Langevin, Paul (1872–1946)

French physicist who contributed to the studies of magnetism and X-ray emissions, especially paramagnetic (weak attractive) and diamagnetic (weak repulsive) phenomena in gases. During World War I he invented an apparatus for locating enemy submarines, which is the basis of modern echolocation techniques.

Langevin was born and educated in Paris and also studied in the UK at the Cavendish Laboratory, Cambridge. He was professor at the Collège de France 1904–09 and from 1909 at the Sorbonne. In 1940, after the start of World War II and the German occupation of France, Langevin became director of the Ecole Municipale de Physique et de Chimie Industrielles, where he had been teaching since 1902, but he was soon arrested by the Nazis for his antifascist views. He escaped to Switzerland in 1944, returning after the liberation of Paris. Langevin suggested 1905 that the alignment of molecular moments in a paramagnetic substance would be random except in the presence of an externally applied magnetic field. He extended his description of magnetism in terms of electron theory to account for diamagnetism, and showed how a magnetic field would affect the motion of electrons in the molecules to produce a moment that is opposed to the field. He was an early supporter of Albert Einstein's theories, and the nuclear institute in Grenoble is named after him.