The paper describes the principle of operation of an arrangement of conductors that is closely coupled to a magnetic film and can act as a signal invertor, modulator or isolator. The transmission characteristic is determined by the currents flowing in control conductors, which alter the film permeability. The net coupling between sections of the input and output conductors can be either positive, negative or zero, depending on the film permeability. The characteristics of the device in practice make it suitable for use as an integral part of film logic circuits.