[1] There followed two years at the Dresden Polytechnikum where he worked as a research assistant with August Toepler[1] (whose design for a mercury vacuum pump he subsequently improved).
He then returned to the 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.
Sources indicate that organisational changes introduced after the PTR came under the control of Emil Warburg (shortly before the outbreak of the First World War) also contributed to Hagen's decision to retire when he did.
[1] 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 (1903),[2] which defines the relationship between optical reflection and electrical conductivity as an approximation in the range of the infrared spectrum.
[9] 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.
[1] Ernst Bessel Hagen's younger brother, Fritz Karl Bessel-Hagen [de] (1856-1945) 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 of Immanuel Kant.