A domain-divided twisted nematic crystal cell and method of fabricating thereof. The main body has two sloping slide portions formed at two adjacent corners thereof. The hearing aid includes a microphone to receive an input signal, a speaker to reproduce the input signal, and a processor. The waveform signal controls the control signal and limits the output power of the power converter. The lubricating oil for a compression refrigerator has a high viscosity index and excellent compatibility under a natural refrigerant atmosphere such as carbon dioxide.