The study of the properties of solid dielectrics showed, some of them are polarized not only by the electric field, but in the process of deformation under mechanical effects on them.

Dielectric polarization under mechanical impact on him is called the direct piezoelectric effect. This has the effect of quartz crystals and all ferroelectrics.

To watch, from crystal cut rectangular parallelepiped, faces which must be oriented a certain way relative to the crystal. Two of its opposite faces covered with the metallic plates A and b with taps for connection to an electrical circuit (rice. 15.26, below is a symbolic representation of a piezoelectric element).

When squeezed, the parallelepiped one face is positively charged, and the other is negative. It turns out, in this case, the density of the polarization charge of the verge is directly proportional to pressure and depends on the size of the box. If the compression is replaced by the stretching of the parallelepiped, the charges at its sides change signs on opposite.

Direct piezoelectric effect can be explained as follows. All piezoelectric crystals have no center of symmetry and consist of positive and negative ions. The positive and negative ions in a crystal to form two independent sublattices, inserted one into another. When the piezoelectric crystal squeeze, these sublattices are shifted relative to each other and one surface of the crystal is positively charged, and the other is negative. Stretching the crystal reverses the shift of the sublattices and the signs of the charges on the surfaces of the crystal to reverse.

In the technique of direct piezoelectric effect is used for the device microphones, adapters (pickups) and t. d.

The piezoelectric crystals is observed and the opposite phenomenon. If the record, cut from a piezoelectric crystal, placed in an electric field, charging metal plates, it is polarized and deformed, for example, compressed. At change of a direction of the external electric field compression plate is replaced by its stretching. Deformation of the crystals during polarization under an external electric field is called inverse piezoelectric effect.

If a plate of a piezoelectric crystal placed in alternating electric field, it will pulsate to the beat of field changes. This is based on the use of reverse piezoelectric effect for obtaining ultrasound, for the device speakers, stabilizers of the vibration frequency and t. d.