A conventional electrical unit (or conventional
unit where there is no risk of ambiguity)
is a unit of measurement in the field of electricity
which is based on the so-called "conventional
values" of the Josephson constant and the
von Klitzing constant agreed by the International
Committee for Weights and Measures (CIPM)
in 1988. These units are very similar in scale
to their corresponding SI units, but are not
identical because of their different definition.
They are distinguished from the corresponding
SI units by setting the symbol in italic typeface
and adding a subscript "90" – e.g., the
conventional volt has the symbol V90 – as
they came into international use on 1 January
1990.
This system was developed to increase the
precision of measurements: The Josephson and
von Klitzing constants can be realized with
great precision, repeatability and ease. The
conventional electrical units have achieved
acceptance as an international standard and
are commonly used outside of the physics community
in both engineering and industry.
The conventional electrical units are "quasi-natural"
in the sense that they are completely and
exactly defined in terms of the universal
constants e, h, and ΔνCs. They represent
a significant step towards using "natural"
fundamental physics for practical measurement
purposes. However, the conventional electrical
units are unlike other systems of natural
units in that the physical constants used
are not set to unity but rather set to fixed
numerical values that are very close to (but
not precisely the same as) those in the SI
system of units.
Several significant steps have been taken
in the last half century to increase the precision
and utility of measurement units:
In 1967, the thirteenth General Conference
on Weights and Measures (CGPM) defined the
second of atomic time in the International
System of Units as the duration of 9192631770
periods of the radiation corresponding to
the transition between the two hyperfine levels
of the ground state of the cesium-133 atom.
In 1983, the seventeenth CGPM redefined the
metre in terms of the second and the speed
of light, thus fixing the speed of light at
exactly 299792458 m/s.
In 1988, the CIPM recommended adoption of
conventional values for the Josephson constant
as exactly KJ-90 = 483597.9×109 Hz/V and
for the von Klitzing constant as exactly RK-90
= 25812.807 Ω as of 1 January 1990.
In 1991, the eighteenth CGPM noted the conventional
values for the Josephson constant and the
von Klitzing constant.
In 2000, the CIPM approved the use of the
quantum Hall effect, with the value of RK-90
to be used to establish a reference standard
of resistance.
In 2018, the twenty-sixth CGPM resolved to
abrogate the conventional values of the Josephson
and von Klitzing constants with the 2019 redefinition
of SI base units.
== Definition ==
Conventional electrical units are based on
defined values of the caesium-133 hyperfine
transition frequency, Josephson constant and
the von Klitzing constant, the first two which
allow a very precise practical measurement
of time and electromotive force, and the last
which allows a very precise practical measurement
of electrical resistance.
The conventional volt, V90, is the electromotive
force (or electric potential difference) measured
against a Josephson effect standard using
the defined value of the Josephson constant,
KJ-90; that is, by the relation KJ = 483597.9
GHz/V90. See Josephson voltage standard.
The conventional ohm, Ω90, is the electrical
resistance measured against a quantum Hall
effect standard using the defined value of
the von Klitzing constant, RK-90; that is,
by the relation RK = 25812.807 Ω90.
Other conventional electrical units are defined
by the normal relationships between units
paralleling those of SI, as in the conversion
table below.
== Conversion to SI units ==
The 2019 redefinition of SI base units defines
all these units in a way that fixes the numeric
values of KJ, RK and ΔνCs exactly, albeit
with values of the first two that differ slightly
from the conventional values. Consequently,
these conventional units all have known exact
values in terms of the redefined SI units.
Because of this, there is no accuracy benefit
from maintaining the conventional values.
== Comparison with natural units ==
Conventional electrical units can be thought
of as a scaled version of a system of natural
units defined as
c
=
e
=
ℏ
=
1.
{\displaystyle c=e=\hbar =1.}
This is a more general (or less specific)
version of either the particle physics "natural
units" or the quantum chromodynamical system
of units but without fixing unit mass.
The following table provides a comparison
of conventional electrical units with other
natural unit systems:
== See also ==
Centimetre–gram–second system of units
ITS-90
