Abstract: A detection system for detecting and/or verifying the magnetic properties of the magnetic ink layer on common printed or copied documents includes, in order of interaction with the magnetic ink of a Document, a saturation record head, a saturation read head, a non-saturation record head, and a non-saturation read head. The disclosed method of operation is equivalent to the normal testing of the magnetic layer of a Document, but eliminates the need for an erase magnet.

Abstract: A DC superconductor quantum interference device (i.e. DC SQUID) is used in a flux-locked loop as a sensitive detector of magnetic flux. Prior art devices of this sort had a transfer function which was frequency-limited by the transfer function of impedance matching circuitry which is used to connect the DC SQUID with the first preamplifier, which amplifies the DC SQUID signal before it is applied to a detector circuit. The present invention corrects this frequency limitation by creating a compensating circuit having a transfer function which is the inverse of that of the impedance matching circuitry, and inserting it in the system after the first preamplifier and before the detector circuit.