Paul Langevin (23.01.1872-19.12.1946) was a French physicist and a member of the Paris Academy of Sciences (1934). He was born in Paris, into the family of a Communard. He graduated from the School of Physics and Chemistry (1891) and the École Normale (1897), after which he worked for a year at the Cavendish Laboratory under J. J. Thomson. In 1902 he received the degree of doctor at the University of Paris. In 1900 he had obtained a post there as a laboratory assistant, and in 1902 he began work at the Collège de France (professor from 1909). At the same time, from 1903, he became professor at the School of Physics and Chemistry, succeeding P. Curie in that post, and from 1925 until the last years of his life he was its director.
His research concerned the ionisation of gases, quantum theory, relativity, ultrasonics and magnetism. Already in his doctoral dissertation “Researches on Ionised Gases” he set out a number of important and original results, in particular the discovery of the existence of heavy ions whose mass exceeds that of ordinary ones by a factor of 1000. His research on magnetism, however, stands foremost in his creative legacy. In 1905 he worked out the thermodynamic and statistical theories of dia- and paramagnetism. He proved the universality of diamagnetism and its connection with the Zeeman effect, and gave a theoretical justification for the independence of diamagnetism of temperature. Langevin's statistical theory of paramagnetism gave a clear molecular picture of the phenomenon and made it possible to calculate the values of the paramagnetic moment and of the magnetic moment of the atoms of the molecules of a substance. Langevin used the statistical method he had developed to construct the theory of the Kerr effect.
He developed (1916) methods of producing ultrashort elastic waves by means of piezoelectric quartz. By making quartz oscillate under the action of an alternating electric field he obtained ultrasonic waves. He was the first to apply his method of producing ultrasonic waves in underwater signalling, in the ultrasonic echo sounder and for the detection of submarines. In 1925 he built a powerful radiator of high-frequency acoustic oscillations, exciting quartz for the first time by a high-voltage alternating current, and designed an underwater ultrasonic quartz transmitter. He is a pioneer of ultrasonics.
His research also concerned electrodynamics and electron and quantum theory. He took an active part in the development of the special theory of relativity. Independently of A. Einstein he established in 1906 the relation between mass and energy, and in 1913 he was the first to arrive at the concept of the mass defect. In 1911, starting from A. Sommerfeld's idea of the quantisation of mechanical action, he showed that it leads to a quantum of magnetic moment — the magneton — and calculated its value.
He was a brilliant populariser and commentator on new ideas in physics, and his lectures had a considerable influence on French physicists. On philosophical questions he held the positions of materialism.
A well-known public figure and an active fighter for peace and justice, he took an active part in the protest over the Dreyfus affair, in the organisation in 1932, together with A. Barbusse and R. Rolland, of the Amsterdam anti-fascist committee and in the creation of the Popular Front in France in 1935, and for more than 20 years he worked in the League of Human Rights (in his last years as its president). He was a sincere friend of the Soviet Union, one of the organisers of the France–USSR society and its chairman (1946).
A member of the Academy of Sciences of the USSR (1929), a Fellow of the Royal Society and others. He created a school of physicists (L. de Broglie, M. de Broglie, F. Joliot-Curie, E. Bauer, P. Biquard, R. Lucas, F. Perrin, J. Solomon and others) [267, 557].