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.