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Chemists' constant hangs in the balance New silicon tally may change Avogadro's number. 22 September 2003 JOANNE BAKER Chemists may be forced to change their value for one of nature's fundamental numbers - Avogadro's constant - following more accurate measurements made using a crystal of pure silicon1. The constant - named after the nineteeth-century Italian physicist Amedeo Avogadro - is used to measure mass based on a fixed number of atoms. It is equal to the number of atoms in 12 grams of carbon-12, the most common form of carbon: at 6.022 x1023, this figure is approaching a million billion billion atoms. If it could be defined more accurately, then a kilogram, the standard unit of mass, could be defined in terms of atoms. At the moment, a kilogram is defined by a standard lump of metal held in a laboratory near Paris, France. Other International Standard (SI) units are now defined in terms of absolute atomic quantities. Examples include the second, which is measured by cycles of radiation emitted by a caesium atom, and the metre, defined as the distance travelled by light in a certain fraction of a second. To get an atomic fix on Avogadro's constant, Peter Becker of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, and colleagues counted atoms in bulk. By smashing X-rays into a silicon cylinder 1.66 metres long, 10 centimetres in diameter and weighing exactly 1 kilogram, the researchers calculated the exact arrangement, and therefore a good approximation of the number, of atoms in the cylinder. The new measurement of Avogadro's number is as accurate as the best previous tally, to one part in ten million - so it won't replace the Paris kilogram yet. But the actual number obtained by the German team disagrees substantially with the old one - by one part in a million. "There could be an error in each of these measurements," says Becker. "But our data have consistency, and there are hints that favour our value." The silicon experiments have been repeated several times, each time giving the same answer. Constant craving Two sets of super-sensitive weighing scales, called a Watt balance, held at national-standards laboratories in the United States and Britain, officially define the existing Avogadro number. But there could be a problem. "The anomaly in the data could be in the original American Watt-balance experiment," says Stuart Davidson, head of mass and density standards at the National Physical Laboratory in Teddington, UK. "We are getting new values with our balance in the UK that agree with the German silicon results." Chemists will have to wait until next year's annual meeting of the International Committee of Weights and Measures (CIPM) to argue for or against the new value for Avogadro's constant. "With so many countries involved, it's not only a scientific problem, it's also cultural," quips Becker. To replace the Paris kilogram, Becker's team will need an accuracy of 1 part in 100 million. Until then, it remains the only basic measuring unit still defined by a unique artefact - a cylinder of platinum and iridium kept at the International Bureau of Weights and Measures (BIPM) near Paris. Nearly 100 copies are stored worldwide, and must be sent to Paris every few years for verification. Joanne Baker is a British Association Media Fellow References Becker, P. et al. Determination of the Avogadro constant via the silicon route. Metrologia, 40, 271 - 287, doi:10.1088/0026-1394/40/5/010 (2003). |Article| � Nature News Service / Macmillan Magazines Ltd 2003
