ACCURATE DETERMINATION OF THE DENSITY OF A CRYSTAL SILICON SPHERE

For an independent determination of the Avogadro constant, the density of a 1-kg crystal silicon sphere was determined by direct measurement s of its mass and volume. A scanning type interferometer was used to m easure the diameters, and the volume was calculated from the mean of u niformly distributed diameters. The sphere was weighed using a balance for the Prototype Kilogram at the NRLM, and the mass was determined a ccurately by direct measurements of the buoyancy force acting on the s phere. The total uncertainty of the density is estimated to be 0.34 pp m. Effects of a thin oxide layer and impurities on the density of a pu re crystal are evaluated.