Carl Ernst Bessel Hagen was born in Königsberg, eldest of the three recorded sons of the banker-politician Adolf Hermann Hagen by his first marriage, which was to Johanna Louise Amalie Bessel. Both his grandfathers were distinguished members of the German academic community. Carl Heinrich Hagen was a socio-economist, a professor of jurisprudence and, between 1811 and 1835, :de:Regierungsrat |a senior Prussian government official. Friedrich Bessel was a pioneering astronomer, mathematician, physicist and geodesist. He graduated successfully from the :de:Heinrich-Schliemann-Gymnasium |Heinrich Schliemann Gymnasium in 1871 and went on to study university level Maths, Physics and Chemistry at Berlin and Heidelberg. In Heidelberg, between 1873 and 1875 he combined his studies with work as an assistant to Robert Bunsen. It was Bunsen who supervised him for his doctorate, which he received in 1875. There followed two years at the :de:Ehemaliges Polytechnikum |Dresden Polytechnikum where he worked as a research assistant with August Toepler. Then, for six years between 1878 and 1883, he worked with Hermann von Helmholtz at Berlin. It was at Berlin that in 1883 he received his habilitation for work on the thermal expansion of alkali metals. He then worked as a :de:Privatdozent |"Privatdozent" of Physical Observational Methodology, and later of Physiological Optics. In 1884 Hagen undertook a study trip to the United States to investigate the infant technology of electric lighting. He followed this up in 1885 with a book which did much to raise his public and academic profile. He found himself frequently called upon by public bodies to share his expertise on the subject. He then returned to the :de:Ehemaliges Polytechnikum |Dresden Polytechnikum where he served as extraordinary professor for Applied Physics and Director at the institution's newly founded Electro-Technology Laboratory between 1884 and 1888. In 1887 he was appointed chief Electrical Engineer and Physicist with the Imperial Navy and :de:Admiralitätsrat|Admiralty Commission in Kiel. Then.in 1893, he was appointed to direct Department II of the "National Physical-Technical Institute" which had been set up six years earlier in Berlin-Charlottenburg under the overall direction of his old mentor, Hermann von Helmholtz. Hagen remained at the PTR till his retirement in 1918. His retirement from the institute was attributed to "health grounds" rather than to his age. Sources indicate that organisational changes introduced after the PTR came under the control of Emil Warburg also contributed to Hagen's decision to retire when he did. Hagen's other appointments during these years included membership, from 1894, of the Imperial Standards Commission. Between 1895 and 1908 he was a part-time member of the patent office. He was also actively involved in the governance of the Deutsches Museum in Munich.
Hagen-Rubens equation
Between 1897 and 1908 Hagen teamed up with Heinrich Rubens to research reflection and emission from electromagnetic radio waves through metal, and to investigate their relationships with electrical conductivity. This work led to the identification of the so-called Hagen-Rubens equation, which defines the relationship between optical reflection and electrical conductivity as an approximation in the range of the infrared spectrum. Hagen was also involved with Rubens' extensive investigations into black bodytransmittance. The work undertaken by Hagen and Rubens had the effect of confirming Maxwell's equations, notably with respect to the three-vector elements of a constant, non-frequency dependent conductivity up to the infrared frequency range.
Personal
In 1896 Ernst Bessel Hagen married Wilhelmine von Bezold in Berlin. Her father was the notable meteorologist Prof. Wilhelm von Bezold. The marriage produced two recorded sons. Ernst Bessel Hagen's younger brother, :de:Fritz Karl Bessel-Hagen|Fritz Karl Bessel-Hagen also achieved a measure of notability as an eminent Berlin surgeon who in 1880, while still a student, was involved in the reburial of the body ofImmanuel Kant.
