A piece of quartz or Rochelle salt which has piezoelectric characteristics. A piezoelectric crystal produces a voltage between two of its opposite surfaces, or faces, when it is bent or compressed. A crystal also distorts when a voltage is placed across two of its opposite faces.

All physical objects have a natural resonant frequency at which they vibrate most easily. A piezoelectric crystal will, when it is placed in an electrical circuit and excited with a pulse of electrical energy, vibrate at its resonant frequency. As the crystal vibrates, it produces an AC voltage whose frequency is the same as the resonant frequency of the crystal.