Historical definitions of the SI base units

When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length, and time. Giorgi later identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units (for temperature, amount of substance, and luminous intensity) were added later.

The early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are mostly interchangeable, but in scientific contexts the difference matters. Mass, strictly the inertial mass, represents a quantity of matter. It relates the acceleration of a body to the applied force via Newton's law, {{nowrap|1=F = m × a}}: force equals mass times acceleration. A force of 1 N (newton) applied to a mass of 1 kg will accelerate it at 1 m/s². This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, and hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is {{nowrap|m × g}}; mass times the acceleration due to gravity, which is 9.81 newtons at the Earth's surface and is about 3.5 newtons at the surface of Mars. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision measurements of a property of a body, and this makes a unit of weight unsuitable as a base unit.{{citation needed|date=January 2024}}

Since 1960 the CGPM has made a number of changes to the SI to meet the needs of specific fields, notably chemistry and radiometry. These are mostly additions to the list of named derived units, and include the mole (symbol mol) for an amount of substance, the pascal (symbol Pa) for pressure, the siemens (symbol S) for electrical conductance, the becquerel (symbol Bq) for "activity referred to a radionuclide", the gray (symbol Gy) for ionising radiation, the sievert (symbol Sv) as the unit of dose equivalent radiation, and the katal (symbol kat) for catalytic activity.{{rp|page=156, 158–159, 165}}{{cite book |author-first=Thomas |author-last=McGreevy |editor-first1=Peter |editor-last1=Cunningham |title=The Basis of Measurement: Volume 2 – Metrication and Current Practice |pages=222–224 |isbn=978-0-948251-84-9 |date=1997 |publisher=Pitcon Publishing (Chippenham) Ltd}}{{rp|221}}

The range of defined prefixes pico- (10⁻¹²) to tera- (10¹²) was extended to quecto- (10⁻³⁰) to quetta- (10³⁰).{{rp|page=152, 158, 164}}

The 1960 definition of the standard metre in terms of wavelengths of a specific emission of the krypton-86 atom was replaced in 1983 with the distance that light travels in vacuum in exactly {{sfrac|{{val|299792458}}}} second, so that the speed of light is now an exactly specified constant of nature.{{citation needed|date=January 2024}}

A few changes to notation conventions have also been made to alleviate lexicographic ambiguities. An analysis under the aegis of CSIRO, published in 2009 by the Royal Society, has pointed out the opportunities to finish the realisation of that goal, to the point of universal zero-ambiguity machine readability.{{citation |last=Foster |first=Marcus P. |date=2009 |title=Disambiguating the SI notation would guarantee its correct parsing |journal=Proceedings of the Royal Society A |volume=465 |issue= 2104|pages=1227–1229 |doi=10.1098/rspa.2008.0343 |bibcode=2009RSPSA.465.1227F |s2cid=62597962 |postscript=.}}

Prior to the 2019 revision of the SI, from 2005 to early 2019, the SI base units were defined as follows.

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{{reflist|refs=

{{cite book |editor1=David B. Newell |editor2=Eite Tiesinga |title=The International System of Units (SI) |url=https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf |access-date=30 November 2019 |publisher=NIST |location=Gaithersburg, MD |date=2019|edition=NIST Special publication 330, 2019}}

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[http://old.iupac.org/publications/books/author/mills.html Quantities Units and Symbols in Physical Chemistry], IUPAC

{{cite book

|url=https://archive.org/details/internationalbur420page |pages=[https://archive.org/details/internationalbur420page/page/238 238]–244 |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.}}

{{cite news|url=https://www.nist.gov/news-events/news/2018/11/historic-vote-ties-kilogram-and-other-units-natural-constants|title=Historic Vote Ties Kilogram and Other Units to Natural Constants|last=Materese|first=Robin|date=16 November 2018|work=NIST|access-date=16 November 2018}}

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{{SI units}}

{{DEFAULTSORT:2005-2019 definitions of the SI base units}}

Category:SI units