| 摘要 |
A quasi-stationary electric field is produced by a field wire which is insulated from ground and extends along a portion of the perimeter to be protected substantially parallel to ground, and which is connected to the output of an oscillator producing an output signal whose wavelength is very long compared to the length of the field wire. The field is sensed by first and second field sensing wires which are insulated from ground and extend substantially parallel to the field wire, on opposite sides of same, which are equidistantly spaced from the field wire and from ground, and which are vertically displaced from the field wire. The changes in the electric field sensed by the sense wires are detected by respective first and second detectors which produce respective first and second electrical analog output signals corresponding to the changes. These first and second electrical analog output signals are combined and processed to produce an output signal whenever neither of the first and second electrical analog output signals is equal to zero and the difference between the first and second analog output signals is greater than a given value, and this combined and processed output signal is used to produce an alarm.
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