These are very useful but expensive electrical switches that can control very large amounts of electrical energy. A relay consists of a switch that is operated by an electrical magnet. There are several types, including normally-open (NO) and normally-closed (NC). In an NO relay, when the magnet is engaged, the metal parts of the switch are pulled together. When the magnet is off the metal parts of the switch are pushed apart. The action is the opposite way around for an NC relay. This is an example of a switch containing moving parts, compared to newer technology solid state devices.