Editing Metric system (section)

=== Realisation ===
{{See also|Realisation (metrology)}}

The base units used in a measurement system must be [[Realisation (metrology)|realisable]]. To that end, the definition of each SI base unit is accompanied by a ''mise en pratique'' (practical realisation) that describes at least one way that the unit can be measured.<ref>{{cite web |url = http://www.bipm.org/en/si/new_si/mise-en-pratique.html |title = What is a ''mise en pratique''? |publisher = [[BIPM]] |year=2011 |access-date = 11 March 2011}}</ref> Where possible, definitions of the base units were developed so that any laboratory equipped with proper instruments would be able to realise a standard without reliance on an artefact held by another country.  In practice, such realisation is done under the auspices of a [[International Organization of Legal Metrology#Conformance Certification|mutual acceptance arrangement]].<ref>{{cite web |url=http://www.oiml.org/maa/ |title=OIML Mutual Acceptance Arrangement (MAA) |publisher=[[International Organization of Legal Metrology]] |access-date=23 April 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130521131225/http://www.oiml.org/maa/ |archive-date=21 May 2013}}</ref>

[[File:Kilometre definition.svg|right|thumb|The [[metre]] was originally defined to be ''one ten millionth'' of the distance between the [[North Pole]] and the [[Equator]] through [[Paris]].<ref name="Alder">{{cite book |last=Alder |first=Ken |title=The Measure of all Things—The Seven-Year-Odyssey That Transformed the World |publisher=Abacus |year=2002 |isbn=978-0-349-11507-8 |location=London}}</ref>]]

In 1791 the commission originally defined the [[metre]] based on the size of the earth, equal to one ten-millionth of the distance from the equator to the North Pole. In the SI, the standard [[metre]] is now defined as exactly {{frac|1|{{val|299,792,458}}}} of the distance that light travels in a [[second]].<ref>{{cite web |url=https://www.bipm.org/en/committees/cg/cgpm/17-1983/resolution-1 |title=17th General Conference on Weights and Measures (1983), Resolution 1. |access-date=17 June 2023}}</ref><ref>{{Cite web|date=20 May 2019|title=Mise en pratique for the definition of the metre in the SI|url=https://www.bipm.org/documents/20126/41489670/SI-App2-metre.pdf/0e011055-9736-d293-5e56-b8b1b267fd68?version=1.8&t=1637238031486&download=false|website=BIPM |access-date=17 June 2023}}</ref> The metre can be realised by measuring the length that a light wave travels in a given time, or equivalently by measuring the wavelength of light of a known frequency.<ref>{{cite conference |last1=Lewis |first1=A. |title=1983 realisation of the metre definition |publisher=National Physical Laboratory |conference=Varenna Summer School |url=https://static.sif.it/SIF/resources/public/files/va2019/Lewis2.pdf |access-date=10 July 2023 |page=15 |date=4 July 2019}}</ref>

The [[kilogram]] was originally defined as the mass of one cubic decimetre of water at 4&nbsp;°C, standardised as the mass of a man-made artefact of platinum–iridium held in a laboratory in France, which was used until a [[2019 revision of the SI|new definition was introduced in May 2019]]. Replicas made in 1879 at the time of the artefact's fabrication and distributed to signatories of the [[Metre Convention]] serve as ''de facto'' standards of mass in those countries. Additional replicas have been fabricated since as additional countries have joined the convention.  The replicas were subject to periodic validation by comparison to the original, called the [[International Prototype of the Kilogram|IPK]]. It became apparent that either the IPK or the replicas or both were deteriorating, and are no longer comparable: they had diverged by 50&nbsp;μg since fabrication, so figuratively, the accuracy of the kilogram was no better than 5 parts in a hundred million or a relative accuracy of {{val|5|e=-8}}. The [[2019 revision of the SI|revision of the SI]] replaced the IPK with an exact definition of the [[Planck constant]] as expressed in SI units, which defines the kilogram in terms of fundamental constants.<ref>{{cite web |date=16 November 2018 |title=The Latest: Landmark Change to Kilogram Approved |url=https://apnews.com/e6991383703e4ad5a9570d97b0e57822 |access-date=17 June 2023 |website=AP News |publisher=Associated Press}}</ref><ref>{{Cite web |date=7 July 2021 |title=Mise en pratique for the definition of the kilogram in the SI |url=https://www.bipm.org/documents/20126/41489673/SI-App2-kilogram.pdf/5881b6b5-668d-5d2b-f12a-0ef8ca437176?version=1.9&t=1637237674882&download=false |access-date=17 June 2023 |website=BIPM}}</ref><ref>{{cite news |last1=Resnick |first1=Brian |title=The new kilogram just debuted. It's a massive achievement. |url=https://www.vox.com/science-and-health/2019/5/17/18627757/kilogram-redefined-world-metrology-day-explained |work=Vox |date=20 May 2019 |access-date=17 June 2023}}</ref>