The first standards for the meter and kilogram were deposited in the National Archives of France in 1799. The kilogram is the only unit in the International System of Units still defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories.
The kilogram is the base unit of mass in the International System of Units (SI). At present, the unit is defined as the mass of the international prototype of the kilogram–a plum-sized cylindrical artifact made from an alloy of platinum and iridium by Johnson Matthey in 1885. The prototype is kept in a safe, with six official copies, in a vault at the International Bureau of Weights & Measures (BIPM) at Sèvres, on the outskirts of Paris. It was designated as the unit of mass in the metric system in 1889 and has been weighed against its copies three times in the past 100 years–in 1890, 1948, and 1992.
A new definition of the kilogram is required, an international team of scientists contends. The current one is imprecise, they say, because it is not linked to an unvarying property of nature. The imprecision injects uncertainties in measurements and physical constants used in calculations and calibrations. They urge a redefinition based on a specific value of either the Planck constant or the Avogadro constant.
A new definition of the kilogram fixed to an invariant of nature would, unlike the Paris prototype, be available to anyone at any time and in any place. The group proposes that the kilogram should be redefined so as to fix its value for all time to a specific value of either the Planck constant or the Avogadro constant, both of which are invariants of nature. The uncertainties of many of the fundamental constants would then immediately be reduced by more than a factor of 10, Mills says.
Redefining the kilogram in this way could have immediate benefits, Quinn says. “For instance, it would improve the precision of certain electrical measurements 50-fold and would enable physicists to make more precise calculations when studying the fundamental quantum properties of matter.”
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Excerpted with permission, Chemical & Engineering News
Copyright © 2005 American Chemical Society