An apparatus used to study the production of ions in the atmosphere by cosmic ray and radioactive bombardment of air molecules. The chamber is an airtight container usually cylindrical in shape and 25–50 L in volume. An insulated electrode is centrally located in the chamber. In operation a potential is applied between the electrode and the chamber wall. The ions produced in the chamber are collected by the electrode and measured by an electrometer. Amplification of ions may occur as the result of gas multiplication.