Green and Blue Field Wires

the ballast resistor is in the coil circuit and has nothing to do with the alternator. In your drawing it is simply a binding point. Follow the blue wire
from ammeter to ignition switch to the binding point on the ballast to a split; one way powers up the regulator and the other goes to the alternator field.
The other side of the field goes back to the regulator along the green line and the regulator grounds it to cause the alternator to charge. When the reg senses voltage above it's set point, it shuts the charge line off by opening the ground circuit. It does this so fast your damped ammeter cannot see it.