an instrument for detecting and measuring the intensity of ionizing radiation. It consists of a gas-filled tube containing a fine wire anode along the axis of a cylindrical cathode with a potential difference of several hundred volts. Any particle or photon which ionizes any number of gas molecules in the tube causes a discharge which is registered by electronic equipment. The magnitude of the discharge does not depend upon the nature or the energy of the ionizing particle

an instrument for detecting and counting ionizing particles that pass through it: it consists of a needlelike electrode inside a hollow metallic cylinder filled with gas which, when ionized by the radiation, sets up a current in an electric field: a refined version (Geiger-Müller counter) with an amplifying system is used for detecting and measuring radioactivity