EL.3 - PIEZOELECTRIC DEMONSTRATOR

When stress is exerted on a piezoelectric crystal, a voltage is generated. There are two models of piezoelectric demonstrators in the lab. In the first one, electrodes link the crystal to an adjustable spark gap. A metal lever-actuated cam lets one aplly the stress to the mounted piezoelectric cell. The cell is made of barium titanate lead zirconate. A spark is generated by pushing the lever. Up to 6000 V can be generated. Releasing it to resume its original form, gives another spark of reverse polarity. The other one consists of a thinly cut and polished crystal attached to a mounted brass disk with two attached leads. A small neon lamp serves as an indicator of electrical flow. Press the crystal against the table top and observe the lamp (direct piezoelectric effect). The movement of the crystal produces about 60 volts of electricity. Touch the clips together and press down on the crystal. Separate the leads before you relieve the pressure on the disk. Release the disk and touch the leads together and listen for the "snap". When the electricity is allowed to flow it produces mechanical movement in the crystal.

The Piezoelectric Principle is used in gas ignitiion systems and phonograph pickups and cyrtal microphones on a smaller magnitude. Conversely, crystals are caused to oscillate by applying voltages to their faces. This is the way transmitter frequencies are controlled. Glow lamps and discharge tubes can be lighted across the terminals. Several kinds of crystals, including tourmaline, Rochelle salt and quartz are piezoelectric. All are single crystals which lack a center of crystal symmetry.