William Henry Bragg

William Henry Bragg: The Late Starter Who Read Crystals

By the age of forty he had published almost nothing. He had spent eighteen years on the other side of the world teaching mathematics and physics to Australian undergraduates, and had arrived in that job at twenty-three knowing, by his own admission, very little physics at all. Then, in 1904, he began to do research — and within three years was a Fellow of the Royal Society, within eleven a Nobel laureate, and within nineteen the director of the Royal Institution. The career of William Henry Bragg is the standing refutation of the idea that physics is a young man's game.

A Cumberland Farm and a Third Wrangler

He was born at Westward in Cumberland on 2 July 1862. His mother died when he was seven and he was brought up by an uncle, passing through the Robert Smyth Academy and then King William's College on the Isle of Man before going up to Trinity College, Cambridge. In 1884 he came out third Wrangler in the mathematical tripos — third in the country in the hardest examination Britain then set.

Twenty-Three, and Almost No Physics

Two years later, aged twenty-three, he was appointed Elder Professor of Mathematics and Physics at the University of Adelaide. The mathematics he had. The physics he did not, and much of his early Australian years went into acquiring it and into building up the university's scientific facilities from very little. In 1889 he married Gwendoline Todd, daughter of the astronomer Charles Todd; they had three children, Gwendolen, Lawrence and Robert. For nearly two decades Bragg was, by the standards of research physics, invisible.

Beginning at Forty-One

The change came in 1904, when he was forty-one, with work on the ionization of gases and the absorption of alpha particles. It was serious experimental physics of the kind then reshaping the understanding of matter, and it was recognised quickly: election to the Royal Society followed within three years. In 1909 he came home to England as professor at the University of Leeds.

Leeds, and the Spectrometer

In 1912 Max von Laue had shown that a crystal will diffract X-rays into a characteristic pattern of spots — a result that simultaneously proved X-rays behave as waves and demonstrated that crystals are built on a regular lattice. Laue had a phenomenon. What nobody yet had was a method.

Bragg supplied one half of it and his elder son the other. Lawrence, then a young man at Cambridge, worked out the relation now universally called Bragg's law: an equation connecting the wavelength of the X-rays, the spacing between the planes of atoms in the crystal, and the angle at which the rays are reflected. Rearranged, it lets you run the argument backwards — measure the angles, know the wavelength, and calculate where the atoms are.

William Henry Bragg built the instrument that made this practical. His X-ray spectrometer allowed the reflections from a crystal face to be measured systematically and precisely rather than photographed and guessed at, and with it the two of them began working out the atomic architecture of real substances, starting with simple salts and small molecules. Between them they turned a curiosity into a technique, and the technique into a science. The 1915 Nobel Prize in Physics went to both, for the analysis of crystal structure by means of X-rays. Lawrence was twenty-five, and remained the youngest laureate in the prize's history until 2014. It is still the only Nobel shared by a father and son for the same work.

1915 was not a good year in the family. Robert Bragg, the younger son, died of wounds at Gallipoli that September.

The Royal Institution

Bragg moved to University College London in 1915 and in 1923 to the Royal Institution as Fullerian Professor, directing it and its Davy Faraday Laboratory for the rest of his life. He built the RI into one of the world's leading centres of crystallography, and he was an unusually gifted expositor — a repeat performer at the Royal Institution Christmas Lectures, with books on sound and light aimed at the general reader. He was knighted in 1920, received the Rumford Medal in 1916, the Copley Medal in 1930 and the Order of Merit in 1931, and served as President of the Royal Society from 1935 to 1940.

Why William Is Called a Genius

The honest account has to begin with the credit question, because it has followed the Bragg name for a century. The law is Lawrence's. His father said so at the time, writing to *Nature* that his son had produced the theory making it possible to calculate the positions of the spots for any arrangement of crystal and plate, and Lawrence said so at the end of his life: "It is sometimes said that my father and I started X-ray analysis together, but actually this was not the case." The world nonetheless kept crediting the older, more famous man — Lawrence was left out of the 1913 Solvay Conference his father attended, and Rutherford's comment to William on their shared Barnard Medal in 1915 was "It is very early for your boy to be getting these distinctions." William himself observed that people were apt to give him first credit on occasions when his son should have it. Lawrence took to using his middle name to escape the confusion.

So the case for William Henry Bragg cannot rest on the theory. It rests on instrumentation and on judgement. The X-ray spectrometer is the reason crystallography became a working discipline rather than a demonstration: a device precise enough to convert a diffraction pattern into a reliable measurement, designed by someone with the rare combination of a Wrangler's mathematics and the hands of a practical experimentalist. And there is the temperament — a man who taught himself an entire subject at twenty-three, began original research at forty-one, and then, at the height of his fame, spent his energy publicly insisting that the central idea belonged to his son. That is not brilliance in the usual sense. It is something scarcer.

Legacy

Bragg died of heart failure in London on 12 March 1942, aged seventy-nine. The method he and Lawrence founded went on to determine the structure of the atom's arrangement in metals, minerals, vitamins, proteins and, in 1953, DNA — work done at institutions the Braggs had shaped, by researchers using the descendants of the spectrometer he built at Leeds. More Nobel Prizes have flowed from X-ray crystallography than from almost any other single technique in the physical sciences. Most of what we know about the shape of the molecular world was read off crystals by the method a Cumberland farm boy started building when he was past forty.

Achievements

• Franklin Medal — 1930
• Honorary Fellow of the Royal Society Te Apārangi
• Fellow of the Royal Society
• Matteucci Medal — 1915
• Faraday Medal — 1936
• Nobel Prize in Physics — 1915
• Notable work: X-ray diffraction
• Held posts at University College London, University of Adelaide and University of Leeds
• Educated at King William's College, Old Grammar School and Trinity College
• Fields of research: chemistry, crystallography, mathematics and physics