Since 1889, the Grand K, a golf ball–size cylinder of platinum alloy, has been the standard for the world’s definition of a kilogram. But last Friday, representatives of 60 countries voted to end the Grand K’s reign. Beginning May 20, 2019, a quantum physics equation will define the measurement rather than a physical object.
Scientists originally defined the kilogram as the mass of a liter of water. But such a comparison proved inconvenient and imprecise, so they forged the Grand K. It has served as the standard of the metric unit of mass for nearly 130 years. A vault in Paris houses the carefully guarded platinum cylinder, which experts take out on rare occasion to compare it with six identical copies kept safe under separate bell jars. Unlocking the vault requires three keys kept in different locations.
Martin Milton, director of the International Bureau of Weights and Measures, said agreeing on the equation that defines the kilogram is “a tribute to man’s ability to collaborate.”
“It’s been called a great work of peace, actually, because it’s one of the areas where all of the states of the world come together with absolutely the same objective,” Milton said. —J.B.