We then attach a wire to the zinc and one to the carbon in the third cell, and we then can obtain from these two wires _only 2 volts_, but 30 ampères. There are, again, many ways of connecting several of these sets together, but it is not intended in this book to go into these at length, for the reason that we only set out to give a simple explanation of the first principles of this subject. We shall therefore only give an illustration of one more method of connecting batteries which will be easy to understand. This is called MULTIPLE SERIES The sketch we have last given shows three batteries connected in multiple. These we will call set No. 1. Now, suppose we take three more batteries exactly similar and connect them together just in the same manner. Let us call this set No. 2. Now take the wire leading from the carbon of set No. 2 and connect it with the wire leading from the zinc of set No. 1. Then take a wire leading from the zinc of set No. 2, and a wire leading from the carbon of set No. 1, and connect them with the lamps or motors. These two sets being connected in multiple series, we shall get 4 volts and 30 ampères. 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.