This may be a litle off topic, but ignition coils are WEIRD, compared to other transformers. I had to work on the ignition on an old farm tractor last week. I always thought that the high voltage (secondary) was from the tip (spark plug wire), to the coil's metal case. It turns out I was wrong. (I am referring to the old coils which are about the size of a slim beer can, and have a pointed top).
After a lot of research, I found that the secondary is measured from the large center terminal, to the ground (-) side of the primary. Once I learned that, it turns out that the secondary is around 500 to 600 ohms, and the primary is around 6 ohms. (I compared this to several other similar coils, and it is about the same).
These ignitions are very simple, but testing them is a challenge, since they do not follow the (regular) transformer wiring. A regular transformer will have 2 wires for the primary, and two or more on the secondary. These transformers only have 3 connections, period....
But this has me puzzled. Since the minus primary connection is used for the secondary high voltage, how can the circuit work???? Ok, my point is this: The coil's minus terminal is connected to the ignition points. The points are making a ON-OFF surge repeatedly. (or creating an AC across the primary). So, how can the coil deliver a spark during the time the points are open, since the secondary is relying on the primary (-) to complete the circuit, which is also being switched on and off by the points. .
This still has me puzzled......
I did find the problem and get the tractor running though. There were actually two problems. The points had welded together, so I replaced them. But the spark was still weak which went back to the ignition resistor. That resistor is supposed to read about 8 volts on the output side, which feeds the coil. I was reading 4.5 volts, with the battery fully charged at around 13.5 volts. So I replaced that resistor as well. Works fine now....