The International System of Units (SI)

1 Introduction
1.1 Historical note
The 9th CGPM† (1948, Resolution 6; CR, 64), instructed the CIPM:
• to study the establishment of a complete set of rules for units of measurement;
• to find out for this purpose, by official inquiry, the opinion prevailing in scientific, technical and educational circles in all countries;
• to make recommendations on the establishment of a practical system of units of measurement suitable for adoption by all signatories to the Meter Convention.

It also laid down, in Resolution 7 (CR, 70), general principles for the writing of unit symbols and listed units which have been assigned special names. The 10th CGPM (1954, Resolution 6; CR, 80), and the 14th CGPM (1971, Resolution 3; CR, 78 and Metrologia , 1972, 8, 36), adopted as base units of this practical system of units, the units of the following seven quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

The 11th CGPM (1960, Resolution 12; CR, 87), adopted the name Syste`me International d’Unite´s (International System of Units), with the international abbreviation SI, for this practical system of units and laid down rules for prefixes, derived units and the former supplementary units, and other matters; it thus established a comprehensive specification for units of measurement. Since then, successive meetings of the CGPM and CIPM have added to, and modified as necessary, the original structure of the SI to take account of advances in science and the needs of users.

The main historical steps which led to these important CGPM decisions may be summarized as follows.
• The creation of the Decimal Metric System at the time of the French Revolution and the subsequent deposition of two platinum standards representing the meter and the kilogram, on 22 June 1799, in the Archives de la Re´publique in Paris can be seen as the first step in the development of the present International System of Units.
• In 1832, Gauss strongly promoted the application of this Metric System, together with the second defined in astronomy, as a coherent system of units for the physical sciences. Gauss was the first to make absolute measurements of the Earth’s magnetic field in terms of a decimal system based on the three mechanical units millimeter, gram and second for,
respectively, the quantities length, mass and time. In later years Gauss and Weber extended these measurements to include electrical phenomena.