Editing Metre Convention (section)

=== New SI ===
{{Main|2019 revision of the SI}}
[[File:Relations between New SI units definitions.svg|thumb|250px|Relations between 2019 definitions of SI units (in colour) and with seven fundamental constants of nature (in grey) with fixed numerical values.]]
After 1960, when the definition of the metre was linked to a particular wavelength of light rather than the international prototype of the metre, the only unit of measure that remained dependent on a particular artefact was the kilogram. Over the years, small drifts which could be as high as {{gaps|20|×|10<sup>−9</sup>}} kilograms per annum in the mass of the international prototype of the kilogram were detected.<ref>
{{cite web
 |url = http://physics.vniim.ru/SI50/files/mohr.pdf
 |author = Peter Mohr
 |title = Recent progress in fundamental constants and the International System of Units
 |work = Third Workshop on Precision Physics and Fundamental Physical Constants
 |date = 6 December 2010
 |access-date = 2 January 2011
 |archive-url = https://web.archive.org/web/20110824105816/http://physics.vniim.ru/SI50/files/mohr.pdf
 |archive-date = 24 August 2011
 |url-status = dead
}}</ref> At the 21st meeting of the CGPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artefact.

Independently of this drift having been identified, the [[Avogadro constant#International Avogadro Coordination|Avogadro project]] and development of the [[Kibble balance|Kibble (or watt) balance]] promised methods of indirectly measuring mass with a very high precision. These projects provided tools that enabled alternative means of redefining the kilogram.<ref name=Crease>
{{cite journal
 |url = http://physicsworld.com/cws/article/indepth/2011/mar/22/metrology-in-the-balance
 |title = Metrology in the balance
 |first1 = Robert P
 |last1 = Crease
 |access-date = 28 June 2012
 |journal = Physics World
 |volume = 24
 |issue = 3
 |pages = 39–45
 |date = 22 March 2011
 |bibcode = 2011PhyW...24c..39C
 |doi = 10.1088/2058-7058/24/03/34
 |archive-date = 27 April 2020
 |archive-url = https://web.archive.org/web/20200427013941/https://physicsworld.com/a/metrology-in-the-balance/
 |url-status = live
}}</ref>

A report published in 2007 by the Consultative Committee for Thermometry to the CIPM noted that their definition of temperature had proved to be unsatisfactory for temperatures below 20&nbsp;K and for temperatures above 1300&nbsp;K. The committee was of the view that the [[Boltzmann constant]] provided a better basis for temperature measurement than did the [[triple point]] of water, as it overcame these difficulties.<ref>
{{cite web
 |url = http://www.bipm.org/wg/CCT/TG-SI/Allowed/Documents/Report_to_CIPM_2.pdf
 |title = Report to the CIPM on the implications of changing the definition of the base unit kelvin
 |author = Fischer, J.
 |date = 2 May 2007
 |access-date = 2 January 2011
 |display-authors = etal
 |archive-date = 23 November 2008
 |archive-url = https://web.archive.org/web/20081123054012/http://www.bipm.org/wg/CCT/TG-SI/Allowed/Documents/Report_to_CIPM_2.pdf
 |url-status = live
}}</ref>

Over the next few years the support for natural constants grew and details were clarified,<ref>
{{cite web
 |url         = http://www.iupap.org/ga/ga26/sunamcorpt.pdf
 |title       = Resolution proposal submitted to the IUPAP Assembly by Commission C2 (SUNAMCO)
 |publisher   = International Union of Pure and Applied Physics
 |year        = 2008
 |access-date  = 8 May 2011
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20110927085329/http://www.iupap.org/ga/ga26/sunamcorpt.pdf
 |archive-date = 27 September 2011
}}</ref><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 = 29 September 2010
 |access-date = 1 January 2011
 |archive-date = 18 May 2020
 |archive-url = https://web.archive.org/web/20200518205720/https://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf
 |url-status = live
}}</ref><ref>
{{cite web
 |url = http://www.bipm.org/utils/common/pdf/24_CGPM_Convocation_Draft_Resolution_A.pdf
 |title = On the possible future revision of the International System of Units, the SI – Draft Resolution A
 |publisher = [[General Conference on Weights and Measures#CIPM|International Committee for Weights and Measures]] (CIPM)
 |access-date = 14 July 2011
 |archive-date = 6 August 2011
 |archive-url = https://web.archive.org/web/20110806053509/http://www.bipm.org/utils/common/pdf/24_CGPM_Convocation_Draft_Resolution_A.pdf
 |url-status = live
}}</ref><ref>
{{cite conference
 |conference = 24th meeting of the General Conference on Weights and Measures
 |title = On the possible future revision of the International System of Units, the SI
 |url = http://www.bipm.org/utils/en/pdf/24_CGPM_Resolution_1.pdf
 |publisher = International Bureau for Weights and Measures
 |location = Saint-Cloud, France
 |date = 21 October 2011
 |access-date = 1 October 2012
 |archive-date = 13 January 2012
 |archive-url = https://web.archive.org/web/20120113072107/http://www.bipm.org/utils/en/pdf/24_CGPM_Resolution_1.pdf
 |url-status = live
}}</ref><ref name="bipm.org"/> until in November 2018, the 26th General Conference on Weights and Measures voted unanimously in favour of revised definitions of the [[SI base unit]]s.<ref>
{{cite web
 |url=https://www.nist.gov/news-events/news/2018/11/historic-vote-ties-kilogram-and-other-units-natural-constants
 |title=Historic Vote Ties Kilogram and Other Units to Natural Constants
 |publisher=NIST
 |date=16 November 2018
 |access-date=16 November 2018
 |archive-date=18 November 2018
 |archive-url=https://web.archive.org/web/20181118014932/https://www.nist.gov/news-events/news/2018/11/historic-vote-ties-kilogram-and-other-units-natural-constants
 |url-status=live
}}</ref><ref name=Milton16>
{{cite conference
 |conference=SIM XXII General Assembly
 |location=Montevideo, Uruguay
 |url=http://www.sim-metrologia.org.br/docs/2016Presentations/BIPM%202016.pdf#page=10
 |conference-url=http://www.sim-metrologia.org.br/docs/2016Presentations/
 |page=10
 |title=Highlights in the work of the BIPM in 2016
 |first=Martin
 |last=Milton
 |date=14 November 2016
 |access-date=21 November 2018
 |archive-url=https://web.archive.org/web/20170901031605/http://www.sim-metrologia.org.br/docs/2016Presentations/BIPM%202016.pdf#page=10
 |archive-date=1 September 2017
 |url-status=dead
}}</ref> The [[2019 revision of the SI]] came into force on the 144th anniversary of the convention, 20 May 2019.<ref name=SI-statement>
{{cite web
 |url = https://www.bipm.org/utils/common/pdf/SI-statement.pdf
 |title = BIPM statement: Information for users about the proposed revision of the SI
 |access-date = 5 May 2018
 |archive-url = https://web.archive.org/web/20180121160000/https://www.bipm.org/utils/common/pdf/SI-statement.pdf
 |archive-date = 21 January 2018
 |url-status = live
}}
</ref>
<ref>
{{cite web
 |url = http://www.bipm.org/en/committees/cipm/meeting/105.html
 |title = Decision CIPM/105-13 (October 2016)
 |access-date = 31 August 2017
 |archive-url = https://web.archive.org/web/20170824095943/http://www.bipm.org/en/committees/cipm/meeting/105.html
 |archive-date = 24 August 2017
 |url-status = live
}}</ref>