To keep the light bulb lit, you would need the alligator clips (wires), the battery, and the the light bulb. When all of these parts of the circuit are connected so that the energy is flowing it is considered a complete or closed circuit. You do not necessarily need to use the switch. The purpose of the switch in this circuit, which is a series circuit, is to turn the electricity on or off. Switches are devices that make it easy to open or close an electrical circuit, therefore turning the flow of electricity on or off.