After Antoine Becquerel's discovery of radioactivity in 1896, there was much speculation as to the nature of the phenomenon. For one thing, it was unclear whether this was an effect produced only by uranium (and, as later discovered by Pierre and Marie Curie, radium), or whether it could be found elsewhere in nature. The two men most responsible for revealing the truth about radioactivity--that is, that it can be found at varying levels nearly everywhere in the universe--were the German physicists Julius Elster and Hans Geitel.

Elster and Geitel met in the physics department of the University of Heidelberg; later they would teach physics together at a local secondary school. While they are most famous for the experiments they performed together, each conducted important research on his own as well. Elster built the first practical photoelectric cell (confirming many of the theories of Heinrich Hertz and James Clerk Maxwell...