Editing Metric system (section)

=== 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|url-access=subscription }}</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|url-access=subscription }}</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}}