The coil formers for the bottles are made to be a sliding fit. One way to do this is to roll each bottle in a couple of thicknesses of paper, cover that with a layer of thin plastic sheet, such as is used for food wrapping, and then wind on two or three layers of glass cloth as shown in Fig. 15.

The power supply stabiliser circuit is shown in Fig. 12. Here the diode D5 will prevent damage if incorrect connection to the power source is made. The supply to the main amplifier and multivibrator is taken from the Zener diode D4, and that for the meter circuit is additionally decoupled by R33 and C21.

Three separate chassis connections, G1-G3, are made between the main amplifier and stabiliser to prevent the possibility of oscillation.


The supply lead from the 12V battery must be screened. The centre wire is positive and terminates at SK5; the negative screen at SK6. Constructional details are given in Fig. l3. Since the magnetometer draws about 750mA during “polarise”, an adequate heavy duty power source must be used.

The circuit for the meter amplifier is given in Fig. 10. Here VR2 acts as a sensitivity control in feeding the complementary pair TR10 and TR11. The meter circuit can be used in conjunction with, or to replace the headset.

The meter needle follows the amplitude of the amplifier output which varies at a rate equal to the difference between the two input frequencies. It is particularly useful with very low difference frequencies.

Constructional details of this module are given in Fig. 11.

Since the amplifier provides high gain, the wiring layout and constructional details of Fig. 9 should be adhered to, to prevent instability. Both of the transformers are contained in Vinkor adjustable pot cores. To wind T1 use 40 s.w.g. enamel covered wire. Slip a couple of inches of fine sleeving over the start to protect the leadout, then wind on seven hundred and fifty turns. Put on another piece of sleeving over the finish leadout and wrap a layer of cellotape round the winding.

Put on two more windings of three hundred and seventy-five turns each and identify the starts and finishes with different coloured sleeving. Wind a layer or two of plastic electrical tape around the completed winding then very carefully assemble the bobbin in the ferrite core. Ensure that nothing gets trapped between the two halves of the core, preventing them mating.

The main amplifier, seen in Fig. 8, serves to increase the level of the precession voltages. A ferrite cored transformer, T1, with the primary centre tapped, is tuned to the required frequency by C4 and C5. The first stage comprising TR5 has a tuned collector load resonant at the same precession frequency.

The output from the secondary of T2 feeds the d.c. coupled amplifier TR6-TR7. This acts, in effect, as a pre-amplifier to the meter circuit, the input for this being taken from M1.

The circuit diagram of the relay driver and multivibrator is given in Fig. 6. Here TR1 and TR2, in modified super alpha configuration, drive the relay. The relay contacts are shown in the quiescent state. A 1W resistor, Rl, can be inserted in the “polarise” circuit to reduce the detector coil current and so cut down on battery consumption. It follows that the higher the value of this resistance the smaller will be the signal presented to the main amplifier, so the choice of value should be made when the unit is completed and tested; 4.7 ohms is a suitable value to start with. “Cut and try” methods should provide balance between a tolerable signal and battery economy.

Now look at the block schematic of the magnetometer in Fig. 5. There are in effect six units comprising: two detector bottles, relay circuit, multivibrator, main amplifier, meter amplifier, and power supply. These form what might be called a de-luxe unit. The relay and meter circuits may be omitted if costs have to be kept down.