The Geiger Muller tube is used to detect and measure radioactivity. It consists of a thin wire down the centre of a metal tube enclosed in a glass container. The apparatus contains gas at low pressure and the whole apparatus is connected to a power supply that produces a strong electric field between the wire and the metal tube.
When an atom undergoes radioactivity decay and produces a particle which passes through the tube, it produces charged ions along it's path. These ions are accelerated i9n the stong magnetic field and produce a current pulse when the hit the anode/cathode. The pulse is detected and advances a countmeter by one OR passed to a ratemeter which counts the number of pulses per second. A loudspeaker can be made to click each time a current pulse is detected.