Cheap talkie amplifier

[KA5PIU]

Hello.

Pre-RADAR aircraft detection.
The funny part is that it worked.
http://www.stonevintageradio.com/items.php?CA=8
Early speakers were little more than earphone elements and excellent acoustic coupling by way of a horn.
Take the modern MP3 player, connect it to a pair of old horn speakers and you will have room filling sound.

[NQ6U]

I once read that during WW I, the French put trained parrots at the top of the Eiffel Tower to detect incoming aircraft.