Piezo-electric phenomena

Page modified on: 10 April 2014


The words 'piezo-electric effect' might sound familiar, but what does it actually mean?

Briefly: when a piezo-electric device is being excited electrically this will cause a particular mechanical movement though, also the inverse is true; thus - when such a device is being forced mechanically as it moves in the way when it is electrically excited - this will cause an equivalent electrical force (vv).

Most of you will once have seen the flash-lighter used for igniting a gas-flame. This is a good example of piezo-electricity. Pushing, thus forcing (squeezing) mechanically a crystal device is resulting in an electrical spark. In electronics we operate quartz crystals as to generate a particular signal at a specific frequency. In normal circumstances, one is trying to limit the mechanical excitation of a piezo-electric device as much as possible. For two reasons: high stress is causing instability of the frequency (drift etc), but also implying the danger that the quartz crystal might destroy itself mechanically.

Somewhere in the first half of the 1990 I met Mr Alblas of Leidsendam, who told me once that he was for some time employed in the 1950 and part of the 1960s at the Dutch PTT quartz laboratory and manufacture. Then he told me that they have made for special purposes ring-quartz apparatus for demonstrating that resonating quartz crystals make real mechanical movements.

Alblas also told me, that the Dutch PTT section: CD OCO was travelling around in the country and were visiting (lower) technical schools as to attract young school boys for becoming engaged at the PTT quartz production (after their graduation).

Believe me, I was tantalised instantly!


This photo was taken in the 1950s and Mr Alblas standing just left of the window. This location was still at the so-called: Radio Laboratorium in the Kazernestraat in the Hague (just under the roof of the building)

For details of those on this picture please click at: Alblas-quartz also describing more on this subject.

Not long thereafter Hans Knap presented his book: Forschungsstelle Langeveld to the public and I was invited to join this event; which was held at Radio Kootwijk. In those days still an operational transmitting site.

During the drinks I had a small conversation with Martin Nieuwenhuizen - then being the Station Manager of Radio Kootwijk. I cannot recollect why, but it definitely happened I told Martin about the quartz apparatus which Mr Alblas had described about. Martin's mouth opened without giving any loudness. After a while he said: now I know what the device in the basement was used for!


Let use now going into details


We are viewing the piezo-electric quartz vibrator mounted inside a glass envelope

First: a quartz ring had to be made, for it one necessitate a rather large quartz block (likely natural quartz); Secondly, the ring was opened by cutting a section out of it. We may consider being it a longitudinal X-mode vibrator, where the vibration movements are entirely in the X-plane of a quartz crystal. Albeit, actually bended in a circular way. Both quartz rod-ends are marked with red colour dots as to increase visualisation of the unique phenomenon.


It proved however, that there are many around who are not familiar to the theory of the three quartz axis involved.


The axis axis x and y are linked onto the piezo-electric phenomenon, whilst the z axis is considered being the optical axis


Though, how does this interact in combination to 'our' ring quartz demonstration?


Shown briefly the ring quartz as it is photographed

With some imagination one can understand the similarity between this drawing and the fundamental forgoing one. Bearing this in mind, it is clear that our ring quartz operates in the so-called longitudinal mode. Hence, entirely along the longitudinal quartz axis.

In daily life most quartz devices used in electrical apparatus are being operated in a particular combination of X and Y mode and their vibration mode hardly will act solely in a single direction; some do bent - others operate in a thickness or, for example, in a sheer mode or that like.  



As to excite the quartz crystal we need electrical electrodes in this case consisting of gold deposited electrodes; visible in the top section of the envelope. These being placed in the centre of the quartz bar, where the mechanical damping is at its minimum


Peter Eijlander very kindly provided a frequency response chart after the films were put on the web the day previously. According his measurement, the centre frequency is 376 Hz; in the YouTube films I mention, from memory, that it is 375 Hz. We may consider that a single hertz may be neglected


The best way bringing the proof that what has been described above is obeying to the piezo-electric laws is showing it on YouTube.

For it I made several video films as to show the phenomena extensively. Some may seemingly be a doublet, but all showing it from different approaches.


Please notice, that some close-up recordings showing interferences, which I believe originate from the stroboscope frequency versus the video camera frame sequence; its automatic circuitry might not coping with the recording circumstances. An indication might be, that the orange stroboscope flashes in some films causing irregular illuminations which are only visible after replaying the video recordings.


Film 0145:    Introduction to the former Dutch PTT demonstration apparatus (1950s). CD OCO might have meant: Centrale Dienst Onderwijs Contacten. Demonstrating that the piezo-electric phenomenon implies also mechanical movements; being made visible by means of a stroboscope

Film 0146:    Viewing the spectacular mechanical movements of the piezo-electric vibrator. The tone-pitch you are listening to is being generated from the electrical vibrations of the ring-quartz. From my memory this is about 375 Hz (376 Hz). That you can see the mechanical movements is only due to the fact that we make use of the orange colour light flashes originating from the stroboscope (GR Strobotac)

Film 0147:    Viewing the piezo-electric phenomenon. Piezo-electricity implies: that an electrical excitation of a quartz or crystal device causes a mechanical response. Whilst, when this device is brought in the same mechanical stress this will causing an electrical force

Film 0148:   Explaining the general apparatus set-up where the two cables being meant for. On the right-hand side for checking an electrical parameter of the complicated quartz oscillator, on the left-hand site is the loudspeaker section. The pitch of the tone originating from the actual quartz frequency and being derived from the quartz oscillator circuitry

Film 0149:    Showing the beautiful way the piezo-electric ring-quartz is responding mechanically on the electrical excitation of the vibrating device. Its actual signal frequency (375 Hz)(376 Hz) is coming out of a loudspeaker which signal originate from the ring-quartz oscillator. The Q-factor is regarded extremely high, I guess between 1,000,0000 - 5,000,000; extreme precautions is taken as to prevent that the mechanical movements becoming too large that both red quartz ends touch together. When this happens the quartz will instantly be destroyed! It takes at least 30 minutes before the quart circuit is generating optimally.