Recently a theory of electrical breakdown in solids has been developed (Fröhlich 1937). This theory is based on the idea that electrical breakdown is a phenomenon due to the acceleration of electrons, as has been suggested by von Hippel (1935) and others. The critical field strength at which the breakdown occurs has been calculated in the following way: In strong external electrical fields, there are always some electrons in the conduction levels of an ionic crystal. These electrons, which are not in thermal equilibrium with the lattice, may be brought into these levels by cold emission or by some similar “pulling out” mechanism. Such an electron will make collisions with the lattice vibrations and thus lose per second a certain energy
B
(
E
), which depends upon its kinetic energy
E
. On the other hand, it will gain per second an energy
A
(
E, F
) from the external field
F
. Now it has been shown in I that
B
decreases but that
A
increases with increasing energy
E
. Thus there exists always an energy
E'
for which
A
=
B
. An electron with energy
E
less than
E'
will, on the average over several collisions, lose energy, whereas an electron with
E
greater than
E'
will gain more and more energy.
Energy distribution and stability of electrons in electric fields