Ernst Hagen (31.01.1851 -15.01.1923) - German physicist. Born in Königsberg. Graduated from the University of Heidelberg (1875). In 1873-75 he worked at the University of Heidelberg, in 1876 — 77 and 1884 —86 at the Dresden Polytechnic, 1878 - 83 at the University of Berlin, 1887 - 92 in Kiel, and in 1893 —1918— he was director of the Physico-Technical Institute (Berlin — Charlottenburg).
His research was mainly on electromagnetism and optics. He studied the absorption of infrared rays by metals. Together with H. Rubens he carried out classic experiments measuring the reflectivity of metals, which confirmed Maxwell's electromagnetic theory of light. He improved Toepler's mercury pump.