A device used to detect and measure ionizing radiation. It consists of a tube containing a low-pressure gas and a cylindrical hollow cathode through the center of which runs a fine-wire anode.

It was first devised in 1908 by the German physicist Hans Geiger (1882-1945).

A potential difference of about 1000 volts is maintained between the electrodes. An ionizing particle or photon passing through a window into the tube will cause an ion to be produced and the high p.d. will accelerate towards its appropriate electrode, causing an avalanche of further ionizations by collision.

The consequent current pulses can be counted in electronic circuits or simply amplified to work a small loudspeaker in the instrument.