NIST SCIENCE BEAT - Physics Laboratory Articles
January 1995

Replace Kilogram Standard? NIST Weighs Options

For 106 years, a single precious platinum-iridium cylinder in Sèvres, France, has served reliably as the kilogram standard. But using a mass standard to calibrate other mass standards has its disadvantages. Le Gran K, as the kilogram standard is sometimes called, is pretty imprecise, when compared to other standards in the International System of Units. It could potentially be damaged or even destroyed. Its mass changes slightly with stray dust particles and when it is cleaned. And it is available in only one laboratory. For some time, scientists have considered replacing the kilogram mass standard with a natural constant. Other base units are defined this way. The meter, for example, is defined in terms of the speed of light. Scientists at the National Institute of Standards and Technology are exploring two possible ways of replacing the kilogram mass standard. Both methods would base the kilogram on the Avogadro constant, which is defined as the number of atoms in 12 grams of pure carbon-12. One method would directly determine Avogadro's number based in part on X-ray measurements in near perfect silicon crystals. Another method would determine Avogadro's number based in part on electrical measurements in a watt-balance experiment. Either method would tie the kilogram to an invariable natural constant and would be accessible to researchers worldwide.

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