Fig. 4.5. Our small-animal exposure assembly. The stalls consist of metal plates sandwiched between
layers of wood. By appropriately energizing or grounding the plates, adjacent electric-field and
control regions can be produced.
In theory, the dielectric constant and the conductivity of the subject determine the strength and
distribution of the electric field that penetrates into its tissue. However, because of the structural
complexity of biological tissue, both constants are point functions that vary with location over cellular
(and smaller) dimensions, and reliable methods for their functional determination under physiologically
realistic conditions do not exist.
If the structural organization of tissue is not considered, it is possible to measure a sort of
average tissue constant-the dielectric constant of a 1-cm. thick plate of brain tissue, for example. Such
data can be used to analyze what might be called the physical reactions of biological tissue- reactions
such as heat production and induced current that do not depend on whether the tissue is alive or dead.
But even the average-value tissue constants are difficult to measure, and, as a result, the reported values
for specific tissues vary over several orders of magnitude (62-64).
The absence of reliable tissue-constant data prevents the calculation of unique, meaningful
internal electric fields. We considered, for example, several of the common physical models for
biological systems, including the sphere, sphere-in-a-sphere, ellipsoid, and rectangular solid (65-68).
Using typical average-value tissue constants, we found that the calculated value of the internal fields
varied by a factor of 2-100 depending upon the assumed values of the tissue constants and the
particular model. When the analysis was broadened to include the transient response, or when more
complicated geometric models were considered, the range of arbitrariness bracketed by the calculations
was even larger. Efforts to take into consideration the point-to-point variations of the constants would
result in even further uncertainty.
Magnetic field
. Figure 4.6 depicts a typical laboratory arrangement for the application of a
magnetic field to a test subject. The current through the coils gives rise to a magnetic field that is
reasonably uniform near the common axis of the coils; the strength of the field, measured in gauss, can
be calculated from the knowledge of the coil current and geometry, and it can be measured by means of
a small calibrated induction coil. A coil-exposure system suitable for use with human subjects is shown
in figure 4.7 (69).
ELECTROMAGNETISM & LIFE - 64