Posted 8/20/2009 05:27:00 PM by Douglas

From the king's foot to a block of metal in Paris. We still have a way to go when it comes to a measurement system that's worth a damn.

More than a century ago, a small metal cylinder was forged in London and sent to a leafy suburb of Paris. The cylinder was about the size of a salt shaker and made of an alloy of platinum and iridium, an advanced material at the time.

In Paris, scientists polished and weighed it carefully, until they determined that it was exactly one kilogram, around 2.2 pounds. Then, by international treaty, they declared it to be the international standard.

Yay, kilogram! But of course, this is a perfect system, right?

As it stands, the entire world's system of measurement hinges on the cylinder. If it is dropped, scratched or otherwise defaced, it would cause a global problem. "If somebody sneezed on that kilogram standard, all the weights in the world would be instantly wrong," says Richard Steiner, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

Yeah, well no. The entire balance of power of the natural universe doesn't give a shit how big this metallic rock is. And there are copies.

For that reason, the official kilogram is kept locked inside a secured vault at the International Bureau of Weights and Measures near Paris. Scientists are so paranoid that they've only taken it out on three occasions: in 1889, 1946 and 1989. Each time, they've compared it to a set of copies. In 1889, the copies and the kilogram weighed the same, but by 1989, they had drifted apart. Based on the data, the kilogram appears to weigh slightly less than the copies.

Not surprising. No matter how good you take care of it, it's going to be off by a molecule or two, and when you're measuring it as precisely as these guys are, it adds up. But interesting that all the copies are increasing. That would imply that this one is decreasing. Any ideas why?

The real crux of this problem is that it's impossible to tell what has changed over the past 120 years. The copies may have grown heavier over time by absorbing air molecules. But it's equally possible that the kilogram is getting lighter. Periodic washings, for example, may have removed microscopic quantities of metal from its surface.

Periodic washing??? In NPR letting summer interns write copy? (oh wait, that's just an art history major). They've only taken it out of it's triple bell housing three times in 120 years. I find it hard to believe that during one of those outings Jean-Michelle would take it down to the car wash.