Normally, it would take an extremely high voltage to cause the
xenon gas inside a flash tube to ionize. Once the tube is conducting, though, it will maintain discharge until the storage capacitor is discharged. To get the tube started, a third trigger electrode is used. On most
xenon tubes, this is a wire wrapped around the glass, or a conductive trace silvered onto it. Cheap disposable cameras just have a piece of copper foil stuck next to the tube. This electrode is wired to a small trigger coil. The trigger coil is a small high-voltage
autotransformer that will step up a pulse of voltage off of the
strobe or flashgun's storage capacitor to about 1,500 volts. A large storage capacitor connected in parallel with the tube's electrodes is charged to 200-500 volts by a small converter. The trigger pulse is carried across the glass by
capacitive coupling, and ionizes the gas enough to start the discharge. The tube will continue to conduct until the
capacitor is mostly discharged.
The trigger coil is fired by a set of contacts attached to the shutter (on a camera flash), or by a small mercury vapor filled bulb which flashes over and becomes conductive once suitable voltage has built up on the storage capacitor to fire the tube.
The schematic for a camera flash trigger circuit is shown below. C1 is the main storage capacitor, and C2 is a small capacitor used to prevent the trigger coil primary from discharging C1. S1 is the shutter switch, which would be replaced by the mercury vapor bulb in a strobe. T1 is the trigger coil.
+
-----------+-------------+----------+
from power | | T1 | |
supply --- C1 $|| | |flash
--- +--$|| | |tube
| | $|| | |
| --- $|| || |
| C2 --- $|| || |
| | $--------|| |
| / S1 trigger | |
-----------+----------+-------------+
-
Note that I said it
mostly discharges the capacitor - if you plan to mess with the innards of a flash gun or strobe light, discharge it first with a test lead or screwdriver. Otherwise, you may get a
nasty little surprise.