Geiger counter, a device used for detecting radioactivity. Radioactive substances such as uranium and plutonium emit charged subatomic particles, and these particles ionise gas held at low pressure in the Geiger counter's cylindrical tube. The charged particles are counted mechanically by the Geiger counter, which then gives a reading either on a dial or audibly by a series of characteristic clicks. The counter is named after the German physicist Hans Geiger (1882-1945), who developed it in the 1920s.