overvoltage and the voltage across the spark gap, Figure 5.9b shows the current flow
through the spark gap. As the voltage across the spark gap is slowly increased
(Figure 5.9a), the gap fires at voltage Vs, bringing down the voltage. That is, at Vs
the gap switches from the insulating state (resistance .10 GV) to the conducting
state (resistance ,0.1 V). The change of state can happen within a fraction of a microsecond.
The voltage Vs (90–300 V) is called the d.c. firing voltage of the gap. Later we
will see that the actual spark overvoltage and the response time of the spark gap depend
upon the rate of increase of the applied voltage across the gap.
During the drop in voltage from Vs, the incremental resistance dV/dI is negative; i.e.
this is a negative resistance region. The current through the gap increases (Figure 5.9b)
and the gap voltage increases slightly to Vgl, the glow voltage. This region is called the
glow region. The glow is produced by a thin layer of excited gas atoms covering part of
the cathode surface and later extending to the whole cathode surface. Maximum current
during the glowregion is between 0.1 and 1.5 Aand the glowvoltage is between 70