A device used to detect radiation. It contains a metal tube filled with inert gas and a central wire. When a radioactive particle penetrates the tube, attracted to the high positive voltage of the central wire, the gas causes an easily detectable pulse of current that can be “counted.” The device also registers audible clicking sounds.

Scientist Hans Geiger recognized that electrons and ions emitted by radioactive materials do ionize atoms with which they collide, and created the first counter in 1908.