I bought one of these many years ago when they were first offered. At a club event we tested about 20 cars and found about half had the polarity backwards and the owners were not aware of it.
Back in high school around 1950 my auto shop teacher showed the class how to determine the polarity by lifting a plug wire and creating a spark and then inserting the tip of a lead pencil into the spark. The side it flashed on told the polarity. I forget which side. Then he would hand the pencil to the class clown and have him do it, resulting in the kid getting shocked.
Most of the coils I've found with DIS and IGN on them were for positive ground systems so there would be no confusion about connecting it up. I've used a pencil to test but have had the Nu-Rex polarity tester for some time. The Nu-Rex is not a new thing. This type of tester has been around for 80 years or more.
On coils marked with + and - this at least shows the polarity for a negative ground system which became the norm after the change over in 1956. There was no need to use DIS and IGN with all domestic systems using negative ground. A person may even run into coils that have no marking at all. More modern coils generally have power applied to the + terminal and the - terminal is connected to the ground breaker points so there is no confusion here. The negative terminal is the one that has the dual connection of both primary and secondary coils since that is the ground side for both coils on negative ground. The secondary coil needs a circuit to function but it will only function on either pulsating direct current or AC current to step up the voltage of the primary. The stepped up voltage goes both ways but is caught by the condenser on the breaker side so the other side has to be the high tension exit when properly connected.
It's all about polarity with inductor coils. They don't care how they are wired since they will function either way but only one way is polarity correct for the way the battery is connected in the system. The primary coil is what makes an electro magnet out of the steel coil core. It can be a magnet either way but only one way is efficient for a spark at the plug since it is important which direction that spark flows between high tension release and ground. The high tension coil or secondary has a ground provided for it either way but the direction the high tension goes is fully dependent on the POLARITY of the magnetic field set up in the primary coil and core before that field collapses as the points open.
