(b. Lόnborg Jutland, Denmark, 9 October 1843; d. Copenhagen, Denmark, 28 November 1917)
Christiansen was the son of Mads Peter Christiansen, a landowner and member of parliament, and Ane Marie Mortensdatter. He obtained the M.S. at the University of Copenhagen in 1866 and then worked for several years as scientific assistant, and subsequently as instructor, at the Polytechnical Institute and the Agricultural College of Copenhagen. In 1886 he was appointed professor of physics at the University of Copenhagen and the Polytechnical Institute.
In 1870 Christiansen discovered the anomalous dispersion of light in solutions of the dyestuff fuchsine. In 1872 he constructed the water-jet pump to produce low pressures. His other experimental work included research on the optical properties of crystals (with Haldor TopsØe), the properties of heat conduction, heat radiation, the movement of air currents through small openings, and friction electricity.
An outstanding teacher, Christiansen exerted great influence on his students, who included Martin Knudsen and Niels Bohr. He himself was strongly influenced by Ludvig Lorenz and played a major role in establishing Lorenz’ influence Christiansen laid the foundation for the study of theoretical physics at the University of Copenhagen; and his textbook in this field was translated into German, English, and Russian.
Christiansen’s Indledning til den matematiske Fysik, 2 vols. (Copenhagen, 1887–1890) was translated into German by J. Möller as Elemente der theoretischen Physik (Leipzig, 1894; 4th ed., 1921), and appeared in English trans. by W. F. Magie as Elements of Theoretical Physics (London, 1897). His other works are listed in Poggendorff, III, 269–270; IV, 247; and V, 221–222.
On his life and work, see H. M. Hansen, in Dansk biografisk leksikon, V (Copenhagen, 1934), 195–198; the obituary by Martin Knudsen, in Festskrift udgivet af KØbenhavns universitet (1918), 119–122; and K. Prytz, in Kongelige Danske Videnskabernes Selskabs Oversigter(1917–1918), 31–54, which includes a complete bibliography of his works .