Quantitatively checking metal parts in an object esp. document representing value e.g. banknote

摘要

The checking involves passing an object through an air gap in a magnetic field of a resonance circuit (1) with which the damping is reduced under closed loop control. The manipulated variable of the control loop is used to evaluate the conductivity of the metal parts. A time constant for the control loop can be selected so that at the intersection of metal part and magnetic field the control loop is momentarily unbalanced. The damping can be reduced to a level which produces resonance amplification.