*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.