Forged against a backdrop of scientific and political upheaval following the French Revolution, a single, small cylinder of platinum-iridium alloy
has laid largely undisturbed for nearly 130 years as the world's benchmark for what, precisely, is a kilogram.
This reference mass is used to calibrate national standards made of platinum-iridium alloy
Experts are willing to make the changes so that it is no longer based on the mass of a solid cylinder of platinum-iridium alloy
that sits beneath three layers of protective glass sealed in a locked vault in Sevres, France.his metal block, known as the International Prototype Kilogram, has been used since it was first registered with the International Bureau of Weights and Measures (BIPM) in 1889 as the definitive unit of mass against which all other kilograms are measured.
The International Prototype Kilogram (IPK) or "Le Grand K," made in the 1880s, is a bar of platinum-iridium alloy
kept in a vault near Paris.
Among global standards for length, mass, time, and other fundamental quantities, only the kilogram remains a physical object--a carefully machined cylinder of platinum-iridium alloy
at the International Bureau of Weights and Measures at Sevres, France.
Careful recalibration against the so-called international prototype of the kilogram -- a lump of platinum-iridium alloy
that sits in a vault at the International Bureau of Weights and Measures in Sevres, France -- showed the U.S.