Dust is affecting this 1889 bar of 90%
platinum and 10% iridium
that currently provides the definition of the kilo
If you’re a weight watcher here’s the good
news, those stubborn kilos on your scales are themselves losing
weight so perhaps you won't need to bother.
France’s Kilogram Artifact is Losing Weight
Specks of dust are the culprit and these are affecting the weight
of the kilo as it is presently defined against an artifact. So for the past 25 years
scientists have been on a mission to find another definition.
The dust in question settles on a slowly deteriorating bar of
90% platinum and 10% iridium stored under glass since 1889 in
a vault at Frances
BIPM-Bureau International des Poids et Mesures. This means
the artifact gains weight and loses it again after a steam bath
and some polishing. But it (and the 80 copies created at the same
time and distributed to signatories of the Metric Treaty) also
appear to be gaining weight at a rate of about 50 micrograms a
century and that phenomenon is unrelated to dust.
Scientists are thus on a quest for a redefinition of the kilogram
which will see it referenced to a constant rather than to the
present artifact (the 1889 bar and its copies some of which says
the New York Times: "were issued to countries that later vanished,
including Serbia and the Dutch East Indies. The Japanese had to
surrender theirs after World War II. Germany has acquired several
including one issued to Bavaria in 1889 and the one that belonged
to East Germany)
Remember trying to get your weary head round the primary school
science drill that the definition of a kilogram was equal
to the mass of its prototype locked away under glass since 1889
in a vault at the International Bureau of Weights and Measures
at Sevres? Well thank your lucky stripes because now the
definition is about to become much more complicated.
Teams of scientists in Paris, Berlin, Washington, Sydney and
elsewhere are hard at work hunting for a new global standard for
the kilogram that will ditch the quaint and easily remembered
glass vault, for a phrase only quantum physicists will really
The issue is even more significant because the current kilo yardstick
also affects the definition of some of the other seven base units
of measure as defined by the International System of Units (SI).
These include the ampere (electric current), mole (amount of substance)
and candela (luminous intensity).
This search for the kilos Holy Grail is complex and even
In one corner is a team set on changing the definition
of the kilogram by referencing it to the mass of an almost perfect
silicon sphere, which can then be translated into abstract mathematical
terms. This team of Sydney scientists and engineers, is backed
by France present keepers of the kilogram and has
and polishing two silvery balls, each weighing precisely one kilogram,
to a level of imperfection of less than 35 millionths of a millimetre.
Using a single crystal of silicon-28 grown by Russian and German
scientists over three years, the objects aim to be not only near-perfect
in roundness, but also in crystal purity and the atomic species.
Because they are made from a stable element, they will not be
affected by the moisture, corrosion and contamination now plaguing
the current kilogram standard. The spheres will be measured for
volume in Australia, Germany and Japan, and then measured for
mass. Belgian scientists will then check the molar mass of the
crystal used to calculate the number of molecules in each sphere.
This video offers a fascinating account:
In the other corner stands the USs National
Institute of Standards and Technology (NIST) which has been widely
misreported as preparing a challenge in the shape
of a formal resolution at next years General Conference
on Weights and Measures over the inconstant kilo. (The US of course
continues to run with an even less dependable Imperial system
of weights and measures shared to a great extent with the
However NISTs Ben Stein has hastened to clarify the widely
published mis-reporting on the Internet, and points out: Earlier
news reports picked up (incorrectly) on our article about the
proposal its a resolution, not a complaint, introduced
by the BIPM-CCU and passed by the CIPM, with international membership
This is not a US proposal, complaint, or concern, but an
international proposal advanced by a group that includes Australia,
France, Mexico and many others. Its a scientific proposal
our article states things in international terms; I welcome
all to read it.
do so here where a release on NISTs website says: The
National Institute of Standards and Technology (NIST) is participating
in a worldwide effort to recommend major revisions to the International
System of Units (SI), the modern metric system that is the basis
of global measurements in commerce, science and other aspects
of everyday life. The new SI, which would be based on seven constants
of nature, would enable researchers around the world to express
the results of measurements at new levels of consistency and accuracy.
In support of a proposal to revise the
International System of Units (SI)
so that it would be based on constants of nature,
NIST efforts such as the watt balance experiment (pictured)
offer new ways of determining an accurate value of the Planck
constanth, thereby contributing to a definition of the kilogram
based on physical constants. Photo Credit Steiner, NIST
So while everyone agrees the definition needs changing
the science world is still uncertain about how to redefine it.
Led by the French standards authority teams of scientists hope
soon to redefine the kilo abstractly and so end the current artifact
As one French report put it: Over the past century, despite
precautions taken to counter the tendency of the metal to leak
a few atoms or to attract dust, a gap of 50 micrograms has widened
between it and its copies lodged around the world. This makes
it vital to establish a real definition for the kilo.
On its website the French standards authority says a twin track
approach is underway. One aims to produce a one kilogram silicon
sphere, and by measuring its volume, to define the kilogram as
the mass of a certain number of atoms. The other is based on the
electromagnetic force generated by passing current through a coil.
Comparing this to the weight of a body would link the kilogram
to a fundamental constant of physics. But the necessary precision
has not yet been attained, and a redefinition of the kilogram
is now unlikely before 2015.
In fact the process is laborious and work has been underway for
some 25 years to switch the definition from a physical model or
artifact to a constant.
Among proposals for a redefinition are: fixing the values to
one of two universal constants Avogadros or Plancks.
The former, says Frances metrology institute, measures
the amount of carbon-12 atoms in 0.012 kg of that element, the
latter explains the sizes of quanta or tiny electromagnetic energy
A team at Université de Versailles Saint-Quentin-en-Yvelines
in association with lInstitut National de Métrologie
says that experiments over the past 100 years have shown that
the standard kilogram drift is of the order of 5 micrograms
per year. The university is part of a project for redefining
the kilogram using a scale called the watt balance
that equalizes mechanical power (MGV) to power supply (ie). The
redefinition was initially due to be approved at the Conférence
Générale des Poids et Mesures (CGPM) the General
Conference on Weights and Measures in 2011 but this now appears
to be delayed.
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