The Air Ion Counter is a handheld meter designed to measure ion density–the number of ions per cubic centimeter (ions/cc) in air. It measures this number separately for positive and negative ions (+ and – ions are usually present simultaneously). This instrument is a true ion density meter, based on a Gerdien Tube (Gerdien Condenser) design, and it contains a fan which pulls air through the meter at a calibrated rate. The standard model measures up to 2 million ions/cc (for comparison, outdoor air typically has 100 to 1000 ions/cc, both + and -). The 2 million range is sufficient for almost all situations except for measuring very close to electric ionizers or strong radioactive sources. For these applications, models with ranges of 20 million and 200 million ions/cc are also available. Such an ion counter will be used for the continuous monitoring of the atmospheric ion density in the ground.