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If you were to pick up a pound of coffee at the store, you could reasonably expect it to weight as much as it did last week or last year or 10 years ago. So you may be surprised to hear that it might not be the case. That's because the pound is defined against the metric system's unit of weight, the kilogram. And the kilogram may be changing. Geoff Brumfiel reports.

GEOFF BRUMFIEL: This story starts with a metal cylinder about the size of a salt shaker. It was forged over 100 years ago in London. From there, it traveled to a suburb of Paris, where it was weighed, polished and locked in a vault. And it's stayed there every since. This little cylinder is the kilogram. I'll say it again: the kilogram. This is the one on which all other kilograms are based. That makes it very valuable.

Mr. RICHARD DAVIS (International Bureau of Weights and Measures): I mean, I picked up its container and I've looked at it, but I have not had the pleasure of actually handling it.

BRUMFIEL: Richard Davis works at the International Bureau of Weights and Measures, where the kilogram is kept. He's been in charge of the cylinder since 1993, but they won't actually let him weigh anything with it. And there's a good reason for that.

Mr. DAVIS: It can be affected by many things. It can get dirty. It needs to be cleaned. You need to watch out about scratching.

BRUMFIEL: The scientists who keep watch are so paranoid that they've only taken it out three times - in 1889, 1946 and 1989. Each time, they used it to check a set of copies. In the beginning, the copies and the kilogram weighed the same. Today, that's no longer the case. Peter Moore is with the National Institute of Standards and Technology, or NIST, in Gaithersburg, Maryland. He studied the measurements.

Dr. PETER MOHR (Physicist, National Institute of Standards and Technology): They don't have a lot of data but it shows that they're drifting apart.

BRUMFIEL: In other words, the weight of the official kilogram seems to be changing. No one really knows why the cylinder might be absorbing air and getting heavier or washing it before its use may have made it lighter or it could be that the copies are changing. There's just no way to tell.

Dr. MOHR: It is the kilogram. There's nothing to compare it to. It is the standard. And so if it's changing, there is no way to know it.

BRUMFIEL: The bottom line is that nobody is sure whether that cylinder in Paris, the official kilogram, weighs the same as it did a hundred years ago. The entire metric system hangs in the balance, well, sort of. The change is small, at most 50 micrograms - that's billionths of a kilogram.

Dr. MOHR: The actually ramifications for a person going into the store and buying something will be negligible. But for very precise work, it makes a difference. And for scientific work, it makes a difference.

BRUMFIEL: And because of this scientists want to throw out the metal cylinder all together and replace it with a reliable number that won't change. To get this number, they've got to weigh the kilogram in a completely different way. They've got to measure its mass without using another weight as a reference. And they've got to do it really carefully. So, NIST has built a special scale. It's kept out back near the woods in the small building whose entry way is covered in little shreds of something black.

Dr. RICHARD STEINER (Physicist, National Institute of Standards and Technology): I've to apologize about the rubber on the floor. We have a raccoon. It gets up into over the rafters there and it pushes the stuff out of the little cracks.

BRUMFIEL: Richard Steiner is in charge of the scale, which is called the watt balance.

Dr. STEINER: You mentioned about being far out. Yes, we are very far out. That we have probably more raccoons come out here than visitors.

BRUMFIEL: Inside of a raccoon free room, it is a two storey contraption. Instead of using weights, it uses electrical and magnetic forces to measure the mass of a metal cylinder - an official copy of the kilogram in Paris.

Dr. STEINER: The fundamental part of this is actually fairly simple. It's basically a very highly calibrated bathroom scale.

BRUMFIEL: Now here's how this thing is going to replace the cylinder with a fixed number. Once Rich Steiner can weigh the kilogram accurately enough, he will be able to relate it to one of the so-called constants of nature - numbers that never change like the speed of light. This will turn the kilogram into a number as permanent as the constants themselves. The challenge is that the cylinder must be weighed with atomic precision. And that's where the watt balance comes in. It's sensitive to as little as 10 billionths of a kilogram.

Dr. STEINER: If you pull the hair out of a person's head and then weighed them, we could tell the difference.

BRUMFIEL: Unfortunately a scale that's sensitive is sensitive to a lot of other things too. Vibrations from passing lawnmowers, for example - the tides, even earthquakes on the other side of the planet upset it, which is why it's taking so long to weigh the kilogram. Steiner has been at it for 16 years. And he considers himself a newcomer to the field.

Dr. STEINER: It's just complicated because we're making mechanical measurements. And we're doing it at the precision of eight digits.

BRUMFIEL: Despite all the problems, Steiner is confident that scientists will be able to redefine the kilogram in five or six more years. When that happens, the little cylinder in Paris will be replaced by a single number, eight digits long. Anybody with a watt balance and a lot of patience will be able to check it for themselves.

Geoff Brumfiel, NPR News.

INSKEEP: And you can weigh Geoff's report against our other science coverage, when you're checking our Web site and the other headlines throughout this day at npr.org.

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