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CHRISTIE, SAMUEL HUNTER (1784–1865), mathematician, son of James Christie the elder [q. v.], was born at 90 Pall Mall, London, on 22 March 1784, and was as a child intimate with Sir Joshua Reynolds. He was educated at Walworth School in Surrey, where his great mathematical abilities were very early developed, and, at the suggestion of Bishop Horsley, his father entered him at Trinity College, Cambridge, where he was admitted a sizar 7 Oct. 1800. In his third year he obtained a scholarship, and in 1805 took his degree of bachelor of arts as second wrangler, having pressed very closely on Turton, afterwards bishop of Ely, who was senior wrangler, and with whom he was bracketed as Smith's prizeman. Christie also threw himself with ardour into all the athletic amusements of the day; he inaugurated the Cambridge University boat club, and became captain of the grenadier company of university volunteers. In 1806 he was appointed third mathematical assistant at the Royal Military Academy at Woolwich. In 1812 he established the system of competitive examinations, but was unable fully to carry out his views in this and in other respects until his advancement to the post of professor of mathematics in 1838. Between 1806 and 1854, when Christie resigned the professor's chair, the Military Academy had been completely transformed owing to his energy. He took an important share in promoting the great advance in magnetical science, which received its impulse from the observations made during the Arctic voyages in 1818 and 1819. The leading idea which runs through his theoretical discussions he first stated as a hypothetical law in a paper published in the Cambridge 'Philosophical Transactions' for 1820. At the meeting of the Royal Society in June 1824 he gave an account of some of his experiments of the effects of temperature upon magnetic forces. He was the first to observe the effect of the slow rotation of iron in producing polarity, and at his suggestion the series of experiments which he originated were repeated by Lieutenant Foster, R.N., during the expedition to the north-west coast of America in 1824 under Captain Parry, with very striking results. In 1833 his paper on the magneto-electric conductivity of various metals was selected by the council of the Royal Society as the Bakerian lecture for that year. In this paper he shows that the conducting power of the several metals varies inversely as the length, and directly as the square of the diameter of the conducting wire. The effect of the solar rays upon the magnetic needle early engaged his attention, and he proved by experiments that the direct effect of the solar rays is definite, and not due to any mere caloric influence. He also suggested that terrestrial magnetism is probably derived from solar influence, but his experiments in this direction leave room for further investigation. Christie appears to have been the first to make use of a torsion balance for the determination of the equivalents of magnetic forces; he also devoted himself to the improvement of the construction of both the horizontal and the dipping needle, and he served constantly upon the compass committee. In the 'Report of the British Association for 1833,' the portion which refers to the magnetism of the earth was drawn up by Christie, and he there again maintained that not only the daily variation, but also the quasi-polarity of the earth, is due to the excitation by the solar heat of electric currents at right angles, or nearly so, to the meridian and he suggests that these currents must be influenced by the continents and seas over which they pass, and also by the chains of mountains. The letter of Baron Humboldt to the president of the Royal Society in 1836 on the establishment of permanent magnetic observatories was referred to Christie and to Mr. Airy, and in consequence of their report the government in 1838 consented to bear the expense of several observatories in various parts of the United Kingdom. He was elected a fellow of the Royal Society on 12 Jan. 1826, and served the office of secretary from 1837 to 1854, when, for the benefit of his health, he went to reside at Lausanne. He was the author of 'Report (with Sir George Airy) upon a Letter on the Phenomena of Terrestrial Magnetism, addressed by M. le Baron de Humboldt to the President of the Royal Society,' 1836, 8vo, and 'An Elementary Course of Mathematics for the use of the Royal Military Academy, and for students in general,' parts i. and ii. 1846, 8vo, part iii. 1847, 8vo, besides fourteen papers in the 'Philosophical Transactions,' and some few contributions to other scientific journals. He died at his residence, Ailsa Villa, Twickenham, on 24 Jan. 1866. He was twice married, first on 12 May 1808 to Elizabeth Theodora, eldest daughter of Charles Claydon, battler of Trinity College, Cambridge. She died on 27 May 1829, and has a monument in All Saints Church, Cambridge. He married secondly, 16 Oct. 1844, Margaret Ellen, daughter of James Malcolm of Killarney.

[Gent. Mag. April 1865, ii. 517–18; Proceedings of Royal Society, vol. xv.; Obituary Notices pp. xi–xiv (1867); Times, 6 Feb. 1865, p. 12 information from the Master of Trin. Coll. Camb.]

G. C. B.