Millimetre
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Millimetre
millimetre
Ruler with millimetre and centimetre marks
General information
Unit systemSI derived unit
Unit ofLength
Symbolmm
Named afterThe metric prefix mille (Latin for "one thousand") and the metre
Conversions
micrometres = 1000 ?m
centimetres cm = 0.1 cm
metres m = 0.001 m
kilometres
inches0.039370 in
feet0.0032808 ft
Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.

The millimetre (international spelling; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.

One millimetre is equal to micrometres or nanometres. Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly 5127 (? 0.03937) of an inch.

## Definition

Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/ of a second".[1] A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/ of a second.

## Informal terminology

A common shortening of millimetre in spoken English is "mil". This can cause confusion since in the United States, "mil" traditionally means a thousandth of an inch.

## Unicode symbols

For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:[2]

• millimetre - SQUARE MM
• square millimetre - SQUARE MM SQUARED
• cubic millimetre SQUARE MM CUBED

In Japanese typography, these square symbols are used for laying out unit symbols without distorting the grid layout of text characters.

## Measurement

On a metric ruler, the smallest measurements are normally millimetres.[3] High-quality engineering rulers may be graduated in increments of 0.5 mm. Digital callipers are commonly capable of reading increments as small as 0.01 mm.[4]

Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths between 30 GHz and 300 GHz for data transmission, in contrast to the 300 MHz to 3 GHz normally used in mobile devices, has the potential to allow data transfer rates of 10 gigabits per second.[5]

The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a thin human hair.[6] A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately 0.1 mm thick.[7]

## References

1. ^ "17th General Conference on Weights and Measures (1983), Resolution 1". International Bureau of Weights and Measures. Retrieved 2013.
2. ^ "CJK Compatibility" (PDF). unicode.org. Retrieved 2013.
3. ^ "How do I read a ruler?". onlineconversion.com. Retrieved 2013.
4. ^ "Accuracy of Calipers". TresnaInstrument.com. Retrieved 2013.
5. ^ Huang, Kao-Cheng; Wang, Zhaocheng (2011). Millimeter Wave Communication Systems. ISBN 9781118102756.
6. ^ "How Small Can the Naked Eye See?". Focus Magazine. Retrieved 2013.
7. ^ Sherlis, Juliya (2001). Elert, Glenn (ed.). "Thickness of a piece of paper". The Physics Factbook. Retrieved .