Le Grand K could be facing retirement at last. For more than a decade, metro- logists have sought to replace the leg- endary standard for the kilogram—a 128-year-old slug of platinum iridium alloy in a Paris vault—with one based
on an immutable constant of nature. This
week in Paris, teams from several countries
presented (almost) final measurements of the
Planck constant, the key to the new standard.
If those numbers agree, the General Conference on Weights and Measures (CGPM) could
redefine the kilogram next year.
“I think it’s highly likely” that the redefinition will proceed, says Barry Wood, a physicist at Canada’s National Research Council
(NRC) in Ottawa. By 1 July, the final numbers
must be accepted for publication.
Metrologists have already redefined another key unit in the International System
of Units (SI), the meter. In 1889, the international community pegged the meter to the
length of a platinum iridium bar. But in 1983,
CGPM defined the speed of light as exactly
299,792,458 meters per second, allowing the
meter to be redefined as the distance light
travels in 1/299,792,458 of a second.
Scientists hope to do something similar
with the kilogram. Because even carefully
tended metal can pick up or shed atoms, the
current definition of the unit drifts over time.
Le Grand K, the last physical artifact used
as a standard, has been taken from its vault
at the International Bureau of Weights and
Measures (BIPM) in Paris only four times, to
calibrate other weights used to “propagate”
the value of the kilogram around the world.
When those comparisons were last made in
2014, most national standards had to be re-
vised downward by about 35 micrograms.
Researchers aim to redefine the kilogram
with an electrical device called a Kibble balance, invented in 1975 by Bryan Kibble, a
physicist at the United Kingdom’s National
Physical Laboratory (NPL) in Teddington,
who died last year. Originally intended to
measure the unit of electrical current, the
ampere, a Kibble balance holds a weight on
one side and has an electric coil in a magnetic
field on the other. When current runs through
the coil, it produces a force that balances the
weight. Adjust the current just right and you
can define the kilogram in terms of a current.
That may not seem much of an improvement, as the kilogram-strength current depends on the specifics of the balance: the
magnetic field strength and the length of
the wire in the coil. But Kibble found a way
around that problem. Moving the coil up and
down in the magnetic field induces a voltage
across the coil that depends on the field and
the wire length in the same way. Kibble realized he could come up with a single equation in which the weight equals the current
in the balance’s “measuring mode” times the
voltage in the “moving mode.” The specifics
of the device drop out, so that any well-made
Kibble balance should give the same result.
The Planck constant, the fundamental
constant of quantum mechanics, enters the
fray through the electrical measurements.
For example, to measure voltages precisely,
physicists use devices called Josephson junctions: rings of superconducting metal, each
crimped at a point with a small, nonsuper-conducting barrier. When radio waves shine
on the ring, a voltage develops across the
barrier that equals the frequency of the radio waves multiplied by the Planck constant.
That enables researchers to measure voltages
in terms of the constant.
It all yields a simple equation that does for
mass what the speed of light did for measurements of length and distance. “If you work
it all out, you basically end up with Planck’s
constant and a couple of frequencies” on
one side and mass on the other, says Ian
Robinson, a physicist at NPL. So knowing the
Current Coil Kg
A balance of forces
In a Kibble balance, the magnetic force on a current-carrying coil offsets the force of gravity on a weight.
By Adrian Cho
Plot to redefine the kilogram nears climax
Metrologists present data needed to switch to standard based on constant of nature