Strictly speaking, there is no armature in an alternator. Instead, there is a rotor with laminated-iron poles and a continuous coil terminated in slip-rings. these can be cylindrical or end-face (a disc surrounded by a concentric ring)
the rotor is surrounded by a stator,-again, laminated-iron, with multiple poles upon which the output-coils are wound..
By energising the rotor ,it will induce a current in the stator windings. the energising input is regulated to give the desired output-voltage at normal operating speeds and to disconnect the machine when output-voltage is less than the connected battery....with a blocking-diode fitted, the main output is safely left connected directly to the battery,as no back-flow can take place.
I assume that in aircraft applications it is disconnected via the master switch and thus fully isolated.
Normally, the battery is used to provide initial excitation current and in solid-state circuits it can be arranged to be the "earthy" side of the warning-lamp. as the machine starts generating, the output opposes the battery-voltage and the light goes outdue to the battery and generator-voltages reducing the current-flow.
in summary, the generating-coils are stationary, the magnet (electromagnet) revolves. output is AC , normally rectified by a diode-pack....regulation of field-current can be solid-state or electro-mechanical.
IMHO, the former is more reliable and i think that early 20th century equipment belongs in the museum.
(ducks incoming)