The spark counter is a device used to detect radioactivity and assess the level of radioactivity. It consists of a fine wire stretched below a sheet of metal qauze. I voltage is applied between the wire and the gauze. The voltage is almost but not quite large enough to cause sparking between the wire and the gauze.
If a radioactive source – radium above is brought close to the gauze, it will cause ionization in the air whenever an atom decays. The ions will reduce the resistance of the air to current and make it a better conductor. When the ions hit the gauze, a pulse of current will be created. This is detected, and indicates that an atom has undergone radioactive decay. Each current pulse can be used to advance a counter by one, or to make a speaker emit a bleep.