THE AVOGADRO CONSTANT

The ubiquitous character of N A has been described by summarizing the results obtained in different periods with different methods and by illustrating in more detail the X-ray crystal-density method. A careful analysis of the quantities involved demonstrates that the search for systematic errors is endless. At present, metrological laboratories are engaged in reducing the relative uncertainty from the 10-7 to the 10-8 level or better. If this could be achieved, a new definition à la Maxwell of the mass unit, and the relevant realization, would be feasible. However, the main justification, to continue devoting human and financial resources, lies within a wider framework. The loop starts with the observation that fundamental constants were the foundations on which Science rested when solid-state physicists, in the forties and fifties, developed the transistor, a significant step in computer technology. The subsequent increasing demand from the micro-electronics industry led to the production of near-perfect silicon monocrystals which were instrumental in the realization of X-ray interferometry; this new kind of interferometry, in its turn, triggered substantial improvements which led to a more accurate value of N A . The loop closes with the observation that N A, as a result of the continuous interaction between Physics and Technology, has contributed to strengthening the edifice of the fundamental constants. © 1995 Società Italiana di Fisica.