metre

length Symbol m and strictly the lower-case form, though used in uppercase conspicuously, in contrast to the traditional m for mile, on British road-signs. (Metric) The base unit for length, defined (1983) as equal to the length of the path travelled by light in a vacuum during a time interval of ¹/_299792458 of a second. The following are among the coherent derived units:

• m·s^-1 for speed;
• m·s^-2 for acceleration;
• m²·s^-1 for kinematic viscosity.

See SI unit for full repertoire.

Using the 1959 definition of the foot, 1 m = ¹/_0.3048 ft = 3.280 839 895~ ft(39.370 078 740~ in).

History

The metre was created in 1793 as the base unit for the metric system, its value being defined on a natural basis as ¹/_10000000 of the length of the quadrant from the North Pole to the Equator along the meridian through Paris (at nominal sea level). The kilometre of 1 000 metres thus became a hundredth of a hundredth of that quadrant, hence, with the concomitant centesimal division of the right angle (see grade), effectively a geographic mile. Its size was set by a physical survey from Dunkirk in northern France to Barcelona in Spain, and recorded in metal standards. A repeat survey gave a discrepant result, prompting definition via a solid physical prototype rather than the natural reference, using the standards already created. Enshrined as a platinum bar, the Mètre des Archives, and with copies provided to standards authorities around the globe, the metre has become the standard of length in virtually all countries. Both the UK and the USA incorporated it in legislation in 1866, but only within a common (not quite identical) non-metric system. The USA adopted it as the basis for defining the foot, etc. (with 1 m = 39.37 in); the UK, though allowing it in contracts, did not make it the fundamental length until 1959, when it set 1 yd = 0.914 4 m exactly, this being a rationalized value adopted by Canada in 1951,Canada: An Act respecting Weights and Measures assented to 20 June 1951 and by the USA and internationally in 1959.

Following the international conference on the metre in 1875, an International Prototype Metre was created in 1879 as a (more elaborately specified) successor to the original prototypes. This survived until 1960, when a natural reference, using light, was established (see below). As developing technology allowed and demanded greater accuracy, older definitions were demonstrated as inadequate, so the definition repeatedly improved. Definition of the angstrom, with an intention of it being exactly 10^-10 m, in 1907 in terms of light (in the form of the red line of cadmium) was a precursor of the modern definition of the metre, and simultaneously an example of the accuracy problem, for it proved to be 1.000 000 2~ times that intention. The definition of the metre in 1960, based on krypton,
[Barrell H. Nature Vol. 189, 195-6 (1961)] was expressly made consistent with the definition of the angstrom, hence deliberately varied the metre by a like proportion.

The current definition, that of 1983, relates the metre to the speed of light, rounding the metre so that the distance travelled by light in 1 second is an integral number of metres, i.e. 299 792 458. Technically this changed the size of the metre again, now at the tenth decimal place. Though far in format from its original geographic definition and not practical for everyday use, this newest definition puts the metre onto a natural base again and into a context that is much more practical to the scientist. As laboratory instruments allow ever more precise measurement of the speed of light, so the metre will change, but only ever more finely.

The speed of light is of great fundamental significance in physics, especially in electromagnetics (see e.m.u. system). It is ironic that, had such a natural base been adopted at the outset, and the metre been defined on a natural base like the light-nanosecond, not only would the pertinent electromagnetic calculations and relationships have involved the simple numerical factor of 10⁹, but also the vital new unit of length would have been effectively a foot, specifically 299.79~ mm (0.983 6~ ft), just over 11 Paris inches of the time, and almost exactly the Roman foot.

The international prototype metre, then its successor definitions, became the standard for the inch, etc., of US-C by the Mendenhall Order of 1893, which declared it equal to 39.37 in, and internationally since 1959 when the international inch was adopted as 25.4 mm, so 1 m = ¹/_0.0254 in = 39.370 078 7~ in. However, it was only in 1964 that the UK formally discarded its prototype yard in favour of that for the metre.UK Weights and Measures Act 1963