French News Online - Features Section

The Dieting Kilogram Fuels A Silicon Mission

France invented the kilo, now it is losing weight!
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 France’s 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 comprehend.

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 kilo’s Holy Grail is complex and even divisive.

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 been grinding 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 US’s 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 year’s 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 UK)!

However NIST’s 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 … it’s 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. It’s a scientific proposal … our article states things in international terms; I welcome all to read it.”

You can do so here where a release on NIST’s 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 anomaly.

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 — Avogadro’s or Planck’s. The former, says France’s 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 packets”.

A team at Université de Versailles Saint-Quentin-en-Yvelines in association with l’Institut 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.

Did You Know?

Further Information:

The Seven Artifacts in the weights and measures family
Background to France’s role in developing international weights and measures
The first standard metric system
Bureau of Weights and Measures (BIPM)
The International System of Units (SI)

Story: Ken Pottinger
editorial@french-news-online.com