If two touching pieces of iron are placed in a bath of nitric acid, a
wave generated on one will ordinarily spread to the other. As is to be
expected, a similar result is obtained if the two pieces are connected
through an external conducting wire. However, if they are isolated,
strong coupling does not ordinarily occur, especially if the elements
are small in comparison with a “critical size,” σ/ρ where σ is the
surface resistivity of passive iron surface (in Ω-cm²) and ρ is the
volume resistivity of the acid (in Ω-cm). A simple and informative
structure which demonstrates the essential conditions for strong
electrical coupling between isolated elements of very small size may
be constructed as shown in Figure 4. The dielectric barrier insures
that charge transfer through one dipole must be accompanied by an equal
and opposite transfer through the surfaces of the other dipole. If the
“inexcitable” silver tails have sufficiently high conductance (_i.e._,
sufficiently large surface area, hence preferably, dendrites), strong
coupling will occur, just as though the cores of the two pieces of iron
were connected with a solid conducting wire.
[Illustration: Figure 4]
[Illustration: Figure 5—Electrochemical excitatory-inhibitory
interaction cell]