# Why the kilogram was redefined: the end of the metal cylinder

> For 130 years the kilogram was a platinum-iridium cylinder in a Paris vault, but measurable drift against its own copies forced metrologists to fix it to the Planck constant in 2019.

*Section: Science — By Amelia Hart (Technology Correspondent) — Published July 11, 2026 — 4 min read*

Canonical URL: https://dailyjunction.co.uk/science/why-the-kilogram-was-redefined-the-end-of-the-metal-cylinder
Tags: metrology, kilogram, si-units, physics, measurement

## Key takeaways

- The International Prototype Kilogram, cast in 1889 from 90% platinum and 10% iridium, diverged from its official copies by roughly 50 micrograms over a century, and no one could say which object had changed.
- In November 2018 the General Conference on Weights and Measures fixed the Planck constant at exactly 6.62607015 x 10^-34 joule-seconds, making the kilogram derivable from that number rather than from an artefact.
- The Kibble balance, invented by Bryan Kibble at the National Physical Laboratory in Teddington in 1975, realises the new definition by balancing a mass against an electromagnetic force measured through quantum-electrical standards.

Until 20 May 2019, the kilogram was a physical object. Not a definition that an object satisfied, but the thing itself: a cylinder of platinum-iridium alloy, 39 millimetres tall and 39 millimetres across, cast in 1889 and kept under three nested bell jars in a basement vault at the International Bureau of Weights and Measures (BIPM) in Sèvres, on the outskirts of Paris. Opening the vault required three keys held by different officials. The International Prototype Kilogram, known in the trade as Le Grand K, was by definition exactly one kilogram. If a fingerprint had been left on it, the mass of everything else in the universe would, formally speaking, have shifted.

The alloy was chosen carefully: 90 per cent platinum for density and inertness, 10 per cent iridium for hardness. Forty official copies were distributed to national laboratories, including Kilogram 18, which served as the United Kingdom's national standard at the National Physical Laboratory (NPL) in Teddington. Every few decades the copies travelled back to Sèvres to be washed with solvent, steam-cleaned and compared against the original on precision balances.

Those comparisons were the problem. The third periodic verification, run between 1988 and 1992, confirmed what earlier measurements had hinted at: the prototype and its copies were drifting apart, by around 50 micrograms over a century — the mass of a grain of sand, spread across a hundred years. Fifty parts per billion sounds trivial, but pharmaceutical dosing, satellite engineering and fundamental physics all chain their mass measurements back to that one object. Worse, the divergence was logically undecidable. Because the IPK defined the kilogram, it could not, by construction, be wrong; the copies had "gained" mass relative to it, but the likelier physical story was that the prototype itself was losing atoms through surface wear and outgassing, or gaining them through contamination, in ways no cleaning protocol could fully control. The world's base unit of mass was a moving target that measurement could not even see moving.

The metre had already shown the way out. Once a platinum bar in the same vault, it was redefined in 1983 as the distance light travels in a vacuum in 1/299,792,458 of a second — anchored to a constant of nature rather than an artefact. The kilogram was the last SI base unit still tied to a lump of metal, and metrologists spent three decades building the machinery to cut the tie.

## Fixing a number instead of guarding an object

The chosen anchor was the Planck constant, h, the quantity from quantum mechanics that relates a photon's energy to its frequency. On 16 November 2018, at the 26th General Conference on Weights and Measures in Versailles, delegates from some 60 states voted to fix its value at exactly 6.62607015 x 10^-34 joule-seconds. Since the joule-second contains kilograms, metres and seconds, and the metre and second were already defined by constants, fixing h pins down the kilogram with no artefact involved. The ampere, kelvin and mole were re-anchored to fixed constants at the same session, completing a redefinition of the SI that had been argued over since the 1990s.

The vote could only happen once two independent experimental routes agreed to within about ten parts per billion. One route counted atoms: the International Avogadro Project ground two of the roundest objects ever manufactured, spheres of isotopically enriched silicon-28, and determined the number of atoms in each from its volume and lattice spacing. The other route was electromechanical, and it is a British invention. The Kibble balance — devised in 1975 by Bryan Kibble at NPL, and renamed in his honour after his death in 2016 — weighs a mass against the force on a current-carrying coil in a magnetic field, then calibrates that electrical measurement through two quantum effects, the Josephson and quantum Hall effects, which express voltage and resistance in terms of h. NPL, the National Research Council of Canada and the American NIST all ran versions of the instrument; the Canadian machine, built around hardware originally constructed at Teddington, produced some of the decisive results.

## What actually changed

For anyone weighing flour or freight, nothing. The new definition was deliberately engineered so that the kilogram's size did not change at the moment of switchover. What changed is the character of the agreement. A national laboratory in any country can now realise the kilogram from first principles by building the apparatus, rather than queueing for access to a guarded cylinder in France. The unit can no longer be scratched, stolen or contaminated, and its stability is that of quantum mechanics itself. Le Grand K remains in its vault at Sèvres, still handled with tongs and reverence — but as a historical object and a check on the new methods, not a definition. After 130 years, humanity stopped trusting a piece of metal and started trusting a number.

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