Résumé
The operational principle of twisted nematic displays involves the dielectric anisotropy of nematics. This crucial property was discovered about a hundred years ago by Jezewski 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. Jezewski 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 Df 1⁄4 fRC fo become audible. This setup is very convenient for demonstration and measurements of all characteristics of the Freedericksz transition driven by magnetic or electric fields