Subclass 1/13: Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems Regulating voltage or current (G05F 1/02 takes precedence; for electric railways B60M 3/02) wherein the variable is actually regulated by the final control device is ac (G05F 1/625 takes precedence) using ferroresonant transformers as final control devices