Cambridge Secondary (7-9). Stage 9. MCE Science. Workbook - Unit 12: Electrical Circuits - Question 6

Use two 1.5 V electric cells in holders, two LED lamps or 3 V lamps, electrical wires and two switches (a breadboard is optional). The cells are in series; each lamp has its own switch in a separate parallel branch, as shown.

Why are the electrical cells connected in series?