Original
date 23 October 2000 --
1st revision 30 September 2002
2nd revision 1 June 2006 ---------
SPARK GAPS
The Spark Gap behaves as a voltage sensitive switch during Tesla Coil operation: open when the Primary Circuit capacitor is charging towards the peak voltage of the mains ac power input , and short when the voltage reaches a voltage near the peak (~10kV) on the capacitor. At this point the energy in the capacitor discharges into the Primary Circuit inductor via the spark gap. This energy oscillates back and forth between the primary circuit capacitor and inductor (rings) at their resonant frequency during the short circuit behaviour of the spark gap. The mechanism for the short circuit in the gap is the ionized gases, which behave as a very good conductor. It allows the Primary Circuit oscillations to occur and to be transferred to the Secondary Circuit. The following picture is one which I built and seems to work well in a low powered Tesla Coil:
It
is desireable for this RF energy to remain in the Secondary
Circuit where the high voltage sparking takes place during each
spark gap firing. Unfortunately, these gases remains ionized for
a short time after the RF energy is transferred to the Secondary
Circuit. The energy will actually reflect back and forth between
the primary and secondary circuits as long as the gap gases are
ionized. This results in a portion of the energy being converted
to heat rather than the sought after sparking. 
The
energy losses are most predominant in single static spark gaps.
Most common of this type are either a pair of small spheres or a
pair of cylinders (copper tube sections)like those at the right
in figure 1. However, these losses can be reduced by a process
called spark gap quenching. Three popular quenching methods which
improve Tesla Coil performance considerably are:
a. High velocity air flow through the gap
b. Rotary spark gaps
c. Multiple spark gaps in series
The
air flow method blows the ionized gases out of the spark gap when
the energy is in the Secondary circuit. Ideally this would occur
right after the first energy transfer. Some Tesla Coilers use
modified vacuum cleaners to do this. 
Rotary spark gaps were used by Radio Amateurs in their transmitters during the early 1900's. To see an example of one click here.
As
you can see, the rotary spark gap consists of a synchronous
motor, stationary and rotating electrodes, and a phase shifting
device as shown in figure 2. at the right. The old Radio Amateur
one was not synchronous so is not suitable for Tesla Coil
application. The syncronous motor is locked to the power line
frequency, usually 1800 RPM for 60Hz lines and 1500 RPM for 50Hz.
The rotating electrodes are mounted and connected together on an
insulated disc attached to the motor shaft, and two stationary
electrodes are the spark gap terminals connected to the Primary
Circuit. The phase shifting device is adjusted to get the
rotating electrodes to pass between the stationary electrodes at
the instant the the AC voltage on the Primary Circuit capacitor
is at its peak. Fine phase adjustments around this point are then
made for the best Tesla Coil sparks.
The
multiple spark gap is basically several narrow spark gaps in
series rather than one wide gap. These devices, called self
quenching spark gaps, were used in the 1900 to 1920 era as spark
gaps for radio transmitters. The gaps were made air tight to
preserve an oxygen free nitrogen dominant gas 
Another method, which Nikola Tesla used, was magnetic quenching. This same method is used in industrial high current relays to quench the arcing across contacts during operation is DC circuits. The magnetic field, generated by a coil in series with the high current circuit, is set at right angles to the direction of the arc across the relay contacts. The coil was referred to as a "blow out" coil if my memory serves me right. I haven't run across magnetic quenching amongst Tesla Coilers yet)
This page is still under construction. More details on spark gaps will be included in its next up-date.
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