(Fr. 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 U.S. 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 U.S. by the secretary of commerce. The European Community decreed the use of SI by 1978.

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.

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 a vacuum during a time interval of 1/299,792,458 of a second.

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.

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 until 1955 as 1/86,400 of a mean solar day (see DAY,) or one complete rotation of the earth on its axis. That year it was redefined as 1/31,556,925.9747 of the solar year in progress at noon on Dec. 31, 1899. 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 again in 1967 in terms of the resonant frequency of the cesium-133 atom; the second is now established as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

Temperature.

The temperature scale adopted by the 1960 conference was based on a fixed temperature point, the triple point of water, at which the solid, liquid, and gas are in equilibrium. The temperature of 273.16 K was assigned to this point. The freezing point of water was designated as 273.15 K, equaling exactly 0° on the Celsius temperature scale. The Celsius scale, which is identical to the centigrade scale, is named for the 18th-century Swedish astronomer Anders Celsius, who first proposed the use of a scale in which the interval between the freezing and boiling points of water is divided into 100 degrees. By international agreement, the term Celsius has officially replaced centigrade.

Other Units.

In SI, the ampere was defined as the constant current that, flowing in two parallel conductors one meter apart in a vacuum, will produce a force between the conductors of 2 × 10^–7 newtons per meter of length.

In 1971 the mole was defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.

The international unit of light intensity, the candela, was defined as 1/60 of the light radiated from a square centimeter of a blackbody, a perfect radiator that absorbs no light, held at the temperature of freezing platinum.

The radian is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.

The steradian is defined as the solid angle that, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.

The SI units for all other quantities are derived from the seven base units and the two supplementary units. Examples of some SI-derived units, expressed in terms of base units, are shown in Table 3.

Some derived units are used so often that they have been assigned special names—usually those of scientists—as shown in Table 4.

One feature of SI is that it is a coherent system; that is, derived units are expressed as products and ratios of the base, supplementary, and other derived units without numerical factors. This results in some units being too large for ordinary use and others too small. To compensate, the prefixes developed for the metric system have been borrowed and expanded. These prefixes, given in Table 5, are used with all three types of units: base, supplementary, and derived. Examples are millimeter (mm), kilometer/hour (km/h), megawatt (MW), and picofarad (pF). Because double prefixes are not used, and because the base unit kilogram already contains a prefix, prefixes are not used with kilogram, although they are used with gram. The prefixes hecto, deka, deci, and centi are used only rarely, and then usually with meter to express areas and volumes.

Because of established usage, the centimeter is retained for body measurements and clothing.

Certain units that are not part of SI are used so widely that it is impractical to abandon them. The units that are accepted for continued use in the U.S. with SI are listed in Table 6.

In cases where their usage is already well established, certain other units are allowed for a limited time, subject to future review: nautical mile, knot, angstrom, standard atmosphere, hectare, and bar.

See also METRIC SYSTEM,; WEIGHTS AND MEASURES,.

For further information on this topic, see the Bibliography, section 366. Measurement.