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 (meter-kilogram-second) system. The international system is commonly referred to throughout the world as SI, after the initials of Système International. The Metric Conversion Act of 1975 commits the United States to the increasing use of, and voluntary conversion to, the metric system of measurement, further defining metric system as the International System of Units as interpreted or modified for the United States by the secretary of commerce.

At the 1960 conference, standards were defined for six base units and for two supplementary units; a seventh base unit, the mole, was added in 1971. The seven base units are listed in Table 1, and the supplementary units are listed in Table 2. The symbols in the last column are not abbreviations (hence, no periods are used), and they are exactly the same in all languages.

II

Length

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

III

Mass

When the metric system was created, the kilogram was defined as the mass of 1 cubic decimeter 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 quantity of mass of the international prototype of the kilogram.

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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 (see Day) or one complete rotation of the earth on its axis. 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 cesium atom—that is, the frequency at which this atom absorbs energy, or 9,192,631,770 hertz (cycles per second).