Peter
I did my project on the piezo-electric effect of crystals. Under this effect, when pressure is applied to crystals, they produce an electrical current. This does not happen with all crystals, though. For an experiment, I tested this effect out to see if it existed and how strong it was.
My hypothesis was that I would only see an effect with Rochelle salt crystals because those are the ones I read about most. I made crystals of Rochelle salt and NaCl, and I bought strong crystals of quartz and calcite. I coated each crystal with Vaseline oil, and then I mounted them on cardboard and attached tinfoil to two ends. The tinfoil I attached to an electrometer to measure electric current. I then took a hammer and struck each crystal several times, observing on the electrometer if there was a significant current released.
The electrometer had so much static on it that, combined with the fact that I didn't use the hammer with exactly the same force every time, it was impossible to measure exactly how much current came from each crystal. I did produce a significant current, 50-100 millivolts above the static level, in each crystal but the NaCl one, though. In fact, the two store bought crystals emitted the strongest currents, but that was most likely because they were so strong that I could hit them harder.
My experiments showed that Rochelle salt, quartz, and calcite crystals all demonstrate the piezo-electric effect; they all emit a current when force is applied. This has important applications. The converse to the effect of emitting a current when struck is that when a current is applied to these crystals, they change shape. This has many applications. Crystals can be made to send and receive pulses, so they can be used in radio, microphones, sonar, and other such instruments. The effect can also be used in ignition systems, as pressure can produce a spark necessary to start an engine or a machine.