You're right, that's an important part of the capacitor's practical function in
the circuit, but not the main function. The reason a car will not run (or run
very well) without the capacitor is because the capacitor serves to "dampen"
the ringinig of the ignition coil, this "ringing" being is a natural effect of
high voltage step-up transformers. On an Oscilloscope, it is seen as a big wave
followed by a bunch of similar patterns, each one a little smaller till it
finially rings itself out. Now, if the point closes again during the decay of
this ringing, there is much less energy available, ie lower spark. Think of
this as a rock thrown in a big pond (ie, no capacitor): the ripples dissipate
slowly across the pond, with the energy dissipated likewise. Using a capacitor
is like the same rock thrown in a small bucket; it dissipates the ripples faster
and energy is more concentrated where it is needed (at the spark plug). Maybe
not the best analogy, but you get the idea... If protecting the points were the
only consideration, a simple inductive bucking-diode would suffice.
Richard
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