Langevin was a student of Perrin in Paris and later worked there with Pierre Curie; in between he spent nearly a year with J J Thomson in Cambridge. His interests in physics were wide-ranging and he became the leading French physicist of his time.
Work on ionized gases led him to study the magnetic properties of gases; most are feebly diamagnetic (repelled by a magnetic field) but ozone (O3) is paramagnetic (weakly attracted into the field). Langevin showed in 1905 that magnetic behaviour could be understood in terms of the electrons present in atoms; electrons had recently (1895) been discovered by Thomson.
In the First World War, he worked on a method for detecting U-boats by echo-sounding, using the reflection of ultrasonic waves (ie sound waves of very high frequency, and not audible). Curie had studied the piezoelectric effect – the small change in the size of some crystals produced by an electric field. Langevin used radio circuitry to produce rapid changes in electric potential in a crystal, so that it vibrated and formed an ultrasonic generator. Reflection of the waves for submarine detection was developed too late for the First World War, but was used in the Second World War as ‘sonar’. It is used also to survey the seabed, to detect fish shoals, and in medical scanning.
Shortly before the Second World War, Langevin worked out how to slow down fast neutrons, a method essential for the later work by others on atomic reactors. After France fell to the Germans in 1940, he was outspoken in his anti-Fascist views and was soon under house arrest; his daughter was sent to Auschwitz and his son-in-law was executed. Langevin escaped to Switzerland and survived to return to his Paris job, as director of a research group.