The Piezoelectric Effect
Certain crystals such as quartz are piezoelectric. That means that when they are compressed or struck, they generate an electric charge. It works the other way as well: If you run an electric current through a piezoelectric crystal, the crystal changes shape slightly. This property makes piezoelectric crystals useful in many applications.
One of the most important uses of piezoelectricity is in quartz clocks and timers. A crystal of quartz will vibrate at a certain rate, depending on its size. As the crystal vibrates back and forth, it generates electrical pulses. A quartz clock uses a small crystal cut to a precise size to keep time. A circuit called an oscillator keeps the quartz crystal vibrating by adding electricity to its pulses. The clock counts the number of pulses the quartz crystal makes and uses that as a basis for measuring seconds, minutes and hours.
Piezoelectric devices can be used both to capture sound and to produce it. Piezoelectric pickups are commonly used for folk guitars and other acoustic instruments. A piezo pickup is a strip of piezoelectric material connected to two wires. The pickup is attached to the instrument. When the instrument is played, the sound makes it vibrate. These vibrations create an electrical current in the piezo pickup, which can be recorded or amplified as sound.
A piezo speaker works in the opposite way. Electricity flows into a sheet of piezoelectric material, making it bend back and forth. This creates pressure waves in the air, which we hear as sound.
One of the most visible applications of piezoelectricity is the piezo lighter. Pretty much any lighter with a push button is powered by piezoelectricity. When you push the button, it makes a small, spring-powered hammer rise off the surface of the piezo crystal. When the hammer reaches the top, it releases and strikes the crystal as the gas is turned on. The impact creates a large voltage across the crystal, which flows into two wires. This voltage is high enough to make a spark between the wires, which ignites the gas. Piezo igniters are used on most gas furnaces and stoves now as well.