Just as a side note....the ballast resistor' purpose is to perform a crude current regulation through the coil that keeps coil heating under control over a wide range of RPM's. The electrical term 'ballast' means just that: a resistive element that varies it's resistance based on the current flowing through them. There were ballast tubes in the old vacuum tube days that did the same kind of thing: current regulation.
The voltage drop from the ballast is just a byproduct of their current regulation. It is not precise current regulation by any means like a solid state regulator, but it does keep the current flow and heating of the coil within a rough range despite wide variations in RPM.