Units of Measurement

Units of measurement serve for the determination of the value of physical quantities. A system of units is a set of rules which defines in a consistent manner the unit of measurement of all quantities used in natural science and technology. The system of units used worldwide is the International System of Units, in French Système International d'Unités (SI). The SI has been introduced by the 11th General Conference on Weights and Measures (CGPM) in 1960. The SI has succeeded a series of systems of units, mainly used in the various fields of natural science, and has subsequently made the complicated conversions between different units superfluous. The International System of Units differentiates between two classes of units: base units and derived units. The base units are:

The derived units are obtained from the base units applying the same algebraic relationships as hold for the respective quantities in the laws of nature. An important property of the International System of Units is coherence, by which is meant that derived units are defined by the multiplication and/or division of the base units without the need for any numerical factors.

An important requirement on the base units is their independence of space and time; i. e., they must be reproducible with the same accuracy at any time in any laboratory. In the attempt to fulfil this requirement, the definitions of the units have already been changed a number of times, and are based today, with the sole exception of the kilogram, no longer on artefacts, but rather on constant properties of nature, which can be experimentally applied harnessed anywhere and at any time.

Switzerland adopted the International System of Units on 1 January 1978. The legal foundation for its application has been laid by the Federal Weights and Measures Law and the Units Ordinance (both available in German, French and Italian only).