Gary..........I've explained this secret trick 1/2 ohm current limiting resistor scheme for the modern 12 volt squarecan ignition coil many times. (it seems at least once a week, grin)
To start with, you need about 15,000 volts to jump yer 0.025" sparkie gap under compression. This is why you just can't take a sparkie outta the engine and say "lookie, I gotts good sparkies, whoops that bytes". The effects of compression makes it harder for the sparkies to jump the gap. ...but... iff'n yer sparkie will jump a 3/16" gap in normal air, yep ya gotts good sparkies alright.
But what does this have to do with ignition coils? Well first off, yer electrical system is 6 or 12 volts and while you can arc-spark with a short circuit, your direct current (DC) battery will never make sparkies under compression. So you use a step-up transformer scheme ...except... transformers don't like direct current because the transformers magnetic iron core saturates and doesn't couple the direct current primary to the secondary. That is majorly profound, and is why alternating current works in transformers.
So why do ignition coil transformers work on direct current? ...because... they want that saturated core's magnetic field ...to... COLLAPSE when the points open!!! The magnetic field's collapse it very sudden and induces the stepped up sparkie voltage in the secondary, which it switched to the correct sparkie by the rotor.
So where are we going with this? Its the electron's flow thru coiled wire that creates a magnetic field. The iron core captures the magnetic field. ...but... the strength of the magnetic field is dependent upon ...are you ready for this?... AMPS!!! ...but... the problem with amps is resistance, which causes HEAT.
So to reduce resistance, increase diameter of wire ...or... shorten the wire making the turns of the coil of wire. ...but wait... its the turns of wire that make the magnetic field strong so shortening the length of wire in a coil is not a viable option to a coil designer. So there is only so much room for all the turns of wire in the squarecan coilbox to increase the magnetic field.
So when they designed the modern 12 volt squrecan frontmount ignition coil, they boo-boo'd. They designed the primary coil turns (and subsequently it total resistance) for standard 12.6 volts of a wet-cell battery. (+10% to charge the battery from whatever yer chargin' it with) ...BIG PROBLEM... is the common in-expensive and popular Delco 10-SI 12 volt alternator internal solidstate voltage regulator is designed for 14.7 volt output and automatically regulates the current pumped out to fill the battery and run all the electricals.
So there you are, pumpin' 14.7 volts into a 12.6 volt coil. Wanna guess whats gonnna happen? TOO MANY AMPS is what is going to happen. Ohms Law still rules (after all these years) So your nice modern 12 volt squarecan ignition coil is baking in its own current and OVERHEATING and melting its internal insulative tars and eventually shorting out some of its windings and causing weak sparkies and hard running. But'll start just fine the next morning after cooling off overnight until it warms up and melts again.
Enter my secret trick current limiting resistor scheme for the modern 12 volt squarecan ignition coil. Back about 1997/8 right after they introduced the 12 volt frontmount coil, (greatest thing since sliced bread) Ford tractor converters (and Ford is the "only one" that uses this squarecan ignition coil) were overjoyed that someone was paying attention to the needs of the tractor marketplace. 12 volt ignition coil, wow, don't need those 12 to 6 volt converting resistor to go with the "infamous ballast resistor" to run on 12 volts, so simple, eh? And then the coil mfgr started getting returns and complaints about coil crapping out...not good!!!
So the coil company (there is only one that I know about) started putting a tiny sheet of toilet paper in the box printed in 6-different languages 'cept 'muricum, recommending using a 0.47 ohm 10W series current limiting resistor to keep from burning out their new modern 12 volt squarecan ignition coil. (instead of changing the internal construction to reduce current draw and subsequent OVERHEATING) But who reads the instructions ennyways, eh?
And thats the long story of my secret trick current limiting resistor scheme for the modern squarecan 12 volt ignition coil used on 9N/2N/8N frontmount ignitions. People can generally get RadioShack p/n 271-131 1-ohm, 10 watt 2-pak resistors and twist the leads in parallel to create a 1/2 ohm 20 watt current limiting resistor. I'm an Electronics Engineer, so I know where I can get real 0.47 ohm resistor, but know what? don't matter, 0.47 is equivalent of 1/2 in the electronics world.........respectfully, Dell, a 12 volt advocate for the right reasons