[steve s]

Here's a nice wiring diagram that some good citizen posted years ago (sorry, I can't identify who deserves the credit.)




Here's a close up of the coil wiring.



The thinner wiring is the secondary winding. If the diagram were more accurate, there would be about 100 times as many turns of it as of the heavier primary wiring.

MOST IMPORTANT: Notice that one end of the secondary winding is connected to one end of the primary winding. This ultimately determines how the polarity at the coil connections determines which way the electrons will be going in the secondary (and in the spark plugs). Here's how--

The goal is to have electrons flow down the center secondary wire that goes to the plugs. BUT, the electron flow induced in the secondary windings will be of the OPPOSITE POLARITY (i.e., direction) as the current in the primary windings. Thus--if you focus on the junction point where the primary and secondary windings connect--in order to get electrons to flow FROM that point, thru the secondary windings, and then on to the plugs, the electron flow in the primary windings must to be TOWARD that point. And, it is that direction of electron flow in the primary windings that you determine when you connect the battery to the coil. The electrons will be going the right way if that primary/secondary junction point is connected to the positive battery terminal, and the other end of the primary to the negative battery terminal, from whence cometh the electrons.

Since that primary/secondary junction point is internal to the coil, you need some external marks to let you know what's what, hence the "+" and "-" notations commonly used. Of course, only one of these connections will be by an actual wire path that you can trace back to the battery; the other will go to the ignition points to ground and back to the battery thru the sheet metal, etc.

On a correctly wired, positive ground Model A, the "+" coil terminal must connect to the ground, or ignition points, side of the circuit. That is the red wire in the diagram. The "-" coil terminal must connect to the "hot" wire coming from the battery via the starter and the ammeter. That is the black wire in the diagram. On a negative ground car, the opposite connections would apply. If you have a coil whose terminals are marked something like "SW" (switch) and "CB" (contact breaker), rather than "+" and "-", you have to know whether that coil was for a negative or positive ground car.
I hope this helps.

Steve