In principle relays are just electromagnets that are used to actuate a switch, this slide presents an illustration of what an actual relay looks like. The operation of a relay starts with a low voltage, which controls the electromagnet coil. When the coil is turned on, it creates a magnetic field that moves the armature. The armature will then cause the contacts to either connect or disconnect, in essence closing or opening the switch depending on the type of relay. Therefore it is possible to apply a small signal, like a small DC voltage, and a small amount of current, and energize the coil. Once that coil is energized, it can control a larger, higher current device of either AC or DC voltage.