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Wikipedia / Metre

The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of a second, where the second is defined by a hyperfine transition frequency of caesium.

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris, setting as that quarter of the Earth's polar circumference.

In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in vacuum in of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency .

Etymology and spelling

The etymological roots of metre can be traced to the Greek verb () ((I) measure, count or compare) and noun () (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (), French (), English (meter for measuring instruments, but metre or meter in poetry) and other languages. The Greek word is derived from the Proto-Indo-European root *meh₁- 'to measure'. In English, the use of the word metre (for the French unit ) began at least as early as 1797.

Metre is the standard spelling of the metric unit for length in all English-speaking nations except the United States and the Philippines, which use meter.

History of definition

SI prefixed forms of metre

SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, astronomical units (), light-years (), or parsecs (), rather than in Mm or larger multiples. "30cm", "30m", and "300m" are more common than "3dm", "3dam", and "3hm", respectively.

The terms micron and millimicron have been used instead of micrometre (μm) and nanometre (nm), respectively, but this practice is discouraged.

Equivalents in other units

Within this table, "inch" and "yard" mean "international inch" and "international yard" respectively, though approximate conversions in the left column hold for both international and survey units.

  1. "≈" means "is approximately equal to";

  2. "=" means "is exactly equal to".

One metre is exactly equivalent to inches and to yards.

A simple mnemonic to assist with conversion is "three 3s": 1metre is nearly equivalent to 3feet inches. This gives an overestimate of 0.125mm.

The ancient Egyptian cubit was about 0.5m (surviving rods are 523–529mm). Scottish and English definitions of the ell (2cubits) were 941mm (0.941m) and 1143mm (1.143m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2m and was standardised at exactly 2m in the mesures usuelles system, such that 1m was exactly toise. The Russian verst was 1.0668km. The Swedish mil was 10.688km, but was changed to 10km when Sweden converted to metric units.

See also

Notes

References

Cited bibliography