Wire spring relay

A wire spring relay is a type of relay, that has springs made from drawn wires of nickel silver, rather than cut from flat sheet metal as in the flat-spring relay. This class of relays provided manufacturing and operating advantages over previous designs. Wire spring relays entered mass production in the early 1950s.

Wire spring relays were the most suitable relays for logic and computing functions. They were used extensively in markers, which were special purpose computers used to route calls in crossbar switch central offices.

Wire spring relays were primarily manufactured by the Western Electric Company for use in electromechanical telephone exchanges in the Bell System. The design was licensed for use around the world, and was commonplace in Japan.

Manufacturing of wire spring relays greatly declined in the late 20th century due to the introduction of digital electronic switching systems that used them in very small numbers.

A relay has two major parts, the electromagnet and the contacts. The electromagnet may have a resistance between 15 and 200 ohms, and is often designed to operate satisfactorily at a common telephony voltage, such as 24 or 48 volts.

The electromagnet can also be modified, by the insertion of metallic slugs (lumps) to create a brief delay before pulling in the contacts (slow operate), or hold the contacts in place briefly after power is removed (slow release).

A wire spring relay typically has many individual contacts. Each contact is either a fixed contact, which does not move, or is a moving contact, and is made from a short piece of wire. The contact points are made from small blocks of precious metals, such as palladium, which are spot-welded to the wire springs. The majority of the wire spring relays manufactured in the 1960s had twelve fixed contacts. A normally open (make) contact, a normally closed (break) contact, or both can be provided for each fixed contact. A moving contact consists of two wires projecting out of the base of the relay, bent slightly inwards in order to exert pressure against the armature.

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