In a landmark decision,
the BIPM's Member States voted on 16 November 2018 to revise the International System of Units (SI), changing the world's definition of the kilogram, the ampère, the kelvin and the mole.
The decision means that all SI units will
now be defined in terms of constants that describe the natural world.
This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.
In the revised SI four of the SI base units
- namely the kilogram, the ampère, the kelvin and the mole – are redefined in terms of constants; the new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively.
Further, the new definitions of all seven base units of the SI are also uniformly expressed using the explicit-constant formulation, and specific mises en pratique will be drawn up to explain the realization of the definitions of each of the base units in a practical way.
The new definitions will come into force on 20 May 2019.
The revised definitions will be based on seven physical constants
the speed of light,
the Avogadro constant)
and are therefore inherently stable. The quantities have
been chosen so that the revised definitions will not need to be modified to accommodate future
improvements in the technologies used to realize them.
will be defined in terms of the Planck constant, guaranteeing long
-term stability of
the SI mass scale. The kilogram can then be realized by any suitable method, (
Kibble (watt) balance or the Avogadro (X
-ray crystal density) method).
and other electrical units,
as practically realized at the highest metrological level,
>will become fully consistent with the definitions of these units. The transition from the 1990
convention to the revised SI will result in small changes to
ated electrical units.
the vast majority of measurement users
, no action need be taken as the volt will change by
about 0.1 parts per million and the ohm will change by even less.
will be redefined with no immediate effect on temperature measurement practice or on
the traceability of temperature measurements, and for most users, it will pass unnoticed.
will be redefined with respect to a specified number of entities (typically atoms or
molecules) and will
depend on the unit of mass, the kilogram. Traceability to the mole
can still be established vi
a all previously employed approaches including, but not limited to, the
use of mass measurements
along with tables of atomic weights and the molar mass constant
will continue to be defined in terms of the hyperfine transition frequency of the
caesium 133 atom. The traceability chain to the second will not be affected. Time and frequency
metrology will not be impacted.
- The metre
in the revised SI will continue to be defined in terms of the speed of light, one of the fundamental constants of physics.
will continue to be defined in terms of cd,
a technical constant for photometry and
will therefore continue to be linked to the watt.