*During a number of the experiments performed in the interior
of the pyramids, low-frequency (2 MHz to 8 MHz) pulses were observed. These
were first misinterpreted to be signals altered by dispersive attenuations
while propagating through 68 m of rock. (See Chapter III for a discussion
of dispersive attenuation.) The low-frequency oscillations were later demonstrated
to be generated by resonances of the wiring along the pyramid corridors.
Careful coupling of the transmitter to the walls of the chamber reduced
the low-frequency signals because it reduced radiation into the air, and
hence excitation of the wiring. This example illustrates the care required
in interpreting electromagnetic soundings near large metallic objects.