Building a better kilogram

Wired reports: [edited]

Has the kilogram gone on a diet? Maybe. For some reason, the official kilo — a 118-year-old lump of metal stored in a vault at the International Bureau of Weights and Measures outside Paris — has slimmed down by as much as 50 micrograms in the past century.

The solution? Build a better kilogram. Researchers at the Australian Commonwealth Scientific and Industrial Research Organization are cutting, grinding, and polishing a boule — a big crystal — of ultrapure silicon 28 into two baseball-sized spheres (one is for double-checking).

Materials scientists are able to measure precisely how many atoms of that silicon isotope are in any given hunk (the completed orb should contain 215 x 1023 atoms).

Creating "the roundest object in the world," says CSIRO engineer Katie Green, means technicians have to worry about only one dimension: diameter. Once finished, it will weigh a perfect kilogram. Give or take an atom.
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