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He returned to Cambridge after the war and graduated in 1921 but, perhaps because of his interrupted studies, he only obtained a Second Class degree in Natural Sciences. However he went on to obtain a doctorate in 1926 and, after being a Fellow of St John's College (1924-27) and Christ's College (1928-29) he was appointed professor of applied mathematics at Manchester.
Hartree held this chair from 1929 to 1937 when he moved to the chair of theoretical physics. After undertaking work with the Ministry of Supply during World War II, he was appointed Plummer Professor of Mathematical Physics at Cambridge. He held this post until his death.
Hartree was basically a theoretical physicist, and he developed powerful methods in numerical analysis. His initial interest in numerical methods arose from his work on anti-aircraft gunnery in 1916-18. However Niels Bohr gave a lecture course in Cambridge in 1921 and Hartree was much influenced, working on applications of numerical methods for integrating differential equations to calculate atomic wave functions.
Hartree learn of a differential analyzer being developed by Vannevar Bush in the USA. This machine, first proposed by Lord Kelvin (William Thomson), performs integration with a wheel rolling on a rotating disk. Hartree visited Boston to learn about the workings of the differential analyzer, then returned to Cambridge and built his own.
The differential analyzer was soon to be replaced by electronic computers and when John Eckert set up ENIAC, Hartree was asked to go to the USA to advise on its use. He showed how to use ENIAC to calculate trajectories of projectiles. In his inaugural address on his appointment to the chair in Cambridge in 1946 he said
It may well be that the high-speed digital computer will have as great an influence on civilization as the advent of nuclear power.In addition to applying numerical methods to ballistics Hartree applied them to the physics of the atmosphere and to hydrodynamics.
He wrote a number of important books including Numerical analysis in 1952. In this book he says
Anyone intending to undertake a serious piece of calculation should realise that adequate checking against mistakes is an essential part of any satisfactory numerical process. No one, and no machine, is infallible, and it may fairly be said that the ideal to aim at is not to avoid mistakes entirely, but to find all mistakes that are made, and so free the work from any unidentified mistakes.References:
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