On the Revision of the International System of Units (SI)
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The International System of Units (SI) was adopted at the 11th meeting of the General Conference of Weights and Measures (CGPM) in 1960, and is a natural outgrowth of the metric system that originated in the 1700s. During this first phase, the SI system consisted of six fundamental units: the kilogram, meter, second, ampere, kelvin, and candela, with another fundamental unit – the mole – being added in 1971.
Metrologists have always sought to replace artificially produced artifacts with natural objects for definitions of units. In 1967, the second was defined by reference to atomic properties – to the period of a transition in cesium-133. In 1983, an SI unit – the unit of length meter – was defined by fixing the exact value of a physical constant – the speed of light in vacuum. Although the ampere, the unit for electric current, was initially defined implicitly, the exact value of the magnetic constant μ0 has been proposed.
By the late 1900s, the unit of mass – the kilogram– was the...
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