metric system (redirected from metric unit)

metric system, system of weights and measures planned in France and adopted there in 1799; it has since been adopted by most of the technologically developed countries of the world. It is based on a unit of length, called the meter (m), and a unit of mass, called the kilogram (kg).

The system has changed somewhat since it was first developed; e.g., the definition of the meter has changed, and the unit for mass is different. The meter was originally intended to be 1-10,000,000 of the distance on the earth's surface between the equator and either pole; however, because of errors in the original survey for determining the meter and because of the impracticality of referring to such a standard, the meter was later redefined in terms of the standard prepared and kept at Sèvres, France, near Paris. Long defined as the distance between two scratches on a bar of platinum-iridium alloy, the meter in 1960 was first redefined in terms of an atomic standard. In 1983 the meter was officially redefined as the distance traveled by light in vacuum during 1-299,792,458 of a second.

The original unit of mass, the gram, was first defined as the mass of pure water at maximum density that would fill a cube whose edges are each 0.01 m. The unit of mass is now the kilogram, defined as the mass of a platinum-iridium cylinder kept at Sèvres. (A gram is now defined as a mass 1-1,000 kg.) Other metric units can be defined in terms of the meter and the kilogram. For example the are, the unit of area, is equal to the area of a square whose edges are each 10 m long. The liter, the metric unit of volume, is equal to the volume of a cube whose edges are each 1-10 m long.

Fractions and multiples of the metric units are related to each other by powers of 10, allowing conversion from one unit to a multiple of it simply by shifting a decimal point, and avoiding the lengthy arithmetical operations required by the English units of measurement. Standard prefixes (found in the table entitled Prefixes for Basic Metric Units) have been accepted for designating multiples and fractions of the meter, gram, are, and other units. Thus, 1,000 grams are a kilogram, 100 ares are a hectare, and 1-100 of a meter is a centimeter.

Several other systems of units based on the metric system have been in wide use. The cgs system is based on the centimeter of length, the gram of mass, and the second of time. The mks system is based on the meter of length, the kilogram of mass, and the second of time. Units in the mks system are larger than the corresponding cgs units. Electric and magnetic units have been defined for both of these systems; in fact, two different sets of electric units are defined in the cgs system. The mks system serves as the basis for the International System of Units, a comprehensive system of units for all physical quantities adopted in 1960 by the 11th General Conference on Weights and Measures.

See also decimal system.

Bibliography

See L. V. Judson, Weights and Measures Standards of the United States: A Brief History (1976; U.S. National Bureau of Standards Special Publication 447); K. Alder, The Measure of All Things (2002).

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metric system

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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A system of weights and measures that uses the gram, meter and liter as its primary units of weight, distance and capacity.

 Metric                English
 1 gram                 .0022046 lb. (.03527 oz.)
 1 decagram (10 gr)     .022046 lb. (.3527 oz.)
 1 hectogram (100 gr)   .22046 lb. (3.527 oz.)
 1 kilogram (1000 gr)  2.2046 lbs. (35.27 oz.)

 1 meter                         39.37 in.
 1 decameter (10 m)              32.8 ft.
 1 hectometer (100 m)           328.08 ft.
 1 kilometer (1000 m)          3280.8 ft.
 1 decimeter (1/10 m)             3.937 in.
 1 centimeter (1/100 m)            .3937 in.
 1 millimeter (1/1,000 m)          .03937 in.
 1 micrometer (1/1,000,000 m)      .00003937 in.
 1 nanometer  (1/1,000,000,000 m)  .00000003937 in.

 1 liter               1.0567 liquid quart