1. For more on Townsend, see Thaddeus J. Trenn, “John Sealy Edward Townsend,” in Charles Coulston Gillispie et al., eds., Complete Dictionary of Scientific Biography (New York: Scribner 2008 and earlier editions), 13:445–447; Alfred von Engel, “John Sealy Edward Townsend. 1868–1957,” Biographical Memoirs of Fellows of the Royal Society 3 (1957), 256–272; Mark McCartney, “John Sealy Edward Townsend,” in Mark McCartney and Andrew Whitaker, eds., Physicists of Ireland (Bristol: Institute of Physics Publishing, 2003), 168–175; Benoit Lelong, “Translating Ion Physics from Cambridge to Oxford: John Townsend and the Electrical Laboratory, 1900–24,” in Robert Fox and Graeme Gooday, eds., Physics in Oxford, 1839–1939 (Oxford: Oxford University Press, 2005); and Brebis Bleaney, “Two Oxford Science Professors, F. Soddy and J. S. E. Townsend,” Notes and Records of the Royal Society of London 56 (2002), 83–88.
2. Other accounts of these experiments and their aftermath (for the most part brief and, occasionally, misleading) include Jost Lemmerich, Science and Conscience. The Life of James Franck (transl. Ann M. Hentschel) (Stanford: Stanford University Press, 2011), esp. chap. 3; Helge Kragh, Niels Bohr and the Quantum Atom (Oxford: Oxford University Press, 2012), esp. 143–146; Josef Kuczera, Gustav Hertz (Leipzig: Teubner, 1985); John Lewis Heilbron, “A History of the Problem of Atomic Structure from the Discovery of the Electron to the Beginning of Quantum Mechanics,” PhD diss., University of California, Berkeley, 1964, esp. 313–319; J. L. Heilbron, “Lectures on the History of Atomic Physics,” in C. Weiner, ed., History of Twentieth Century Physics (New York: Academic Press, 1977), 40–107, esp. 75–77; Giora Hon, “Franck and Hertz versus Townsend: A Study of Two Types of Experimental Error,” Historical Studies in the Physical and Biological Sciences 20 (1989), 79–106; Giora Hon, “From Propagation to Structure: The Experimental Technique of Bombardment as a Contributing Factor to the Emerging Quantum Physics,” Physics in Perspective 5 (2003), 150–173; Giora Hon and Bernard R. Goldstein, “Centenary of the Franck-Hertz Experiments,” Annalen der Physik. 525 (2013), A179–A183; and for an especially thorough treatment, Henry Gilbert Small, “The Helium Atom in the Old Quantum Theory,” PhD diss., University of Wisconsin, 1971, esp. 45–56 and 214–257.
3. Hertz’s essay, “Zur Geschichte unserer Versuche über den Energieaustausch zwischen langsamen Elektronen und Atomen,” translated as “On the History of our Experiments on the Energy Exchange between Slow Electrons and Atoms” and cited hereafter as “History,” is in the James Franck Papers, Box 24, Folder 11, Special Collections Research Center, University of Chicago Library, hereafter cited as JFP. The correspondence between Hertz and Robert Platzman may be found there and in Box 24, Folder 3. In the translation, I have numbered the paragraphs for ease of reference.
4. Lemmerich, Franck (ref. 2), 307–309. See also Hertz to Franck, letter of January 5, 1964 and Franck to Hertz, letter of February 3, 1964, in JFP, Box 3, Folder 13.
5. R. W. Pohl, “Von den Studien- und Assistentenjahren James Francks,” Physikalische Blätter 28 (1972), 542–544. See also Thomas S. Kuhn, “Memorandum for Confidential File,” Emil Warburg file, Archives for the History of Quantum Physics (Niels Bohr Library and Archives, American Institute of Physics, College Park, MD; Walter Library, University of Minnesota; and elsewhere); hereafter cited as AHQP.