The SI has an official status in most countries, including the United States, Canada, and the United Kingdom, although these three countries are amongst a handful of nations that, to various degrees, also continue to use their customary systems. The International System of Units, or SI, : 123 is a decimal and metric system of units established in 1960 and periodically updated since then. Principles Countries using the metric (SI), imperial, and US customary systems as of 2019. The system was published in 1960 as a result of an initiative that began in 1948, so it is based on the metre–kilogram–second system of units (MKS) rather than any variant of the CGS. 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 original motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them. The last artefact used by the SI was the International Prototype of the Kilogram, a cylinder of platinum–iridium. 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. A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit. 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. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units. Prior to 2019, h, e, k, and N A were not defined a priori but were rather very precisely measured quantities. The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant K cd. These defining constants are the speed of light in vacuum c, the hyperfine transition frequency of caesium Δ ν Cs, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant N A, and the luminous efficacy K cd. Since 2019, the magnitudes of all SI units have been defined by declaring that seven defining constants have certain exact numerical values when expressed in terms of their SI units. The SI is intended to be an evolving system units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. power-of-ten) multiples and sub-multiples of the coherent unit. Since the sizes of coherent units will be convenient for only some applications and not for others, the SI provides twenty-four prefixes which, when added to the name and symbol of a coherent unit produce twenty-four additional (non-coherent) SI units for the same quantity these non-coherent units are always decimal (i.e. The seven base units and the 22 coherent derived units with special names and symbols may be used in combination to express other coherent derived units. Twenty-two coherent derived units have been provided with special names and symbols. These are called coherent derived units, which can always be represented as products of powers of the base units. The system can accommodate coherent units for an unlimited number of additional quantities. The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity). : 123 Established and maintained by the General Conference on Weights and Measures (CGPM ), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce. The International System of Units, internationally known by the abbreviation SI (for Système International), : 125 : iii is the modern form : 117 of the metric system and the world's most widely used system of measurement. Please discuss this issue on the article's talk page. Please consider splitting content into sub-articles, condensing it, or adding subheadings. This article may be too long to read and navigate comfortably.
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