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I know I'm going to regret answering your question but here goes. This is a LONG reply. Your question is one that is often very hotly debated! The ingition coil is in reality an alternating current device. It is a very specific type of TRANSFORMER called an auto transformer. In the case of our old tractors the auto transformer (or coil) has a low voltage primary that is connected one end to the battery, the other to the distributor. You are correct, this primary winding does not care which direction the current flows through it! It is just mandatory that current does flow through it. When the current is interrupted, the magnetic field collapses and induces a voltage in the secondary that is OPPOSITE in polarity to the primary's voltages and currents. This is one of the laws of transformer action. Whatever happens in the primary is balanced (power wise) but opposite in polarity in the secondary. We put into the primary a low voltage high current and we get out of the secondary a high voltage low current(this is the balance I mentioned) but what comes out the secondary is opposite in polarity to the primary. So how does this have anything to do with wiring a coil to a tractor? I'm going to ask you to picture in your mind (or draw it on paper) a stout spring wound with #9 wire that has perhaps 10 turns to it laying on the table in front of you. To the right end of this stout spring is a fine wire screen door spring. Lots of coils or turns and much finer wire! The battery is connected to the left end of the coarse spring which represents the primary winding. The points and condenser are connected to the junction point between the two springs. The High voltage lead out of the coil to the distributor is connected to the right end of the screen door spring which represents the secondary winding. This is the correct way to connect a coil to the tractor. If instead we connect the battery to the point between the springs, that puts the points and condenser at the left end of the primary winding. This wiring WILL work! The only problem is that this forces the spark current (what we really want out of the coil) to go through BOTH the primary and secondary windings to get to the spark plug. Since thatever happens in the secondary also happens in the primary BUT IN REVERSE POLARITY we get a weaker spark with the coil wired in this manner because the primary and secondary voltages are opposite in polarity and they fight each other. How much weaker the spark is is a function of the winding of the coil. Generally the difference is around 4 to 5 thousand volts less when wired incorrectly. I know I can't just hold a spark plug wire away from the plug or ground and see the difference in length of the spark or feel the difference either. They both smart! So there's just one last question..... Did the manufacturer of the coil label the case + at the left end of our table top example or did they label this point as -? Generally the post labelled - is the junction between the primary and secondary which is the distributor side of the coil. You're most apt to see this reversal on old coils. There are exceptions to this example. The Wico distributor made for Deere's Pony engines is nothing like this example at all! Most 2 cylendar motorcycles and today's GM (probably others) use this Pony style exception. They're most easily spotted by noting that there are 2 high voltage wires comming from each coil. There is one more test that can be performed that involves placing a lead pencil point in the middle of a spark and watching which side of the pencil has the yellow spark. I'll have to look that one up to adequately describe how to set up that experiment, I won't trust my memory to that one!
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