Piezoelectricity

Squeeze a small crystal of quartz between your fingers with enough force, and it will generate a measurable electric voltage across its surface, entirely without any battery, wiring, or external power source involved. This phenomenon, called piezoelectricity, occurs only in certain crystalline materials whose internal atomic structure lacks a center of symmetry, meaning the arrangement of positive and negative ions within the crystal isn't perfectly balanced around a central point. In an undisturbed piezoelectric crystal, the positive and negative charges are still distributed in a way that cancels out overall, producing no net external voltage. But when mechanical stress deforms the crystal's shape, even by an amount too small to see with the naked eye, it shifts the relative positions of those positive and negative ions, and because the structure already lacked that central symmetry, the shift doesn't cancel out evenly the way it would in a symmetric material. Instead, one face of the crystal ends up with a slight excess of positive charge and the opposite face with a slight excess of negative charge, creating a measurable voltage between them. Critically, this effect also works in reverse: applying an external voltage to a piezoelectric crystal causes it to physically deform by a tiny, precise amount, which is the exact principle exploited in quartz watches, where an applied voltage makes a quartz crystal vibrate at an extremely stable, predictable frequency that the watch uses to keep accurate time. The same reversible relationship between mechanical stress and electrical charge also underlies ultrasound imaging devices, which use rapid applied voltage pulses to make a crystal vibrate and emit sound waves, then switch to reading the voltage the crystal generates when the returning echoes strike it.