BASIC MAGNETOMETER

This phenomena can now be used to make a ferrous metal detector. Two such bottles filled with distilled water are spaced about six feet apart. The longitudinal axes of the bottles lie east-west. The coils wound round the bottles are connected in series and a current passed through them. After three seconds, the current is cut off and an amplifier connected across the coils. If the
intensity of the earth’s field is the same at each bottle, the precession frequency at each will be the same, and the signals from both coils of equal frequency.

If, on the other hand, the field intensities were different due to some local magnetic disturbance, then the signal frequencies will differ, and the difference can be detected. The amplifier input signal is then the sum of two signals of slightly different frequencies and the
output will be a signal of a third frequency which is half the sum of the two input frequencies. The amplitude of this is modulated at a fourth frequency equal to the difference between the
original two input frequencies. This composite output signal can be heard in a headset as a note of about 2kHz with a marked quaver. This is illustrated in Fig. 4. The greater the difference in the field intensities between the two bottles, the faster will be the quaver. It only remains to
reach for the shovel and see what is causing the magnetic difference.

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