales ¹ Normal human body temperature is 36. ... Cover of brochure The International System of Units. ... Measurement is the determination of the size or magnitude of something. ... In thermodynamics, temperature is the physical property of a system that underlies the common notions of hot and cold —something that is hotter has the greater temperature. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... In thermodynamics, temperature is the physical property of a system that underlies the common notions of hot and cold —something that is hotter has the greater temperature. ... This article is about the technical details of the thermodynamic concept of temperature. ... Absolute zero is the point on the thermodynamic (absolute) temperature scale where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. ...

Definition of the kelvin

The unit “kelvin” and its scale are, by international agreement, defined by two points: absolute zero, and the triple point of specially prepared ocean water. Absolute zero is defined as being precisely 0 K and –273.15 °C. Absolute zero is where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. At absolute zero, matter contains no heat energy. Also, the triple point of water is defined as being precisely 273.16 K and 0.01 °C. This definition does three things: 1) it fixes the magnitude of the kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water; 2) it establishes that one kelvin has precisely the same magnitude as a one degree increment on the Celsius scale; and 3) it establishes the difference between the two scales’ null points as being precisely 273.15 kelvins (0 K = –273.15 °C and 273.16 K = 0.01 °C). Temperatures in kelvin can be converted to other units per the table at top right. Absolute zero is the point on the thermodynamic (absolute) temperature scale where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. ... In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... // Overview of VSMOW VSMOW, or Vienna Standard Mean Ocean Water, is a standard defined in 1968 by the International Atomic Energy Agency. ... Kinetic energy (also called vis viva, or living force) is energy possessed by a body by virtue of its motion. ... Fig. ... In physics, heat is defined as energy in transit. ... A degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701-1744), who first proposed a similar system in 1742. ...

Some key temperatures relating temperatures on the Kelvin and Celsius scales are shown in the below table.

^A For Vienna Standard Mean Ocean Water at one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. // Overview of VSMOW VSMOW, or Vienna Standard Mean Ocean Water, is a standard defined in 1968 by the International Atomic Energy Agency. ...

SI prefixed forms of kelvin

SI prefixes are often employed to denote decimal multiples and submultiples of the kelvin. The most commonly used factors of kelvin are listed below in bold.^[1] An SI prefix is a prefix that can be applied to an SI unit to form a decimal multiple (supramultiple or submultiple). ...

Typographical conventions

Uppercase/Lowercase and Plural Form Usage

When reference is made to the unit kelvin (either a specific temperature or a temperature interval), kelvin is always spelled with a lowercase k unless it is the first word in a sentence. When reference is made to the Kelvin scale, use an uppercase K.

Until the 13th General Conference on Weights and Measures (CGPM) in 1967-1968, this unit was called a degree just as other temperature degrees are, distinguished from other degrees with the adjective “Kelvin” in “degrees Kelvin,” or often as “degrees absolute” (which was more ambiguous, since it could also refer to degrees Rankine). When the units were degrees, the plural was formed by adding an “s” to degree and like other adjectives in English, the adjective identifying the scale was unchanged in the plural. After the name change, the plural of kelvin is “kelvins.”^[2] When reference is made to the “Kelvin scale,” the word “kelvin”—which is normally a noun—functions adjectivally to modify the noun “scale” (like “Georgia peach”) and is capitalized. The Conférence générale des poids et mesures (General Conference on Weights and Measures or CGPM) is one of the three organizations established to maintain the SI system under the terms of the Metre Convention (1875). ... An adjective is a part of speech which modifies a noun, usually describing it or making its meaning more specific. ...

Formating and Typestyle for the K Symbol

The kelvin symbol is always a roman (non-italic) capital K since the lowercase version is the SI prefix for 1 × 10³. The admonition against italicizing the symbol K applies to all SI unit symbols; only symbols for variables and constants (e.g. P = pressure, and c = 299,792,458 m/s) are italicized in scientific and engineering papers. As with most other SI unit symbols (angle symbols, e.g. 45° 3′ 4″, are the exception,) there is a space between the numeric value and the kelvin symbol (e.g. “99.987 K”). For more information on conventions used in technical writing, see the informative SI Unit rules and style conventions by the NIST. An SI prefix is a prefix that can be applied to an SI unit to form a decimal multiple (supramultiple or submultiple). ... The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ...

The Special Unicode Kelvin Sign

Unicode, which is an industry standard designed to allow text and symbols from all of the writing systems of the world to be consistently represented and manipulated by computers, includes a special “kelvin sign” at U+212A. One types  K  when encoding this special kelvin character in a Web page. Its appearance is similar to an ordinary uppercase K. To better see the difference between the two, below in maroon text is the kelvin character followed immediately by a simple uppercase K: Because of technical limitations, some web browsers may not display some special characters in this article. ... WWWs historical logo designed by Robert Cailliau The World Wide Web (WWW or simply the Web) is a global, read-write information space. ...

KK

When viewed on computers that properly support Unicode, the above line appears as follows (size may vary):

There is a noticeable difference between the two. However, if the computer being used to view a particular Web page doesn’t support the Unicode kelvin sign character (K), it may be canonically decomposed by the browser into U+004B (uppercase K) and the two would appear identical. In still other computers, the kelvin symbol is mapped incorrectly and produces an odd character. In Unicode, canonical decomposition is a process of converting composite characters into canonically-ordered strings of simpler characters using Unicode canonical mappings. ... Icons for Web browser shortcuts on an Apple computer (Safari, Internet Explorer, and Firefox). ...

Accordingly, for Web use, it is better to use the simple uppercase K to represent the kelvin symbol so it can be properly viewed by the widest possible audience.

Why technical articles use a mix of Kelvin and Celsius scales

In science (especially) and in engineering, the Celsius and Kelvin scales are often used simultaneously in the same article (e.g. “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”) This issue had been addressed by Resolution 3 of the 13th CGPM (1967/68) which stated that “a temperature interval may also be expressed in degrees Celsius.” Yet the practice of simultaneously using both “°C” and “K” remains widespread throughout the technical world as the use of prefixed forms of “°C” (such as “µ°C”) has not been well–adopted.

Because the kelvin is an individual unit of measure, it is particularly well-suited for expressing temperature intervals: differences between temperatures or their uncertainties (e.g. “Agar exhibited a melting point hysteresis of 25 kelvins,” and “The error was 10 millikelvins”). Of course, the kelvin is also used to express specific temperatures along its scale (e.g. “The triple point of hydrogen is 13.8033 kelvin”). Note however that to avoid confusion, the singular form of kelvin (no “s” at the end) is preferred when referring to a particular temperature on the scale; when referring to temperature intervals, the plural form is preferred.

Intervals and specific temperatures on the Kelvin scale both utilize the same symbol (e.g. “Agar exhibited a melting point hysteresis of 25 K,” and “The triple point of hydrogen is 13.8033 K”). Thus, wherever ambiguity might exist due to this dual use of the symbol within a document, the full unit name in its plural form, kelvins, is preferred when expressing intervals (e.g. “The helium temperature was 650 mK… and our uncertainty with these other measurements was 15 millikelvins.”)

This practice of using both “°C” and “K” is usually avoided in non-technical articles intended for the general public where both the kelvin and its symbol, K, are not well recognized and could be confusing.

History

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow. William Thomson, 1st Baron Kelvin, GCVO, OM, PC, PRS (26 June 1824–17 December 1907) was an Irish-Scottish mathematical physicist, engineer, and outstanding leader in the physical sciences of the 19th century. ... Various rulers or governments of Europe, of Japan bestow or recognise the title of baron. ... The Kelvin is Glasgows second river after the River Clyde. ... The University of Glasgow, founded in 1451, is the largest of the three universities in Glasgow, Scotland. ...

Comparison of temperature scales

See also

Absolute zero is the point on the thermodynamic (absolute) temperature scale where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. ... A degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701-1744), who first proposed a similar system in 1742. ... Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... In physics, heat is defined as energy in transit. ... The International Temperature Scale of 1990 (ITS-90) is an equipment calibration standard for making measurements on the kelvin and Celsius temperature scales. ... Rankine is a now rarely used temperature scale named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859. ... This article is about the technical details of the thermodynamic concept of temperature. ... In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ...

References

1. ^ The term “most commonly used” is based on those with more than 500 Google hits on the name.
2. ^ Webster's 11th Collegiate; NIST SP 811

External links

• BIPM brochure on the kelvin
• Barry N. Taylor, Guide for the Use of the International System of Units (SI), Washington, D.C.:Government Printing Office, 1995 (html version, pdf version)