METRIC SYSTEM

international 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, by the late 20th century, was used officially in almost all nations. The French Revolution of 1789 provided the opportunity to pursue the frequently discussed idea of replacing the confusing welter of traditional but illogical units of measure with a rational system based on multiples of 10. In 1791 the French National Assembly directed the French Academy of Sciences 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 of the Earth, measured around the poles of the meridian passing through Paris. An arduous six-year survey to determine the arc of the meridian from Barcelona, Spain, to Dunkirk, Fr., eventually yielded a value of 39.37008 inches for the new unit to be called the metre, from Greek metron, meaning measure. All other 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 (one-thousandth of a cubic metre). Greek prefixes were established for multiples of 10, ranging from pico- (one-trillionth) to tera- (one trillion) and including the more familiar micro- (one-millionth), milli- (one-thousandth), centi- (one-hundredth), and kilo- (one thousand). 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, but 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 Svres, 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, upon the archive standards rather than directly upon 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 Svres. 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 (q.v.).