Editing International System of Units (section)

== History ==

[[File:Alter Grenzstein Pontebba 01.jpg|thumb|upright|[[Boundary marker|Stone marking]] the [[Austria-Hungary|Austro-Hungarian]]/Italian border at [[Pontebba]] displaying [[myriametre]]s, a unit of 10&nbsp;km used in [[Central Europe]] in the 19th century (but since [[deprecation|deprecated]])<ref name="Europa1842">{{cite web |url=http://www.spasslernen.de/geschichte/groessen/mas1.htm |date= 1 May 2009 |website=spasslernen |title=Amtliche Maßeinheiten in Europa 1842 |language=de |trans-title=Official units of measure in Europe 1842 |access-date=26 March 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120925070621/http://www.spasslernen.de/geschichte/groessen/mas1.htm |archive-date= 25 September 2012 }} Text version of Malaisé's book: {{cite book |url=https://archive.org/details/bub_gb_TQgHAAAAcAAJ |title=Theoretisch-practischer Unterricht im Rechnen |publisher=Verlag des Verf. |language=de |trans-title=Theoretical and practical instruction in arithmetic |author-first= Ferdinand von |author-last=Malaisé |place=München |date=1842 |pages=307–322 |access-date=7 January 2013}}</ref>]]
{{broader|History of the metric system}}

=== CGS and MKS systems ===

{{See also|CGS system of units}}
[[File:US National Length Meter.JPG|thumb|right|Closeup of the National Prototype Metre, serial number 27, allocated to the United States]]

The concept of a system of units emerged a hundred years before the SI.
In the 1860s, [[James Clerk Maxwell]], [[William Thomson, 1st Baron Kelvin|William Thomson]] (later Lord Kelvin), and others working under the auspices of the [[British Association for the Advancement of Science]], building on previous work of [[Carl Gauss]], developed the [[centimetre–gram–second system of units]] or cgs system in 1874. The systems formalised the concept of a collection of related units called a ''coherent'' system of units. In a coherent system, ''base units'' combine to define ''derived units'' without extra factors.<ref name=NIST330/>{{rp|page=2}} For example, using metre per second is coherent in a system that uses metre for length and second for time, but kilometre per hour is not coherent. The principle of coherence was successfully used to define a number of units of measure based on the CGS, including the [[erg]] for [[energy]], the [[dyne]] for [[force]], the [[barye]] for [[pressure]], the [[poise (unit)|poise]] for [[dynamic viscosity]] and the [[stokes (unit)|stokes]] for [[kinematic viscosity]].<ref name="BIPMCentenary">{{cite book |url=https://archive.org/details/internationalbur420page |page=[https://archive.org/details/internationalbur420page/page/12 12] |title=The International Bureau of Weights and Measures 1875–1975: NBS Special Publication 420 |date=20 May 1975 |editor-last1=Page
| editor-first1=Chester H. |editor-last2=Vigoureux |editor-first2=Paul |publisher=[[National Bureau of Standards]] | location=Washington, D.C.}}</ref>

=== Metre Convention ===

{{Main|Metre Convention |MKS system of units}}
A French-inspired initiative for international cooperation in [[metrology]] led to the signing in 1875 of the [[Metre Convention]], also called Treaty of the Metre, by 17 nations.{{efn|Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States, and Venezuela.}}<ref name="Alder">{{cite book |title=The Measure of all Things – The Seven-Year-Odyssey that Transformed the World |author-last=Alder |author-first=Ken |date=2002 |publisher=Abacus |location=London |isbn=978-0-349-11507-8}}</ref>{{rp|pages=353–354}} The [[General Conference on Weights and Measures]] (French: {{lang|fr|Conférence générale des poids et mesures}} – CGPM), which was established by the Metre Convention,<ref name="BIPMCentenary"/> brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements.<ref name=Giunta/>{{rp|page=37}}<ref>{{Cite book |last=Quinn |first=Terry J. |title=From artefacts to atoms: the BIPM and the search for ultimate measurement standards |date=2012 |publisher=Oxford University Press |isbn=978-0-19-530786-3 |location=New York Oxford}}</ref> Initially the convention only covered standards for the metre and the kilogram. This became the foundation of the MKS system of units.<ref name=NIST330/>{{rp|page=2}}

=== Giovanni Giorgi and the problem of electrical units ===

At the close of the 19th century three different systems of units of measure existed for electrical measurements: a [[CGS-based system for electrostatic units]], also known as the Gaussian or ESU system, a [[CGS-based system for electromechanical units]] (EMU), and an International system based on units defined by the Metre Convention<ref>{{cite book |title=Weights, Measures and Units |url=https://archive.org/details/dictionaryofweig0000fenn |url-access=registration |author-first=Donald |author-last=Fenna |at=International unit |isbn=978-0-19-860522-5 |publisher=[[Oxford University Press]] | date=2002}}</ref> for electrical distribution systems. Attempts to resolve the electrical units in terms of length, mass, and time using [[dimensional analysis]] was beset with difficulties – the dimensions depended on whether one used the ESU or EMU systems.<ref name="Maxwell2">{{cite book |title=A treatise on electricity and magnetism |volume=2 |author-first=J. C. |author-last=Maxwell |date=1873 |publisher=Clarendon Press |location=Oxford |url=https://archive.org/stream/electricandmag02maxwrich |pages=242–245 |access-date=12 May 2011}}</ref> This anomaly was resolved in 1901 when [[Giovanni Giorgi]] published a paper in which he advocated using a fourth base unit alongside the existing three base units. The fourth unit could be chosen to be [[electric current]], [[voltage]], or [[electrical resistance]].<ref name="IECGiorgi">{{cite web |url=http://www.iec.ch/about/history/beginning/giovanni_giorgi.htm |title=In the beginning...: Giovanni Giorgi |publisher=[[International Electrotechnical Commission]] | access-date=5 April 2011 |archive-date=15 May 2011 |archive-url=https://web.archive.org/web/20110515134553/http://www.iec.ch/about/history/beginning/giovanni_giorgi.htm |url-status=dead}}</ref>

Electric current with named unit 'ampere' was chosen as the base unit, and the other electrical quantities derived from it according to the laws of physics. When combined with the MKS the new system, known as MKSA, was approved in 1946.<ref name=NIST330/>

=== 9th CGPM, the precursor to SI ===

In 1948, the 9th CGPM commissioned a study to assess the measurement needs of the scientific, technical, and educational communities and "to make recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention".<ref>{{cite web|url=http://www.bipm.org/en/CGPM/db/9/6/|title= Resolution 6 of the 9th CGPM|website=BIPM |access-date=22 August 2017 |date=1948 |url-status=dead |archive-url=https://web.archive.org/web/20170822153835/http://www.bipm.org/en/CGPM/db/9/6/ |archive-date= 22 August 2017  }}</ref> This working document was ''Practical system of units of measurement''. Based on this study, the 10th CGPM in 1954 defined an international system derived six base units: the metre, kilogram, second, ampere, degree Kelvin, and candela.

The 9th CGPM also approved the first formal recommendation for the writing of symbols in the metric system when the basis of the rules as they are now known was laid down.<ref>{{cite web |url=http://www.bipm.org/en/CGPM/db/9/7/ |title=Resolution 7 of the 9th meeting of the CGPM (1948): Writing and printing of unit symbols and of numbers |date=1948 |access-date=6 November 2012 |publisher=[[International Bureau of Weights and Measures]] |url-status=dead |archive-url=https://web.archive.org/web/20130514091113/http://www.bipm.org/en/CGPM/db/9/7/ |archive-date= May 14, 2013 }}</ref> These rules were subsequently extended and now cover unit symbols and names, prefix symbols and names, how quantity symbols should be written and used, and how the values of quantities should be expressed.<ref name="SIBrochure"/>{{rp|pages=104,130}}

=== Birth of the SI ===

The 10th CGPM in 1954 resolved to create an international system of units<ref name=Giunta>{{Cite book |last=Giunta |first=Carmen J. |url=https://link.springer.com/10.1007/978-3-031-28436-6 |title=A Brief History of the Metric System: From Revolutionary France to the Constant-Based SI |date=2023 |publisher=Springer International Publishing |isbn=978-3-031-28435-9 |series=SpringerBriefs in Molecular Science |location=Cham |language=en |doi=10.1007/978-3-031-28436-6|bibcode=2023bhms.book.....G |s2cid=258172637 }}</ref>{{rp|page=41}} and in 1960, the 11th CGPM adopted the ''International System of Units'', abbreviated SI from the French name {{lang|fr|Le Système international d'unités}}, which included a specification for units of measurement.<ref name="SIBrochure"/>{{rp|page=110}}

The [[International Bureau of Weights and Measures]] (BIPM) has described SI as "the modern form of metric system".<ref name="SIBrochure"/>{{rp|page=95}} In 1971 the [[mole (unit)|mole]] became the seventh base unit of the SI.<ref name=NIST330/>{{rp|page=2}}

=== 2019 redefinition ===

[[File:Unit relations in the new SI.svg|thumb |right |Reverse dependencies of the SI base units on seven [[physical constant]]s, which are assigned exact numerical values in the [[2019 revision of the SI|2019 redefinition]]. Unlike in the previous definitions, the base units are all derived exclusively from constants of nature. Here, <math>a \rightarrow b</math> means that <math>a</math> is used to define <math>b</math>.]]
{{Main|2019 revision of the SI}}

After the [[history of the metre|metre was redefined]] in 1960, the [[International Prototype of the Kilogram]] (IPK) was the only physical artefact upon which base units (directly the kilogram and indirectly the ampere, mole and candela) depended for their definition, making these units subject to periodic comparisons of national standard kilograms with the IPK.<ref name="NPL kg">{{cite web |title=Redefining the kilogram |url=http://www.npl.co.uk/educate-explore/redefining-the-kilogram/|publisher=UK National Physical Laboratory |access-date=30 November 2014 |url-status=live |archive-url= https://web.archive.org/web/20141227083141/http://www.npl.co.uk/educate-explore/redefining-the-kilogram/ |archive-date= Dec 27, 2014 }}</ref> During the 2nd and 3rd Periodic Verification of National Prototypes of the Kilogram, a significant divergence had occurred between the mass of the IPK and all of its official copies stored around the world: the copies had all noticeably increased in mass with respect to the IPK. During ''extraordinary verifications'' carried out in 2014 preparatory to redefinition of metric standards, continuing divergence was not confirmed. Nonetheless, the residual and irreducible instability of a physical IPK undermined the reliability of the entire metric system to precision measurement from small (atomic) to large (astrophysical) scales.<ref>{{Cite web |date=12 May 2018 |title=A Turning Point for Humanity: Redefining the World's Measurement System |url=https://www.nist.gov/si-redefinition/turning-point-humanity-redefining-worlds-measurement-system |access-date=16 January 2024 |website=NIST |language=en}}</ref>
By avoiding the use of an artefact to define units, all issues with the loss, damage, and change of the artefact are avoided.<ref name=SIBrochure9thEd/>{{rp|page=125}}

A proposal was made that:<ref>{{cite web |title=Appendix 1. Decisions of the CGPM and the CIPM |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-App4-EN.pdf |page=188 |publisher=[[BIPM]] | access-date=27 April 2021}}</ref>
* In addition to the speed of light, four constants of nature – the [[Planck constant]], an [[elementary charge]], the [[Boltzmann constant]], and the [[Avogadro constant]] – be defined to have exact values
* The [[International Prototype of the Kilogram]] be retired
* The current definitions of the kilogram, ampere, kelvin, and mole be revised
* The wording of base unit definitions should change emphasis from explicit unit to explicit constant definitions.

The new definitions were adopted at the 26th CGPM on 16 November 2018, and came into effect on 20 May 2019.<ref>{{cite web |url=http://www.bipm.org/cc/TGFC/Allowed/Minutes/CODATA_Minutes_14-BIPM-public.pdf |title=Report on the Meeting of the CODATA Task Group on Fundamental Constants |date=3–4 November 2014 |publisher=[[BIPM]] | author-first=B. |author-last=Wood |page=7 |quote=[BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion.}}</ref> The change was adopted by the European Union through Directive (EU) 2019/1258.<ref>{{cite web |url=https://eur-lex.europa.eu/eli/dir/2019/1258/oj |title=Commission Directive (EU) 2019/1258 of 23 July 2019 amending, for the purpose of its adaptation to technical progress, the Annex to Council Directive 80/181/EEC as regards the definitions of SI base units |author=<!--Not stated--> |date=23 July 2019 |website=[[Eur-Lex]] | access-date=28 August 2019}}</ref>

Prior to its redefinition in 2019, the SI was defined through the seven base units from which the derived units were constructed as products of powers of the base units. After the redefinition, the SI is defined by fixing the numerical values of seven defining constants. This has the effect that the distinction between the base units and derived units is, in principle, not needed, since all units, base as well as derived, may be constructed directly from the defining constants. Nevertheless, the distinction is retained because "it is useful and historically well established", and also because the [[ISO/IEC 80000]] series of standards, which define the [[International System of Quantities]] (ISQ), specifies base and derived quantities that necessarily have the corresponding SI units.<ref name="SIBrochure9thEd" />{{rp|page=129}}