Dallas Ft. Worth - Washington D.C. - Seattle - New Orleans
The prime sensory instrument of any ghost investigation is an EMF meter. I feel there is more confusion and BS about the use of these things than any other piece of equipment except perhaps a camera. The main reason why is because it can be very hard to tell what you are reading and whether it is actually anomalous or not.
After some experimentation, I feel that the Multidetector Profi (Pro Edition) is about the best you can use for the money. Yes, it is just over a $100, but well worth it if you are the lead tech in a group. I have close-ups of both the Profi and Standard Multidetectors above for reference. Both have adjustable settings for between E (Electric) and H (Magnetic) plus a range switch from X1 to X1000. For quick reference 1 mG = 100 nT .
The difference between the two is that the Profi has a filter function (notice the VLF and ELF settings) plus an audible alarm and X10 more sensitivity. The VLF allegedly will screen out 60 Hz noise so prevalent in many buildings (and even outdoors). My studies have found that it DOES make a difference, but not a total one. I often will still get readings from 60 Hz power even with it set to VLF (500 Hz - 400 KHz ). My guess is either the filtering is not perfect, and/or power lines have harmonics which extend beyond the 60 Hz range.
For reference on the Profi, each LED light at the X100 setting represents 1 milligauss. On the standard, each LED light at the X10 setting represents 1 milligauss. Many in the ghost hunting business feel that a reading between 2-8 mG could indicate the presence of something supernatural.
My personal opinion is that any UNEXPLAINABLE reading could be an alert to paranormal activity. What you are looking for is:
1) A reading as above (could be 2, might be 20 mG) that cannot be explained by a local power source. For instance, if I was walking on an allegedly haunted battlefield far from any power lines and my EMF meter started reading 1, or even 15 mG, I would be very inclined to think something was up. But check the Solar activity for the day before you jump to conclusions. Be sure and practice with a meter in your own home so you become familiar with what will cause an indication.
2) A reading which changes or fluctuates for no accountable reason. Standing in a room, the reading is 2 mG, then it spikes to 9, and back down to 2. Could be something there...
3) Normal background readings which DROP for no explainable cause can be the indication that something is sucking up energy to manifest.
4) To always try correlating the reading with some other phenomena. A cold spot, orbs on film, or even a funny feeling can lend weight to a strange EMF indication.
5) To experiment with different settings and meters. The guide table shown above is not some law written in stone; use whatever configuration gives you best results. Try different brands and models of EMF meters to see what works for you.
This is one of my favorite toys, an EMF meter mounted on a holder. I have four of these that I deploy in an array as a form of energy trap. Works very well in front of a camera. The calibration is for 1-10 mG. Kudos to Chris Moseley of Dagulf's Ghost again for the holder idea. (the ones I got are small acrylic picture frames from Office Depot for less than $2 each) Slap on a 9v battery with electrical tape or velcro and it's ready to rock. The only problem is that the original supplier of these beauties (Goldmine Electronics) has run out, and you will only find them further out in the supply chain. They were sold on Ghost Safari until they went offline, but you can check on EBay, though they seem to be gone there too .
Incidentally, all of the aforementioned EMF detectors are what as known as SINGLE-AXIS detectors, which simply means that they only measure fields in one dimension. So the orientation of the sensing probe/antenna will affect the reading. The TriField Meters are famous for being TRIPLE-AXIS meters wherein they measure in all three dimensions at once. There are supposedly DOUBLE-AXIS meters, but they are quite rare.
TRIFIELD NATURAL EM METER
This is also the EMF meter of choice by many ghost hunters. Reviews are located in the SPI store.
Here is a company in Dallas which specializes in the use of EMF meters as well as digital video graphics: