rapidly changing magnetic field induces a sharp voltage spike in the primary winding. This voltage spike is several hundred volts negative with respect to the +12volt applied to the battery side of the primary and because the coil is acting as a transformer, a pulse of up to 20,000 volts appears across the secondary winding. The voltage required to fire the sparkplug determines the maximum voltage which actually appears on the High Tension terminal and this can range from 5000 to over 15000 volts, depending on the plug gaps and the conditions inside the combustion chamber. If a sparkplug lead happens to be disconnected, the secondary voltage can rise to the maximum possible figure and cause unnecessary strain on the insulation of the coil and the high voltage leads.

The voltage spike in the primary will strike an energy wasting arc across the just-opened points unless something is done about it. This is one function of the capacitor connected across the points, an electrical device which has the ability to accept an inflow of current until it is fully

charged. Coil current continues to flow briefly into the uncharged capacitor while the points move apart to the point where an arc can't be struck by the few hundred volts of the primary spike. Once the capacitor is fully charged, the current flow stops and the magnetic field can collapse. The voltage spike in the primary rapidly rises to its peak value, inducing the multi-thousands of volt pulse in the secondary. Without the capacitor, the points will be rapidly eroded  and the collapsing magnetic field will expend a good part of its energy as a few dozen volts maintaining the unwanted arc across the points."

The capacitor is actually "charged" after the points open, not before and acts to "absorb" current, rather than as a current source.

From Gregory Wells                                   800-331-2193 x103

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