How does a relay work?

How does a relay work? — CS, Fairfax, Virginia

A relay is an electromagnetically operated switch. It contains a coil of wire that acts as an electromagnet. Since electric currents are magnetic, this coil of wire develops north and south magnetic poles whenever current passes through it. A metal core is often placed inside the coil of wire to enhance its magnetism. Adjacent to the coil of wire is a moveable piece of iron. While iron normally appears nonmagnetic when it’s by itself, it becomes highly magnetic whenever it’s exposed to a nearby magnetic pole. The iron piece becomes magnetic as current flows through the coil and the two are attracted toward one another. As the iron piece shifts toward the coil, it moves various electric contacts that are attached to it. These contacts close some circuits while opening others. The coil remains magnetic and continues to hold the iron piece near it until current stops flowing through the coil. When the current does stop, the coil loses its magnetism and so does the iron piece. A spring in the relay then pulls the two apart and the electric contacts return to their original positions.

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