<P> Chemically, quartz is a compound called silicon dioxide . Many materials can be formed into plates that will resonate . However, quartz is also a piezoelectric material: that is, when a quartz crystal is subject to mechanical stress, such as bending, it accumulates electrical charge across some planes . In a reverse effect, if charges are placed across the crystal plane, quartz crystals will bend . Since quartz can be directly driven (to flex) by an electric signal, no additional transducer is required to use it in a resonator . Similar crystals are used in low - end phonograph cartridges: The movement of the stylus (needle) flexes a quartz crystal, which produces a small voltage, which is amplified and played through speakers . Quartz microphones are still available, though not common . </P> <P> Quartz has a further advantage in that its size does not change much as temperature fluctuates . Fused quartz is often used for laboratory equipment that must not change shape along with the temperature . A quartz plate's resonance frequency, based on its size, will not significantly rise or fall . Similarly, since its resonator does not change shape, a quartz clock will remain relatively accurate as the temperature changes . </P> <P> In the early 20th century, radio engineers sought a precise, stable source of radio frequencies and started at first with steel resonators . However, when Walter Guyton Cady found that quartz can resonate with less equipment and better temperature stability, steel resonators disappeared within a few years . Later, scientists at NIST (then the U.S. National Bureau of Standards) discovered that a crystal oscillator could be more accurate than a pendulum clock . </P> <P> The electronic circuit is an oscillator, an amplifier whose output passes through the quartz resonator . The resonator acts as an electronic filter, eliminating all but the single frequency of interest . The output of the resonator feeds back to the input of the amplifier, and the resonator assures that the oscillator "howls" with the exact frequency of interest . When the circuit starts up, even a single shot can cascade to bringing the oscillator at the desired frequency . If the amplifier is too perfect, the oscillator will not start . </P>

What is a quartz crystal in a watch