Système International d'Unités

The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units (ampere, kelvin, second, metre, kilogram, candela, mole) and a set of twenty decimal prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units for other common physical quantities like lumen, watt, etc.

The base units, except for one, are derived from invariant constants of nature, such as the speed of light and triple point of water, which can be readily observed and measured with great accuracy. The kilogram standard of mass is a physical artefact certified in 1889, consisting of a cylinder of platinum-iridium, which ostensibly has the same mass (weight) as one litre of water at the freezing point. Its stability has been a matter of significant concern, resulting in recent proposals to change the definition to one derived from some invariant constant of nature.

Derived units may be defined in terms of base units or other derived units. They are adopted to facilitate measurement in diverse fields of endeavor. 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. The last new derived unit was defined in 1999.

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