The metre (Commonwealth English [1]) or meter (American English [2]) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light [3] in absolute vacuum during a time [4] interval of 1/299,792,458 of a second [5].

SI prefixes applied to the metre

Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).

The metre may be used with SI prefixes as shown.

Multiple	Name	Symbol		Multiple	Name	Symbol
100	metre	m
101	decametre	dam		10−1	decimetre	dm
102	hectometre	hm		10−2	centimetre	cm
103	kilometre	km		10−3	millimetre	mm
106	megametre	Mm		10−6	micrometre	µm
109	gigametre	Gm		10−9	nanometre	nm
1012	terametre	Tm		10−12	picometre	pm
1015	petametre	Pm		10−15	femtometre	fm
1018	exametre	Em		10−18	attometre	am
1021	zettametre	Zm		10−21	zeptometre	zm
1024	yottametre	Ym		10−24	yoctometre	ym

Conversions

1 metre is equivalent to:

• exactly 1/0.9144 yards [6] (approximately 1.0936 yards)
• exactly 1/0.3048 feet [7](approximately 3.2808 feet)
• exactly 10000/254 inches [8] (approximately 39.370 inches)

History

See History at [9]

Timeline of definition

• May 8, 1790 — The National Assembly (French Revolution) [10] decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.

• March 30, 1791 — The French National Assembly accepts the proposal by the French Academy of Sciences [11] that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian (geography)[12] along a quadrant (one-fourth the polar circumference of the earth).

• 1795 — Provisional metre bar constructed of brass [13].

• December 10, 1799 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.

• September 28, 1889 — The first Conférence Générale des Poids et Mesures [14] defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium [15], measured at the melting point of ice.

• October 6, 1927 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmospheric pressure [16] and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.

• October 20, 1960 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths [17] in vacuum of the electromagnetic radiation [18] corresponding to the transition between the 2p¹⁰ and 5d⁵ quantum levels of the krypton-86 atom.

• October 21, 1983 — The seventeenth CGPM defines the length to be distance travelled by light [19] in vacuum during a time interval of 1/299 792 458 of a second [20].

See also

• Metric system
• SI
• SI prefix
• Conversion of units for comparisons with other units
• Orders of magnitude (length)
• Speed of light

External links

• History of the metre at the U.S. National Institute of Standards and Technology (NIST)
• Timeline of history of the metre at the NIST
• Bureau International des Poids et Measures - Lengths

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