The kilogram has been transformed as new definition takes hold

Photographer Matt Roth
New York Times/Redux/Eyevine

THERE’S a new way to weigh. For over a century, the kilogram was defined by a metal cylinder in a French vault. Now, this key unit of mass is defined using the Planck constant, a fundamental figure in physics.

The Planck constant relates a photon’s energy to its frequency. This incredibly small number has to be measured by a very sensitive Kibble balance (pictured above) that uses a powerful magnetic field to do its work.

Researchers will now be able to weigh things accurately without flying to France to compare them with the cylinder, known as Le Grand K or the International Prototype Kilogram (pictured left). If it lost mass, perhaps by being scratched, or gained some through a fingerprint, say, the definition of the kilogram would alter. By comparing Le Grand K with copies that are distributed globally, it was clear that, despite the efforts to protect the precious cylinders, their mass did change.

At last, the kilogram will join science’s other units in becoming universal and unchanging.