the direct piezoelectric effect
The direct piezoelectric effect is the mechanical-to-electrical half of piezoelectricity: squeeze the crystal and it produces electric charge. This is the effect the Curie brothers first observed in 1880, and it is the one at work whenever a piezoelectric acts as a sensor or a spark generator — press it, and out comes a voltage.
Apply a mechanical stress T to a poled piezoceramic and it develops an electric displacement (a surface charge per unit area) proportional to the stress: D = d times T, where d is the piezoelectric charge coefficient in coulombs per newton. Physically the stress shoves the positive and negative sublattices by different amounts in the non-centrosymmetric crystal, so bound charge of one sign appears on one face and the opposite sign on the other. If you leave the electrodes open-circuit instead of measuring charge, that charge sits across the material's own capacitance and shows up as a voltage; the relevant number there is the piezoelectric voltage coefficient g, equal to d divided by the permittivity. A tiny force on a stiff ceramic can generate a surprisingly large open-circuit voltage — enough, in a gas igniter, to break down a centimetre of air.
The direct effect powers gas igniters, accelerometers and vibration sensors, force and pressure gauges, contact microphones, the receive side of sonar and ultrasound, and piezoelectric energy harvesters that scavenge power from vibration. An honest limitation defines how it is used: the charge it generates slowly leaks away through any finite resistance, so a piezoelectric sensor responds to changing or dynamic loads but cannot faithfully measure a truly static (DC) force — hold a constant weight on it and the signal decays to zero. That is why piezo sensors are read with charge amplifiers and are the wrong choice for measuring a steady, unchanging load.
A piezoelectric gas igniter works entirely by the direct effect: a spring-loaded hammer strikes a small PZT cylinder, the sudden stress generates several thousand volts across it in an instant, and that voltage jumps a spark gap to light the gas — no battery, ever.
Force in, charge out: the direct effect makes piezoelectrics generators and sensors, but only for changing loads, since the charge cannot be held.
A piezoelectric sensor cannot measure a true static force. The generated charge bleeds off through finite leakage resistance, so a held, unchanging load reads as a signal that decays toward zero — piezos are inherently dynamic sensors.