by Malcolm Holser
The way the ignition works is this: You connect a coil to some electricity, and it builds up a magnetic field. You stop feeding the electricity, and the field collapses, inducing flow. The voltage, in this open circuit, builds rapidly toward infinity and eventually it must go somewhere, and arcs across whatever is handy.
Relays do this, and generate a back-spike when they are disengaged, arcing across the contacts of the controlling switch. Often, a "diode clamp" will be placed across the relay coil to prevent this. (always in electronics that cannot tolerate voltage spikes).
The ignition coil is really two coils intertwined, with the leads to one going to the spark plug, and the leads to the other going to the points. Your points close, and a magnetic field builds up. They must stay closed long enough (they DWELL closed for a bit!) to energize the field. They open, and the field collapses. The inductance of the collaspsing field does not care which of the two intermingled coils created it -- it dumps into both, but because of the way they are wound, it mostly dumps into the spark-plug side. Enough gets into the points-side, though, that there will be a spark across them as well. The condensor is there to absorb this. Without a condensor, your ignition will work fine, but your points will very rapidly pit because of the arcing, and may weld themselves together. Alway replace the condensor with the points -- it is more common that a poor condensor has caused the points to fail than the points wearing on their own.
Electronic ignitions use big transistors to turn on and off the one side of the ignition coil and internally damp this feed-back voltage with big diodes, a far better solution, but not one available in the 1800's when Benz invented the spark-plug. Many newer ignitions augment the energizing current to the coil with a Capacitance-Discharge (a capacitor is charged and then quickly dumped to the coil). This allows the coil to charge quickly, and the "dwell" becomes less critical. A condensor and capacitor are the same thing, though, and this use may have led to thinking that the capacitor in a normal system boosts the spark voltage. It might, but the effect would be minimal.
Even though the points stay open for quite some time, the spark only occurs the instant they open. A little basic electrical theory:
The ignition coil is a step-up transformer. There's a 12V coil, wound around many, many, many fine turns of wire which are connected to the high voltage wire of the coil. Transformers work on a *changing* magnetic field. While the points are closed, the field is steady, so no voltage comes out of the coil. The instant the points are opened, the field created by the 12V current collapses suddenly, producing a high voltage in the fine windings, and a short, low-current, high voltage spark. No matter how long the points are open (or closed) the spark only lasts an instant. The reason the cap needs to be on right is because you want the spark to have to jump as small a gap as possible, so less voltage is lost crossing it.
Incidentally, the collapsing magnetic field also induces a current in the 12V windings. This would normally arc across the points, pitting them and shortening their life. This is why you have a condensor, to absorb this unwanted 'inductive kick.'