And so he should. He’s allowed a little sentimentality: Abbott, who works at the National Institute of Standards and Technology, is one of three designated keepers of the US kilogram standard. The lab maintains the standard using a collection of platinum-iridium cylinders stored in an underground lab in Maryland, all replicas of the IPK. All scales manufactured in the US have to be calibrated using some method that traces back to these weights. Your bathroom scale was calibrated by a weight whose mass was confirmed via another weight, and so on, where the last weight in the calibration chain is perching in a bell jar in Abbott’s lab.

A month ago at his lab in Maryland, Abbott showed me the weights. He handled the weights sweetly, almost like an owner tending to his pets. The first time he ever picked up one of the kilogram replicas, to place it inside a machine, was 12 years ago. The proper protocol involves grabbing them with a pair of tongs covered in soft chamois leather and coated in lint paper. “I was so scared,” he says. “It was like if someone had said, ‘Why don’t you take my Ferrari for a ride?’” The IPK’s home lab in France sells kilogram cylinders at around $85,000 apiece, depending on the price of platinum. Platinum iridium is an extremely hard material and difficult to scratch, but “it makes you paranoid,” he says.

Abbott also has to monitor the weights, to check that their masses stay the same over time. In the lab, he has developed a nearly obsessive attention to cleanliness and frequently reminds his colleagues to change their gloves. “If your gloves are dirty, and you pick up a tool, whatever’s on the gloves are going to go on the tool. And that means it could get on the mass and change its weight,” he says. “You have to remember where your hands have been, and what they’ve touched.” His vigilance has kept the cylinders largely safe from mishaps. “One time I dropped one of the masses rather hard [inside a machine], and it fell over,” says Abbott. “I was worried about that, but it didn’t hurt anything.”

He knows the weights well enough to have favorites: K4 and K79, whose numbers signify the order in which they were manufactured. “They’re just so stable over the years, so I really like them,” he says. “When you measure their mass, they really don’t change.”

K4, along with another cylinder named K20, are the most historic items in the collection: both are 130-year-old platinum iridium cylinders that are replicas of Le Grand K. “They’re brothers, cut from the same bar of platinum iridium,” says Abbott. Periodically, he or one of his colleagues have to hand-carry them to France, to check if their masses have fluctuated against the one true kilogram. There, they reunite the cylinders, one per trip, with its brother at its home French lab, which compares their weights.

Abbott has made the trip once, in 2011. “It’s a real cloak-and-dagger affair,” he says. He treated the kilogram like a precious carry-on. Using the tongs, he placed it in a custom-built container, a tiny covered platform with ungreased screws that squeak when you fiddle with them. Then he wrapped it in bubble paper and stuck it inside a camera bag. To keep customs and TSA officials’ grubby hands from opening the container, the director of NIST wrote him an official letter describing the mission to accompany the kilogram.

On the plane, Abbott kept the kilogram next to him on the seat for the whole ride. He even took it to the bathroom with him. “I didn’t want to be the one known for losing the kilogram,” says Abbott.