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{{about|the unit of temperature}}{{2019 SI redefinition}}{{pp-move-indef}}{{Use dmy dates|date=August 2011}}{{Use British English|date=November 2018}}

The kelvin is the base unit of temperature in the International System of Units (SI), having the unit symbol K. It is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).The kelvin is defined by fixing the numerical value of the Boltzmann constant {{Mvar|k}} to 1.380 649×10−23 Jâ‹…K−1. This unit is equal to kgâ‹…m2â‹…s−2â‹…K−1, where the kilogram, metre and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition.WEB,weblink BIPM - SI Brochure,, 2019-08-01, Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously, such as the IPK, whose mass diverged over time from the original value.One kelvin is equal to a change in the thermodynamic temperature {{Mvar|T}} that results in a change of thermal energy {{Mvar|kT}} by 1.380 649×10−23 J.WEB,weblink Mise en pratique, BIPM, The Kelvin scale fulfills Thomson's requirements as an absolute thermodynamic temperature scale. It uses absolute zero as its null point.Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree. The kelvin is the primary unit of temperature measurement in the physical sciences, but is often used in conjunction with the degree Celsius, which has the same magnitude.


{{see also|Thermodynamic temperature#History}}File:Lord Kelvin photograph.jpg|thumb|upright|Lord Kelvin, the namesake of the unit]]In 1848, William Thomson, who later was made Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby "infinite cold" (absolute zero) was the scale's null point, and which used the degree Celsius for its unit increment. Kelvin calculated that absolute zero was equivalent to −273 Â°C on the air thermometers of the time.JOURNAL, Lord Kelvin, William, On an Absolute Thermometric Scale, Philosophical Magazine, October 1848,weblink 2008-02-06, no,weblink" title="">weblink 1 February 2008, dmy-all, This absolute scale is known today as the Kelvin thermodynamic temperature scale. Kelvin's value of "−273" was the negative reciprocal of 0.00366—the accepted expansion coefficient of gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.In 1954, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to exactly 273.16 kelvins.WEB, Resolution 3: Definition of the thermodynamic temperature scale, Resolutions of the 10th CGPM, Bureau International des Poids et Mesures,weblink 1954, 2008-02-06, no,weblink" title="">weblink 23 June 2007, dmy-all, In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K. Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction {{sfrac|273.16}} of the thermodynamic temperature of the triple point of water."WEB,weblink Resolution 4: Definition of the SI unit of thermodynamic temperature (kelvin), 1967, Resolutions of the 13th CGPM, Bureau International des Poids et Mesures,weblink" title="">weblink 15 June 2007, no, 2008-02-06, dmy-all, In 2005, the Comité International des Poids et Mesures (CIPM), a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition specified as Vienna Standard Mean Ocean Water.WEB, Unit of thermodynamic temperature (kelvin), SI Brochure, 8th edition, Section,weblink Bureau International des Poids et Mesures, 1967, 2008-02-06, yes,weblink" title="">weblink 26 September 2007, On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. With this change the triple point of water became an empirically determined value of approximately 273.16 kelvin. For legal metrology purposes, the new definition officially came into force on 20 May 2019, the 144th anniversary of the Metre Convention.{{citation|title=Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)|url=}}

Usage conventions

When spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm (e.g. "the triple point of water is not exactly 273.16 kelvins"WEB, Rules and style conventions for expressing values of quantities, SI Brochure, 8th edition, Section,weblink Bureau International des Poids et Mesures, 1967, 2012-08-27, no,weblink" title="">weblink 16 July 2012, dmy-all, ). When reference is made to the "Kelvin scale", the word "kelvin"—which is normally a noun—functions adjectivally to modify the noun "scale" and is capitalized. 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").WEB, SI Unit rules and style conventions,weblink National Institute of Standards and Technology, September 2004, 2008-02-06, no,weblink" title="">weblink 5 February 2008, dmy-all, WEB, Rules and style conventions for expressing values of quantities, SI Brochure, 8th edition, Section 5.3.3,weblink Bureau International des Poids et Mesures, 1967, 2015-12-13, no,weblink" title="">weblink 23 September 2015, dmy-all, Before the 13th CGPM in 1967–1968, the unit kelvin was called a "degree", the same as with the other temperature scales at the time. It was distinguished from the other scales with either the adjective suffix "Kelvin" ("degree Kelvin") or with "absolute" ("degree absolute") and its symbol was °K. The latter term (degree absolute), which was the unit's official name from 1948 until 1954, was ambiguous since it could also be interpreted as referring to the Rankine scale. Before the 13th CGPM, the plural form was "degrees absolute". The 13th CGPM changed the unit name to simply "kelvin" (symbol: K).JOURNAL, Barry N. Taylor, Guide for the Use of the International System of Units (SI), Special Publication 811, .PDF, National Institute of Standards and Technology,weblink 2008, 2011-03-05, no,weblink" title="">weblink 3 June 2016, dmy-all, The omission of "degree" indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales (although the Rankine scale continued to use "degree Rankine"), but rather an absolute unit of measure which can be manipulated algebraically (e.g. multiplied by two to indicate twice the amount of "mean energy" available among elementary degrees of freedom of the system).

Use in conjunction with degrees Celsius

(File:CelsiusKelvinThermometer.jpg|right|thumb|upright|A thermometer calibrated in degrees Celsius (left) and kelvins (right))In science and engineering, degrees Celsius and kelvins are often used simultaneously in the same article, where absolute temperatures are given in degrees Celsius, but temperature intervals are given in kelvins. E.g. "its measured value was {{val|0.01028|u=degC}} with an uncertainty of 60 ÂµK".{{cn|date=September 2019}}This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing relative temperatures, and the magnitude of the degree Celsius is exactly equal to that of the kelvin.WEB, Units with special names and symbols; units that incorporate special names and symbols, SI Brochure, 8th edition, Section 2.2.2, Table 3,weblink Bureau International des Poids et Mesures, 2006, 2016-06-27, yes,weblink" title="">weblink 18 June 2007, dmy-all, Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states "a temperature interval may also be expressed in degrees Celsius",WEB, Resolution 3: SI unit of thermodynamic temperature (kelvin), Resolutions of the 13th CGPM,weblink Bureau International des Poids et Mesures, 1967, 2008-02-06, no,weblink" title="">weblink 21 April 2007, dmy-all, the practice of simultaneously using both °C and K is widespread throughout the scientific world. The use of SI prefixed forms of the degree Celsius (such as "µ°C" or "microdegree Celsius") to express a temperature interval has not been widely adopted.{{cn|date=September 2019}}

2019 redefinition

In 2005 the CIPM embarked on a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann's constant takes the exact value {{val|1.3806505|e=-23|u=J|up=K}}.WEB,weblink Draft Chapter 2 for SI Brochure, following redefinitions of the base units, Ian Mills, CCU, 29 September 2010, 2011-01-01, no,weblink" title="">weblink 10 January 2011, dmy-all, The committee had hoped that the programme would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger programme.PRESS RELEASE,weblink General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram., General Conference on Weights and Measures, Sèvres, France, 23 October 2011, 25 October 2011, no,weblink" title="">weblink 9 February 2012, dmy-all, The redefinition was further postponed in 2014, pending more accurate measurements of Boltzmann's constant in terms of the current definition,WEB
, Report on the Meeting of the CODATA Task Group on Fundamental Constants
, 3–4 November 2014
, B.
, Wood
, 7
, [BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion.
, no
,weblink" title="">weblink
, 13 October 2015
, dmy-all
but was finally adopted at the 26th CGPM in late 2018, with a value of {{mvar|k}} = {{val|1.380649|e=-23|u=J|up=K}}.JOURNAL
, The CODATA 2017 values of h, e, k, and NA for the revision of the SI
, Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants
, D B, Newell, F, Cabiati, J, Fischer, K, Fujii
, S G, Karshenboim, H S, Margolis, E, de Mirandés
, P J, Mohr, F, Nez, K, Pachucki, T J, Quinn
, B N, Taylor, M, Wang, B M, Wood, Z, Zhang
, Metrologia, 55, 1, 29 January 2018
, 10.1088/1681-7575/aa950a, free
, From a scientific point of view, the main advantage is that this will allow measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant. It also has the philosophical advantage of being independent of any particular substance. The challenge was to avoid degrading the accuracy of measurements close to the triple point. From a practical point of view, the redefinition will pass unnoticed; water will still freeze at 273.15 K (0 °C),WEB,weblink Updating the definition of the kelvin, International Bureau for Weights and Measures (BIPM), 2010-02-23, no,weblink" title="">weblink 23 November 2008, dmy-all, and the triple point of water will continue to be a commonly used laboratory reference temperature.The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of {{val|1.38064903|(51)|e=-23|u=J|up=K}}, with a relative standard uncertainty of {{val|3.7|e=-7}}. Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now {{val|273.1600|(1)|u=K}}.

Practical uses


Colour temperature

{{see also|Stefan–Boltzmann constant}}The kelvin is often used as a measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about {{val|4000|u=K}} appear reddish, whereas those above about {{val|7500|u=K}} appear bluish. Colour temperature is important in the fields of image projection and photography, where a colour temperature of approximately {{val|5600|u=K}} is required to match "daylight" film emulsions. In astronomy, the stellar classification of stars and their place on the Hertzsprung–Russell diagram are based, in part, upon their surface temperature, known as effective temperature. The photosphere of the Sun, for instance, has an effective temperature of {{val|5778|u=K}}.Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide is that higher colour temperature produces an image with enhanced white and blue hues. The reduction in colour temperature produces an image more dominated by reddish, "warmer" colours.

Kelvin as a unit of noise temperature

In electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate noise floor, i.e. the noise temperature. The so-called Johnson–Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise.

Unicode character

The symbol is encoded in Unicode at code point {{unichar|212A|kelvin sign}}. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using {{unichar|004B|latin capital letter k}} instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: {{unichar|2126|ohm sign}}, {{unichar|212A|kelvin sign}}, and {{unichar|212B|angstrom sign}}. In all three instances, the regular letter should be used."BOOK, The Unicode Standard, Version 8.0, August 2015, The Unicode Consortium, Mountain View, CA, USA, 978-1-936213-10-8, 22.2,weblink 6 September 2015, no,weblink" title="">weblink 6 December 2016, dmy-all,

See also



External links

  • JOURNAL, Bureau International des Poids et Mesures, The International System of Units (SI) Brochure, 8th Edition, International Committee for Weights and Measures,weblink 2006, 2008-02-06,
{{Scales of temperature}}{{SI units}}

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