Voltaic pile
Note how the discs follow a pattern: first a disc made from one of the metals, then a cardboard or leather disc, then a disc made from the other kind of metal. Then the same kinds of discs, in the same order, again and again. In the picture, you can see how one "set" of zinc disc, cardboard disc and copper disc has been "pushed" a little to the right. Such a "set" is called a cell: the voltaic pile in the picture has 6 cells. The two different metal discs in a cell will each have a voltage across it; about 1.1 volts. Stacking cells in a pile adds to the voltage of the cells, and from the discs at the top and bottom of the voltaic pile in the picture, there is a voltage of 6 × 1.1 volts = 6.6 volts.