Metrology - Kilogram faces last significant birthday at 125 [News Briefing]

THE KILOGRAM is the last SI unit still defined by an artefact rather than a physical constant, represented since September 1899 by a 4cm cylinder made of 90 per cent platinum and 10 per cent iridium. Called the International Prototype Kilogram (IPK), it is stored in the basement of the International Bureau of Weights and Measures in S¿¿vres, France. Over the last 125 years, however, comparisons between the IPK and its six official copies have shown their masses slowly diverging - presumed to be due to wear and chemical interaction with the atmosphere - prompting calls to redefine the unit in terms of a fundamental constant of nature.