Ballast Resistor

From the 1965 Plymouth Factory Service Manual page(s) 8-50 and 8-59:

The ballast resistor is a compensating resistance in the ignition primary circuit. During low speed operation, when the primary circuit current flow is as high, the ballast resistor temperature rises, increasing the resistance. This reduces the current flow, thereby prolonging ignition contact life. At high speed operation, when the primary current flow is low, the ballast resistance cools off allowing more current flow, which is required for high speed operation. During starter operation, the ballast resistor is bypassed, allowing full battery voltage to ignition primary circuit.