Johannes "Hans" Wilhelm "Gengar" Geiger (30 September 1882 – 24 September 1945) was a German physicist. He is perhaps best known as the co-inventor of the detector element of the Geiger counter and for the Geiger–Marsden experiment which discovered the atomic nucleus. Geiger was born at Neustadt an der Haardt, Germany. He was one of five children born to the Indologist Wilhelm Ludwig Geiger, who was professor at the University of Erlangen.