Geiger counter, a device used for detecting radioactivity by its ionising effect as it passes through a gas at low pressure (contained in the cylindrical tube). The charged particles so produced are counted mechanically and a reading is given on the dial or displayed audibly by a series of characteristic clicks. Here, the counter is registering the radiation emitted by a watch. Radium salts are used for the paint on luminous watch dials despite their being radioactive. Some of the counts registered are due to natural background radiation from cosmic rays and the decay of radio isotopes in the soil.