(b. St. Gallen, Switzerland, 4 March 1862; d. Zurich, Switzerland, 8 October 1940)
Emden received his doctorate at Strasbourg in 1887 as a student of the physicist August Kundt. In 1907 he became an assistant professor (extraordinarius) of physics and meteorology at the Technische Hochschule in Munich. He held that position until 1928, when he became an assistant professor for astrophysics at the University of Munich. His work Gaskugeln, Anwendungen der mechanischen Wärmetheorie auf kosmologische und meteorologische Probleme (Leipzig, 1907) was epoch-making; all subsequent textbooks on astrophysics have been based on it. He was therefore entrusted with writing the article on the thermodynamics of celestial bodies in the Enzyklopädie der mathematischen Wissenschaften.
Emden introduced the concept of “polytropic change of state,” although he did not need to display the radiation pressure explicity in his calculations. Nevertheless, this pressure appeared mathematically in the exponents of the polytropic lines. Emden was also the first to give a derivation for radiative equilibrium for nondiscernible particles, that is, photon statistics. He thereby became a precursor in the use of the Bose-Einstein statistics. In further works he dealt with astronomical refraction, the thermodynamics of the atmosphere, and propagation of sound in the atmosphere. In addition, he contributed significantly to the development of the theory of balloon flight.
In 1920 he became a member of the Bavarian Academy of Sciences. From 1916 on, formal difficulties kept him from obtaining German citizenship. Consequently, he at least avoided financial losses upon his dismissal when the Nazis came into power in 1933. He died in Zurich, in his native Switzerland, on 8 October 1940.
Emden’s works include, in addition to those cited in the text, “Theoretische Grundlagen der Ballonführung,” in Illustrierte aeronautische Mitteilungen (1901); Grundlagen der Ballonführung (Leipzig, 1910); “Abnorme Horbarkeit,” in Sitzungsberichte der Bayerischen Akademie der Wissenschaften zu München (1916), 113–123; “Zur Thermo-dynamik der Atmosphäre,” in Meteorologische Zeitschrift, 33 (1916), 351–360; 35 (1918), 13–29, 74–81, 114–123; 40 (1923), 171–177; “Lichtquanten,” in Physikalische Zeitschrift, 22 (1921), 513–517; “Astronomische Refraktion,” in Astronomische Nachrichten, 219 (1923), 45–56; “Strahlungsgleichgewicht,” in Zeitschrift für Physik, 23 (1924). 176–213; and “Freiballon,” in Handbuch der Experimentalphysik, IV, pt. 3 (1930). 115–131.
J. O. Fleckenstein