Capacitance-based verification device for a security thread embedded within currency paper

摘要

A verification device for a security thread embedded within currency paper includes a pair of capacitive plates having applied thereto time-varying electronic signals that are 180 DEG out of phase. Disposed between the plates is a sensor element comprising a cylindrical wire dimensionally equal to the security thread. A guard electrode surrounds the sensor; the guard, sensor, and plates comprising a capacitance bridge. When the security thread in the currency is adjacent to the sensor and in proximity thereto, a capacitance imbalance occurs in the bridge. Electronic circuitry is responsive to this imbalance and provides an indication of a valid security thread embedded within the proffered currency paper.