ME TRO LO G Y
Alas, poor kilogram!
Your 50-parts-in-a-billion fluctuations will not be missed
Photo credit: NIST
Advanced measuring instruments like the NIST-4 Kibble balance have enabled a redefinition of SI units like the kilogram. The quadrennial General Conference on
the copies and original. The physical
of the triple point of water, but it s not
Weights and Measures (GCWM) doesn’t usually
substance of the IPK was also subject to
possible to prepare two exactly identical
make the evening news, but it got a mention
natural degrading processes, resulting in
formulations of the isotopic mixture
on Friday 16 November when it redefined the
extremely small fluctuations in weight, as
of water required for this test. New
definition of four commonly used units of
small as 50 parts in a billion. While these are
advances in acoustic thermometry
measurement, including the kilogram. Also
not significant fluctuations if you’re weighing
have resulted in extremely accurate
affected were the ampere, kelvin and mole, so
meat at the butcher’s, they become extremely
measurements of the Boltzmann
that altogether four of the seven measurement
important if you are, for example, conducting
units that comprise the International System
experiments at a subatomic level.
of Units (SI) were changed. Instead of linking the SI units to physical
constant. jjThe ampere will now be measured
Fundamental forces of nature remain
according to the elementary electrical
consistent and can be measured far more
charge. Where the past definition relied
artefacts, the revision ties their values
precisely, hence the GCWM’s decision
on the charge in newtons between
to fundamental physical constants that
to adopt the following as the constants
two wires containing 1 ampere each
allow for far more precise measurements.
underlying SI measurements:
and separated by a distance of 1 metre,
Since 1889, the kilogram, for example, was
jjThe kilogram will be linked to Planck’s
the new definition is refreshingly
linked to the weight of a platinum/iridium
constant. The Planck constant relates a
straightforward: 1 ampere equals
compound cylinder referred to as the
light particle s energy, and hence mass, to
International Prototype of the Kilogram (IPK),
its frequency. New measuring technology
colloquially known as Le Grande K. While the
like NIST-4 Kibble balance can measure
constant. In the present metric system,
original was carefully housed in a sealed
the constant with incredible accuracy.
a mole is the amount of substance that
glass container in a suburb of Paris, copies
1 coulomb per second. jjThe mole will be linked to the Avogadro
jjThe kelvin will be linked to the
contains as many elementary entities
of the IPK were distributed to measurement
Boltzmann constant. This constant
as there are atoms in 0.012 kilograms
associations around the world.
relates temperature to energy by means
of carbon-12. The Avogadro constant is
of statistical mechanics. Previously,
expressed as the number of molecules in
the kelvin was measured as a fraction
one gram-molecule of oxygen. M
Eventually it was discovered that there were miniscule variations in weight between
The new defining constants of the International System of Units Defining constant Hyperfine transition frequency of Cs
8
LMS Issue 6 | 2018
Symbol
Numerical value
Unit
∆vCs
9 192 631 770
Hz
Speed of light in a vacuum
c
299 792 458
m s-1
Planck constant
h
6.626 070 15 x 10-34
Elementary charge
e
1.602 176 634 x 10
Boltzmann constant
k
1.380 649 x 10-23
J K-1
Avogadro constant
NA
6.022 140 76 x 1023
mol-1
Luminous efficacy
Kcd
683
lm W-1
-19
J Hz-1 C
w w w.lm s m a g a z i n e .c o. z a
