Bearing down on the kilogram standard.

Since 1889, a single platinum-iridium bar has lain sealed in an airtight bell jar in the International Bureau of Weights and Measures in Sevres, France.

Nicknamed "Le Grand K," this bar constitutes the one and only true kilogram.

Of all the standard international units of measure, the kilogram remains the only one whose definition relies on a physical artifact. All other units - of time, length, or electric charge - have their definitions rooted in constants of nature, such as the speed of light or atomic vibrations.

As part of an international effort, researchers at the National Institute of Standards and Technology (NIST) want to redefine the kilogram in a way that will make the standard absolute, unchanging, and accessible to anyone, anywhere - liberating Le Grand K from its heavy burden as standard-bearer.

"One problem is that the current standard tends to drift a little bit," says Batty N. Taylor, a physicist at NIST. "The kilogram has varied by as much as .05 parts per million in the last 100 years." The causes of that variance remain unknown, though Taylor believes that "outgassing, absorption, or just dirt accumulation and cleaning" may be responsible.

The platinum-iridium bar presents other disadvantages. It is inaccessible to researchers, can be reproduced only with difficulty, and could be damaged or destroyed.

To remedy these problems, researchers want to define the kilogram as a function of the Avogadro constant, which measures the number of molecules (6.023 x [10.sup.23]) present in a gas occupying 22.41 liters at fixed temperature and pressure. Currently, Avogadro's number is rooted in the exact number of atoms present in 12 grams of the isotope carbon-12.

"By definition, the Avogadro number relates macroscopic masses to atomic measurements," Taylor says. "That makes it appealing as a basis for defining the kilogram."

But creating a reliable, accurate, easily reproducible standard has proved trickier than expected. An apparatus must consistently reproduce a kilogram with an uncertainty approaching one-billionth.

According to Taylor, scientists worldwide are exploring five possible kilogram definitions. Currently, two methods lead in accuracy. The first, called the moving coil watt balance method, relates electrical energy to mechanical power at the quantum level. Invented by B. P. Kibble at England's National Physical Laboratory, this method offers a precise value of the Planck constant, from which one derives Avogadro's number.

A second approach, the X-ray crystal density method, relies on mass and density measurements of silicon atoms in a pure crystal. Researchers in Germany, Japan, Belgium, and the United States are refining the accuracy of this technique, whose uncertainty hovers near one-millionth.

In Japan and Russia, scientists are levitating masses with superconductors; in Germany, experimenters are debugging a vacuum Faraday system, whereby gold atoms beamed onto a collector yield an electric constant, from which an Avogadro number can be derived. A final approach, the volt balance method, has scientists in France, Australia, and Yugoslavia measuring minute differences in electric potential as a way to generate an Avogadro number indirectly.

Still, official adoption of a new standard lingers on the horizon. "With a lot of luck," says Taylor, "we might see a change within the decade."