The intensity of the electrostatic field around a
conductor is non-uniform. It has its maximum
strength at the surface of the conductor and its
intensity diminishes inversely as the distance from
the center of the conductor. Hence, as the voltage
level in the conductor is raised, the critical field
strength is approached and the initial discharges
occur only at or very near to the conductor surface.
For the positive half cycle, the electron avalanches
move toward the conductor and continue to grow
until they hit the surface. For the negative half
cycle, the electron avalanches move away from the
conductor surfaces toward a weaker field and cease
to advance when the field becomes too weak to
accelerate the electrons to ionizing velocity.