[marc hildebrant]

Joe K,

Actually, the third brush regulates the amount of current only as the Model A generator is a constant current generator. Thus, for a constant current device, the voltage rises or falls to make the amount of current equal to what the third brush is set to. That's why the voltage rises to such a high level when the generator's output is disconnected...it's trying to push current into an open circuit and only knows to keep raising the amount.

The battery keeps the voltage to the proper value and the ammeter shows how much current is going into the battery.

Sorry, but I don't have a better explanation.

Marc