Abstract: A system for actuating an alarm device secreted in a packet of bills when the packet is carried outwardly through a transmitted field at an exit from protected premises, in which such actuation is prevented when the packet is carried into the field near the exit and then back again into the protected premises. This is accomplished by constituting the field of two different field regions, an inner field region and an outer field region, having different identifying characteristics which are distinguishable by electronic circuitry in the packet, these field regions being disposed so that a person removing the packet from the protected premises must pass along an egress path extending in sequence through the inner field region and the outer field region.

Abstract: A method and apparatus for measuring the individual strengths of two superimposed vector fields at different frequencies by sensing the three orthogonal directional components of the field, sampling these three components in rapid repetitive sequence, squaring the amplitudes of the resulting samples, filtering the squared samples to select the second harmonics of those field frequency components to be measured, amplitude-detecting the filtered signals, and detecting the square root of the amplitudes of the detected signals. Such method and system eliminates the need to provide highly accurately matched components for each separate frequency channel, as is required in previously-known systems.