The most common control device used to activate or deactivate electrical equipment is a
relay. A relay is a remotely actuated switch, and it is usually switched on/off by switching
power to the relay coil. The coil, when activated, sets up an electromagnetic field that
pulls the switch closed. A relay’s switch points are usually called ‘contacts’ or ‘poles,’ and
it is quite common to provide more than one set of contacts. For example, to switch 240
VAC circuits, a 2-contact relay (commonly called a double-pole relay) is necessary.