View source for Metre

{{Short description|SI unit of length}}
{{Infobox unit
| name     = metre
| standard = SI
| quantity = [[Physics:Length|length]]
| symbol   = m<ref>
  {{cite web
  | url=http://physics.nist.gov/cuu/Units/meter.html
  | title=Base unit definitions: Meter
  | publisher=[[Organization:National Institute of Standards and Technology|National Institute of Standards and Technology]]
  | access-date=2010-09-28
  }}</ref>
| dimension = L
| units1 = SI units
| inunits1 =
  {{unbulleted list
  | {{val|1000|ul=mm}}
  | {{val|0.001|ul=km}}
  }}
| units2   = [[Imperial units|Imperial]]/US units
| inunits2 =
  {{unbulleted list
  | ≈&thinsp;{{val|1.0936|u=[[Yard|yd]]}}
  | ≈&thinsp;{{val|3.2808|u=[[Foot (unit)|ft]]}}
  | ≈&thinsp;{{val|39.37|u=[[Inch|in]]}}
  }}
| units3   = Nautical units
| inunits3 = ≈&thinsp;{{val|0.00053996|u=[[Nautical mile|nmi]]}}
| image=Metric seal.svg
| caption=Seal of the International Bureau of Weights and Measures (BIPM) – Use measure (Greek: {{lang|grc|ΜΕΤΡΩ ΧΡΩ}})
}}

The '''metre''' (or '''meter''' in [[Social:American and British English spelling differences#-re, -er|American spelling]]; symbol: '''m''') is the [[SI base unit|base unit]] of [[Physics:Length|length]] in the [[International System of Units]] (SI). Since 2019 the metre has been defined as the length of the path travelled by light in vacuum during a time interval of {{sfrac|{{val|299792458}}}} of a [[Second|second]], where the second is defined by a [[Physics:Caesium standard|hyperfine transition frequency of caesium]].<ref name="SIBrochure9thEd">
{{citation
 |author=International Bureau of Weights and Measures
 |title=The International System of Units (SI)
 |date=20 May 2019
 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf
 |archive-url=https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232
 |url-status=live
 |edition=9th
 |isbn=978-92-822-2272-0
 |archive-date=18 October 2021
}}</ref>

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the [[Astronomy:Equator|equator]] to the [[Earth:North Pole|North Pole]] along a [[Great circle|great circle]], so the [[Earth:Earth's circumference|Earth's polar circumference]] is approximately {{val|40000|u=km}}.

In 1799, the metre was redefined in terms of a prototype metre bar, the bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of [[Physics:Krypton-86|krypton-86]]. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of [[Physics:Proper length|proper length]]. From 1983 until 2019, the metre was formally defined as the length of the path travelled by [[Physics:Light|light]] in vacuum in {{sfrac|1|{{gaps|299|792|458}}}} of a [[Second|second]]. After the [[2019 redefinition of the SI base units|2019 redefinition of the SI base units]], this definition was rephrased to include the definition of a second in terms of the caesium frequency {{math|Δ''&nu;''<sub>Cs</sub>}}. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures {{val|40007.863|u=km}}, a change of 0.022% from the original value of exactly {{val|40000|u=km}}, which also includes improvements in the accuracy of measuring the circumference.

== Spelling ==
''Metre'' is the standard spelling of the metric unit for length in all English-speaking nations; the exceptions are the United States<ref>
{{cite web
 |url=https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf
 |title=The International System of Units (SI) – NIST
 |publisher=[[Organization:National Institute of Standards and Technology|National Institute of Standards and Technology]]
 |location=US
 |date=26 March 2008
 |quote=The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary. Thus the spellings 'meter', 'liter', 'deka', and 'cesium' are used rather than 'metre', 'litre', 'deca', and 'caesium' as in the original BIPM English text.
}}</ref><ref>The most recent official brochure about the International System of Units (SI), written in French by the ''{{lang|fr|Bureau international des poids et mesures}}'', [[Organization:International Bureau of Weights and Measures|International Bureau of Weights and Measures]] (BIPM) uses the spelling ''metre''; an English translation, included to make the SI standard more widely accessible also uses the spelling ''metre'' (BIPM, 2006, p. 130''ff''). However, in 2008 the U.S. English translation published by the U.S. [[Organization:National Institute of Standards and Technology|National Institute of Standards and Technology]] (NIST) chose to use the spelling ''meter'' in accordance with the United States Government Printing Office Style Manual. The Metric Conversion Act of 1975 gives the Secretary of Commerce of the US the responsibility of interpreting or modifying the SI for use in the US. The Secretary of Commerce delegated this authority to the Director of the National Institute of Standards and Technology (Turner). In 2008, NIST published the US version (Taylor and Thompson, 2008a) of the English text of the eighth edition of the BIPM publication ''{{lang|fr|Le Système international d'unités}} (SI)'' (BIPM, 2006). In the NIST publication, the spellings "meter", "liter" and "deka" are used rather than "metre", "litre" and "deca" as in the original BIPM English text (Taylor and Thompson (2008a), p. iii). The Director of the NIST officially recognised this publication, together with Taylor and Thompson (2008b), as the "legal interpretation" of the SI for the United States (Turner). Thus, the spelling ''metre'' is referred to as the "international spelling"; the spelling ''meter'', as the "American spelling".</ref><ref>
{{cite web
 |url=http://www.metricationmatters.com/docs/Spelling_metre_or_meter.pdf
 |title=Spelling metre or meter
 |first=Pat |last=Naughtin
 |website=Metrication Matters
 |year=2008
 |access-date=2017-03-12
 |archive-url=https://web.archive.org/web/20161011100154/http://www.metricationmatters.com/docs/Spelling_metre_or_meter.pdf
 |archive-date=11 October 2016|url-status=usurped}}</ref><ref>{{cite web |url= http://grammarist.com/spelling/meter-metre/
 |title=Meter vs. metre
 |website=Grammarist
 |date=21 February 2011
 |access-date=2017-03-12
}}</ref> and the Philippines,<ref>The Philippines uses English as an official language and this largely follows American English since the country became a colony of the United States. While the law that converted the country to use the [[Metric system|metric system]] uses ''metre'' (Batas Pambansa Blg. 8) following the SI spelling, in actual practice, ''meter'' is used in government and everyday commerce, as evidenced by laws (''kilometer'', Republic Act No. 7160), Supreme Court decisions (''meter'',  G.R. No. 185240), and national standards (''centimeter'', PNS/BAFS 181:2016).</ref> which use ''meter''.

Measuring devices (such as [[Engineering:Ammeter|ammeter]], [[Engineering:Speedometer|speedometer]]) are spelled "-meter" in all variants of English.<ref>
{{cite encyclopedia
 |url=http://dictionary.cambridge.org/results.asp?searchword=ammeter
 |title=Cambridge Advanced Learner's Dictionary
 |year=2008
 |publisher={{wipe|Cambridge University Press}}
 |access-date=2012-09-19
}}, s.v. ammeter, meter, parking meter, speedometer.</ref> The suffix "-meter" has the same Greek origin as the unit of length.<ref>
{{cite encyclopedia
 |title=American Heritage Dictionary of the English Language
 |edition=3rd
 |year=1992
 |location=Boston
 |publisher=Houghton Mifflin
}}, s.v. meter.</ref><ref>
{{cite web
 |title=-meter – definition of -meter in English
 |url=https://en.oxforddictionaries.com/definition/-meter
 |archive-url=https://web.archive.org/web/20170426153254/https://en.oxforddictionaries.com/definition/-meter
 |url-status=dead
 |archive-date=26 April 2017
 |publisher=Oxford Dictionaries
}}</ref>

== Etymology ==
The etymological roots of ''metre'' can be traced to the Greek verb {{lang|grc|μετρέω}} ({{transliteration|grc|metreo}}) (to measure, count or compare) and noun {{lang|grc|μέτρον}} ({{transliteration|grc|metron}}) (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin ({{lang|la|metior, mensura}}), French ({{lang|fr|mètre, mesure}}), English and other languages. The Greek word is derived from the Proto-Indo-European root ''*meh₁-'' 'to measure'. The motto {{lang|grc|ΜΕΤΡΩ ΧΡΩ}} ({{transliteration|grc|metro chro}}) in the seal of the [[Organization:International Bureau of Weights and Measures|International Bureau of Weights and Measures]] (BIPM), which was a saying of the Greek statesman and philosopher [[Biography:Pittacus of Mytilene|Pittacus of Mytilene]] and may be translated as "Use measure!", thus calls for both measurement and moderation. The use of the word ''metre'' (for the French unit {{lang|fr|mètre}}) in English began at least as early as 1797.<ref name="Oxford">Oxford English Dictionary, Clarendon Press 2nd ed. 1989, vol. IX p. 697 col. 3.</ref>

== History of definition <span class="anchor" id="History"></span><span class="anchor" id="Definition"></span> ==
{{Main|History of the metre}}
{{Overly detailed|section|nosplit=1|details=|date=September 2023}}

=== Universal measure: the metre linked to the figure of the Earth ===
[[File:Obs-Paris-meridienne.jpg|thumb|The Meridian room of the Paris Observatory (or Cassini room): the [[Paris meridian|Paris meridian]] is drawn on the ground.|left]]

The Scientific revolution began with Copernicus' work. Building on this,  Galileo discovered [[Physics:Gravitational acceleration|gravitational acceleration]] to explain the fall of bodies at the surface of the Earth.<ref>{{Cite web |title=Museo Galileo - In depth - Gravitational acceleration |url=https://catalogue.museogalileo.it/indepth/GravitationalAcceleration.html |access-date=2023-12-29 |website=catalogue.museogalileo.it}}</ref> He also observed the regularity of the period of swing of the [[Physics:Pendulum|pendulum]] and that this period depended on the length of the pendulum.<ref>{{Cite web |title=Museo Galileo - In depth - Pendulum |url=https://catalogue.museogalileo.it/indepth/Pendulum.html |access-date=2023-12-29 |website=catalogue.museogalileo.it}}</ref>

[[Kepler's laws of planetary motion]] served both to the discovery of [[Physics:Newton's law of universal gravitation|Newton's law of universal gravitation]] and to the determination of the distance from Earth to the Sun by Giovanni Domenico Cassini.<ref>{{Cite web |title=M13. From Kepler's Laws To Universal Gravitation – Basic Physics |url=https://www.basic-physics.com/m13-from-keplers-laws-to-universal-gravitation/ |access-date=2023-12-30 |language=en-US}}</ref><ref>
{{cite book
 |title=L'exploration du système solaire
 |last=Bond, Peter
 |date=2014
 |publisher=De Boeck
 |others=Dupont-Bloch, Nicolas.
 |isbn=9782804184964
 |edition=[Édition française revue et corrigée]
 |location=Louvain-la-Neuve
 |pages=5–6
 |oclc=894499177
}}</ref> They both also used a determination of the size of the Earth, then considered as a sphere, by Jean Picard through [[Triangulation (surveying)|triangulation]] of [[Paris meridian]].<ref name=":3" /><ref name="Levallois" /> In 1671, Jean Picard also measured the length of a [[Seconds pendulum|seconds pendulum]] at Paris Observatory and proposed this unit of measurement to be called the astronomical radius (French: ''Rayon Astronomique'').<ref>{{Cite book |last=Picard |first=Jean (1620–1682) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/btv1b7300361b |title=Mesure de la terre [par l'abbé Picard] |date=1671 |pages=3–5 |language=EN}}</ref>{{sfn|Bigourdan|1901|pp=8,158–159}} In 1675, [[Company:Tito Livio Burattini|Tito Livio Burattini]] suggested the term ''{{Lang|it|metro cattolico}}'' meaning universal measure for this unit of length, but then it was discovered that the length of a seconds pendulum varies from place to place.<ref name="Earth-1911" /><ref>{{Cite book |last1=Poynting |first1=John Henry |url=https://archive.org/details/bub_gb_TL4KAAAAIAAJ |title=A Textbook of Physics |last2=Thomson |first2=Joseph John |date=1907 |publisher=C. Griffin |pages=[https://archive.org/details/bub_gb_TL4KAAAAIAAJ/page/n30 20] |language=en}}</ref><ref>{{cite web |title=Science. 1791, l'adoption révolutionnaire du mètre |url=https://www.humanite.fr/science-1791-ladoption-revolutionnaire-du-metre-702009 |website=humanite.fr |access-date=2021-08-03 |language=fr |date=25 March 2021}}</ref><ref>{{cite book |last1=Lucendo |first1=Jorge |title=Centuries of Inventions: Encyclopedia and History of Inventions |date=23 April 2020 |publisher=Jorge Lucendo |page=246 |url=https://books.google.com/books?id=4l3eDwAAQBAJ&pg=PT246 |access-date=2021-08-02 |language=en}}</ref>

[[File:Repsold.jpg|thumb|[[Physics:Gravimeter|Gravimeter]] with variant of Repsold–Bessel pendulum]]

[[Biography:Christiaan Huygens|Christiaan Huygens]] found out the [[Physics:Centrifugal force|centrifugal force]] which explained variations of gravitational acceleration depending on latitude.<ref>{{Cite web |last=Silas |first=Walter |date=2022-10-30 |title=Centrifugal force Vs centripetal force |url=https://probingphysics.com/centrifugal-force-vs-centripetal-force/ |access-date=2023-12-30 |website=Probing the Universe |language=en-US}}</ref><ref>{{Cite web |title=Gravity: Notes: Latitude Dependent Changes in Gravitational Acceleration |url=https://pburnley.faculty.unlv.edu/GEOL452_652/gravity/notes/GravityNotes18LatitudeVariations.htm |access-date=2023-12-30 |website=pburnley.faculty.unlv.edu}}</ref> He also mathematically formulated the link between the length of the [[Physics:Pendulum|simple pendulum]] and gravitational acceleration.<ref name="Perrier-1935" /> According to Alexis Clairaut, the study of variations in gravitational acceleration was a way to determine the [[Earth:Figure of the Earth|figure of the Earth]], whose crucial parameter was the [[Flattening|flattening]] of the [[Earth:Earth ellipsoid|Earth ellipsoid]]. In the 18th century, in addition of its significance for [[Earth:Cartography|cartography]], [[Geodesy|geodesy]] grew in importance as a means of empirically demonstrating the [[Physics:Gravity|theory of gravity]], which [[Biography:Émilie du Châtelet|Émilie du Châtelet]] promoted in France in combination with [[Biography:Gottfried Wilhelm Leibniz|Leibniz's]] mathematical work and because the [[Astronomy:Earth radius|radius of the Earth]] was the unit to which all celestial distances were to be referred. Indeed, Earth proved to be an [[Spheroid|oblate spheroid]] through geodetic surveys in [[Earth:French Geodesic Mission to the Equator|Ecuador]] and [[Earth:French Geodesic Mission to Lapland|Lapland]] and this new data called into question the value of [[Astronomy:Earth radius|Earth radius]] as Picard had calculated it.<ref name="Perrier-1935">
{{cite journal
 |last=Perrier |first=Général
 |date=1935
 |title=Historique Sommaire De La Geodesie
 |url=https://www.jstor.org/stable/43861533
 |journal=Thalès
 |volume=2 |pages=117–129, p. 128
 |issn=0398-7817
 |jstor=43861533
}}</ref><ref>
{{cite book
 |last=Badinter |first=Élisabeth
 |url=https://www.worldcat.org/oclc/1061216207
 |title=Les passions intellectuelles
 |date=2018 |publisher=Robert Laffont
 |others=Normandie roto impr.
 |isbn=978-2-221-20345-3
 |location=Paris
 |oclc=1061216207
}}</ref><ref>
{{cite journal
 |last=Touzery |first=Mireille
 |date=2008-07-03
 |title=Émilie Du Châtelet, un passeur scientifique au XVIIIe siècle
 |url=https://journals.openedition.org/histoire-cnrs/7752
 |journal=La revue pour l'histoire du CNRS
 |language=fr
 |issue=21
 |doi=10.4000/histoire-cnrs.7752
 |issn=1298-9800
 |doi-access=free
}}</ref><ref name="Earth-1911">
{{cite EB1911
 |wstitle=Earth, Figure of the
 |volume= 8
 |pages=801–813
 |short=1
}}</ref><ref name="Levallois" />

After the [[Earth:Anglo-French Survey (1784–1790)|Anglo-French Survey]], the French Academy of Sciences commissioned an expedition led by [[Biography:Jean Baptiste Joseph Delambre|Jean Baptiste Joseph Delambre]] and [[Biography:Pierre Méchain|Pierre Méchain]], lasting from 1792 to 1798, which measured the distance between a belfry in Dunkirk and Montjuïc castle in [[Place:Barcelona|Barcelona]] at the [[Earth:Longitude|longitude]] of the Paris Panthéon. When the length of the metre was defined as one ten-millionth of the distance from the [[Earth:North Pole|North Pole]] to the [[Astronomy:Equator|Equator]], the flattening of the Earth ellipsoid was assumed to be {{Sfrac|1|334}}.<ref>{{Cite book |last=Capderou |first=Michel |url=https://books.google.com/books?id=jRQXQhRSrz4C |title=Satellites : de Kepler au GPS |date=2011-10-31 |publisher=Springer Science & Business Media |isbn=978-2-287-99049-6 |pages=46 |language=fr}}</ref><ref>{{Cite web |last=Ramani |first=Madhvi |title=How France created the metric system |url=http://www.bbc.com/travel/story/20180923-how-france-created-the-metric-system |access-date=2019-05-21 |website=www.bbc.com |language=en}}</ref><ref name="Levallois">{{Cite web |last=Levallois |first=Jean-Jacques |date=1986 |title=La Vie des sciences |url=https://gallica.bnf.fr/ark:/12148/bpt6k5470853s |access-date=2019-05-13 |website=Gallica |pages=262, 285, 288–290, 269, 276–277, 283 |language=FR}}</ref><ref>Jean-Jacques Levallois, La méridienne de Dunkerque à Barcelone et la détermination du mètre (1792 – 1799), Vermessung, Photogrammetrie, Kulturtechnik, 89 (1991), 375-380.</ref><ref name="Levallois-1991">{{Cite journal |last=Zuerich |first=ETH-Bibliothek |year=1991 |title=La méridienne de Dunkerque à Barcelone et la déterminiation du mètre (1972-1799) |url=https://dx.doi.org/10.5169/seals-234595 |language=FR |pages=377–378 |doi=10.5169/seals-234595 |access-date=2021-10-12 |website=E-Periodica}}</ref><ref name="Martin-2008">{{Cite journal |last1=Martin |first1=Jean-Pierre |last2=McConnell |first2=Anita |date=2008-12-20 |title=Joining the observatories of Paris and Greenwich |url=https://royalsocietypublishing.org/doi/10.1098/rsnr.2008.0029 |journal=Notes and Records of the Royal Society |language=en |volume=62 |issue=4 |pages=355–372 |doi=10.1098/rsnr.2008.0029 |s2cid=143514819 |issn=0035-9149}}</ref>

In 1841, Friedrich Wilhelm Bessel using the [[Least squares|method of least squares]] calculated from several [[Earth:Arc measurement|arc measurement]]s a new value for the flattening of the Earth, which he determinated as {{Sfrac|1|299.15}}.<ref name=":2">{{Cite web |last=von Struve |first=Friedrich Georg Wilhelm |date=July 1857 |title=Comptes rendus hebdomadaires des séances de l'Académie des sciences / publiés... par MM. les secrétaires perpétuels |url=https://gallica.bnf.fr/ark:/12148/bpt6k30026 |access-date=2021-08-30 |website=Gallica |pages=509, 510 |language=EN}}</ref><ref name="Viik-2006">{{Cite news |last=Viik |first=T |date=2006 |title=F. W. Bessel and geodesy |pages=10, 6 |work=Struve Geodetic Arc, 2006 International Conference, The Struve Arc and Extensions in Space and Time, Haparanda and Pajala, Sweden, 13–15 August 2006 |citeseerx=10.1.1.517.9501}}</ref><ref name=":1">{{Cite journal |last=Bessel |first=Friedrich Wilhelm |date=1841-12-01 |title=Über einen Fehler in der Berechnung der französischen Gradmessung und seineh Einfluß auf die Bestimmung der Figur der Erde. Von Herrn Geh. Rath und Ritter Bessel |url=https://ui.adsabs.harvard.edu/abs/1841AN.....19...97B |journal=Astronomische Nachrichten |volume=19 |issue=7 |pages=97 |bibcode=1841AN.....19...97B |doi=10.1002/asna.18420190702 |issn=0004-6337}}</ref> He also devised a new instrument for measuring gravitational acceleration which was first used in {{wipe|Switzerland}} by Emile Plantamour, [[Biography:Charles Sanders Peirce|Charles Sanders Peirce]] and Isaac-Charles Élisée Cellérier (8.01.1818 – 2.10.1889), a [[Place:Geneva|Geneva]]n mathematician soon independently discovered a mathematical formula to correct [[Observational error|systematic errors]] of this device which had been noticed by Plantamour and Adolphe Hirsch.<ref>{{citation-attribution|{{Cite book|url=http://www.rac.es/ficheros/Discursos/DR_20080825_173.pdf|title=Discursos leidos ante la Real Academia de Ciencias Exactas Fisicas y Naturales en la recepcion pública de Don Joaquin Barraquer y Rovira|last=Ibáñez e Ibáñez de Ibero|first=Carlos|publisher=Imprenta de la Viuda e Hijo de D.E. Aguado|year=1881|location=Madrid|pages=70–78}}}}</ref><ref>{{Cite journal |date=1880 |title=Rapport de M. Faye sur un Mémoire de M. Peirce concernant la constance de la pesanteur à Paris et les corrections exigées par les anciennes déterminations de Borda et de Biot |url=https://gallica.bnf.fr/ark:/12148/bpt6k3047v/f1457.image.r=1880%201880 |journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences |volume=90 |pages=1463–1466 |access-date=2018-10-10 |via=Gallica}}</ref> This allowed Friedrich Robert Helmert to determine a remarkably accurate value of  {{Sfrac|1|298.3}} for the flattening of the Earth when he proposed his [[Earth:Earth ellipsoid|ellipsoid of reference]] in 1901.<ref name="Enc. Universalis-1996">{{Cite book |title=Encyclopedia Universalis |publisher=Encyclopedia Universalis |year=1996 |isbn=978-2-85229-290-1 |pages=320, 370. Vol 10 |oclc=36747385}}</ref> This was also the result of the [[Metre Convention]] of 1875, when the metre was adopted as an international scientific unit of length for the convenience of continental European geodesists following the example of Ferdinand Rudolph Hassler.<ref name="Brunner-1857">{{Cite web |last=Brunner |first=Jean |date=1857-01-01 |title=Appareil construit pour les opérations au moyen desquelles on prolongera dans toute l'étendue de l'Espagne le réseau trigonométrique qui couvre la France in Comptes rendus hebdomadaires des séances de l'Académie des sciences / publiés... par MM. les secrétaires perpétuels |url=https://gallica.bnf.fr/ark:/12148/bpt6k3001w |access-date=2023-08-31 |website=Gallica |pages=150–153 |language=FR}}</ref><ref name="Pérard-1957">{{Cite web |last=Pérard |first=Albert |date=1957 |title=Carlos Ibáñez e Ibáñez de Ibero (14 avril 1825 – 29 janvier 1891), par Albert Pérard (inauguration d'un monument élevé à sa mémoire) |url=https://www.academie-sciences.fr/pdf/eloges/ibanez_notice.pdf |website=Institut de France – Académie des sciences |pages=26–28}}</ref><ref>Adolphe Hirsch, ''Le général Ibáñez notice nécrologique lue au comité international des poids et mesures, le 12 septembre et dans la conférence géodésique de Florence, le 8 octobre 1891'', Neuchâtel, imprimerie Attinger frères.</ref><ref>{{Cite web |last=Wolf |first=Rudolf |date=1891-01-01 |title=Histoire de l'appareil Ibañez-Brunner in Comptes rendus hebdomadaires des séances de l'Académie des sciences / publiés... par MM. les secrétaires perpétuels |url=https://gallica.bnf.fr/ark:/12148/bpt6k3068q |access-date=2023-08-31 |website=Gallica |pages=370–371 |language=FR}}</ref><ref name="Clarke-1873">{{Citation |last=Clarke |first=Alexander Ross |title=XIII. Results of the comparisons of the standards of length of England, Austria, Spain, United States, Cape of Good Hope, and of a second Russian standard, made at the Ordnance Survey Office, Southampton. With a preface and notes on the Greek and Egyptian measures of length by Sir Henry James |periodical=Philosophical Transactions |volume=163 |page=463 |year=1873 |place=London |doi=10.1098/rstl.1873.0014 |doi-access=free}}</ref><ref name="BEG-1868">{{Cite book |url=http://gfzpublic.gfz-potsdam.de/pubman/item/escidoc:108187:4/component/escidoc:272449/Generalbericht.mitteleurop%C3%A4ische.Gradmessung%201867.pdf |title=Bericht über die Verhandlungen der vom 30. September bis 7. October 1867 zu BERLIN abgehaltenen allgemeinen Conferenz der Europäischen Gradmessung |publisher=Central-Bureau der Europäischen Gradmessung |year=1868 |location=Berlin |pages=123–134 |language=german}}</ref>

==== Meridional definition ====
In 1790, one year before it was  ultimately decided that the metre would be based on the [[Earth:Meridian arc#Quarter meridian|Earth quadrant]] (a quarter of the [[Earth:Earth's circumference|Earth's circumference]] through its poles), Talleyrand proposed that the metre be the length of the seconds pendulum at a [[Earth:Latitude|latitude]] of 45°. This option, with one-third of this length defining the [[Foot (unit)|foot]], was also considered by Thomas Jefferson and others for redefining the yard in the United States shortly after gaining independence from the British Crown.<ref>{{Cite web |title=The seconds pendulum |url=https://www.roma1.infn.it/~dagos/history/sm/node3.html |access-date=2023-10-06 |website=www.roma1.infn.it}}</ref><ref>{{cite book|last=Cochrane|first=Rexmond|title=Measures for progress: a history of the National Bureau of Standards|chapter-url=http://nvl.nist.gov/nvl2.cfm?doc_id=505 |year=1966 |publisher=U.S. Department of Commerce |page=532 |chapter=Appendix B: The metric system in the United States |access-date=2011-03-05 |archive-url=https://web.archive.org/web/20110427023306/http://nvl.nist.gov/nvl2.cfm?doc_id=505|archive-date=2011-04-27|url-status=dead}}</ref>

Instead of the seconds pendulum method, the commission of the French Academy of Sciences – whose members included [[Biography:Jean-Charles de Borda|Borda]], [[Biography:Joseph-Louis Lagrange|Lagrange]], [[Biography:Pierre-Simon Laplace|Laplace]], [[Biography:Gaspard Monge|Monge]] and [[Biography:Marquis de Condorcet|Condorcet]] – decided that the new measure should be equal to one ten-millionth of the distance from the [[Earth:North Pole|North Pole]] to the [[Astronomy:Equator|Equator]], determined through measurements along the meridian passing through Paris. Apart from the obvious consideration of safe access for French surveyors, the Paris meridian was also a sound choice for scientific reasons: a portion of the quadrant from Dunkirk to Barcelona (about 1000&nbsp;km, or one-tenth of the total) could be surveyed with start- and end-points at sea level, and that portion was roughly in the middle of the quadrant, where the effects of the Earth's oblateness were expected not to have to be accounted for. Improvements in the measuring devices designed by Borda and used for this survey also raised hopes for a more accurate determination of the length of this meridian arc.<ref name="Larousse" /><ref name="RNMF">{{Cite web |title=L'histoire des unités {{!}} Réseau National de la Métrologie Française |url=https://metrologie-francaise.lne.fr/fr/metrologie/histoire-des-unites |access-date=2023-10-06 |website=metrologie-francaise.lne.fr}}</ref><ref>{{Cite book |last1=Biot |first1=Jean-Baptiste (1774–1862) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k1510037p |title=Recueil d'observations géodésiques, astronomiques et physiques, exécutées par ordre du Bureau des longitudes de France en Espagne, en France, en Angleterre et en Écosse, pour déterminer la variation de la pesanteur et des degrés terrestres sur le prolongement du méridien de Paris... rédigé par MM. Biot et Arago,... |last2=Arago |first2=François (1786-1853) Auteur du texte |date=1821 |pages=viii–ix |language=EN}}</ref><ref name="Débarbat-1799" /><ref name="Martin-2008" />

The task of surveying the Paris meridian arc took more than six years (1792–1798). The technical difficulties were not the only problems the surveyors had to face in the convulsed period of the aftermath of the French Revolution: Méchain and Delambre, and later [[Biography:François Arago|Arago]], were imprisoned several times during their surveys, and Méchain died in 1804 of yellow fever, which he contracted while trying to improve his original results in northern Spain. In the meantime, the commission of the French Academy of Sciences calculated a provisional value from older surveys of 443.44 lignes. This value was set by legislation on 7 April 1795.<ref name="Larousse" /><ref name="RNMF" /><ref name="Débarbat-1799" /><ref name=":0">{{Cite book |last=Delambre |first=Jean-Baptiste (1749–1822) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k110160s |title=Grandeur et figure de la terre / J.-B.-J. Delambre ; ouvrage augmenté de notes, de cartes et publié par les soins de G. Bigourdan,... |date=1912 |pages=202–203, 2015, 141–142, 178 |language=EN}}</ref><ref>{{Cite web |title=Comprendre – Histoire de l'observatoire de Paris - Pierre-François-André Méchain |url=https://promenade.imcce.fr/fr/pages2/297.html |access-date=2023-10-15 |website=promenade.imcce.fr}}</ref>

In 1799, a commission including [[Biography:Johann Georg Tralles|Johan Georg Tralles]], Jean Henri van Swinden, [[Biography:Adrien-Marie Legendre|Adrien-Marie Legendre]] and Jean-Baptiste Delambre calculated the distance from Dunkirk to Barcelona using the data of the [[Triangulation (surveying)|triangulation]] between these two towns and determined the portion of the distance from the North Pole to the Equator it represented. Pierre Méchain's and Jean-Baptiste Delambre's measurements were combined  with the results of the [[Earth:French Geodesic Mission to the Equator|Spanish-French geodetic mission]] and a value of {{Sfrac|1|334}} was found for the Earth's flattening. However, French astronomers knew from earlier estimates of the Earth's flattening that different meridian arcs could have different lengths and that theire curvature could be irregular. The distance from the North Pole to the Equator was then extrapolated from the measurement of the Paris meridian arc between Dunkirk and Barcelona and was determined as 5 130 740 toises. As the metre had to be equal to one ten-millionth of this distance, it was defined as 0.513074 toise or 3 feet and 11.296 lines of the Toise of Peru, which had been constructed in 1735 for the [[Earth:French Geodesic Mission to the Equator|French Geodesic Mission to the Equator]]. When the final result was known, a bar whose length was closest to the meridional definition of the metre was selected and placed in the National Archives on 22 June 1799 (4 messidor An VII in the Republican calendar) as a permanent record of the result.<ref name="Clarke-1867" /><ref name="Levallois" /><ref name="Larousse" /><ref name="Débarbat-1799">{{Cite web |last=Suzanne |first=Débarbat |title=Fixation de la longueur définitive du mètre |url=https://francearchives.gouv.fr/fr/pages_histoire/39436 |access-date=2023-10-06 |website=FranceArchives |language=fr}}</ref><ref>{{Cite web |title=Histoire du mètre {{!}} Métrologie |url=https://metrologie.entreprises.gouv.fr/fr/point-d-histoire/histoire-du-metre |access-date=2023-10-06 |website=metrologie.entreprises.gouv.fr}}</ref><ref name="Débarbat-2019" /><ref>{{Cite book |last1=Delambre |first1=Jean-Baptiste (1749–1822) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k110604s |title=Base du système métrique décimal, ou Mesure de l'arc du méridien compris entre les parallèles de Dunkerque et Barcelone. T. 1 / , exécutée en 1792 et années suivantes, par MM. Méchain et Delambre, rédigée par M. Delambre,... |last2=Méchain |first2=Pierre (1744–1804) Auteur du texte |date=1806–1810 |pages=93–94, 10 |language=EN}}</ref>

==== Early adoption of the metre as a scientific unit of length: the forerunners ====
[[File:HasslerCollection 001.jpg|left|thumb|Triangulation near New York City, 1817]]

In 1816, Ferdinand Rudolph Hassler was appointed first Superintendent of the Survey of the Coast. Trained in geodesy in Switzerland, France and {{wipe|Germany}}, Hassler had brought a standard metre made in Paris to the United States in 1805. He designed a baseline apparatus which instead of bringing different bars in actual contact during measurements, used only one bar calibrated on the metre and optical contact. Thus the metre became the unit of length for geodesy in the United States.<ref>{{Cite book |last1=American Philosophical Society. |url=https://www.biodiversitylibrary.org/item/26092 |title=Transactions of the American Philosophical Society |last2=Society |first2=American Philosophical |last3=Poupard |first3=James |date=1825 |volume=new ser.:v.2 (1825) |location=Philadelphia [etc.] |pages=234–278}}</ref><ref name="Cajori-1921" /><ref name="Clarke-1873" />

In 1830, Hassler became head of the Office of Weights and Measures, which became a part of the Survey of the Coast. He compared various units of length used in the {{wipe|United States}} at that time and measured [[Physics:Thermal expansion|coefficients of expansion]] to assess temperature effects on the measurements.<ref name="Parr-2006" />

In 1832, [[Biography:Carl Friedrich Gauss|Carl Friedrich Gauss]] studied the [[Earth:Earth's magnetic field|Earth's magnetic field]] and proposed adding the [[Second|second]] to the basic units of the metre and the [[Kilogram|kilogram]] in the form of the [[Centimetre–gram–second system of units|CGS system]] ([[Centimetre|centimetre]], [[Gram|gram]], second). In 1836, he founded the Magnetischer Verein, the first international scientific association, in collaboration with [[Biography:Alexander von Humboldt|Alexander von Humboldt]] and [[Biography:Wilhelm Eduard Weber|Wilhelm Edouard Weber]]. The coordination of the observation of geophysical phenomena such as the Earth's magnetic field, [[Astronomy:Lightning|lightning]] and gravity in different points of the globe stimulated the creation of the first international scientific associations. The foundation of the Magnetischer Verein would be followed by that of the Central European Arc Measurement (German: ''[[Organization:International Association of Geodesy|Mitteleuropaïsche Gradmessung]]'') on the initiative of Johann Jacob Baeyer in 1863, and by that of the International Meteorological Organisation whose president, the Swiss meteorologist and physicist, Heinrich von Wild would represent [[Place:Russian Empire|Russia]] at the [[Organization:International Committee for Weights and Measures|International Committee for Weights and Measures]] (CIPM).<ref name="Débarbat-2019">{{Cite journal |last1=Débarbat |first1=Suzanne |last2=Quinn |first2=Terry |date=2019-01-01 |title=Les origines du système métrique en France et la Convention du mètre de 1875, qui a ouvert la voie au Système international d'unités et à sa révision de 2018 |journal=Comptes Rendus Physique |series=The new International System of Units / Le nouveau Système international d’unités |volume=20 |issue=1 |pages=6–21 |doi=10.1016/j.crhy.2018.12.002 |bibcode=2019CRPhy..20....6D |s2cid=126724939 |issn=1631-0705|doi-access=free }}</ref><ref name="Enc. Universalis-1996" /><ref>{{Cite web |date=2015-12-08 |title=History of IMO |url=https://public-old.wmo.int/en/about-us/who-we-are/history-IMO |archive-url=https://web.archive.org/web/20231218170901/https://public-old.wmo.int/en/about-us/who-we-are/history-IMO |url-status=dead |archive-date=18 December 2023 |access-date=2023-10-07 |website=public.wmo.int |language=en}}</ref><ref>{{Cite web |title=Wild, Heinrich |url=https://hls-dhs-dss.ch/articles/028982/2014-11-11/ |access-date=2023-10-07 |website=hls-dhs-dss.ch |language=de}}</ref><ref name="Von Wild-1903">''Heinrich VON WILD (1833–1902)'' in COMlTÉ INTERNATIONAL DES POIDS ET MESURES. PROCÈS-VERBAUX DES SÉANCES. DEUXIÈME SÉRIE. TOME II. SESSION DE 1903. pp. 5–7.</ref><ref name="Quinn-2012" />

In 1834, Hassler, measured at Fire Island the first baseline of the Survey of the Coast, shortly before Louis Puissant declared to the French Academy of Sciences in 1836 that Jean Baptiste Joseph Delambre and Pierre Méchain had made errors in the [[Earth:Arc measurement|meridian arc measurement]], which had been used to determine the length of the metre. Errors in the method of calculating the length of the [[Paris meridian]] were taken into account by Bessel when he proposed his [[Earth:Earth ellipsoid|reference ellipsoid]] in 1841.<ref>{{Cite book |last1=Hassler |first1=Harriet |url=http://archive.org/details/ferdinandrudolph1068hass |title=Ferdinand Rudolph Hassler (1770–1843) |last2=Burroughs |first2=Charles A. |date=2007 |others=NIST Research Library |pages=51–52}}</ref><ref name="Lebon-1899" /><ref>{{Cite book |last=Puissant |first=Louis (1769–1843) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k5323385b |title=Nouvelle détermination de la distance méridienne de Montjouy à Formentera, dévoilant l'inexactitude de celle dont il est fait mention dans la base du système métrique décimal, par M. Puissant,... lu à l'Académie des sciences, le 2 mai 1836 |language=EN}}</ref><ref name="Viik-2006" /><ref name=":1" />

[[File:Appareil Ibáñez.jpg|thumb|Ibáñez apparatus calibrated on the metric Spanish Standard and used at Aarberg, in canton of Bern, {{wipe|Switzerland}}]]

[[Astronomy:Egyptian astronomy|Egyptian astronomy]] has ancient roots which were revived in the 19th century by the modernist impetus of Muhammad Ali who founded in Sabtieh, Boulaq district, in [[Place:Cairo|Cairo]] an Observatory which he was keen to keep in harmony with the progress of this science still in progress. In 1858, a Technical Commission was set up to continue, by adopting the procedures instituted in Europe, the cadastre work inaugurated under Muhammad Ali. This Commission suggested to Viceroy Mohammed Sa'id Pasha the idea of buying geodetic devices which were ordered in France. While Mahmud Ahmad Hamdi al-Falaki was in charge, in Egypt, of the direction of the work of the general map, the viceroy entrusted to Ismail Mustafa al-Falaki the study, in Europe, of the precision apparatus calibrated against the metre intended to measure the geodesic bases and already built by Jean Brunner in Paris. Ismail Mustafa had the task to carry out the experiments necessary for determining the expansion coefficients of the two platinum and brass bars, and to compare the Egyptian standard with a known standard. The Spanish standard designed by Carlos Ibáñez e Ibáñez de Ibero and Frutos Saavedra Meneses was chosen for this purpose, as it had served as a model for the construction of the Egyptian standard. In addition, the Spanish standard had been compared with [[Biography:Jean-Charles de Borda|Borda]]'s double-toise N° 1, which served as a comparison module for the measurement of all geodesic bases in France, and was also to be compared to the Ibáñez apparatus. In 1954, the connection of the southerly extension of the [[Earth:Struve Geodetic Arc|Struve Geodetic Arc]] with an arc running northwards from {{wipe|South Africa}} through [[Place:Egypt|Egypt]] would bring the course of a major [[Earth:Meridian arc|meridian arc]] back to land where [[Biography:Eratosthenes|Eratosthenes]] had founded [[Geodesy|geodesy]].<ref>{{Cite book |last=Jamʻīyah al-Jughrāfīyah al-Miṣrīyah |url=http://archive.org/details/bulletindelasoc00almgoog |title=Bulletin de la Société de géographie d'Égypte |date=1876 |publisher=[Le Caire] |others=University of Michigan |pages=6–16}}</ref><ref>{{Cite book |last=texte |first=Ismāʿīl-Afandī Muṣṭafá (1825–1901) Auteur du |url=https://gallica.bnf.fr/ark:/12148/bpt6k840511v |title=Notes biographiques de S. E. Mahmoud Pacha el Falaki (l'astronome), par Ismail-Bey Moustapha et le colonel Moktar-Bey |date=1886 |pages=10–11 |language=EN}}</ref><ref>{{Cite book |last=texte |first=Ismāʿīl-Afandī Muṣṭafá (1825-1901) Auteur du |url=https://gallica.bnf.fr/ark:/12148/bpt6k62478474 |title=Recherche des coefficients de dilatation et étalonnage de l'appareil à mesurer les bases géodésiques appartenant au gouvernement égyptien / par Ismaïl-Effendi-Moustapha, ... |date=1864 |language=EN}}</ref><ref>{{Cite news |title=Nomination of the STRUVE GEODETIC ARC for inscription on the WORLD HERITAGE LIST |pages=40, 143–144 |url=https://whc.unesco.org/uploads/nominations/1187.pdf}}</ref><ref name="Soler-1997" />

[[File:Britannica_Figure_of_the_Earth.jpg|thumb|'''West Europe–Africa Meridian-arc''': a meridian arc extending from the Shetland Islands, through Great Britain, France and Spain to El Aghuat in Algeria, whose parameters were calculated from surveys carried out in the mid to late 19th century. It yielded a value for the equatorial radius of the earth ''a'' = 6 377 935 metres, the ellipticity being assumed as 1/299.15. The radius of curvature of this arc is not uniform, being, in the mean, about 600 metres greater in the northern than in the southern part. Greenwich meridian is depicted rather than [[Paris meridian|Paris meridian]].|left]]

Seventeen years after Bessel calculated his [[Earth:Earth ellipsoid|ellipsoid of reference]], some of the meridian arcs the German astronomer had used for his calculation had been enlarged. This was a very important circumstance because the influence of errors due to [[Physics:Vertical deflection|vertical deflection]]s was minimized in proportion to the length of the meridian arcs: the longer the meridian arcs, the more precise the image of the [[Earth:Earth ellipsoid|Earth ellipsoid]] would be.<ref name=":2" /> After [[Earth:Struve Geodetic Arc|Struve Geodetic Arc]] measurement, it was resolved in the 1860s, at the initiative of Carlos Ibáñez e Ibáñez de Ibero who would become the first president of both the [[Organization:International Association of Geodesy|International Geodetic Association]] and the [[General Conference on Weights and Measures|International Committee for Weights and Measure]], to remeasure the arc of meridian from Dunkirk to Formentera and to extend it from Shetland to the Sahara.<ref>J. M. López de Azcona, "Ibáñez e Ibáñez de Ibero, Carlos", ''Dictionary of Scientific Biography'', vol. VII, 1–2, Scribner's, New York, 1981.</ref><ref>{{Cite book |last=commission |first=Internationale Erdmessung Permanente |url=https://play.google.com/store/books/details?id=M1PnAAAAMAAJ |title=Comptes-rendus des séances de la Commission permanente de l'Association géodésique internationale réunie à Florence du 8 au 17 octobre 1891 |date=1892 |publisher=De Gruyter, Incorporated |isbn=978-3-11-128691-4 |pages=23–25, 100–109 |language=fr}}</ref><ref name="CEM-2013">{{Cite web |title=El General Ibáñez e Ibáñez de Ibero, Marqués de Mulhacén |url=https://www.e-medida.es/numero-4/el-general-ibanez-e-ibanez-de-ibero-marques-de-mulhacen/}}</ref><ref name="Soler-1997">{{Cite journal |last=Soler |first=T. |date=1997-02-01 |title=A profile of General Carlos Ibáñez e Ibáñez de Ibero: first president of the International Geodetic Association |url=https://doi.org/10.1007/s001900050086 |journal=Journal of Geodesy |language=en |volume=71 |issue=3 |pages=176–188 |citeseerx=10.1.1.492.3967 |doi=10.1007/s001900050086 |bibcode=1997JGeod..71..176S |s2cid=119447198 |issn=1432-1394}}</ref> This did not pave the way to a new definition of the metre because it was known that the theoretical definition of the metre had been inaccessible and misleading at the time of Delambre and Mechain arc measurement, as the [[Geoid|geoid]] is a ball, which on the whole can be assimilated to an oblate [[Spheroid|spheroid]], but which in detail differs from it so as to prohibit any generalization and any extrapolation from the measurement of a single meridian arc.<ref name="Levallois-1991" /> In 1859, Friedrich von Schubert demonstrated that several meridians had not the same length, confirming an hypothesis of [[Biography:Jean le Rond d'Alembert|Jean Le Rond d’Alembert]]. He also proposed an ellipsoid with three unequal axes.<ref>{{Citation |last=Historische Commission bei der königl. Akademie der Wissenschaften |title=Schubert, Theodor von |date=1908 |url=https://de.wikisource.org/wiki/ADB:Schubert,_Theodor_Friedrich_von |work=Allgemeine Deutsche Biographie, Bd. 54 |pages=231 |access-date=2023-10-01 |series=Allgemeine Deutsche Biographie |edition=1. |place=München/Leipzig |publisher=Duncker & Humblot}}</ref><ref>{{Cite web |last=D'Alembert |first=Jean Le Rond |title=Figure de la Terre, in Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers, par une Société de Gens de lettres |url=https://artflsrv04.uchicago.edu/philologic4.7/encyclopedie0922/navigate/6/2075 |access-date=2023-10-01 |website=artflsrv04.uchicago.edu}}</ref> In 1860, Elie Ritter, a mathematician from [[Place:Geneva|Geneva]], using Schubert's data computed that the Earth ellipsoid could rather be a spheroid of revolution accordingly to [[Biography:Adrien-Marie Legendre|Adrien-Marie Legendre]]’s model.<ref>{{Cite book |last1=Société de physique et d'histoire naturelle de Genève. |url=https://www.biodiversitylibrary.org/item/41152 |title=Memoires de la Société de physique et d'histoire naturelle de Genève. |last2=Genève |first2=Société de physique et d'histoire naturelle de |date=1859 |publisher=Georg [etc.] |volume=15 |location=Geneve |pages=441–444, 484–485}}</ref> However, the following year, resuming his calculation on the basis of all the data available at the time, Ritter came to the conclusion that the problem was only resolved in an approximate manner, the data appearing too scant, and for some affected by [[Physics:Vertical deflection|vertical deflection]]s, in particular the latitude of Montjuïc in the French meridian arc which determination had also been affected in a lesser proportion by systematic errors of the [[Engineering:Repeating circle|repeating circle]].<ref>{{Cite book |last1=Société de physique et d'histoire naturelle de Genève. |url=https://www.biodiversitylibrary.org/item/50016 |title=Memoires de la Société de physique et d'histoire naturelle de Genève. |last2=Genève |first2=Société de physique et d'histoire naturelle de |date=1861 |publisher=Georg [etc.] |volume=16 |location=Geneve |pages=165–196}}</ref><ref name="Schiavon-2004" /><ref name="Levallois-1991" />{{Blockquote|text=The definition of the length of a metre in the 1790s was founded upon Arc measurements in France and Peru with a definition that it was to be 1/40 millionth of the circumference of the earth measured through the poles. Such were the inaccuracies of that period that within a matter of just a few years
more reliable measurements would have given a different value for the definition of this international standard. That does not invalidate the metre in any way but highlights the fact that continuing improvements in instrumentation made better measurements of the earth’s size possible.|title=Nomination of the STRUVE GEODETIC ARC for inscription on the WORLD HERITAGE LIST|source=p. 40}}

[[File:Struve Geodetic Arc-zoom-en.svg|thumb|Struve Geodetic Arc]]

It was well known that by measuring the latitude of two stations in [[Place:Barcelona|Barcelona]], Méchain had found that the difference between these latitudes was greater than predicted by direct measurement of distance by triangulation and that he didn't dare to admit this inaccuracy.<ref>{{Cite web |title=c à Paris ; vitesse de la lumière ... |url=http://expositions.obspm.fr/lumiere2005/triangulation_plus.html |access-date=2021-10-12 |website=expositions.obspm.fr}}</ref><ref>{{Cite book |last=Jouffroy |first=Achille de (1785-1859) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k6338674m |title=Dictionnaire des inventions et découvertes anciennes et modernes, dans les sciences, les arts et l'industrie.... 2. H–Z / recueillis et mis en ordre par M. le marquis de Jouffroy ; publié par l'abbé Migne,... |date=1852–1853 |pages=419 |language=EN}}</ref><ref name=":0" /> This was later explained by clearance in the central axis of the [[Engineering:Repeating circle|repeating circle]] causing wear and consequently the [[Zenith|zenith]] measurements contained significant systematic errors.<ref name="Schiavon-2004">Martina Schiavon. La geodesia y la investigación científica en la Francia del siglo XIX : la medida del arco de meridiano franco-argelino (1870–1895). ''Revista Colombiana de Sociología'', 2004, Estudios sociales de la ciencia y la tecnologia, 23, pp. 11–30.</ref> [[Astronomy:Polar motion|Polar motion]] predicted by [[Biography:Leonhard Euler|Leonard Euler]] and later discovered by Seth Carlo Chandler also had an impact on accuracy of latitudes' determinations.<ref>{{Cite journal |last1=Yokoyama |first1=Koichi |last2=Manabe |first2=Seiji |last3=Sakai |first3=Satoshi |date=2000 |title=History of the International Polar Motion Service/International Latitude Service |journal=International Astronomical Union Colloquium |language=en |volume=178 |pages=147–162 |doi=10.1017/S0252921100061285 |issn=0252-9211|doi-access=free }}</ref><ref name="Perrier-1935" /><ref>{{Cite web |title=Polar motion {{!}} Earth's axis, wobble, precession {{!}} Britannica |url=https://www.britannica.com/science/polar-motion |access-date=2023-08-27 |website=www.britannica.com |language=en}}</ref><ref name="Torge-2016">{{Cite journal |last=Torge |first=Wolfgang |date=2016 |editor-last=Rizos |editor-first=Chris |editor2-last=Willis |editor2-first=Pascal |title=From a Regional Project to an International Organization: The "Baeyer-Helmert-Era" of the International Association of Geodesy 1862–1916 |url=https://link.springer.com/chapter/10.1007/1345_2015_42 |journal=IAG 150 Years |series=International Association of Geodesy Symposia |language=en |location=Cham |publisher=Springer International Publishing |volume=143 |pages=3–18 |doi=10.1007/1345_2015_42 |isbn=978-3-319-30895-1}}</ref> Among all these sources of error, it was mainly an unfavourable [[Physics:Vertical deflection|vertical deflection]] that gave an inaccurate determination of Barcelona's [[Earth:Latitude|latitude]] and a metre "too short" compared to a more general definition taken from the average of a large number of arcs.<ref name="Levallois-1991" />

As early as 1861, Johann Jacob Baeyer sent a memorandum to the King of [[Place:Prussia|Prussia]] recommending international collaboration in [[Place:Central Europe|Central Europe]] with the aim of determining the shape and dimensions of the Earth. At the time of its creation, the association had sixteen member countries: [[Place:Austrian Empire|Austrian Empire]], Kingdom of Belgium, {{wipe|Denmark}}, seven German states (Grand Duchy of Baden, [[History:Kingdom of Bavaria|Kingdom of Bavaria]], Kingdom of Hanover, Mecklenburg, [[History:Kingdom of Prussia|Kingdom of Prussia]], [[Place:Kingdom of Saxony|Kingdom of Saxony]], Saxe-Coburg and Gotha), [[History:Kingdom of Italy|Kingdom of Italy]], Netherlands, [[Place:Russian Empire|Russian Empire]] (for {{wipe|Poland}}), [[History:Union between Sweden and Norway|United Kingdoms of Sweden and Norway]], as well as {{wipe|Switzerland}}. The [[Organization:International Association of Geodesy|Central European Arc Measurement]] created a Central Office, located at the Prussian Geodetic Institute, whose management was entrusted to Johann Jacob Baeyer.<ref>{{Cite journal |last=Levallois |first=J. J. |date=1980-09-01 |title=Notice historique |url=https://doi.org/10.1007/BF02521470 |journal=Bulletin géodésique |language=fr |volume=54 |issue=3 |pages=248–313 |doi=10.1007/BF02521470 |bibcode=1980BGeod..54..248L |s2cid=198204435 |issn=1432-1394 }}</ref><ref name="Torge-2016" />

Baeyer's goal was a new determination of anomalies in the shape of the Earth using precise triangulations, combined with gravity measurements. This involved determining the [[Geoid|geoid]] by means of gravimetric and leveling measurements, in order to deduce the exact knowledge of the terrestrial spheroid while taking into account local variations. To resolve this problem, it was necessary to carefully study considerable areas of land in all directions. Baeyer developed a plan to coordinate geodetic surveys in the space between the parallels of Palermo and Freetown Christiana ({{wipe|Denmark}}) and the meridians of [[Place:Bonn|Bonn]] and Trunz (German name for Milejewo in {{wipe|Poland}}). This territory was covered by a triangle network and included more than thirty observatories or stations whose position was determined astronomically. Bayer proposed to remeasure ten arcs of meridians and a larger number of arcs of parallels, to compare the curvature of the meridian arcs on the two slopes of the [[Place:Alps|Alps]], in order to determine the influence of this mountain range on [[Physics:Vertical deflection|vertical deflection]]. Baeyer also planned to determine the curvature of the seas, the [[Place:Mediterranean Sea|Mediterranean Sea]] and [[Place:Adriatic Sea|Adriatic Sea]] in the south, the North Sea and the [[Place:Baltic Sea|Baltic Sea]] in the north. In his mind, the cooperation of all the States of [[Place:Central Europe|Central Europe]] could open the field to scientific research of the highest interest, research that each State, taken in isolation, was not able to undertake.<ref>{{Cite journal |last=Zuerich |first=ETH-Bibliothek |title=Exposé historique des travaux de la commission géodésique suisse de 1862 à 1892 |url=https://doi.org/10.5169/seals-88335 |access-date=2023-10-11 |website=E-Periodica |date=1892 |language=fr |doi=10.5169/seals-88335}}</ref><ref name="Quinn-2019" />

{{wipe|Spain}} and Portugal joined the [[Organization:International Association of Geodesy|European Arc Measurement]] in 1866. French Empire hesitated for a long time before giving in to the demands of the Association, which asked the French geodesists to take part in its work. It was only after the Franco-Prussian War, that Charles-Eugène Delaunay represented {{wipe|France}} at the Congress of [[Place:Vienna|Vienna]] in 1871. In 1874, [[Biography:Hervé Faye|Hervé Faye]] was appointed member of the Permanent Commission which was presided by Carlos Ibáñez e Ibáñez de Ibero.<ref name="Lebon-1899">{{Cite book |last=Lebon |first=Ernest (1846–1922) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k949666 |title=Histoire abrégée de l'astronomie / par Ernest Lebon,... |date=1899 |pages=168–171 |language=EN}}</ref><ref>{{Cite journal |last1=Drewes |first1=Hermann |last2=Kuglitsch |first2=Franz |last3=Adám |first3=József |last4=Rózsa |first4=Szabolcs |date=2016 |title=The Geodesist's Handbook 2016 |url=http://link.springer.com/10.1007/s00190-016-0948-z |journal=Journal of Geodesy |language=en |volume=90 |issue=10 |pages=914 |doi=10.1007/s00190-016-0948-z |bibcode=2016JGeod..90..907D |s2cid=125925505 |issn=0949-7714}}</ref><ref name="CEM-2013" /><ref name="BEG-1868" />

The International Geodetic Association gained global importance with the accession of {{wipe|Chile}}, Mexico and {{wipe|Japan}} in 1888; {{wipe|Argentina}} and {{wipe|United States}} in 1889; and [[Place:British Empire|British Empire]] in 1898. The convention of the International Geodetic Association expired at the end of 1916. It was not renewed due to the [[Social:World War I|First World War]]. However, the activities of the [[Astronomy:International Latitude Service|International Latitude Service]] were continued through an {{Lang|fr|Association Géodesique réduite entre États neutre}} thanks to the efforts of H.G. van de Sande Bakhuyzen and Raoul Gautier (1854–1931), respectively directors of [[Organization:Leiden Observatory|Leiden Observatory]] and Geneva Observatory.<ref name="Soler-1997" /><ref name="Torge-2016" />

==== International prototype metre bar ====
[[File:US National Length Meter.JPG|thumb|Closeup of National Prototype Metre Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) in collaboration with Johnson Mattey and given to the United States, which served as the standard for American cartography from 1890 replacing the Committee Meter, an authentic copy of the ''Mètre des Archives'' produced in 1799 in Paris, which Ferdinand Rudolph Hassler had brought to the United States in 1805|left]]

After the {{wipe|French Revolution}}, Napoleonic Wars led to the adoption of the metre in Latin America following independence of Brazil and [[Place:Hispanic America|Hispanic America]], while the [[History:American Revolution|American Revolution]] prompted the foundation of the Survey of the Coast in 1807 and the creation of the [[Organization:National Institute of Standards and Technology|Office of Standard Weights and Measures]] in 1830. In [[Earth:Continental Europe|continental Europe]], Napoleonic Wars fostered German nationalism which later led to unification of Germany in 1871. Meanwhile, most European countries had adopted the metre. In the 1870s, [[Place:German Empire|German Empire]] played a pivotal role in the unification of the metric system through the [[Organization:International Association of Geodesy|European Arc Measurement]] but its overwhelming influence was mitigated by that of neutral states. While the German astronomer [[Biography:Wilhelm Julius Foerster|Wilhelm Julius Foerster]], director of the Berlin Observatory and director of the German Weights and Measures Service boycotted the Permanent Committee of the Internation Metre Commission alongside with the Russian and Austrian representatives in order to promote the foundation of a permanent [[Organization:International Bureau of Weights and Measures|International Bureau of Weights and Measures]], the German born, Swiss astronomer, Adolphe Hirsch conformed to the opinion of Italy and Spain to create, in spite of French reluctance, the International Bureau of Weights and Measures in France as a permanent institution at the disadventage of the ''Conservatoire national des Arts et Métiers''.<ref name="Quinn-2019" /><ref name="Von Wild-1903" /><ref>{{Cite web |date=30 March 1875 |title=Bericht der schweizerischen Delegierten an der internationalen Meterkonferenz an den Bundespräsidenten und Vorsteher des Politischen Departements, J. J. Scherer in Erwin Bucher, Peter Stalder (ed.), Diplomatic Documents of Switzerland, vol. 3, doc. 66, dodis.ch/42045, Bern 1986. |url=https://dodis.ch/42045 |website=Dodis}}</ref>

At that time, [[Unit of measurement|units of measurement]] were defined by primary [[Standard (metrology)|standard]]s, and unique artifacts made of different [[Chemistry:Alloy|alloy]]s with distinct coefficients of [[Physics:Thermal expansion|expansion]] were the legal basis of units of length. A wrought iron ruler, the Toise of Peru, also called ''Toise de l'Académie'', was the French primary standard of the toise, and the metre was officially defined by an artifact made of platinum kept in the National Archives. Besides the latter, another platinum and twelve iron standards of the metre were made by [[Company:Étienne Lenoir (instrument maker)|Étienne Lenoir]] in 1799. One of them became known as the ''Committee Meter'' in the United States and served as standard of length in the United States Coast Survey until 1890. According to geodesists, these standards were secondary standards deduced from the Toise of Peru. In Europe, except Spain, surveyors continued to use measuring instruments calibrated on the Toise of Peru. Among these, the toise of Bessel and the apparatus of Borda were respectively the main references for geodesy in [[Place:Prussia|Prussia]] and in {{wipe|France}}. These measuring devices consisted of bimetallic rulers in platinum and brass or iron and zinc fixed together at one extremity to assess the variations in length produced by any change in temperature. The combination of two bars made of two different metals allowed to take [[Physics:Thermal expansion|thermal expansion]] into account without measuring the temperature. A French scientific instrument maker, Jean Nicolas Fortin, had made three direct copies of the Toise of Peru, one for Friedrich Georg Wilhelm von Struve, a second for Heinrich Christian Schumacher in 1821 and a third for Friedrich Bessel in 1823. In 1831, [[Company:Henri-Prudence Gambey|Henri-Prudence Gambey]] also realized a copy of the Toise of Peru which was kept at Altona Observatory.<ref name="Wolf 1882 20, 32">{{Cite book |last=Wolf |first=M. C |url=https://www.worldcat.org/oclc/16069502 |title=Recherches historiques sur les étalons de poids et mesures de l'observatoire et les appareils qui ont servi a les construire. |date=1882 |publisher=Gauthier-Villars |location=Paris |pages=7–8, 20, 32 |language=French |oclc=16069502}}</ref>{{sfn|Bigourdan|1901|pp=8,158–159,176–177}}<ref name="Quinn-2012">{{Cite book |last=Quinn |first=T. J. |url=https://www.worldcat.org/oclc/861693071 |title=From artefacts to atoms : the BIPM and the search for ultimate measurement standards |date=2012 |isbn=978-0-19-990991-9 |location=Oxford |pages=20, 37–38, 91–92, 70–72, 114–117, 144–147, 8 |oclc=861693071}}</ref><ref name="Clarke-1867">{{Cite journal |last1=Clarke |first1=Alexander Ross |last2=James |first2=Henry |date=1867-01-01 |title=X. Abstract of the results of the comparisons of the standards of length of England, France, Belgium, Prussia, Russia, India, Australia, made at the ordnance Survey Office, Southampton |url=https://royalsocietypublishing.org/doi/10.1098/rstl.1867.0010 |journal=Philosophical Transactions of the Royal Society of London |volume=157 |page=174 |doi=10.1098/rstl.1867.0010 |s2cid=109333769}}</ref><ref name="NIST Special Publication">{{Cite book |url=https://play.google.com/store/books/details?id=NiEEAQAAIAAJ |title=NIST Special Publication |date=1966 |publisher=U.S. Government Printing Office |pages=529 |language=en}}</ref><ref>{{Cite web |title=Borda et le système métrique |url=https://mesurelab.fr/wp/metrologie/histoire-de-la-metrologie/borda-et-le-systeme-metrique/ |access-date=2023-08-29 |website=Association Mesure Lab |language=fr-FR}}</ref><ref name="Viik-2006" /><ref name="Clarke-1873" /><ref name="Brunner-1857" />

[[File:Metric standards Rijksmuseum.jpg|thumb|Historic Dutch replicas of metric standards in the collection of Rijksmuseum, Amsterdam: iron metre with case constructed by Étienne Lenoir in 1799, copper grave kilogram with case (1798), copper volume measures (1829)]]

In the second half of the 19th century, the creation of the [[Organization:International Association of Geodesy|International Geodetic Association]] would mark the adoption of new scientific methods.<ref>{{Cite journal |last=Zuerich |first=ETH-Bibliothek |date=1892 |title=Exposé historique des travaux de la commission géodésique suisse de 1862 à 1892 |url=https://doi.org/10.5169/seals-88335 |language=de |doi=10.5169/seals-88335 |access-date=2023-08-29 |website=E-Periodica}}</ref> It then became possible to accurately measure parallel arcs, since the difference in longitude between their ends could be determined thanks to the invention of the electrical telegraph. Furthermore, advances in [[Metrology|metrology]] combined with those of [[Physics:Gravimetry|gravimetry]] have led to a new era of [[Geodesy|geodesy]]. If precision metrology had needed the help of geodesy, the latter could not continue to prosper without the help of metrology. It was then necessary to define a single unit to express all the measurements of terrestrial arcs and all determinations of the [[Physics:Gravitational acceleration|gravitational acceleration]] by means of pendulum.<ref>Carlos Ibáñez e Ibáñez de Ibero, ''Discursos leidos ante la Real Academia de Ciencias Exactas Fisicas y Naturales en la recepcion pública de Don Joaquin Barraquer y Rovira'', Madrid, Imprenta de la Viuda e Hijo de D.E. Aguado, 1881, p. 78</ref><ref name="Clarke-1867" />

In 1866, the most important concern was that the Toise of Peru, the standard of the toise constructed in 1735 for the [[Earth:French Geodesic Mission to the Equator|French Geodesic Mission to the Equator]], might be so much damaged that comparison with it would be worthless, while Bessel had questioned the accuracy of copies of this standard belonging to Altona and Koenigsberg Observatories, which he had compared to each other about 1840. This assertion was particularly worrying, because when the primary Imperial [[Yard|yard]] standard had partially been destroyed in 1834, a new standard of reference was constructed using copies of the "Standard Yard, 1760", instead of the pendulum's length as provided for in the Weights and Measures Act of 1824, because the pendulum method proved unreliable. Nevertheless Ferdinand Rudolph Hassler's use of the metre and the creation of the Office of Standard Weights and Measures as an office within the Coast Survey contributed to the introduction of the [[Metric Act of 1866]] allowing the use of the metre in the United States, and preceded the choice of the metre as international scientific unit of length and the proposal by the [[Organization:International Association of Geodesy|European Arc Measurement]] (German: ''Europäische Gradmessung'') to establish a "European international bureau for weights and measures".<ref name="Wolf 1882 20, 32" /><ref name=":5">{{Cite web |title=Metric Act of 1866 – US Metric Association |url=https://usma.org/laws-and-bills/metric-act-of-1866#locale-notification |access-date=2021-03-15 |website=usma.org}}</ref><ref name="BEG-1868" /><ref name="Quinn-2019">{{Cite journal |last=Quinn |first=Terry |date=2019 |title=Wilhelm Foerster's Role in the Metre Convention of 1875 and in the Early Years of the International Committee for Weights and Measures |journal=Annalen der Physik |language=en |volume=531 |issue=5 |pages=2 |bibcode=2019AnP...53100355Q |doi=10.1002/andp.201800355 |issn=1521-3889 |s2cid=125240402|doi-access=free }}</ref><ref name="Clarke-1867" /><ref>{{Cite journal |last=Bessel |first=Friedrich Wilhelm |date=1840-04-01 |title=Über das preufs. Längenmaaß und die zu seiner Verbreitung durch Copien ergriffenen Maaßregeln. |url=https://ui.adsabs.harvard.edu/abs/1840AN.....17..193B |journal=Astronomische Nachrichten |volume=17 |issue=13 |pages=193 |bibcode=1840AN.....17..193B |doi=10.1002/asna.18400171302 |issn=0004-6337}}</ref><ref>{{Cite book |last=Britain |first=Great |url=https://books.google.com/books?id=qKZFAAAAcAAJ&q=yard+pendulum&pg=PA759 |title=The Statutes of the United Kingdom of Great Britain and Ireland |date=1824 |language=en}}</ref><ref name="Guillaume-1916">{{Cite journal |last=Guillaume |first=Ed. |date=1916-01-01 |title=Le Systeme Metrique est-il en Peril? |url=https://ui.adsabs.harvard.edu/abs/1916LAstr..30..242G |journal=L'Astronomie |volume=30 |pages=244–245 |bibcode=1916LAstr..30..242G |issn=0004-6302}}</ref><ref name=":4" />

[[File:Metre alloy.jpg|thumb|Creating the metre-alloy in 1874 at the Conservatoire des Arts et Métiers. Present Henri Tresca, George Matthey, Saint-Claire Deville, and Debray.|left]]

In 1867 at the second General Conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth.<ref name="Hirsch-1891">{{Cite web |last=Hirsch |first=Adolphe |date=1891 |title=Don Carlos Ibanez (1825–1891) |url=http://www.bipm.org/utils/common/pdf/obituaries/1891_CIPM_ES_IBANEZ-Don-Carlos.pdf |access-date=2017-05-22 |website=Bureau International des Poids et Mesures |pages=4, 8}}</ref><ref>{{Cite web |title=BIPM – International Metre Commission |url=http://www.bipm.org/en/measurement-units/history-si/international-metre-commission.html |access-date=2017-05-26 |website=www.bipm.org}}</ref><ref name="IAG">{{Cite web |title=A Note on the History of the IAG |url=http://www.iag-aig.org/index.php?tpl=text&id_c=80&id_t=143 |access-date=2017-05-26 |website=IAG Homepage}}</ref> According to a preliminary proposal made in [[Place:Neuchâtel|Neuchâtel]] the precedent year, the General Conference recommended the adoption of the metre in replacement of the toise of Bessel, the creation of an International Metre Commission, and the foundation of a World institute for the comparison of geodetic standards, the first step towards the creation of the [[Organization:International Bureau of Weights and Measures|International Bureau of Weights and Measures]].<ref name="Ross-James">{{Cite journal |last1=Ross |first1=Clarke Alexander |last2=James |first2=Henry |date=1873-01-01 |title=XIII. Results of the comparisons of the standards of length of England, Austria, Spain, United States, Cape of Good Hope, and of a second Russian standard, made at the Ordnance Survey Office, Southampton. With a preface and notes on the Greek and Egyptian measures of length by Sir Henry James |journal=Philosophical Transactions of the Royal Society of London |volume=163 |pages=445–469 |doi=10.1098/rstl.1873.0014 |doi-access=free}}</ref><ref name="Hirsch-1891" /><ref name="IAG" /><ref name="Brunner">{{Cite web |last=Brunner |first=Jean |date=1857 |title=Comptes rendus hebdomadaires des séances de l'Académie des sciences / publiés... par MM. les secrétaires perpétuels |url=https://gallica.bnf.fr/ark:/12148/bpt6k3001w |access-date=2019-05-15 |website=Gallica |pages=150–153 |language=FR}}</ref><ref>{{cite book |last=Guillaume |first=Charles-Édouard |title=La Création du Bureau International des Poids et Mesures et son Œuvre |publisher=Gauthier-Villars |year=1927 |location=Paris |page=321 |trans-title=The creation of the International Bureau of Weights and Measures and its work}}</ref>

Hassler's metrological and geodetic work also had a favourable response in Russia.<ref name="Parr-2006">{{Cite journal |last=Parr |first=Albert C. |date=2006-04-01 |title=A Tale About the First Weights and Measures Intercomparison in the United States in 1832 |url=https://www.nist.gov/publications/tale-about-first-weights-and-measures-intercomparison-united-states-1832 |journal=Journal of Research of the National Institute of Standards and Technology |language=en |volume=111 |issue=1 |pages=31–32, 36 |doi=10.6028/jres.111.003 |pmc=4654608 |pmid=27274915 |via=NIST}}</ref><ref name="Cajori-1921">{{Cite journal |last=Cajori |first=Florian |date=1921 |title=Swiss Geodesy and the United States Coast Survey |url=https://www.jstor.org/stable/6721 |journal=The Scientific Monthly |volume=13 |issue=2 |pages=117–129 |bibcode=1921SciMo..13..117C |issn=0096-3771}}</ref> In 1869, the Saint Petersburg Academy of Sciences sent to the French Academy of Sciences a report drafted by Otto Wilhelm von Struve, Heinrich von Wild and Moritz von Jacobi inviting his French counterpart to undertake joint action to ensure the universal use of the [[Metric system|metric system]] in all scientific work.<ref name="Guillaume-1916" />

In the 1870s and in light of modern precision, a series of international conferences was held to devise new metric standards. When a conflict broke out regarding the presence of impurities in the metre-alloy of 1874, a member of the Preparatory Committee since 1870 and Spanish representative at the [[Metre Convention|Paris Conference]] in 1875, Carlos Ibáñez e Ibáñez de Ibero intervened with the French Academy of Sciences to rally France to the project to create an International Bureau of Weights and Measures equipped with the scientific means necessary to redefine the units of the [[Metric system|metric system]] according to the progress of sciences.<ref name="NIST-2003">National Institute of Standards and Technology 2003; Historical context of the SI: Unit of length (meter)</ref><ref name="Pérard-1957" /><ref name="Quinn-2012" /><ref>{{Citation |last=Dodis |first=Diplomatische Dokumente der Schweiz {{!}} Documents diplomatiques suisses {{!}} Documenti diplomatici svizzeri {{!}} Diplomatic Documents of Switzerland {{!}} |title=Bericht der schweizerischen Delegierten an der internationalen Meterkonferenz an den Bundespräsidenten und Vorsteher des Politischen Departements, J. J. Scherer |date=1875-03-30 |url=https://dodis.ch/42045 |access-date=2021-09-20 |publisher=Diplomatische Dokumente der Schweiz {{!}} Documents diplomatiques suisses {{!}} Documenti diplomatici svizzeri {{!}} Diplomatic Documents of Switzerland {{!}} Dodis |language=fr}}</ref>

The [[Metre Convention]] (''Convention du Mètre'') of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: ''{{lang|fr|Bureau International des Poids et Mesures}}'') to be located in Sèvres, France. This new organisation was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation distributed such bars in 1889 at the first [[General Conference on Weights and Measures]] (CGPM: ''{{lang|fr|Conférence Générale des Poids et Mesures}}''), establishing the ''International Prototype Metre'' as the distance between two lines on a standard bar composed of an alloy of 90% [[Chemistry:Platinum|platinum]] and 10% [[Chemistry:Iridium|iridium]], measured at the melting point of ice.<ref name="NIST-2003" />

=== Metrology and paradigm shift in physics ===
[[File:Komplet invarskih žica.png|thumb|Invar wire baseline apparatus]]

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 [[Physics:Temperature measurement|thermometry]] work led to the discovery of special alloys of iron–nickel, in particular [[Chemistry:Invar|invar]], whose practically negligible coefficient of expansion made it possible to develop simpler baseline measurement methods, and for which its director, the Swiss physicist [[Biography:Charles Édouard Guillaume|Charles-Edouard Guillaume]], was granted the Nobel Prize in Physics in 1920. Guillaume's Nobel Prize marked the end of an era in which [[Metrology|metrology]] was leaving the field of [[Geodesy|geodesy]] to become a [[Engineering:Technology|technological]] application of [[Physics:Physics|physics]].<ref>{{Cite web |title=BIPM – la définition du mètre |url=https://www.bipm.org/fr/measurement-units/history-si/evolution_metre.html |access-date=2019-05-15 |website=www.bipm.org}}</ref><ref>{{Cite journal |date=1934-12-01 |title=Dr. C. E. Guillaume |journal=Nature |language=en |volume=134 |issue=3397 |pages=874 |doi=10.1038/134874b0 |bibcode=1934Natur.134R.874. |s2cid=4140694 |issn=1476-4687|doi-access=free }}</ref><ref>{{Cite journal |last=Guillaume |first=C.-H.-Ed |date=1906-01-01 |title=La mesure rapide des bases géodésiques |journal=Journal de Physique Théorique et Appliquée |volume=5 |pages=242–263 |url=https://zenodo.org/record/2007289 |doi=10.1051/jphystap:019060050024200}}</ref>

In 1921, the Nobel Prize in Physics was awarded to another Swiss scientist, [[Biography:Albert Einstein|Albert Einstein]], who had questioned the [[Physics:Luminiferous aether|luminiferous aether]] in 1905, just as [[Biography:Isaac Newton|Newton]] had questioned [[Physics:Mechanical explanations of gravitation|Descartes' Vortex theory]] in 1687.<ref>{{Cite web |last=Huet |first=Sylvestre |title=Einstein, le révolutionnaire de la lumière |url=https://www.liberation.fr/week-end/2005/02/12/einstein-le-revolutionnaire-de-la-lumiere_509445/ |access-date=2023-10-07 |website=Libération |language=fr}}</ref><ref>{{Cite book |last=Ferreiro |first=Larrie D. |url=https://play.google.com/store/books/details?id=p-Y3DgAAQBAJ |title=Measure of the Earth: The Enlightenment Expedition That Reshaped Our World |date=2011-05-31 |publisher=Basic Books |isbn=978-0-465-02345-5 |pages=19–23 |language=en}}</ref><ref name=":3">{{Cite web |title=Lettres philosophiques/Lettre 15 - Wikisource |url=https://fr.wikisource.org/wiki/Lettres_philosophiques/Lettre_15 |access-date=2023-10-07 |website=fr.wikisource.org |language=fr}}</ref>

Furthermore, [[Physics:Special relativity|special relativity]] changed conceptions of [[Time|time]] and [[Physics:Mass|mass]], while [[Physics:General relativity|general relativity]] changed that of [[Space|space]]. According to Newton, space was [[Euclidean geometry|Euclidean]], infinite and without boundaries and bodies gravitated around each other without changing the structure of space. [[Physics:Introduction to general relativity|Einstein's theory of gravity]] states, on the contrary, that the mass of a body has an effect on all other bodies while modifying the structure of space. A massive body induces a curvature of the space around it in which the path of light is inflected, as was demonstrated by the displacement of the position of a star observed near the Sun during an eclipse in 1919.<ref>Stephen Hawking, Paris, Dunod, 2003, 2014, 929 <abbr>p.</abbr>, p. 816–817</ref>

==== Wavelength definition ====
In 1873, [[Biography:James Clerk Maxwell|James Clerk Maxwell]] suggested that light emitted by an element be used as the standard both for the unit of length and for the second. These two quantities could then be used to define the unit of mass.<ref>{{cite book |url=https://archive.org/details/electricandmagne01maxwrich |title=A Treatise On Electricity and Magnetism |first=James Clerk |last=Maxwell |publisher=MacMillan and Co. |location=London |volume=1 |year=1873 |page=3}}</ref> About the unit of length he wrote:

{{blockquote|text=In the present state of science the most universal standard of length which we could assume would be the wave length in vacuum of a particular kind of light, emitted by some widely diffused substance such as sodium, which has well-defined lines in its spectrum. Such a standard would be independent of any changes in the dimensions of the earth, and should be adopted by those who expect their writings to be more permanent than that body.|author=James Clerk Maxwell|title=''[[Physics:A Treatise on Electricity and Magnetism|A Treatise on Electricity and Magnetism]]''|source=3rd edition, Vol. 1, p. 3}}

[[Biography:Charles Sanders Peirce|Charles Sanders Peirce]]’s work promoted the advent of American science at the forefront of global metrology. Alongside his intercomparisons of artifacts of the metre and contributions to gravimetry through improvement of reversible pendulum, Peirce was the first to tie experimentally the metre to the wave length of a spectral line. According to him the standard length might be compared with that of a wave of light identified by a line in the [[Astronomy:Sunlight|solar spectrum]]. Albert Michelson soon took up the idea and improved it.<ref name=":4">{{Cite journal |last=Crease |first=Robert P. |date=2009-12-01 |title=Charles Sanders Peirce and the first absolute measurement standard |url=https://doi.org/10.1063/1.3273015 |journal=Physics Today |volume=62 |issue=12 |pages=39–44 |doi=10.1063/1.3273015 |bibcode=2009PhT....62l..39C |s2cid=121338356 |issn=0031-9228}}</ref><ref>{{Cite journal |last=Lenzen |first=Victor F. |date=1965 |title=The Contributions of Charles S. Peirce to Metrology |url=https://www.jstor.org/stable/985776 |journal=Proceedings of the American Philosophical Society |volume=109 |issue=1 |pages=29–46 |jstor=985776 |issn=0003-049X}}</ref>

In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular [[Wavelength|wavelength]] of [[Physics:Light|light]] as a standard of length. By 1925, [[Astronomy:Interferometry|interferometry]] was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new [[International System of Units]] (SI) as equal to {{val|1650763.73}} [[Wavelength|wavelength]]s of the [[Orange (colour)|orange]]-[[Red|red]] emission line in the [[Physics:Electromagnetic spectrum|electromagnetic spectrum]] of the [[Physics:Krypton-86|krypton-86]] [[Physics:Atom|atom]] in [[Physics:Vacuum|vacuum]].<ref name="Marion">{{cite book |last=Marion |first=Jerry B. |title=Physics For Science and Engineering |year=1982 |publisher=CBS College Publishing |isbn=978-4-8337-0098-6 |page=3}}</ref>

==== Speed of light definition ====
To further reduce uncertainty, the 17th CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the [[Second|second]] and the [[Physics:Speed of light|speed of light]]:<ref name="Res1" /><ref>{{Cite web|last=BIPM|date=20 May 2019|title=Mise en pratique for the definition of the meter 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}}</ref>
:: The metre is the length of the path travelled by light in vacuum during a time interval of {{gaps|1|/|299|792|458}} of a second.

This definition fixed the speed of light in [[Physics:Vacuum|vacuum]] at exactly {{val|299792458}} metres per second<ref name="Res1">{{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=2022-12-07}}</ref> (≈{{val|300000|u=km/s}} or ≈1.079 billion km/hour<ref>The exact value is {{val|299792458|u=m/s}} = {{val|1079252848.8|u=km/h}}.</ref>). An intended by-product of the 17th CGPM's definition was that it enabled scientists to compare lasers accurately using frequency, resulting in wavelengths with one-fifth the uncertainty involved in the direct comparison of wavelengths, because interferometer errors were eliminated. To further facilitate reproducibility from lab to lab, the 17th CGPM also made the iodine-stabilised [[Physics:Helium–neon laser|helium–neon laser]] "a recommended radiation" for realising the metre.<ref name="recommendations-2" /> For the purpose of delineating the metre, the BIPM currently considers the HeNe laser wavelength, {{nowrap|''λ''{{sub|HeNe}}}}, to be {{val|632.99121258|u=nm}} with an estimated relative standard uncertainty (''U'') of {{val|2.1|e=-11}}.<ref name="recommendations-2" /><ref name="uncertainty">The term "relative standard uncertainty" is explained by NIST on their web site: {{cite web |title=Standard Uncertainty and Relative Standard Uncertainty |work=The NIST Reference on constants, units, and uncertainties: Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html |publisher=NIST |access-date=2011-12-19}}</ref><ref>National Research Council 2010.</ref>

This uncertainty is currently one limiting factor in laboratory realisations of the metre, and it is several orders of magnitude poorer than that of the second, based upon the caesium fountain [[Engineering:Atomic clock|atomic clock]] ({{nowrap|1=''U'' = {{val|5|e=-16}}}}).<ref>National Institute of Standards and Technology 2011.</ref> Consequently, a  realisation of the metre is usually delineated (not defined) today in labs as {{val|1579800.762042|(33)}} wavelengths of helium–neon laser light in vacuum, the error stated being only that of frequency determination.<ref name="recommendations-2">{{cite web |title=Iodine (&lambda; ≈ 633 nm) |publisher=BIPM |url=http://www.bipm.org/utils/common/pdf/mep/M-e-P_I2_633.pdf |work=Mise en Pratique |year=2003 |access-date=2011-12-16}}</ref> This bracket notation expressing the error is explained in the article on measurement uncertainty.

Practical realisation of the metre is subject to uncertainties in characterising the medium, to various uncertainties of interferometry, and to uncertainties in measuring the frequency of the source.<ref name="Beers2" /> A commonly used medium is air, and the [[Organization:National Institute of Standards and Technology|National Institute of Standards and Technology]] (NIST) has set up an online calculator to convert wavelengths in vacuum to wavelengths in air.<ref name="NIST_calculator">The formulas used in the calculator and the documentation behind them are found at {{cite web |url=http://emtoolbox.nist.gov/Wavelength/Documentation.asp |title=Engineering metrology toolbox: Refractive index of air calculator |date=23 September 2010 |publisher=NIST |access-date=2011-12-16}} The choice is offered to use either the [http://emtoolbox.nist.gov/Wavelength/Edlen.asp modified Edlén equation] or the [http://emtoolbox.nist.gov/Wavelength/Ciddor.asp Ciddor equation]. The documentation provides [http://emtoolbox.nist.gov/Wavelength/Documentation.asp#EdlenorCiddor a discussion of how to choose] between the two possibilities.</ref> As described by NIST, in air, the uncertainties in characterising the medium are dominated by errors in measuring temperature and pressure. Errors in the theoretical formulas used are secondary.<ref name="errors">{{cite web |url=http://emtoolbox.nist.gov/Wavelength/Documentation.asp#UncertaintyandRangeofValidity |title=§VI: Uncertainty and range of validity |work=Engineering metrology toolbox: Refractive index of air calculator |date=23 September 2010 |publisher=NIST |access-date=2011-12-16}}</ref>

By implementing a refractive index correction such as this, an approximate realisation of the metre can be implemented in air, for example, using the formulation of the metre as {{val|1579800.762042|(33)}} wavelengths of helium–neon laser light in vacuum, and converting the wavelengths in vacuum to wavelengths in air. Air is only one possible medium to use in a realisation of the metre, and any partial vacuum can be used, or some inert atmosphere like helium gas, provided the appropriate corrections for refractive index are implemented.<ref name="Dunning">{{cite book |title=Atomic, molecular, and optical physics: electromagnetic radiation, Volume 29, Part 3 |chapter=Physical limits on accuracy and resolution: setting the scale |chapter-url=https://books.google.com/books?id=FV4Y39AGYuYC&pg=PA316 |page=316 |first1=F. B. |last1=Dunning |first2=Randall G. |last2=Hulet |isbn=978-0-12-475977-0 |publisher=Academic Press |year=1997 |quote=The error [introduced by using air] can be reduced tenfold if the chamber is filled with an atmosphere of helium rather than air.}}</ref>

The metre is ''defined'' as the path length travelled by light in a given time, and practical laboratory length measurements in metres are determined by counting the number of wavelengths of laser light of one of the standard types that fit into the length,{{#tag:ref|The BIPM maintains a list of recommended radiations on their web site.<ref name="recommendations-1">{{cite web |title=Recommended values of standard frequencies |url=http://www.bipm.org/en/publications/mep.html |publisher=BIPM |date=9 September 2010 |access-date=2012-01-22}}</ref><ref>National Physical Laboratory 2010.</ref>}} and converting the selected unit of wavelength to metres. Three major factors limit the accuracy attainable with laser [[Astronomy:Interferometry|interferometers]] for a length measurement:<ref name="Beers2">
A more detailed listing of errors can be found in {{cite web |work=NIST length scale interferometer measurement assurance; NIST document NISTIR 4998 |title=§4 Re-evaluation of measurement errors |first1=John S |last1=Beers |first2=William B |last2=Penzes |url=https://www.nist.gov/calibrations/upload/4998.pdf  |access-date=2011-12-17 |date=December 1992 |pages=9 ''ff'' }}
</ref><ref name="Webster2">Zagar, 1999, pp. 6–65''ff''.</ref>
* uncertainty in vacuum wavelength of the source,
* uncertainty in the refractive index of the medium,
* [[Least count|least count]] resolution of the interferometer.

Of these, the last is peculiar to the interferometer itself. The conversion of a length in wavelengths to a length in metres is based upon the relation
: <math> \lambda = \frac{c}{n f} ,</math>
which converts the unit of wavelength ''&lambda;'' to metres using ''c'', the speed of light in vacuum in m/s. Here ''n'' is the [[Physics:Refractive index|refractive index]] of the medium in which the measurement is made, and ''f'' is the measured frequency of the source. Although conversion from wavelengths to metres introduces an additional error in the overall length due to measurement error in determining the refractive index and the frequency, the measurement of frequency is one of the most accurate measurements available.<ref name="Webster2" />

The CIPM issued a clarification in 2002:
{{Blockquote|text=Its definition, therefore, applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored (note that, at the surface of the Earth, this effect in the vertical direction is about 1 part in {{val|e=16}} per metre). In this case, the effects to be taken into account are those of special relativity only.}}

=== Timeline ===
{|class="wikitable"
!width=130pt|Date!!Deciding body!!Decision
|-
| 8 May 1790||French National Assembly||The length of the new metre to be equal to the length of a [[Physics:Pendulum|pendulum]] with a half-period of one [[Second|second]].<ref name="Larousse">{{Cite book |last=Larousse |first=Pierre |url=https://gallica.bnf.fr/ark:/12148/bpt6k205363w |title=Grand dictionnaire universel du XIXe siècle : français, historique, géographique, mythologique, bibliographique.... T. 11 MEMO-O / par M. Pierre Larousse |date=1866–1877 |pages=163 |language=EN}}</ref>
|-
|30 Mar 1791||French National Assembly||Accepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to one ten-millionth of the length of a great circle [[Circular sector|quadrant]] along the Earth's meridian through Paris, that is the distance from the equator to the north pole along that quadrant.{{sfn|Bigourdan1901|pp=20–21}}
|-
|1795||colspan=2|Provisional metre bar made of brass and based on [[Paris meridian|Paris meridan]] arc (French: ''Méridienne de France'') measured by Nicolas-Louis de Lacaillle and Cesar-François Cassini de Thury, legally equal to 443.44 [[Line (unit)|lines]] of the ''toise du Pérou'' (a standard French unit of length from 1766).<ref name="Larousse" /><ref name="Levallois" /><ref name="Wolf">{{Cite book |last=Wolf |first=Charles (1827–1918) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k9807509c |title=Recherches historiques sur les étalons de poids et mesures de l'Observatoire et les appareils qui ont servi à les construire / par M. C. Wolf... |date=1882 |pages=C.38–39, C.2–4 |language=FR}}</ref><ref name="entreprises">{{Cite web |title=Histoire du mètre |url=https://www.entreprises.gouv.fr/metrologie/histoire-metre |access-date=2019-05-16 |website=Direction Générale des Entreprises (DGE) |language=fr}}</ref>  [The line was 1/864 of a ''toise''.]
|-
|10 Dec 1799||French National Assembly||Specifies the platinum metre bar, presented on 22 June 1799 and deposited in the National Archives, as the final standard. Legally equal to 443.296 lines on the ''toise du Pérou''.<ref name="entreprises" />
|-
|24–28 Sept 1889||1st [[General Conference on Weights and Measures]] (CGPM)||Defines the metre as the distance between two lines on a standard bar of an alloy of [[Chemistry:Platinum|platinum]] with 10% [[Chemistry:Iridium|iridium]], measured at the melting point of ice.<ref name="entreprises" /><ref>{{Cite web|url=https://www.bipm.org/utils/common/pdf/CGPM/CGPM1.pdf|title=CGPM : Compte rendus de la 1ère réunion (1889).|website=BIPM}}</ref>
|-
|27 Sept – 6 Oct 1927||7th CGPM||Redefines the metre as the distance, at {{convert|0|C|K|abbr=on|lk=on}}, between the axes of the two central lines marked on the prototype bar of platinum–iridium, this bar being subject to one standard [[Physics:Atmospheric pressure|atmosphere of pressure]] and supported on two cylinders of at least 10&nbsp;mm (1&nbsp;cm) diameter, symmetrically placed in the same horizontal plane at a distance of 571&nbsp;mm (57.1&nbsp;cm) from each other.<ref>{{Cite web|url=https://www.bipm.org/utils/common/pdf/CGPM/CGPM7.pdf|title=CGPM : Comptes rendus de le 7e réunion (1927)|page=49}}</ref>
|-
|14 Oct 1960||11th CGPM||Defines the metre as {{val|fmt=spaces| 1650763.73}} [[Wavelength|wavelength]]s in [[Physics:Vacuum|vacuum]] of the [[Physics:Electromagnetic radiation|radiation]] corresponding to the transition between the 2p{{sup|10}} and 5d{{sup|5}} quantum levels of the [[Chemistry:Krypton|krypton]]-86 [[Physics:Atom|atom]].{{sfn|Judson|1976}}
|-
|21 Oct 1983||17th CGPM||Defines the metre as the length of the path travelled by [[Physics:Light|light]] in vacuum during a time interval of {{sfrac|299 792 458}} of a [[Second|second]].<ref>Taylor and Thompson (2008a), Appendix 1, p. 70.</ref><ref>{{cite web |url=https://www.nationalgeographic.org/thisday/oct21/meter-redefined/ |title=Meter is Redefined |publisher=National Geographic Society |location=US |access-date=2019-10-22 }}</ref>
|-
|2002||[[Organization:International Committee for Weights and Measures|International Committee for Weights and Measures]] (CIPM)||Considers the metre to be a unit of [[Physics:Proper length|proper length]] and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by [[Physics:General relativity|general relativity]] to be negligible with respect to the uncertainties of realisation".<ref name="taylor2008a77">Taylor and Thompson (2008a), Appendix 1, p. 77.</ref>
|}

{| class="wikitable" style="margin:0 auto;"
|+Definitions of the metre since 1795{{sfn|Cardarelli|2003}}
! Basis of definition
! Date
! Absolute<br>uncertainty
! Relative<br>uncertainty
|-
<!--
REMOVED THIS ENTRY AS THIS INFO IS SCREWED UP. BESSEL WOULD HAVE BEEN 8 YEARS OLD WHEN CARRYING OUT THIS. NEEDS TO BE LOOKED AT.
| {{sfrac|10 000 000}} part of the [[Circular sector|quadrant]] along the meridian, astronomical measure by Bessel (443.44 lines)
| 1792
| {{val|0.5|–|0.1|u=mm}}
| {{val|e=-4}}
|-
-->
| {{sfrac|{{val|10000000}}}} part of the [[Circular sector|quadrant]] along the meridian, measurement by [[Biography:Jean Baptiste Joseph Delambre|Delambre]] and [[Biography:Pierre Méchain|Méchain]] (443.296 lines)
| 1795
| {{val|500|–|100|u=um}}
| {{val|e=-4}}
|-
| First prototype ''{{lang|fr|[[History of the metre#Mètre des Archives|Mètre des Archives]]}}'' platinum bar standard
| 1799
| {{val|50|–|10|u=um}}
| {{val|e=-5}}
|-
| Platinum–iridium bar at melting point of ice (1st CGPM)
| 1889
| {{val|0.2|–|0.1|u=um}}
| {{val|e=-7}}
|-
| Platinum–iridium bar at melting point of ice, atmospheric pressure, supported by two rollers (7th CGPM)
| 1927
| n.a.
| n.a.
|-
| [[Physics:Hyperfine structure|Hyperfine]] atomic transition; {{val|1650763.73}} wavelengths of light from a specified transition in [[Physics:Krypton-86|krypton-86]] (11th CGPM)
| 1960
| {{val|4|u=nm}}
| {{val|4e-9}}<ref>[http://www.bipm.org/en/CGPM/db/17/1/ Definition of the metre] Resolution 1 of the 17th meeting of the CGPM (1983)</ref>
|-
| Length of the path travelled by light in vacuum in {{sfrac|{{val|299792458}}}} second (17th CGPM)
| 1983
| {{val|0.1|u=nm}}
| {{val|e=-10}}
|}

== Early adoptions of the metre internationally ==
In France, the metre was adopted as an exclusive measure in 1801 under the Consulate. This continued under the [[Place:First French Empire|First French Empire]] until 1812, when Napoleon decreed the introduction of the non-decimal ''[[Mesures usuelles|mesures usuelles]]'', which remained in use in France up to 1840 in the reign of Louis Philippe.<ref name="Larousse" /> Meanwhile, the metre was adopted by the Republic of Geneva.<ref>{{cite web |title=Metrisches System |url=https://hls-dhs-dss.ch/articles/013754/2014-05-22/ |access-date=2021-12-15 |website=hls-dhs-dss.ch |language=de }}</ref> After the joining of the canton of Geneva to {{wipe|Switzerland}} in 1815, Guillaume Henri Dufour published the first official Swiss map, for which the metre was adopted as the unit of length.<ref>{{cite web |title=Kartografie |url=https://hls-dhs-dss.ch/articles/008258/2014-11-26/ |access-date=2021-12-13 |website=hls-dhs-dss.ch |language=de }}</ref><ref>{{Cite journal|last=Dufour|first=G.-H.|date=1861|title=Notice sur la carte de la Suisse dressée par l'État Major Fédéral|url=https://www.persee.fr/doc/globe_0398-3412_1861_num_2_1_7582|journal=Le Globe. Revue genevoise de géographie|volume=2|issue=1|pages=5–22|doi=10.3406/globe.1861.7582}}</ref>

=== Adoption dates by country ===
* {{wipe|France}}: 1801–1812, then 1840<ref name="Larousse" />
* Republic of Geneva, Switzerland: 1813<ref name="Dubler-2014">{{cite web |title=Metrisches System |url=https://hls-dhs-dss.ch/de/articles/013754/2014-05-22/ |access-date=2021-12-09 |website=hls-dhs-dss.ch |language=de }}</ref>
* Kingdom of the Netherlands: 1820
* Kingdom of Belgium: 1830
* {{wipe|Chile}}: 1848
* [[Place:Kingdom of Sardinia (1720–1861)|Kingdom of Sardinia]], Italy: 1850
* {{wipe|Spain}}: 1852
* Portugal: 1852
* Colombia: 1853
* Ecuador: 1856
* Mexico: 1857
* {{wipe|Brazil}}: 1862
* {{wipe|Argentina}}: 1863
* Italy: 1863
* {{wipe|United States}}: 1866<ref name=":5" />
* [[Place:German Empire|German Empire]], {{wipe|Germany}}: 1872
* Austria, 1875
* {{wipe|Switzerland}}: 1877<ref name="Dubler-2014" />

== SI prefixed forms of metre ==
{{Main|Orders of magnitude (length)}}
[[SI prefix]]es can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, [[Astronomy:Astronomical unit|astronomical unit]]s (149.6&nbsp;Gm), [[Astronomy:Light-year|light-year]]s (10&nbsp;Pm), or [[Astronomy:Parsec|parsec]]s (31&nbsp;Pm), rather than in Mm, Gm, Tm, Pm, Em, Zm or Ym; "30&nbsp;cm", "30&nbsp;m", and "300&nbsp;m" are more common than "3&nbsp;dm", "3&nbsp;dam", and "3&nbsp;hm", respectively.

The terms ''micron'' and ''millimicron'' have been used instead of ''micrometre'' (μm) and ''nanometre'' (nm), respectively, but this practice is discouraged.<ref>Taylor & Thompson 2003, p. 11.</ref>

{{SI multiples
| unit=metre
| symbol=m
| xd=[[Decimetre|decimetre]]
| xda=[[Decametre|decametre]]
| xc=[[Centimetre|centimetre]]
| xh=[[Hectometre|hectometre]]
| xm=[[Millimetre|millimetre]]
| xk=[[Kilometre|kilometre]]
| xmc=[[Micrometre|micrometre]]
| xn=[[Nanometre|nanometre]]
| xp=[[Picometre|picometre]]
| xf=[[Femtometre|femtometre]]
}}

== Equivalents in other units ==
{| class=wikitable style="margin:0 auto;"
|-
! colspan="5" style="text-align:left;"|Metric unit<br/>expressed in non-SI units
! colspan="4" style="text-align:left;"|Non-SI unit<br/>expressed in metric units
|-
| 1 metre ||≈ ||style="text-align:right;"|1.0936 ||[[Yard|yard]]||
| 1 yard||= ||style="text-align:right;"|0.9144 ||metre
|-
| 1 metre ||≈ ||style="text-align:right;"|39.370 ||[[Inch|inch]]es||
| 1 inch||= ||style="text-align:right;"|0.0254 ||metre
|-
| 1 [[Centimetre|centimetre]] ||≈ ||style="text-align:right;"|{{val|0.39370}} ||inch||
| 1 inch||= ||style="text-align:right;"|2.54 ||centimetres
|-
| 1 [[Millimetre|millimetre]] ||≈ ||style="text-align:right;"|{{val|0.039370}} ||inch||
| 1 inch||= ||style="text-align:right;"|25.4 ||millimetres
|-
| 1 metre ||= ||style="text-align:right;"|10{{sup|10}}||[[Ångström|ångström]]||
| 1 ångström||= ||style="text-align:right;"|10{{sup|−10}} ||metre
|-
| 1 [[Nanometre|nanometre]] ||= ||style="text-align:right;"|10||ångström||
| 1 ångström||= ||style="text-align:right;"|100 ||[[Picometre|picometre]]s
|}
Within this table, "inch" and "yard" mean "international inch" and "international yard"<ref>Astin & Karo 1959.</ref> respectively, though approximate conversions in the left column hold for both international and survey units.
: "≈" means "is approximately equal to";
: "=" means "is exactly equal to".

One metre is exactly equivalent to {{sfrac|5 000|127}}{{nbsp}}inches and to {{sfrac|1 250|1 143}}{{nbsp}}yards.
<!-- 1 metre ≈ 39.370 078 740 157 5 in / or 39.375 in = 1000.125 mm // what is this for? -->

A simple [[Biology:Mnemonic|mnemonic]] to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3{{nbsp}}[[Foot (unit)|feet]] {{frac|3|3|8}}{{nbsp}}inches. This gives an overestimate of 0.125{{nbsp}}mm.

The ancient Egyptian [[Cubit|cubit]] was about 0.5{{nbsp}}m (surviving rods are 523–529{{nbsp}}mm).<ref>Arnold Dieter (1991). [https://books.google.com/books?id=DU04vCP_TFAC ''Building in Egypt: pharaonic stone masonry'']. Oxford: Oxford University Press. {{ISBN|978-0-19-506350-9}}. p.251.</ref> Scottish and English definitions of the [[Ell|ell]] (two cubits) were 941{{nbsp}}mm (0.941{{nbsp}}m) and 1143{{nbsp}}mm (1.143{{nbsp}}m) respectively.<ref>{{Cite web |url=http://www.dsl.ac.uk/getent4.php?plen=7441&startset=10747969&query=ELL&fhit=ell&dregion=form&dtext=snd#fhit |title=Dictionary of the Scots Language |access-date=2011-08-06 |archive-url=https://web.archive.org/web/20120321184808/http://www.dsl.ac.uk/getent4.php?plen=7441&startset=10747969&query=ELL&fhit=ell&dregion=form&dtext=snd#fhit |archive-date=2012-03-21 |url-status=dead }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=-BHnAAAAMAAJ&pg=PA221 |title=The Penny Magazine of the Society for the Diffusion of Useful Knowledge |publisher=Charles Knight |pages=221–22 |date=1840-06-06 |df=dmy-all}}</ref> The ancient Parisian ''toise'' (fathom) was slightly shorter than 2{{nbsp}}m and was standardised at exactly 2{{nbsp}}m in the [[Mesures usuelles|mesures usuelles]] system, such that 1{{nbsp}}m was exactly {{frac|1|2}}{{nbsp}}toise.<ref name=H&H>{{cite web |url = https://archive.org/details/outlinesofevolut00halluoft/page/66 |title = Outlines of the evolution of weights and measures and the metric system |first1 = William  |last1 = Hallock |first2 = Herbert T |last2 = Wade |publisher = The Macmillan Company |year = 1906 |pages = 66–69|location = London}}</ref> The Russian [[Verst|verst]] was 1.0668{{nbsp}}km.{{sfn|Cardarelli|2004}} The [[Scandinavian mile|Swedish mil]] was 10.688{{nbsp}}km, but was changed to 10{{nbsp}}km when Sweden converted to metric units.<ref>{{cite encyclopedia |url=https://snl.no/mil |title=Mil |encyclopedia=Store norske leksikon |first=Knut |last=Hofstad |access-date=2019-10-18 |df=dmy-all}}</ref>

== See also ==
* [[Physics:ISO 1|ISO 1]]{{spaced ndash}}standard reference temperature for length measurements
* [[Metric prefix]]
* [[Vertical position]]

== Notes ==
{{Reflist|30em}}

== References ==
{{refbegin}}
* {{Anchor|Alder2002}}{{cite book |title=The Measure of All Things : The Seven-Year Odyssey and Hidden Error That Transformed the World |first=Ken |last=Alder |year=2002 |publisher=Free Press |location=New York |isbn=978-0-7432-1675-3 |url-access=registration |url=https://archive.org/details/measureofallthin00alde }}
* {{Anchor|AstinKaro1959}}Astin, A. V. & Karo, H. Arnold, (1959), [http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf ''Refinement of values for the yard and the pound''], Washington DC: National Bureau of Standards, republished on  National Geodetic Survey web site and the Federal Register (Doc. 59–5442, Filed, 30 June 1959)
* {{Anchor|BarbrowJudson1976}}{{cite book | title=Weights and Measures Standards of the United States, a brief history | first=Lewis V. | last=Judson | others=Derived from a prior work by Louis A. Fisher (1905) | editor-first=Louis E. | editor-last=Barbrow | publisher=US Department of Commerce, National Bureau of Standards | location=US | date=1976-10-01 | orig-year=1963<!-- 1963-03 --> | id=NBS Special Publication 447; NIST SP 447; 003-003-01654-3 | lccn=76-600055 | doi=10.6028/NBS.SP.447}}
* {{Cite book |last=Bigourdan |first=Guillaume |url=https://archive.org/details/lesystmemtri00bigo|title=Le système métrique des poids et mesures ; son établissement et sa propagation graduelle, avec l'histoire des opérations qui ont servi à déterminer le mètre et le kilogramme |trans-title=The metric system of weights and measures; its establishment and gradual propagation, with the history of the operations which served to determine the meter and the kilogram |date=1901 |publisher=Gauthier-Villars |location=Paris}}
* {{Anchor|Guedj2001}}{{cite book |title=La Mesure du Monde |trans-title=The Measure of the World |first=Denis |last=Guedj |translator-first=Art |translator-last=Goldhammer |publisher=University of Chicago Press |location=Chicago |year=2001}}
* {{Anchor|Cardarelli2003}}{{cite book |title=Encydopaedia of scientific units, weights, and measures: their SI equivalences and origins |last=Cardarelli |first=François |year=2003 |publisher=Springer-Verlag London Limited |isbn=978-1-85233-682-0 |chapter=Chapter 2: The International system of Units |chapter-url=http://www.francoiscardarelli.ca/PDF_Files/ESU_Sample_Chapter_Section_1_2.pdf#page=6 |at=Table 2.1, p. 5 |quote=Data from Giacomo, P., Du platine à la lumière [From platinum to light], ''Bull. Bur. Nat. Metrologie'', '''102''' (1995) 5–14. |access-date=2017-01-26 |url=https://archive.org/details/encyclopaediaofs0000card }}
* {{cite book |last=Cardarelli |first=F. |year=2004 |title=Encyclopaedia of Scientific Units, Weights and Measures: Their SI Equivalences and Origins |publisher=Springer |edition=2nd |pages=[https://archive.org/details/encyclopaediaofs0000card/page/120 120]–124 |isbn=1-85233-682-X |url=https://archive.org/details/encyclopaediaofs0000card |url-access=registration}}
* {{anchor|nistmetre}}[http://physics.nist.gov/cuu/Units/meter.html ''Historical context of the SI: Meter'']. Retrieved 26 May 2010.
* {{Anchor|NIST2011}}National Institute of Standards and Technology. (27 June 2011). ''[https://www.nist.gov/pml/div688/grp50/primary-frequency-standards.cfm NIST-F1 Cesium Fountain Atomic Clock]''. Author.
* {{Anchor|NPL2010}}National Physical Laboratory. (25 March 2010). ''[http://www.npl.co.uk/science-technology/time-frequency/optical-frequency-standards-and-metrology/research/iodine-stabilised-lasers Iodine-Stabilised Lasers]''. Author.
* {{Anchor|NRC2010}}{{cite web|url=http://www.nrc-cnrc.gc.ca/eng/projects/inms/si-length.html |publisher=National Research Council Canada |title=Maintaining the SI unit of length |date=5 February 2010 |archive-url=https://web.archive.org/web/20111204014454/http://www.nrc-cnrc.gc.ca/eng/projects/inms/si-length.html |archive-date=4 December 2011 |url-status=dead }}
* {{Anchor|PH-BatasPambansa8}}Republic of the Philippines. (2 December 1978). ''[http://www.chanrobles.com/bataspambansabilang8.htm Batas Pambansa Blg. 8: An Act Defining the Metric System and its Units, Providing for its Implementation and for Other Purposes]''. Author.
* {{Anchor|PH-RA7160}}Republic of the Philippines. (10 October 1991). ''[https://www.officialgazette.gov.ph/downloads/1991/10oct/19911010-RA-7160-CCA.pdf Republic Act No. 7160: The Local Government Code of the Philippines]''. Author.
* {{Anchor|PH-GR185240}} Supreme Court of the Philippines (Second Division). (20 January 2010). ''[https://web.archive.org/web/20180427202441/http://sc.judiciary.gov.ph/jurisprudence/2010/january2010/185240.htm G.R. No. 185240]''. Author.
* {{Anchor|taylor2008a}}Taylor, B.N. and Thompson, A. (Eds.). (2008a). [https://web.archive.org/web/20171120061639/https://www.nist.gov/sites/default/files/documents/2016/12/07/sp330.pdf ''The International System of Units (SI)'']. United States version of the English text of the eighth edition (2006) of the International Bureau of Weights and Measures publication ''Le Système International d' Unités (SI)'' (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
* {{Anchor|taylor2008b}}Taylor, B.N. and Thompson, A. (2008b). [http://physics.nist.gov/cuu/pdf/sp811.pdf ''Guide for the Use of the International System of Units''] (Special Publication 811). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 23 August 2008.
* {{Anchor|turner}}Turner, J. (deputy director of the National Institute of Standards and Technology). (16 May 2008). [https://www.nist.gov/pml/wmd/metric/upload/FRN_Vol_73_No_96_16May2008_SI_Interpretation.pdf "Interpretation of the International System of Units (the Metric System of Measurement) for the United States"]. ''Federal Register'' Vol. 73, No. 96, p.{{nbsp}}28432–28433.
* {{Anchor|Zagar1999}}Zagar, B.G. (1999). [https://books.google.com/books?id=VXQdq0B3tnUC&pg=PT164#PPT160,M1 Laser interferometer displacement sensors] in J.G. Webster (ed.). ''The Measurement, Instrumentation, and Sensors Handbook''. CRC Press. {{ISBN|0-8493-8347-1}}.
{{refend}}

{{SI units}}
{{SI units of length}}

[[Category:SI base units]]

{{Sourceattribution|Metre}}