Redefining the Kilogram is in the Works

The NIST-4 Kibble balance, an electromagnetic weighing machine that is used to measure Planck's constant, and in turn, redefine the kilogram. (Jennifer Lauren Lee / NIST)

Scientists Are About to Redefine the Kilogram and Shake Up Our System of Measures

by Jay Bennett

Locked in a vault that requires three keys to open, in the town of Sèvres just to the southwest of Paris, there is a kilogram. Actually, it’s The Kilogram, the International Prototype of the Kilogram (IPK), the kilogram against which all other kilograms must take their measure, Le Grand K. This cylinder of platinum-iridium alloy sits under three protective glass bells, in a temperature- and humidity-controlled environment, in a safe along with six official copies, in the underground vault of Sèvres.

“If you were to drop it, it would still be a kilogram, but the mass of the whole world would change,” says Stephan Schlamminger, a physicist with the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.

The IPK only emerges from its vault every 40 years or so, when the golf-ball-sized ingot, exactly a kilogram by definition since 1889, is used to calibrate copies that are shared with countries around the world. But there is a problem. In the vault with the IPK are six témoins, or “witnesses”—the official copies. Over the years, as evidenced by the rare occasions when Le Grand K and its witnesses have been measured, the mass of the IPK has “drifted.”

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