Sir William Henry Bragg

The English physicist Sir William Henry Bragg (1862-1942) was the founder of the science of crystal-structure determination by x-ray diffraction methods. He received the Nobel Prize in physics jointly with his son, William L. Bragg, in 1915.

William Henry Bragg was born on July 2, 1862, at Westward, Cumberland, England. He attended King's College, Isle of Man, and Trinity College, Cambridge, where he took honors in mathematics in 1884. A year later he became professor of mathematics and physics at the University of Adelaide, Australia. He married there.

In time Bragg turned to experimentation, and soon after W. C. Roentgen's momentous discovery of x-rays in 1895, Bragg set up and experimented with an x-ray tube. In the next few years he did basic work and published papers on radioactivity, the range of alpha-particles and their power to ionize gases, and the behavior of secondary electrons, particularly those produced by gammarays. This work led him to form his views on the nature of x-rays.

Bragg returned to England in 1908 as Cavendish professor of physics at Leeds. Four years later Max von Laue, W. Friedrich, and P. Knipping discovered the diffraction of x-rays by a crystal. Bragg, in a simple reinterpretation of Laue's elegant mathematical theory, looked upon the interaction as a reflection of the waves of a narrow incident beam from a large number of equally spaced parallel planes of atoms. The Bragg equation embraces both the corpuscular and wave theories of x-rays and relates the x-ray wavelength, the angle of reflection, and the spacing of the planes. Bragg immediately saw the importance of the discovery and was able, with his son, to determine the exact arrangement of atoms or ions in crystals of a variety of simple substances such as common salt, diamond, and copper. Previously the arrangements of atoms in the elements and their compounds were inferred by indirect chemical methods, partly dependent on molecular weights observed in the gaseous state or solution.

After World War I Bragg moved to London. First at University College and then as director of the Royal Institution, he was responsible for the spread of crystalstructure methods, and the sciences of metallurgy and mineralogy, both predominantly concerned with solids, were reborn. Physicists could calculate properties of solids on the basis of atomic positions in the ideal crystal. Interpretation now extends to the endless variety of less perfectly ordered structures which make up the world of fibers, polymers, liquid crystals, and other aggregates of atoms, ions, and molecules, including proteins, enzymes, viruses, and other materials of life.

Bragg was president of the Royal Society from 1935 to 1940. He was kind and fatherly, admirable in the Royal Institution Christmas Lectures for juveniles, and an interpreter of science to the general public. He died in London on March 12, 1942.

Further Reading

A brief but adequate biography and an account of Bragg's work is in Nobel Foundation, Nobel Lectures, Including Presentation Speeches and Laureates' Biographies: Physics, 1901-1921 (1967). His life and work are discussed in Bernard Jaffe, Chemistry Creates a New World (1957); George Gamow, Biography of Physics (1961); and R. Harré, ed., Scientific Thought (1969).

Additional Sources

Caroe, G. M., William Henry Bragg, 1862-1942: man and scientist, Cambridge; New York: Cambridge University Press, 1978.

Jenkin, John., The Bragg family in Adelaide: a pictorial celebration, Australia: University of Adelaide Foundation in conjunction with La Trobe University, 1986. □

Bragg crystal spectrometer An instrument that uses a crystal to split X-rays according to energy, just as a prism splits visible light into the colours of the spectrum. It is named after the British physicist William Henry Bragg (1862–1942). The device has the highest resolution of any X-ray spectrometer, but suffers from low efficiency and can operate only in a very narrow band. It has been used to best effect in studies of the Sun, but has also been flown on X-ray satellites such as the Einstein Observatory.