Petr Leonidovich Kapitza (born 9.07.1894) is a Soviet physicist and an academician (1939; corresponding member 1929). He was born in Kronstadt. He graduated from the Petrograd Polytechnic Institute (1918) and stayed on to work in A. F. Ioffe's department. In 1921 he was sent on a scientific mission to England, where he worked at the Cavendish Laboratory. In 1924—32 he was deputy director of the Cavendish Laboratory, and in 1930 —34 director of the Mond Laboratory of the Royal Society and professor (in 1929 he was elected a Fellow of the Royal Society). After his return to the USSR he organised the Institute of Physical Problems in Moscow, of which he was director in 1935—46 and has been director since 1955. In 1939—46 he was professor at Moscow University, and from 1947 at the Moscow Institute of Physics and Technology.
His research concerned nuclear physics, the physics and engineering of superstrong magnetic fields, the physics and engineering of low temperatures, high-power electronics and the physics of high-temperature plasma. In 1920, together with N. N. Semenov, he proposed a method of determining the magnetic moment of an atom, which was realised in 1922 in the experiments of O. Stern and W. Gerlach. In 1923 he was the first to place a Wilson cloud chamber in a strong magnetic field and to observe the curvature of the tracks of alpha particles.
In 1924 he proposed a new method of producing pulsed superstrong magnetic fields (with a field strength of up to 500,000 oersted). Having obtained record values of the magnetic field, he studied its effect on various physical properties of matter. In 1928 he established the law that the electrical resistance of a number of metals grows linearly with the magnetic field strength (Kapitza's law). He created new methods of liquefying hydrogen and helium and designed new types of liquefier (piston, expansion-engine and turbo-expander plant). In 1934 he built a helium liquefier of the expansion-engine type with an output of 2 l/h, and in 1939 a low-pressure plant for the industrial production of oxygen from air. Kapitza's turbo-expander forced a revision of the principles on which the refrigeration cycles used for liquefying and separating gases are built, and this substantially changed the development of oxygen production technology throughout the world.
Having developed the technique of producing liquid helium, he studied its properties. In a series of experiments he showed that below the critical temperature (2.19 K) the viscosity of liquid helium becomes extremely small (the discovery of the superfluidity of helium II), and he made a thorough study of the properties of liquid helium in this new state, showing in particular that it consists of two components, a superfluid one and a normal one. This research stimulated the development of the quantum theory of liquid helium worked out by L. D. Landau. In 1941 he observed a temperature jump at the “solid body — liquid helium” interface (the Kapitza temperature jump). In 1978, for fundamental research in the field of low-temperature physics, he was awarded the Nobel Prize.
In the post-war period Kapitza's attention turned to high-power electronics. He developed the general theory of magnetron-type electron devices and built continuous-wave magnetron generators, the planotron and the nigotron. He put forward a hypothesis about the nature of ball lightning. He detected experimentally (1959) the formation of a high-temperature plasma in a high-frequency discharge. His research also concerned the history of physics and the organisation of science.
Twice Hero of Socialist Labour (1945, 1974) and twice winner of the USSR State Prize (1941, 1943). A member of many foreign academies of sciences and scientific societies. The M. V. Lomonosov Gold Medal (1959), the M. Faraday (1942), B. Franklin (1944), N. Bohr (1964) and E. Rutherford (1966) medals, the F. Simon Prize (1973) and others. Editor-in-chief of the “Journal of Experimental and Theoretical Physics” (from 1955).