If a circuit has 2 batteries, you can connect them using a series connection. In a series connection, the negative (-) end of one battery must be connected to the positive (+) end of the other battery. For the light to go on, electricity must flow from one end of the connected batteries, through the light bulb and the switch, to the other end.
The switch breaks the circuit when it is turned off and completes it when it is turned on.
