System of units

The International System of Units(abbreviated SI) is the modern form of the metric system, according to a resolution of the International Bureau of Weights and Measures. It is generally a system of units of measurement devised around seven SI base units and the convenience of the number ten. The older metric system included several groups of units. The SI was established in 1960, based on the metre-kilogram-second system, rather than the Centimetre–gram–second system of units system, which, in turn, had a few variants. The SI is declared as 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, which is used both in everyday commerce and in science.Definitions of the basic units can be found at the sites bottom, 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, the definition of the metre fixes the speed of light in vacuum c₀, 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 bottom table. Since the electric constant is related to μ₀ by ε₀ = 1/μ₀c₀², it too is known exactly.

The desire for international cooperation on metrology led to the signing in 1875 of the Metre Convention, a treaty that established three international organizations to oversee the keeping of metric standards:

• General Conference on Weights and Measures – a meeting every four to six years of delegates from all member states;
• International Bureau of Weights and Measures - an international metrology centre in France; and
• International Committee for Weights and Measures —an administrative committee that meets annually at the BIPM.

The history of the metric system has seen a number of variations, and has spread around the world, to replace many traditional 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 promote a worldwide measurement system. As a result, the 9th General Conference on Weights and Measures, in 1948, asked the International Committee for Weights and Measures 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 kelvin), and candela. In 1960, the 11th CGPM named the system the International System of Units. The seventh base unit, the mole mole, was added in 1971 by the 14th CGPM.

One of the CIPM committees, the CCU, has proposed a number of changes to the definitions of the base units used in SI.

The International System of Units consists of a set of units together with a set of SI. The units are divided into two classes—base units and derived units. There are seven SI, each representing, by convention, different kinds of physical quantities.

In addition to the SI units, there is also a set of non-SI units accepted for use with SI, which includes some commonly used non-coherent units such as the litre.