Ignition Coils - 
All coils sold are plainly marked "for use with an external resistor only" 
or "for use without an external resistor only".

The former is a 14v coil that allows full voltage only during cranking to 
assist starting in cold weather. Use it without the required resistor and 
you will be replacing points every 15 minutes.

The latter is a 12v coil. Use it with a resistor and you'll be lucky if the 
engine runs at all.
Dec 01


Ballast Resistor-
The purpose of the ballast resistor is to reduce the current thru the
primary circuit.
Voltage is also reduced, but it is more of an unintended result when one
applies Ohms law to a simple series circuit.
The coil doesn't need the voltage dropped since when the circuit opens 
and the coil fires there is an approximaate 300 volt kick mutually induced 
back into the primary circuit.
Current does need to be controlled however.  Modern electronic ignition 
systems use a current limiting circuit as part of the ignition module, 
GM HEI and Ford TFI are good examples.



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When cranking the engine, the starter load causes the battery voltage to
drop down to about 9V. Since this is insufficient to power a 12V coil, a
(approx) 9V coil is used. 
When the engine is started, battery voltage goes back to 12V, which would
fry the 9V parts.
The resistor (which is bypassed during cranking) drops the V back to a
safe level.
You will probably notice a wire going to one of the terminals on the
starter solenoid. This is what bypasses the resistor during cranking.

Some other earlier systems used a resistance wire instead. When cold, the
wire would pass the full battery V. As the wire warmed, its resistance
would increase and thus drop the voltage. I guess the logic was that a
warm engine could start on 9V.
These could cause a lot of problems on hard to start engines. You would be
cranking away and the voltage to the coil is slowly dropping from 9V to
6?.