International System of Units

It is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

The current way of defining the SI is a result of a decades-long move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions.

A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit.

One problem with artefacts is that they can be lost, damaged, or changed; another is that they introduce uncertainties that cannot be reduced by advancements in science and technology.

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, 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.

The values assigned to these constants were fixed to ensure continuity with previous definitions of the base units.

[3]: 6 Twenty-two coherent derived units have been provided with special names and symbols as shown in the table below.

[1]: 140 According to the SI Brochure,[1]: 148  unit names should be treated as common nouns of the context language.

This means that they should be typeset in the same character set as other common nouns (e.g. Latin alphabet in English, Cyrillic script in Russian, etc.

For example, in English and French, even when the unit is named after a person and its symbol begins with a capital letter, the unit name in running text should start with a lowercase letter (e.g., newton, hertz, pascal) and is capitalised only at the beginning of a sentence and in headings and publication titles.

[4]: iii Symbols of SI units are intended to be unique and universal, independent of the context language.

[1]: 147 In the United States, the guideline produced by the National Institute of Standards and Technology (NIST)[11]: 37  clarifies language-specific details for American English that were left unclear by the SI Brochure, but is otherwise identical to the SI Brochure.

[5]: 111  Various consultative committees of the CIPM decided in 2016 that more than one mise en pratique would be developed for determining the value of each unit.

Nevertheless, with this nearly universal level of acceptance, the SI "has been used around the world as the preferred system of units, the basic language for science, technology, industry, and trade.

[23] The ISQ is formalised, in part, in the international standard ISO/IEC 80000, which was completed in 2009 with the publication of ISO 80000-1,[24] and has largely been revised in 2019–2020.

[25] The SI is regulated and continually developed by three international organisations that were established in 1875 under the terms of the Metre Convention.

[27] The brochure leaves some scope for local variations, particularly regarding unit names and terms in different languages.

In the 1860s, James Clerk Maxwell, 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 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 for dynamic viscosity and the stokes for kinematic viscosity.

[f][30]: 353–354 The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention,[29] 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.

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.

[4] 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".

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.

[5]: 110 The International Bureau of Weights and Measures (BIPM) has described SI as "the modern form of metric system".

[38] 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.

Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives.

The CIPM recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI,[5] including the hour, minute, degree of angle, litre, and decibel.

Organisations Standards and conventions [1] This article incorporates text from this source, which is available under the CC BY 3.0 license.