Here is the same diagram with the switch closed.
Everything with yellow is system voltage.
With the switch on, but engine not running, some current current flows through the field. If the battery is 12.5 Volts, the blue wire will be close to 12.5 Volts. The green wire will be less because power is used by the windings generating the magnetic currrent.

If the engine was running, the ammeter would be centered (or show charging if the battery needed charging). The system voltage would be 14 Volts more or less, and would come from the alternator output (Bat) stud. The blue wire would be the same. The green wire would be noticibly less than 14 Volts if current was flowing (when power is needed), and 14 Volts if no power was needed.