We have completed maintenance on DiscoverMagazine.com and action may be required on your account. Learn More

A Kilogram Just Ain't What It Used To Be

The archetypal kilogram is losing mass, and no one knows why.

By Miranda Marquit
Feb 13, 2008 6:00 AMNov 12, 2019 6:40 AM


Sign up for our email newsletter for the latest science news

Physicists are getting worked up about something that should have been settled long ago: Just how massive is a kilo­gram? Most units of scientific measure are now defined not by physical objects but by universal constants. A meter, for example, is the distance traveled by light in a vacuum in 1/299,792,458 of a second. The kilogram is a holdout, still defined by a cylinder of platinum-iridium kept in a vault near Paris since 1889. And the cylinder is losing weight—a grain of salt’s worth so far—demonstrating the need for a unit based on a physical constant.

Two major standardization strategies have surfaced. One proposes a numerical unit built on Avogadro’s number, derived from the number of atoms in 12 grams of carbon-12. The other approach makes use of Planck’s constant and the Watt balance, which measures the electromagnetic force needed to hold a kilogram.

For Richard Davis at the official bureau for measurements in France, there is no clear win­ner; he believes both methods are too pricey for most labs. But until researchers find a simple, reproducible means, we’re stuck with a hunk of platinum and iridium that is ever-so-slowly getting lighter.

1 free article left
Want More? Get unlimited access for as low as $1.99/month

Already a subscriber?

Register or Log In

1 free articleSubscribe
Discover Magazine Logo
Want more?

Keep reading for as low as $1.99!


Already a subscriber?

Register or Log In

More From Discover
Recommendations From Our Store
Shop Now
Stay Curious
Our List

Sign up for our weekly science updates.

To The Magazine

Save up to 40% off the cover price when you subscribe to Discover magazine.

Copyright © 2024 Kalmbach Media Co.