Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Patent
1977-12-22
1978-11-21
Tokar, M.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
324 05R, G01R 3308
Patent
active
041268237
ABSTRACT:
A magnetic field detector for determining the presence of a magnetic field disturbance in the vicinity of the apparatus senses the difference in magnetic fields at two spaced-apart locations. A pair of field sensors, each including a body of polarizable liquid and a probing coil, are connected in series and provide an output signal that is produced in the coils by the free precession of the protons of the liquid bodies in the earth's magnetic field. A timer circuit controls a relay to alternately connect the magnetometer coils to a source of polarizing current during a polarizing period, and to detection circuitry during a listening period. The detection circuitry includes a high gain signal channel comprising a multi-stage, high input impedance preamplifier and a bandpass amplifier tuned to the frequency range of the signals generated in the probing coils. The output signal from the high gain signal channel is applied to a rectifying circuit that rectifies the amplified free precession signals at a selected bias level to produce a signal that reduces to zero as the free precession signals in the coils decay. Filtering and amplifying of the rectified signal is provided, with the signal being finally applied to a meter operable to follow a low frequency beat signal and provide for the observation and timing of the length of the interval between the beginning of the listening period and the occurrence of the first subsequent minimum value of the beat signal. The time interval is marked by deflections of the meter movement and characterizes the size and distance of a detected object.
REFERENCES:
patent: 3081428 (1963-03-01), Fowler
patent: 3257608 (1966-06-01), Bell et al.
patent: 3441838 (1969-04-01), Salvi
patent: 3495160 (1970-02-01), Raffini et al.
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