BY R. T. GLAZEBROOK, M.A., F.R.S. FELLOW OF TRINITY COLLEGE, AND W. N. SHAW, M. A. FELLOW OF EMMANUEL COLLEGE Demonstrators at the Cavendish Laboratory, Cambridge THIRD EDITION LONDON LONGMANS, GREEN, AND CO. AND NEW YORK: 15 EAST 16th STREET PREFACE. THIS book is intended for the assistance of Students and Teachers in Physical Laboratories. The absence of any book covering the same ground made it necessary for us, in conducting the large elementary classes in Practical Physics at the Cavendish Laboratory, to write out in MS. books the practical details of the different experiments. The increase in the number of well-equipped Physical Laboratories has doubtless placed many teachers in the same position as we ourselves were in before these books were compiled; we have therefore collected together the manuscript notes in the present volume, and have added such general explanations as seemed necessary. In offering these descriptions of experiments for publication we are met at the outset by a difficulty which may prove serious. The descriptions, in order to be precise, must refer to particular forms of instruments, and may therefore be to a certain extent inapplicable to other instruments of the same kind but with some difference, perhaps in the arrangement for adjustment, perhaps in the method of graduation. Spherometers, spectrometers, and kathetometers are instruments with which this difficulty is particularly likely to occur. With considerable diffidence we have thought it best to adhere to the precise descriptions referring to instruments in use in our own Laboratory, trusting that the necessity for adaptation to corresponding instruments used elsewhere will not seriously impair the usefulness of the book. Many of the experiments, however, which we have selected for description require only very simple apparatus, a good deal of which has in our case been constructed in the Laboratory itself. We owe much to Mr. G. Gordon, the Mechanical Assistant at the Cavendish Laboratory, for his ingenuity and skill in this respect. Our general aim in the book has been to place before the reader a description of a course of experiments which shall not only enable him to obtain a practical acquaintance with methods of measurement, but also as far as possible illustrate the more important principles of the various subjects. We have not as a rule attempted verbal explanations of the principles, but have trusted to the ordinary physical text-books to supply the theoretical parts necessary for understanding the subject; but whenever we have not been able to call to mind passages in the text-books sufficiently explicit to serve as introductions to the actual measurements, we have either given references to standard works or have endeavoured to supply the necessary information, so that a student might not be asked to attempt an experiment without at least being in a position to find a satisfactory explanation of its method and principles. In following out this plan we have found it necessary to interpolate a considerable amount of more theoretical information. The theory of the balance has been given in a more complete form than is usual in mechanical text-books; the introductions to the measurement of fluid pressure, thermometry, and calorimetry have been inserted in order to accentuate certain important practical points which, as a rule, are only briefly touched upon; |