The primary object of our research is the control and determination of
dynamic behavior in response to electrical stimulation in close-packed
aggregates of small pellets submerged in electrolyte. Typically, the
aggregate contains (among other things) iron and the electrolyte
contains nitric acid, this combination making possible the propagation
of electrochemical surface waves of excitation through the body of
the aggregate similar to those of the Lillie iron-wire nerve model.
The iron pellets are imbedded in and supported by a matrix of small
dielectric (such as glass) pellets. Furthermore, with the addition
of soluble salts of various noble metals to the electrolyte, long
interstitial dendritic or fibrous structures of the second metal can
be formed whose length and distribution change by electrodeposition in
response to either internal or externally generated fields.
[Illustration: Figure 1—Test chamber and fluid exchanger]
Coupling between isolated excitable (iron) sites is greatly affected
by the fine structure and effective bulk resistivity of the glass and
fluid medium which supports and fills the space between such sites.
In general (see Section 3, following) it is necessary, to promote
strong coupling between small structures, to impede the “short-circuit”
return flow of current from an active or excited surface, through
the electrolyte and back through the dendritic structure attached
to the same excitable site. This calls for control (increase) of
the bulk resistivity, preferably by means specifically independent
of electrolyte composition, which relates to and affects surface
phenomena such as recovery (_i.e._, the “refractory” period). Figure 2
illustrates the way in which this is being done, _i.e._, by appropriate
choice of particle size distributions. The case illustrated shows
the approximate proper volume ratios for maximum resistivity in a
two-size-phase random mixture of spheres.