From 1884 to 1918, he was a faculty member of the University of Strasbourg and was nominated as an assistant professor on 27 September 1884.
After the end of World War I and the occupation of Alsace-Lorraine by France, Cohn and his family were expelled from Strasbourg on the Christmas Eve of 1918.
He resigned from the Deutsche Physikalische Gesellschaft (DPG) together with other physicists like Richard Gans, Leo Graetz, George Jaffé, Walter Kaufmann, in protest at the despotism of the Nazi regime.
Because of his Jewish descent he found himself forced to emigrate to Switzerland because of the pressure under the Nazi regime.
[1] At the beginning of the 20th century, Cohn was one of the most respectable experts in the area of theoretical electrodynamics.
Cohn's electrodynamics of moving bodies was based on the assumption that light travels within the Earth's atmosphere with a constant velocity - however, his theory suffered from internal failures.
Another weak point stems from the fact, that his concept was formulated without the use of atoms and electrons.
As a heuristic concept this can be described as a material "aether", but in Cohn's opinion this would be only "metaphorical" and would not affect the consequences of his theory.
For instance, local time was described by him as a consequence of the assumption that light propagates in spherical waves with constant velocity in all directions (a similar definition was already given by Poincaré in 1900).
In optics, however, we define these identical moments of time by assuming, that the propagation takes place in spherical waves for every relatively resting and isotropic medium.
[8]He also illustrated the effects of length contraction and time dilation by using moving rods and clocks.
are those time intervals indicated by an "initially correctly ticking" clock, after it was inserted into the system and accordingly has changed its rate.