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Piezoelectric materials produce a voltage difference between sides, when subjected to a stress (pressure).  Pushing on them produces a voltage difference between sides.  Actually they are more sensitive in the resistance mode, where stress produces a large change in resistance.  They may be used to measure atmospheric pressure differences as a common microphone or as a pressure sensor.  With a mass attached, they may be used  to detect accelerations, like the detectors in cell phones that keep projected images upright in the Earth’s gravitational field.

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