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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