EMF - Electromagnetic Field

In day-to-day use, electromagnetic field (EMF) meters are used to diagnose problems with electrical wiring and power lines and to get readings on working appliances. Ghost hunters use the meters to look for spikes in the EMF signal, with the spikes suggesting a change in electrical current, and thus a spirit being. 

EMF fields are present in the world around us, and if you subscribe to the assumption that ghosts contain some electrical residue, EMF meters would be a good way to detect their presence.