The EMF meter is an important piece of hunter equipment that measures EMFs (Electromagnetic fields), commonly given off by spirits. This helps hunters find if the room is haunted, or sometimes where the spirit's body is. Although, if there are nearby power lines, the EMF meter is rendered useless and its readings unreliable. When EMF gets high, it makes noise.
Physical Appearance
The EMF meter first appeared in Phantom Traveler when Dean made his own out of an old tape player. That creation was never seen again. They later switched to the one they use now, which is small and more unnoticeable.
EMF meters don't always look the same. The Ghostfacers used a more squarism one, and a bit larger than Sam and Dean's. Fake ones have even been made in The Real Ghostbusters, when fans pretended to be Sam and Dean.