Micrometre

The micrometre (Commonwealth spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known by the non-SI term micron, is a unit of length in the International System of Units (SI) equalling 1e-6metre (SI standard prefix "micro-" = e-6); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001mm, or about 0.00004inch). 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 (0.000000001m). The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, and for grading wool by the diameter of the fibres. The width of a single human hair ranges from approximately 20 to 200μm. Between 1 μm and 10 μm: 1–10 μm – length of a typical bacterium 3–8 μm – width of strand of spider web silk 5 μm – length of a typical human spermatozoon's head 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 17 to 181 μm – diameter of human hair 70 to 180 μm – thickness of paper 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. 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. Additionally, 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.