View source for SI base unit

{{Short description|One of the seven units of measurement that define the metric system}}
{{Use dmy dates|date=May 2023}}
[[File:SI base units.svg|thumb|right|266px|
{{Center|The seven SI base units}}
{|
|- 
! {{nowrap|Symbol}} !! Name !! Base quantity
|-
| style="text-align: centre;" | s   || [[second]] || [[time]]
|-
| style="text-align: centre;" | m   || [[metre]] || [[length]]
|-
| style="text-align: centre;" | kg  || {{nowrap|[[kilogram]]}} || [[mass]]
|-
| style="text-align: centre;" | A   || [[ampere]] || [[electric current]]
|-
| style="text-align: centre;" | K   || [[kelvin]] || [[thermodynamic temperature]]
|-
| style="text-align: centre;" | mol || [[mole (unit)|mole]] || [[amount of substance]]
|-
| style="text-align: centre;" | cd  || [[candela]] || [[luminous intensity]]
|-
|}
]]

The '''SI base units''' are the standard [[units of measurement]] defined by the [[International System of Units]] (SI) for the seven [[base quantities]] of what is now known as the [[International System of Quantities]]: they are notably a basic set from which all other [[SI units]] can be [[SI derived unit|derived]]. The units and their physical quantities are the [[second]] for [[time]], the [[metre]] (sometimes spelled meter) for [[length]] or [[distance]], the [[kilogram]] for [[mass]], the [[ampere]] for [[electric current]], the [[kelvin]] for [[thermodynamic temperature]], the [[Mole (unit)|mole]] for [[amount of substance]], and the [[candela]] for [[luminous intensity]]. The SI base units are a fundamental part of modern [[metrology]], and thus part of the foundation of modern science and technology.

The SI base units form a set of mutually independent dimensions as required by [[dimensional analysis]] commonly employed in science and technology.{{citation needed|date=January 2020}}

The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter.  For example, the ''metre'' has the symbol m, but the ''[[kelvin]]'' has symbol K, because it is named after [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] and the ''[[ampere]]'' with symbol A is named after [[André-Marie Ampère]].

== Definitions ==
On 20 May 2019, as the final act of the [[2019 revision of the SI]], the [[International Bureau of Weights and Measures|BIPM]] officially introduced the following new definitions, replacing the [[2005–2019 definitions of the SI base units|preceding definitions of the SI base units]].
{{clear}}
{| class="wikitable"
|+style="font-size:largerfont-weight:bold;"| SI base units
!Name
!style="text-align: center"| Symbol
! Measure
! Post-2019 formal definition<ref name="SI 9th edition">{{Cite web|url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf|title=The International System of Units (SI), 9th Edition|date=2019|publisher=Bureau International des Poids et Mesures}}</ref>
! Historical&nbsp;origin / justification
! [[Dimensional analysis|Dimension{{br}}symbol]]
|-
| [[second]]
|style="text-align: center"| s
| [[time]]
| "The second, symbol s, is the SI unit of [[time]]. It is defined by taking the fixed numerical value of the caesium frequency, ∆''ν''<sub>Cs</sub>, the [[Caesium standard|unperturbed ground-state hyperfine transition frequency of the caesium 133 atom]], to be {{val|9192631770}} when expressed in the unit Hz, which is equal to s<sup>−1</sup>."<ref name="SI 9th edition"/>
| The day is divided into 24 hours, each hour divided into 60 minutes, each minute divided into 60 seconds.<br/>A second is {{nowrap|1 / (24 × 60 × 60)}} of the [[day]]. Historically, a day was defined as the [[mean solar day]]; i.e., the average time between two successive occurrences of local apparent solar [[noon]].
|style="text-align:center"| T
|-
| [[metre]]
|style="text-align: center"| m
| [[length]]
| "The metre, symbol m, is the SI unit of [[length]]. It is defined by taking the fixed numerical value of the [[speed of light in vacuum]] ''c'' to be {{val|299792458}} when expressed in the unit {{nowrap|m s<sup>−1</sup>}}, where the second is defined in terms of [[caesium standard|∆''ν''<sub>Cs</sub>]]."<ref name="SI 9th edition"/>
| 1&nbsp;/&nbsp;{{val|10000000}} of the distance from the [[Earth]]'s equator to the North Pole measured on the [[Paris meridian|meridian arc through Paris]].
|style="text-align:center"| L
|-
| [[kilogram]]
|style="text-align: center"| kg
| [[mass]]
| "The kilogram, symbol kg, is the SI unit of [[mass]]. It is defined by taking the fixed numerical value of the [[Planck constant]] ''h'' to be {{val|6.62607015|e=-34}} when expressed in the unit {{nowrap|J s}}, which is equal to {{nowrap|kg m<sup>2</sup> s<sup>−1</sup>}}, where the metre and the second are defined in terms of ''c'' and ∆''ν''<sub>Cs</sub>."<ref name="SI 9th edition"/>
| The mass of one [[litre]] of [[water]] at the temperature of melting ice. A litre is one thousandth of a cubic metre.
|style="text-align:center"| M
|-
| [[ampere]]
|style="text-align: center"| A
| [[electric current]]
| "The ampere, symbol A, is the SI unit of [[electric current]]. It is defined by taking the fixed numerical value of the [[elementary charge]] ''e'' to be {{val|1.602176634|e=-19}} when expressed in the unit C, which is equal to {{nowrap|A s}}, where the second is defined in terms of ∆''ν''<sub>Cs</sub>."<ref name="SI 9th edition"/>
| The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of [[silver nitrate]].
|style="text-align:center"| I
|-
| [[kelvin]]
|style="text-align: center"| K
| [[thermodynamic temperature]]
| "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 {{val|1.380649|e=-23}} when expressed in the unit {{nowrap|J K<sup>−1</sup>}}, which is equal to {{nowrap|kg m<sup>2</sup> s<sup>−2</sup> K<sup>−1</sup>}}, where the kilogram, metre and second are defined in terms of ''h'', ''c'' and ∆''ν''<sub>Cs</sub>."<ref name="SI 9th edition"/>
| The [[Celsius scale]]: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0&nbsp;K is [[absolute zero]]).
|style="text-align:center"| Θ
|-
| [[Mole (unit)|mole]]
|style="text-align: center"| mol
| [[amount of substance]]
| "The mole, symbol mol, is the SI unit of [[amount of substance]]. One mole contains exactly {{nowrap|6.022 140 76 × 10<sup>23</sup>}} elementary entities. This number is the fixed numerical value of the [[Avogadro constant]], ''N''<sub>A</sub>, when expressed in the unit mol<sup>−1</sup> and is called the [[Avogadro number]].{{pb}}
The amount of substance, symbol ''n'', of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles."<ref name="SI 9th edition"/>
| [[Atomic weight]] or [[molecular weight]] divided by the [[molar mass constant]], 1&nbsp;g/mol.
|style="text-align:center"| N
|-
| [[candela]]
|style="text-align: center"| cd
| [[luminous intensity]]
| "The candela, symbol cd, is the SI unit of [[luminous intensity]] in a given direction. It is defined by taking the fixed numerical value of the [[luminous efficacy]] of monochromatic radiation of frequency {{val|540|e=12|u=Hz}}, ''K''<sub>cd</sub>, to be 683 when expressed in the unit [[Lumen (unit)|lm]] W<sup>−1</sup>, which is equal to {{nowrap|cd [[steradian|sr]] W<sup>−1</sup>}}, or {{nowrap|cd sr kg<sup>−1</sup> m<sup>−2</sup> s<sup>3</sup>}}, where the kilogram, metre and second are defined in terms of ''h'', ''c'' and ∆''ν''<sub>Cs</sub>."<ref name="SI 9th edition"/>
| The [[candlepower]], which is based on the light emitted from a burning candle of standard properties.
|style="text-align:center"| J
|-
|}

== 2019 revision of the SI ==
{{Main|2019 revision of the SI}}
[[File:Unit relations in the old SI.svg|thumb|upright=1.35|The [[International System of Units|SI system]] after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, the [[metre]] is defined as the distance travelled by [[light]] in a specific fraction of a [[second]]), with the constants of nature and artefacts used to define them (such as the mass of the [[IPK]] for the kilogram).]]
[[File:Unit_relations_in_the_new_SI.svg|thumb|upright=1.35|New SI: Dependence of base unit definitions on [[physical constant]]s with fixed numerical values and on other base units that are derived from the same set of constants. Arrows are shown in the opposite direction compared to typical [[dependency graph]]s, i.e. <math>a \rightarrow b</math> in this chart means <math>b</math> depends on <math>a</math>.]]

New base unit definitions were adopted on 16 November 2018, and they became effective on 20 May 2019. The definitions of the base units have been modified several times since the [[Metre Convention]] in 1875, and new additions of base units have occurred. Since the redefinition of the metre in 1960, the kilogram had been the only base unit still defined directly in terms of a physical artefact, rather than a property of nature. This led to a number of the other SI base units being defined indirectly in terms of the mass of the same artefact; the [[Mole (unit)|mole]], the [[ampere]], and the [[candela]] were linked through their definitions to the mass of the [[International Prototype of the Kilogram]], a roughly golfball-sized [[platinum]]–[[iridium]] cylinder stored in a vault near Paris.

It has long been an objective in [[metrology]] to define the kilogram in terms of a [[Physical constant|fundamental constant]], in the same way that the metre is now defined in terms of the [[speed of light]]. The 21st [[General Conference on Weights and Measures]] (CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram". Two possibilities attracted particular attention: the [[Planck constant]] and the [[Avogadro constant]].

In 2005, the [[International Committee for Weights and Measures]] (CIPM) approved preparation of new definitions for the kilogram, the ampere, and the kelvin and it noted the possibility of a new definition of the mole based on the Avogadro constant.<ref>{{cite web|author=94th Meeting of the [[International Committee for Weights and Measures]]|date=2005|url=http://www.bipm.org/utils/en/pdf/CIPM2005-EN.pdf|title=Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants|archive-url=https://web.archive.org/web/20110807052257/http://www.bipm.org/utils/en/pdf/CIPM2005-EN.pdf |archive-date=2011-08-07}}</ref> The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.<ref>23rd [[General Conference on Weights and Measures]] (2007). [http://www.bipm.org/utils/en/pdf/Resol23CGPM-EN.pdf Resolution 12: On the possible redefinition of certain base units of the International System of Units (SI)].</ref>

In a note to the CIPM in October 2009,<ref>{{cite web|url = http://www.bipm.org/cc/CIPM/Allowed/98/CIPM2009_49_TIMING_THE_NEW_SI.pdf | title = Thoughts about the timing of the change from the Current SI to the New SI| author = Ian Mills, President of the CCU | publisher = CIPM | date = October 2009 | access-date =2010-02-23}}</ref> Ian Mills, the President of the CIPM ''Consultative Committee – Units'' (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed [[New SI definitions|new definition]].  He urged the CIPM to accept the proposed changes in the definition of the ''kilogram'', ''ampere'', ''kelvin'', and ''mole'' so that they are referenced to the values of the fundamental constants, namely the [[Planck constant]] (''h''), the [[elementary charge]] (''e''), the [[Boltzmann constant]] (''k''), and the [[Avogadro constant]] (''N''<sub>A</sub>).<ref name="draft">
{{cite web
 |url = http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf
 |title = Draft Chapter 2 for SI Brochure, following redefinitions of the base units
 |author = Ian Mills
 |publisher = CCU
 |date = 2010-09-29
 |access-date = 2011-01-01
}}</ref> This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy.

== See also ==
* [[International vocabulary of metrology]]
* [[International System of Quantities]]
* [[Non-SI units mentioned in the SI]]
* [[Metric prefix]]
* [[Physical constant]]

== References ==
{{reflist|30em}}

== External links ==
* [http://www.bipm.org/ International Bureau of Weights and Measures]
* [http://www.npl.co.uk/reference/measurement-units/si-base-units/the-mole National Physical Laboratory]
* [http://physics.nist.gov/cuu/Units/current.html NIST – SI]

{{SI units}}
{{SI base quantities}}

{{DEFAULTSORT:Si Base Unit}}
[[Category:SI units|Base unit]]
[[Category:SI base units| ]]
[[Category:SI base quantities|Base unit]]
[[Category:Dimensional analysis]]