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 metreEdit

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 | |
---|---|---|---|---|---|---|

10^{0}
| metre
| m | ||||

10^{1}
| decametre | dam | 10^{−1}
| decimetre | dm | |

10^{2}
| hectometre | hm | 10^{−2}
| centimetre | cm | |

10^{3}
| kilometre | km | 10^{−3}
| millimetre | mm | |

10^{6}
| megametre | Mm | 10^{−6}
| micrometre | µm | |

10^{9}
| gigametre | Gm | 10^{−9}
| nanometre | nm | |

10^{12}
| terametre | Tm | 10^{−12}
| picometre | pm | |

10^{15}
| petametre | Pm | 10^{−15}
| femtometre | fm | |

10^{18}
| exametre | Em | 10^{−18}
| attometre | am | |

10^{21}
| zettametre | Zm | 10^{−21}
| zeptometre | zm | |

10^{24}
| yottametre | Ym | 10^{−24}
| yoctometre | ym |

## ConversionsEdit

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 Edit

See History at [9]

### Timeline of definition Edit

- 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 2p
^{10}and 5d^{5}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 Edit

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

## External links Edit

- 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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