How and Why the Kilogram Value Changed

At this year’s General Conference of Weights and Measures, a collective of 60 countries voted to change the kilogram’s value. Up until this vote, the kilogram was based on a platinum and iridium fragment, called the “Le Grande K,” which is stored in a chamber in the International Bureau of Weights and Measures in Sèvres, France. Over time, Big K seemed to be losing weight—or its replicas around the world (which researchers elsewhere used) were somehow heavier. This meant that a decision was made: “rather than basing the unit on this physical object, henceforth, the measure will be based on a fundamental factor in physics known as Planck’s constant.” Read more about the change at National Geographic.