Though metrologists chose the Planck constant as the basis for redefinition, other constants of nature can also be used, if for no other reason than to provide a check that the Planck constant definition is correct. For decades, an international collaboration has been working on a complementary strategy for defining the kg that involves counting the number of atoms in an almost perfectly round, softball-sized sphere of silicon-28. The goal: a more precise determination of the Avogadro constant ( N A), which relates the number of atoms or molecules in a substance to its mass. Alternative Recipe for the Kg. NIST's Carl Williams with a silicon sphere, not one of the two used in the Avogadro project. In the international system of measurement (SI), the Avogadro constant ( N A) is technically defined as the number of carbon-12 atoms in 12 grams of carbon-12. Equal to about 6.022 x 10 23 'constituents' (usually atoms or molecules) per mole, with moles being the standard unit to describe an amount of a substance, the Avogadro constant is used to make the huge numbers used in chemistry more manageable. A mole of any substance — hydrogen molecules, carbon atoms, golf balls — equals 6.022 x 10 23 of its constituents (in this case hydrogen molecules, carbon atoms, golf balls).

Close-up of the sphere. Click image for larger version.Since the constant's definition depends on a substance's mass, scientists reasoned that they could exploit this relationship by working backwards, from a well-defined N A to the kilogram. Rasterlink Pro 5 Activation Key.

First, though, they will need to define N A with greater precision — with a relative uncertainty of just 20 parts per billion, or 2.0 x 10 -8 — in order for a new kg definition based on the Avogadro constant to compete in accuracy and reliability with the current standard. Vogue Prima Vista Motorhomes For Sale. Beginning in the early 1990s, members of a worldwide collaboration called the International Avogadro Coordination (IAC) — now known as the International Avogadro Project — have been working to describe this number with sufficient resolution. Mitsubishi Plc Instruction Set Manual Manual. Instead of a chunk of carbon-12 atoms, they use a uniform crystal of silicon-28, carefully machined into spheres with a mass of 1 kg based on the current definition.