The world is derived from the Greek world which means to squeeze or press. It is usually described as the generation of electricity as a result of mechanical pressure. "When mechanical pressure is applied to any one of these materials, the crystalline structure generates a voltage which is proportional to the pressure." On the other hand when an electric field is applied, the structure changes shape creating dimensional alterations in the material only if the lattice owns no centre of symmetry, the phenomenon being called as piezoelectricity. This property is found in certain classes of crystalline materials including natural crystals of Quartz, Rochelle salt and tourmaline plus manufactured ceramics such as barium and lead
This special crystal structure has no centre of symmetry. This unit cell has a centre of symmetry. When unstressed the centre of the mass of the negative charges at the centre as shown. There is therefore no net polarization in the unit cell and p=0 when a mechanical force is applied to the cell in which the second row of charges is a line of symmetry. The distance between the second and third; the centre of mass of the negative charges still coincides with the positive charge. Hence, there is no change in the electric. This result is generally true for all crystal that has a centre of symmetry.