The Sciences

The Kilogram Isn't What It Used to Be—It's Lighter

Within a high-security, climate-controlled vault in France, the perfect kilogram is getting ever so slightly less massive—and no one knows why.

By Dava SobelMar 7, 2009 6:00 PM


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Sèvres*, France—What I love best about the kilogram is its tangibility, its solid, sculpted form of shiny platinum and iridium. I’m referring to not just any kilogram but the quintessential one that resides here—the actual International Prototype Kilogram, or IPK, created in 1879 as the official standard of mass. It’s a smooth cylinder of alloy, only an inch and a half high and an inch and a half in diameter. Though petite, the IPK is necessarily dense; it weighs 2.2046 pounds. If you went to pick it up, you might think someone had cemented it to the tabletop for a prank. Even if you knew what to expect, its compact heft would still boggle your senses.


Of course, they won’t let you pick it up. They won’t even let you anywhere near it. If you touched it—if you so much as breathed on it—you would change its mass, and then where would we be? That’s why the IPK leads such a sheltered life. It is kept under a triple bell jar inside a temperature- and humidity-controlled vault in a secure room within the Parc de Saint-Cloud enclave of the International Bureau of Weights and Measures, or BIPM (Bureau International des Poids et Mesures). Thus protected, it reigns over a world’s worth of measurement. Every hill of beans, every lump of coal, every milligram of medication—in short, every quantity of any substance that can be weighed—must be gauged against this object. The IPK is, in and of itself, the International System of Units’ definition of mass. Through a complex dissemination protocol, the essence of the kilogram is transferred from the IPK to its counterparts at standards laboratories around the world, and from there to centers of industry and scientific research, ending up in grocery stores, post offices, and bathrooms everywhere. Although I have come to pay my re-spects to the IPK, I am denied even a glimpse of the thing. Nor can I see one of its six official copies, for these reside alongside the prototype in guarded seclusion. I must content myself with replicas —with the working standards that fill the ultraclean laboratory of Richard Davis, an American physicist in Paris who for the past 15 years has headed the Mass Section at the BIPM. Gloved for work, Davis wears a lab coat over his street clothes, blue paper bootees over his shoes, and a net over his hair. Around him kilogram weights of various shapes and materials sit on colored plates under glass bell jars, like an assortment of fine cheeses. They have been delivered here from other countries to be reckoned in comparison with the IPK. “That one belongs to Ireland,” Davis says, indicating a stainless-steel kilogram on a red dish. Member states—signatories to the Meter Convention—pay dues to the BIPM that cover the cost of periodic checks on their national reference standards. It takes a minimum of four days to calibrate a single kilogram according to the BIPM’s cautious regimen of repeated comparison weighings. Visiting kilograms could theoretically go home after a week, but they typically stay in the lab for months, allowing the time it takes them to become thermally stable in their new surroundings, undergo cleaning by the BIPM method, and prove themselves, through repeated trials, to be worthy ambassadors of mass. Given the uncertainty, however minuscule, in every measurement, such repetitions are essential before these national standards can leave with a calibration certificate stating how they compare with the IPK, along with a precise correction factor. En route to or from Paris, the visiting kilograms disdain ordinary transport. Zeina Jabbour, group leader of Mass & Force at the National Institute of Standards and Technology in Gaithersburg, Maryland, recently brought two of the four U.S. kilograms here for calibration. She carried one herself in a specially designed case inside a padded camera bag that was all but handcuffed to her wrist, and she entrusted the other to a colleague who flew on a different plane. (“That way, if something happened to one of us.”) Soon after her flight touched down at Charles de Gaulle, she grabbed a taxi straight to the BIPM on the other side of the city for a handover directly to Davis. * The dateline incorrectly read, "Sèvres Cedex, France." Return to the corrected dateline. Before picking up a kilogram with a pair of widemouthed forceps called lifters, Davis flicks off suspected specks of dust with a fine-tipped brush. (“My wife paints.”) He has modified the artist’s brush for his purposes by degreasing its fibers and covering its metal ferrule with plastic, “so if you accidentally hit the kilogram, you won’t scratch it.” On a balance precise to 10 decimal places, a scratch counts. Davis tests the Irish kilogram in a sealed chamber against three BIPM working standards that are also made of stainless steel. He doesn’t weigh it against the platinum-iridium standard, since stainless weights are only one-third as dense, and therefore three times as large, displacing a much greater quantity of air. “You’d have to make an air buoyancy correction that would amount to almost a tenth of a gram,” he explains. “That is huge.” Although Davis serves as the IPK’s official guardian, even he rarely sees the original prototype, which is too precious and vulnerable to damage to remain in constant use. Over the course of its century-plus lifetime, the IPK has emerged only three times to serve “campaigns” of active duty, most recently in 1988–1992, when it participated in a formal verification of all kilogram prototypes belonging to the 51 Meter Convention member states. On that occasion, however, the IPK itself was found wanting. Despite all the protective protocols and delicate procedures, it had mysteriously changed. No one can say whether the IPK has lost weight (perhaps by the gradual escape of gases trapped inside it from the start) or if most of the prototypes have gained (possibly by accumulating atmospheric contaminants). The difference is approximately 30 micrograms —30 billionths of a kilogram—in a hundred years. (Imagine 30 cents out of a $10 million stack of pennies.) This alarming show of instability is driving global efforts to redefine the kilogram, so that mass need not depend on the safety or stability of some manufactured item stored in a safe. In fact, more than mass hangs in the balance, for the kilogram is tied to three other base units of the International System of Units (SI), namely the ampere, the mole, and the candela. Several more quantities—including density, force, and pressure—are in turn derived from the kilogram. Other 19th-century artifacts of measurement have long since been retired in favor of fundamental constants of nature. In 1983, for example, the platinum-iridium bar that described the length of the meter yielded to a new benchmark: A meter is now defined as the distance light travels in a vacuum in 1/299,792,458 second (a second being the time it takes an atom of cesium-133 to vacillate 9,192,631,770 times between the two hyperfine levels of its ground state). These figures fail to give the average person any real feel for the quantities in question, but to a metrologist —one who specializes in the science of measurement—such equivalences rooted in physics have the advantage of permanence and reproducibility. One invariant vying to replace the IPK is Planck’s constant, which could be determined via an experimental device called a watt balance. Alternatively, researchers may successfully express mass in terms of Avogadro’s number (which is tied to the unchanging mass of individual atoms), provided they can count the atoms in a crystal of silicon-28. But neither of these complex, costly endeavors is likely to yield a new standard in time for the next meeting of the General Conference of Weights and Measures, scheduled for 2011. For now, the International Prototype Kilogram stands firm on metrology’s last frontier.

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