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By prospect   March 2005

The ever-shrinking kiloThe kilogram is losing weight. Despite the fact that it never leaves a secure safe in Paris, the 90 per cent platinum and 10 per cent iridium cylinder created in 1889 as the standard definition of a kilogram has lost around 50 micrograms since being cast. No one knows why, but more importantly, no one can come up with a better way of defining the kilo, the only one of physics’s seven base units to be defined in reference to a material artefact rather than a fundamental constant (like the speed of light, used to define the second).…

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