Measurement of Mass

Since 1889, the standard kilogram was defined as the mass of a specific, carefully preserved cylinder of platinum-iridium alloy, kept under controlled conditions in a special glass case at the International Bureau of Weights and Measures. This definition, based on a single physical artefact, was replaced on 20 May 2019, because meticulous long-term measurements showed the prototype cylinder's mass was very slowly changing — the artefact was found to be picking up microscopic particles of dirt over time and to be affected by the surrounding atmosphere, both of which shift its mass by tiny but measurable amounts. A reference standard for the world's mass measurements that can drift is clearly unsatisfactory.

The new definition of the kilogram is instead based on the Planck constant ($h$), tied to it through electromagnetism: an electric current passed through a coil can be used to make an electromagnet, and an electromagnet can be made strong enough to attract magnetic materials — the same effect used, at industrial scale, in cranes that lift heavy pieces of iron or steel. The modern kilogram is realised in the laboratory by finding the precise electric current needed through an electromagnet so that the magnetic pull it produces exactly balances (pulls down one side of an extremely sensitive balance to counteract) a known reference mass of 1 kg on the other side — and, working through the physics of that balance, this current requirement is expressed exactly in terms of the fixed value of the Planck constant. Because $h$ is a true constant of nature, this new definition of the kilogram can never drift, unlike the old artefact-based one. …