Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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Introduction to Electricity 29
Figure 2-15.
Photoelectric cell sensor.
Neon Lamp
Figure 2-16.
Piezoelectric effect.
Squeeze a Rock Get a Volt
Hard Rubber Cushion
Barium Titanate
Hard Rubber Cushion
In 1880, the French chemists Pierre and Marie
Curie discovered the phenomenon of piezoelectricity,
which means, “electricity through pressure.”
They found that when pressure is applied to
a crystal of quartz, tourmaline, or Rochelle salt, a
Figure 2-17. Piezoelectric knock sensor. (GM Service
and Parts Operations)
voltage is generated between the faces of the
crystal. Although the effect is only temporary,
while the pressure lasts, it can be maintained by
alternating between compression and tension.
Piezoelectricity is put to practical use in phonograph
pickups and crystal microphones, where
mechanical vibrations (sound waves) are converted
into varying voltage signals. Similar applications
are used in underwater hydrophones and
piezoelectric stethoscopes.
Applying a high-frequency alternating voltage
to a crystal can create a reverse piezoelectric
effect. The crystal then produces mechanical
vibrations at the same frequency, which are called
ultrasonic sound waves because they are beyond