The "Avogadro" problem: summary of tests with null results

Experiments for the re-definition of the unit of mass by the X-ray crystal density method appear extremely difficult and time-consuming, and no laboratory has reached uncertainties below one part in 10/sup 7/. At present, success is additionally hampered by an unexpected imperfection of the Japanese N/sub A/ crystal, which has not yet totally identified and which causes a spread of the measured data over a range of more than 3 parts in 10/sup 6/ thus pointing to a density lower than that expected by all those involved. Recent progress in the development of moving coil watt balances, a competitive experiment, is, however, motivating: the measurement facilities for N/sub A/ are currently improved and at the same time any density influences on the molar volume are being investigated and proved.