Both of these "authorities" are incorrect. Modern WTMD's use multiple coils of wire which form the feedback of an oscillator circuit, which operates at a frequency somewhere between audio and very-low-frequency radio. 30-150Khz is typical. The reason for multiple coils is to create "zones" so they can pinpoint (sic) the area of disturbance. When metal comes within the field of a coil, it changes the frequency of the oscillator. Iron and ferrous metals cause the frequency to decrease, brass and non-ferrous metals increase the frequency. When the logic detects a change of frequency a certain amount (sensitivity) from the "set" frequency, it makes an alarm. This is the principle behind the White's metal detector, for finding coins on the beach. The frequencies involved and the power level have been universally agreed to be absolutely safe to human tissue, although it is remotely possible to upset electronic devices, like first-generation pacemakers. I hope this clears this up.