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# meter

The meter (abbreviation, m; the British spelling is metre) is the International System of Units (SI) unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1/299,792,458 (3.33564095 x 10-9) of a second. The meter was originally defined as one ten-millionth (0.0000001 or 10-7) of the distance, as measured over the earth's surface in a great circle passing through Paris, France, from the geographic north pole to the equator.

One meter is a little more than three English feet, or about 39.37 inches. One foot is approximately 0.3048 meter. There are about 1609 meters in a statute mile. The official span was at one time formally defined as the separation between two scratches on a platinum bar in Paris. This was, of course, intended mainly for show, and not for use in the laboratory.

Power-of-10 prefix multipliers facilitate the derivation of other, often more convenient, distance units from the meter. One centimeter (cm) is equal to 0.01 m, one millimeter (mm) is equal to 0.001 m, and one kilometer (km) is equal to 1000 m. These units are found in nonscientific as well as scientific literature. Smaller units are the realm of the scientist and engineer. One micrometer (symbolized ?m or ?), also called a micron, is equal to 0.000001 (10-6) m. One nanometer (nm) is equal to 10-9 m. One Angström unit (symbolized Ä) is equal to 10-10 m, or 0.1 nm.

The meter and its kin are used to specify the wavelengths of EM fields. The so-called radio spectrum occupies an informally defined range of wavelengths from roughly a millimeter (microwaves) to several tens of kilometers (myriametric waves). A 3-m radio wave falls near the middle of the standard FM (frequency modulation) broadcast band; a 300-m radio wave is near the middle of the standard AM (amplitude-modulation) broadcast band. The range of visible light wavelengths is from approximately 390 nm (violet) to 770 nm (red). The speed of EM-field propagation in a vacuum, to nine significant figures, is 2.99792458 x 108 meters per second. In this sense, the meter can be derived from the second if the latter unit has been previously defined in absolute terms; one meter is the distance a ray of light travels through a vacuum in 3.33564095 x 10-9 second.

In engineering applications, and also in an everyday sense, the term meter refers to any instrument used to measure the magnitude of a quantity. Examples include the volume-unit (VU) meter in home audio systems, the ammeter to measure electric current, and the kilowatt-hour meter to measure electrical energy consumed over a period of time.

Also see International System of Units (SI) and prefix multipliers.

This was last updated in September 2005

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