In 1911 Geiger and John Mitchell Nuttall discovered the Geiger–Nuttall law (or rule) and performed experiments that led to Rutherford's atomic model.

[6] In 1912, Geiger was named head of radiation research at the Physikalisch-Technische Reichsanstalt (Imperial Physical Technical Institute – PTR) in Charlottenburg.

In 1936 he took a position with Technische Universität Berlin where he continued to research cosmic rays, nuclear fission, and artificial radiation until his death in 1945.

[2] Beginning in 1939, after the discovery of atomic fission, Geiger was a member of the Uranium Club, the German investigation of nuclear weapons during World War II.

[7] Although Geiger signed a petition against the Nazi government's interference with universities, he provided no support to colleague Hans Bethe (winner of the 1967 Nobel Prize in Physics) when he was fired for being Jewish.