Experimental Elasticity

EXPERIMENTAL ELASTICITY A MANUAL FtR THE LABtRATtRY G. F. C. SEA. RLE, Sc. 19., F. R. S. UNIVERSITY LECTURER IN EXPERIMENTAL PHYSICS AND DEMONSTRATOR IN EXPERIMENTAL PHYSICS AT THE CAVENDISH LABORATORY SECtNB EBITI0N CAMBRIDGE at the University Press 1933 First Edition 1908 Reprinted 1920 Second Edition 1933 PRINTED IN GREAT BRITAIN PREFACE nPHE present volume has its origin in the manuscript notes --which I have prepared from time to time for the use of the students attending my class in practical physics at the Cavendish Laboratory. When, in 1890, I was appointed to my present post of Demonstrator in Experimental Physics, I found that the then existing text-books of practical physics did not entirely meet the needs of the students, partly because they did not, as a rule, show how the formulae required in the experi mental work are derived from the principles of the subject. The students themselves added to the difficulty, for their ideas as to those principles were often indistinct. I was thus led to devise some experiments intended to illustrate principles as simply and directly as possible. I also wrote notes explaining how the necessary formulae are obtained from the principles involved in those experiments and describing in detail how the practical work is to be conducted. The students showed very kind appreciation of these earlier notes and thus I was encouraged to prepare others this work has proved so interesting that I have continued it, as opportunities have occurred, with the result that at the present time the students attending my practical class rely mainly upon these manuscript notes for the necessary instructions. Many of the students have made almost complete copies ofsome hundreds of pages of manuscript and have perhaps learned vi PREFACE more in that way than by merely reading a printed book con taining the same matter. But the plan of using manuscript notes has numerous disadvantages. For instance, the limited number of copies of any one manuscript makes it difficult to arrange for more than two or three students to do the same experiment at one time and often prevents the students from preparing them selves beforehand for the experiments assigned to them. There is, besides, the risk of the loss or the destruction of the manuscripts themselves. For the safety of the manuscripts I have relied on the consideration of the students and this has hardly ever failed. To throw together into a small volume the manuscripts dealing with one branch of physics would seem an easy task. But the result would hardly be satisfactory, for some of the earlier manuscripts require revision in the light of later experience, while many of the manuscripts contain mathematical arguments which are repeated in others of the series. This repetition was necessary for the practical working of the class but would be intolerable in a book. For these and other reasons I decided that it would be more satisfactory to rewrite the whole of the manuscripts, and to arrange the material, with additions, in the form of a series of small text books, in which a fairly full account of the mathematical treatment should accompany a detailed description of the experimental work. To make a beginning, the present volume is published and this, I hope, will be followed in a few months by a similar volume on Experimental Optics. I hope, if life and health be given me, to complete the scheme bywriting volumes on Mechanics, on Electricity and Magnetism and on Heat and Sound. The present volume cannot lay claim to any sort of com pleteness. Its purpose is simply to give the substance of my PREFACE yii course of instruction in the subject in a form which may be useful to students at the Cavendish Laboratory and elsewhere. The first chapter contains an account of the elements of the mathematical theory of elasticity, with one or two necessary propositions in thermodynamics...