PLANCK, MAX ), German physicist, was born at Kiel on April 23, 1858. He studied at the universities of Munich and Berlin, became an assistant in Munich University, and was subsequently appointed a professor in Kiel (1885) and in Berlin (1889). Planck devoted himself to the study of theoretical physics, and in particular to that of thermodynamics. His lec tures on this subject appeared in book form and were translated into English, French and Russian. He gained international fame by his Law of Radiation (1901), which asserts that the energy of radiation is emitted and absorbed in integral multiples of certain indivisible "quanta" of energy, which depend on the frequency of oscillation of the electrons. In 1912 Planck extended this "quantum theory" (q.v.) to all kinds of energy and added the assumption that only emission proceeds discontinuously, in quanta, while absorption is continuous. On this assumption he was able
to derive the distribution of energy in the spectrum of black body radiation. The quantum theory has been developed and modified by him and many other physicists but the foundation was laid by Planck's work on black-body radiation. In 1918 he was awarded the Nobel Prize for physics. Planck became editor of the Annalen der Physik. He was elected a foreign member of the Royal Society in 1926.