SECURELY tucked away inside a French vault is a lump of metal known as the International Prototype. A mixture of platinum and iridium, it was made in the 1880s to define the mass of a kilogram.
But work by a team of Australians could help pave the way for the retirement of this century- old prototype, as weight and measurement experts across the globe work towards a more scientific definition of the kilogram.
The project requires the development of perfect silicon spheres, and optical engineers at CSIRO's Australian Centre for Precision Optics — considered world leaders in the craft — are doing their part.
Scientists will use the spheres to determine how many silicon atoms make up a kilogram, and this will be used as the new definition — bringing the kilogram into line with other base units such as the metre and the second, which are all defined by physical constants.
"It's really an atom-counting exercise … and we'll come up with a new definition of the kilogram based on atoms, rather than based on the thing in Paris," explained Walter Giardini, of Australia's National Measurement Institute.
CSIRO's optical engineers will form two perfect spheres from a 20-centimetre cylinder of exceptionally pure silicon that arrived in Australia last night. The silicon, which has taken three years to produce, was made in Russia and grown into a near-perfect crystal in Germany.
The precision optics centre, located in the Sydney suburb of Lindfield, has already made about a dozen spheres for what is known as the Avogadro Project — with the most perfect sphere so far just 35 nanometres away from being perfectly round.
This means the diameter of the sphere varied by an average of only 35 millionths of a millimetre, making it a top contender for the title of the roundest object in the world.
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