The 17th Conférence Générale des Poids et Mesures 1983 adopted the following definition of the meter:
The meter (m) is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Pierre-François-André Méchain (1744 – 1804)
This definition of the meter assigns a fixed value to the light velocity – the speed of light in vacuum is exactly 299 792 458 metres per second,
c0 = 299 792 458 m/s.
More detailed information on the practical realisation of the meter can be found on our page „Length“.
The original definition and the first realisation of the meter date back to the time of the French Revolution. At that time, many different local length units were seriously hampering trade which even affected the upcoming Industrial Revolution. It was in the spirit of the French Revolution to look for a length unit that was equal for all people.
Therefore it was decided to use a certain fraction of an earth meridian (i.e. the Earth’s circumference from pole to pole) as a length standard. 1792 P. F. A. Méchain and J. B. J. Delambre began to measure the meridian that runs through Paris between Dunkirk and Barcelona. The commotions of the French Revolution delayed this work so that it could only be completed after six years.
The length of the meridian was deduced from the length of this section and it was decided that from now on one ten-millionth of a quadrant of the earth’s meridian corresponds to one meter (thus the circumference of the Earth is approximately 40 000 km). By the way, the data lying behind the meter also led to the development of modern methods for the evaluation of measurement data (C. F. Gauß, A. M. Legendre).
Jean-Baptiste Joseph Delambre (1749 – 1822)
1799 a platinum bar, corresponding to the determined length of the meter became the official French standard (“Mètre des Archives”). At the end of the 19th century (1st Conférence Générale des Poids et Mesures 1889) a bar of a platinum-iridium alloy (10% Iridium) became the new prototype of the meter. The Mètre des Archives served as a reference. Copies (likewise platinum-iridium) for the member states of the Meter Convention (among them also the former Austria-Hungary) were produced to serve as their national standards.
Shortly after (1892-1893), the first successful experiments were made at BIPM to determine the length of the meter in terms of multiple wavelength of monochromatic light. But it was not until 1960 that a definition of the meter was adopted based on the wavelength of monochromatic light.
As a definition the radiation corresponding to a transition between specified energy levels of the krypton 86 isotope (86Kr) was chosen. The wavelength of this radiation multiplied by 1 650 763.73 gives one meter.
At about the same time, the laser was invented and new, even more precise methods of length measurement became feasible. This led to the redefinition of the meter, valid since 1983, as the length of the path travelled by light in vacuum during a specific fraction of the second. In the 1970s painstaking measurements determined the light velocity in vacuum very precisely to define the value given above. The second is the base unit that can be realised with today’s highest possible precision. At present, primarily the frequency of laser radiation is determined for length measurements and out of it the wavelength is determined from light velocity. Here stabilised lasers are used as standards for frequency and wavelength. Nevertheless, the original prototype of the meter 1889 is still being kept at BIPM in Paris.
It should be mentioned that at the time of the French Revolution an alternative approach for the definition of the meter was discussed. The meter should be defined as the length of a pendulum with the frequency of one beat per second. This definition was also based on the second but it was rejected in favour of the meridian definition at that time.