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Relays are devices used to make or break connections in an electric circuit. When a relay switch opens or closes depends on the conditions of the circuit. Timing relays, for instance, are designed to open or close a circuit after a given period of time. They enable users to set an on and off cycle for any given electrical device, or for a particular component inside an electrical device. Other relays respond to the specific conditions inside of a given circuit.

Traditional relays react to variations in current, and will open or close a switch when the current reaches a certain voltage. These devices utilize the magnetic fields generated by electric current; in a relay, the current eventually increases the magnetic field enough to attract an armature linked to a contact. This contact shifts position accordingly, thereby opening or closing a circuit. The contact can perform operations in the same or another electric circuit. Depending on the type of relay involved, the contact may return to its original position after the current is off or after the circuit achieves another set of conditions.

Numerous types of relays are available, depending on the application. Alternating relays automatically switch between multiple devices to reduce the possibility of overuse and to minimize wear on any given machine. Overload relays will help control current in high-amp-rated motors and similar systems. Miniature relays are also manufactured for applications where size is a major concern, while solid state relay provides a reliable, durable component with no moving parts.

Of the relays available, there a wide range of designs. For example, reed relays contain a pair of open contacts inside a vacuum tube, which is normally surrounded by a coil. The contacts make the circuit when the current running through the coil provides a suitable magnetic field. Specialized reed relays included mercury-wetted contacts used for precision applications. Multiple contact relays were common in industrial functions, where they provided numerous automation possibilities, although new technology has outdated many of these devices.