I wrote:
>generating a high voltage in the primary circuit (the points side of the
>circuit and a much higher one in the secondary -- the spark plug
>side.  The high voltage in the primary, call it 75 volts for ballpark,
>charges the capacitor which takes up some of the energy that was stored in
>the coil -- then discharges shoves it back into the coil again, expanding
>the magnetic field and generating another fast spark.  The energy bounces
>back and forth between coil and capacitor, each time losing some into
>another spark, until it's no longer enough to cause the plug to fire.  A
>while later the points be either leading or trailing edge of whatever
>timing device is used.

I should have also said that this oscillating condition is secondary -- the
major reason for the capacitor is that if it weren't there, the sudden
voltage spike as the points open and the coil field starts to collapse,
will generate enough spark between the opening points to vaporize some of
the tungsten point material and strike an arc.  Unlike a spark, an arc can
carry heavy current over long distances at fairly low voltages, by moving
metal from one end to the other.  This has the twin effect of drastically
reducing the energy available to create the spark, and also rapidly
wrecking the points as a mountain builds up on one side and a valley on the
other.  How much good the secondary oscillations contribute I'm not sure --
but without the capacitor to prevent the arc from forming (by absorbing the
current during the time the points are opening far enough so the spark
can't jump them)

david

--
David Beierl -- dbeierl@attglobal.net