<VV> Coil volts

[Padgett]

>the coil hot side should receive a full 12 (or whatever the battery  tests 
>at)
>minus a little bit for wire runs, during cranking, since the full  voltage
>should be supplied via the starter solenoid. When the starter stops, and  the
>engine runs, it should drop to about 9-10 volts.

Well not quite. The reason the ignition is designed for 9-10 v is when the 
starter puts its load on the system (a BIG load), the voltage is going to 
drop considerably but not below 9 v. So by designing the ignition for 9 v 
you have the hot spark needed for starting. After the engine starts and the 
starter disengages, the charging system returns to normal so the resistor 
is necessary to keep from overdriving the coil.

Transistor ignitions usually perform this juggling act internally so no 
ballast resistor (which is really just wasting power) is necessary.

Padgett