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
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.
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Copyright Taylor & Francis Ltd 1997
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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
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Cover design by Youngs Design in Production
To Christine and Kenneth
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.
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.
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.
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
Facsimile: J.J.Thomson, The Relation between the Atom and the Charge of Electricity Carried byit, Philosophical Magazine, 40 (1895) 51127.
5 X-rays and Cathode Rays (18951900) 87
Research Students 87
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.
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.
7 Subsequent Developments 185
Elementary Particle Physics 185
The Electronics Revolution 188
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