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2 The Way the Earth Works: Plate Tectonics
3 Patterns in Nature: Minerals
4 Up From the Inferno: Magma and Igneous Rocks
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6 Change in the Solid State: Metamorphic Rocks
7 The Wrath of Vulcan: Volcanic Eruptions
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10 Deep Time: How Old is Old?
11 A Biography of Earth
12 Riches in Rock: Energy and Mineral Resources
13 Unsafe Ground: Landslides and Other Mass Movements
14 Streams and Floods: The Geology of Running Water
15 Restless Realm: Oceans and Coasts
16 A Hidden Reserve: Groundwater
17 Dry Regions: The Geology of Deserts
18 Amazing Ice: Glaciers and Ice Ages
19 Global Change in the Earth System

Keeping Time with Quartz

by Stephen Marshak
Overview Image

Quartz Crystal

Credit: Jay Schomer

Crystals of quartz have a property known as piezo­electricity. This means that when you apply pressure to a quartz crystal, silicon and oxygen ions shift and cause positive and negative charges to appear on opposite crystal faces. The reverse of this effect also occurs, so that if positive and negative charges are applied, the crystal contracts slightly. Because of this property, a quartz crystal can be made to alternately expand and contract (that is, oscillate) by passing an alternating electrical current (one in which the flow direction of electrons rapidly reverses back and forth) through it. Every crystal has a characteristic frequency, measured in oscillations per second, at which it likes to vibrate. The frequency depends on the size of the crystal, just as the frequency (pitch) of a vibrating bell depends on the size of the bell. When the frequency of the alternating electrical current applied to a crystal comes close to the crystal's characteristic frequency, the crystal "locks" the electrical current to its frequency. The regularity of this oscillation provides a basis for keeping time. Oscillating quartz crystals thus provide the heart of a quartz watch.

How is a quartz watch made? First, the watchmaker obtains tiny quartz fragments, made by fracturing larger crystals. The crystals are so small that they oscillate at frequencies of between 100,000 and 2.5 million times per second when a current from a small battery passes through them. Devices called "frequency dividers" reduce the vibration to a few beats per second, and these beats either drive gears that then turn the hands of the watch, or control the digital image that appears on the watch face.

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