Magazine article Science News

A Step Closer to an Atomic-Based Kilogram?

Magazine article Science News

A Step Closer to an Atomic-Based Kilogram?

Article excerpt

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. …

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