Length (the metre, m)
SI base unit of length
Current Definition
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m/s, where the second is defined in terms of ΔvCs.
History
| Year | Definition of the metre |
|---|---|
| 1795 | The metre was defined as 1/40 000 000 part of the circumference of the earth, using the distance measured from Dunkirk to Barcelona (France). |
| 1799 | The definition of the metre was changed to the Metre des Archives, an artefact of rectangular cross-section which is constructed to be one ten-millionth of a quadrant of the Earth. |
| 1889 | The metre was defined as the length of the international prototype of metre (IPM) which was a "X" cross-section bar made of platinum-iridium. |
| 1960 | The metre was redefined in terms of the wavelength of the orange-red line of krypton-86 in vacuum. "The metre is the length equal to 1 650 763,73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom." |
| 1983 | The metre was redefined by the speed of light in vacuum during a time interval of 1/299 792 458 of a second. Iodine stabilized Helium-Neon laser was recommended for realizing the metre. |
| 2019 | The definition of the metre, albeit unchanged, was re-phrased as follows The metre is defined by taking the fixed numerical value of the speed of light in vacuum to be 299 792 458 when expressed in the unit ms-1, where the second is defined in terms of the caesium frequency ΔvCs |
Reference
- The International System of Units (SI) – Base Unit
- The ninth SI Brochure.