Metre

Metre (American English: Meter) is the base unit of length in the International System of Units (SI). It is defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.

History

The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length.

Definition

The metre is currently defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. This definition fixes the speed of light in vacuum at exactly 299,792,458 metres per second (≈186,282 miles per second).

SI prefixes

SI prefixes can be used to denote decimal multiples and submultiples of the metre, as follows:

• Kilometre (km): 1,000 metres
• Hectometre (hm): 100 metres
• Decametre (dam): 10 metres
• Metre (m): 1 metre
• Decimetre (dm): 0.1 metres
• Centimetre (cm): 0.01 metres
• Millimetre (mm): 0.001 metres
• Micrometre (µm): 0.000001 metres
• Nanometre (nm): 0.000000001 metres

See also

• International System of Units
• Kilometre
• Centimetre
• Millimetre
• Micrometre
• Nanometre

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