Originally Posted by tclederman
A "hotter spark" for starting and a "lower/cooler spark" for driving.

It's actually "the same spark" in both situations. When the starter is operating (particularly in very cold or very hot conditions) the load of the starter drops the battery voltage to approximately 9-9.5 volts. With the engine running and the generator/alternator charging, the "battery" voltage is actually the voltage produced by the charging system- - - - -14.5 volts or so. A 5 volt difference in what is applied to the the primary coil winding would create vastly different current flow across the points. The simple fix is to create a coil winding that will produce a hot spark at the lowest expected voltage, and protect the points with a resistor to reduce the current flow when the voltage is at its high limit. The bypass switch allow full (but low) battery voltage to be applied to the coil during cranking.

There's nothing exotic about it- - - -just good engineering to assure quick starts from good spark plug voltage when the battery is under load, and protect the points the rest of the time.
Jerry


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