Agree with all the posts on this page
-residual magnetism helps starts things off
-if the dynamo output is below the regulation voltage, then the regulator dumps power into the field coil - creating a stronger field - and thus the output increases
-as the revs increase, the dynamo output also rises. Above the regulation voltage, and the regulator is cutting power to the field coil, reducing the magnetic field and thus reducing the dynamo output
I used voltage to see what the field coil was doing but current, as posted above, is what the regulator actually controls. I find either is useful for fault diagnosis ( but not calibration ! )