While I'm partial to the wristwatch or pocket watch suggestion, crystal radio sets for receiving AM radio broadcasts were popular from the 1920s up until transistors came around. No external power was required. This example has a built in earpiece.
The wires serve as/or antenna and ground. Connecting them was important depending on how strong the signal was and/or how distant the transmitting antenna was. For the two I actually tried out it was necessary to focus pretty intently on hearing the broadcast through a small earpiece.