The Kilogram is Losing Weight

More than 100 years ago a salt shaker sized object made of platinum and iridium was forged in London. This was then shipped to Paris where it was shaped and polished and carefully weighed until it was exactly one kilogram (about 2.2 pounds). By international treaty, this then became the international for the exact weight of a kilogram. The problem is, the mass of the cylinder may be changing.

Read more