Jeffcott, Henry Homan (1877–1937), engineer, was born 6 February 1877 at Muff Lodge, Co. Donegal, second son of William Jeffcott, land agent and magistrate, and his wife Ellen (née Roche?). He was educated at the High School, Dublin, and TCD (1895–9), where he was first mathematics scholar, a gold medallist, and senior moderator in mathematics. In 1900 he returned, after a short period in London, to Dublin University and entered the engineering school, obtaining an engineering degree in 1902 as well as the MacCullagh prize for a treatise on the theory of electricity. After graduation he worked as an assistant engineer for Siemens Brothers at Woolwich and Stafford as well as in the workshops of Sir W. G. Armstrong at Whitworth & Co. Ltd in Manchester, before being appointed head of the metrology department of the National Physical Laboratory, Teddington, Middlesex (1905–10).
He returned to Ireland in 1910 as professor of engineering at the Royal College of Science in Dublin, a position he held until 1922, serving as dean of faculty (1914–22). During the first world war he was responsible for the manufacture of munitions in the college workshops, involving the construction and design of turret lathes and the adaptation of existing lathes by novel methods. In 1918 he was appointed secretary to the Water Power Resources of Ireland sub-committee and took an active part in collaboration with Sir John P. Griffith (qv) in the preparation of a scheme for the development of the water power of the River Liffey. During the war the Royal Society of London asked Jeffcott to try to resolve a problem that had come to light with an aspect of the accepted theory of rotor dynamics. Jeffcott's classic paper on lateral vibration was published in the Philosophical Magazine in 1919. In it he described the dynamics of rotating shafts, and posited a hypothetical model, still known as ‘the Jeffcott rotor’, which was a major advance in the explanation of rotor dynamic behaviour. He was awarded an honorary D.Sc. from Dublin University in 1922 and was appointed secretary of the Institution of Civil Engineers in the same year. He had some success as an inventor, most notably with the Jeffcott direct-reading tachometer, and contributed papers on water resources and other engineering projects to contemporary scientific journals, including the Proceedings of the Royal Dublin Society. He died 29 June 1937 at his home in Walton-on-Thames, Surrey. A memorial service was attended by a large number of prominent people.
He married Louise E. Howard Lang, daughter of George Lang; they had one son, who died in infancy.