piezoelectricity
/ pee-AY-zoh-ee-lek-TRISS-ih-tee /
Take a certain kind of crystal — quartz is the classic one — and squeeze it gently. A small voltage appears across its faces, as if pressing it wrung out a trickle of electricity. Let go and the voltage vanishes; squeeze the other way and the voltage flips sign. This direct trade between squeezing and voltage is piezoelectricity.
It works because in these crystals the positive and negative charges are arranged just lopsidedly enough that bending the lattice shifts them apart, creating polarization, which shows up as voltage on the surfaces. The effect runs both ways: not only does pressure make voltage, but applying a voltage makes the crystal change shape ever so slightly. Only crystals lacking a center of symmetry can do this — squeezing a symmetric one just compresses it evenly with nothing left over.
This matters because piezoelectricity quietly powers countless gadgets: the spark in a gas lighter, the timekeeping crystal in a watch, ultrasound scanners, and microphones. A common misconception is that any crystal will do it; in fact most won't, because most have the symmetric arrangement that produces no net shift. The honest caveat is that the voltages from squeezing are tiny and the motions from applied voltage are minuscule — useful precisely because they are so reliable and fast, not because they are large.
Click an electric gas lighter and a tiny hammer strikes a piezoelectric crystal, jolting it so hard that the voltage spikes high enough to leap across an air gap as a spark. No battery is involved — the energy of your thumb's click is converted straight into the firing spark.
A gas lighter's spark is a piezoelectric crystal turning a sharp tap into a high voltage.
Piezoelectricity needs a crystal with no center of symmetry; all ferroelectrics are piezoelectric, but many piezoelectrics — like quartz — are not ferroelectric.