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Historical Definitions of the Meter

• The definition of the meter has changed over the years since the metric system was first introduced in 1793.

• The actual length of the meter has not changed.

• New definitions have been introduced to make it easier to measure the meter's length more precisely.

• Currently the meter has a relative uncertainty of plus or minus 2.5 × 10^-11.

• Here is a historical summary:

  Year	Definition
  1793	1/10,000,000 of the distance from north pole to equator.
  1795	Provisional meter bar constructed from brass.
  1799	Prototype meter bars constructed from platinum.
  1889	International prototype meter bar constructed from platinum-iridium. It was an X-shaped cross section bar.
  1906	1,000,000/0.64384696 wavelengths of the red line of the cadmium spectrum.
  1960	1,650,763.73 wavelengths of radiation emitted during the transition between levels 2p10 and 5d5 of the krypton-86 atom.
  1983	Length traveled by light in a vacuum during 1/299,792,458 of a second.