'Le Grand K' has a weight problem Kilo: A platinum cylinder near Paris is the world's standard for the kilogram. Unfortunately, it's beginning to slip -- 20 billionths of a gram annually.

August 27, 1998|By Michael Stroh | Michael Stroh,SUN STAFF

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.

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