Wire Spring Relay - Physical Description

Physical Description

A wire spring relay has two major parts, the electromagnet and the contacts. The electromagnet can have a resistance of 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, or hold the contacts in place briefly after power is removed.

A wire spring relay typically has many contacts, each plated with precious metals such as palladium. 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 majority of the wire spring relays manufactured in the 1960s had twelve fixed contacts. Each fixed contact, in the shape of a square box, is mounted on the end of a thick wire stem. A "make" contact, a "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.

Rough adjustments can be made to the fixed contacts as a whole by inserting a screwdriver blade into a slot in the front support structure and twisting as appropriate. This usually suffices to cause the contacts to make and break when they should. In some cases a special tool, known as a "spring bender" to adjust the bend of the springs may be applied to adjust individual fixed or moving contacts.