Volume of the Avogadro sphere: diameter determination with the spheres interferometer

For the new determination of Avogadro's constant, N/sub A/, one part of the measuring task consists in the high accuracy determination of the volume of a silicon sphere with a mass of 1 kg and a diameter of about 90 mm. An interferometer with spherical reference surfaces has been designed for this specific purpose. The measuring principle of the interferometer and first measurements on silicon spheres are described.