1 : micrometre
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A micrometre (American spelling: micrometer), symbol µm is an SI unit of length. It is defined as one millionth of a metre (1 × 10-6 m), equivalent to one thousandth of a millimetre. The symbol µ is the "micro sign"; it is derived from the Greek letter mu (μ). The micrometre is a common unit of measurement for wavelengths of infrared radiation.
Some people (especially in astronomy and the semiconductor business) use the old name micron and/or the solitary symbol µ, for the same thing, even though it is officially discouraged.
micron is also used as a unit of pressure (of a vacuum, since it is used to measure a very small pressure), an abbreviated form of micrometre of mercury (µmHg). It is equal to a millitorr, or 1/1000th of a torr.
A micrometer is the name of a tool that measures the thickness of objects. In American English, the spelling of the tool and the unit are identical.
See also: 1 E-6 m (a list of objects on the order of one to ten micrometres)
nanometre << micrometre << millimetre
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