Editing Metre Convention (section)

== Post-1875 developments ==
The science of metrology has progressed vastly since 1875. In particular the treaty was amended in 1921 with the result that many other international organizations have a forum within the CIPM to ensure harmonization of measurement standards across many [[Academic discipline|disciplines]]. In addition, what were originally conceived as standards for the purposes of trade have now been extended to cover a large number of aspects of human activity including medicine, science, engineering and technology.

=== Extensions to the treaty (1921) and development of the SI ===
{{main|International System of Units}}

The metre convention was originally drawn up with the main purpose of providing standards of length and mass only.  Standards relating to other quantities were under the control of other bodies – time was measured by [[astronomer]]s, electrical units by a series of ad-hoc international conferences,<ref>
{{cite web
 |title = System of Measurement Units
 |date = 24 April 2012
 |url = http://www.ieeeghn.org/wiki/index.php/System_of_Measurement_Units
 |access-date = 1 October 2012
 |publisher = [[IEEE]]
 |archive-date = 21 August 2014
 |archive-url = https://web.archive.org/web/20140821025655/http://www.ieeeghn.org/wiki/index.php/System_of_Measurement_Units
 |url-status = live
}}</ref> and other physical standards and concepts were maintained or defined by international bodies such as [[International Congress of Applied Chemistry]] or the [[International Union of Pure and Applied Physics]].

In 1901 [[Giovanni Giorgi|Giorgi]] published a proposal for building a coherent set of units based on four base units – the metre, kilogram, second and one electrical unit ([[ampere]], [[volt]] or [[ohm]]). In 1921 the convention was extended to permit the promotion of standards relating to any physical quantity which greatly increased the scope of the CIPM's remit and implicitly giving it freedom to exploit Giorgi's proposals. The 8th CGPM (1933) resolved to work with other international bodies to agree to standards for electrical units that could be related back to the international prototypes.<ref>
{{cite web
 |url = http://www.bipm.org/en/CGPM/db/8/10/
 |title = Résolution 10 de la 8e réunion de la CGPM (1933) – Substitution des unités électriques absolues aux unités dites " internationales "
 |trans-title = Resolution 10 of the 8th meeting of the CGPM (1933) – Substitution of the so-called "International" electrical units by absolute electrical units
 |language = fr
 |year = 1935
 |publisher = Bureau International des Poids et Meseures
 |access-date = 2 October 2012
 |archive-date = 26 November 2020
 |archive-url = https://web.archive.org/web/20201126185709/https://www.bipm.org/en/CGPM/db/8/10/
 |url-status = live
}}</ref> This was agreed in principle by the International Electrotechnical Commission at its congress in Brussels in 1935 subject to the choice of the fourth unit being agreed with, amongst others, the appropriate consultative committee of the CIPM.<ref>
{{cite journal
 |first = Arthur E
 |last = Kennelly
 |title = Adoption of the Metre-Kilogram-Mass-Second (M.K.S.) Absolute System of Practical Units by the International Electrotechnical Commission (I.E.C.), Bruxelles, June 1935
 |journal = Proceedings of the National Academy of Sciences of the United States of America
 |date = 21 October 1935
 |volume = 21
 |number = 10
 |pages = 579–583
 |doi=10.1073/pnas.21.10.579|pmid = 16577693
 |bibcode = 1935PNAS...21..579K
 |pmc = 1076662
 |doi-access = free
}}</ref>

In 1948, three years after the end of [[World War II]] and fifteen years after the 8th CGPM, the 9th CGPM was convened. In response to formal requests made by the International Union of Pure and Applied Physics and by the French Government to establish a practical system of units of measure, the CGPM requested the CIPM to prepare recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention.<ref>
{{cite conference
 |url = http://www.bipm.org/en/CGPM/db/9/6/
 |title = Resolution 6 – Proposal for establishing a practical system of units of measurement
 |conference = 9th Conférence Générale des Poids et Mesures (CGPM)
 |date = 12–21 October 1948
 |access-date = 8 May 2011
 |archive-date = 14 May 2013
 |archive-url = https://web.archive.org/web/20130514081152/http://www.bipm.org/en/CGPM/db/9/6/
 |url-status = live
}}</ref> At the same time the CGPM formally adopted a recommendation for the writing and printing of unit symbols and of numbers.<ref>
{{cite conference
 |url = http://www.bipm.org/en/CGPM/db/9/7/
 |title = Resolution 7 – Writing and printing of unit symbols and of numbers
 |conference = 9th Conférence Générale des Poids et Mesures (CGPM)
 |date = 12–21 October 1948
 |access-date = 8 May 2011
 |archive-date = 28 July 2012
 |archive-url = https://web.archive.org/web/20120728105127/http://www.bipm.org/en/CGPM/db/9/7/
 |url-status = live
}}</ref> The recommendation also catalogued the recommended symbols for the most important [[MKS system of units|MKS]] and [[Centimetre gram second system of units|CGS]] units of measure and for the first time the CGPM made recommendations concerning derived units.

The CIPM's draft proposal, which was an extensive revision and simplification of the metric unit definitions, symbols and terminology based on the MKS system of units, was put to the 10th CGPM in 1954. In the proposal the CIPM recommended that the ampere be the base unit from which electromechanical standards would be derived. After negotiations with the CIS and IUPAP, two further base units, the degree kelvin and the candela were also proposed as base units.<ref>
{{cite conference
 |url = http://www.bipm.org/en/CGPM/db/10/6/
 |title = Resolution 6 – Practical system of units
 |conference = 10th Conférence Générale des Poids et Mesures (CGPM)
 |date = 5–14 October 1954
 |access-date = 8 May 2011
 |archive-date = 1 April 2021
 |archive-url = https://web.archive.org/web/20210401081328/https://www.bipm.org/en/CGPM/db/10/6/
 |url-status = live
}}</ref> The full system and name "Système international d'unités" were adopted at the 11th CGPM.<ref>
{{cite conference
 |url = http://www.bipm.org/en/CGPM/db/11/12/
 |title = Resolution 12 – Système international d'unités
 |conference = 11th Conférence Générale des Poids et Mesures (CGPM)
 |date = 11–20 October 1960
 |access-date = 8 May 2011
 |archive-date = 14 May 2013
 |archive-url = https://web.archive.org/web/20130514081801/http://www.bipm.org/en/CGPM/db/11/12/
 |url-status = live
}}</ref> During the years that followed the definitions of the base units and particularly the {{lang|fr|mise en pratique}}<ref>
{{cite web
 |url = http://www.bipm.org/en/si/si_brochure/appendix2/
 |title = Practical realisation of the definitions of some important units
 |work = SI brochure, Appendix 2
 |publisher = BIPM
 |date = 9 September 2010
 |access-date = 5 May 2011
 |archive-date = 10 August 2014
 |archive-url = https://web.archive.org/web/20140810093615/http://www.bipm.org/en/si/si_brochure/appendix2/
 |url-status = live
}}</ref> to realize these definitions have been refined.

The formal definition of International System of Units (SI) along with the associated resolutions passed by the CGPM and the CIPM are published by the BIPM on the Internet and in brochure form at regular intervals. The eighth edition of the brochure ''Le Système international d'unités – The International System of Units'' was published in 2006.<ref>{{SIbrochure8th}}</ref>

=== Mutual Recognition Arrangements (CIPM-MRA) ===
[[File:CIPM-MRA-logo.jpg|thumb|Logo used by laboratories that have been accredited under the CIPM MRA scheme]]
During the 1940s, the United States government recognized the benefits of its suppliers keeping [[quality control]] records in respect of manufactured goods that would provide traceability of the process. This process was formalized by the British Government and in 1979 as the quality control standard [[BS 5750|BS&nbsp;5750]]. In 1987 BS&nbsp;5750 was adopted by [[International Organization for Standardization|ISO]] as the basis for [[ISO 9000]].<ref>
{{cite web
 |url = http://www.bsieducation.org/Education/HE/quality-management/quality-management.shtml
 |title = BSI Education: Quality Management Systems
 |access-date = 2 October 2012
 |year = 2008
 |publisher = [[British Standards Institution]] (BSI)
 |archive-date = 25 February 2012
 |archive-url = https://web.archive.org/web/20120225061350/http://www.bsieducation.org/Education/HE/quality-management/quality-management.shtml
 |url-status = live
}}</ref> ISO 9000 is a general purpose quality control standard which works in conjunction industry-specific standards: for example ISO 15195:2003 which gives the specific requirements for reference measurement laboratories in laboratory medicine.<ref>
{{cite web
 |url = http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=38363
 |title = ISO 15195:2003 – Laboratory medicine – Requirements for reference measurement laboratories
 |publisher = [[International Organization for Standardization]] (ISO)
 |access-date = 2 October 2012
 |archive-date = 8 May 2016
 |archive-url = https://web.archive.org/web/20160508155108/http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=38363
 |url-status = live
}}</ref>

International trade is hampered by one country not recognising the quality controls in place in other countries – often due to standards being different or being incompatible with each other. At the 20th CGPM (1995), it was recognized that although ad-hoc recognition of instrument calibration between cooperating countries had been taking place for a hundred years, a need had arisen for a more comprehensive agreement. Consequently, the CIPM was mandated to investigate the setting up of a [[Mutual Recognition Agreement]] in respect of instrument calibration. Any such agreement would require the keeping of records that could demonstrate the traceability of calibrations back to the base standards. Such records would be recorded within an ISO&nbsp;9000 framework. Four years later, in 1999 the text of the CIPM-MRA was agreed at the 21st CGPM.<ref>
{{cite web
 |url = http://www.bipm.org/en/cipm-mra/historical.html
 |title = Historical development of the CIPM MRA
 |access-date = 2 October 2012
 |publisher = [[International Bureau of Weights and Measures|Bureau International des Poids et Mesures]]
 |archive-date = 19 September 2012
 |archive-url = https://web.archive.org/web/20120919144100/http://www.bipm.org/en/cipm-mra/historical.html
 |url-status = live
}}</ref><ref>
{{cite web
 |url = http://www.bipm.org/en/cipm-mra/mra_online.html
 |title = Text of the CIPM MRA
 |access-date = 15 October 2012
 |publisher = [[International Bureau of Weights and Measures|Bureau International des Poids et Mesures]]
 |year = 2003
 |archive-date = 19 September 2012
 |archive-url = https://web.archive.org/web/20120919143925/http://www.bipm.org/en/cipm-mra/mra_online.html
 |url-status = live
}}</ref>

The CIPM-MRA scheme is to catalogue the capabilities of National Measurement Institutes (NMIs) such as [[National Institute of Standards and Technology|NIST]] in the United States or the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] in Britain whose calibration procedures have been peer-assessed. The essential points of CIPM-MRA are:<ref>
{{cite web
 |url = http://www.bipm.org/en/cipm-mra/objectives.html
 |title = The essential points of the CIPM MRA
 |publisher = [[International Bureau of Weights and Measures|Bureau International des Poids et Mesures]]
 |access-date = 2 October 2012
 |archive-date = 19 September 2012
 |archive-url = https://web.archive.org/web/20120919144035/http://www.bipm.org/en/cipm-mra/objectives.html
 |url-status = live
}}</ref>
* The agreement is only open to countries that have signed the Metre Convention, either as full or as associate members.
* A country may have more than one NMI, though only one NMI is chosen as the signatory organization.
* The measurement capabilities of NMIs will be peer-reviewed at regular intervals and each NMI will recognize the measurement capabilities of other NMIs.
* The BIPM maintains a publicly available database of the measurement capabilities of each NMI.
* NMIs

Subsequent to launch of the CIPM MRA and in response to a [[European Community]] [[Directive (European Union)|directive]] on {{lang|la|in vitro}} medical devices,<ref>{{CELEX|31998L0079|text=Directive 98/79/EC of the European Parliament and of the Council of 27 October 1998 on in vitro diagnostic medical devices}}</ref> the [[Joint Committee for Traceability in Laboratory Medicine]] (JCTLM) was created in 2002 through a Declaration of Cooperation between the International Committee of Weights and Measures (CIPM), the [[International Federation of Clinical Chemistry and Laboratory Medicine]] (IFCC), and the [[International Laboratory Accreditation Cooperation]] (ILAC).<ref>
{{cite web
 |url = http://www.bipm.org/en/committees/jc/jctlm/
 |title = JCTLM: Joint Committee for Traceability in Laboratory Medicine
 |publisher = [[International Bureau of Weights and Measures|Bureau International des Poids et Mesures]]
 |access-date = 2 October 2012
 |archive-date = 16 May 2012
 |archive-url = https://web.archive.org/web/20120516010132/http://www.bipm.org/en/committees/jc/jctlm/
 |url-status = live
}}</ref> The joint committee provides a forum for the harmonization of standards of the various participants.

=== Coordination of International Atomic Time ===
With the advent of the [[atomic clock]] it has been possible to define and measure [[International Atomic Time]] with sufficient precision that variations in the [[Earth's rotation]] can be detected. The [[International Earth Rotation and Reference Systems Service|International Earth Rotation Service]] monitors these changes relative to the stars at regular intervals and proposes leap seconds as and when these are needed. Currently there are over 200 atomic clocks in over 50 national laboratories around the world and the BIPM, in terms of the mandate given to it under the Metre Convention, coordinates the various atomic clocks.<ref>
{{cite web
 |url = http://www.bipm.org/en/scientific/tai/tai.html
 |title = International Atomic Time
 |publisher = [[International Bureau of Weights and Measures|Bureau International des Poids et Mesures]]
 |access-date = 21 October 2012
 |archive-url = https://web.archive.org/web/20110110113226/http://www.bipm.org/en/scientific/tai/tai.html
 |archive-date = 10 January 2011
 |url-status = dead
}}</ref>

=== 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>