The initial electron, which lost most of its velocity
in the collision, and the electron knocked out of
the air molecule, which also has a low velocity, are
both accelerated by the electric field, and at the
next collision, each electron is capable of ionizing
an air molecule. After the second collision, there
are now four electrons to proceed, and so on, the
number of electrons doubling after each collision.
All this time, the electrons are all advancing toward
the positive electrode and after many collisions,
their number has grown enormously. This is
the process by which the so-called electron avalanche
is built up, each avalanche being initiated
by a single free electron which finds itself in an
intense electrostatic field.