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Special Issue on Physical Units

The concept of the physical unit is key to the understanding of physics, for it is the link between the theoretical relationships which describe physical quantities and the measurements which confirm or confound their predictions. No topic can be more suitable for treatment in a special issue of Metrologia, for the journal's field is measurement: fine measurements, measurements of fundamental quantities, the relationships between measurements, the processes and conventions which allow measurements to be exchanged, duplicated and confirmed. And if physical quantities are to be measured, reference must be made to the units in which they are expressed. Units, in turn, serve no function in isolation. Used individually, units can convey only the simplest of ideas. When organized, however, so that they are part of a system which is logically coherent, which permits the transfer of measurements from one place to another or from one area of science to another and is recognized universally by those whose work depends on unambiguous statements of quantity, they represent a tool of remarkable power with which to express quantities in a concise and consistent way. This issue describes the units of the Système International d'Unités (SI). It does this in a series of essays, each of which treats the SI in a way which reflects the personal interests and viewpoint of the author. Individual articles deal with the definition and description of units, their historical development and their application in scientific, legal, business, regional and international affairs. There is no pretence that this is a textbook on the SI, for the topic is not covered exhaustively, completely or even uniformly: the purpose of these essays is to show that the SI is ubiquitous, touching many aspects of human endeavour; that it is useful, serving the interests of science, technology and commerce; and that it is not closed, the underlying principles of how to describe, select, define and disseminate the base and derived units of physics still being open to debate. Giving weight and coherence to this issue is the collective experience of the authors. Individually, they have made important contributions to measurement, have taken part in the operations of national standards laboratories and in the work in which such laboratories collaborate. They are members of the committees which negotiate terms for the use and recognition, at national, regional and international level, of the units which industrial laboratories maintain. Just under half of them are members of the Comité Consultatif des Unites (CCU), the committee to which the Comité International des Poids et Mesures (CIPM), itself turns when it looks for advice on issues affecting the use of units. This issue deals with topics which are central to the decisions of the CIPM. This is an appropriate place, therefore, to mention the close interest which two members of the Comité International have taken in Metrologia and the unfailing support they have given it: Professor Dieter Kind and Professor Jan de Boer. As President of the Comité International, Professor Kind greatly influenced the decision that the Bureau International des Poids et Mesures should purchase Metrologia and did much to create the spirit of independent commentary in which the journal operates. Now that his retirement from his principal post as President of the Physikalisch-Technische Bundesanstalt, Braunschweig, approaches, I take this opportunity, on behalf of Metrologia, its readers and its Editorial Board, to wish him a happy retirement and hope that in the future he will still find time to take an interest in the affairs of the journal. Professor de Boer's contribution to measurement is long and distinguished. His membership of the CIPM began in 1954 and he was its Secretary from 1962 to 1989. He thus took part in the debate which resulted in the decision to launch Metrologia in 1965. As the sole person to hold office as President of the CCU, he writes with unique authority in the article which opens this issue. He was a contributor to the first volume of Metrologia, and I hope that his most recent contribution marks the beginning of a new and distinguished series.