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Atom counting helps kilogram watch its weight

At a conference at the Royal Society in London this week, researchers discussed ways to replace the ageing chunk of metal that serves as the world's standard kilogram with a kilogram based on a fundamental constant.

How do we measure the kilogram now?

The kilogram is the last unit to use a physical artifact as its standard. Called the international prototype kilogram, the standard is a platinum-iridium alloy cylinder that has been kept in a safe at the headquarters of the International Bureau of Weights and Measures (BIPM) in Sèvres, near Paris, since 1889.

Why not keep using that prototype?

Its mass is changing, and scientists don't know why. Its mass now varies by as much as 50 micrograms from the "national copies" that have been distributed to metrology labs worldwide.

The mass change may result from the gradual loss of hydrogen that was blasted into the alloy when the cylinder was forged, or perhaps from the use of coarse emery paper and cleaning fluids to scrub it in the past, says Terry Quinn, former head of the BIPM.

"Is the prototype kilogram losing mass? Or are the national copies getting heavier? The answer is that we just don't know – there is nothing more stable to compare it with. So we need something better," says BIPM physicist Michael Stock.

What approach to metrologists want to take instead?

In October, the General Conference on Weights and Measures in Paris is expected to begin the process of changing the definition of the kilogram to one based on fundamental constants like Avogadro's constant, the number of atoms in a mole, and the Planck constant, which relates the energy of a photon or particle to its frequency. If everyone can agree on the technologies to do this, the redefinition process should be completed by 2015.

Other units have already been redefined in similar ways. The metre was once defined as the length of a slice of the Earth's circumference, but now it is defined as the distance light travels in a certain period of time. And the second, once defined as a fraction of a day, is now defined as the duration of a precise number of transitions in a caesium atom.

How will the kilogram be defined in future?

There are two rival ways vying for prominence. One method defines the kilogram as the mass of a certain number of silicon-28 atoms. The second uses a mass-measurement device called a "Watt balance" to define the kilogram in terms of voltage and current.

How can anyone count silicon atoms?

It's all revealed in this week's edition of the journal Physical Review Letters. A team led by metrologist Peter Becker of the Federal Institute of Physical and Technical Affairs in Braunschweig, Germany, reveals a breakthrough in an attempt to measure the number of atoms in a silicon sphere, which has let them compute Avogadro's constant to unprecedented accuracy.

Previously, they used lasers to scan a pure silicon sphere, thereby measuring its volume. Then they used X-ray diffraction to estimate the volume each atom occupies. That should have led to an accurate count of the number of atoms in the sphere, but it didn't because the silicon-28-based crystal contained a high number of silicon-29 and silicon-30 isotopes.

Using a technique called isotope dilution mass spectrometry, Becker and colleagues have now made a sphere in which the unwanted Si-29 and Si-30 isotopes were almost completely depleted. As a result, they have calculated Avogadro's constant with an uncertainty of 3.0 x 10-8.

Becker told New Scientist this week that if they can refine their technique a little further, dropping the measurement uncertainty to 2.0 x 10-8, they reckon they will be ready to stake a claim to a method that is good enough to define the kilogram as the mass of a set number of silicon-28 atoms.

What about the Watt balance method?

A Watt balance is essentially a scale with the prototypical kilogram on one side and an electromagnetic field pulling on the other. Ultrasensitive electronics, such as Josephson junctions, could be used to stably and reproducibly calculate the current and voltage required to balance the kilogram. The method would define the kilogram in relation to the Planck constant.

Metrology institutes around the world are trying to perfect the Watt balance method, but each experiment has its own levels of uncertainty that must be overcome. Canada's National Research Council, for instance, has just spotted a systematic error in a UK-supplied balance that it needs to fix.

Does that mean there's still a lot of arguing to come before the prototype kilogram is replaced?

Yes. Terry Quinn, former head of the International Bureau of Weights and Measures, expects the "temperature to rise quite a bit" as the runners and riders jockey for position.


The current definition of a kilogram (Image: BIPM)

The current definition of a kilogram (Image: BIPM)

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