Air ion detectors are a tool used by some ghost hunters which may help aid the ghost hunter as an early warning signal in detecting possible paranormal activity which is about to occur.
The Air Ion Counter is a hand-held meter designed to measure ion density which is 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).
There are some theories behind this tool when it comes to hunting ghosts.
Theory 1 - It is believed that spirits need to draw energy to manifest themselves therefore a spirit may draw positive or negative ions out of the atmosphere to manifest themselves which would be detected by the air ion detector.
Theory 2 - In theory it is believed that if a spirit is present it would affect the natural and/or man-made ions in the air. If this is true, then the air ion detector would be able to indicate and detect this.