An electrical relay is a switch that is under the control of another circuit. A classic example is the system used to start a car — when someone turns an ignition key, the ignition does not interact directly with the car battery. Instead, it activates a relay, which passes the signal on so that the car can start. There are a number of reasons for setting the system up that way.
The earliest electrical relays were developed in the 1830s, as people began to recognize that such switches could be extremely useful. Historically, they were often made with electromagnets, which continue to be used today, although for some applications, solid state relays are preferred. They key difference between electromagnetic and solid state options is that electromagnetic relays have moving parts and solid state relays do not. Electromagnets also conserve more energy than their solid state counterparts do.
One of the reasons an electrical relay is such a popular tool for electricians and engineers is that it can control electrical output that is higher than the electrical input it receives. In the example discussed above, if the ignition is connected directly to the battery, heavy duty insulated wiring would be needed to connect the steering column to the battery, and the ignition switch would also need to be much more robust. By using a relay, relatively lightweight wiring can be used, saving space and increasing vehicle safety.
A variety of circuits can be connected to this type of switch. Relays can be used as amplifiers for electrical energy, as in the car example, and they can also connect to things like alarm switches, activating when a circuit is broken to trigger an alarm. Many electrical failsafe systems use relays that turn on or off in response to things like a current overload, irregular current, and other issues which may arise. They trip to shut the system down until the problem can be addressed.
Simple electrical relay kits for people interested in electricity and how it works are available at many science stores, usually designed to be used with small batteries for safety. More advanced systems are available for people who want to work directly with wall current to build relay switches, and industrial versions designed to handle heavy duty power are available for a variety of applications, from controlling railroad signaling devices to managing the power load in a factory.