Reed switch with coil. The reed switch, inside the coil of wire, is an electrical component that is placed in a circuit to allow the circuit to be switched on and off by completing or breaking the circuit. The switch is activated by magnetism, here by an electromagnetic coil. The electricity that flows through the coil magnetizes whatever is inside it. In this case, it magnetizes two metal contacts inside the switch. These contacts attract each other, bend, and touch. This completes the circuit. When the electromagnet is switched off, the magnetism is lost, and the contacts spring apart. For internal details, see T355/242.