The phenomenon of ferroresonance

The presence of an iron-cored inductor in an a.c. circuit can in certain circumstances result in phenomena known as `ferroresonance¿, and these phenomena have been the subject of much study. Over the years, ferroresonant circuits have been used in power-system applications such as airport lighting and servomotors, and in the early 1950s were considered as a serious rival to thermionic valves for use as switching devices in early computers.