Hidden in a vault outside Paris, vacuum-sealed under three bell jars, sits a palm-sized metal cylinder known as “Le Grand K.” Forged in 1879 from an alloy of platinum and iridium, it was hailed as the perfect kilogram—the gold standard by which other kilograms would be judged. However, it is wasting away, and scientists are scrambling for a more reliable standard. Part of the "Uno Dos of Trace" series.
