kilogram
Massexact by definition
The kilogram is the SI base unit of mass. Since 20 May 2019 it has been defined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule seconds, with the metre and second already fixed by the speed of light and the caesium transition. Its conversion factor here is 1 because every other mass unit on this page is ultimately referred back to it.
| 1 kg | 1 kg |
The Kilogramme des Archives, a platinum cylinder deposited in 1799, was replaced in 1889 by the International Prototype of the Kilogram, a platinum-iridium cylinder kept near Paris and known as Le Grand K. For 130 years the unit was literally that object. Comparisons with its official copies drifted by roughly 50 micrograms over a century, and nobody could say which had changed, which is what finally motivated a definition built from constants of nature.
About the kilogram
The kilogram was the last SI base unit tied to a manufactured object, and the story of getting rid of that object is the best short illustration of what a unit definition is for. Until 2019 the kilogram was Le Grand K, a platinum-iridium cylinder in a vault at Sèvres. If it gained a fingerprint's worth of mass, then by definition every other mass in the universe got slightly lighter. Periodic comparisons against its sister copies showed a spread of tens of micrograms accumulating over a century, and because the prototype defined the unit there was no experiment that could say whether the prototype or the copies had moved.
The replacement fixes the Planck constant at exactly \(h = 6.62607015 \times 10^{-34}\) J·s. Since a joule is \(\mathrm{kg\,m^2\,s^{-2}}\), fixing \(h\) together with the already-fixed metre and second pins down the kilogram. In practice a Kibble balance realises it by balancing a weight against an electromagnetic force whose electrical power is measured through the Josephson and quantum Hall effects, both of which depend on \(h\). The number 6.62607015 × 10⁻³⁴ was chosen so that the new kilogram agreed with the old prototype to within the measurement uncertainty of the day, about one part in 10⁸. Nothing in your kitchen changed.
One consequence worth carrying: mass is not weight. A kilogram is a quantity of matter and it is the same on the Moon, while the weight of that kilogram, the force it presses down with, is \(mg\) and falls from about 9.81 N on Earth to 1.6 N on the Moon. Scales that read in kilograms are measuring force and quietly dividing by an assumed \(g\).