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On Time with Quartz

Chances are your watch says "quartz" on it. It may have been quite inexpensive, yet it keeps time much more accurately than even the most expensive spring-driven watch of a decade or more ago. What is a quartz watch, and why is it so much more accurate than the older kinds of watches?

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Each quartz watch contains a thin wafer of quartz sliced along a particular crystallographic orientation. The quartz acts as a "pacemaker" to keep the hands or the electronic display precisely on time. The property of quartz that makes this work is called piezoelectricity, which means that pressure applied to a crystal creates an electric current. Quartz is used to make gauges that measure high pressures (for instance, the water pressure on a deep-ocean submersible vessel).

The greater the pressure, the stronger the electric current. With tension (outward pulling), the electric current flows in the opposite direction. Electrons move one way in an electric circuit if the quartz crystal is under pressure and the other way if the quartz is under tension.

If you reverse the process and apply an electric current to the quartz crystal, the crystal changes shape slightly. More precisely, it expands and compresses; and it will do so extremely rapidly and with remarkable regularity. The expansions and compressions are tiny vibrations that occur at the rate of about 100,000 vibrations per second. Thus for each minute that your watch runs, the quartz wafer in it shrinks and expands 6 million times (the electricity for this is supplied by the watch's battery).

These vibrating crystals are so accurate that they are off by no more than one vibration out of 10 billion. Precision-manufactured quartz clocks used in observatories lose or gain no more than one second every ten years. Your watch is probably not that accurate because of imperfections in the mechanical parts or the electronic circuitry. But even if your watch is a few seconds off each month, it is a vast improvement over the windup watches, even the finest of which are now obsolete.

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