A Concise Summary Of The International System Of Units

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A concise summary
of the International System of Units,
SI
the
Metrology is the science of measurement, embracing all measurements,
made at a known level of uncertainty, in any field of human activity.
Table 1 The seven base units of the SI
The Bureau International des Poids et Mesures,
the BIPM, was established by Article 1 of the
Convention du Mètre, on 20 May 1875, and is
Quantity
charged with providing the basis for a single,
coherent system of measurements to be used
Unit, symbol:
definition of unit
throughout the world. The decimal metric system,
length
dating from the time of the French Revolution,
metre, m:
The metre is the length of the path travelled by light in vacuum
was based on the metre and the kilogram. Under
during a time interval of 1/299 792 458 of a second.
the terms of the 1875 Convention, new inter-
It follows that the speed of light in vacuum, c
, is 299 792 458 m/s exactly.
national prototypes of the metre and kilogram were
0
mass
made and formally adopted by the first Conférence
Générale des Poids et Mesures (CGPM) in 1889.
kilogram, kg:
The kilogram is the unit of mass; it is equal to the mass
Over time this system developed, so that it now
of the international prototype of the kilogram.
includes seven base units. In 1960 it was decided
It follows that the mass of the international prototype of the kilogram,
m(K ), is always 1 kg exactly.
at the 11th CGPM that it should be called the
Système International d’Unités, the SI (in English:
time
the International System of Units). The SI is not
second, s:
The second is the duration of 9 192 631 770 periods of
static but evolves to match the world’s increas-
the radiation corresponding to the transition between the two hyperfine
ingly demanding requirements for measure-
levels of the ground state of the caesium 133 atom.
ments at all levels of precision and in all areas of
It follows that the hyperfine splitting in the ground state of
science, technology, and human endeavour. This
the caesium 133 atom, v(hfs Cs), is 9 192 631 770 Hz exactly.
document is a summary of the SI Brochure, a
electric current
publication of the BIPM which is a statement of
ampere, A:
The ampere is that constant current which, if maintained
the current status of the SI.
in two straight parallel conductors of infinite length, of negligible circular
cross-section, and placed 1 metre apart in vacuum, would produce between
The seven base units of the SI, listed in Table 1,
these conductors a force equal to 2 × 10
–7
newton per metre of length.
provide the reference used to define all the meas-
It follows that the magnetic constant, µ
, also known as the permeability
urement units of the International System. As
0
of free space is 4 × 10
H/m exactly.
−7
science advances, and methods of measurement
thermodynamic temperature
are refined, their definitions have to be revised.
kelvin, K:
The kelvin, unit of thermodynamic temperature, is the fraction
The more accurate the measurements, the greater
1/273.16 of the thermodynamic temperature of the triple point of water.
the care required in the realization of the units of
It follows that the thermodynamic temperature of the triple point of water,
measurement.
, is 273.16 K exactly.
T
tpw
amount of substance
mole, mol:
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.
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.
It follows that the molar mass of carbon 12, M(
12
C), is 12 g/mol exactly.
luminous intensity
candela, cd:
The candela is the luminous intensity, in a given direction,
of a source that emits monochromatic radiation of frequency
540 × 10
hertz and that has a radiant intensity in that direction
12
of 1/683 watt per steradian.
It follows that the spectral luminous efficacy, K, for monochromatic
The international prototype of the kilogram, K , the only
radiation of frequency 540 × 10
Hz is 683 lm/W exactly.
12
remaining artefact used to define a base unit of the SI.

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