Crystal radio receiver
The Crystal radio receiver was first built c1900 by Greenleaf Whittier Pickard, who used crystalline minerals to detect radio signals. A semiconductive mineral crystal, usually lead sulfide (Galena) or cadmium sulfide was fixed inside a brass cup and the radio operator found the loudest signal by touching the wire, called a cat's whisker, to various points on the surface of the crystal. The radio was entirely powered from the radio waves it received. Often the most expensive part was the large antenna required.
People first built and used simple and inexpensive crystal radio sets without batteries or electrical power. Even though vacuum tube radios were common following World War I, crystal radios remained popular, especially among beginning amateur radio enthusiasts, Boy Scouts and school children, who built crystal radios to learn basic electronics and communication.
During the Great Depression parents would build a crystal radio detector from inexpensive galena crystal and a safety pin. After this detector was connected to iron bedsprings (which doubled as an antenna) and grounded to household cold-water pipes, a youngster needed only inexpensive headphones. GIs during World War II constructed similar radios from rusty razor blades and pencil lead, the iron oxide crystals of the rust replacing the galena crystal and the graphite of the pencil lead substituting for the safety-pin wire. These crude, but functional, radios were nicknamed foxhole radios. A modern design for a "trash radio" is constructed from a tin can and some wire.
A hundred years after their first use, hobbyists still build and tinker with -- and listen to -- crystal radios constructed from just a few parts. The most common modern design uses a coil for a tuner, and a diode instead of a crystal. The output is usually from an earplug. A lengthy antenna wire (15m (40ft) or more) is still helpful to get good sound.