The Kilogram's Journey: From a Platinum Cylinder to the Planck Constant

The Original Kilogram: One Liter of Water (1795)

When the French Revolutionary government created the metric system in 1795, the kilogram was defined with elegant simplicity: it was the mass of one cubic decimeter (one liter) of pure water at its maximum density, which occurs at 4 °C. The prefix "kilo-" denoted one thousand grams, where one gram was one cubic centimeter of water at the same temperature.

This water-based definition had genuine appeal. Water is universally available, chemically pure water behaves predictably, and linking mass to volume meant the system was self-consistent: a liter of water weighs exactly a kilogram. Any nation could, in principle, realize the standard independently.

In practice, it was nearly impossible to maintain. Weighing water precisely requires knowing its temperature to fractions of a degree, its pressure (which affects density), and its isotopic purity (heavy water, D₂O, is 11% denser than ordinary water). At 4 °C and standard atmospheric pressure, one liter of pure water has a mass of 0.999972 kg — close to one kilogram, but not exactly. The definition was technically sound; the realization was not.

A physical artifact would be needed.

The International Prototype Kilogram (Le Grand K, 1889)

In 1889, the First General Conference on Weights and Measures (CGPM) ratified the International Prototype Kilogram — a cylinder of 90% platinum and 10% iridium, manufactured by the London firm Johnson Matthey. Platinum-iridium was chosen for hardness, corrosion resistance, and polishability. The cylinder's dimensions — 39 mm tall, 39 mm in diameter — minimize surface area for a given volume, reducing contamination risk.

Le Grand K was stored under three nested glass bell jars in a vault at the BIPM in Sèvres, requiring three keys held by three separate officials. Six Official Copies remained at the BIPM; 40 national copies were distributed to member nations (Copy #4 to the USA, Copy #18 to the UK, Copy #12 to Russia).

By definition, Le Grand K weighed exactly 1 kilogram. Always. Without exception. This was not a measurement — it was the definition.

The Problem: Le Grand K Was Changing

The fatal flaw of an artifact-based kilogram emerged during periodic verification campaigns, when national copies were returned to Sèvres and compared against Le Grand K and the Official Copies using mass comparators resolving a few micrograms.

The 1988–1992 third verification campaign revealed the problem clearly. The six Official Copies at the BIPM showed masses diverging from Le Grand K by up to 50 µg over the preceding century — some heavier, some lighter. Because Le Grand K was by definition exactly 1 kg, it was impossible to determine which objects were changing or in which direction.

A drift of 50 µg in 100 years is 5 × 10⁻⁸ — negligible in everyday life, but critical for semiconductor manufacturing, pharmaceutical dosing, and precision engineering, where mass traceability at better than 10 µg is required. More fundamentally: a unit defined by a specific object in a vault in France is not a universal standard. The kilogram was the last SI base unit still tied to an artifact, and metrologists spent decades working toward a solution.

The Watt Balance (Kibble Balance) Approach

The path to a constant-based kilogram ran through quantum mechanics. In 1975, Bryan Kibble at the UK National Physical Laboratory proposed an experiment linking mechanical power to electrical power with extraordinary precision, using the quantum Hall effect and the Josephson effect. The device became known as a watt balance — renamed the Kibble balance after Kibble's death in 2016.

The operating principle: a coil moving through a magnetic field at velocity v generates voltage V = BLv. The same coil carrying current I experiences force F = BIL. Setting the two operating modes equal links mechanical power (force × velocity) to electrical power (voltage × current), giving m = V × I / (g × v). The Josephson effect quantizes voltage in terms of h/e, and the quantum Hall effect quantizes resistance in terms of h/e², so both measured electrical quantities connect directly to the Planck constant h.

Multiple Kibble balance experiments at NIST (USA), NPL (UK), PTB (Germany), and BIPM converged on a consistent value of h across instruments sharing no common components. By 2017, the agreement was sufficient to proceed with redefinition.

The 2019 Redefinition via Planck's Constant

On 20 May 2019, the International System of Units was revised. The kilogram was redefined by fixing the numerical value of the Planck constant at exactly:

h = 6.62607015 × 10⁻³⁴ J·s = 6.62607015 × 10⁻³⁴ kg·m²·s⁻¹

This value was chosen to be consistent with the best pre-revision measurements of h relative to Le Grand K. The kilogram is now the mass that makes this equation true, given the fixed definitions of the meter and second. No artifact is required. Any laboratory with a Kibble balance, laser interferometer, and quantum electrical standards can realize the kilogram independently.

The definition also simultaneously redefined three other SI base units: the ampere (via the elementary charge), the kelvin (via the Boltzmann constant), and the mole (via the Avogadro constant). The second and meter retained their definitions unchanged.

What Changed in Practice? (Almost Nothing)

For anyone outside a national metrology laboratory, the 2019 redefinition changed nothing. Every scale calibrated before May 2019 against mass standards traceable to Le Grand K remained calibrated to the same accuracy afterward. The numerical value of h was chosen so that 1 kg_new = 1 kg_old to within 10 parts per billion — the uncertainty of the best Kibble balance measurements at the time.

What changed was the foundation. Le Grand K is now a historical artifact rather than the master reference. If it were destroyed, the kilogram could be reconstructed from h. If Le Grand K's mass drifts in the future, it no longer drags the definition of mass with it.

For weight conversions and specific pairs like kilograms to pounds, the conversion factor (1 kg = 2.20462262185 lb) remains unchanged — fixed by the International Yard and Pound Agreement of 1959.

Timeline: Key Dates in Kilogram History

The kilogram's journey from a bucket of water to a quantum constant is a story about the limits of physical objects as standards — and about the century-long project to replace them with something more permanent. The Planck constant was always there; it took until 2019 to measure it precisely enough to use it as a definition.