NOEL KING, HOST:
All right. Here's a question. How much is a kilogram?
STEVE INSKEEP, HOST:
The dad joke answer to that question is, it weighs about one kilogram. A more useful answer is that a kilogram is a little over two pounds.
KING: But if you're making precise scientific measurements, you need to know exactly how much. And the prototype kilogram, which was used to determine the exact measurement, has changed. Jon Pratt is with the National Institute of Standards and Technology.
JON PRATT: It's kind of a big deal. Kilogram was the last artifact or the last object-based standard in the measurement system. And so it's a wistful moment.
KING: That object was a hunk of metal called Le Grand K. It was housed in a vault outside of Paris, and it was the official definition of the kilogram. Now, it was reassuring to have solid metal. But scientists say it was, ever so gradually, losing weight. It lost some atomic particles over centuries. Now, that's a tiny change in the object's mass, but it's enough to matter. So last year, scientists and policymakers from around the world got together, and they voted to redefine the kilogram. That decision took effect yesterday.
INSKEEP: The new definition is rather hard to explain, but it's now based on physics and fundamental, unchanging constants of nature.
PRATT: We're saying goodbye to a sort of 16th-century approach to things and trying to pull all of measurement into at least the 20th century.
INSKEEP: The caretaker of Le Grand K - yes, the hunk of metal had a caretaker. That caretaker says it will now be an historical artifact.
(SOUNDBITE OF LYMBYC SYSTYM'S "DIFFERENTIAL") Transcript provided by NPR, Copyright NPR.