Electromagnetic Fields (EMF) exist naturally, all around us, every day. It doesn't take a ghost to create one, nor even to disturb what's generally there.
So why are ghost hunters so obsessed with monitoring such fluctuations in the force?
EMF Meters read the levels of such energy all around. Anything around 2-7 miliGauss could arguably be a ghost passing by, but only as it's so low it's hard to pin-point another cause.
Mostly paranormal investigators use EMF Meters to determine what natural sources might be around to suggest a haunting. High levels of EMF in a location have been known to cause hallucinations, feelings of dread, paranoia and other emotional side-effects often associated with seeing a ghost.
Did the hair stand up on the back of your neck? Then it might be a spirit; or it might be a mobile 'phone tower sitting on top of the nearest hill. A thunderstorm is a great natural source of electromagnetic energy too. Remember how strange the air feels during one. Any building with a build up of this energy would replicate the same sensation.
If you go crying ghost, then a Physicist turns up to discover that there's a generator sitting in the corner, you're going to look very silly. Get an EMF Meter to determine what isn't a haunting, so that you're on safer ground when you do actually find a ghost.