For energy stored in the inductance, wL = 1/2 LIexp2 where the energy is taken for the existing current at the instant the points open (L di/dt). The resistor value, inductor primary resistance, and the battery voltage set the I (amperage) part for a given L (inductance) and point dwell (time the points are closed causing currint to flow through the coil), regardless of the location of the resistor (inside or outside the coil's case).
The resistor is sized to control the current through the points to around 4-5 amperes (value derived from MF service manual).
The coil cares not if 6 or 12 volts is running the circuit. It reacts to current. The secondary of the coil (actually high voltage transformer) rises as fast as the leakage inductance of the coil will allow up to a max of around 18kv (prior to electronic ignitions) or to whatever part of that is needed to make the plug gap break over.
The purpose of the capacitor (condenser...condenses the voltage spike) across the points is to limit the rate of rise and thus ultimate value of the inductive kick generated across the points when they open to reduce pitting and provide for longer point contact life. I think that by eliminating, or at least reducing the sparking you reduce the O3 (ozone) that would be generated in a spark which helps to keep the point material where it was initially located, not on the other contact, aka pitting.
It has been rumored that when 12v systems became common a lot of 6v coils were used with the internal resistor making a 12v coil. But JD's pics may knock the wind out of some of that.
I remember Ford cars having the external resistor bypassed for ease of starting.
Oh well. That's the way I remember it and it has been awhile.
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Today's Featured Article - Talk of the Town: The Saga of Grandpa's Tractor - by The following saga is from the Tractor Talk Discussion Forum. Someone. The saga starts with the following message: Hey guys I have a decision to make. I know what you all will probably suggest and it will probably agree with me way down inside, but here it is. I have a picture blown up and framed in my "tractor room" of a Farmall M. It was my Grandpa's tractor, of which whom I never got to meet. He froze to death getting this tractor out of the barn to pull a truck out of the ditch before I was born. Anyway my dad and aunt had to sell it at the auction,
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