Paul Langevin (1872-1946), French physicist. Langevin (right) studied under Perrin and Pierre Curie, and later worked with J.J.Thompson at Cambridge. His first research was in the magnetic properties of gases. In 1905, Langevin described magnetism in terms of electrons (which Thompson had discovered in 1895). During the First World War he worked on a method for detecting submarines by means of ultrasonic waves. Although his work was not finished by the end of the war, his invention was used extensively in the Second World War, and was known as sonar. Persecuted for his outspoken criticism of fascism, Langevin escaped to Switzerland, returning to Paris after the war.