The 12-volt breaker point ignitions often use a ballast resistor like the white one shown here. (Note: Crimp connectors are inappropriate for primary circuits.) This is the ballast for GM trucks built in the '50s and '60s. Ford and others have used a resistance wire to the primary side of the coil. When wiring, do not eliminate resistance circuits. Know the voltage requirements of the coil, and check available voltage. Some aftermarket coils require a full 12 volts.