Okay, so not exactly what the OP asked for, but here's Dave's 30-Minute Tractor Physics Sunday lesson:
So the reason why there's an importance... is that in a battery-coil ignition system, DC is applied to the PRIMARY winding (which as Les noted out, is a few ohms, with 12v across it, and several amperes of current), which creates a magnetic field around the iron core, and the secondary winding (spark plug side).
This causes a magnetic field to build around the core, and while this happens fast, it doesn't happen instantly... the delay in building of the magnetic field is called 'inductive reactance'... the rate at which an electromagnetic element can respond to change.
The primary coil winding is a long piece of copper wire wrapped around that core. Wire carries electricity fairly well, but not perfectly... there's a certain amount of resistance to a piece of wire, and the shorter, the wire is, or the thicker it is, the less resistance it has.
As Les's demonstration points out:
A 3 ohm coil has half the resistance of a 6 ohm coil, hence, the 3 ohm coil will pass twice the current that a 6 ohm coil can.
A 1.5 ohm coil will pass twice as much current as a 3.0 ohm coil.
And then there's this point- a 1.5 ohm coil will build a magnetic field in about half the time, as a 3.0 ohm coil.
And the other point- pushing power through a resistance generates heat... and the more current you flow through a resistor, the more heat it generates.
Once the magnetic field has fully built, the only thing that is happening, is the coil is doing nothing more than getting hot...
But when the breaker points open, the magnetic field around the primary winding (the 12v side) collapses. When that field collapses, magnetic lines of force cut across the primary and secondary windings really fast (not instant, but close), and those magnetic lines cause a whole lot of voltage to be induced ONTO the primary and secondary windings. That voltage goes looking for someplace to go.
On the secondary (spark plug) side, which has about 1000 times as many turns, the voltage is about 20,000x the voltage as the PRIMARY side... it goes looking for a path, and if we did everything 'right' (cleaned the rotor, cap, had good wires, and no carbon-fouled plugs), it jumps across a gap at a spark plug. If we didn't do it right, it takes the first most convenient detour back, through the block, or rotor, or a carbon-track in the cap, or through your hand (when you're holding a wire). [don't try to tell me that you haven't participated in an ignition detour...]
Now, when that field collapses, the PRIMARY side (12v) is also taking a hit. If all you have is breaker points, that collapsing 'hit' jumps the gap in the breaker points, burning them a little bit, and 'biting' your 12v battery a little bit, and finding it's way through contacts of the ignition switch, and through the starter solenoid/switch and brushes, through the generator cutout, alternator regulator, and alternator diode stack.
This is where a little 'protection' comes in handy. Put a capacitor between the coil primary and ignition points... just enough to absorb the 'bang' of that inductive collapse, and the primary side is snubbed down so as to not hammer the points (and everything else), and if it's REALLY 'tuned' right, will cause the coil to oscillate a little, to make a really hot spark. Oh, and rename this capacitor to 'condenser', for an 'old time radio' feel.
The coil's resistance is something you need to know not just for troubleshooting, but for matching... because of something called 'dwell'.
Dwell is the amount of time that the contact points are closed, and current is flowing, before they open to generate a spark... y'know, that waiting period I was mentioning above when the only thing that's happening... is the coil getting HOT.
The DWELL time of a coil is a percentage... the amount of time ON, versus off. A coil that's on for a long time before firing, experiences more DWELL than an engine that has a very short time.
With mechanical breaker point ignition, the points are operated by a mechanical cam lobe... which means an engine that is running fast, has the same dwell as one running slow. Dwell can be altered by changing shape of the cam, but the percentage stays the same regardless of speed... until, of course, you're trying to spin it so fast that the point spring can't keep up, and the points 'float'.
...but in an electronic ignition, the electronics manage dwell. It can turn the coil on for a longer percentage of time at high speed, and a lesser at low speed. At high speed, that assures that you'll get a good strong spark at high speed, but when slowed way down, the dwell can be shortened up to keep from overheating the coil.
And then there's cold starting. When you crank the engine, the starter motor is chewing up a considerable amount of the battery's power... it's totally normal to see system voltage to drop from 13ish volts down to 9 volts during cranking. 9v to a 12v coil is a pretty darned big drop, especially when cold starting is the time when you need a good hot (magneto-with-impulser-like) snap.
So the engineering solution is to WIND THE COIL FOR HOT SPARK AT 9V.
It generates a nice hot snappy spark at 9v...
And once you release the switch, system voltage jumps up to 13ish, which promptly incinerates the coil internally, right?
So to restrain it from self-immolation, there's a device in the wiring called a 'ballast resistor'... it's just a resistor, and the intention of it, is to prevent the coil from carrying so much current that it releases it's internal magic goo onto your frame rail, setting your soybeans alight, and leaving you stranded on the far edge of the south 40 amidst harvest.
When you turn the key, or press the starter button, there's an extra contact, and a wire going straight to the coil, that bypasses the ballast resistor, so whatever the full battery cranking voltage is available, can get to the coil. Once the starter is released, the ignition ON circuit powers the coil through the resistor.
And this, is the most common reason why, when you try to start an engine, it'll crank and fire... but as soon as you release the key or switch, the engine stalls (the RUN contact, or ballast resistor, or wiring on that branch has a failure), or when you CRANK the engine, it won't fire, but as soon as you release the KEY, or if you hand-turn the engine with the key on, it'll fire just fine... ballast resistance and switch is fine, but the cranking bypass circuit is not.
Feel free to review your notes, I'll take questions after the break, quiz will be on Wednesday, test on Friday.
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