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SI units

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ampere

Overview page. Subjects: Chemistry.

Symbol A. The SI unit of electric current. The constant current that, maintained in two straight parallel infinite conductors of negligible cross section placed one metre apart in a vacuum,...

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angstrom

Overview page. Subjects: Science and Mathematics.

Unit of length equal to 10-10 m, used in measuring electromagnetic radiation including visible light and X-rays. Replaced in SI units by the nanometre (10 Å = 1 nm).

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bar

Overview page. Subjects: Earth Sciences and Geography — Ecology and Conservation.

A c.g.s. unit of pressure equal to 106 dynes per square centimetre or 105 pascals (approximately 750 mmHg or 0.987 atmosphere). The millibar (100 Pa) is commonly used in meteorology.

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becquerel

Overview page. Subjects: Science and Mathematics — Medicine and Health.

n. the SI unit of activity of a radioactive source, being the activity of a radionuclide decaying at a rate of one spontaneous nuclear transition per second. It has replaced...

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binary prefixes

Overview page. Subjects: Computing — Physics.

A set of prefixes for binary powers designed to be used in data processing and data transmission contexts. They were suggested in 1998 by the International Electrotechnical Commission (IEC)...

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caesium clock

Overview page. Subjects: Chemistry — Physics.

An atomic clock that depends on the energy difference between two states of the caesium–133 nucleus when it is in a magnetic field. In one type, atoms of caesium–133 are irradiated with...

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calorie

Overview page. Subjects: Medicine and Health — Science and Mathematics.

The quantity of heat required to raise the temperature of 1 gram of water by 1°C (1 K). The calorie, a c.g.s. unit, is now largely replaced by the joule, an SI unit. 1 calorie = 4.186 8...

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candela

Overview page. Subjects: Chemistry.

Symbol Cd. The SI unit of luminous intensity equal to the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 Hz and has a radiant...

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c.g.s. units

Overview page. Subjects: Chemistry — Biological Sciences.

A system of units based on the centimetre, gram, and second. Derived from the metric system, it was badly adapted to use with thermal quantities (based on the inconsistently defined...

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coherent units

Overview page. Subjects: Chemistry — Physics.

A system of units of measurement in which derived units are obtained by multiplying or dividing base units without the use of numerical factors. SI units form a coherent system; for example...

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