Editing SI base unit (section)

== 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
|-
|}