^^ I was just gonna say that about Chevy ballast wire.....
The single ballast resistor and the low resistance side of the dual ballast is a form of current regulator. (The term 'ballast' used used to mean just that in older electronics.) The resistance actually varies with the amount of average current through the ballast.
This allows the coil current to be set nominally higher than what the coil would survive to counteract the natural drop of average coil current at high RPM's. The ballast will then lower the average coil current at low RPM to keep the coil from burning up. It also puts extra current in the coil when cold for better cold start spark energy.
BTW the old Ford V8 flatheads did not have a ballast; the static coil current was set for adequate sustained spark output at high RPM's, and depended on the regular interruption of the coil current by the points with the engine running to control the average coil current. If you left the key in RUN but the engine not running, the coil would burn out in short order. A ballasted system was a innovation 'back in the day' to cure this problem.
The main ballast in a Mopar (single or dual ballast) should be 0.5 to 0.6 ohms cold, and a bit over 2 ohms hot. There are a lot of other ballast resistors sold that have higher resistances and they will badly weaken the coil spark. (I have to wonder how many coils were replaced and system converted because of the wrong ballast.) The BWD RU19 is a pretty close equivalent sold in box stores; it has about the same cold resistance but a hot resistance of 3-3.5 ohms. The most common Mopar single ballast is PN 2095501; if you can find one, they work noticeably better in the single ballast systems with a stock type coil., especially on cold starts.
