The document discusses the piezoelectric effect, which is the generation of an electric charge in certain solid materials in response to applied mechanical stress. Piezoelectric materials exhibit this effect, developing a separation of electric charges. Examples include quartz and Rochelle salt. Piezoelectric transducers use this effect to measure physical quantities like force, pressure, or strain. They have advantages like high frequency response, ruggedness, and not requiring external power, but their output is low and impedance is high. Applications include dynamic measurement and studying high speed phenomena.