The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent:
| metre, m |
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The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.
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| kilogram, kg |
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The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
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| second, s |
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The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
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| ampere, A |
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The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.
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| kelvin, K |
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The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
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| mole, mol |
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- The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
- When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
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| candela, cd |
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The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
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