The kilogram as a physical object

Also known as Le Grand K, International Prototype of the Kilogram, IPK

Cause of death

Killed by a successor

Made by

International Bureau of Weights and Measures

The Planck constant does not sit in a vault in Paris slowly changing mass.

For 130 years the kilogram was a specific lump of platinum-iridium in a vault outside Paris, and everything anyone weighed anywhere on Earth was ultimately compared to it. The problem was that it drifted: official copies diverged from the original by tens of micrograms, and there was no way to know which had changed. In 2019 the kilogram was redefined in terms of the Planck constant, and the last physical artefact was removed from the SI system.