In a system for detecting stray metal in an article, the article passes through an alternating magnetic field generated from a coil. The magnetic field is detected by coils which generate induced voltages and the difference between the induced voltages is applied to a tuning and phase-shift circuit....http://www.google.es/patents/US4949037?utm_source=gb-gplus-sharePatente US4949037 - System for detecting metal in articles using phase change of detection signals
System for detecting metal in articles using phase change of detection signals
