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.

The relay is used to switch the series wound coils L1, L2 from the “polarise” condition to “detect”. In the “polarise” condition the relay passes current through the coils via terminal B1 at the stabiliser. On “detect” the relay switches the precession frequencies produced at the coils to the main amplifier input terminals Al, A2. Detection of the modulation envelope is then provided for, both aurally and visually, by the headphones X1 and meter M1.

The relay can be made to operate manually by the pushswitch S1 or automatically when coupled via S2 to the multivibrator; this switch does in fact select the mode.

The multivibrator switches the relay on for about three seconds and off for the same period. This continuous operation is particularly useful when searching. The manual option is used when setting up or detecting a very small requency difference.