[JSeery]

Ballast resistor is to reduces the voltage/current to the original coil. The Ford coil was designed to operate on the lower voltage. One advantage is the resistance of a ballast resistor changes as it heats up, so there is more voltage at startup. Then the resistor heats up and reduces the voltage/current to the circuit.

The circuit breaker protects the lighting circuits. In the years prior to the introduction of the circuit breaker a fuse was used. Part of the advantage to using a circuit breaker is that the circuit can rest with a minor overload. When a fuse blows that is it until you replace the fuse. A circuit breaker trips when it overheats, once it cools down it will reset.