Hot Spark!!!!!! The energy that is dispersed in the spark across the plug gap is stored in the inductor (coil) as 1/2 L [inductance of the primary in this case (7 mh for a number) x actual current flowing at the instant the points open squared].
So for a 7 mh coil and 4 amps you have 1/2 (7 exp-3) x 4 exp 2 for 56 millijoules....(mj defined as a volt x amp x second and is a measure of energy. Get it going via pulses per unit of time and you are consuming energy at some rate which is work, measured in watts or horse power, 746 w/hp)
But for the same coil and 5 amps you have 88 mj, some 60% more energy.
The voltage across the coil both primary and secondary per the turns ratio will rise to infinity theoretically when the current attempts to stop due to the points opening, in an attempt to keep it moving.....nature of the beast. The actual breakdown value of the plug depends on the pressure on the plug from it's environment and the gap. The "heat" of the spark is the energy in the arc (volt-amp-period) across the gap and that comes from the coil. So low current, exponential loss of energy. Question is, how much does it take to fire the plug in a given engine at a given time that contains enough energy to light off the available fuel air mixture present at the gap?
On the resistor getting hot, that's the reason it's built the way it is. It's in a high wattage package with provisions for mounting to a heat sink...a piece of sheet metal on the tractor somewhere.....sorry heat sink but better than nothing.
Talking about coils getting hot from running, what always got me was the mounting of the coil right on the side of the engine block. Convenient, yes, but there are better places that are cooler.....course the fan blows across it and I guess that makes it ok.
Mr. JMOR sir, got time for those pictures of the points with and without the capacitor (condenser)?
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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