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Spirit box

A modified radio, also called a ghost box, that ghost hunters use to try to hold live conversations with spirits.

What users say it does
HuffPost reported that Frank Sumption's ghost box was claimed to let users speak with spirits in real time, with spirit voices said to come through its constant scan of AM radio. Wikipedia says Sumption claimed the spirit world gave him the design.
What it measures
Skeptical Inquirer describes a radio with its scan-lock turned off, so it keeps sweeping frequencies at a set rate and puts out erratic white noise broken by brief scraps of speech or music from stations it passes.

Frank Sumption, an EVP enthusiast, built the first ghost box in 2002, according to HuffPost and Wikipedia. Wikipedia describes it as a white noise generator joined to an AM receiver that was altered to sweep the band and pick out split-second bits of sound.

Brian Dunning, on Skeptoid, says the box produces what you'd get by recording a radio while twisting the tuning knob: random noise from every station passed.

Sources