Theory #1: When a ghost manifests it may give off an Electro Magnetic Field (EMF). EMF meters can detect electro magnetic fields.
Theory #2: When a ghost manifests, it needs energy to due so. Some people believe that a ghost can draw the energy from a high electro magnetic field (EMF), which can then cause it to manifest.
NOTE: SAFETY WARNING
There have been studies done that state a reading of 0mg or 2mg are considered safe levels of continuous exposure to EMF. Living in areas of high exposure to EMF (3mg or higher) can lead to some of the problems as follows: childhood leukemia, brain cancer, miscarriages, fetal abnormalities, nausea, migraines, fatigue, feelings of being watched or followed, hallucinations, depression, etc. If you are hypersensitive to Electro Magnetic Fields (EMF) then a continuous field of 1mg can affect you.
Some of the meters that we use to detect EMF are listed below: