|Physics Dept. Demonstration Manual|
To show that a piezoelectric crystal subject to stress produces a voltage.
To show that when an alternating voltage is applied to a piezoelectric crystal the crystal vibrates in such a way that it may serve as an audio speaker.
Do NOT manually flex the piezoelectric crystal while
the crystal is connected to the function generator. The crystal can produce
voltages high enough to damage circuit components within the function
A: Manual Flexing
The neon bulb should be connected to the output terminals of the piezoelectric crystal. Hold the crystal assembly in both hands with your fingers under the lucite base and both thumbs on the brass cover of the crystal (that is, hold in an underhand hamburger grip). Gradually press down with your thumbs and note which pin of the neon bulb produces light. Gradually reduce the thumb pressure and note that the opposite pin in the neon bulb produces light.
The above process has demonstrated that flexing the crystal produces a voltage and that the polarity of the voltage changes with the direction of flexing.
Connect the function generator to the banana jack terminals of the crystal assembly. (The neon bulb can remain in place. The generator voltage is not sufficient to fire the bulb hence the presence of the bulb will have no effect.)
The recommended settings are:
Turn on the function generator and adjust it's amplitude to achieve a suitable sound level. One could also adjust the frequency dial over the audio range. The crystal's efficiency is at a maximum at about 3000 hz. One may have to adjust the function generator amplitude as other frequencies are used.