April – George Ellery Hale and collaborators publish their discovery that the magnetic polarity of sunspot pairs reverses on an 11-year solar cycle and that the polarity varies by hemisphere, which becomes known as Hale's law.[1][2]
James Jeans discovers that the dynamical constants of motion determine the distribution function for a system of particles.
Betz's law is published by German physicist Albert Betz, indicating the maximum power that can be extracted from the wind, independent of the design of a wind turbine in open flow.
^Hale, George E.; Ellerman, Ferdinand; Nicholson, S. B.; Joy, A. H. (April 1919). "The Magnetic Polarity of Sun-Spots". The Astrophysical Journal. 49: 153. Bibcode:1919ApJ....49..153H. doi:10.1086/142452.
^Charbonneau, P.; White, O. R. (1995-04-18). "Hale's Sunspot Polarity Law". www2.hao.ucar.edu. High Altitude Observatory. Archived from the original on 2021-08-19. Retrieved 2021-08-20.
^Dyson, F. W.; Eddington, A. S.; Davidson, C. R. (1920). "A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Solar eclipse of May 29, 1919". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 220 (571–581): 291–333. Bibcode:1920RSPTA.220..291D. doi:10.1098/rsta.1920.0009. Paper received October 30, read November 6, published April 27, 1920.
^Verhandlungen der Deutschen Physikalischen Gesellschaft.; Mehra, Jagdish; Rechenberg, Helmut (1982). The Historical Development of Quantum Theory. Vol. 1, Part 1: The Quantum Theory of Planck, Einstein, Bohr and Sommerfeld 1900–1925: its Foundation and the Rise of Its Difficulties. Springer. p. 330. ISBN 978-0-387-95174-4.
^hirschfeld.in-berlin.de, The first Institute for Sexual Science.
^Famous GLBT & GLBTI People – Dr. Magnus Hirschfeld stonewallsociety.
^Grossmann, Atina. Reforming Sex. Oxford University Press, 1995.