PREFACE

ΤΟ

THE FOURTH

EDITION.

THE issue of a new edition affords us the opportunity of making some alterations and additions which the experience of ourselves or our successors at the Cavendish Laboratory has shewn to be desirable.

The development of physical science on the lines. indicated by the principle of the conservation of energy has made more conspicuous the importance of experimental Dynamics as the basis of experimental physics, so that some considerable space has been given to that branch of the subject, and a good deal of attention has been devoted to the geometrical representation of rates of variation, especially as illustrating the determination of the velocity and acceleration of a body the position of which is known for successive instants of time. Geometrical representation has, indeed, been kept in view throughout.

The advances that have been made in the sciences of magnetism and electro-magnetism have also necessitated some considerable additions. The chapter on magnetism

has been enlarged, and a chapter on electro-magnetic induction has been added.

It has been thought better not to disturb the numbering of the sections, and the new sections have therefore been separately numbered A-Z and I to e.

In the preparation of this edition we are greatly indebted both to Mr. H. F. Newall, who was demonstrator when the apparatus for many of the new sections was first set up, and also especially to Mr. G. F. C. Searle, of Peterhouse, upon whose version of the Laboratory MSS. the text of many of the new sections depends. Mr. Searle, besides contributing the section on the dynamical equivalent of heat, has also been good enough to revise the whole of the proof sheets and to give us the advantage of his experience in the Laboratory by making numerous valuable suggestions.

Many of the original drawings for the figures were made for us by Mr. Hayles, the Lecture Assistant at the Laboratory.

January 6, 1893.

R. T. GLAZEBROOK.

W N. SHAW.

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;