Subscribe now

Physics

The kilogram has been transformed as new definition takes hold

This picture shows the Kibble balance – the new way to measure the kilogram, as it joins other units in the SI measurement system which are pegged to physical constants

By Leah Crane

20 May 2019

New Scientist Default Image

Matt Roth/New York Times/Redux/Eyevine

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.

New Scientist Default Image

Matt Roth/New York Times/Redux/Eyevine

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.

Topics: