On certain older ignition systems on older stuff they still maintained a 6 volt coil using a ballast resister usually rated at 1.6 ohms to drop the voltage to about 6 or 7 volts going to the coil on a 12 volt system .What happens if you don't use a dropping resister is the points flash over causing misfires .
I recently discovered this on my old Ferguson tractor when I installed new points over looking no resister .Just a lapse of memory on my part ,should have known that already .