This circuit is sensitive to low frequency electromagnetic radiation and will detect for example hidden wiring or the field that encompasses a transformer. Pickup is by a radial type inductor, used as a probe which responds well to low frequency changing magnetic and electric fields. Ordinary headphones are used to for detection.
http://www.zen22142.zen.co.uk/Circuits/Misc/emf.htm
Here is an interesting circuit for a magnetic proximity switch which can be used in various applications. The magnetic proximity switch circuit, in principle, consists of a reed switch at its heart.
http://www.electronic-circuits-diagrams.com/sensorsimages/sensorsckt1.shtml
locates stray electromagnetic fields, response from 50Hz to 100kHz, will detect both audio and RF signals
http://www.mitedu.freeserve.co.uk/Circuits/Testgear/emmeter.htm
determining direction using the earth's weak magnetic field
http://www.edn.com/archives/1996/031496/06df2.htm
Complete design details for an antigravity relay to levitate small iron parts.
http://www.coilgun.info/levitation/home.htm
25-1000 MHz with the NIST dipole-detector
http://www.scott-inc.com/html/nist.htm