Opening address

Abstract

Ladies and GentlemenMy cordial thanks to you for participating in our workshop and to all those who have sponsored it.When in 1957 I attended the International Congress on Fundamental Constants held in Turin on the occasion of the first centenary of the death of Amedeo Avogadro, I did not expect that about thirty-five years later a small but representative number of distinguished scientists would meet here again, to discuss how to go beyond the sixth decimal figure of the Avogadro constant.At that time, the uncertainty of the value of this constant was linked to the fourth decimal figure, as reported in the book by DuMond and Cohen. The progress made in the meantime is universally acknowledged to be due to the discovery of x-ray interferometry. We are honoured that one of the two founding fathers, Prof. Ulrich Bonse, is here with us, but we regret that the other, Prof. Michael Hart, is not present. After Bonse and Hart's discovery, the x-ray crystal density method triggered, as in a chain reaction, the investigation of two other quantities related to the Avogadro constant—density and molar mass. Scientists became, so to speak, resonant and since then have directed their efforts, just to mention a few examples, to producing near-perfect silicon spheres and determining their density, to calibrating, with increasing accuracy, mass spectrometers, and to studying the degree of homogeneity of silicon specimens.Obviously, I do not need to explain to you why the Avogadro constant is important. I wish, however, to underline that it is not only because of its position among fundamental constants, as we all know very well itsdirect links with the fine structure constant, the Boltzmann and Faraday constants, the h/e ratio, but also because when a new value of NA is obtained, the whole structure of the fundamental constants is shaken to a lesser or greater extent.Let me also remind you that the second part of the title of this workshop concerns the silicon representation of the mole. Most of you, I presume, are neo-Pythagoreans, and consequently believe that a new definition and, maybe, a new realization of the unit of mass will be based on a number of atoms of silicon, a view which will certainly lead you to cross swords with the "electrical party".The importance of NA is also linked to the considerable and far-reaching return in other scientific and industrial fields. Finally, let me add that, ethically, the work of many persons all over the world and the money and energy they spend in order to add a decimal figure, may be an example of commitment to be given to our students.Last but not least, my warm thanks to the Director of the Istituto di Metrologia "G Colonnetti", where the experiment has been in progress since 1971, and to all the researchers involved in this work. I do hope that the National Council of Research will continue to support this important project.While wishing you a pleasant stay in Turin, I express the hope that our meeting will prove a fruitful opportunity for discussion and exchange of views.