Scientist builds toward weighty endeavor Scale could check kilogram accuracy

GAITHERSBURG — GAITHERSBURG -- When Paul T. Olsen finishes tinkering with his ultrasensitive scale, the fate of the kilogram could hang in the balance.

The soft-spoken, 55-year-old scientist at the National Institute of Standards and Technology (NIST) has spent 12 years building an elaborate two-story contraption, one of the most sensitive scales in existence.


If he can improve the accuracy of the device tenfold in the next couple of years, he and his NIST colleagues plan to use it to help decide whether a 1-kilogram platinum-iridium cylinder, kept in a vault in Paris, is gaining or losing weight.

Since 1889, that cylinder has served as the standard the metric-speaking world uses to calibrate the scales that weigh everything from steel to salmon to subatomic particles. But there is growing concern among scientists that it may be giving off or absorbing gases, causing changes in its weight of up to 3 parts in 100 million.


That's not much. If the U.S. population varied by the same factor, it would gain or lose no more than 8 people a year. But the idea that the standard kilogram is growing or shrinking, even by tiny amounts, drives some scientists crazy.

"If you buy a pound of hamburger in a grocery store, you could care less," Dr. Olsen admitted. "But if you work in basic research, it does matter. There are some things that you'd just like to know what they really are."

There are some other more-or-less practical benefits, other scientists say. For one, a more precise kilogram would permit scientists to "weigh" subatomic particles, such as the proton and electron, with better accuracy.

Dr. Olsen's scale appears to be a hodgepodge of superconducting magnets, copper coils, a finely balanced flywheel, four computers, lasers and other high-tech gear. A lot of the apparatus was assembled by the scientist in the basement of his home in Frederick.

It measures the weight of an object by determining with great precision the amount of power it takes to generate the electric field needed to counterbalance the object.

To protect against electromagnetic interference, the scale is housed in a building on the NIST campus built without iron or steel doors, windows or nails. To minimize vibration, Dr. Olsen conducts experiments by remote control from outside the room.

The scale is capable of generating a magnetic field about 2,000 times stronger than Earth's. "If it was on now, it would neutralize the magnetic stripe on all your credit cards," said Dr. Olsen, crouching in a plywood closet housing the upper half of his instrument.

The scale will be ready for the kilogram testing experiment in about two years.


"I think you have to be a bit of a strange bird to be willing to work at something long enough so you can measure it in a part in 10 million," he said. "You can't do that overnight."

During the 20th century, scientists have refined the second and the meter -- as technology has advanced.

The clock's second was once calculated as a rough fraction of a solar day. Since 1964 it's been defined by the oscillations of the cesium-133 atom in an atomic clock at the U.S. Naval Observatory in Washington. The meter was first a fraction of the Earth's circumference, but went through several standards. Since 1983, it has been the distance light travels in a vacuum in 1/299,792,458 of a second.

But the kilogram remains a stubborn, literal "artifact," in the jargon of NIST scientists. And an artifact, like any physical object, is subject to change.

Originally, the kilogram was defined as the weight of a liter of water at 4 degrees centigrade, but the value varied depending on the level of contamination by minerals or organic material. So in 1889 the platinum-iridium cylinder was adopted.

It is brought out of its vault every few years to use to compare with other standard kilogram cylinders maintained by governments worldwide.


There are three such kilogram cylinders in the United States, all of them at the National Institute of Standards and Technology. Dr. Olsen keeps one of them in a safe.

Dr. Olsen and his colleagues won't weigh the Paris kilogram directly. Instead, they plan to make a series of measurements of Dr. Olsen's kilogram, which has been carefully matched with its Paris twin over the years.

By comparing the latest, ultrasensitive data from Dr. Olsen's scale with past efforts in Paris to weigh the kilogram there, NIST experimenters hope to determine if the world's standard is drifting.