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Vol. 32 No. 6
November-December 2010


50th Anniversary of the SI: The International System of Units

by Ian M. Mills

The year 2010 is the 50th anniversary of the adoption of the name “International System of Units,” and the symbol “SI” (taken from the French words Système international d’unités), by the General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures) at its meeting in 1960. This 11th meeting of the General Conference, held 11–20 October 1960, was the formal beginning of the SI. The success of the SI in providing internationally agreed standards for the worlds of science, engineering, and technology is an occasion to be celebrated.

The BIPM was originally established in 1875 by the Metre Convention to ensure world-wide unification of measurements. At its first meeting, in 1889, the General Conference decided definitions of the metre and the kilogram in terms of prototype artifacts. Definitions of the electrical units based on the ampere were added at the 9th meeting of the General Conference, in 1948, in agreement with the International Electrochemical Commission. At the same conference the candela was adopted as the name of the unit of luminous intensity in place of the older “new candle.” The definition of the kelvin was added at the 10th meeting of the General Conference, in 1954, and the formal definition of the second was added by the CIPM in 1956. Finally, at its 11th meeting, the General Conference in 1960 laid the foundation for the complete system of units, built upon the six base units used at that time and including multiple and sub-multiple prefixes, and adopted the name International System of Units by which the entire system is known today. The definition of the mole for the quantity amount of substance, the seventh base unit, was added at the 14th meeting of the General Conference, in 1971.

There have been many developments to the system during the 50 years from 1960 to 2010. The definitions of many of the base units have been revised to meet new developments in the experimental basis of metrology, and to ensure the coherence of the entire system. These changes have been faithfully recorded in successive editions of the SI Brochure, prepared by the Consultative Committee for Units (CCU) and published by the BIPM at irregular intervals.

The brochure has grown from the 36 pages of the original first edition to the 200 pages of the latest 8th edition published in 2006. The 8th edition is also accompanied for the first time by the four-page Concise Summary of the SI. These publications are available free on the BIPM website <www.bipm.org/en/si> and are now accepted around the world as the reference standard for all quantitative measurements.

. . . it is a living system, forever being revised and improved in a variety of ways . . .

The SI has now reached maturity as the ultimate international reference for all standards of measurement. But it is a living system, forever being revised and improved in a variety of ways to recognize our developing abilities in making measurements. At present, there are active plans to revise the definitions of four of the seven base units—the kilogram, ampere, kelvin, and the mole—as we strive to achieve a more uniform system in which the definitions of all units are referenced to the fundamental constants of nature.

Happy 50th birthday to the SI! May it continue to prosper in the hands of the metrologists of the world.

Ian Mills <i.m.mills@reading.ac.uk> is a professor at the School of Chemistry, University of Reading, Reading, UK.

Reprinted with permission from the BIPM Key Comparison Database (KCDB) Newsletter, No. 13, June 2010 <http://kcdb.bipm.org/NL/13/NL_13_June10.html#text11>.


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