Function of Ballast Resister

What function does the ballast resister have on a 12v system on a MF 35 gas tractor? And, what is the consequence of not having one?  August 7, 2016.

The original ignition coils on Ferguson tractors have an internal resistor, and do not require an external ballast resistor. If you purchase a replacement coil without an internal resistor you have to place a ballast resistor in series with the coil (between the ignition switch and the coil). If you do not install a ballast resistor with a coil without an internal resistor you will eventually burn out the coil.

As I understand it, the resistor lowers the voltage going to the coil when the tractor is running. Because of the nature of how resistors operate, when the voltage going to the coil drops as a result of the starter being engaged, the resistor does not reduce the voltage going to the coil, and the coil receives adequate voltage to create a proper spark.