Relays are among the oldest and most heavily used electronic components in the world. The most common design for a relay is that of an electromagnetic switch that can change its open or closed state with the application of a current to a coil.
Relays are commonly used to isolate circuits, particularly in cases when one of the circuits is a control circuit and the other one is the controlled circuit.
The long history of relays extends all the way back to the days of telegraphs, when they were used in a repeater function. They continue to be utilized by the telephone industry in exchange facilities. Later on, relays found a place in the very first computers, where they were utilized as a means to perform logical operations.
Relays are still common today, being used on everything from household appliances to the largest industrial equipment in the world. Their flexibility as electronic components makes them consistently useful. Most relays are mechanical, though there are ones that are completely solid-state, that have no moving parts involved in their operation, and that allow a very compact design to be achieved in some devices.
With the addition of solid-state relays to the available tools, the usefulness of relays has only expanded. Today, these parts are so common that they can oftentimes be harvested from old electronic devices and reused on other electronic devices, as a great many basic circuits call for them to be installed for safety or functionality purposes