Bragg, William Henry

(b. Westward, Cumberland, England, 2 July 1862; d. London, England, 12 March 1942)

physics.

Born on his father’s farm near Wigton, Bragg was the eldest child of Robert John Bragg, former officer in the merchant marine, and Mary Wood, daughter of the vicar of the parish of Westward. His mother died when he was seven. A bachelor uncle, William Bragg, a pharmacist and the dominant member of the family, then took his namesake to live with him. After six years Bragg’s father removed his son from the uncle’s house in Market Harborough (50 miles northeast of Cambridge) and sent him to King William’s College, a public school on the Isle of Man. Bragg continued, however, to return to Market Harborough during vacations even after he had gone up to Cambridge, and to look forward to his uncle’s pride in his accomplishments.

Bragg was always at the top of his school class, quiet and rather unsocial but tall, strong, and good at competitive sports. Having outstripped his schoolmates, he made little progress in his final year, 1880–1881. “But a much more effective cause for my stagnation was the wave of religious experience that swept over the upper classes of the school during that year…. we were terribly frightened and absorbed; we could think of little else.”¹ The mature Bragg preserved his composure by refusing to take literally the biblical threat of eternal damnation, although he retained his faith and his abhorrence of atheism.

Bragg entered Trinity College on a minor scholarship, obtaining a major scholarship the following year. Beginning his work at Cambridge in the long vacation, July and August 1881, he went up every “long” afterward. Under Routh’s coaching he read mathematics, and only mathematics, “all the morning, from about five to seven in the afternoon, and an hour or so every evening” for three years, coming out third wrangler in Part I of the mathematical tripos in 1884. “I never expected anything so high…. I was fairly lifted into a new world. I had new confidence: I was extraordinarily happy”² Bragg obtained first-class honors in Part III of the mathematical tripos in 1885, and left Cambridge at the end of that year upon being appointed to succeed Horace Lamb as professor of mathematics and physics at the University of Adelaide. Although in his last year at Cambridge Bragg attended lectures by J. J. Thomson at the Cavendish Laboratory, at the time of his appointment his physical studies had not included any electricity; he subsequently attempted Maxwell’s Treatise only after reading more elementary texts.

At Cambridge, Bragg published nothing; in his first eighteen years at Adelaide (1886–1904) he published three minor papers on electrostatics and the energy of the electromagnetic field. Rather, his efforts were invested in the development of a marvelously, indeed beguilingly, simple and comprehensible style of public and classroom exposition, in the affairs of his university, and in those of the Australian Association for the Advancement of Science. One of the Australian notables by virtue of his office, in 1889 he married the daughter of the postmaster and government astronomer, Charles Todd, and fell in with the extensive but relaxed social life and out-of-doors recreations. His elder son, William Lawrence, caddied for his father, a fine golfer; his daughter was a devoted companion.

This is not the sort of life that brings election to the Royal Society of London (1907), the Bakerian lectureship (1915), the Nobel Prize in physics (1915), the Rumford Medal of the Royal Society (1916), sixteen honorary doctorates (1914–1939), presidency of the Royal Society (1935–1940), and membership in numerous foreign academies, including those of Paris, Washington, Copenhagen, and Amsterdam. The new life began, at age forty-one, in 1903–1904.

In 1903 Bragg was once again president of Section A (astronomy, mathematics, and physics) of the Australasian Association for the Advancement of Science. His presidential address, delivered at Dunedin, New Zealand, on 7 January 1904, was entitled “On Some Recent Advances in the ory of the Ionization of Gases”³ Conscious that he was addressing Rutherford’s “friends and kindred,” and possibly stimulated by the unavoidable comparison between his won accomplishments and those of the younger man, Bragg gave a highly critical review of the field, finding fault with much of the work that had been done and with many of the assumptions upon which it rested. The most damaging criticism was directed toward the work on the scattering and absorption of the ionizing radiations (α, β, and γ rays) by matter—the atoms of which Bragg, following the most modern views, supposed to consist of “thousands of electrons.” The absorption of the particulate β and α rays had, unjustifiably, been assumed analogous to the exponential decrease in intensity of a wave traversing an absorbing medium; moreover, the analogy confounded energy flux and particle flux. “The exponential law is not applicable to this kind of radiation…. ‘Amount of radiation’ is not a term with definite meaning.”⁴ If an exponential law seemed to hold, that was only because of the superposition of a variety of factors—principally the broad spectrum of initial velocities of the particles and the scattering of the particles by the absorber.

In the spring of 1904, “through the generosity of a constant friend of the University of Adelaide”⁵ and with the assistance of R. D. Kleeman, Bragg began experiments on the absorption of α particles emitted by a radium bromide source. Early in September, Bragg and Kleeman reported the results of a rather thorough investigation that combined simple experiments and highly ingenious analysis.⁶ The α particles fell into a few groups, each of which had a definite range, and thus a definite initial velocity. Each group corresponded to a different radioactive species in the source, so that the measurement of α particle ranges soon became an invaluable tool in identifying radio-active substances.

For the next two and a half years, until the spring of 1907. Bragg followed up this line of investigation very vigorously, publishing a paper every few months. Then as in 1903–1904, a highly critical review paper (“On the Properties and Natures of Various Electric Radiations”)⁷ heralded a reorientation of his interest. Again the title was a misnomer, for the main point of the paper was to present arguments supporting “the possibility that the γ and X rays may be of a material nature.” specifically neutral pairs consisting of an electron and an α particle. (This was a full year before Rutherford and Geiger found the α particle to be doubly charged.)

As early as January 1904, Bragg had expressed doubts about the identity of γ rays and X rays—the latter just then being rather convincingly shown to have many properties of transverse ether pulses.⁸ Now, considering γ and X rays to be of the same nature, he declared the evidence in favor of the ether pulse theory to be “overrated,” and emphasized that the ory was unable to account for the large quantity of energy and momentum that remained in the ray regardless of the distance from its source, and that could all be delivered to a single electron. During the following five years Bragg backed off somewhat from this concrete model of the γ ray, emphasizing its “corpuscular” rather than its “material” nature.⁹ but did not abandon the general concept of an electron-with-its-charge-neutralized until after the discovery of X-ray diffraction in 1912. Thus, initially without being aware of the views of Einstein and Stark, Bragg became the first, and remained the foremost, English-language advocate of a view of X rays that stressed their “quantal” properties.¹⁰

Barkla answered Bragg’s challenge,¹¹ and their exchange of body blows over the distribution of scattered X rays initiated a continuing feud. Thereafter, Bragg’ experiments, and the controversy, focused upon a remarkable inference that Bragg drew from his neutral-pair theory: the ionization accompanying the passage of X rays and γ rays through matter is not produced by the direct action of these rays, but is entirely a secondary effect occurring only after the ray has been converted into a high-speed electron (through removal of the neutralizing positive charge).

Until the spring of 1911 the available data were ambiguous, and the opponents numerous.¹² However, the first result to come out of C. T. R. Wilson’s cloud chamber was a clear demonstration that the exposure of a gas to a beam of X rays did not produce a diffuse homogeneous fogging, but a large number of short wiggly lines; ionization occurred only along the path of the photoelectron.¹³ Bragg’s inference became—and has remained—the accepted view of the interaction of high-frequency radiation with matter. And yet, just as Bragg’s contention was receiving striking experimental support, the ory from which he derived it seemed to be decisively refuted by the discovery of an interference phenomenon accompanying the passage of an X-ray beam through a crystal.

But to pick up the biographical thread: Bragg’s star rose rapidly after his first publications. In 1907, nominated by Horace Lamb and supported by Rutherford, he was elected to the Royal Society of London; in 1908 he was appointed Cavendish professor of physics at the University of Leeds, returning to England in March 1909. The first year or two at Leeds were not happy ones; the removal from Australia, the lack of solid scientific results, and the sniping criticism of his work by Barkla undermined Bragg’s self-esteem. Things brightened in 1911–1912 with the vindication of his views on ionization by X rays, and of his views on the scattering of β and α particles (in this latter question he was closely allied with Rutherford against J.J. Thomson).¹⁴ After completing a detailed account of his researches and views, Studies in Radioactivity (1912), Bragg was on the lookout for a new problem.¹⁵

During the summer and fall of 1912 the Laue-Friedrich-Knipping phenomenon was, naturally, the subject of discussion. After some initial success in construing the photographs on the corpuscular hypothesis,¹⁶ Bragg and his son convinced themselves that a wave interpretation was unavoidable. This transition was smoothed by the instrumentalist epistemology that Bragg had adopted in the course of his corpuscular hypothesis campaign: “Theories were no more… than familiar and useful tools.”¹⁷ Early in November, William Lawrence, working at the Cavendish, showed how the Laue phenomenon might be regarded as a reflection of electromagnetic radiation in the incident beam from those planes in the crystal that were especially densely studded with atoms, and he derived the famous Bragg relation, nλ = 2d sin 0, connecting the wavelength of the X ray with the glancing angle at which such a reflection could occur.¹⁸

The younger Bragg’s paper was entitled “The Diffraction of Short Electromagnetic Waves”—not “X rays”—for he wished to hold open the possibility that X rays (as known especially by their ionizing properties) were nevertheless his father’s corpuscles, the diffracted-reflected entity affecting the photographic plates being merely the Bremsstrahlung necessarily accompanying the stopping of cathode rays in the X-ray tube. Despite the uncertainty whether the reflected rays could ionize—and even despite some counter evidence—Bragg’s epistemology did not allow him to see the issue any longer as either/or: “The problem then becomes, it seems to me, not to decide between then becomes, it seems to me, not to decide between two theories of X-rays, but to find, as I have said elsewhere, one theory which possesses the capacities of both.”¹⁹

In January 1913 Bragg succeeded in detecting the reflected rays with an ionization chamber,²⁰ and by March he had constructed the first X-ray spectrometer. Initially Bragg used it to investigate the spectral distribution of the X rays, relations between wavelength and Planck’s constant, the atomic weight of emitter and absorber, and so on.²¹ But very quickly he adopted his son’s interest in the inversion of the Bragg relation: using a known wavelength in order to determine d, the distances between the atomic planes, and thus the structure, of the crystal mounted in the spectrometer. Apart from specifying general symmetry conditions, before June 1912 it had not been possible to give the actual arrangement of the constituent atoms of any crystal. Laue’s assignment of a simple cubic lattice to zinc sulfide had been corrected by William Lawrence to face-centered cubic, and he went on to analyze the crystal structure of the alkali halides on the basis of “Laue diagrams” that he had made at Cambridge. The spectrometer first served to confirm these structures and to determine the absolute values of the lattice spacings, and then was applied to more difficult cases.²² By the end of 1913 the Braggs had reduced the problem of crystal structure analysis to a standard procedure.

In 1915 Bragg moved to London as Quain professor of physics at University College, and throughout the war he continued to direct some crystal analyses. He had, however, already become involved in war work, and it soon took almost all of his time. Bragg was a member of the panel of scientific experts attached to the Central Committee of the Board of Invention and Research, an institution created by Lord Fisher in July 1915 to aid the navy by screening inventions and sponsoring research. In April 1916, with the title of resident director of research and a staff of two physicists and a mechanic, Bragg was installed at the Naval Experiment Station at Hawkcraig to work on submarine detection. No satisfactory cooperation could be obtained here because of intraservice rivalry between Fisherites and anti-Fisherites, and at the end of 1916 the work was transferred to Harwich, with much loss of time and momemtum.²³ “It was,” Andrade opined, “probably in acknowledgement of his war work, as well as of his scientific eminence, that Bragg was made a C.B.E. in 1917 and was knighted as a K.B.E. in 1920.”²⁴

“The outbreak of war,” Bragg asserted in 1920, “practically put a stop to the work with the spectroscope [i.e., X-ray spectrometer], which had been commenced in England, and we have fallen behind other countries which have been able to push on with it.”²⁵ Of the two sorts of work—measuring λ (or, more generally, the properties of X rays and the X-ray spectra emitted by atoms) and measuring d (or, more generally, the structure of various crystals)—the first and more fundamental task, although pioneered by Barkla, Bragg, and Moseley, was largely abandoned in Britain after the war. Bragg assumed his duties at the University of London and began gathering a research school about himself. In 1923, when Bragg became head of the Royal Institution, this young and energetic group was installed in the previously moribund Davy-Faraday Research Laboratory. Their work, following a tacit agreement between Bragg and his son, was confined to the analysis of organic crystals. And in this field, which has now become so fundamental to molecular biology, Bragg put Britain way out in front.²⁶

Bragg was president of the Royal Society at a very difficult time (1935–1940). He was one of the three Britons who had been members of the Deutsche Physikalische Gesellschaft since before World War I, and he now welcomed “certain ambiguous advances from learned bodies in Nazi Germany, and he did his best to further ostensible plans for an understanding between the two countries which, in his goodness of heart, he took at their face value.”²⁷ Then the Royal Society was caught in the cross currents of agitation over the study of the social relations of science and the assertion of the social responsibility of science. Finally, there was the war, with its innumerable committees and councils—and air raids. Bragg’s mind remained keen, even for scientific questions, but his energy began to fail. On 10 March 1942 he “had to take to his bed: two days later he was dead.”²⁸

NOTES

1. Autobiographical note quoted by Bragg and Caroe, pp. 171–172.

2.Ibid., p. 173.

3.Report of the Australasian A.A.S. (Dunedin), 10 (1904), 47–77.

4.Ibid., p. 69. Bragg’s critique, contrary to the usual account, was not limited to (and thus not derived from the peculiar constitution of) the α particle.

5. Bragg, Studies in Radioactivity (London, 1912), p. 5.

6. Bragg, “On the Absorption of X-rays, and on the Classification of the X-rays of Radium,” in Philosophical Magazine, 6th ser., 8 (Dec. 1904), 719–725; Bragg and Kleeman. “On the lonization Curves of Radium,” ibid., 726–738. Dated 8 September 1904.

7.Philosophical Magazine, 6th Ser., 14 (Oct. 1907), 429–449. Read before the Royal Society of South Australia, 7 May and 4 June 1907.

8. See the article on Barkla in D.S.B.

9. Bragg, “The Consequences of the Corpuscular Hypothesis of γ and X-rays, and the Range of β Rays,” in Philosophical Magazine, 6th Ser., 20 (Sept. 1910), 385–416; Studies in Radio-activity.

10. Not everyone shut his mind to this new gospel: “Personally, I have long been a convert to Professor Bragg’s views on the nature of X-rays…” H. L. Callendar, “Presidential Address, Section A,” in Report of the British A.A.S. (Dundee, 1912), p. 396. Cf. Russell McCormmach, “J.J. Thomson and the Structure of Light,” in British Journal for the History of Science, 3 (1967), 362–387.

11. See the article on Barkla in D.S.B.

12. Bragg, in Philosophical Magazine, 6th ser., 20 (Sept. 1910), 385–416; 22 (July 1911), 222–223; and 23 (Apr. 1912), 647–650.

13. C. T. R. Wilson, “On a Method of Making Visible the Paths of lonising Particles Through a Gas,” in Proceedings of the Royal Society of London, 85A (9 June 1911), 285–288. Received 19 April 1911.

14. J. L. Heilbron, “The Scattering of α and β Particles and Rutherford’s Atonm,” in Archive for History of Exact Sciences, 4 (1968), 247–307.

15. R. A. Millikan, Autobiography (London, 1951), pp. 95, 99.

16. Bragg, “X-rays and Crystals,” in Nature, 90 (24 Oct. 1912), 219; dated 18 October. P. P. Ewald, “William Henry Bragg and the New Crystallography,” in Nature, 195 (28 July 1962), 320–325. P. Forman, “On the Discovery of the Diffraction of X-rays by Crystals: Why Munich, Which X-rays?,” in Acts du XII^e Congrès International d’Histoire des Sciences, Paris, 1968.

17. Bragg, “Radiations Old and New,” in Report of the British A.A.S. (Dundee, 1912), pp. 750–753.

18. W. L. Bragg, “The Diffraction of Short Electromagnetic Waves by a Crystal,” in Proceedings of the Cambridge Philosophical Society, 17 (14 Feb. 1913), 43–57, Read 11 November 1912.

19. Bragg, “X-rays and Crystals,” in Nature, 90 (28 Nov. 1912), 360–361. The “elsewhere” may refer to Studies in Radioactivity, p. 192.

20. Bragg, “X-rays and Crystals,” in Nature, 90 (23 Jan. 1913), 572. Dated 17 January.

21. W. H. Bragg and W. L. Bragg, “The Reflection of X-rays by Crystals,” in Proceedings of the Royal Society of London, 88A (1 July 1913), 428–438, received 7 April 1913; W. H. Bragg, “The Reflection of X-rays by Crystals (II),” ibid., 89A (22 Sept. 1913), 246–248, received 21 June 1913.

22. W. H. Bragg and W. L. Bragg, “The Structure of Diamond,” ibid. (22 Sept. 1913), 277–291, received 30 July.

23. J. J. Thomson, Recollections and Reflections (New York, 1937), pp. 207–210.

24. Andrade, “William Henry Bragg 1869–1942,” p. 284.

25. Bragg, as president of the Physical Society of London, opening a discussion on X-ray spectra, in Proceedings of the Physical Society of London, 33 (1920), 1.

26. Articles by J. M. Robertson, J. D. Bernal, and K. Londsdale, in P. P. Ewald, ed., Fifty Years of X-Ray Diffraction (Utrecht, 1962).

27. Andrade, op. cit., p. 290.

28.Ibid.

BIBLIOGRAPHY

I. Original Works. An excellent chronological bibliography prepared by K. Lonsdale is appended to the article by Andrade (see below). Bragg’s papers at the Royal Institution, London, include a collection of offprints, research notebooks covering 1903–1913, summaries of some literature read, a few MS drafts (notably that of Studies in Radioactivity), miscellaneous scientific correspondence after 1920, a very little scientific correspondence before 1920, autobiographical notes on his youth, and family correspondence. Bragg’s correspondence with Rutherford, some fifty letters written between 1904 and 1915, is in the Rutherford papers at the Cambridge University Library; copies are available at the Royal Institution and at McGill University, Montreal. The locations of thirty-one letters from Bragg to other correspondents are given in T. S. Kuhn et al., Sources for History of Quantum Physics (Philadelphia, 1967), p. 26.

II. Secondary Literature. Works on Bragg are E. N. da C. Andrade, “William Henry Bragg 1869–1942,” in Obituary Notices of Fellows of the Royal Society of London, 4 (1943), 277–300; Sir Lawrence Bragg and Mrs. G. M. Caroe (Gwendolen Bragg), “Sir William Bragg, F.R.S.,” in Notes and Records of the Royal Society of London, 16 (1961), 169–182; and Who Was Who 1941–1950 (London, 1952), p. 134.

Paul Forman

Bragg, William Henry

ENGLISH PHYSICIST
1862–1942

Sir William Henry Bragg was born on July 2, 1862, near Wigton in the northwest of England, the son of an officer in the merchant navy. He attended King William's College on the Isle of Man, before studying for the mathematical degree at Trinity College, Cambridge, in 1884. Two years later he

was appointed professor of mathematics and physics at the University of Adelaide in South Australia. It was not until nearly twenty years later that Bragg began serious scientific research, concentrating first on α -particles and then x rays, in which he was critical of certain aspects of the then accepted theories on both. The significance of his work was such that it warranted his return to England, where he was appointed professor of physics at the University of Leeds in 1909. In 1912 to 1913, working with his son William Lawrence Bragg, a research student at the Cavendish Laboratory, Cambridge, he discovered how to use x rays to determine the molecular structure of crystals. This turned out to be one of the key scientific discoveries of the twentieth century for which the two shared the Nobel Prize for physics in 1915.

During World War I, Bragg moved to University College, London, and worked for the admiralty, developing the submarine detection systems ASDIC (Allied Submarine Detection Investigation Committee) and SONAR (Sound Navigation and Ranging). After the war Bragg became disenchanted with University College, and following the death of James Dewar in 1923, he moved to the Royal Institution. There Bragg created a major British center for x-ray crystallography. The scientists at the laboratory established a distinctive approach to x-ray crystallography that later formed the basis of the British school of molecular biology. Bragg trained such figures as Kathleen Lonsdale, the first woman fellow of the Royal Society ), J. D. Bernal (who went to Birkbeck College), W. T. Astbury (University of Leeds), and, of course, William Henry Bragg's son, (William) Lawrence Bragg. The Braggs came to a tacit agreement that the work in the Royal Institution would concentrate on organic crystals, whereas Lawrence's independent efforts would explicitly focus on minerals.

Bragg, knighted for his work during World War I, played a major role in scientific popularization and administration during the interwar period. He was one of the earliest scientists to take advantage of the new medium of radio that he used to full effect to emphasize the value of science for society at large and for industry in particular. From 1935 to 1940 he served as president of the Royal Society, in which capacity he played a major role in helping scientists fleeing from fascist regimes to find employment, in establishing the committee that became the scientific advisory committee to the war cabinet, and in determining what scientific resources would be available for the looming conflict with Germany. During the blitz (the German bombing of London during World War II), the Royal Institution was a designated bomb shelter, and often Bragg would go down to the shelter at night to help boost the morale of people taking refuge there, actions for which he is still remembered. He died, in office, on March 10, 1942.

see also Bragg, William Lawrence; Dewar, James; Lonsdale, Kathleen.

Frank A. J. L. James

Bibliography

Andrade, E. N. da C. (1943). "William Henry Bragg 1862–1942." Obituary Notices of Fellows of the Royal Society 4:277–300.

Caroe, G. M. (1978). William Henry Bragg, 1862–1942. Man and Scientist. Cambridge, U.K.: Cambridge University Press.

Home, R. W. (1984). "The Problem of Intellectual Isolation in Scientific Life: W. H. Bragg and the Australian Scientific Community, 1886–1909." Historical Records of Australian Science 6:19–30.

Hughes, Jeff (2002). "Craftsmanship and Social Service: W. H. Bragg and the Modern Royal Institution." In "The Common Purposes of Life": Science and Society at the Royal Institution of Great Britain, ed. Frank A. J. L. James. Aldershot, U.K.: Ashgate, pp. 225–247.

Jenkin, John (1986). The Bragg Family in Adelaide: A Pictorial Celebration. Adelaide, South Australia: University of Adelaide Foundation.

Quirke, Viviane (2002). "'A Big Happy Family': The Royal Institution under William and Lawrence Bragg and the History of Molecular Biology." In "The Common Purposes of Life": Science and Society at the Royal Institution of Great Britain, ed. Frank A. J. L. James. Aldershot, U.K.: Ashgate, pp. 249–271.