http://www.techlib.com/electronics/induction.html
The induction receiver shown below is very sensitive and can serve a variety of
purposes. It is excellent for tracing wiring behind walls, receiving audio from an
induction transmitter, hearing lightning and other electric discharges, and monitoring a
telephone or other device that produces an audio magnetic field ("telephone pickup
coil").
(linked from http://soundandmusic.org/resources/artists-toolkit/how-tos/fieldrecording )
