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For scientists, kilogram loses mass appeal
Standard used to define unit since late 1800s is becoming less precise
11/18/2002
On the outskirts of Paris, in a locked vault to which only three people have the key, lies a treasure worth more than its weight in gold. It's even worth more than its weight in platinum and iridium, which is what it's made of. The squat metal cylinder weighs exactly 1 kilogram, as it should. It is the world's definition of mass, the standard kilogram against which all others are judged. But now "le grand K," as the kilogram is known, is putting itself out of date. Since it was cast in the late 1800s, it has changed mass ever so slightly, drifting by a few millionths of a gram per year when compared with six copies made at the same time.
And that just won't do, physicists say. "It's scientifically very unsatisfactory to have a mass standard that changes in mass," said Paul De Bi�vre, a standards expert at the European Commission's Institute for Reference Materials and Measurements in Geel, Belgium. It's time for a new kilogram standard, researchers say � one that won't depend on the vagaries of a single chunk of metal. So physicists are striving to replace le grand K with a fundamental physical measurement to last forever. Scientists have done so for other important units of measure. A second, for instance, is defined as 9,192,631,770 periods of a flickering between two levels of a cesium-133 atom. A meter is the distance light travels in a vacuum during 1/299,792,458ths of a second. (It used to be the distance between two scratches on a certain platinum-iridium bar, which is kept next to le grand K at the International Bureau of Weights and Measures, or BIPM, near Paris.) To fix the kilogram, one group of physicists is trying to define mass based on voltage, resistance and other electromagnetic measurements. A second group wants to make a perfect sphere of silicon; by counting the number of atoms in it more accurately than ever, the scientists hope to arrive at a new mass standard. One of these ideas � or both, or neither � may replace le grand K in the next decade or two. "At least we can do no worse than it's been for the last 100 years," said Richard Steiner, a physicist at the National Institute of Standards and Technology in Gaithersburg, Md. Why it's needed "The thing that unites us in the world is the definition of the units," Dr. De Bi�vre said. The metric system, used almost everywhere in the world except for the United States, was born during the French Revolution in an effort to unify the many weights and measures used at the time. In 1875, 17 nations signed the Metre Convention and adopted the metric standards.
Today, 51 countries have signed on to the International System of Units, or SI, after its French acronym. It defines seven basic units: meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for the amount of a substance and candela for luminous intensity. Over the years, all units but the kilogram have received a physical definition that doesn't depend on a thing. The kilogram is last because it's not an easy thing to define. It was first conceived of as the mass of a cubic decimeter of water at 4 degrees Celsius. (In English units, a kilogram equals roughly 2.2 pounds.) Today, the kilogram is whatever the mass of le grand K is. It's a squat metal cylinder, about an inch and a half high and wide, made of 90 percent platinum and 10 percent iridium. That mixture is particularly stable and dense, making it a good candidate for a lasting mass standard. In 1889, keepers of le grand K made six copies, which are kept in the same temperature- and humidity-controlled vault as the original. Most of the countries that signed the Metre Convention have their own copy of the kilogram, which they occasionally send to Paris for calibration. Le grand K comes out of its vault only once every few decades � "only when there's a scientific reason to think that your uncertainty is too high," said Richard Davis, head of the mass section at the international standards bureau. It hasn't been out since 1992, when it was cleaned with a chamois cloth coated with solvents, then steamed with doubly distilled water. Nobody knows why the mass of le grand K fluctuates compared with its copies, but some scientists think it might be absorbing atmospheric contaminants. Even weighing it is a delicate job, as bumping it against the scale flakes off tiny pieces, Dr. Davis said. The new efforts aim to replace the kilogram standard with one that varies by less than 1 part in 100 million each year. For one group of scientists, that means defining the kilogram as a certain number of atoms of a particular element. Working backward "You essentially add up this huge number of atoms in the crystal just by knowing what the spacing between the atoms is," Dr. Davis said.
Once the scientists know how many atoms are in 1 kilogram, that number could redefine the kilogram standard. The team works with the element silicon, fashioning spherical crystals a bit bigger than a billiard ball. "All of this has to be done very carefully" because of the precision required, said Dr. De Bi�vre, one of the project's leaders. "No national institute can do all of what is needed." Labs in Australia, Belgium, Germany, Italy, Japan and the United States share the tasks of creating and polishing 1-kilogram spheres of silicon, then measuring their physical properties. For years, the project was foiled by naturally occurring holes that dotted the silicon crystal like missing gumballs in a gum dispenser. Only recently have scientists realized that the presence of such holes could explain why silicon spheres made by different labs appear to have different densities. The delay did have one spinoff, said physicist Frans Spaepen of Harvard University: Computer chip manufacturers now know exactly how many holes riddle silicon crystals. The next step for the team is to make a sphere out of pure silicon-28. Until now, the scientists have used a mixture of the three naturally occurring silicon isotopes � silicon-28, -29 and -30 � which are forms of the element with different masses. Scientists at the Institute for Crystal Growth in Berlin will soon make test crystals of silicon-28 to see how the material behaves. The project could have its kilogram replacement as soon as six years from now, Dr. De Bi�vre said. By then, the competing technique may also have an answer. This second device, known as a "watt balance," was invented in the 1970s by Bryan Kibble of England's National Physical Laboratory. The apparatus works by balancing the force of gravity pulling down on a 1-kilogram mass against an upward-pulling magnetic force. The device can indirectly define the kilogram because all the other units measured � such as time, length, voltage and resistance � are already precisely defined. The idea "has a beautiful exactness about it," Dr. Kibble said. But it's not so easy to pull off. Scientists have built two big watt balances � a two-story one at the U.S. standards lab in Gaithersburg and a room-sized one in England. Both devices work pretty well, but so far neither is accurate enough to do better than le grand K. The British watt balance reported in 1988 that it had some encouraging results, which were confirmed a decade later by the American one. But now the British team has redone its experiment and reached a different conclusion. "We're still not there," Dr. Kibble said. The American team has also torn down and rebuilt its watt balance. Preliminary results are expected by the end of this year, said Dr. Steiner. "Back in '98 when we agreed with them [the British lab], it looked real neat," he said. "That's why everybody is looking to see, once we've rebuilt our system, will we get the same number that we did four years ago?" Other approaches It's not clear which, if any, of these new approaches will finally go to the BIPM as the kilogram standard. The teams see themselves as improving precision physics, not as competing for an international prize. "If the purpose of all this work was to beat others, I would stop instantly," said Dr. De Bi�vre of the silicon sphere project. Eventually, he said, the watt balance and silicon sphere ideas might be used to cross-check each other. Whatever the new kilogram standard turns out to be, it must hold true for the next 100, or 1,000, or 10,000 years for measurers. It mustn't depend on a single thing sitting in a vault in Paris. And it must be reproducible anywhere in the world. "It doesn't matter, you don't have to have it at the BIPM," said Dr. Davis. "Anyone could have one." E-mail [EMAIL PROTECTED]
Krishna Kambhampaty
[EMAIL PROTECTED]
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