This basic
oscillator will detect the Earth magnetic field. The ferrite rod and coil are taken from
an old Medium Wave receiver and a small magnet is glued at one end. Tune to a medium wave
commercial station until you hear a beat note. Any movement of the ferrite rod will
produce an audible note that depends on the prevailing Earth magnetic field. Screening is
essential. Use a plastic box padded, on the inside, with copper wires running parallel to
the rod and grounded in one place only. A small hole is made in the box in order to adjust
the trimmer capacitor with a plastic screwdriver. An American equivalent to the BC337
could be the 2N2369A but I did not try it out.
