metric system

a decimal system of weights and measures, adopted first in France but now widespread, universally used in science, mandatory for use for all purposes in a large number of countries, and favored for use in most (as in the U.S.). The basic units are the meter (39.37 inches) for length and the gram (15.432 grains) for mass or weight. Derived units are the liter (0.908 U.S. dry quart, or 1.0567 U.S. liquid quart) for capacity, being the volume of 1000 grams of water under specified conditions; the are (119.6 square yards) for area, being the area of a square 10 meters on a side; and the stere (35.315 cubic feet) for volume, being the volume of a cube 1 meter on a side, the term "stere," however, usually being used only in measuring firewood.

Names for units larger and smaller than these are formed from the above names by the use of the following prefixes: kilo-, 1000; hecto-, 100; deka-, 10; deci-, 0.1; centi-, 0.01; milli-, 0.001. To these are often added: tera-, one trillion; giga-, one billion; mega-, one million. With the addition of basic physical units it is now officially known by the French name Le Système International d'Unités (abbreviation SI) or in English as the International System of Units.

[1860-65]

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International decimal system of weights and measures, based on the metre (m) for length and the kilogram (kg) for mass, originally adopted in France in 1795.

All other metric units were derived from the metre, including the gram (g) for weight (1 cc of water at its maximum density) and the litre (l, or L) for capacity (0.001 cu m). In the 20th century, the metric system became the basis for the International System of Units, which is now used officially almost worldwide.

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▪ measurement

       Metric system Metric systeminternational decimal system of weights and measures, based on the metre for length and the kilogram for mass, that was adopted in France in 1795 and is now used officially in almost all countries.

      The French Revolution of 1789 provided an opportunity to pursue the frequently discussed idea of replacing the confusing welter of thousands of traditional units of measure with a rational system based on multiples of 10. In 1791 the French National Assembly directed the French Academy of Sciences (Sciences, Academy of) to address the chaotic state of French weights and measures. It was decided that the new system would be based on a natural physical unit to ensure immutability. The academy settled on the length of 1/10,000,000 of a quadrant of a great circle (great circle route) of the Earth, measured around the poles of the meridian passing through Paris. An arduous six-year survey led by such luminaries as Jean Delambre (Delambre, Jean-Baptiste-Joseph), Jacques-Dominique Cassini (Cassini, Dominique, comte de), Pierre Mechain (Mechain, Pierre), Adrien-Marie Legendre (Legendre, Adrien-Marie), and others to determine the arc of the meridian from Barcelona, Spain, to Dunkirk, France, eventually yielded a value of 39.37008 inches for the new unit to be called the metre, from Greek metron, meaning “measure.”

      By 1795 all metric units were derived from the metre, including the gram for weight (one cubic centimetre of water at its maximum density) and the litre for capacity (1/1,000 of a cubic metre). Greek prefixes were established for multiples of 10, myria (10,000), kilo (1,000), hecto (100), and deca (10), while Latin prefixes were selected for the submultiples, milli (0.001), centi (0.01), and deci (0.1). Thus, a kilogram equals 1,000 grams, a millimetre 1/1,000 of a metre. In 1799 the Metre and Kilogram of the Archives, platinum embodiments of the new units, were declared the legal standards for all measurements in France, and the motto of the metric system expressed the hope that the new units would be “for all people, for all time.”

      Not until 1875 did an international conference meet in Paris to establish an International Bureau of Weights and Measures. The Treaty of the Metre signed there provided for a permanent laboratory in Sèvres, near Paris, where international standards are kept, national standard copies inspected, and metrological research conducted. The General Conference of Weights and Measures, with diplomatic representatives of some 40 countries, meets every six years to consider reform. The conference selects 18 scientists who form the International Committee of Weights and Measures that governs the bureau.

      For a time, the international prototype metre and kilogram were based, for convenience, on the archive standards rather than directly on actual measurement of the Earth. Definition by natural constants was readopted in 1960, when the metre was redefined as 1,650,763.73 wavelengths of the orange-red line in the krypton-86 spectrum, and again in 1983, when it was redefined as the distance traveled by light in a vacuum in 1/299,792,458 second. The kilogram is still defined as the mass of the international prototype at Sèvres.

      In the 20th century the metric system generated derived systems needed in science and technology to express physical properties more complicated than simple length, weight, and volume. The centimetre-gram-second (CGS) and the metre-kilogram-second (MKS) systems were the chief systems so used until the establishment of the International System of Units.

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