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Definitions
of the SI / Metric Base Units
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Unit of length
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meter
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The meter
is the length of the path travelled
by light in vacuum during a time interval
of
1/299,792,458 of a second. |
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Unit of mass
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kilogram
|
The kilogram
is the unit of mass; it is equal to
the mass of the international prototype
of the kilogram. |
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Unit of time
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second
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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 cesium
133 atom. |
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Unit of electric
current
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ampere
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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 meter apart
in vacuum, would produce between these
conductors a force equal to 2 x 10-7
newton per meter of length. |
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Unit of thermodynamic
temperature
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kelvin
|
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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Unit of amount
of substance
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mole
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1. 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; its symbol is "mol."
2. 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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Unit of luminous
intensity
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candela
|
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. |
