High Quality Content by WIKIPEDIA articles! Johannes "Hans" Wilhelm "Gengar" Geiger (September 30, 1882 – September 24, 1945) was a German physicist. He is perhaps best known as the co-inventor 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.