Modern definitions of the metre
The definition of the metre went through several phases.
- In 1791 the metre was defined as 1/10,000,000 of the Earth's quadrant between the pole and the equator along the meridian running through Paris.
- In 1889 the International Bureau of Weights and Measures derived the international prototype metre from that definition as the distance between two lines on a standard bar of 90% platinum and 10% iridium. Several platinum-iridium metre bars were produced and one of these (number 6) replaced the Mètre des Archives in Paris to become the International Prototype Metre. The remaining bars were distributed to the representative nations. Lots were drawn; Britain received bar number 16.
The alloy from which the bar was made proved to be exceptionally stable. The same cannot be said of the Imperial Standard Yard of 1885, which was made of base metal and shrank at the rate of one part per million in about 20 years.
- In 1960 the International System of Units (SI) made use of advances in the techniques of light wave measurement and defined the metre as equal to 1,650,763.73 wavelengths of the orange-red line in the spectrum of the krypton-86 atom in vacuum.
- In 1983 the General Conference on Weights and Measures made use of advances in laser technology and redefined the metre as the distance travelled by light in vacuum in one 299,792,458th of one second.