Goodbye, Le Grand K: How the Kilogram Was Redefined in 2019
Until 2019, the definition of the kilogram was an object. Every kilogram in the world — on every bathroom scale, in every laboratory, behind every price per kilo at the supermarket — ultimately traced back to a single small cylinder of metal kept under three glass bell jars in a vault in Sèvres, just outside Paris. If that cylinder gained or lost a few atoms, the kilogram itself changed.
On 20 May 2019 that changed. The kilogram is now defined by a constant of nature. Here is how and why it happened.
From a litre of water to a metal cylinder
When the metric system was created in revolutionary France in the 1790s, the kilogram was defined as the mass of one litre (one cubic decimetre) of water. That was elegant but impractical, because the mass of water depends on its temperature and purity. In 1799 a platinum standard, the Kilogramme des Archives, was made to embody it.
After the Treaty of the Metre in 1875, a new standard was manufactured from an alloy of 90% platinum and 10% iridium — dense, hard and highly resistant to corrosion. In 1889 it was formally adopted as the International Prototype of the Kilogram. It is a cylinder about 39 mm tall and 39 mm across, roughly the size of a golf ball. Metrologists nicknamed it “Le Grand K”. Dozens of official copies were made and sent to countries that had signed the treaty.
The problem with an object
An artefact definition has unavoidable weaknesses. It can be damaged, contaminated or lost. It can only be in one place, so every comparison requires carefully transporting precious copies. And, as it turned out, it does not stay perfectly constant.
Over the 20th century, periodic comparisons showed that Le Grand K and its official copies were drifting apart — by around 50 micrograms over a hundred years, roughly the mass of a small grain of sand. Nobody could say whether the prototype was losing mass or the copies were gaining it, because by definition the prototype always weighed exactly one kilogram. That uncertainty was a real problem for the most precise science and technology, and it also affected other units, such as the ampere, that depended on the kilogram.
Defining units by constants of nature
The solution had already been applied to other units. In 1967 the second was redefined by the frequency of radiation from caesium atoms (exactly 9,192,631,770 cycles). In 1983 the metre was redefined using the speed of light, fixed at exactly 299,792,458 metres per second. Anyone with the right equipment can realise these units anywhere, without visiting a vault.
For the kilogram, scientists chose the Planck constant, a fundamental number from quantum physics that links energy to frequency. Once its value is fixed exactly, and the second and metre are already defined, the kilogram follows from it.
The vote: 16 November 2018
At the 26th General Conference on Weights and Measures (CGPM) in Versailles, representatives of member states voted unanimously to redefine four base units of the International System of Units at once: the kilogram, the ampere, the kelvin and the mole. The new definitions took effect on 20 May 2019, World Metrology Day, chosen because it is the anniversary of the signing of the Treaty of the Metre in 1875.
| Unit | Now defined by fixing | Exact value |
|---|---|---|
| kilogram | Planck constant h | 6.626 070 15 × 10⁻³⁴ J·s |
| ampere | Elementary charge e | 1.602 176 634 × 10⁻¹⁹ C |
| kelvin | Boltzmann constant k | 1.380 649 × 10⁻²³ J/K |
| mole | Avogadro constant Nₐ | 6.022 140 76 × 10²³ mol⁻¹ |
How you actually weigh a kilogram now
Fixing a constant on paper is only useful if laboratories can turn it into a real mass. Two very different experiments made this possible, and their agreement gave scientists the confidence to make the change.
The first is the Kibble balance, originally called the watt balance, conceived by Bryan Kibble at the UK’s National Physical Laboratory in the 1970s. It balances the weight of a test mass against an electromagnetic force produced by a coil in a magnetic field. Using quantum electrical standards, that force can be measured in terms of the Planck constant, so the balance effectively weighs a mass using h.
The second is the X-ray crystal density method, developed through the international Avogadro project. Researchers made almost perfect spheres of highly enriched silicon-28 and counted the atoms in them by measuring the sphere’s volume and the spacing of its crystal lattice with extraordinary accuracy. Knowing the number of atoms leads to the Avogadro constant and, from it, the Planck constant.
Does any of this change my scale?
No. The new definition was carefully chosen so that one kilogram on 20 May 2019 had the same mass as one kilogram the day before. Your kitchen scale, a bag of flour, and the conversion of 1 pound = 0.45359237 kilograms are all unaffected. Le Grand K still exists and is still kept at the International Bureau of Weights and Measures; it has simply become a very well-studied piece of metal rather than the definition of a unit.
What changed is long-term stability. The kilogram can no longer drift, and any sufficiently equipped laboratory in the world can realise it independently. For the first time since the metric system was conceived, all seven SI base units are defined by constants of nature — fulfilling the 18th-century dream of a measurement system “for all people, for all time”.
Sources and further reading
- BIPM, The International System of Units (SI Brochure), 9th edition (2019)
- Resolution 1 of the 26th CGPM (2018), “On the revision of the International System of Units”
- NIST, “Kilogram: Introduction” and Kibble balance publications