The metre (Commonwealth English[]) or meter (American English ]) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light []in absolute vacuum during a time []interval of 1/299,792,458 of a second[].
SI prefixes applied to the metreEdit
The metre may be used with SI prefixes as shown.
1 metre is equivalent to:
- exactly 1/0.9144 yards[] (approximately 1.0936 yards)
- exactly 1/0.3048 feet[](approximately 3.2808 feet)
- exactly 10000/254 inches[] (approximately 39.370 inches)
See History at []
Timeline of definition Edit
- May 8 1790 — The National Assembly (French Revolution) []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 []that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian (geography)[] along a quadrant (one-fourth the polar circumference of the earth).
- 1795 — Provisional metre bar constructed of brass[].
- 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 [] defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium [], 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 [] 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 [] in vacuum of the electromagnetic radiation [] 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 [] in vacuum during a time interval of 1/299 792 458 of a second [].
See also Edit
- Metric system
- SI prefix
- Conversion of units for comparisons with other units
- Orders of magnitude (length)
- Speed of light
- 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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