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How can a constant of nature replace a kilogram of metal?
For more than a century, the kilogram depended on a single platinum–iridium cylinder kept near Paris. Adopted in 1889, this was the international prototype of the kilogram: its mass defined the unit, rather than merely exemplifying it. The arrangement created a . If the prototype’s mass changed, there was no independent way to establish that change within the definition itself. On 20 May 2019, a revised International System of Units came into force, replacing this material reference with a fixed numerical value for Planck’s constant: exactly 6.62607015 × 10⁻³⁴ joule seconds. Because the joule incorporates the kilogram, fixing the constant’s value, together with existing definitions of the metre and second, anchors mass to an feature of nature rather than to a unique object.
Turning that definition into an actual measurement requires a practical . One approach uses a Kibble balance, which links the mechanical power associated with a mass in a gravitational field to electrical power. Its weighing and moving measurements allow the apparatus to eliminate a difficult-to-measure combination of magnetic field and wire geometry from the calculation. Quantum electrical standards then connect the result to Planck’s constant. This does not mean that every laboratory needs such an instrument: calibrated weights still the unit through chains of comparisons. The distinction is fundamental. A damaged reference weight can now be replaced without changing what a kilogram means. Nevertheless, an definition does not guarantee flawless measurements; uncertainty remains a property of the experimental process, not a defect in the unit.