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Definition of Kilogram to Change

The unit of mass will no longer be defined by a lone physical object but instead in terms of a universal constant.

Nov 9, 2018
Ashley P. Taylor

ABOVE: A copy of the cylinder of platinum-iridium that has been used to define the kilogram
WIKIMEDIA, NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Next Friday (November 16), representatives of 57 countries will assemble at a conference in Versaille, France, to vote on changing the definition of the kilogram, The Guardian reports. The proposed change is to define the kilogram in terms of Planck’s constant, which relates the energy of a photon to the frequency of its waves, rather than on a singular piece of metal.

It was France’s King Louis XVI who, in the late 1700s, got a group of scientists together to devise international units of measure. Out of this meeting, the kilogram—or at least the idea of a standard unit of mass—was born

For more than a century, the kilogram has been defined by the weight of a cylinder of platinum-iridium at the international bureau of weights and measures in Sèvres, France. The mass of the cylinder—nicknamed Le Grand K—is constant, but its weight is not. It grows with the accumulation of pollution from the air and shrinks ever so slightly with each cleaning.

The lump of metal in France is a matter of international significance. National metrology labs around the world have copies of it, base national mass-measurement systems on those copies, and, every 40 years or so, send the copies back to France to check them against the prototype.

Stephan Schlamminger, a physicist at the US National Institute of Standards and Technology, tells The Guardian that he believes the new change fulfills the original principles of international units of measure. “The idea was to have a measurement system for all times and for all people. They fell short on the kilogram. It has these problems with stability, so it is not for all times, and it is locked in a safe, so it is not for all people,” Schlamminger says. “Planck’s constant never changes, so it is the same for all time. And its value is woven into the fabric of the universe, so it is there for everyone.”

The vote is expected to be positive, the formality at the end of a long debate. If it is, the new system will go into effect on May 20, 2019.

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