<VV> Re: Spark gaps, how they work

[Frank DuVal]

After some private replies asking for more explanation I'll try somemore 
bad writing:

The first time I heard of the spark gap test I thought it was bogus. 
But, it really works on point style ignition. The theory should also 
work on electronic ignition, but the voltages are already higher, so 
probability of damage to ignition components or personal injury is much 
greater. An HEI system nornally fires through .060" gap and hurts when 
it finds you. With a .250" gap or greater, it will find you with 
shocking resulys. A good working  points ignition system can fire a 1/2" 
gap with a blue spark. The color and length of spark is a good indicator 
of voltage available. This would be a typical old school ignition test 
at cranking speed for no- start troubleshooting.  Jim  Burkhard had a 
good explanation in a later VV message. The coil is the generator of 
spark voltage. The windings turn ratio determines the operating voltage. 
But the voltage is generated by the collapsing magnetic field exciting 
electrons in the windings. So, the primary winding forms a magnetic 
field with the points closed. When the points open, the collapsing 
magnetic field sweeps by the windings, like a moving armature sweeps by 
field winding magnetic field in a generator, and this voltage at the 
tower terminal will rise until it finds a return path to ground, the 
other terminal of the high voltage coil winding. The saturation level of 
the coil limits the amount of energy available to raise the voltage.  
When the spark gap fires the energy is diisapated, and the voltage goes 
away. The size of the spark gap  is relative to the ionization voltage 
of air.  The larger the gap, the  higher the voltage. The more small  
gaps in series, the  larger the gap appears to the coil output, as the 
voltage has to ionize all the gaps at the same time to fire.

Frank DuVal