The "reference kilogram" is a lump of metal in France — the kilo being the only metric measurement not based on some objective standard. A project is underway to replace the kilo with something independently reproducible: perfectly spherical balls of based on Carbon-12. Incidentally, these new kilos would also be "the roundest objects in the world."
Fabrication of a Silicon SphereBy definition, an Avogadro number of Carbon-12 atoms weigh exactly 12 grams. As such, the kilogram could bedefined as the mass of 1000/12 * Avogadro's number of Carbon-12 atoms. The Avogadro constant itself is obtained from the ratio of the molar mass to the mass of an atom. For a crystalline structure such as silicon, the atomic volume is obtained from the lattice parameter and the number of atoms per unit cell. The atomic mass is then the product of the volume and density.
The Avogadro Project involves an international collaboration between laboratories in Germany, Italy, Belgium, Japan, Australia and USA. Currently the Avogadro constant is known to an uncertainty of approximately 0.1 ppm. It is hoped that the uncertainty will be reduced to 0.01 ppm after a further five years.
In determining the Avogadro constant, the preferred method has been to use one of the high-precision spheres fabricated here at the ACPO. These come in the form of a highly polished 1 kg single crystal silicon sphere, fabricated with a roundness in range of 60 nm. Silicon is used because of its well known crystal structure, stability and its relative ease of use. The volume is determined from the measurement of the silicon sphere's diameter and roundness. Accurate measurement of the mass then allows the density to be derived.
(via JWZ)
