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The international prototype of the kilogram is made of the metals platinum and iridium and sets the standard weight for the kilogram. (BIPM / AFP / Getty Images)

Is the Kilogram Gaining Weight?

Scientists are trying to stop this standard unit of measurement from getting bigger

By Sara Goudarzi | null null , null

The kilogram has been putting on weight! Since 1875, a metal cylinder in a vault near Paris, France, has been used as the international standard for the weight of a kilogram. But over the years, this cylinder has gained small amounts of mass, and scientists are now trying to shrink it back down to size.

Mass is the amount of matter in an object. Made of two metals, platinum and iridium, the cylinder—known as the international prototype of the kilogram (IPK)—has been used to compare the mass of other objects. Kilograms are considered the standard unit of measurement that people use to weigh everything in our universe.

“We’re only talking about a very small change—less than 100 micrograms,” says Peter Cumpson of Newcastle University in England. A small grain of sand, for example, is about 600 micrograms. “But mass is such a fundamental unit that even this very small change is significant,” says Cumpson.

The change in weight is especially important in industries like medicine, where every microgram needs to be measured just right.

How did the weight gain happen? The metals that the cylinder is made from have been weighed down with molecules from the air, like mercury and hydrocarbons (compounds made up of hydrogen and carbon). There are 40 replicas of the IPK around the world, and scientists have discovered that all the kilogram prototypes are gaining weight at different rates.


This doesn’t mean that the definition of the kilogram will change. But the International Committee for Weights and Measures, which takes care of the prototype, still wants to solve the IPK’s weight problem.

So scientists are putting the kilogram on a “diet.” They are exposing the IPK to ultraviolet (UV) light and ozone—a molecule with three oxygen atoms—to strip it of contamination. This mixture of ozone and UV light is like a very strong dose of sunlight.

“What we have done at Newcastle is effectively give these surfaces a suntan,” Cumpson says.

Scientists know that the solution is only temporary: The IPK will put on weight again. They are still searching for a permanent way to standardize this important unit of mass.

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