Units of measurement are necessary to describe physical quantities. They are internationally (previously nationally or even regionally) agreed comparative quantities with a fixed value that is repeatable at any time. Today’s system of units of measurement (the International System of Units – abbreviated in all languages as “SI”) was introduced in 1960 within the scope of the Metre Convention. At first intended for science, technology and education, the system has meanwhile gained general acceptance in commerce and all other areas of social life. The SI distinguishes between two categories of units: base units and derived units. The subsequent seven base units exist since 1971:
The derived units are formed from the base units by algebraic operations (multiplication and division) based on the laws of nature for the relevant quantities.
In this connection, it is essential that no other proportionality factor than 1 is included (coherent system of units). Some derived units have been given special names: e.g. volt, hertz, joule. It should be possible to realize the base units in an adequate laboratory at any time. Consequently, their definitions relate to invariable properties of nature (atomic properties and fundamental constants) with the exception of the unit of mass. Only the kilogram is still represented by an international prototype. The disadvantage of such prototypes is that they are exposed to environmental influences and its entailed changes and, moreover, that they are not freely available.
The International System of Units was introduced in Austria with the amendment of the Metrology Act 1973.