Editing Metre Convention (section)

=== Reference standards ===
[[Image:Platinum-Iridium meter bar.jpg|right|thumb|220px|Historical ''[[international prototype of the metre]]'', made of an alloy of platinum and iridium, that was the standard from 1889 to 1960.]]Although the new standard metre had the same value as the old metre, it had an "X" cross-section designed by [[Henri Tresca]] rather than a rectangular cross-section as this reduced the flexing when taking measurements. Moreover, the new bar, rather than being exactly one metre in length was a little longer than one metre and had lines engraved on them that were exactly one metre apart.<ref>
{{cite book |first1=Thomas |last1=McGreevy |title=The Basis of Measurement: Volume 1 – Historical Aspects |pages=150–151 |isbn=0-948251-82-4 |year=1995 |publisher=Pitcon Publishing (Chippenham) Ltd}}
</ref> The London firm [[Johnson Matthey]] delivered 30 prototype metres and 40 prototype kilograms. At the first meeting of the [[CGPM]] in 1889 bar No.&nbsp;6 and cylinder No.&nbsp;X were chosen by lot as the international prototypes. The remainder were either kept as BIPM working copies or distributed by lot to member states as national prototypes.<ref>
{{cite journal
 |last1= Jabbour
 |first1= Z.J.
 |last2= Yaniv
 |first2= S.L.
 |year= 2001
 |title= The Kilogram and Measurements of Mass and Force
 |journal= J. Res. Natl. Inst. Stand. Technol.
 |volume= 106
 |issue= 1
 |pages= 25–46
 |url= http://nvl.nist.gov/pub/nistpubs/jres/106/1/j61jab.pdf
 |access-date= 28 March 2011
 |doi= 10.6028/jres.106.003
 |pmid= 27500016
 |pmc= 4865288
 |archive-url= https://web.archive.org/web/20110604144310/http://nvl.nist.gov/pub/nistpubs/jres/106/1/j61jab.pdf
 |archive-date= 4 June 2011
 |url-status= dead
}}</ref>

The comparison of the new prototypes of the metre with each other involved the development of special measuring equipment and the definition of a reproducible temperature scale. The BIPM's [[Temperature measurement|thermometry]] work led to the discovery of special alloys of iron–nickel, in particular [[invar]], whose practically negligible coefficient of expansion made it possible to develop simpler baseline measurement methods,<ref name="Guillaume-1906">{{Cite journal |last=Guillaume |first=C.-H.-Ed |date=1906-01-01 |title=La mesure rapide des bases géodésiques |url=https://zenodo.org/record/2007289 |journal=Journal de Physique Théorique et Appliquée |volume=5 |pages=242–263 |doi=10.1051/jphystap:019060050024200}}</ref> and for which its director, the Swiss physicist [[Charles Édouard Guillaume]], was granted the [[Nobel Prize in Physics]] in 1920. Guillaume's Nobel Prize marked the end of an era in which [[metrology]] was leaving the field of [[geodesy]] to become an autonomous [[Branches of science|scientific discipline]] capable of redefining the metre through [[Technology|technological]] applications of [[physics]].<ref>{{Cite web |title=BIPM – la définition du mètre |url=https://www.bipm.org/fr/measurement-units/history-si/evolution_metre.html |archive-url=https://web.archive.org/web/20170430075245/http://www.bipm.org/fr/measurement-units/history-si/evolution_metre.html |archive-date=30 April 2017 |access-date=2019-05-15 |website=www.bipm.org}}</ref><ref name="BIPM-150">{{Cite web |title=History – The BIPM 150 |url=https://thebipm150.org/history/ |access-date=2025-01-24 |language=}}</ref> On the other hand, the foundation of the United States Coast and Geodetic Survey by Ferdinand Rudolph Hassler paved the way to a new definition of the metre, with [[Charles Sanders Peirce]] being the first to experimentally link the metre to the wavelength of a spectral line. [[Albert A. Michelson]] soon took up the idea and improved it.<ref name="Crease-2009">{{Cite journal |last=Crease |first=Robert P. |date=2009-12-01 |title=Charles Sanders Peirce and the first absolute measurement standard |url=https://pubs.aip.org/physicstoday/article/62/12/39/390647/Charles-Sanders-Peirce-and-the-first-absolute |journal=Physics Today |volume=62 |issue=12 |pages=39–44 |bibcode=2009PhT....62l..39C |doi=10.1063/1.3273015 |issn=0031-9228}}</ref>

The prototype metre was retained as the international standard until 1960 when the metre was redefined in terms of the wavelength of the orange-red line of [[krypton-86]]. The current definition of the metre is "the length of the path travelled by light in vacuum during a time interval of 1/{{val|299792458}} of a second".

On 16 November 2018, the 26th [[General Conference on Weights and Measures]] (CGPM) voted unanimously in favour of revised definitions of some [[SI base unit]]s, in particular the kilogram.<ref>{{Cite news|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|last=Materese|first=Robin|date=16 November 2018|work=NIST|access-date=17 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> The new definitions came into force on 20 May 2019, but did not change the metre.<ref name=SI-statement/><ref name=bipm>[http://www.bipm.org/en/committees/cipm/meeting/105.html "Decision CIPM/105-13 (October 2016)"] {{Webarchive|url=https://web.archive.org/web/20170824095943/http://www.bipm.org/en/committees/cipm/meeting/105.html |date=24 August 2017 }}. The day is the 144th anniversary of the Metre Convention.</ref>