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{{alius|longitudine|Metrum (discretiva)}}
[[Fasciculus:Platinum-Iridium meter bar.jpg|thumb|''Metrum ordiale,'' Platinum-Iridium. Definitio metri usque ad annum [[1960]].]]
'''Metrum'''<ref name=”Schlickum”>Schlickum, O. (1879). ''Lateinisch-deutsches Special-Wörterbuch der pharmazeutischen Wissenschaften.''  Leipzig: Ernst Günther’s Verlag.</ref> ({{pns|i|n|metrum}}; [[Lingua Graeca|Graece]]: ''μέτρον,'' via [[Francogallice]] ''mètre'') est [[mensura principalis]] ([[Anglice]]: ''base unit''<ref>[https://en.wikipedia.org/wiki/Base_unit_(measurement) Vide commentarium in vicipaedia Anglica.]</ref>) [[longitudo|longitudinis]], una ex septem principalibus [[physica]]e mensuris in [[Systema Internationale|Internationali Unitarum Systemate]] ([[Anglice]]: ''International System of Units''). Definitio, primum excogitatum ut 10 000 000 pars spatii ab [[aequator]]e ad [[Polus Septentrionalis|Polum Septentrionalem]], continenter expolitur ut auctum [[metrologia]]e scitae implicet. Post die [[21 Octobris]] [[1983]], metrum definitur ut spatium a [[celeritas lucis|luce transitum]] in [[vacuum|vacuo]] 299 792 458 parte [[secundum|secundi]].<ref name="Res1">17th General Conference on Weights and Measures (1983), decretum primum.</ref>

== Definitio ==
=== Vetus ===
[[Fasciculus:Kilometre definition.svg|thumb|Prima definitio metri]]
Anno [[1793]] metrum primum definitum fit. Definitio erat metrum esse partem <math>1/40000000</math> meridiani orbis, qui [[Lutetia]]m percurrit. Anno [[1795]] primum exemplar in metallo orichalco factum et anno [[1799]] exemplar finale, quod "metrum ordiale" nominatur, in metallo [[platinum|platino]] funditum est. 

Mensurae exactiores telluris in [[saeculum 19|saeculo undevicensimo]] effectae monstrabant priorem mensuram falsam fuisse. Novum exemplar necessarium erat. Hoc anno, [[1889]], ab officiis mundi ponderum et mensuris in metallis [[Platinum|platino]] (90%) et [[iridium|iridio]] (10%) funditum est.

Ab anno [[1960]] usque ad annum [[1983]] metrum erat pars producti <math>1650763,73</math> longitudinis undae in vacuo quae atomus Cryptonis-86 inter affectiones 5d5 et 2p10 mutans generat.

=== Hodierna ===
Ab anno [[1983]], metrum definitur ut spatium quod [[lux|lumen]] in vacuo in parte <math>1/299792458</math> [[secundus|secundi]] percurrit, ut [[celeritas lucis|velocitas lucis]] subtiliter definita sit sequenti modo
:<math> c = 299792458 \;\mathrm{m}/\mathrm{s}</math>

== Variae Longitudines ==
: [[chiliometrum]] = 1000 metrorum
: [[decimetrum]] = 1/10 metri
: [[centimetrum]] = 1/100 metri
: [[millimetrum]] = 1/1000 metri
: [[micrometrum]] (''micron'') = 1/1 000 000 metri
: [[nanometrum]] = 10<sup>-9</sup> metri
: [[picometrum]] = 10<sup>-12</sup> metri
: [[femtometrum]] = 10<sup>-15</sup> metri

{{NexInt}}
* [[Index magnitudinum physicarum]]
* [[Mensura metrica]]
* [[Spatium]]

== Notae ==
<references/>

== Bibliographia ==
* {{Anchor|beers1992}}Beers, J. S. et W. B. Penzes. [[1992]]. [http://ts.nist.gov/MeasurementServices/Calibrations/upload/4998.pdf NIST Length Scale Interferometer Measurement Assurance.] National Institute of Standards and Technology 4998.
* {{Anchor|cgpm1983}}Bureau International des Poids et Mesures. 17th General Conference on Weights and Measures. (1983). [http://www.bipm.org/en/CGPM/db/17/1/ Resolution 1.] 
* {{Anchor|bipm2006}}Bureau International des Poids et Mesures. (2006). [http://www1.bipm.org/utils/common/pdf/si_brochure_8.pdf ''The International System of Units (SI)'']
** [http://www.bipm.org/en/si/si_brochure/ HTML version]
* Bureau International des Poids et Mesures. (n.d.). [http://www.bipm.fr/en/convention/resolutions.html ''Resolutions of the CGPM''] (search facility).
* Bureau International des Poids et Mesures. (n.d.). [http://www1.bipm.org/en/si/history-si/evolution_metre.html ''The BIPM and the evolution of the definition of the metre''].
* Layer, H. P. [[2008]]. [https://web.archive.org/web/20080610151558/http://www.mel.nist.gov/div821/museum/length.htm ''Length—Evolution from Measurement Standard to a Fundamental Constant'']. Gaithersburg, MD: National Institute of Standards and Technology.
* Mohr, P., Taylor, B. N., et David B. Newell. (28 Decembris 2007). [http://physics.nist.gov/cuu/Constants/codata.pdf ''CODATA Recommended Values of the Fundamental Physical Constants: 2006'']. Gaithersburgi Terrae Mariae: National Institute of Standards and Technology.
* National Institute of Standards and Technology. (December 2003). [http://physics.nist.gov/cuu/Units/index.html ''The NIST Reference on Constants, Units, and Uncertainty: International System of Units (SI)''].
** [http://physics.nist.gov/cuu/Units/units.html ''SI base units''].
** [http://physics.nist.gov/cuu/Units/current.html ''Definitions of the SI base units''].
** {{anchor|nistmetre}}[http://physics.nist.gov/cuu/Units/meter.html ''Historical context of the SI: Metre''].
* National Research Council Canada. (16 May 2008). [https://web.archive.org/web/20081220062439/http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_si_e.html ''Optical Frequency - Maintaining the SI Metre''].
* {{anchor|penzes2005}}Penzes, W. (29 December 2005). [https://web.archive.org/web/20080916064126/http://www.mel.nist.gov/div821/museum/timeline.htm ''Time Line for the Definition of the Meter'']. Gaithersburg, MD: National Institute of Standards and Technology,  Precision Engineering Division.
* {{Anchor|taylor2008a}}Taylor, B.N. and Thompson, A. (Eds.). (2008a). [https://web.archive.org/web/20160603215953/http://physics.nist.gov/Pubs/SP330/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.
* {{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.
* Tibo Qorl. [[2005]]. [http://histoire.du.metre.free.fr/en/ The History of the Meter] (conversus a Sibille Rouzaud).
* {{Anchor|turner}}Turner, J. (Deputy Director of the National Institute of Standards and Technology). (16 Maii 2008).[http://ts.nist.gov/WeightsAndMeasures/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;28432-3.
* Zagar, B. G. [[1999]]. [http://books.google.com/books?id=VXQdq0B3tnUC&pg=PT164#PPT160,M1 "Laser interferometer displacement sensors."] In  ''The Measurement, Instrumentation, and Sensors Handbook,'' ed. J.G. Webster (CRC Press, ISBN 0849383471).

{{Mensurae longitudinis}}
{{Unitates SI}}
[[Categoria:Mensura]]
[[Categoria:Unitates physicae]]
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{{Myrias|Physica}}