PARIS — A kilogram just isn't what it used to be. The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously ...

For more than a century, a single metal cylinder in a Paris vault quietly defined what a kilogram was. After 130 years, scientists deliberately killed that object’s authority and rebuilt the standard ...

Scientists and policy makers from around 60 nations voted unanimously Friday to redefine the kilogram. The decision at the General Conference on Weights and Measures in Versailles, France was greeted ...

But the kilogram prototype, known as “Le Grand K,” was made by humans and is subject to all our limitations. It is inaccessible — the safe containing the cylinder can be opened only by three ...