The unit of length is the meter (or metre), defined by the distance, at 0 °C under normal atmospheric pressure, between the axes of the two central lines marked on the bar of platinum-iridium (90% platinum and 10% iridium) kept at the Bureau International des Poids et Mesures and declared Prototype of the meter by the 1st Conférence Générale des Poids et Mesures (CGPM), this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.
The 1889 definition of the meter, based on the international prototype of platinum-iridium, was replaced by the 11th General Conference on Weights and Measures (CGPM) (1960) using a definition based on the wavelength of krypton 86 radiation. This change was adopted in order to improve the accuracy with which the definition of the meter could be realized, the realization being achieved using an interferometer with a traveling microscope to measure the optical path difference as the fringes were counted. In turn, this was replaced in 1983 by the 17th CGPM (1983/84) that specified the current definition, as follows:
The meter is the length of the path traveled by light in vacuum during a time interval of 1/299.792.458 of a second.
It follows that the speed of light in the vacuum is precisely 299.792.458 meters per second, \(c_0\) = 299.792.458 m/s.
The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under conditions specified in 1889.
The modern meter was defined in the 17th General Conference of Weights and Measures held on 20 October 1983. According to this, the meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
This standard is technologically more accurate and feasible when compared to the red–orange radiation of a krypton 86 atom and can be realized in practice through the use of an iodine-stabilized helium–neon laser. The reproducibility of the modern meter is found to be 3 parts in 1011, which could be compared to measuring the earth’s mean circumference to an accuracy of about 1 mm.