PRECISION VOLUME MEASUREMENTS ON SILICON SPHERES

In the determination of the Avogadro constant N-A, use is made of sili con spheres as volume-density standards, where volume measurements pla y a critical role, After various changes in the experimental apparatus and in the measurement procedure. neu volume measurements have been m ade on the two IMGC spherical standards, Present results are confirmin g the 1994 results within 0.15x10(-6) in volume, Preliminary data are also presented on the thermal expansivity of four silicon spheres, The average linear coefficient of thermal expansion at 20 degrees C is al pha = 2.567 x 10(-6) /K-1, with a sphere-to-sphere variability of up t o 2.5 x 10(-8)/K-1.