McClelland, John Alexander (1870–1920), physicist, was born in Dunalis in the parish of Dunboe, near Coleraine, Co. Londonderry, and was baptised on 1 December 1870, youngest among eleven children of William McClelland, a presbyterian farmer, and Margaret McClelland (née Morrison). As a young boy, he showed no interest in farming, but instead was educated from 1885 at Coleraine Academical Institution and then attended QCG. The decision to study in Galway may have been influenced by the fact that the professor of physics there was Alexander Anderson (qv), who was from the same part of north Co. Londonderry. McClelland was an outstanding student, graduating BA (1892), with first-class honours and gold medal in physics, and MA (1893).
In 1895 he won a research fellowship from the RUI and also an 1851 exhibition scholarship, with which he transferred (1896) to Trinity College, Cambridge, to pursue further research under J. J. Thomson and C. T. R. Wilson at the Cavendish laboratory. Röntgen rays had just been discovered, and the Cavendish was in the forefront of exciting new research into radiation and the effects of electricity on gases. McClelland's work was reported to the Royal Society in 1896 and 1897, and he went on to develop a novel method of studying the mobility of ions in gases given off from incandescent metals and flames. This work was reported in the Philosophical Magazine in July 1898 and in the Proceedings of the Cambridge Philosophical Society in December 1899. He received a research BA degree from Cambridge in 1897.
In 1900 he returned to Ireland as professor of physics at UCD, then based in very inadequate premises in what later became Newman House, St Stephen's Green. In 1901 he became a fellow of the RUI. He gave evidence to the royal commission on university education; his views were influential when it was decided to establish the NUI. In 1908 he was appointed to the chair of experimental physics at UCD in the new NUI, and (in the better equipped premises of the superseded RUI) developed a remarkable research school, which included students such as Felix Hackett (qv), J. J. Dowling (d. 1960), J. J. Nolan (qv), P. J. Nolan (qv), and J. J. McHenry (1897–1976). He put a great deal of work into planning an up-to-date physics building in Earlsfort Terrace, though – as a result of financial constraints – only part of it was constructed.
His work resulted in some fundamental discoveries on ionising radiation from radioactive materials; he published on these in the Transactions of the Royal Dublin Society. In 1904 he showed that radium emanation (later known to be the gas radon) had no electrical charge, and also indicated that gamma rays did not carry a negative charge, as other researchers had suggested. The following year he began to study the nature of secondary radiation, recognising that it was constituted of negatively charged particles. Of particular importance was his discovery that secondary radiation from the elements increases with the atomic weight of the substance being bombarded. His work seemed to indicate that secondary radiation resulted from atomic disintegration of the target substance, but it was difficult to interpret his results at a time when scientists were still working with an incomplete understanding of the nature of matter and of radiation. He published ten papers in the Proceedings of the Royal Irish Academy (1911–19) on atmospheric electricity and the ionisation of gases; work on the electrical charge of raindrops and of aerosols, partly carried out in collaboration with his students, established UCD's considerable reputation in atmospheric research.
McClelland had a sustained commitment throughout his career to the cause of education, acting as a commissioner of national education (1909–20), a member of the NUI senate (1908–20), and a member of the governing body of UCD (1908–20). He is credited with having had considerable influence on shaping the new college. His other public roles included serving as a member of the committee of education of the Department of Agriculture and Technical Instruction (1909–20), and from 1915 as Irish representative on the UK Advisory Council for Scientific and Industrial Research, to help develop connections between science and industry. In this capacity he worked on questions affecting Irish industries (especially on scientific aspects of the exploitation of peat) and supported the proposed development of a school of research in Ireland.
For twelve years he was a trustee of St Andrew's College, Dublin, a secondary school. He served as secretary (from 1906) and member of the council of science of the RIA (1907–20), and was made an FRS (1909). He was a member of the RDS, and served on its council of science. In 1903, at his suggestion, the RDS bought sixty milligrammes of radium bromide to enable him to continue his researches on radiation; this work antedated the establishment by the RDS in 1914 of the Irish Radium Institute, where pioneering work on therapeutic uses of radium was carried out. In 1906 McClelland was awarded the degree of D.Sc. (h.c.) by the RUI. He was awarded an honorary D.Sc. from the University of Dublin (1917), and the Boyle medal of the RDS (1918) in recognition of his services to science. He died 13 April 1920, aged 49, of rheumatic fever, possibly exacerbated by overwork, at his residence at Rostrevor, Orwell Road, Rathgar, Dublin.
He married (1901) Sarah Josephine (‘Ina’), daughter of Joseph Esdale, a builder in Coleraine; they had two sons and three daughters. Their elder son, Alexander, became a civil engineer, and was head of the Safety in Mines Board at the Royal School of Mines in London.