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Summary Article: Langevin, Paul (1872-1946) from The Hutchinson Dictionary of Scientific Biography

Place: France

Subject: biography, physics

French physicist who invented the method of generating ultrasonic waves that is the basis of modern echolocation techniques. He was also the first to explain paramagnetism and diamagnetism, by which substances are either attracted to or repulsed by a magnetic field. Langevin was the leading mathematical physicist of his time in France, and contributed greatly to the dissemination and development of relativity and modern physics in general in his country.

Langevin was born in Paris on 23 January 1872. He attended the Ecole Lavoisier and the Ecole de Physique et de Chimie Industrielles, where he was supervised by Pierre Curie during his laboratory classes. In 1891 he entered the Sorbonne, but his studies were interrupted for a year in 1893 by his military service. In 1894 Langevin entered the Ecole Normale Supérieure, where he studied under Jean Perrin. Langevin won an academic competition which in 1897 enabled him to go to the Cavendish Laboratories at the University of Cambridge for a year. There he studied under J J Thomson, and met scientists such as Ernest Rutherford, Charles Wilson, and John Townsend.

Langevin received his PhD in 1902 for work done partly at Cambridge and partly under Curie on gaseous ionization. He spent a great deal of time in Perrin's laboratory, and was caught up in the excitement accompanying the early years of study on radioactivity and ionizing radiation. Langevin joined the faculty of the Collège de France in 1902, and was made professor of physics there in 1904, a post he held until 1909 when he was offered a similar position at the Sorbonne.

During World War I Langevin contributed to war research, improving a technique for the accurate detection and location of submarines, which continued to be developed for other purposes after the war. He became a member of the Solvay International Physics Institute in 1921, and was elected president of that organization in 1928. 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 outspoken anti-fascist views. He was first imprisoned in Fresnes, and later placed under house arrest in Troyes. The execution of his son-in-law and the deportation of his daughter to Auschwitz (which she survived) forced Langevin to escape to Switzerland in 1944. He returned to Paris later that year and was restored to the directorship of his old school, but died soon after in Paris on 19 December 1946.

Langevin's early work at the Cavendish Laboratories and at the Sorbonne concerned the analysis of secondary emission of X-rays from metals exposed to radiation, and Langevin discovered the emission of secondary electrons from irradiated metals independently of Georges Sagnac (1869-1928). He also studied the behaviour of ionized gases, being interested in the mobility of positive and negative ions, and in 1903 he published a theory for their recombination at different pressures. He then turned to paramagnetic (weak attractive) and diamagnetic (weak repulsive) phenomena in gases. Curie had demonstrated experimentally in 1895 that the susceptibility of a paramagnetic substance to an external magnetic field varies inversely with temperature. Langevin produced a model based on statistical mechanics to explain this in 1905. He suggested that the alignment of molecular moments in a paramagnetic substance would be random in the absence of an externally applied magnetic field, but would be non-random in its presence. The greater the temperature, however, the greater the thermal motion of the molecules and thus the greater the disturbance to their alignment by the magnetic field.

Langevin further postulated that the magnetic properties of a substance are determined by the valence electrons, a suggestion which influenced Niels Bohr in the construction of his classic model describing the structure of the atom. He was able to extend his description of magnetism in terms of electron theory to account for diamagnetism. He showed how a magnetic field would affect the motion of electrons in the molecules to produce a moment that is opposed to the field. This enabled predictions to be made concerning the temperature-independence of this phenomenon and furthermore to allow estimates to be made of the size of electron orbits.

Langevin became increasingly involved with the study of Albert Einstein's work on Brownian motion and on space and time. He was a firm supporter of the theory of the equivalence of energy and mass. Einstein later wrote that Langevin had all the tools for the development of the special theory of relativity at his disposal before Einstein proposed it himself and that if he had not proposed the theory, Langevin would have done so.

During World War I, Langevin took up the suggestion that had been made by several scientists that the reflection of ultrasonic waves from objects could be used to locate them. He used high-frequency radio circuitry to oscillate piezoelectric crystals and thus obtain ultrasonic waves at high intensity, and within a few years had a practical system for the echo location of submarines. This method has become the basis of modern sonar and is used for scientific as well as military purposes.

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