The plan and purpose of this book are outlined in chapter 1, and what is said there need not be repeated here. A preface does, however, offer the opportunity of acknowledging the help I have received during its preparation, and indeed over the years before it was even conceived. I cannot begin to guess how far my understanding and opinions have benefited from the good fortune that has allowed me to spend so much of my working life in the Cavendish Laboratory, surrounded by physics and physicists of the highest quality, and by students who at their best were at least the equal of their teachers. For the resulting gifts of learning, casually offered and accepted for the most part, and now untraceable to their source, I extend belated thanks. More specifically, in the preparation of the material I have relied heavily on Douglas Stewart for the construction of apparatus, Christopher Nex for programming the computer to draw many of the diagrams (and incidentally for two notes on his bassoon in fig. 5.8), Gilbert Yates for help in the theory and practice of electronics, and Shirley Fieldhouse for typing and retyping of the manuscript. My thanks to each of them for much willing and expert help, and to the following who by discussion and criticism, by assistance in carrying out experiments, or by providing material for diagrams have given support and comfort on the way: Dr J. E. Baldwin, Mr W. E. Bircham, Prof. J. Clarke, Dr M. H. Gilbert, Prof. O. S. Heavens, Dr R. E. Hills, Dr I. P. Jones, Mr P. Jones, Mr P. Martel, Prof. D. H. Martin, Mr P. D. Maskell, Dr P. J. Mole, Mr E. Puplett, Dr G. P. Wallis and Mr D. K. Waterson.