The example was one of the first cases when I realized you can feed an impulse back and if not careful, you can damage the amplifier, because CURRENT increases each time the feedback loops through the coils of the system. Guitarists frequently use this effect to create a sustaining sound using the feedback phenomena. Yes, there is an external source of power for the amplifier, but if someone cares to measure the input and output currents they will find that it increases each time if the guitar is close enough to the speaker, thus increasing the amplitude of the signal.
The example above is a great way to show that with relatively simple means.
The example above is a great way to show that with relatively simple means.