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development of opposite electrical charges on different parts of a crystal that is subjected to temperature change. First observed (1824) in quartz, pyroelectricity is exhibited only in crystallized nonconducting substances having at least one axis of symmetry that is polar (that is, having no centre of symmetry, the different crystal faces occurring on opposite ends). Portions of the crystal with the same symmetry will develop charges of like sign. Opposite temperature changes produce opposite charges at the same point; i.e., if a crystal develops a positive charge on one face during heating, it will develop a negative charge there during cooling. The charges gradually dissipate if the crystal is kept at a constant temperature.