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 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 f_o, 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 Fréedericksz transition driven by magnetic or electric fields.

中文翻译：

---

Fréedericksz空中过渡

扭曲向列型显示器的工作原理涉及向列型的介电各向异性。杰夫斯基和卡斯特（Jeżewskiand Kast）于一百年前发现了这一关键特性，他们使用了一种所谓的谐振方法，其中LC储能电路的频率由填充有向列液晶的电容器的电容来设定。Jeżewski和Kast观察到，在电容器上施加磁场后，谐振频率发生了变化。他们将介电常数的相应变化解释为是由于磁场使分子重新定向所致。在这里，我们描述了该实验的现代，简单和低成本版本。代替使用真空灯的LC振荡器，我们使用运算放大器RC振荡器，其中扭曲的向列显示起着电容器的作用。f _RC由在双边带模式（DSB）下运行的软件定义的无线电检测到。通过适当调整接收频率f _o，即使$\Delta F=F_{\mathrm{[R}C}-F_{Ø}$变得可听。此设置非常便于演示和测量由磁场或电场驱动的Fréedericksz跃迁的所有特性。