An electrical relay is a type of heavy-duty, remote-control switch able to handle high-current accessories, yet capable of being actuated by substantially less current. Relays install between the ...

As you can see in the schematic the 12V battery (or use other voltages) is connected with the plus at terminal T1 and minus at terminal T2. The contact 1 and 4 are connected together as 2 and 3 are ...