Micrometre
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The micrometre (Commonwealth English) or micrometer (American English) (SI symbol: μm) is a unit of length in the International System of Units (SI) equalling Template:Val (SI standard prefix "micro-" = Template:Val); that is, one millionth of a metre (or one thousandth of a millimetre, Template:Val, or about Template:Val).<ref name="Encyclopædia Britannica Online">Template:Cite encyclopedia</ref>
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 (Template:Val).
The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria,<ref name="Encyclopædia Britannica Online" /> and for grading wool by the diameter of the fibres.<ref name="NSW Department of Education and Communities">Template:Cite web</ref> The width of a single human hair ranges from approximately 20 to Template:Val.
Examples
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Between 1 μm and 10 μm:
- 1–10 μm – length of a typical bacterium
- 3–8 μm – width of strand of spider web silk<ref>Template:Cite web</ref>
- 5 μm – length of a typical human spermatozoon's head<ref>Template:Cite journal</ref>
- 6–8 μm – diameter of a typical red blood cell<ref>Template:Cite web</ref>
- 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<ref name="ISTAG">Template:Cite web</ref>
- 17 to 181 μm – diameter of human hair<ref name="Physics Factbook">The diameter of human hair ranges from 17 to 181 μm. Template:Cite web</ref>
- 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.<ref name="BIPM">BIPM - Resolution 7 of the 13th CGPM 1967/68), "Abrogation of earlier decisions (micron, new candle.)"</ref><ref>Template:Cite journal</ref> 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.
Template:Anchor In American English, the use of micron may help to differentiate the unit from the micrometer, a measuring device, because the unit's name in American spelling is a homograph of the device's name. In spoken English, they are distinguished by pronunciation, as the name of the measuring device is stressed on the second syllable (Template:IPAc-en Template:Respell), whereas the unit name places the stress on the first syllable (Template:IPAc-en Template:Respell).<ref>Template:Cite LPD</ref>
The plural of micron is normally microns, though micra was occasionally used before 1950.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>
Symbol
The official symbol for the SI prefix micro- is a Greek lowercase mu.<ref>Template:Cite web</ref> Unicode has inherited Template:Unichar from ISO/IEC 8859-1, distinct from the code point Template:Unichar. According to the Unicode Consortium, the Greek letter character is preferred,<ref>Template:Cite web</ref> 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 Template:Font. For example, "15 μm" would appear as "Template:Font". This gave rise in early word processing to substituting just the letter Template:Serif for the symbol if the Greek letter μ was not available, as in "Template:Serif".<ref>Template:Cite book</ref>
The Unicode CJK Compatibility block contains square forms of some Japanese katakana measure and currency units. Template:Unichar corresponds to Template:Lang Template:Transliteration.