Measurements of electric potential variation by piezoelectricity of granite

From laboratory experiments, we observed that unloading stress on a piece of granite induced a pulsed electric potential variation, which decays exponentially with a time constant ranging from 1.5 to 4.0 s. The electric potential variation depends on the magnitude and the sustaining time of applied stress, indicating that the variation is due to piezoelectricity and migration of bound charges that compensate the piezoelectric polarization. An apparent piezoelectric coefficient of granite was obtained to be about 1.4 × 10−15 C/N, three orders of the magnitude smaller than that of a single crystal of quartz.