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International System of Units

The '''International System of Units''' (abbreviated '''SI''' from the French language|French ''''Bureau International des Poids et Mesures) is the modern form of the metric system and is generally a system devised around the convenience of the number ten. It is the world's most widely used Systems of measurement|system of measurement, both in everyday commerce and in science.Official BIPM definitionsAn extensive presentation of the SI units is maintained on line by NIST, including a diagram of the interrelations between the derived units based upon the SI units. Definitions of the basic units can be found on this site, as well as the CODATA report listing values for special constants such as the electric constant, the magnetic constant and the speed of light, all of which have defined values as a result of the definition of the metre and ampere.

In the International System of Units (SI) (BIPM, 2006), the definition of the meter fixes the speed of light in vacuum ''c''₀, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) μ₀, and the definition of the mole fixes the molar mass of the carbon 12 atom M(¹²C) to have the exact values given in the table 1, p.7. Since the electric constant (also called the permittivity of vacuum) is related to μ₀ by ε₀ = 1/μ₀c₀², it too is known exactly.

– CODATA report The older metric system included several groups of units. The SI was developed in 1960 from the old Mks system of units|metre-kilogram-second (mks) system, rather than the centimetre gram second system of units|centimetre-gram-second (cgs) system, which, in turn, had a few variants. Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improves. The system is nearly universally employed, and most countries do not even maintain official definitions of any other units. A notable exception is the United States, which continues to use United States customary units|customary units in addition to SI. In the United Kingdom, metrication|conversion to metric units is government policy, but the transition is not yet complete. Those countries that still recognise non-SI units (e.g., the US and UK) have redefined their traditional non-SI units in SI units.

Realisation of units

It is important to distinguish between the definition of a unit and its realisation. The definition of each base unit of the SI is carefully drawn up so that it is unique and provides a sound theoretical basis upon which the most accurate and reproducible measurements can be made. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. A description of how the definitions of some important units are realised in practice is given on the BIPM website.SI ''Practical Realization'' brochure A coherent SI derived unit can be expressed in SI base units with no numerical factor other than the number 1.Ambler Thompson and Barry N. Taylor, (2008), ''Guide for the Use of the International System of Units (SI)'', (Special publication 811), Gaithersburg, MD: National Institute of Standards and Technology, p. 3, footnote 2. The coherent SI derived unit of resistance, the ohm, symbol Ω, for example, is uniquely defined by the relation Ω = m²·kg·s⁻³·A⁻², which follows from the definition of the quantity electrical resistance. However, any method consistent with the laws of physics could be used to realise any SI unit.

History

The metric system was conceived by a group of scientists (among them, Antoine Lavoisier|Antoine-Laurent Lavoisier, who is known as the "father of modern chemistry") who had been commissioned by Louis XVI of France to create a unified and rational system of measures. After the French Revolution, the system was adopted by the new government. On August 1, 1793, the National Convention adopted the new decimal "metre" with a provisional length as well as the other decimal units with preliminary definitions and terms. On April 7, 1795 (''Loi du 18 germinal, an III'') the terms "''gramme''" and "''kilogramme''" replaced the former terms "gravet" (correctly "milligrave") and "grave (mass)|grave". On December 10, 1799 (a month after coup of 18 Brumaire|Napoleon's coup d'etat), the metric system was definitively adopted in France. The Metric system#History|history of the metric system has seen a number of variations, whose use has spread around the world, to replace many traditional Systems of measurement|measurement systems. At the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to metrication|promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities. Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units that were recommended are the metre, kilogram, second, ampere, degree Kelvin (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the ''International System of Units'', abbreviated SI from the French name: . The seventh base unit, the mole (unit)|mole, was added in 1971 by the 14th CGPM.

Future development

ISO 31 contains recommendations for the use of the International System of Units; for electrical applications, in addition, IEC 60027 has to be taken into account. As of 2008, work is proceeding to integrate both standards into a joint standard ISO/IEC 80000|''Quantities and Units'' in which the quantities and equations used with SI are to be referred as the '''International System of Quantities''' (ISQ).SI Brochure

Units

The international system of units consists of a set of units together with a set of SI prefix|prefixes. The units of SI can be divided into two subsets. There are seven SI base unit|base units: Each of these base units represents, at least in principle, Dimensional analysis|different kinds of physical quantities. From these seven base units, several SI derived unit|other units are derived. In addition to the SI units, there is also a set of non-SI units accepted for use with SI.