Metrologists to Redefine the Kilogram

The weight of a kilogram is changing—and metrologists want it to stop. The kilogram isn’t just an ephemeral standard of measurement; it’s a physical cylinder of platinum-iridium alloy, cast in 1879 and squirreled away under lock and key at the International Bureau of Weights and Measures in Sèvres, France, the Economist explains. Its mass is the basis for the kilogram. The problem: When weighed against its copies, discrepancies of up to 69 micrograms have been noted.

That’s smaller than a grain of sand, but it’s not good enough for metrologists. In a world where precise measurements are possible, a changing kilogram isn’t good enough. So they’re hoping to redefine the kilogram in terms of natural constants, as most other forms of measurement now are. There was once a platinum-iridium meter standard, too, for instance, but now a meter is defined as 1/299,792,458 of the distance light travels in a second in a vacuum. Metrologists hope to define the kilogram in terms of Planck’s constant, but they haven’t yet perfected the calculation necessary—they expect to do so within a few years. (More measurement stories.)