Editing Metric system (section)

==History==
{{Main|History of the metric system}}
[[File:BIPM courtyard.jpg|thumb|upright=1.2|[[Pavillon de Breteuil]], Saint-Cloud, France, the home of the metric system since 1875]]
The [[French Revolution]] (1789–99) enabled France to reform its many outdated systems of various local weights and measures. In 1790, [[Charles Maurice de Talleyrand-Périgord]] proposed a new system based on natural units to the [[National Assembly (France)|French National Assembly]], aiming for global adoption. With the [[United Kingdom of Great Britain and Ireland|United Kingdom]] not responding to a request to collaborate in the development of the system, the [[French Academy of Sciences]] established a commission to implement this new standard alone, and in 1799, the new system was launched in France.<ref name="McGreevy v1">{{cite book |title=The Basis of Measurement: Volume 1—Historical Aspects |isbn=978-0-948251-82-5 |publisher=Picton Publishing |location=Chippenham |year=1995 |first1=Thomas |last1=McGreevy |editor1-first=Peter |editor1-last=Cunningham}}</ref>{{rp|145–149}}

A number of different metric systems have been developed, all using the ''Mètre des Archives'' and ''Kilogramme des Archives'' (or their descendants) as their base units, but differing in the definitions of the various derived units.

{| class="wikitable floatright" style="font-size: 95%; width:230px;"
|+Variants of the metric system<!-- without gravitational ones, mechanical units-->
! Measure
! SI/MKS
! {{abbrlink|CGS|Centimetre–gram–second system of units}}
! {{abbrlink|MTS|Metre–tonne–second system of units}}
|-
|[[distance]]
| metre {{br}}(m)
| centimetre {{br}}(cm)
| metre {{br}}(m)
|-
|[[mass]]
| kilogram {{br}}(kg)
| gram {{br}}(g)
| [[tonne]] {{br}}(t)
|-
|[[time]]
| second {{br}}(s)
| second {{br}}(s)
| second {{br}}(s)
|-
|[[velocity]]
| m/s
| cm/s
| m/s
|-
|[[acceleration]]
| m/s<sup>2</sup>
| [[Gal (unit)|gal {{br}}(Gal)]]
| m/s<sup>2</sup>
|-
|[[force]]
| [[newton (unit)|newton (N)]]
| [[dyne]] {{br}}(dyn)
| [[sthene]] {{br}}(sn)
|-
|[[pressure]]
| [[pascal (unit)|pascal (Pa)]]
| [[barye]] {{br}}(Ba)
| [[pièze]] {{br}}(pz)
|-
|[[Power (physics)|energy]]
| [[joule]] {{br}}(J)
| [[erg]] {{br}}(erg)
| kilojoule {{br}}(kJ)
|-
|[[Power (physics)|power]]
| [[watt]] {{br}}(W)
| erg/s {{br}}(erg/s)
| kilowatt {{br}}(kW)
|-
|[[viscosity]]
| Pa⋅s
| [[Poise (unit)|poise]] {{br}}(P)
| pz⋅s
|}

=== 19th century ===
In 1832, Gauss used the astronomical second as a base unit in defining the gravitation of the Earth, and together with the milligram and millimetre, this became [[Gaussian units|the first system of mechanical units]]. He showed that the strength of a magnet could also be quantified in terms of these units, by measuring the oscillations of a magnetised needle and finding the quantity of "magnetic fluid" that produces an acceleration of one unit when applied to a unit mass.<ref>{{Cite journal |last=O'Hara |first=James Gabriel |date=1983 |title=Gauss and the Royal Society: The Reception of His Ideas on Magnetism in Britain (1832-1842) |url=https://www.jstor.org/stable/531344 |journal=Notes and Records of the Royal Society of London |volume=38 |issue=1 |pages=17–78 |doi=10.1098/rsnr.1983.0002 |jstor=531344 |s2cid=145724822 |issn=0035-9149}}</ref><ref>{{Cite journal |last=Van Baak |first=D. A. |date=October 2013 |title=Re-creating Gauss's method for non-electrical absolute measurements of magnetic fields and moments |url=https://pubs.aip.org/aapt/ajp/article/81/10/738-744/1057517 |journal=American Journal of Physics |language=en |volume=81 |issue=10 |pages=738–744 |doi=10.1119/1.4816806 |bibcode=2013AmJPh..81..738V |issn=0002-9505}}</ref> The [[centimetre–gram–second system of units]] (CGS) was the first coherent metric system, having been developed in the 1860s and promoted by Maxwell and Thomson. In 1874, this system was formally promoted by the [[British Association for the Advancement of Science]] (BAAS).<ref name=SI_1_8>{{SIBrochure8th|page=109}}</ref> The system's characteristics are that density is expressed in {{nowrap|g/cm<sup>3</sup>}}, force expressed in [[dyne]]s and mechanical energy in [[erg]]s. Thermal energy was defined in [[calorie]]s, one calorie being the energy required to raise the temperature of one gram of water from 15.5&nbsp;°C to 16.5&nbsp;°C. The meeting also recognised [[Centimetre gram second system of units|two sets of units for electrical and magnetic properties]] – the electrostatic set of units and the electromagnetic set of units.<ref>{{cite book |title = Reports on the Committee on Standards of Electrical Resistance – Appointed by the British Association for the Advancement of Science |chapter-url = https://archive.org/stream/reportscommitte00maxwgoog |chapter = First Report – Cambridge 3 October 1862 |pages = 1–3 |first1 = William |last1 =Thomson |first2 =James Prescott |last2 =Joule |first3 = James Clerk |last3 =Maxwell |first4 =Flemming |last4 =Jenkin |editor1-first = Flemming |editor1-last =Jenkin |location = London |year =1873 |access-date = 12 May 2011}}</ref>

The CGS units of electricity were cumbersome to work with. This was remedied at the 1893 International Electrical Congress held in Chicago by defining the "international" ampere and ohm using definitions based on the [[metre]], [[kilogram]] and [[second]], in the [[International System of Electrical and Magnetic Units]].<ref>{{cite web |url = http://physics.nist.gov/cuu/Units/ampere.html |title = Historical context of the SI—Unit of electric current (ampere) |publisher = The NIST Reference on Constants, Units and Uncertainty |access-date = 10 April 2011}}</ref> During the same period in which the CGS system was being extended to include electromagnetism, other systems were developed, distinguished by their choice of coherent base unit, including the [[History of the metric system#QES|Practical System of Electric Units]], or QES (quad–eleventhgram–second) system, was being used. Here, the base units are the quad, equal to {{val|e=7|u=m}} (approximately a quadrant of the Earth's circumference), the eleventhgram, equal to {{val|e=-11|u=g}}, and the second.  These were chosen so that the corresponding electrical units of potential difference, current and resistance had a convenient magnitude.{{refn|{{citation |author=James Clerk Maxwell |year=1954 |orig-year=1891 |title=A Treatise on Electricity & Magnetism |volume=2 |edition=3rd | publisher=[[Dover Publications]]}}}}{{rp|268}}{{refn|name="Carron Babel"|{{cite arXiv |last=Carron |first=Neal |eprint=1506.01951 |title= Babel of Units. The Evolution of Units Systems in Classical Electromagnetism |class= physics.hist-ph |date=2015 }}}}{{rp|17}}

=== 20th century ===
In 1901, [[Giovanni Giorgi]] showed that by adding an electrical unit as a fourth base unit, the various anomalies in electromagnetic systems could be resolved. The metre–kilogram–second–[[coulomb]] (MKSC) and metre–kilogram–second–[[ampere]] (MKSA) systems are examples of such systems.<ref name=IECGiorgi>{{cite web|url = http://www.iec.ch/about/history/beginning/giovanni_giorgi.htm|title = In the beginning... Giovanni Giorgi|year = 2011
|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><ref name=":1">{{Cite journal |last=Jayson |first=Joel S. |date=January 2014 |title=The Daniell cell, Ohm's law, and the emergence of the International System of Units |url=https://pubs.aip.org/aapt/ajp/article/82/1/60-65/1058039 |journal=American Journal of Physics |language=en |volume=82 |issue=1 |pages=60–65 |arxiv=1512.07306 |doi=10.1119/1.4826445 |bibcode=2014AmJPh..82...60J |s2cid=119278961 |issn=0002-9505}}</ref>

The [[metre–tonne–second system of units]] (MTS) was based on the metre, [[tonne]] and second – the unit of force was the [[sthène]] and the unit of pressure was the [[pièze]]. It was invented in France for industrial use and from 1933 to 1955 was used both in France and in the [[Soviet Union]].<ref name=ieeeghn>{{cite web |url=http://www.ieeeghn.org/wiki/index.php/System_of_Measurement_Units |title = System of Measurement Units |work = IEEE Global History Network |publisher= [[Institute of Electrical and Electronics Engineers]] (IEEE) |access-date = 21 March 2011}}</ref><ref>{{cite web |url = http://www.hydrelect.info/articles.php?lng=fr&pg=3 |title = Notions de physique – Systèmes d'unités |language = fr |trans-title=Symbols used in physics – units of measure |access-date = 21 March 2011 |publisher = Hydrelect.info}}</ref> [[Gravitational metric system]]s use the [[kilogram-force]] (kilopond) as a base unit of force, with mass measured in a unit known as the [[Gravitational metric system|hyl]], ''Technische Masseneinheit'' (TME), mug or [[slug (unit)|metric slug]].<ref>
{{cite web
 |url = http://www.numericana.com/answer/units.htm#slug
 |title = Final Answers
 |first1 = Gérard P
 |last1 = Michon
 |publisher = Numericana.com
 |date = 9 September 2000
 |access-date = 11 October 2012
}}</ref> Although the CGPM passed a resolution in 1901 defining the standard value of [[Standard gravity|acceleration due to gravity]] to be 980.665&nbsp;cm/s<sup>2</sup>, gravitational units are not part of the [[International System of Units]] (SI).<ref>{{cite web
|url = http://www.bipm.org/en/CGPM/db/3/2/
|title = Resolution of the 3rd meeting of the CGPM (1901)
|publisher = General Conference on Weights and Measures
|access-date = 11 October 2012}}</ref>

===Current===
The International System of Units is the modern metric system. It is based on the metre–kilogram–second–ampere (MKSA) system of units from early in the 20th century.<ref name=SI_units /> It also includes numerous coherent derived units for common quantities like power (watt) and irradience (lumen). Electrical units were taken from the International system then in use.  Other units like those for energy (joule) were modelled on those from the older CGS system, but scaled to be coherent with MKSA units. Two additional base units – the ''kelvin'', which is equivalent to degree Celsius for change in thermodynamic temperature but set so that 0&nbsp;K is [[absolute zero]], and the ''candela'', which is roughly equivalent to the [[international candle]] unit of illumination – were introduced. Later, another base unit, the ''mole'', a unit of [[amount of substance]] equivalent to the [[Avogadro number]] number of specified molecules, was added along with several other derived units.<ref name="Gold Book mole">{{Cite book |url=https://goldbook.iupac.org/terms/view/M03980 |title=IUPAC – mole (M03980) |author=IUPAC Gold Book |publisher=[[International Union of Pure and Applied Chemistry]]|doi=10.1351/goldbook.M03980 |s2cid=241546445 }}</ref>

The system was promulgated by the General Conference on Weights and Measures (French: ''Conférence générale des poids et mesures'' – CGPM) in 1960.  At that time, the metre was redefined in terms of the wavelength of a spectral line of the [[krypton-86]] atom (krypton-86 being a stable isotope of an inert gas that occurs in undetectable or trace amounts naturally), and the standard metre artefact from 1889 was retired.<ref name=":0">{{cite book|last1=Urone |first1=Peter Paul |last2=Hinrichs |first2=Roger |last3=Dirks |first3=Kim |last4=Sharma |first4=Manjula |title=College Physics |publisher=OpenStax |isbn=978-1-947172-01-2 |year=2020 |url=https://openstax.org/details/books/college-physics}}</ref>{{rp|16}}

Today, the International system of units consists of 7 base units and innumerable coherent derived units including 22 with special names. The last new derived unit, the ''katal'' for catalytic activity, was added in 1999. All the base units except the second are now defined in terms of exact and invariant constants of physics or mathematics, barring those parts of their definitions which are dependent on the second itself. As a consequence, the speed of light has now become an exactly defined constant, and defines the metre as {{frac|299,792,458}} of the distance light travels in a second. The kilogram was defined by a [[International Prototype of the Kilogram|cylinder of platinum-iridium alloy]] until a [[2019 revision of the SI#Kilogram|new definition]] in terms of  [[Physical constant|natural physical constants]] was adopted in 2019. As of 2022, the range of decimal prefixes has been extended to those for 10<sup>30</sup> (''quetta–'') and 10<sup>−30</sup> (''quecto–'').<ref>{{cite web |url=https://www.theregister.com/2022/11/22/new_si_prefixes_clear_the/|title=New SI prefixes clear the way for quettabytes of storage|date=22 November 2022|publisher=The Register|access-date=23 Nov 2022|df=dmy-all}}</ref>