This is called connecting in multiple series, and may be extended indefinitely with any number of batteries.
We should add that one of the elements in a battery is called "positive," and the other "negative."
THE EDISON PRIMARY BATTERY
As this type of battery will work efficiently on either open or closed circuit, we have thought best to describe it separately at this place, in order not to confuse your ideas while reading about batteries generally.
The type of cell we will now describe was originated by an inventor named Lalande, and was known by that name; but it has been greatly improved and rendered more efficient by Edison, and is now manufactured and sold by him under the name of the Edison Primary Battery.
Before describing the cell itself, let us consider the action that takes place in a battery of this kind.
If certain metals are placed in a suitable solution, and are connected together, outside of the solution, by wires, vigorous chemical action will take place at the surfaces of the metals, and electrical energy will be produced. The plates must be of different metals, and the solution should be one that will dissolve neither of them except when an electric current is allowed to flow.
One of the metals is usually zinc, which is gradually eaten away or dissolved by the solution while the battery is delivering electrical energy. It is the chemical combination of the zinc and the solution that produces this energy, which leaves the zinc in the form of an electric current, and passes through the solution to the other metal, out of the cell to the wire, and thence back by another wire to the zinc, where it is once more started on its circuit.
At the surface of the other metal, which may be, and frequently is, copper, small bubbles of the gas called hydrogen are produced. This gas rises to the surface of the liquid and gradually passes off into the air. But its presence offers resistance to the passage of the current; so that generally there is associated with the copper a supply of the gas oxygen. Oxygen and hydrogen are always very eager to mix with each other, and, therefore, when the hydrogen bubbles appear they are quickly taken up by the oxygen near by. The mixture of these two gases forms water, which becomes part of the solution. All of this happens so quickly that the hydrogen cannot be perceived so long as there is any oxygen left in the copper-oxide plate.