The K II Meter measures electromagnetic fields in milliGauss and is often seen on Ghost Hunters. The one you see Jason and Grant use is the first one that came out and why they had to use a coin to engage the on/off button to hold it down.
The new KII's do not need that adjustment as the on and off button will just stay down once pushed. You can find these at various ghost hunting equipment stores online. You can also find these with small infrared lights on the side and other variations.
The K II seemed to be touted as one of the better electromagnetic field detectors in the previous years and better than the Ghost Meter. Many believed the Ghost Meter in comparison would pick up cell phone energy but I did not experience that at all. In fact, I almost felt that the K II can be too exclusive of some frequencies that it may not pick up on things as well as the Ghost Meter. I seem to get better interaction especially with children spirits with the Ghost Meter as the bar flashes so brightly compared to the K II. I do like using the K II as a backup however.
One thing to note when you go on any of the ghost hunting events, is that you should put your name on ALL of your equipment! This is very important. I had left my first K II at the Widow's Cottage I believe while in Gettysburg and it was not to be found thereafter, so we believe someone picked it up and kept it, unfortunately. So I had to order a second one. I did order it from the link above and it came very quickly!! Thanks Chris!
Overall, I do like the K II as I use it in conjunction with the Ghost Meter, pointing it in another direction to help pinpoint the location of energies or spirits that may try to communicate with me. It's fairly simple to use, easy on/off button and you hold it steady in front of you and move it slowly throughout a room or location to get a base reading first. That will allow you to locate any energy pockets of high EMFs where perhaps powerlines may be, etc etc. Then, it is best to set it down on something sturdy and level. I like to use it in doorways or thresholds much like the Ghost Meter but anywhere like a table or the floor where it won't be stepped on, or some place that is steady. It's a great steadfast and basic tool to have.