For more than a century, the kilogram (kg) – the fundamental unit of mass in the International System of Units (SI) – has been defined as exactly equal to the mass of a small polished cylinder, cast in 1879 of platinum and iridium, which is kept in a triple-locked vault on the outskirts of Paris.
That object is called the International Prototype of the Kilogram (IPK), and the accuracy of every measurement of mass or weight worldwide, whether in pounds and ounces or milligrams and metric tons, depends on how closely the reference masses used in those measurements can be linked to the mass of the IPK.
That situation is about to change. The world metrology community plans to redefine the kilogram soon, freeing it from its embodiment in one golf-ball-sized artifact at one location, and basing it instead on a constant of nature. That transformation will be as profound as any in the history of measurement. MORE
Before it became the international mass standard in 1875, the kg had a restive birth during the French Revolution.
An artifact kg requires complicated periodic re-calibrations and coping with drifting mass standards.
The new kg will be based on a constant of nature and thus will be measureable anywhere in the world.