Magnetic Latching Crossbar Switches: A New Development in Magnetic Properties of Tool Steel

Since the introduction of the dial-type telephone switching systems, switching devices such as relays and electromagnets have become the most essential and widely used of all the components in the telephone central office. Many notable improvements on these switching devices have made it possible for the telephone systems to grow and serve the increasing population of customers. The advancements on these devices have dealt largely with their sensitivity and speed of operation, contact switching capacity, service life and reliability. In contrast to these improvements, however, it appears that very little has been done to save operating power by utilizing residual magnetic energy to effectivel}' hold the electromagnets in the operated position without continuous current drain. Recently, however, a new electromagnet core design that provides 1351 J 308 TIIE BELL SYSTEM TECHNICAL JOURNAL, S E P T E M B E R 1 9 0 0 this function was developed for crossbar switch hold magnets. This utilizes a new combination of magnetic properties that have been found to exist in high carbon steel after it has undergone a suitable hardening heat-treating cycle. There are no mechanical locking features associated with this new magnetic latching hold-magnet design. The magnetic latching force developed at the termination of the short electrical operating pulse is obtained solely by the efficient use of the residual magnetic induction and coercive force properties of the new magnet core. To restore the electromagnet to its nonoperated position, it is only necessary to re-energize the magnet coil with another short pulse of lower current strength and opposite polarity.