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Electron diffraction chez Thomson: early responses to quantum physics in Britain

Published online by Cambridge University Press:  05 February 2010

Max Planck Institute for the History of Science, Boltzmannstraße 22, 14195Berlin, Germany. Email:


In 1927, George Paget Thomson, professor at the University of Aberdeen, obtained photographs that he interpreted as evidence for electron diffraction. These photographs were in total agreement with de Broglie's principle of wave–particle duality, a basic tenet of the new quantum wave mechanics. His experiments were an initially unforeseen spin-off from a project he had started in Cambridge with his father, Joseph John Thomson, on the study of positive rays. This paper addresses the scientific relationship between the Thomsons, father and son, as well as the influence that the institutional milieu of Cambridge had on the early work of the latter. Both Thomsons were trained in the pedagogical tradition of classical physics in the Cambridge Mathematical Tripos, and this certainly influenced their understanding of quantum physics and early quantum mechanics. In this paper, I analyse the responses of both father and son to the photographs of electron diffraction: a confirmation of the existence of the ether in the former, and a partial embrace of some ideas of the new quantum mechanics in the latter.

Research Article
Copyright © British Society for the History of Science 2010

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1 Those writing in support of G.P. Thomson's application were E.C. Pearce, master of Corpus Christi College; William Spens, fellow, tutor and director in science, Corpus Christi; Ernest Rutherford, director of the Cavendish Laboratory; Horace Lamb, former professor in Manchester, retired in Cambridge since 1920; Alex Wood, university lecturer in experimental physics, fellow and tutor of Emmanuel College; R.A. Herman, university lecturer in mathematics, fellow of Trinity College; B. Melvill Jones, Francis Mond Professor of Aeronautics in Cambridge; C.T.R. Wilson, Solar Physics Observatory, Cambridge; and R.T. Glazebrook, who had been a demonstrator in the Cavendish before being appointed director of the National Physical Laboratory. These letters are all kept in the Special Libraries & Archives of the University of Aberdeen.

2 Letters by Alex Wood and R.T. Glazebrook, 21 July 1922, Special Libraries & Archives of the University of Aberdeen.

3 G.P. Thomson, application letter to the chair in Aberdeen, Special Libraries & Archives of the University of Aberdeen.

4 The biographical approach to an entity such as the electron has been worked in detail by T. Arabatzis in his recent ‘biography’ of the electron. However, his work does not include any reference to the dual aspect, wave and particle, of the electron. See Theodore Arabatzis, Representing Electrons: A Biographical Approach to Theoretical Entities, Chicago: University of Chicago Press, 2006. For more general considerations on the use of the biographical discourse applied to scientific entities see Lorraine Daston (ed.), Biographies of Scientific Objects, Chicago: University of Chicago Press, 2000. In this paper, however, I do not consider the electron as an active character in the story but only insofar as it binds together the scientific careers of J.J. and G.P. Thomson.

5 For recent studies on the discovery of the electron see George E. Smith, ‘J.J. Thomson and the electron, 1887–1889’, in Jed Z. Buchwald and Andrew Warwick (eds.), Histories of the Electron, Cambridge, MA: MIT Press, 2001, pp. 21–76; Isobel Falconer, ‘Corpuscles and electrons’, in Buchwald and Warwick (op. cit.), pp. 77–100; and Helge Kragh, ‘The electron, the protyle, and the unity of matter’, in Buchwald and Warwick (op. cit.), pp. 195–226; Theodore Arabatzis, ‘Rethinking the “discovery” of the electron’, Studies in the History and Philosophy of Modern Physics (1996) 27, pp. 405–435; Falconer, Isobel, ‘Corpuscles, electrons and cathode rays: J.J. Thomson and the “Discovery of the Electron”’, BJHS (1987) 20, pp. 241276.CrossRefGoogle Scholar

6 See Isobel Falconer, ‘Theory and experiment in J.J. Thomson's work on gaseous discharge’, PhD dissertation, Cambridge, 1985; and Edward Arthur Davis and Isobel Falconer, J.J. Thomson and the Discovery of the Electron, London: Taylor & Francis, 1997. See also Heilbron, John, ‘J.J. Thomson and the Bohr atom’, Physics Today (1977) 30, pp. 2330.CrossRefGoogle Scholar

7 See Andrew Warwick, Masters of Theory: Cambridge and the Rise of Mathematical Physics, Chicago: University of Chicago Press, 2003.

8 See Dong-Won Kim, Leadership and Creativity: A History of the Cavendish Laboratory, 1871–1919, Dordrecht: Kluwer, 2002.

9 Robert John Strutt, The Life of Sir J.J. Thomson, Cambridge: Cambridge University Press, 1942.

10 George Paget Thomson, J.J. Thomson and the Cavendish Laboratory in His Day, London: Nelson, 1964.

11 Falconer, Isobel, ‘J.J. Thomson's work on positive rays’, Historical Studies in the Physical Sciences (1988) 18, pp. 265310.CrossRefGoogle Scholar

12 Warwick, op. cit. (7), Epilogue.

13 The expression was used by Rutherford in a conversation with Lodge. See J. Arthur Hill (ed.), Letters from Sir Oliver Lodge: Psychical, Religious, Scientific and Personal, London: Cassell, 1932, p. 224. Quoted in Noakes, Richard, ‘Ethers, religion and politics in late-Victorian physics: beyond the Wynne thesis’, History of Science (2005) 63, pp. 415455CrossRefGoogle Scholar, p. 445.

14 A biographical sketch of G.P. Thomson can be found in P.B. Moon, ‘George Paget Thomson’, Biographical Memoirs of the Fellows of the Royal Society (1977) 23, pp. 265–310. In this section, I partly follow this sketch and the autobiographical notes of G.P., kept in the archives of Trinity College, Cambridge, on which Moon relied to write this biographical memoir.

15 Joseph John Thomson, Recollections and Reflections, London: Bell, 1936, p. 34. The expression means that J.J. never left Cambridge for more than a few weeks and that he was in the university every single academic term since his arrival in Cambridge.

16 G.P. Thomson papers (subsequently GP). Trinity College Archive, A2, 6.

17 See Strutt, op. cit. (9), Chapter 7.

18 GP, A2, 16.

19 Moon, op. cit. (14), p. 531.

20 GP, A2, 18.

21 Warwick, op. cit. (7), pp. 260–261.

22 Oral interview with G.P. Thomson, Archive for the History of Quantum Physics, Tape T2, side 2, 1.

23 J.J. Thomson, op. cit. (15), p. 39.

24 See Joseph Larmor, Aether and Matter, Cambridge: Cambridge University Press, 1900; and Joseph John Thomson, Conduction of Electricity through Gases, Cambridge: Cambridge University Press, 1903.

25 Oral interview with G.P. Thomson, Archive for the History of Quantum Physics, Tape T2, side 2, 2.

26 Warwick, op. cit. (7). See also Andrew Warwick, ‘Cambridge mathematics and Cavendish physics: Cunningham, Campbell and Einstein's relativity, 1905–1911. Part I: the uses of theory. Part II: comparing traditions in Cambridge Physics’, Studies in the History and Philosophy of Science (1992) 23, pp. 625–656; (1993) 24, pp. 1–25.

27 Warwick, op. cit. (7), pp. 396–397, argues in the following way: ‘What I have sought to demonstrate … is that adherence to the E[lectromagnetic] T[heory of] M[atter] in Cambridge after 1900 was not just a product of dogmatic belief in the ether's existence and of hostility to an alternative theory that dismissed the ether as superfluous. As far as Larmor's students were concerned, his work stood as a comprehensive and progressive addition to a research tradition in Cambridge that stretched back to Maxwell himself. Their commitment was not simply to the notion of an ether, but to a sophisticated conceptual structure and range of practical calculating techniques that were gradually acquired through years of coaching and problem solving. As they acquired these skills, the ether became an ontological reality that lent meaning both to the idea of an ultimate reference system and to the application of dynamical concepts to electromagnetic theory.’

28 As Warwick himself points out in the Epilogue to his book, an analysis of the reception of early quantum physics in Cambridge similar to his study of relativity is still to be done. In this paper I only take J.J. Thomson as a paradigmatic case of Cambridge physics in the early 1910s, since he certainly influenced the early career of G.P. Thomson.

29 Joseph John Thomson, The Structure of Light: The Fison Memorial Lecture, Cambridge: Cambridge University Press, 1925, p. 15.

30 Bruce R. Wheaton, The Tiger and the Shark: Empirical Roots of Wave–Particle Dualism, Cambridge: Cambridge University Press, 1983.

31 See Falconer, ‘Corpuscles, electrons and cathode rays’, op. cit. (5).

32 Joseph John Thomson, ‘The relation between the atom and the charge of electricity carried by it’, Philosophical Magazine (1895) 40, pp. 511–544, p. 512.

33 Joseph John Thomson, Electricity and Matter, London: Archibald Constable, 1906.

34 Joseph John Thomson, ‘Presidential address’, in Report of the British Association for the Advancement of Science, Winnipeg, 1909, London, 1909, pp. 21–23.

35 According to McCormmach, Joseph Larmor was the first British scientist to react, in 1902, to Planck's hypothesis. See Russell McCormmach, ‘J.J. Thomson and the structure of light’, BJHS (1967) 3, pp. 362–387, p. 375.

36 McCormmach, op. cit. (35), pp. 375–376. For these different models, see Wheaton, op. cit. (30), especially Chapters 4 and 6.

37 See the Cambridge University Reporter. In 1919 Darwin offered a course on ‘Quantum theory and origin of spectra’. This course changed to ‘Recent developments on spectrum theory’ the following year, and a joint course on isotopes with Aston in 1921. In 1922 Fowler gave his first special course on ‘The theory of quanta’.

38 G.P. Thomson, op. cit. (10), p. 70.

39 Kim, op. cit. (8), p. 129.

40 Kim, op. cit. (8), Chapter 5.

41 Falconer, op. cit. (11), develops a very fine analysis of J.J. Thomson's work on positive rays, on which much in the following paragraphs is based.

42 See Falconer, op. cit. (11). For J.J.'s interests in chemistry see Jaume Navarro, ‘Imperial incursions in late-Victorian Cambridge: J.J. Thomson and the domains of the physical sciences’, History of Science (2006) 44, pp. 469–495.

43 Kim, op. cit. (8), pp. 169–174.

44 See also Jeff Hughes, ‘Redefining the context: Oxford and the wider world of British physics, 1900–1940’, in Robert Fox and Graeme Gooday (eds.), Physics in Oxford 1839–1939: Laboratories, Learning and College Life, Oxford: Oxford University Press, 2005, pp. 267–300, p. 276: ‘From around 1910, then, the Cavendish was being partially eclipsed by the development of Rutherford's research school at Manchester and the growth of “modern” physics at Leeds, London, Oxford, and elsewhere. In some ways it was a victim of its own success … [According to Bragg] there were too many students chasing too few ideas for research and too little apparatus. Thomson's own research on positive rays was in the doldrums, and the temper of the Cavendish seemed to have changed: it had lost the cohesiveness, excitement, and tightness of direction it had possessed.’

45 George Paget Thomson, ‘Charles Galton Darwin’, Biographical Memoirs of Fellows of the Royal Society (1963) 9, pp. 69–85, p. 70.

46 Graeme K. Hunter, Light Is a Messenger: The Life and Science of William Lawrence Bragg, Oxford: Oxford University Press, 2004, p. 21.

47 George Paget Thomson, Applied Aerodynamics, London: Hodder & Stoughton, 1920.

48 Oral interview with G.P. Thomson, Archive for the History of Quantum Physics, Tape T2, side 2, 12.

49 Joseph John Thomson, Rays of Positive Electricity and their Application to Chemical Analyses, London: Longmans, Green, 1921 (1st edn. 1913).

50 G.P. Thomson, op. cit. (10), p. 137.

51 Falconer, op. cit. (11), gives a detailed account of the divergence of J.J. Thomson's and Aston's ideas on positive rays.

52 For instance, the only paper in the Proceedings of the Royal Society on positive rays in the early 1920s is one in which Lord Rayleigh uses a technique similar to the one used by J.J. Thomson to interpret a photograph from an aurora borealis: ‘A photographic spectrum of the aurora of May 13–15, 1921, and laboratory studies in connection with it’, Proceedings of the Royal Society of London (1922) 101, pp. 114–124.

53 J.J. Thomson, op. cit. (49), Preface.

54 Thomson, George Paget, ‘A note on the nature of the carriers of anode rays’, Proceedings of the Cambridge Philosophical Society (1920) 20, pp. 210211Google Scholar; idem, ‘The spectrum of hydrogen positive rays’, Philosophical Magazine (1920) 40, pp. 240–247; idem, ‘The application of anode rays to the investigation of isotopes’, Philosophical Magazine (1921) 42, pp. 857–867; idem, ‘The scattering of hydrogen positive rays, and the existence of a powerful field of force in the hydrogen molecule’, Proceedings of the Royal Society (1922) 102, pp. 197–209.

55 Warwick, op. cit. (7), pp. 325–333. It was this group, and William Niven in particular, who got J.J. Thomson interested in Maxwell's Treatise during his undergraduate years.

56 Here I want to emphasize that G.P.'s laboratory in Aberdeen eventually became an extension only of J.J. Thomson's room at the Cavendish. Historians of science have analysed the transfer of elements of what has been called the ‘Cavendish style’ by research students appointed professors in other universities, such as Rutherford in McGill, Langevin in the Collège de France, or Townsend in Oxford. In these cases, one can compare the regimes implemented in the new research groups with the Cavendish regime; but with Aberdeen this is not possible, since there was never such a thing as a research group during G.P.'s tenure. See Benoit Lelong, ‘Translating ion physics from Cambridge to Oxford: John Townsend and the electrical laboratory, 1900–24’, in Fox and Gooday, op. cit. (44), pp. 209–232; John L. Heilbron, ‘Physics at McGill in Rutherford's time’, in Mario Bunge and William R. Shea (eds.), Rutherford and Physics at the Turn of the Century, Folkestone: Dawson, 1979, pp. 42–73.

57 Thomson, George Paget, ‘The scattering of positive rays of hydrogen’, Philosophical Magazine (1926) 1, pp. 961977Google Scholar; idem, ‘The scattering of positive rays by gases’, Philosophical Magazine (1926) 2, pp. 1076–1084.

58 George Paget Thomson, ‘An optical illusion due to contrast’, Proceedings of the Cambridge Philosophical Society (1926) 23, pp. 419–421, p. 421.

59 The letters from his wife, Kathleen, are kept in a special folder in the G.P. Thomson archives at Trinity College, Cambridge. GP A14 A.

60 We find G.P. Thomson giving a presentation in the Kapitza Club on 7 February and 2 August 1927, and on 30 July 1929. He was also present on 10 March 1928. See Churchill Archives, CKFT 7/1.

61 de Broglie, Louis, ‘A tentative theory of light quanta’, Philosophical Magazine (1924) 47, pp. 446458.Google Scholar This paper was communicated by Ralph Fowler.

62 De Broglie, op. cit. (61), p. 450.

63 For this process, see V.V. Raman and Paul Forman, ‘Why was it Schrödinger who developed de Broglie's ideas?’, Historical Studies in the Physical Sciences (1969) 1, pp. 291–314.

64 Topper, David R., ‘“To reason by means of images”: J.J. Thomson and the mechanical picture of Nature’, Annals of Science (1980) 37, pp. 3157.CrossRefGoogle Scholar

65 Thomson, Joseph John, ‘A suggestion as to the structure of light’, Philosophical Magazine (1924) 48, pp. 737746Google Scholar; and idem, op. cit. (29).

66 Thomson, George Paget, ‘Early work in electron diffraction’, American Journal of Physics (1961) 29, pp. 821825CrossRefGoogle Scholar, p. 821.

67 Oral interview with G.P. Thomson, Archive for the History of Quantum Physics, Tape T2, side 2, 8.

68 In his reconstruction of the events, G.P. presented a slightly different version of the facts. G.P. Thomson, op. cit. (66), p. 821: ‘At that time we were all thinking of the possible ways of reconciling the apparently irreconcilable. One of these ways was supposing light to be perhaps particles after all, but particles which somehow masqueraded as waves; but no one could give any clear idea as to why this was done. The first suggestion I ever heard which did not stress most of all the behaviour of the radiation came from the younger Bragg, Sir Lawrence Bragg, who once said to me that he thought the electron was not so simple as it looked, but never followed up this idea. However, it made a considerable impression on me, and it pre-disposed me to appreciate de Broglie's first paper in the Philosophical Magazine of 1924.’

69 See Arturo Russo, ‘Fundamental research at Bell Laboratories: the discovery of electron diffraction’, Historical Studies in the Physical Sciences (1981) 12, pp. 117–160, especially pp. 141–144.

70 George Paget Thomson, ‘A physical interpretation of Bohr's stationary states’, Philosophical Magazine (1925) 1, pp. 163–164, p. 163.

71 G.P.'s article, op. cit. (70), p. 164, only studies the hydrogen atom and ‘a simple extension of the above accounts also for the stationary states of ionized helium, and gives approximately the energy of the K ring of electrons’.

72 GP, F4, 7.

73 Expression used by Sir Oliver Lodge. See Hill, op. cit. (13), p. 225. See also Jeff Hughes, ‘“Modernists with a vengeance”: changing cultures of theory in nuclear science, 1920–1930’, Studies in the History and Philosophy of Modern Physics (1998) 29, pp. 339–367.

74 GP, A6, 7.

75 GP, C24, 13.

76 The published autobiographical accounts are the following: G.P. Thomson, op. cit. (66); and an extended version of it in George Paget Thomson, ‘The early history of electron diffraction’, Contemporary Physics (1968) 9, pp. 1–15. Moon's biographical sketch of G.P. Thomson is only a transcription of some paragraphs from these accounts. See Moon, op. cit. (14). See also his autobiographical notes in Trinity College, Cambridge.

77 Born's paper had a strong impact on many of those present, but especially on the American physicist working at the Bell laboratories, Clinton J. Davisson, when he heard that the anomalous results he had been obtaining in experiments on electron dispersion with his colleague Lester H. Germer might be signs of electron diffraction. That branch of the story, which was studied in detail by historian of science Arturo Russo, ends with the confirmation of electron diffraction in the Bell laboratories and the sharing of the Nobel Prize with G.P. Thomson for their experimental proof of de Broglie's principle. At the time of his first experiments, however, Thomson was not fully aware of Davisson's project. Born also mentioned the experiments of the young German physicist Walter M. Elsasser, who had unsuccessfully tried to detect diffraction patterns in the passage of an electron beam through a metallic film. See Russo, op. cit. (69).

78 G.P. Thomson, op. cit. (76), p. 7. These results were published in Nature: E.G. Dymond, ‘Scattering of electrons in helium’, Nature (1926) 118, pp. 336–337.

79 In Cambridge, P.M.S. Blackett had also tried to obtain evidence of electron diffraction, but gave up after a few months. See Mary Jo Nye, Blackett, Physics, War, and Politics in the Twentieth Century, Cambridge, MA: Harvard University Press, 2004, p. 46.

80 G.P. Thomson, op. cit. (66), p. 823.

81 G.P. Thomson, op. cit. (76), p. 7.

82 George Paget Thomson and Alexander Reid, ‘Diffraction of cathode rays by a thin film’, Nature (1927) 119, p. 890.

83 George Paget Thomson, ‘The diffraction of cathode rays by thin films of platinum’, Nature (1927) 120, p. 802; ‘Experiments on the diffraction of cathode rays’, Proceedings of the Royal Society (1928) 117, pp. 600–609; ‘Experiments on the diffraction of cathode rays. II’, Proceedings of the Royal Society (1928) 119, pp. 651–663; ‘Experiments on the diffraction of cathode rays. III’, Proceedings of the Royal Society (1929) 125, pp. 352–370.

84 GP, A6, 10/3.

85 I do not mean to say here that there is a parallel between the story of the Braggs, father and son, and the story of the Thomsons, also father and son. I only suggest that one can easily suppose that G.P.'s friendship with Bragg was a natural channel for him to follow closely the developments on X-rays.

86 See Hunter, op. cit. (46), pp. 70 and 104. Unfortunately, I have found no evidence of conversations between G.P. Thomson and W.L. Bragg on this matter in the summer of 1926.

87 Darwin came back to Cambridge after the war and was made a fellow of Christ's College while G.P. was a fellow in Corpus Christi. On Darwin see G.P. Thomson, op. cit. (45).

88 The following anecdote helps to illustrate the importance of electromagnetic deflection. Probably around the beginning of March 1928, he also had the opportunity to discuss his experimental results with Schrödinger himself as the latter recalled in 1945: ‘After mentioning briefly the new theoretical ideas that came up in 1925/26, I wish to tell of my meeting you in Cambridge in 1927/28 (I think it was in 1928) and of the great impression the marvellous first interference photographs made on me, which you kindly brought to Mr. Birthwistle's house, where I was confined with a … cold. I remember particularly a fit of scepticism on my side (“And how do you know it is not the interference pattern of some secondary X-rays?”) which you immediately met by a magnificent plate, showing the whole pattern turned aside by a magnetic field.’ Schrödinger to G.P. Thomson, 5 February 1945, GP, J105, 4. The exact date can be traced by the minutes of the Kapitza Club, which says that Schrödinger gave a paper to the club on 10 March 1928. See Churchill Archives, CKFT, 7/1.

89 G.P. Thomson, ‘Experiments I’, op. cit. (83), p. 608.

90 G.P. Thomson, ‘Experiments I’, op. cit. (83), pp. 608–609.

91 Oral interview with G.P. Thompson, Archive for the History of Quantum Physics, Tape T2, side 2, 15.

92 G.P. Thomson, op. cit. (45), p. 81.

93 See Jaume Navarro, ‘J.J. Thomson on the nature of matter: corpuscles and the continuum’, Centaurus (2005) 47, pp. 259–282.

94 Joseph John Thomson, ‘Waves associated with moving electrons’, Philosophical Magazine (1928) 5, pp. 191–198, p. 191.

95 Joseph John Thomson, ‘Electronic waves and the electron’, Philosophical Magazine (1928) 6, pp. 1254–1281, p. 1259.

96 J.J. Thomson, op. cit. (95), p. 1254. J.J.'s model for the electron sphere would soon be expressed in terms only of what he came to call ‘granules’, particles ‘having the same mass μ, moving with the velocity of light c, and possessing the same energy μc 2’. See Joseph John Thomson, ‘Atoms and electrons’, Manchester Memoirs (1930–1931) 75, pp. 77–93, p. 86.

97 Joseph John Thomson, Beyond the Electron, Cambridge: Cambridge University Press, 1928, p. 9.

98 J.J. Thomson, op. cit. (97), p. 22.

99 J.J. Thomson, op. cit. (97), p. 23.

100 J.J. Thomson, op. cit. (97), p. 31.

101 J.J. Thomson, op. cit. (97), p. 34.

102 Joseph John Thomson, Tendencies of Recent Investigations in the Field of Physics, London: British Broadcasting Corporation, 1930, pp. 26–27.

103 Oral interview with G.P. Thomson, Archive for the History of Quantum Physics, Tape T2, side 2, 9: ‘Well, I think he was very pleased [with my developments], largely because it was in the family.’

104 Joseph John Thomson, ‘Electronic waves’, Philosophical Magazine (1939) 27, pp. 1–33.

105 Thomson and Reid, op. cit. (82), and Churchill Archives, CKFT 7/1.

106 G.P. Thomson, op. cit. (83).

107 George Paget Thomson, ‘The waves of an electron’, Nature (1928) 122, pp. 279–282, p. 281.

108 George Paget Thomson, The Wave Mechanics of Free Electrons, New York: McGraw Hill, 1930, p. 11.

109 G.P. Thomson, op. cit. (108), p. 12.

110 G.P. Thomson, op. cit. (108), p. 282.

111 For a thorough analysis of the problems with beta decay and the conservation of energy see Carsten Jensen, Controversy and Consensus: Nuclear Beta Decay, 1911–1934, Basel: Birkhäuser, 2000.

112 On Darwin's ideas on the conservation of energy see Klaus Stolzenburg (ed.), Niels Bohr, Collected Works, vol. 5, Amsterdam: North-Holland, 1984, pp. 13–19, pp. 67–69, pp. 81–83 and pp. 317–319; and Jørgen Kalckar (ed.), Niels Bohr, Collected Works, vol. 6, Amsterdam: North-Holland, 1985, pp. 91–99, pp. 305–319 and pp. 347–349.

113 George Paget Thomson, ‘On the waves associated with β-rays, and the relation between free electrons and their waves, Philosophical Magazine (1929) 7, pp. 405–417, p. 410.

114 G.P. Thomson, op. cit. (113), p. 415.

115 See George Paget Thomson, ‘The disintegration of radium E from the point of view of wave mechanics’, Nature (1928) 121, pp. 615–616: the apparent non-conservation of energy ‘is to be expected on the new wave mechanics, if the ejection of a β-particle is produced by anything like a sudden explosion. In such a case one would expect that the wave-group which accompanies, and on some views actually constitutes, the electron, would be of the nature of a single pulse, that is, the damping factor of the amplitude would be of the order of the wave-length. Such a wave-group, being very far from monochromatic, would spread rapidly lengthwise owing to the large dispersion of the phase waves, and so the distance within which the electron may occur becomes large, implying a marked “straggling” in velocity. Similarly, if the waves pass through a magnetic field, which is for them a refracting medium, the group will split into monochromatic waves going in different directions, just as white light is split up by a prism. Thus an observer who forms the magnetic spectrum of the β-rays will find electrons in places corresponding to paths of various curvatures, that is, he will find a spectrum continuous over a wide range.’

116 In Jaume Navarro, ‘“A dedicated missionary”: Charles Galton Darwin and the new quantum mechanics in Britain’, Studies in the History of Modern Physics (2009) 40, pp. 316–326, I argue that Darwin's approach to theoretical quantum mechanics can be traced back to his early training in the Cambridge Mathematical Tripos, which, following Warwick's analysis (op. cit. (7)), provided physicists and mathematical physicists with an epistemological and ontological framework that was at odds with the so-called Copenhagen interpretation of quantum mechanics, but which resonated very well with the continuous ontology of Schrödinger's approach.

117 For standard account of this episode in the history of quantum mechanics see Jagdish Mehra and Helmut Rechenberg, The Historical Development of Quantum Theory, vol. 6, part 1, New York: Springer, 2000. See also Helge Kragh, Quantum Generations, Princeton: Princeton University Press, 1999, Chapter 11. For a critical assessment of the equivalence between Heisenberg and Schrödinger's approach see F.A. Muller, ‘The equivalence myth of quantum mechanics. Part I’, Studies in History and Philosophy of Modern Physics (1997) 28, pp. 35–61.

118 Charles Galton Darwin, ‘The wave equations of the electron’, Proceedings of the Royal Society (1928) 118, pp. 654–680, 654. See also idem, The New Conceptions of Matter, London: Bell, 1931, p. 124.

119 Charles Galton Darwin, ‘Collision problem in wave mechanics’, Proceedings of the Royal Society (1929) 124, pp. 375–394, pp. 391–392.

120 Darwin, op. cit. (119), pp. 393–394: ‘The subworld of ψ expresses in its own way everything that happens; but it is a dead world, not involving definite events, but instead the potentiality for all possible events. It becomes animated by our consciousness, which so to speak cuts sections of it when it makes observations. These observations are described in a language and by means of rules which are foreign to the subworld; the quantum itself enters for the first time … whereby we can talk of atoms, electrons and light-quanta.’

121 G.P. Thomson, op. cit. (108), p. 12. My emphasis.

122 Darwin, The New Conceptions of Matter, op. cit. (118), p. 107.

123 George Paget Thomson, ‘New discoveries about electrons’, The Listener (1929) 1, pp. 219–220, p. 220.

124 One might want to ask how influential in this story was J.J.'s wife, G.P.'s mother, also trained as a physicist. We have no evidence of her playing any active role in the scientific life of the family, but even if new evidence proves otherwise, that would not invalidate the arguments in this paper, but only complement them. For a gender-based analysis of scientific families see Helena M. Pycior, Nancy G. Slack and Pnina G. Abir-Am, Creative Couples in the Sciences, New Brunswick: Rutgers University Press, 1996.

125 Jeff Hughes uses the Aberdeen example for a different purpose – to play down the overall importance of the Cavendish in the 1920s and 1930s. See Hughes, op. cit. (44), p. 283: ‘At Aberdeen, the work of J.J.'s son George Thomson on positive rays in hydrogen led to the elaboration of electron diffraction, for which he shared the 1937 Nobel Prize for physics – aptly reminding us that although we tend to associate the canonical achievements of modern British physics with the Cavendish, they often emerged elsewhere.’

126 See Falconer, ‘Corpuscles, electrons and cathode rays’, op. cit. (5).