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Why the Kilogram Keeps Getting Heavier

You may not be the only one who’s put on a few over the years.

Metrologists—folks who are downright fanatical about weights and measures—have discovered the international standard used to define the kilogram has gained tens of micrograms from surface contamination over the decades.

The standard, a cylinder of platinum and platinum-iridium alloy called the “international prototype,” has been one of the world’s seven standard base units of measurements since the late 1800s. And while a few micrograms of weight gain isn’t exactly going to rival the results of you scarfing down a diet of Krispy Kremes, it matters when measuring critical items like restricted radioactive material.

A new cleaning method using ozone and ultraviolet light may help scrub away most of an IPK’s excess weight, but not all of it. Future measurement standards may forsake the metal cylinders for technology such as electromagnetism that could produce a constant amount of force equal to the desired weight.