Editing Metrology (section)

=== Definition of units ===
The [[International System of Units]] (SI) defines seven base units: [[length]], [[mass]], [[time]], [[electric current]], [[thermodynamic temperature]], [[amount of substance]], and [[luminous intensity]].<ref name="NIST_SI">{{cite web|title=SI base units|url=http://physics.nist.gov/cuu/Units/units.html|website=The NIST Reference on Constants, Units, and Uncertainty|publisher=National Institute of Standards and Technology|access-date=15 February 2017|url-status=live|archive-url=https://web.archive.org/web/20170119053614/http://physics.nist.gov/cuu/Units/units.html|archive-date=19 January 2017}}</ref> By convention, each of these units are considered to be mutually independent and can be constructed directly from their defining constants.<ref name="SI 9th edition">{{SIbrochure9th}}</ref>{{rp|129}} All other SI units are constructed as products of powers of the seven base units.<ref name="SI 9th edition"/>{{rp|129}}

{| class="wikitable"
|-
|+SI base units and standards
|-
! Base quantity !! Name !! Symbol !! Definition
|-
| Time || [[second]]|| s || The duration of 9192631770 periods of the radiation corresponding to the transition between the two [[Hyperfine structure|hyperfine]] levels of the [[ground state]] of the [[Isotopes of caesium#Caesium-133|caesium-133]] atom<ref name="SI 9th edition"/>{{rp|130}}
|-
| Length || [[metre]]|| m || The length of the path travelled by light in a [[vacuum]] during a time interval of 1/299792458 of a second<ref name="SI 9th edition"/>{{rp|131}}
|-
| Mass || [[kilogram]]|| kg || Defined ([[2019 revision of the SI|as of 2019]]) by "... taking the fixed numerical value of the [[Planck constant]], ''h'', to be {{val|fmt=commas|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>}} ..."<ref name="SI 9th edition"/>{{rp|131}}
|-
| Electric current || [[ampere]]|| A || Defined (as of 2019) by "... taking the fixed numerical value of the [[elementary charge]], ''e'', to be {{val|fmt=commas|1.602176634|e=-19}} when expressed in the unit C, which is equal to {{nowrap|A s}} ..."<ref name="SI 9th edition"/>{{rp|132}}
|-
| Thermodynamic temperature || [[kelvin]]|| K || Defined (as of 2019) by "... taking the fixed numerical value of the [[Boltzmann constant]], ''k'', to be {{val|fmt=commas|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>}} ..."<ref name="SI 9th edition"/>{{rp|133}}
|-
| Amount of substance || [[Mole (unit)|mole]] || mol || Contains (as of 2019) "... exactly {{val|fmt=commas|6.02214076|e=23}} 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> ..."<ref name="SI 9th edition"/>{{rp|134}}
|-
| Luminous intensity || [[candela]] || cd || The luminous intensity, in a given direction, of a source emitting monochromatic radiation of a frequency of {{val|540|e=12|u=Hz}} with a radiant intensity in that direction of 1/683 watt per [[steradian]]<ref name="SI 9th edition"/>{{rp|135}}
|}

Since the base units are the reference points for all measurements taken in SI units, if the reference value changed all prior measurements would be incorrect. Before 2019, if a piece of the international prototype of the kilogram had been snapped off, it would have still been defined as a kilogram; all previous measured values of a kilogram would be heavier.<ref name=BGtoM>{{cite web|last1=Goldsmith|first1=Mike|title=A Beginner's Guide to Measurement|url=http://www.npl.co.uk/upload/pdf/NPL-Beginners-Guide-to-Measurement.pdf|publisher=National Physical Laboratory|access-date=16 February 2017|url-status=live|archive-url=https://web.archive.org/web/20170329111015/http://www.npl.co.uk/upload/pdf/NPL-Beginners-Guide-to-Measurement.pdf|archive-date=29 March 2017}}</ref> The importance of reproducible SI units has led the BIPM to complete the task of defining all SI base units in terms of [[physical constant]]s.<ref name=redef>{{cite web|title=On the future revision of the SI|url=http://www.bipm.org/en/measurement-units/rev-si/|publisher=Bureau International des Poids et Mesures|access-date=16 February 2017|url-status=dead|archive-url=https://web.archive.org/web/20170215111649/http://www.bipm.org/en/measurement-units/rev-si/|archive-date=15 February 2017}}</ref>

By defining SI base units with respect to physical constants, and not artefacts or specific substances, they are realisable with a higher level of precision and reproducibility.<ref name = redef/> As of the revision of the SI on 20 May 2019 the [[kilogram]], [[ampere]], [[kelvin]], and [[Mole (unit)|mole]] are defined by setting exact numerical values for the [[Planck constant]] (''{{Math|h}}''), the [[elementary electric charge]] (''{{Math|e}}''), the [[Boltzmann constant]] (''{{Math|k}}''), and the [[Avogadro constant]] ({{Math|''N''<sub>A</sub>}}), respectively. The [[second]], [[metre]], and [[candela]] have previously been defined by physical constants (the [[caesium standard]] (Δ''ν''<sub>Cs</sub>), the [[speed of light]] (''{{Math|c}}''), and the [[luminous efficacy]] of {{val|540|e=12|u=Hz}} visible light radiation (''K''<sub>cd</sub>)), subject to correction to their present definitions. The new definitions aim to improve the SI without changing the size of any units, thus ensuring continuity with existing measurements.<ref name=Kuehne>
{{cite web
 |first=Michael|last=Kühne
 |title=Redefinition of the SI
 |url=http://www.its9.org/symposium_program.html#SI_Redefinition_Keynote_Abstract
 |work=Keynote address, ITS<sup>9</sup> (Ninth International Temperature Symposium)
 |location=Los Angeles
 |access-date=1 March 2012|date=22 March 2012
 |publisher=NIST
 |archive-url=https://web.archive.org/web/20130618064512/http://www.its9.org/symposium_program.html|archive-date=18 June 2013
 |url-status=dead
}}</ref><ref name="SI 9th edition"/>{{rp|123,128}}