Avogadro Project silicon sphere. Silicon sphere in a measuring machine being used as part of the International Avogadro Project. The project aims to find a new way of defining the SI unit of mass, the kilogram. It is possible to define the kilogram as a fixed number of atoms of a particular substance, thus relating the kilogram to an atomic mass. Silicon is a good candidate for this approach because it can be grown as a large single crystal, in a very pure form. Photographed at the National Physical Laboratory, Teddington, UK.