The operational principle of twisted nematic displays involves the dielectric anisotropy of nematics. This crucial property was discovered about a hundred years ago by Jeżewski and Kast who used a so-called resonance method in which the frequency of an LC tank circuit was set by the capacitance of a capacitor filled with a nematic liquid crystal. Jeżewski and Kast observed that the resonance frequency changed upon application of a magnetic field to the capacitor. They interpreted the corresponding change in the dielectric permittivity as being due to reorientation of molecules by the magnetic field. Here, we describe a modern, simple, and low-cost version of this experiment. Instead of the LC oscillator working with vacuum lamps, we use an op-amp RC oscillator in which a twisted nematic display plays the role of the capacitor. For the purpose of classroom demonstrations, the oscillator frequency fRC is detected by a software-defined radio operating in the double-side band mode (DSB). Upon an appropriate tuning of the reception frequency fo, even small changes of become audible. This setup is very convenient for demonstration and measurements of all characteristics of the Fréedericksz transition driven by magnetic or electric fields.
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June 2021
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June 01 2021
Fréedericksz transition on air
Juliette Plo;
Juliette Plo
ENS Paris-Saclay
, 4 avenue des Sciences, 91190 Gif sur Yvette, France
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Dihya Sadi;
Dihya Sadi
ENS Paris-Saclay
, 4 avenue des Sciences, 91190 Gif sur Yvette, France
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Elio Thellier;
Elio Thellier
ENS Paris-Saclay
, 4 avenue des Sciences, 91190 Gif sur Yvette, France
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Pawel Pieranski;
Pawel Pieranski
a)
Laboratoire de Physique des Solides, Université Paris-Saclay
, 91405 Orsay, France
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Mehdi Zeghal;
Mehdi Zeghal
Laboratoire de Physique des Solides, Université Paris-Saclay
, 91405 Orsay, France
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Patrick Judeinstein
Patrick Judeinstein
Laboratoire Léon Brillouin, CNRS-CEA, Université Paris-Saclay
, 91191 Gif-sur-Yvette, France
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a)
Electronic mail: [email protected]
Am. J. Phys. 89, 603–611 (2021)
Article history
Received:
September 18 2020
Accepted:
December 31 2020
Citation
Juliette Plo, Dihya Sadi, Elio Thellier, Pawel Pieranski, Mehdi Zeghal, Patrick Judeinstein; Fréedericksz transition on air. Am. J. Phys. 1 June 2021; 89 (6): 603–611. https://doi.org/10.1119/10.0003350
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