'Measuring the World: From Material to Ethereal'

article by KENNETH CHANG here.

LOCKED in a vault in Paris is a cylinder about the size of a plum. Its mass is exactly one kilogram. It is the kilogram.

For 116 years, this cylinder made of platinum and iridium has been the world's defining unit of mass. It's an easy concept to understand.

Scientists at the National Institute of Standards and Technology in Gaithersburg, Md., announced last month significant progress toward supplanting this cylinder. Their concept is not so easy to understand.

It's a two-story-tall contraption that looks one part Star Trek, one part Wallace and Gromit. Briefly put, it measures the power needed to generate an electromagnetic force that balances the gravitational pull on a kilogram of mass.

"It's such a very complicated thing that's hard to explain," said Richard Steiner, the physicist in charge of the project. He has been working on this "electronic kilogram" machine for more than a decade.