If you open a standard 9 volt battery, you find these tiny cylinders inside. Why?
Standard electric cells (AA, AAA, C, and D cells) only produce 1.5 volts. To get batteries with higher voltages, they use several cells in series. For a nine volt battery, they use six 1.5 volt cells. (6 X 1.5 = 9) If you open up one of those expensive six volt lantern batteries, you will find 4 D cells inside.