This is Townsend's breakdown criterion. It does not give
any information about the temporal growth of the processes
leading to breakdown. It is, however, inherent in the
Townsend theory that many generations of electron avalanches
are required to build up a breakdown.
The second Townsend coefficient -y is very sensitive to
electrode conditions and gas impurities, and it will only
be well defined under carefully controlled laboratory conditions.
Equation (2) is, therefore, of little use to the highvoltage
design engineer. Schumann [10] has suggested that
the Townsend criterion for breakdown in air at atmospheric
pressure may be written as