EMF

EMF Meter Electromagnetic field meters (also called EMF detectors). Meters that measure the derivative (rate of change) of the surrounding magnetic field; thus, these meters are sensitive to changing magnetic fields (fields with a frequency above 0 hertz). Most EMF meters are most sensitive to frequencies of about 30-10,000 hertz, while some have a wider response. Paranormal investigators and ghost hunters use EMF meters while conducting investigations. Unusual activity in the form of high readings on these meters signifies that there is some type of paranormal activity, such as the presence of a ghost or other supernatural creature. – Pad of Definitions (1.04 Phantom Traveler), Official Website

EMF (Electromagnetic Field) A physical field that is produced by electrically charged objects and which affects the behaviour of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction, one of the four fundamental forces of nature. The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. – Pad of Definitions (2.18 Hollywood Babylon), Official Website

from 1.04 Phantom Traveler:

Sam: What is that?
Dean: It's an EMF meter. It reads electromagnetic frequencies.
Sam: Yeah, I know what an EMF meter is, but why does that one look like a busted up old walkman?
Dean : 'Cause that's what I made it out of.

Not supernatural readings from the EMF Meter

Any device that uses electricity emits some EMF. Most of them are shield and are too low to cause problems. But power lines and transformers are sources of Extremely Low Frequency (ELF) that can jam the readings of the EMF meter from other, say more supernatural, sources.

from 1.17 Hell House
Sam: You got somethin’?
Dean: Yeah. The EMF’s no good.
Sam: Why?
Dean: I think that thing’s still got a little juice in it. It’s screwin’ with all the readings.
Sam: Yeah, that’d do it.

from 4.11 Family Remains
Sam: Needle's all over the place.
Dean: Yeah -- Power lines.

Episodes featuring the EMF meter

• 1.04 Phantom Traveler
• 1.11 Scarecrow
• 1.14 Nightmare
• 1.17 Hell House
• 2.02 Everybody Loves a Clown
• 2.06 No Exit
• 2.07 The Usual Suspects
• 2.13 Houses of the Holy
• 2.18 Hollywood Babylon
• 3.05 Bedtime Stories
• 3.14 Long-Distance Call
• 4.02 Are You There, God? It's Me, Dean Winchester
• 4.06 Yellow Fever
• 4.08 Wishful Thinking
• 4.11 Family Remains
• 4.13 After School Special
• 5.02 Good God Y'All - no EMF m,eter, but when Castiel reveals that [{dean's Amulet]] will glow hot in the presence of God, Sam calls it a 'God EMF'
• 5.19 Hammer Of The Gods
• 6.02 Two And A Half Men
• 6.14 Mannequin 3: The Reckoning