Memory circuits in the form of latches can also be created by having a relay contact complete the circuit of its own coil when operated. The relay will then latch and store the state to which it was driven. With this capability, relays were used to create special purpose computers for telephone switches in the 1930s. These designs were converted, starting in the 1950s, to wire spring relays, making them faster and more reliable. The majority of wire spring relays were used in 5XB switches.
Most wire spring relays have a permalloy core, and require continuous power to maintain state. Some have a steel core, making them magnetically latching relays, similar to the ferreed and remreed types of reed relay.
Reed relays are smaller and cheaper, thus better suited to data storage. They were used in conjunction with wire spring relays, for example to store digits for sending to other crossbar switching offices. In a multi-frequency sender (the part of a switch which sends routing information about outgoing calls over trunk lines), for example, wire spring relays direct the dialed digits one at a time from reed relay packs to frequency generators, under sequential control of logic implemented with wire spring relays. At the other end, similar relays steered the incoming digits from the tone decoder to a reed relay memory. In such uses two-out-of-five codes and similar schemes checked for errors at both