# Kelvin

Kelvin | |
---|---|

Unit system | SI base unit |

Unit of | Temperature |

Symbol | K |

Named after | William Thomson, 1st Baron Kelvin |

The **Kelvin scale** is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The **kelvin** (symbol: K) is the base unit of temperature in the International System of Units (SI).

Until 2018, the kelvin was defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01 °C or 32.018 °F).^{[1]} In other words, it was defined such that the triple point of water is exactly 273.16 K.

On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. For legal metrology purposes, the new definition will officially come into force on 20 May 2019 (the 130th anniversary of the Metre Convention).^{[2]}

The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907), who wrote of the need for an "absolute thermometric scale". 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. The definition implies that absolute zero (0 K) is equivalent to −273.15 °C (−459.67 °F).

### History

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 °C on the air thermometers of the time.^{[3]} 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.^{[4]}

In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.^{[5]} 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 1/273.16 of the thermodynamic temperature of the triple point of water."^{[1]}

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.^{[6]}

In 2018, Resolution A of the 26th CGPM adopted a significant redefinition of SI base units which included redefining the Kelvin in terms of a fixed value for the Boltzmann constant of 649×10^{−23} J/K. 1.380

### 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 exactly 273.16 kelvins"^{[7]}). 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").^{[8]}^{[9]}

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).^{[10]} 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

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 28 °C with an uncertainty of 60 µK." 0.010

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.^{[11]} Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states "a temperature interval may also be expressed in degrees Celsius",^{[5]} 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.

### 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 6505×10^{−23} J/K. 1.380^{[12]} 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.^{[13]}

The redefinition was further postponed in 2014, pending more accurate measurements of Boltzmann's constant in terms of the current definition,^{[14]} but was finally adopted at the 26th CGPM in late 2018, with a value of k = 649×10^{−23} J/K. 1.380^{[12]}^{[15]}

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),^{[16]} 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 64903(51)×10^{−23} J/K, with a relative standard uncertainty of 1.380×10^{−7}. 3.7^{[15]} Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now . 273.1600(1) K

### Practical uses

from kelvins | to kelvins | |
---|---|---|

Celsius | [°C] = [K] − 273.15 | [K] = [°C] + 273.15 |

Fahrenheit | [°F] = [K] × ^{9}⁄_{5} − 459.67 |
[K] = ([°F] + 459.67) × ^{5}⁄_{9} |

Rankine | [°R] = [K] × ^{9}⁄_{5} |
[K] = [°R] × ^{5}⁄_{9} |

For temperature intervals rather than specific temperatures,1 K = 1 °C = ^{9}⁄_{5} °F = ^{9}⁄_{5} °RComparisons among various temperature scales |

#### 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 radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about appear reddish, whereas those above about 4000 K appear bluish. Colour temperature is important in the fields of image projection and 7500 Kphotography, where a colour temperature of approximately is required to match "daylight" film emulsions. In 5600 Kastronomy, 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 . 5778 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 U+212A K KELVIN SIGN. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using
U+004B K
LATIN CAPITAL LETTER K instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters:
U+2126 Ω
OHM SIGN,
U+212A K
KELVIN SIGN, and
U+212B Å
ANGSTROM SIGN. In all three instances, the regular letter should be used."^{[17]}

### See also

### Other Languages

- Afrikaans
- Alemannisch
- العربية
- Aragonés
- Asturianu
- Azərbaycanca
- বাংলা
- Bân-lâm-gú
- Беларуская
- Беларуская (тарашкевіца)
- Български
- བོད་ཡིག
- Bosanski
- Brezhoneg
- Català
- Чӑвашла
- Čeština
- Cymraeg
- Dansk
- Deutsch
- Eesti
- Ελληνικά
- Español
- Esperanto
- Euskara
- فارسی
- Fiji Hindi
- Føroyskt
- Français
- Frysk
- Furlan
- Gaeilge
- Galego
- ગુજરાતી
- 한국어
- Հայերեն
- हिन्दी
- Hrvatski
- Bahasa Indonesia
- Interlingua
- Íslenska
- Italiano
- עברית
- ಕನ್ನಡ
- Къарачай-малкъар
- ქართული
- Қазақша
- Kiswahili
- Kreyòl ayisyen
- Kurdî
- Кыргызча
- Latina
- Latviešu
- Lëtzebuergesch
- Lietuvių
- Limburgs
- Lingála
- La .lojban.
- Lumbaart
- Magyar
- Македонски
- മലയാളം
- मराठी
- مصرى
- Bahasa Melayu
- Mirandés
- Монгол
- Nederlands
- नेपाली
- 日本語
- Нохчийн
- Nordfriisk
- Norsk
- Norsk nynorsk
- Occitan
- Oʻzbekcha/ўзбекча
- ਪੰਜਾਬੀ
- پنجابی
- Piemontèis
- Plattdüütsch
- Polski
- Português
- Română
- Русский
- Scots
- Sicilianu
- Simple English
- Slovenčina
- Slovenščina
- Soomaaliga
- Српски / srpski
- Srpskohrvatski / српскохрватски
- Suomi
- Svenska
- தமிழ்
- తెలుగు
- ไทย
- Türkçe
- Українська
- اردو
- Vèneto
- Tiếng Việt
- West-Vlams
- Winaray
- 吴语
- 粵語
- Žemaitėška
- 中文