Production of electricity by pressure:

Certain crystals and semiconductors produce an emf between two opposite faces when the mechanical pressure on them is either increased or decreased (the polarity of the emf is reversed when the pressure changes from an increase to a decrease).  This emf is known as the piezoelectric emf.

This effect is used in a number of devices including semi-conductor strain gauges and vibration sensors.  As the mechanical pressure on the crystal is altered, a varying voltage which is related to the pressure is produced by the crystal.  The voltage can be as small as a fraction of a volt or as large as several thousand volts depending on the crystal material and the pressure. Aircraft systems employing the piezoelectric effect generally only produce very small emf's, the very high voltages produced by materials such as lead zirconate titanate are used in ignition systems for gas ovens and gas fires.