Early VW's used an ignition system based on the Kettering patents from the 1920's, in which the battery voltage was stepped up to several thousand volts through the use of a transformer, the thing we call the coil. But transformers only work when the voltage is changing. (Alternating current 'changes' 120 times a second (ie, 60 cycles) so transformers work just fine.) To use a step-up transformer in a car you'll need something to cause the voltage to change. Just turning it on and off will work. Henry Ford used a magnetic-reed oscillator, a kind of vibrating switch. Boss Kettering (he ended up running General Motors) had the genius to connect the ignition transformer through a mechanical switch driven by the engine. Opening and closing the switch provided the changing voltage needed to make the transformer work. The switch of course is the ignition points.
How the Coil Works
The reason a transformer works lies in the physical properties of electrical current. When a current flows through a conductor it generates a magnetic field around the conductor. Conversely, when a conductor is moved through a magnetic field, a voltage will be induced in the conductor. A transformer takes advantage of those principles of inductance by winding one coil over the top of another. At low frequencies you can focus or concentrate the magnetic field by winding the coils around an iron core. And since you can't move the coils relative to one another, the changing voltage in the primary winding serves as the 'movement' needed to induce a voltage in the secondary winding. And as you've probably guessed, the voltage in either winding is proportional to the number of coils in the inductor; if there are more turns in the secondary, its induced voltage will be higher than the voltage in the primary. But there's no such thing as a free lunch, the total amount of energy remains the same. If you pump in 120 watts (that is, ten amps at twelve volts) to develop, say, 30,000 volts in the secondary (about what you need to jump the gap of a spark plug under worst-case conditions) the amperage can't be more than about three-thousandths of an amp (.003). Actually, things never work out that neatly in reality because there are losses in the coil's iron core, etc.
Why The Capacitor is Needed
Those same principles of inductance create a kind of paradox, because when you stop feeding juice to the coil, that is, when the points open and the magnetic field collapses, inducing the 30,000 volt current in the secondary, it also induces a current in the primary as well! It's not very much because there are only a few windings in the primary, but it's enough to jump a small air-gap, such as the one between the just-opening points in the distributor. That tiny spark is enough to erode metal away from the points and if there is any oily vapor inside the distributor, any oil on the points will become carburized; you'll 'burn' the points.
To keep your points from burning as they open and close, you'll need to provide something to absorb that spike of counter-current, something more attractive, electrically speaking, than the air-gap between the points. That's a job for a capacitor. To the counter-current, the capacitor looks like a black hole, an attractive one. The spike of current dives right in. And the points don't burn.
The points have a tough job, switching up to eight amps of current many times per second at highway speed. Indeed, as engine speed increases the efficiency of your ignition system decreases, thanks to heating problems and fundamental electrical laws. This declining efficiency has a serious effect on your spark voltage and results in poor high-speed performance, incomplete combustion and a host of other ills.
But us humans are tricky rascals. To see how tricky, read the articles covering electronic ignition systems.
-Bob Hoover
-1995
(Ed. Note: This article deliberately ignores more sophisticated resonant frequency explanations of the coil-condenser relationship as being unnecessary with regard to vehicle maintenance.)
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