Editing SI base unit (section)

== 2019 revision of the SI ==
{{Main|2019 revision of the SI}}
[[File:Unit relations in the old SI.svg|thumb|upright=1.35|The [[International System of Units|SI system]] after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, the [[metre]] is defined as the distance travelled by [[light]] in a specific fraction of a [[second]]), with the constants of nature and artefacts used to define them (such as the mass of the [[IPK]] for the kilogram).]]
[[File:Unit_relations_in_the_new_SI.svg|thumb|upright=1.35|New SI: Dependence of base unit definitions on [[physical constant]]s with fixed numerical values and on other base units that are derived from the same set of constants. Arrows are shown in the opposite direction compared to typical [[dependency graph]]s, i.e. <math>a \rightarrow b</math> in this chart means <math>b</math> depends on <math>a</math>.]]

New base unit definitions were adopted on 16 November 2018, and they became effective on 20 May 2019. The definitions of the base units have been modified several times since the [[Metre Convention]] in 1875, and new additions of base units have occurred. Since the redefinition of the metre in 1960, the kilogram had been the only base unit still defined directly in terms of a physical artefact, rather than a property of nature. This led to a number of the other SI base units being defined indirectly in terms of the mass of the same artefact; the [[Mole (unit)|mole]], the [[ampere]], and the [[candela]] were linked through their definitions to the mass of the [[International Prototype of the Kilogram]], a roughly golfball-sized [[platinum]]–[[iridium]] cylinder stored in a vault near Paris.

It has long been an objective in [[metrology]] to define the kilogram in terms of a [[Physical constant|fundamental constant]], in the same way that the metre is now defined in terms of the [[speed of light]]. The 21st [[General Conference on Weights and Measures]] (CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram". Two possibilities attracted particular attention: the [[Planck constant]] and the [[Avogadro constant]].

In 2005, the [[International Committee for Weights and Measures]] (CIPM) approved preparation of new definitions for the kilogram, the ampere, and the kelvin and it noted the possibility of a new definition of the mole based on the Avogadro constant.<ref>{{cite web|author=94th Meeting of the [[International Committee for Weights and Measures]]|date=2005|url=http://www.bipm.org/utils/en/pdf/CIPM2005-EN.pdf|title=Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants|archive-url=https://web.archive.org/web/20110807052257/http://www.bipm.org/utils/en/pdf/CIPM2005-EN.pdf |archive-date=2011-08-07}}</ref> The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.<ref>23rd [[General Conference on Weights and Measures]] (2007). [http://www.bipm.org/utils/en/pdf/Resol23CGPM-EN.pdf Resolution 12: On the possible redefinition of certain base units of the International System of Units (SI)].</ref>

In a note to the CIPM in October 2009,<ref>{{cite web|url = http://www.bipm.org/cc/CIPM/Allowed/98/CIPM2009_49_TIMING_THE_NEW_SI.pdf | title = Thoughts about the timing of the change from the Current SI to the New SI| author = Ian Mills, President of the CCU | publisher = CIPM | date = October 2009 | access-date =2010-02-23}}</ref> Ian Mills, the President of the CIPM ''Consultative Committee – Units'' (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed [[New SI definitions|new definition]].  He urged the CIPM to accept the proposed changes in the definition of the ''kilogram'', ''ampere'', ''kelvin'', and ''mole'' so that they are referenced to the values of the fundamental constants, namely the [[Planck constant]] (''h''), the [[elementary charge]] (''e''), the [[Boltzmann constant]] (''k''), and the [[Avogadro constant]] (''N''<sub>A</sub>).<ref name="draft">
{{cite web
 |url = http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf
 |title = Draft Chapter 2 for SI Brochure, following redefinitions of the base units
 |author = Ian Mills
 |publisher = CCU
 |date = 2010-09-29
 |access-date = 2011-01-01
}}</ref> This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy.