Fig. 4.6. Arrangement for application of magnetic field.
Since tissue is magnetically transparent, an applied magnetic field completely penetrates the
subject's tissues, where it induces an electric field that is proportional to its rate of change. The induced
electric field does not depend on the tissues' electrical constants, but it does depend on the shape of the
test subject and on its position within the subject (68). For these reasons we again find that the concept
of dose is not well defined, and that the applied field, therefore, is the appropriate dependent parameter
in magnetic field studies.
Electromagnetic radiation
. The electric and magnetic fields depicted in figures 4.4 and 4.6 are,
in a sense, bound to their respective sources: the electric field is associated with the voltage on the
metal plates, and the magnetic field arises from the current in the coil. Electromagnetic radiation is a
propagating physical entity consisting of inseparable electric and magnetic fields. It has its origins in
electronic transitions in a source-typically an antenna-with which it has no physical link once it is
liberated. The plane wave, in which the electric and magnetic fields are orthogonal to one another and
to the direction of propagation of the wave, is the simplest and most important type of electromagnetic
radiation. The power density, P, of a plane wave is measured in terms of power per unit of area
traversed. We shall express it in microwatts per square centimeter (µW/cm2). The relation between P
and the strength of the electric field, E, (in v/m) is:
Fig. 4.7. The large-coil exposure system at the Naval Aerospace Medical Research Laboratory.
(Reproduced, by permission, from D. E. Beischer, et al., Exposure of man to magnetic fields
ELECTROMAGNETISM & LIFE - 65