Found a page on the web that gives a good explanation of ballast resistors. It states there that "Due to a high temperature coefficient of the iron-wire resistor, the hot resistance is about three times of the cold resistance."
See:
http://what-when-how.com/automobile/...ms-automobile/
Scroll down to:
Output Control Ballast Resistor.
It also states: "The resistor can be installed either internally in the coil or externally in the circuit."