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The kelvin is the
base unit
base unit
of
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI), having the unit symbol K. It is named after the British scientist,
Belfast Belfast ( ; , ) is the capital and largest city of Northern Ireland, standing on the banks of the River Lagan on the east coast. It is the 12th-largest city in the United Kingdom and the second-largest on the island of Ireland. It had a popul ...

Belfast
-born
Glasgow University , image_name = University_of_Glasgow_Coat_of_Arms.jpg , image_size = 150px , latin_name = Universitas Glasguensis , motto = la, Via, Veritas, Vita ''Via et veritas et vita'' (, ) is a Latin language, Latin phrase meaning "the way and the t ...
engineer and physicist
William Thomson, 1st Baron Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British , and born in . at the for 53 years, he did important work in the of electricity and formulation of the first and second , and did much to unify the emergi ...
(1824–1907). The Kelvin scale fulfills Thomson's requirements as an
absoluteAbsolute may refer to: Companies * Absolute Entertainment, a video game publisher * Absolute Radio, (formerly Virgin Radio), independent national radio station in the UK * Absolute Software Corporation, specializes in security and data risk managem ...
thermodynamic temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from Kinetic theory of gases, kinetic theory or statistical mechanics. A thermodynamic temperature reading of zero is of particular importance for the third law of therm ...

thermodynamic temperature
scale Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
; it uses
absolute zero Absolute zero is the lowest limit of the thermodynamic temperature Thermodynamic temperature is the measure of ''absolute temperature'' and is one of the principal parameters of thermodynamics. A thermodynamic temperature reading of zero deno ...
as its null point (i.e. low
entropy Entropy is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics ...

entropy
). Unlike degree
Fahrenheit The Fahrenheit scale ( or ) is a Scale of temperature, temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he ...

Fahrenheit
and degree
Celsius The degree Celsius is a unit of temperature on the Celsius scale, a temperature scale Scale of temperature is a methodology of calibrating the physical quantity temperature in metrology. Empirical scales measure temperature in relation to conv ...

Celsius
, the kelvin is not referred to or written as a ''degree''. The kelvin is the primary unit of temperature measurement for the
physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical sciences". ...
, however it is often used in conjunction with degree Celsius, which has the same magnitude. The Kelvin scale was developed over 100 years after those other two temperature scales, and Celsius still remains the dominant scale in common use around the world. The kelvin is now defined by fixing the numerical value of the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor In mathematics, two varying quantities are said to be in a Binary relation, relation of proportionality, Multiplication, multiplicatively connected to a Constant (mathematics), c ...
to . Hence, one kelvin is equal to a change in the
thermodynamic temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from Kinetic theory of gases, kinetic theory or statistical mechanics. A thermodynamic temperature reading of zero is of particular importance for the third law of therm ...

thermodynamic temperature
that results in a change of
thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
by  J. The relation between kelvin and
Celsius The degree Celsius is a unit of temperature on the Celsius scale, a temperature scale Scale of temperature is a methodology of calibrating the physical quantity temperature in metrology. Empirical scales measure temperature in relation to conv ...

Celsius
is ''T''K = ''t''°C + 273.15; therefore, on the Kelvin scale, pure water freezes at 273.15 K and boils at 373.15 K in 1
atm ATM or atm often refers to: * Atmosphere (unit) or atm, a unit of atmospheric pressure * Automated teller machine, a cash dispenser or cash machine ATM or atm may also refer to: Computing * ATM (computer), a ZX Spectrum clone developed in Mos ...
.


History

In 1848, William Thomson, who was later ennobled as
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of ...

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 °C using the air thermometers of the time. This absolute scale is known presently 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 The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures The International Bureau of Weights and Measures (fr ...
(CGPM) gave the Kelvin scale its modern definition by designating the
triple point of water In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is governed ...
as its second defining point and assigned its temperature to exactly 273.16 kelvin. 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 of the thermodynamic temperature of the
triple point of water In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is governed ...
." In 2005, the Comité International des Poids et Mesures (
CIPM The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the (BIPM), the established in 1875 under the terms of the through which member states act together on ...
), 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 Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ...
. On 16 November 2018, a new definition was adopted, in terms of a fixed value of the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor In mathematics, two varying quantities are said to be in a Binary relation, relation of proportionality, Multiplication, multiplicatively connected to a Constant (mathematics), c ...
. With this change the triple point of water became an empirically determined value of approximately 273.16 kelvin. For
legal metrology 290px, alt=Man in white standing in front of a large machine, A scientist stands in front of the Microarcsecond Metrology (MAM) testbed. Metrology is the scientific study of measurement ' Measurement is the number, numerical quantification (s ...
purposes, the new definition officially came into force on 20 May 2019, the 144th anniversary of the
Metre Convention The Metre Convention (french: link=no, Convention du Mètre), also known as the Treaty of the Metre, is an international treaty A treaty is a formal, legally binding written agreement between actors in international law International ...

Metre Convention
.


Usage conventions

According to the
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation An intergovernmental organization (IGO) is an organization composed primarily of sovereign state ...
, when spelled or spoken, the unit is pluralised using the same grammatical rules as for other
SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system The metric system is a system of measurement A syste ...
such as the
volt The volt is the derived unit for electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work (physics), work energy needed to move a ...

volt
or
ohm The ohm (symbol: Ω) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, ...

ohm
(e.g., "... the triple point of water is exactly 273.16 kelvins"). 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"). (The style guide for
CERN The European Organization for Nuclear Research (french: Organisation européenne pour la recherche nucléaire), known as CERN (; ; derived from the name ), is a European research organization that operates the largest particle physics laborat ...
, however, specifically says to always use "kelvin", even when plural.) Before the 13th CGPM in 1967–1968, the unit kelvin was termed 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 __NOTOC__ The Rankine scale () is an absolute scale An absolute scale is a system of measurement that begins at a minimum, or zero point, and progresses in only one direction. An absolute scale differs from an arbitrary, or "relative", scale, whic ...
. Before the 13th CGPM, the plural form was "degrees absolute". The 13th CGPM changed the unit name to simply "kelvin" (symbol: K). 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).


2019 redefinition

In 2005 the
CIPM The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the (BIPM), the established in 1875 under the terms of the through which member states act together on ...
began a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous method. In particular, the committee proposed
redefining the kelvin
redefining the kelvin
such that
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor In mathematics, two varying quantities are said to be in a Binary relation, relation of proportionality, Multiplication, multiplicatively connected to a Constant (mathematics), c ...
takes the exact value . 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. The redefinition was further postponed in 2014, pending more accurate measurements of Boltzmann's constant in terms of the current definition, but was finally adopted at the 26th CGPM in late 2018, with a value of  =  For scientific purposes, the main advantage is that this allows 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 unit J/K is equal to kg⋅m2⋅s−2⋅K−1, where the
kilogram The kilogram (also kilogramme) is the base unit of mass Mass is the physical quantity, quantity of ''matter'' in a physical body. It is also a measure (mathematics), measure of the body's ''inertia'', the resistance to acceleration (change ...
,
metre The metre ( Commonwealth spelling) or meter (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English o ...
and
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
are defined in terms of the
Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature an ...
, the
speed of light The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
, and the duration of the
caesium-133 Caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golde ...

caesium-133
ground-state hyperfine transition respectively. Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously, such as the
International Prototype of the Kilogram The International Prototype of the Kilogram (referred to by metrologists as the IPK or Le Grand K; sometimes called the '' ur-kilogram,'' or ''urkilogram,'' particularly by German-language authors writing in English) is an object that was used to ...
, whose mass diverged over time from the original value. The challenge was to avoid degrading the accuracy of measurements close to the triple point. For practical purposes, the redefinition was unnoticed; water still freezes at 273.15 K (0 °C), and the triple point of water continues 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 , with a relative standard uncertainty of . Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now .


Practical uses


Colour temperature

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 A black body or blackbody is an idealized physical object, physical body that absorption (electromagnetic radiation), absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence (optics), angle of incidence. The ...

black body
radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about appear reddish, whereas those above about appear bluish. Colour temperature is important in the fields of image projection and
photography Photography is the art Art is a diverse range of (products of) human activities Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and int ...

photography
, where a colour temperature of approximately is required to match "daylight" film emulsions. In
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, the
stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectr ...
of stars and their place on the
Hertzsprung–Russell diagram The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The ...
are based, in part, upon their surface temperature, known as
effective temperatureThe effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physik ...
. The photosphere of the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
, for instance, has an effective temperature of . 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

For electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate
noise floor In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored. In radio com ...
, 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 The Boltzmann constant ( or ) is the proportionality factor In mathematics, two varying quantities are said to be in a Binary relation, relation of proportionality, Multiplication, multiplicatively connected to a Constant (mathematics), c ...
and can be used to determine the noise temperature of a circuit using the
Friis formulas for noise Friis formula or Friis's formula (sometimes Friis' formula), named after Danish-American electrical engineer Harald T. Friis, is either of two formulas used in telecommunications engineering to calculate the signal-to-noise ratio of a multistage a ...
.


Unicode character

The symbol is encoded in
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
at code point . However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using instead; that is, a normal capital . "Three letterlike symbols have been given canonical equivalence to regular letters: , , and . In all three instances, the regular letter should be used."


See also

*
Comparison of temperature scales This is a collection of temperature conversion formulas and comparisons among eight different temperature scales, several of which have long been obsolete. Celsius Fahrenheit Kelvin Rankine Delisle Sir Isaac Newton's Degr ...

Comparison of temperature scales
*
International Temperature Scale of 1990The International Temperature Scale of 1990 (ITS-90) is an equipment calibration standard specified by the International Committee of Weights and Measures (CIPM) for making measurements on the Kelvin The kelvin is the SI base unit, base unit of ...
*
Negative temperature Certain systems can achieve negative thermodynamic temperature; that is, their temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the sou ...


References


External links

* {{CGS units 1848 introductions Scottish inventions SI base units UCUM base units William Thomson, 1st Baron Kelvin Scales of temperature