J.J. Thompson And The Discovery Of The Electron

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  • J.J.Thomson and the Discovery of the Electron

  • Thomson in the 1920s

  • Page of cathode ray manuscript in Thomsons own hand

    iii

  • J.J.Thomson and the Discovery of theElectron

    E.A.DAVIS and I.J.FALCONER

  • UK Taylor & Francis Ltd., 1 Gunpowder Square, London EC4A 3DEUSA Taylor & Francis Inc., 1900 Frost Road, Suite 101, Bristol, PA 19007

    This edition published in the Taylor & Francis e-Library, 2005.

    To purchase your own copy of this or any of Taylor & Francis or Routledges collection of thousands of eBooks please go towww.eBookstore.tandf.co.uk.

    Copyright Taylor & Francis Ltd 1997

    All rights reserved. No part of this publication may be reproduced, stored in aretrieval system, or transmitted, in any form or by any means, electronic,

    electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise,without the prior permission of the copyright owner.

    British Library Cataloguing in Publication DataA catalogue record for this book is available from the British Library

    ISBN 0-203-48409-6 Master e-book ISBN

    ISBN 0-203-79233-5 (Adobe eReader Format)ISBN 0-7484-0696-4 cased

    0-7484-0720-0 paper

    Library of Congress Cataloging Publication Data are available

    Cover design by Youngs Design in Production

  • To Christine and Kenneth

  • Contents

    Foreword ix

    Preface xxi

    1 Formative Years 1

    Education at Owens College 1

    Cambridge and the Mathematics Tripos 6

    2 Early Research 9

    Analytical Dynamics 9

    Electromagnetic Mass 12

    College LectureshipVortex Atoms 12

    The Cavendish Laboratory 14

    Facsimiles: J.J.Thomson, On the Electric and Magnetic Effects Produced by the Motion ofElectrified Bodies, Philosophical Magazine, 11 (1881), 22934.

    20

    J.J.Thomson, Treatise on the Motion of Vortex Rings, 1882, pp. 12 and 11424. 25

    3 Cavendish Professorship: First Years 35

    Appointment as Cavendish Professor 35

    Gaseous Discharge 36

    The Changing Nature of the Cavendish 41

    Facsimiles: J.J.Thomson, On the Theory of the Electric Discharge in Gases, PhilosophicalMagazine, 15 (1883) 42734.

    47

    J.J.Thomson, Some Experiments on the Electric Discharge in a Uniform Electric Field, with someTheoretical Considerations about the Passage of Electricity Through Gases, Proceedings of theCambridge Philosophical Society, 5 (1886) 3919.

    52

    4 Gaseous Discharges: Further Developments (18911895) 60

    Grotthus Chains and Faraday Tubes of Force 60

    Electrodeless Discharges 65

  • Electrolysis of Steam and Condensation 67

    Electricity of Drops 69

    The Gyroscopic Atom and Chemical Combinations 70

    Summary 72

    Facsimile: J.J.Thomson, The Relation between the Atom and the Charge of Electricity Carried byit, Philosophical Magazine, 40 (1895) 51127.

    74

    5 X-rays and Cathode Rays (18951900) 87

    Research Students 87

    X-rays 89

    The Cathode Ray Controversy 96

    Resolution of the Cathode Ray Problem 97

    Measurement of the Charge 101

    Corpuscles versus Electrons 105

    Acceptance of Subatomic Particles 106

    Facsimiles: J.J.Thomson, Cathode Rays, Proceedings of the Royal Institution (1897), pp. 114. 110

    J.J.Thomson, Cathode Rays, Philosophical Magazine, 44 (1897) 296314. 121

    J.J.Thomson, On the Masses of the Ions in Gases at Low Pressures, Philosophical Magazine, 48(1899) 54767.

    135

    6 Later Years 151

    The Electromagnetic View of Matter and the Thomson Atom 151

    Positive Rays 157

    The Structure of Light 161

    Views on Physical Theories 165

    Last 20 Years 166

    Facsimile: J.J.Thomson, The Structure of the Atom, Proceedings of the Royal Institution, 18 (1905)115.

    171

    7 Subsequent Developments 185

    Elementary Particle Physics 185

    The Electronics Revolution 188

    Index 191

    viii

  • ForewordDavid Thomson

    The Masters Lodge at Trinity College in Cambridge faces onto the Great Court of grand and pleasingproportions and is backed by its walled garden leading down to the River Cam. It has been the serene homefor more than three hundred years of great academic figures. Sir Joseph Thomson, my grandfather and thelast Master to hold the position for life, died there in August 1940 at the age of 83. He and his wife Rosehad lived in the Lodge, together with their daughter Joan, for 22 years, presiding over the grandest ofCambridge Colleges with a simple wisdom, tact and humour that complemented his achievements as aphysicist of super-eminence, as Lloyd George once described him.

    1940 was a bleak moment for the nation and scarcely an appropriate time to assess the place in science ofJ.J. Thomson, the initials by which he was invariably known rather than the title Sir Joseph. The mainperiod of his creativity lay roughly between the years 1890 and 1910; the high point, which gave him fame,was his work with cathode rays leading in 1897 to his theory about corpuscles smaller than atoms, later tobe called electrons, and subsequent confirmation of these theoretical ideas with proof of the particlesexistence, which many had doubted, by a series of brilliant experiments.

    The setting for these discoveries was the Cavendish Laboratory of which J.J. was professor for the longspan of 35 years. Apart from his own ground-breaking work which took the first steps into the subatomicphysics of the twentieth century, he directed, recruited and inspired a succession of brilliant young menround him at the Cavendish who were to share this work and later to create whole new fields of science. It isprobable that there has never been such a successful and sustained record of new scientific discovery in onelaboratory across such a broad field as that which happened under J.J.s leadership of the Cavendishlaboratory from 1884 to 1919. It can claim to be the first real School of Physics in the modern sense, and inhis time it was generally regarded as the leading experimental laboratory of physics in the scientific world.

    In 1997 we are celebrating the centenary of J.J.s first exposition of his thesis that a cathode ray particleis more than 1000 times lighter than the lightest chemical atom and a universal constituent of matter. It isalso an opportunity, now long overdue, to re-assess J.J.s own reputation and place in the history of science,judged against the developments throughout the twentieth century both in pure and applied science whichbuilt on the foundations of his work and that of the group round him at the Cavendish.

    At J.J.s death his great period of work had ceased a whole generation earlier. He was certainly not a manwith only one discovery to his name, as his subsequent work on positive rays leading to the discovery withAston of non-radioactive isotopes, and the constant ferment of his intellectual energy amply proved. However,by the time the First World War started in 1914, when he was nearing sixty, his main work had beenachieved.

    The effect of that war on the laboratory practically brought original research to an end. The youngerscientists, including J.J.s son, soon found themselves in France, fighting at the Front. J.J. himself was

  • drawn into War Committee work and also became President of the Royal Society. Shortly before the end ofthe war Montagu Butler, then Master of Trinity, died and J.J. was appointed to succeed. He did not thereforetake up an active research role when peace time conditions returned and he retired officially from theCavendish in 1919, though retaining an honorary professorship and space for his own research. His mostfamous pupil and collaborator, Ernest Rutherford, succeeded him and J.J. became increasingly a seniorspokesman of Science. He did however continue to produce new work, much of it based round universaltheories of the ether, harking back to some of his earliest concepts.

    In 1940 then, the formal honours were paid at his death. He was a national figure. In the popular mindthe man who split the atom, he was the senior member of the Order of Merit, the epitome of Cambridgescience and something of a legend, in Cambridge at least, as a much admired, if sometimes absent-minded,genius.

    His ashes were buried in Westminster Abbey and the stone above them lies in the shadow of IsaacNewtons memorial and next to that of Rutherford, who had predeceased him, sadly early, three yearsbefore.

    Trinity did not lack for great names to succeed him. After science, it was the turn of history. Theappointment at Trinity is a royal one on the Prime Ministers recommendation, and the choice most aptlywas George Trevelyan. To quote Trevelyan, a mans reputation with the world at large is usually at itslowest exactly a hundred years after his birth, especially in the realm of letters. It was to be true ofTrevelyan himself, and in the realm of science equally true of his predecessor at Trinity. In both cases thetide is turning.

    J.J.s background was Manchester; liberal, mercantile Manchester, where his family had lived for threegenerations. His great-grandfather came from Scotlandhence the Scottish spelling of Thomsonandstarted a small business selling antiquarian books, occasionally acting as a publisher too. It must haveprospered sufficiently to give a reasonable living and a house with servants in the middle-class suburb ofCheetham. Socially this would have put the family in the merchant middle class, somewhat below thedoctors, solicitors and clergy of the professional classes.

    J.J.s father, also Joseph, died aged 38 when the boy was only 16, and the only other child, Frederick,four years younger. Though not much seems to be remembered of t