A step closer to an atomic-based kilogram?

Among global standards for length, mass, time, and other fundamental quantities, only the kilogram remains a physical object--a carefully machined cylinder of platinum-iridium alloy at the International Bureau of Weights and Measures at Sevres, France.

Because the cylinder collects dust and in other ways slightly changes mass with age--and might be lost or stolen--the stewards of the world's measurement system are eagerly seeking a new standard based on a carbon atom's mass or other fixed quantity of the atomic world (SN: 9/24/94, p. 199; 1/28/95, p. 63).

Toward that goal, researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., report that by weighing the kilogram against an electromagnetic force, they have measured a fundamental physical quantity, known as the Planck constant, with twice the precision of previous experiments. The Planck constant serves numerous roles in quantum mechanics, including setting limits on how much can be simultaneously known about a particle's momentum and position.

Although the new measurement falls short by a factor of 10 of the accuracy needed to redefine the kilogram, it takes an important step in that direction, metrologists say. "We make a connection between the atomic world and the macroscopic world for the property of mass," says Edwin R. Williams, leader of the NIST team. The researchers linked those worlds by balancing an exact copy of the French kilogram against a force already defined by atomic-scale quantities. They used a balance so delicate that it is housed in its own building and operated outside regular office hours to avoid vibrations.

Reporting in the Sept. 21 PHYSICAL REVIEW LETTERS, the NIST group gives a new value for the Planck constant of 6.62606891 X [10.sup.-34] Joule-seconds, with an uncertainty of 87 parts per billion, less than half the uncertainty of prior measurements. The improvement in the Planck constant--the first in a decade--will also benefit other fundamental scientific constants, whose official values will soon be revised, Williams says. The Planck constant underlies a half-dozen of them and could serve as the basis for redefining the kilogram in terms of absolute physical quantities.

The NIST measurement is "of great importance to basic physics and metrology," says Erich Braun of the German standards institute, the Physikalisch-Technische Bundesanstalt in Braunschweig. A team there is trying to redefine the kilogram by counting the number of atoms in a kilogram of silicon.

So far, their precision cannot match either NIST or the National Physical Laboratory in Teddington, England, where the balance method originated. Improvements already installed at the English balance and expected next year at NIST promise to improve precision to the point that the atomic kilogram could become a reality.