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micrometre
micrometre
	A 6 μm diameter carbon filament above a 50 μm diameter human hair
General information
Unit system	SI
Unit of	length
Symbol	μm
Conversions
1 μm in ...	... is equal to ...
SI base units	10−6 m
Natural units	1.8897×104 a0
imperial/US units	3.9370×10−5 in

The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures;^[1] SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron,^[2] is a unit of length in the International System of Units (SI) equalling 1×10⁻⁶ metre (SI standard prefix "micro-" = 10⁻⁶); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.00004 inch).^[1]

The nearest smaller common SI unit is the nanometre, equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre (0.000000001 m).

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria,^[1] and for grading wool by the diameter of the fibres.^[3] The width of a single human hair ranges from approximately 20 to 200 μm.

Examples

Between 1 μm and 10 μm:

1–10 μm – length of a typical bacterium
3–8 μm – width of strand of spider web silk^[4]
5 μm – length of a typical human spermatozoon's head^[5]
10 μm – size of fungal hyphae
about 10 μm – size of a fog, mist, or cloud water droplet

Between 10 μm and 100 μm:

about 10–12 μm – thickness of plastic wrap (cling wrap)
10 to 55 μm – width of wool fibre^[6]
17 to 181 μm – diameter of human hair^[7]
70 to 180 μm – thickness of paper

SI standardization

The term micron and the symbol μ were officially accepted for use in isolation to denote the micrometre in 1879, but officially revoked by the International System of Units (SI) in 1967.^[8] This became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960.

In the SI, the systematic name micrometre became the official name of the unit, and μm became the official unit symbol.

In American English, the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is often stressed on the second syllable (/maɪˈkrɒmɪtər/ my-KROM-it-ər), whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable (/ˈmaɪkroʊmiːtər/ MY-kroh-meet-ər).

The plural of micron is normally microns, though micra was occasionally used before 1950.^[9]^[10]^[11]

Symbol

The official symbol for the SI prefix micro- is a Greek lowercase mu.^[12] Unicode has inherited U+00B5 µ MICRO SIGN from ISO/IEC 8859-1, distinct from the code point U+03BC μ GREEK SMALL LETTER MU. According to the Unicode Consortium, the Greek letter character is preferred,^[13] but implementations must recognize the micro sign as well for compatibility with legacy character sets. Most fonts use the same glyph for the two characters.

Before desktop publishing became commonplace, it was customary to render the symbol μ in texts produced with mechanical typewriters by combining a slightly lowered slash with the letter u. For example, "15 μm" would appear as "15_/um". This gave rise in early word processing to substituting just the letter u for the symbol if the Greek letter μ was not available, as in "15 um".^[14]

The Unicode CJK Compatibility block contains square forms of some Japanese katakana measure and currency units. U+3348 ㍈ SQUARE MIKURON corresponds to ミクロン mikuron.

Notes and references

^ ^a ^b ^c "micrometre". Encyclopædia Britannica Online. Retrieved 18 May 2014.
^ "Writing with SI (Metric System) Units". NIST. 13 January 2010.
^ "Wool Fibre". NSW Department of Education and Communities. Archived from the original (Word Document download) on 17 June 2016. Retrieved 18 May 2014.
^ Ramel, Gordon. "Spider Silk". Archived from the original on 4 December 2008. Retrieved 14 December 2008. A typical strand of garden spider silk has a diameter of about 0.003 mm ... Dragline silk (about .00032 inch (.008 mm) in Nephila)
^ Smith, D.J.; Gaffney, E.A.; Blake, J.R.; Kirkman-Brown, J.C. (25 February 2009). "Human sperm accumulation near surfaces: a simulation study" (PDF). Journal of Fluid Mechanics. 621. Cambridge University Press: 295. Bibcode:2009JFM...621..289S. doi:10.1017/S0022112008004953. S2CID 3942426. Archived from the original (PDF) on 6 November 2013.
^ "Fibreshape applications". IST - Innovative Sintering Technologies Ltd. Retrieved 4 December 2008. Histogram of Fiber Thickness [micrometre]
^ The diameter of human hair ranges from 17 to 181 μm. Ley, Brian (1999). Elert, Glenn (ed.). "Diameter of a human hair". The Physics Factbook. Retrieved 8 December 2018.
^ BIPM - Resolution 7 of the 13th CGPM 1967/68), "Abrogation of earlier decisions (micron, new candle.)"
^ Proceedings of the Royal Society of Queensland. Part I. Vol. XIX. H. Pole & Co. 1907 – via Google Books.
^ Bigalow, Edward Fuller; Agassiz Association (1905). The Observer. Vol. 7–8 – via Google Books.
^ 10 micra/10 microns (Start at 1885; before that, the word "micron", singular or plural, was rare)
^ "Prefixes of the International System of Units". International Bureau of Weights and Measures. Archived from the original on 23 May 2018. Retrieved 9 May 2016.
^ Beeton, Barbara; Freytag, Asmus; Sargent, Murray III (30 May 2017). "Unicode Technical Report #25". Unicode Technical Reports. Unicode Consortium. p. 11.
^ John C. Mutchler, ed. (1999). The American Directory of Writer's Guidelines: A Compilation of Information for Freelancers from More Than 1,300 Magazine Editors and Book Publishers (2 ed.). Quill Driver Books. p. 47. ISBN 978-1-884956-08-9.