First of all, there is no evidence that a spirit in any way creates or alters an EM Field. That is pure Hollywood. So any indication you get with a meter is indicative of an EM Field, but not a ghost.
These meters are used by investigators to rule out mundane causes that could result if false positives. For instance if you detect a strong field that is precisely where NOT to place your recorder if you are doing EVPs.
Some use EMF meters, I personally don't. Instead I use two EMF monitors. First I scan using a dynamic monitor. This allows me to hear the EM Field on headphones rather than simply see a meter reading. EM Fields follow the invesrse square law of physics. That simply means that the farther you are from the source the weaker the field becomes (The exact ratio can be calculated but for illustration the exact values aren't neccessary.) Point is since we don't ordinarily know exactly where the source originates we can't know its absolute strength. It may be a weak field nearby, or a strong field originating some distance away. The meter only tells you the field it encounters, not its absolute strength.
By hearing the field you can learn some things about its characteristics. 60 Hz hum means a power line nearby, (50 in some countries), a buzz usually indicates some form of ionizing radiation like a florescent light, a whistle might be a switch mode power supply in a computer. And of course, with an RF sniffer you can hear radio waves if that is the source.
Once the scan is complete I switch to a static monitor. That simply keeps track on the baseline and alerts me if something changes. Maybe a solar flare, or radio transmitter kicks on. Anything that may cause a change in the backgrpound of the area under investigation.
So you see there is quite a bit to EMF monitoring, but it has nothing to do with detecting ghosts.
Edited by CaveRat, 20 October 2009 - 05:05 PM.
2
) and I just wanted some input on what type to buy (I believe the correct one to buy is an EMF Tri-Field) and where a good place would be to get one. Also, as I noticed prices differed on the internet, what's the price range I should be looking at? Or, would a local store in my area carry one? If so, what type of store? Electicians-R-Us, maybe? 
