A kilogram was once defined by a physical cylinder kept in a vault in France.

The longer version

For more than a century the world's reference mass was a single cylinder of platinum alloy stored under bell jars. Copies were distributed to other countries, and periodically they were compared with the original. Tiny differences appeared over time, probably from surface contamination and handling.

That drift was a problem, because a definition should not change. The unit has since been redefined using a fundamental constant of nature, so any laboratory with the right equipment can realise it directly. The old cylinder remains a historical object rather than a standard, and its story shows how measurement moved from artefacts to constants.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.