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, anormally 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.
The moving contacts are held away from the fixed contacts by a phenolic paper pattern called a "card". By changing the depth of the cuts on this form, the contacts can be made to make or break earlier or later than others. This can be used to transfer electrical control or power from one source to another by having a "make" contact operate before the corresponding "break" contact does.
For the stored program control exchanges of the early 1970s, many relays were made with steel cores that remained magnetized after current ceased to flow in the winding. This magnetic latching feature, different from the use of slugs to delay relay operation, was used in the arrays of reed relays that switched connection paths in the early models of electronic switching systems. A miniature wire spring relay was also produced, starting in approximately 1974 as part of the 1A redesign of the 1ESS switch.