Piezoelectric earphone:   The crystal earphone or piezoelectric earphone is made of materials that expand or contract as electric current flows though them.  Ceramics, quartz, ceramic coated brass... The crystal earphones are extremely sensitive and need very little current to produce sound.  Sound can be produced by grounding one lead and scratching the other on a table.  These ear pieces are perfect for the low current induced in these simple radios.  The excerpt below is from another site where soldiers built a simple speaker.  (Science Toys) "But what about the earphone? Richard Lucas, who was a POW in Vietnam, built a radio in camp and was also able to improvise an earphone. He writes: Four nails were bound together with cloth from our clothes. Wire was obtained from wire used around the camp which I might add wasn't coated with varnish. It was bare wire, so we wound a layer and, using a candle, we dripped wax over the turns, which were spaced as closed as possible without shorting out (not touching). We repeated this process over and over again until we had about 10 layers of wire, which were insulated from each other layer by a strip of cloth and wax. Then we put this in a piece of bamboo and adjusted it so it was about a 1/32 of an inch from the end. A tin can lid was positioned over the coil of wire and nails. Then connecting it to our "foxhole radio" (basic design as yours) we could here about three radio stations. Our antenna was the barbwire around the camp and the ground was wire laid along the ground to make up the ground. Best listening was at night and it had to be pretty quiet because the earphone was pretty weak. If we had a magnet to set up a bias on the coil, the volume would have been a lot louder." And Mike Barnard points out that "the headphones were almost always acquired from a tank crew's radio operator, and often one side of the headphone was cannibalized for wire to wind the tuning coil while the other was used for listening."  ".(http://bizarrelabs.com/foxhole.htm)