meaning of a metric prefix, such as “milli-”.

For a variety of temperatures in kelvins, visit temperature: an SI Tour

The unit of thermodynamic temperature in SI, 20 May 2019 – present, one of the base units. Symbol K.

The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant *k* to be 1.380 649 × 10⁻²³ when expressed in the unit J K⁻¹, which is equal to kg m² s⁻² K⁻¹, where the kilogram, metre and second are defined in terms of *h, c* and Δν_{cs}.

Resolutions Adopted.

Resolution 1, appendix 3.

26 CGPM, Versailles 13-16 November 2018.

BIPM.

Here *h* is the Planck constant, *c* is the speed of light, and Δν_{cs} refers to the frequency of light emitted by a cesium atom under specified conditions.

Users of the kelvin will not notice any effect from the redefinition, either in the lab or commerce. It is part of the effort to base SI on physical constants, to lay a foundation for future developments.

When first defined by the 10ᵗʰ CGPM in 1954 (Resolution 3), the kelvin's symbol was “°K” and its name was the “degree Kelvin”. The 13th CGPM (Resolution 3, 1967), concerned that this usage encouraged a false distinction between the unit of thermodynamic temperature and the unit of temperature interval, which are actually the same thing, changed the symbol to “K” and the name to “kelvin”, without “degree” and without capitalizing “kelvin”. They did, however, permit continued use of the old name and symbol for the time being.

In 1980, responding to a request from the Consultative Committee on Units, the International Committee for Weights and Measures (the CIPM) voted to forbid further use of the “°K” symbol and the term “degree Kelvin”. See K.

At its 24ᵗʰ meeting (Paris, October 2011), the CGPM decided to declare in advance its intention to make the numerical value of the Boltzmann constant a matter of definition, rather than something to be determined experimentally. The new value was to be *exactly* 1.380 6X × 10⁻²³ joule per kelvin, where X stood for one or more yet to be determined digits.

Fixing the value of the Boltzmann constant fixes the size of the kelvin, since the meter, kilogram and second are already fixed. (In terms of SI base units, the Boltzmann constant is meter² kilogram second⁻² kelvin⁻¹, which algebraically, yields the size of the kelvin.) The meter is already defined by the speed of light, the second by the frequency of light emitted by cesium atoms, and in the 2019 round of redefinitions the kilogram was defined by specifying the value of the Planck constant. The redefinition of the kelvin does not depend at all on measuring the triple point of water, whose temperature will now be determined experimentally.

The kelvin is also the basis of the Celsius temperature scale. The temperature in kelvins can be found by adding 273.15 to the temperature in degrees Celsius.

The unit of thermodynamic temperature, a base unit in SI, = the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (but see below). The triple point of water is the unique combination of temperature and pressure at which water exists simultaneously as liquid, solid and gas. It can be reproduced (as of 2006) with a variation of 0.000 050 K or less.

As instrumentation improved, discrepancies between measurements at national labs revealed a problem with this definition of the kelvin. It turns out that the triple point of water depends significantly on the isotopic composition of the water. In 2005, at its 94th meeting, the CIPM formally recommended¹ a specific isotopic composition for the water to be used, namely:

- 0.000 155 76 mole of ²H per mole of ¹H
- 0.000 379 9 mole of ¹⁷O per mole of ¹⁶O, and
- 0.002 005 2 mole of ¹⁸O per mole of ¹⁶O:

This composition is one used by the International Atomic Energy Agency, called “Vienna Standard Mean Ocean Water,” which the IAEA supplies as a reference standard. The use of this standard was recommended by the International Union of Pure and Applied Chemistry (IUPAC).²

The kelvin is named for the British physicist William Thomson, ennobled as first Baron Kelvin of Largs (as much for industrial as scientific accomplishments, but the first British scientist to receive a peerage). Thomson took the name Kelvin from that of a small river near Glasgow.

1. Recommendation 2 (CI-2005)

2. J. R. de Laeter, J. K. Bohlke, P. De Bièvre, H. Hidaka, H. S. Peiser, K. J. R. Rosman and P. D. P. Taylor.

Atomic weights of the elements. Review 2000 (IUPAC Technical Report).

*Pure and Applied Chemistry*, vol. **75**, no. 6, pages 683-799, (2003).

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Last revised: 26 April 2019.