A crystal radio is a primitive form of radio set that you can build at home. Because of the small currents involved, you can't use ordinary headphones with a crystal set -- you'll need special sensitive earphones instead.
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Crystal Radio Sets
Unlike conventional radio sets that require batteries or another power source, a crystal radio uses only the energy of radio waves themselves. A crystal set needs no batteries and has few components. You can build one using household materials.
The trickiest part of a crystal radio is often the earpiece. Crystal sets do not give out an electrical signal because they rely on the energy of radio waves -- conventional headphones don't work with them. An old-style telephone handset can work but the best option is a piezoelectric earbud. Certain materials produce tiny movements when stimulated by small electrical currents, which is called the piezoelectric effect. Unfortunately, piezo earbuds are hard to find.
Piezoelectric elements are seldom used in earphones now but they are used to make motion sensors. These larger elements are almost as sensitive as an old-style piezo earbud. You can fit two piezoelectric elements in a pair of ear protectors to make homemade headphones for your crystal radio experiments.
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