The simplest definition for the relay is that it is an electrically operated switch. The device is used where it is necessary to control a circuit by a low-power signal or where a number of circuits have to be controlled by one signal.
Relays are controlled by electric current, magnetism or temperature. There are two types of relays – electromechanical and solid state. In the former, contacts are opened and closed using a magnetic force. With solid state relays, switching is done electronically.
Electromechanical relays are general purpose relays. They are more common because they have lower cost, and can switch from AC or DC with equal ease.
The relay was invented in 1835 by Joseph Henry. He created it in order to improve his version of the electrical telegraph.
Relays are used for many purposes. Their application can be seen in digital computers, telephone exchanges, and automation systems.
