SURVEILLANCE SYSTEM HAVING MAGNETO-MECHANICAL MARKER

摘要

The marker is for a surveillance system defining an interrogation zone. A field generator provides a magnetic filed of variable frequency within the zone. A marker is secured to an article to be taken across the zone. The marker has an elongated ductile strip of magnetostictive ferromagnetic material. The marker resonates mechanically at a frequency within the range of the incident magnetic field. A hard ferromagnetic element is disposed adjacent the strip and upon being magnetised biases the strip and arms if to resonate at that frequency.