International System of Units

I

Introduction

International System of Units (French, Le Système International d’Unités), name adopted by the Eleventh General Conference on Weights and Measures, held in Paris in 1960, for a universal, unified, self-consistent system of measurement units based on the MKS (metre-kilogram-second) system. The international system is commonly referred to throughout the world as SI, the acronym for Système International.

At the 1960 conference, standards were defined for six base units and two supplementary units; a seventh base unit, the mole, was added in 1971. The class of supplementary units was abrogated in 1995 and the supplementary units merged with the derived units. The seven base units are listed in Table 1. The symbols in the last column are the same in all languages.

II

Length

The metre had its origin in the metric system. By international agreement, the standard metre had been defined as the distance between two fine lines on a bar of platinum-iridium alloy. The 1960 conference redefined the metre as 1 650 763.73 wavelengths of the reddish-orange light emitted by the isotope krypton 86. The metre was again redefined in 1983 as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second, which is the present definition.

III

Mass

When the metric system was created, the kilogram was defined as the mass of 1 cubic decimetre of pure water at the temperature of its maximum density (4.0° C; 39.2° F). A solid cylinder of platinum was carefully made to match this quantity of water under the specified conditions. Later it was discovered that a quantity of water as pure or as stable as required could not be provided. Therefore the primary standard of mass became the platinum cylinder, which was replaced in 1889 by a platinum-iridium cylinder of similar mass. Today this cylinder still serves as the international kilogram, and the kilogram in SI is defined as a mass equal to that of the International Prototype Kilogram.

IV

Time

For centuries, time has been universally measured in terms of the rotation of the Earth. The second, the basic unit of time, was defined as 1/86 400 of a mean solar day or one complete rotation of the Earth on its axis in relation to the Sun. Scientists discovered, however, that the rotation of the Earth was not constant enough to serve as the basis of the time standard. As a result, the second was redefined in 1967 in terms of the resonant frequency of the caesium atom, that is, the frequency at which this atom absorbs energy. The formal definition reads: “The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.”