Freedericksz Transition On Air

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 socalled 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 f(RC) 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 Delta f = f(RC) - f(o) become audible. This setup is very convenient for demonstration and measurements of all characteristics of the Freedericksz transition driven by magnetic or electric fields. (C) 2021 Published under an exclusive license by American Association of Physics Teachers.