Researchers at the German National Metrology Institute in Braunschweig said they will use a 10-centimetre diameter pure silicon sphere to establish a better standard measurement than the existing one.

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Currently, a kilogramme is defined as the mass of a platinum-alloy cylinder that is stored in a heavily guarded vault outside Paris, but which is slowly losing weight and therefore becoming less reliable.

The new silicon sphere is very special indeed and cost two million euros (about R25m) to make. Manufactured over five years in Russia, Australia and Germany it weighs as close as is possible to one kilogramme, is a perfect sphere and is 99.99 percent made out of an isotope of silicon called silicon 28.

How it will be done
Scientists in Braunschweig will now begin the painstaking task of performing thousands of experiments on the sphere to count the number of silicon atoms it is made of.

This will tell then how many silicon atoms there are in a kilogramme, and knowing this will negate the need to have a kilogramme measure in a vault in Paris or elsewhere, says Arnold Nicolaus, project coordinator.

"Afterwards we can destroy the sphere," Nicolaus said. "If we then say that we have done it with the best precision, then we know how many atoms are in a silicon (sphere) that - compared to the mass standard - is closest to one kilogramme.

"Then we know so many atoms are needed for a silicon artefact to make a one kilogramme mass piece."

Finished by end of year
The German team is working in cooperation with a group of Japanese researchers who will work with an identical silicon sphere.

Peter Becker, also involved in the project, said he hoped the German-Japanese team could finish their work by the end of next year because two other international groups are also working on the same problem using different approaches.

He said any eventual adjustment to the kilogramme measure would be so subtle that it would only affect scientists, not bathroom scales. – (Sapa)

April 2008

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