  To base the kilogram definition on the atomic mass of the silicon 28 atom,
the present relative uncertainty of the silicon 28 lattice parameter must
lowered to 3E-9. To achieve this goal, a new experimental apparatus capable of
a centimetre measurement-baseline has been made at the INRIM. The comparison
between the determinations of the lattice parameter of crystals MO*4 of INRIM
and WASO4.2a of PTB is intended to verify the measurement capabilities and to
assess the limits of this experiment.
