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Kilogram Sphere

July 16, 2007

Scientists are setting a new standard for weight.


A weighty feat of engineering. I’m Bob Hirshon and this is Science Update.

For over a century, a lump of metal in Paris has served as the international standard for the kilogram. But the metal’s true weight has fluctuated slightly over time. Now, scientists are finally making a more precise standard. Optical physicist Katie Green and her colleagues at Australia’s national science agency will create a ball of highly stable silicon.

We know a lot about silicon, and the atoms, and the structure of the material. So we can count how many atoms are in the ball, and basically define the kilogram in terms of a number of atoms.

To make the math work, the sphere must be flawlessly round. So Green’s team will spend the rest of the year smoothing and polishing, using the latest imaging technology to spot every microscopic bump. I’m Bob Hirshon, for AAAS, the science society.