Emil Khristianovich Lenz (24.02.1804— 10.02.1865) was a Russian physicist and a member of the St Petersburg Academy of Sciences (1834). He was born in Dorpat (now Tartu). He studied at the University of Dorpat. In 1836 he took charge of the chair of physics and physical geography at St Petersburg University; from 1840 he was dean of the faculty of physics and mathematics and from 1863 rector. He also taught at the Naval Cadet Corps (1835—41), at the Mikhailovsky Artillery Academy (1848 — 61) and at the Pedagogical Institute (1851-59).
His main work was in the field of electromagnetism. In 1833 he established the rule for determining the direction of the induced electromotive force (Lenz's law), and in 1842 (independently of J. Joule) the law of the thermal action of an electric current (the Joule — Lenz law). Together with B. S. Jacobi he was the first to develop methods of calculating the electromagnets in electrical machines, and he established the existence in them of the “armature reaction”. He discovered the reversibility of electrical machines.
He studied how the resistance of metals depends on temperature.
His work was also in the field of geophysics. In 1823 — 26 he took part in a voyage round the world on the sloop Predpriyatiye and carried out important geophysical research. Lenz's pupils included M. P. Avenarius, F. F. Petrushevsky, F. N. Shvedov, R. E. Lenz, A. I. Savelyev, N. P. Sluginov and others.