Magnetic Design of Relays

The complex switching equipment which handles the telephone traffic in automatic central offices is built up of simple component elements, of which the great majority are telephone relaj'S. The large investment in these relays, of which tens of millions are made each year, has led to intensive effort to construct and use them as cheaply as possible, so that they will perform their function at a minimum over-all cost to the telephone system and thus to the subscriber. As the costs of use vary with the efficiency and speed, maximum economy requires the solution of technical problems in magnet design as well as the related problems of mechanical design for economy in manufacture. As a result, the technology of magnet design is under constant study at Bell Telephone Laboratories, directed to increased understanding of magnet performance 23 04 T H E BELL SYSTEM TECHNICAL J O U R N A L , JANUARY J 9 5 4 and to its improvement. This article gives the background of this technology as it applies to the dc telephone relay. A telephone relay is an electromagnetic switch which actuates metallic contacts in response to signals applied to its coil. Its characteristics as a component of switching circuits are defined primarily by (1) the contact assembly, (2) the coil resistance, (3) the current flow requirements for operation, holding, and release, and (4) the operate and release times. The design of the contact assembly determines the number and nature of the contacts, the sequence of their operation, their current carrying capacity, and their reliability and life.