The present invention features a magnetic field compensation system for suppressing alternating or time-varying magnetic fields in a large volume of space. A magnetic sensor detects within a predetermined range of frequencies and generates a signal. A coil or pair of Helmholtz drive coils is positioned...http://www.google.com.au/patents/US5465012?utm_source=gb-gplus-sharePatent US5465012 - Active feedback system for suppression of alternating magnetic fields
