The separation between first and second points is determined. At the first point are first, second and third mutually orthogonal coils that are excited in sequence, so that first, second and third magnetic fields are sequentially derived. At the second point are first, second and third mutually orthogonal magnetic field sensors that respond to the first, second and third magnetic fields, respectively to derive responses indicative of the magnetic fields coupled to them. The derived responses are combined in accordance with: ##EQU1## to derive the separation magnitude, where: V11, V12 and V13 are responses of the first, second and third sensors while the first coil is excited; V21, V22 and V23 are responses of the first, second and third sensors while the second coil is excited; V31, V32 and V33 are responses of the first, second and third sensors while the third coil is excited. A variable gain element responds to the responses derived from the sensors for selectively modifying the amplitude of signals transduced by the sensors. An analog to digital converter responds to the selectively modified signals as derived by the variable gain element for deriving a multi-bit digital output signal having a predetermined optimum range. In response to the magnitude of the multi-bit digital output signal the gain of the variable gain element is controlled to maintain the multi-bit digital output signal in the range.

An apparatus for measuring a weak magnetic field in a background D.C. magnetic field such as the earth's magnetic field or the like includes a magnetic pickup portion that is supported by a support mechanism which does not impede minute movement in the x- and y-axis direction. A pair of A.C. linear gradient magnetic fields whose directions of gradients are parallel to the x-axis and y-axis, respectively, are used for obtaining reference signals for sensing the position of the pickup portion. The frequencies of the A.C. linear gradient magnetic fields are set to be sufficiently higher than the frequency of the weak magnetic field that is to be measured. The A.C. linear gradient magnetic fields are provided by A.C. current flowing in two pairs of linear conductors which are perpendicular to each other.

The present invention features a magnetic field compensation system for suppressing alternating or time-varying magnetic fields in a large volume of space. A magnetic sensor detects within a predetermined range of frequencies and generates a signal. A coil or pair of Helmholtz drive coils is positioned into a volume containing the magnetic sensor. A preamplifier is connected to the magnetic sensor for amplifying the signal. A signal processor is connected to the preamplifier for receiving the signal and for providing phase correction and gain. The signal processor has both an averaging and an absolute peak detector for determining magnetic field average and peak excursions. A power amplifier is connected to the coil or pair of Helmholtz drive coils and to the signal processor for receiving and amplifying the processed signal and for applying it to the coil or pair of Helmholtz drive coils.

The present invention provides a power frequency, i.e., time-varying, magnetic field (PF-NF) detector system for characterizing the operational condition of a remote facility responsive to time-varying magnetic fields generated by electrical transmission lines associated with the remote facility. The PF-MF detector system advantageously includes a PF-MF sensor which generates N time-varying magnetic field data sets, and a PF-MF analyzer which generates an operational condition assessment responsive to the N time-varying magnetic field data sets, wherein N is an integer greater than or equal to 1. Additionally, the PF-MF detector system includes an accumulator which stores and forwards the N time-varying magnetic field data sets to the PF-MF analyzer via a communications channel. A method for characterizing the operational condition of inaccessible electrical equipment responsive to the time-varying magnetic field generated by a transmission line are also described.

A device for unlocking a lid provided in a vehicle includes an unlocking actuator unit, a magnetic sensor unit, a signal processing unit, and a controller means. A transmitter unit with a power source generates and transmits a specific magnetic unlocking signal which is received by the magnetic sensor unit, hence, the unlocking actuator is controlled through the signal processing means and the controller means and, hence, the lid is unlocked.