From the Pad of Definitions
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.