The kilogram used to live in a vault

From a cylinder to a constant of nature

The old kilogram was defined by the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder kept under controlled conditions near Paris. National standards were compared with it, and working weights were calibrated through further comparisons. That chain made the cylinder’s mass the reference point for measurements around the world.

The weakness was not that the prototype was constantly changing. It was that its mass could only be known by comparing it with other objects—and the prototype itself. If it changed slightly, there was no independent way to tell whether the cylinder had shifted or its comparison copies had. Any drift could therefore ripple through the calibration chain.

Turning a constant into a kilogram

Since 2019, the kilogram has been defined by assigning an exact value to the Planck constant, a fundamental quantity in quantum physics. A number alone cannot weigh a sack of flour, so laboratories use precision instruments to connect that constant to an actual mass.

One such instrument is a Kibble balance. It compares the downward pull on a test mass with an electromagnetic force. In a carefully controlled setup, the balance also measures electrical effects that can be related to the Planck constant using quantum electrical standards. When the measurements are combined, the instrument can determine the mass without treating any particular lump of metal as the ultimate reference.

A stable definition, realized in laboratories

Fixing the constant does not mean every scale suddenly measures mass directly from quantum physics. Most scales are still calibrated through ordinary reference weights. The difference is at the top of the chain: national measurement laboratories can establish the kilogram using the fixed constant, then pass that standard on to industry and commerce.

The cylinder remains a remarkable historical artifact, but it no longer gets the final say. The kilogram now rests on a definition that does not depend on whether one carefully guarded object gains or loses a trace of material.