Electrical and magnetic effects in crystals

Noncentrosymmetric crystals may generate measurable anisotropic electrical effects in response to external forces snch as temperatnre changes 

FIGURE 5.17. Measurement of the piezoelectric effect. A crystal is placed between metal plates and these are struck with a mallet. The changes developed as a result cause a current to flow. 

Cady in World War II realized that such a mechanical resonance of a vibrating crystal could be used in frequency control. This discovery had an important influence on radio communications.Alternating electric fields, such as those generated by the radio tubes of the time, were applied to plates of piezoelectric crystals and the expansions and contractions of the plates were caused to react on electrical circuits. If the natural frequency of the mechanical vibration of the quartz plate coincided with the frequency of oscillation of the electric circuit, resonance between the two took place and energy was acquired by the mechanical oscillators. Later. Rochelle salt and barium titanate, which are each both ferroelectric and piezoelectric, were used. ° In ferroelectric crystals, the polarization or dipole moment is reversed or reoriented upon application of an electric field. Ferroelasticity is another property displayed by some crystals in which stress can cause the interconversion between two stable orientational states. These physical properties of crystals are of great use in modern technology. 

The pyroelectric effect results from the electric charge separation resulting from the stress caused by the temperature change. A small potential difference, sometimes too small to measure, develops across 

FIGURE 5,18. The pyroelectric effect. Charges develop on opposite faces on heating a pyroelectric crystal. These changes can be located by suitable powders, as shown. 

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