Under three vacuum-sealed bell jars, inside a locked safe, deep within a vault two stories below a secure building outside Paris, rests a tiny plug of metal that weighs heavily on the minds of scientists around the world. The French call the object "Le Grand K," and for good reason. Since 1883, this platinum cylinder about the size of a film canister has been the official kilogram, the standard by which the world measures mass and weight under the metric system.

Even in the United States, where public exposure to the metric system ends at the schoolhouse door, Le Grand K is king. The weight of the U.S. pound is based on it. And the weight of everything from coffee beans to cars is measured on scales whose calibration can be traced back to the Paris kilogram through a chain of official heirs.

But now researchers want to dethrone Le Grand K and send it to the scrap heap. The reason: The official kilogram has a wee weight problem.

Over the years, scientists have removed it from its airless cocoon just three times to be weighed and cleaned. The last time, in 1989, they discovered Le Grand K's mass was changing by about 20 billionths of a gram per year -- which adds up roughly to the weight of a grain of pepper every decade.

That may not sound like much, but it could mean big trouble ahead.

"The kilogram and other fundamental units of measure are the foundation of all science, technology and commerce," says physicist Richard Deslattes of the National Institute of Standards and Technology (NIST) in Gaithersburg, where the official U.S. kilogram copy is kept. "If someone were to get their hands on these things and change their values, the universe would be a madhouse."

That's one reason scientists dislike the Paris kilogram. Although it has survived two world wars at the International Bureau of Weights and Measures in Sevres, they still worry that it could be lost or stolen.

Worse yet, microscopic particles of anything from a scientist's sweat to Parisian pollution can damage the platinum. The kilogram's caretakers suspect that some or all of these are behind its current troubles. Even the exquisitely delicate procedure used to clean the kilogram can muck up its mass.

But the ultimate problem is that the Paris kilogram is a thing. It's the sole survivor of an era when weights and measures were based on objects -- an era that most metrologists would like to put behind them.

Using things as standards of measurement is fraught with peril. In 1120, for example, King Henry I of England decreed that the yard should be equal to the distance from the tip of his nose to the end of his outstretched arm, which made international trade problematic. In Saxon England, the barleycorn was the legal unit of weight. Merchants could make more money by soaking their barleycorns in water.

Something unpleasant?

"How do you know the kilogram ain't getting heavier or doing something else unpleasant? You don't as long as you've defined the unit as the mass of that object sitting over there in Paris," says Deslattes.

Now scientists hope to do with the kilogram what they have already done with the meter, the second, and the other basic units of measure: yoke their values not to erratic things but to immutable natural phenomena.

Consider the second. For centuries it was defined as 1/86,400 of a solar day. But then astronomers discovered the planet is actually slowing down, losing a second every few thousand years. So in 1967, they defined a second as the time it takes a cesium-133 atom to vibrate exactly 9,192,631,770 times. Today, the best cesium atomic clocks lose one second in 1.4 million years.

Likewise, the meter was once defined as the distance between two hash marks on a platinum bar, an object as unstable as the kilogram cylinder. In 1983, scientists voted to make it equal to the distance light travels in a vacuum in 1/299,792,458 of a second.

Had scientists not taken this bull by the horns, "much of the modern technology that we take for granted could not have been developed," says physicist Richard Davis at the International Bureau of Weights and Measures in Sevres, France.

Without atomic clocks, space travel would be impossible, since no mechanical chronometer is precise enough to guide spacecraft to the outposts of the solar system. The Global Positioning System -- the web of satellites that tells everyone from soldiers to pleasure boaters precisely where on Earth they are -- wouldn't work either.

Industry at risk

Industry would eventually suffer if fundamental measurements were less stable and precise. High-tech electronics firms such as Intel Corp. already carve transistors into silicon slabs just a few hundred atoms across. Even factories that make car engines talk about measurements in millionths of an inch.

"Every time measurement technology has advanced," says Deslattes, "it spurs innovation."

But the kilogram has been a stubborn holdout, defying the efforts to develop an accurate replacement.

"There are hard experiments, and then there are these things," says Deslattes, who has been searching for a substitute measure of the kilogram on and off since the 1960s.

In July, scientists gathered in Washington to tackle the problem again. Not surprisingly, they have split into two opposing camps.

The "atom counters" want to redefine the kilogram as the weight of a specific number of silicon atoms -- a method not much different in principle from trying to guess how many marbles are inside a fish bowl. In this case, the fish bowl is a 1-kilogram sphere of crystal silicon.

So far, the atom counters have been stymied by atomic irregularities in silicon itself -- not to mention the sheer scale of the enterprise: the mass of each silicon atom is incredibly small and there are so many of them.

"Even if you were able to count a million atoms each second, it would take much longer than the age of the universe to reach the number in one kilogram," says Davis.

The force

That's why the "force measurers," as the second group is known, are taking a different approach, one that many scientists are betting on.

They want to create an electromagnetic force that exactly balances the pull of gravity on a kilogram. By generating that force, they would be able to reproduce the kilogram any time.

To that end, scientists at NIST in Gaithersburg and a laboratory in England are each building giant pan balances much like the kind used in an old-fashioned apothecary. The one at NIST is almost two stories tall. Like a regular scale, the Watt balance, as the device is known, has two suspended pans that tip toward the side with the heavier load.

Fixed below each pan is a cylindrical electromagnet. Scientists place a 1-kilogram bar of gold on one side, then adjust the electromagnetic current on the opposite side to balance it precisely. Whatever combination of voltage and current does the trick would determine the new definition of the kilogram.

It sounds simple, but researchers have spent more than 20 years just getting their numbers into the ballpark. The Watt balance is nearly as capricious as the Paris kilogram, vulnerable to moisture in the air, vibration from passing cars, and the gravitational tug of the moon.

Still, when they presented their latest results last month in Washington, NIST scientists had measured the kilogram to seven decimal places. Unfortunately, they need eight.

So now the U.S. team is building a cavernous vacuum chamber for the Watt balance to eliminate the remaining environmental influences. If it works and other labs can reproduce the results, NIST scientists think they might be able to topple Le Grand K in three years.

They may be just in the nick of time.

Says Deslattes: "I have a feeling that from the aging cadre who are working on this problem, if we don't see some progress in the next few years, I don't know if we can beat the kilogram."

Pub Date: 8/27/98