Field Induction Responses

When a field variation occurs outside an electrical conductor, such as the geomagnetic storm field above the conducting Earth, a secondary field is induced to flow in the conductor. Scientists call the source variation the external field and call the induced field the internal field. The longer the oscillation period of the source field, the deeper that this field penetrates into the conducting material. The science of physics tells us that, by nature, with every changing magnetic field there exists a companion changing electric field. We call the pair electromagnetic (e.m. or em) fields. At the surface of the conducting Earth, magnetic field sensors detect the sum of the external (source) and the internal (induced) magnetic fields. A pair of electric field probes, such as lead electrodes buried in the ground at a set distance apart, show a voltage difference that can measure the electric counterpart of the magnetic changes (see Section 5.1.3, p. 127).

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