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ION–SOLVENT INTERACTIONS 181

1. The time between application of an electric field sufficiently high to cause the
breakdown phenomena and their occurrence is about
2. Bright, white light is emitted.
3. The most singular and characteristic phenomenon is that called streamers. The
electrodes concerned emit a piercing series of filaments (Fig. 2.71) that spread
out across the solution from the electrode. The filaments turn out to be
low-density tubules through the liquid and fade away rapidly after breakdown.
These low-density regions in the dielectric remind one ofthe branchlike growth
oftrees (Fig. 2.71). The heads of the streamers travel at high velocity, reaching
in some systems 1 % of the velocity of light.
4. Although older books give tables of the breakdown voltage for a series of
different liquids, it is not helpful to describe dielectric breakdown phenomena
in terms of the voltage applied between two metal plates. The phenomenon
depends on the strength of the electric field (volts per unit distance), not upon
volts. It is also not enough to state a field strength as the volts between two
electrodes immersed in the dielectric divided by the distance between the
plates. This is because the phenomenon is known to be critically associated
with the interfacial regions of the cathode and anode concerned. In such
regions, however, there are huge discontinuities in field strength. A total of
30,000 V applied over 10 cm may cause dielectric breakdown, but this may
not be directly related to the apparent field across the whole liquid
between the plates. It may depend upon the field very near the
surface or even the over a few nanometers, which can be calculated
to occur at the tips of the spikelike micropromontories that exist on many real
surfaces.
5. An increase in pressure applied to the liquid suppresses the formation of
streamers, which are the most telling sign of breakdown.
6. The critical applied volts (and the associated electric field at the interfaces
concerned) that cause breakdown depend upon the conductance of the solution
(Fig. 2.72).

2.21.2. Mechanistic Thoughts
Dielectric breakdown is a phenomenon of practical importance; it is by no means
only an academic puzzle. One can appreciate this through the analogy between
mechanical strength and dielectric strength. Any mechanical structure collapses if too
great a stress is placed upon it. Correspondingly, any substance can withstand an
electric field only to a certain degree of field strength, after which it ceases to remain
an insulator and opens the flood gates for electrons to come across; as a result, the
resistance to the applied potential undergoes a catastrophic decline. For example, a
condenser may be used with a water dielectric to store great amounts of electrical
energy to activate electromagnetic weapons. It then becomes important to know, and