The Metre

The metre is the base unit of length in the International System of Units (SI). It's definition has been periodically refined to reflect advances in science and technology. Since 1983, it is defined as the distance travelled by light in vacuum inof a second.

In the eighteenth century, there were two favoured approaches to the definition of the metre – one in terms of the period of a standard pendulum, the other defining the metre as one ten-millionth of the length of the distance from the Equator to the North Pole.

In the 1870s, a series of international conferences was held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. The organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice. The original prototype is still kept at the BIPM.

In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. This was done in 1960 when the metre in was redefined in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

To further reduce uncertainty, the seventeenth CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light:

The metre is the length of the path travelled by light in vacuum during a time interval ofof a second.

This definition fixed the speed of light in a vacuum at precisely 299,792,458 metres per second. Although the metre is now defined as the distance travelled by light in a given time, actual laboratory realisations of the metre are still delineated by measuring the wavelength of laser light of a standard type, using interferometry to effectively count the number of wavelengths in a metre. Three major factors limit the accuracy attainable with laser interferometers.

• Uncertainty in vacuum wavelength of the source,

• Uncertainty in the refractive index of the medium,

• Laser count resolution of the interferometer.