The Negative Ion Detector (NID) Meter has until recently been an expensive piece of equipment for amateur ghost hunters, until the advent of the Spectral Electronics NID Meter.
So what does an NID Meter do?
Basically negative ion fields can be created naturally by electrical discharges, like lightning. Negative Ion Fields can effect the human body in a series if ways which include nausea, headaches, dizziness and disorientation but also create feelings of happiness or sadness, symptoms like these are often experienced by paranormal investigations and are often associated with the presence of Ghosts. The NID Meter is designed to detect NI Fields and pose a possible explanation for some experiences encountered by Paranormal Investigators.
Ghosts have often been associated with Electro Magnetic Fluctuations (EMF) but some paranormal groups are also working on the theory that ghosts create or use NI Fields to manifest/communicate. I think it will be hard to differentiate between a naturally occurring NI Field and an NI Field created by a ghost, only exhaustive base line tests prior to an investigation will allow an investigator to reduce the odds in detecting natural or ghost Ni Fields.
Good piece of equipment and rapidly becoming an essential tool of any ghost hunter!