Geiger-Müller (GM) counter /gī″gər mil″ər/ [Hans Geiger, German physicist, 1882–1945; Walther Müller, 20th-century German physicist; Fr, conter, to tell] , an electronic device that indicates the level of radioactivity of a substance by counting the number of ionizing subatomic particles emitted by the substance. As the particles pass through a gas-filled tube inside the counter, they ionize the gas and cause an electric discharge. The tube cannot identify the type or energy of a particle. Also called Geiger counter.
