National Geographic : 2009 Oct
TECHNOLOGY PHOTO: MARC STEINMETZ A Grander K What is a kilogram? It's 2.2 pounds, of course. Or is it? The kilo is the only basic international standard pegged to a physical object---a 120-year-old platinum-iridium cylinder kept in a vault outside Paris and known as Le Grand K. In recent years scientists noticed slight variations in the cylinder's weight. They've gone into high gear to redefine the kilo as a universal constant based on nature instead of an object vulnerable to distortion. Physicists in some countries are analyzing near-perfect spheres of pure silicon crystal that will allow them to count the number of atoms in a kilo for the first time. Other scientists are measuring the kilo in terms of gravity and magnetism. Once results are confirmed, an international committee will make the final decision on redefinition, perhaps as soon as 2011. "It'll be a grand thing," says physicist Richard Steiner, who leads the American effort. Then Le Grand K can gain or lose as much weight as it wants. ---Hannah Bloch German physicist Arnold Nicolaus wants to count the atoms in this silicon sphere to set a standard for the kilogram. STANDARD SOLUTIONS Unlike the kilogram, the other six basic international units---meter, second, ampere, kelvin, mole, and candela---are defined by universal measures. The meter is defined by the length light travels in 3.3 nanoseconds.