Another ignition source is the piezoelectric effect, though this also
has many other important applications. Piezoelectricity was discovered
in 1880 by Pierre and Paul-Jacques Curie, they found that when they compressed
certain types of crystals, including quartz and Rochelle salt (potassium sodium tartrate tetrahydrate), along certain axes, a voltage
was produced on the surface of the crystal. This effect is only observed
in crystals whose unit cells do not have a centre of symmetry, e.g. the
zinc blende form of ZnS. When the crystal is stressed its structure distorts,
and if the structure is asymmetrical a charge separation develops and a
potential difference is generated. The voltage may be discharged as a spark,
and the spark used as an ignition source. This form of ignition is found
in modern gas ovens and gas lighters.
Devices were also invented as a specialized and controllable ignition
source for systems, such as the spark plug in a car. The spark plug was
invented by Robert Bosch and is a device that fits into the cylinder head
of an internal-combustion engine. The spark plug is composed of two electrodes
separated by an air gap, across which current from a high-tension ignition
system discharges, to form a spark for fuel ignition. Spark-gap length
affects the energy of the spark, and the shape of the insulator affects
the temperature of operation.
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