The International System of Units
And its base units
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International System of Units 1 1 20 20 30 35 37 39 39 42 51 69 75 78 82 86 90 90 93
Metre Convention General Conference on Weights and Measures International Bureau of Weights and Measures International Committee for Weights and Measures
SI base unit Metre Kilogram Second Ampere Kelvin Mole Candela
SI derived unit Units accepted for use with SI
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International System of Units
For a topical guide to this subject, see Outline of the metric system. The International System of Units (abbreviated SI from French: Le Système international d'unités) is the modern form of the metric system. It comprises a coherent system of units of measurement built around seven base units, 22 named and an indeterminate number of unnamed coherent derived units, and a set of prefixes that act as decimal-based multipliers. The standards, published in 1960, are based on the metre-kilogram-second system, rather than the centimetre-gram-second system, which, in turn, had several variants. The SI has been declared to be an evolving system; thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves. SI is the world's most widely used system of measurement, used in both everyday commerce and science.
The seven SI base units and the interdependency of their definitions. Clockwise from top: kelvin (temperature), second (time), metre (length), kilogram (mass), candela (luminous intensity), mole (amount of substance) and ampere (electric current).
The system has been nearly globally adopted. Only Burma, Liberia and the United States have not adopted SI units as their official system of weights and measures. In the United States metric units are not commonly used outside of science, medicine and the government; however, United States customary units are officially defined in terms of SI units. The United Kingdom has officially adopted a partial metrication policy, with no intention of replacing imperial units entirely. Canada has adopted it for most purposes, but imperial units are still legally permitted and remain in common use throughout a few sectors of Canadian society, particularly in the buildings, trades and railways sectors.
The metric system was first implemented during the French Revolution (1790s) with just the metre and kilogram as standards. In the 1860s British scientists, working through the British Association for the Advancement of Science laid the foundations for a coherent system based on length, mass and time, but the inclusion of electrical units into the system was hampered until 1900 when Giorgi identified the need to define an electrical quantity alongside the original three quantities. Meanwhile, in 1875, the Treaty of the Metre passed custodianship of the prototype kilogram and metre from French to international control. In 1921 the Treaty was extended to include all physics measurements
International System of Units and in 1948 an overhaul of the metric system was set in motion which resulted in the publication of the International System of Units in 1960.
The metric system was developed from 1791 onwards by a group of scientists commissioned by the Assemblée nationale and Louis XVI of France to create a unified and rational system of measures. The group, which included Antoine-Laurent Lavoisier (the "father of modern chemistry") and the mathematicians Pierre-Simon Laplace and Adrien-Marie...
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