piezoelectric effect In crystals such as quartz, mechanical stress along one axes produces electric potential difference along another axis. This phenomenon is called piezoelectric effect, and crystals showing such effects are called piezoelectric crystals. The converse effect that is electrical potential producing strain in the crystal also occurs. The piezoelectric crytals are widely used for controlling frequency of an oscillators as in clocks. Slices are cut from the crystal as shown in the fig. p3. The dimension of the slice determining the resonant frequency. The resonant frequency is a function of temperature. A zero temperature coefficient of frequency is achieved over usual temperature range, by selecting angle intermediate to the x and y cuts.