Crystal earpiece
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A Crystal Earphone (more properly called a piezoelectric earphone, pronounced pee-zo) is made of a material that changes its shape when connected to a source of electricity. Some crystals such as quartz and Rochelle salt are piezoelectric. Some ceramics (such as those made with barium titanate) are also piezoelectric. A common piezoelectric earphone is made of a disk of brass that is coated with barium titanate ceramic. When electricity is connected to it, the ceramic bends the brass disk, and we can hear the vibrations this causes in the air.
