Sir William Henry Bragg & Sir William Lawrence Bragg

1862-1942

English Physicist

1890-1971

English Physicist

The Braggs, William Lawrence and William Henry, studied the diffraction of the then newly discovered x rays by crystalline solids. Their studies confirmed that interatomic distances could be accurately determined by this technique. X-ray diffraction has come to be accepted as the most accurate method of determining the structures of molecules and complex crystals.

William Henry Bragg attended Cambridge University, graduating with honors in 1885 and moving to Australia to accept a teaching post at the University of Adelaide. In Australia he married, his wife giving birth to William Lawrence in 1890. At Adelaide he developed a reputation as an outstanding teacher and public speaker, and busied himself with the affairs of the Australian Association for the Advancement of Science. His research output was meager, however, not at all consistent with the eminence he would eventually achieve.

A turning point in William Henry Bragg's career occurred in 1904, when Bragg, serving a second term as the president of the astronomy, mathematics, and physics section of the Australian Association, was called on to make a presidential address. Perhaps concerned that his audience would be comparing him with New Zealander Ernest Rutherford, who had achieved substantial fame for research into radioactivity as a much younger man, Bragg chose to present a highly critical review of the current understanding of alpha, beta, and gamma rays. He also began publishing papers on this area every few months. In 1907 he was elected a fellow of the Royal Society and in 1908 was recalled to England as Cavendish Professor at the University of Leeds.

William Lawrence Bragg was quickly recognized as a child prodigy and entered the University of Adelaide at 15, graduating three years later with an honors degree. He then entered Trinity College in Cambridge to work under C.T.R. Wilson (1869-1959), the Scottish physicist who would become known as the inventor of the cloud chamber.

The discovery in 1912 by German physicist Max von Laue (1879-1960) and his assistants that x rays scattered by a thin crystal formed a characteristic pattern of spots on photographic film naturally attracted the senior Bragg's attention. While William Henry had believed that x rays were some sort of material particle, he and his son concluded that the experiments could only be explained as the scattering of a wave. They then derived what has come to be known as the Bragg relation. It was for this work that father and son shared the Nobel Prize for physics in 1915. Both Braggs remind active in research throughout their careers, both being knighted for their contributions. William Henry served for five years (1935-1940) as president of the Royal Society. William Lawrence became director of the Cavendish Laboratory in 1938, serving until 1953.

The technique of x-ray diffraction made it possible to determine the arrangement of atoms within a crystal. With a bit of mathematical elaboration, to take into account the different efficiencies with which different atoms would scatter x rays, it became possible to determine the arrangement of atoms even in very complicated molecules, provided that the substance could be crystallized. Even with a partially ordered sample, some useful information can be gained. X-ray diffraction data obtained by Rosalyn Franklin (1920-1958) at the Cavendish would prove critical in the determination of the structure of DNA. Today the technique, with computer controlled data collection and analysis, has become one of the mainstays of biomolecular research.

DONALD R. FRANCESCHETTI