by Adrian Rice

© Oxford University Press 2004 All rights reserved

**Henrici, Olaus Magnus Friedrich Erdmann** (1840-1918), mathematician, was born in Meldorf, Danish Holstein, the son of Theodor Henrici. Between the ages of nine and sixteen he attended the *Gymnasium* in Meldorf, before working for three years as an apprentice engineer at Flensburg. When he was nineteen he went to the Karlsruhe *Polytechnicum,* where, following the advice of Rudolf Clebsch, he studied mathematics. From the age of twenty-two he was a student of Ludwig Otto Hesse at Heidelberg, where he obtained his doctorate, and then moved to Berlin, where he studied under Karl Weierstrass and Leopold Kronecker, leading mathematicians there. He then became a *Privatdozent* at Kiel University but, being unable to support himself, moved to London in 1865.

Struggling to earn a living, Henrici provided for himself by teaching elementary mathematics to schoolboys, an experience that was to prove invaluable to his subsequent career by developing his power of expression. He obtained an introduction from Hesse to James Joseph Sylvester which enabled him to become acquainted with many of the foremost British mathematicians of the day, including Arthur Cayley, William Kingdon Clifford, and Thomas Archer Hirst, then professor of pure mathematics at University College, London. Henrici became Hirst's assistant and, in 1869, was appointed mathematics professor at Bedford College for Women.

At Easter 1870 Henrici stood in for Hirst who had fallen ill; a few months later he was appointed the new professor at University College, following Hirst's resignation. He held the chair of pure mathematics for ten years. In 1879 Clifford, the professor of applied mathematics and mechanics, died, and Henrici acted as a temporary replacement before transferring to the applied chair the following year. He introduced projective geometry, vector analysis, and graphical statics into the University College mathematics syllabus--a radical departure from the analytically biased Cambridge-style course previously taught.

In 1884 Henrici left University College to become professor of mechanics and mathematics at the newly founded Central Technical College (later part of Imperial College) in South Kensington. There he set up an innovative laboratory of mechanics upon which many others were later based. Being highly proficient in the construction of models and apparatus, Henrici soon filled it with a myriad of machines and devices, including his 'harmonic analyser'. This modified version of a machine originally invented by Lord Kelvin to obtain Fourier coefficients mechanically was perhaps his most original piece of work.

Henrici was a great perfectionist and would publish only when he was entirely convinced the work was satisfactory. Consequently, his publications are not voluminous, but the quality is very high. Among the most significant, a short book, *Congruent Figures* (1878), attempted to familiarize students with the new projective methods he was introducing. A second volume dealing with similar figures was planned but never written. Similarly, another small book, *Vectors and Rotors* (1903), based on his lectures to first-year students, dwelt only on the elementary part of the subject, being intended as the first part of a more comprehensive work which was never completed. Nevertheless, Henrici can be given the credit for introducing vectorial analysis into English mathematical teaching, making much use of it in his classes, where his success is confirmed by student accounts of 'the singular lucidity of his teaching' (Hill, xlix) and the 'masterly ease and freedom' (Bellot, 322) of his exposition.

Other notable works are two papers read before the London Mathematical Society on the theory of discriminants and singularities of envelopes of series of curves *(Proceedings of the London Mathematical Society,* 1st ser., 2, 1869). He also contributed lengthy and perceptive articles to the eleventh edition (1910-11) of the *Encyclopaedia Britannica* on calculating machines, Euclidean geometry, projective geometry, and projection, as well as shorter pieces on descriptive geometry and perspective.

In 1874, Henrici was elected a fellow of the Royal Society, and he served on its council in 1882-3. He was also president of the London Mathematical Society from 1882 to 1884. On 15 August 1877 he married Helen Stodart Kennedy of Stepney Green, daughter of John Kennedy, a Congregational minister. Their son, Major Ernst Olaf Henrici of the Royal Engineers, collaborated on Henrici's final published work in 1912, a paper on the theory of measurement by metal tapes and wires in catenary, in which formulae were given to facilitate accurate measurements when using a catenary on any sizeable slope. Henrici retired in 1911 and moved to Chandler's Ford, Hampshire, where he died at his home, Hiltingbury Lodge, on 10 August 1918. He was survived by his wife.

ADRIAN RICE

**Sources **

M. J. M. Hill, *Proceedings of the London Mathematical Society,* 2nd ser., 17 (1918), xlii-xlix

H. H. Bellot, *University College, London, 1826-1926* (1929), 322

A. R. Hall, *Science for industry: a short history of the Imperial College of Science and Technology and its antecedents* (1982)

W. P. Ker, ed., *Notes and materials for the history of University College, London: faculties of arts and science* (1898)

M. J. M. Hill, 'Some account of the holders of the chair of pure mathematics from 1828 to the present time', 1924, UCL, materials for the history of UCL, Mem. 2A/19, fols. 5-6

M. J. Tuke, *A history of Bedford College for Women, 1849-1937* (1939), 344

*WWW, 1916-28*

d. cert.

m. cert.

election certificate, RS

**Archives **

Sci. Mus.

**Likenesses **

photograph, UCL *[see illus.]*

photograph, Sci. Mus., Robert Tucker collection

**Wealth at death **

£2716 *8s. 3d.:* probate, 25 Oct 1918, *CGPLA Eng. & Wales*

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