Relay

   A  relay is a switching device for opening and closing one or more electrical circuits on receipt of an electrical signal.
    A simpel kind of relay consists of two parts, a switch in a highvoltage circuit which is operated by a spring loaded armature and a low voltage electro-magnetic  circuit.
    The electro-magnet comprises  a soft iron core shaped like a horseshoe. Each arm of the core has a coil wound around it. The coils are connected to each other and to a battery through an on/off switch.
   When the switch is closed, the electro-magnet is energized, and the armature is attached  towards it. This closes the contacts and enables the current to flow through the high voltage circuit. When the switch is opened, a spring pulls the armature away  from the magnet, and the contacts open again.