Fasten a piece of copper wire from the carbon of the first cell to the zinc of the second. If you have three cells, fasten another piece of wire from the carbon of the second cell to the zinc of the third, as shown in Figure 111. Fasten one end of a copper wire to the zinc of the first cell and the other end of this wire to one binding post of an electric bell. Fasten one end of another piece of copper wire to the carbon of the third cell, if you have three, and touch the other end of this wire to the free binding post of the electric bell. If you have everything connected rightly, the bell should ring. [Illustration: FIG. 111. A wet battery of three cells connected to ring a bell.] DIFFERENT KINDS OF BATTERIES. There are many different kinds of batteries. The one you have just made is a simple one frequently used for doorbells. Other batteries are more complicated. Some are made with copper and zinc in a solution of copper sulfate; some, even, are made by letting electricity from a dynamo run through a solution from one lead plate to another until a chemical substance is stored on one of them; then, when the two lead plates are connected by a wire, the electrons run from one to the other. This kind of battery is called a _storage battery_, and it is much used in submarines and automobiles. [Illustration: FIG. 112. A battery of three dry cells.] But all the different batteries work on the same general principle: A chemical solution divides into two parts, one with many electrons and the other with a less number. One part of the solution gathers on one pole (piece of metal in the solution) and charges it positively; the other part gathers on the other pole and charges it negatively. Then the electricity flows from one pole to the other. POSITIVE AND NEGATIVE POLES. Before people knew anything about electrons, they knew that electricity flowed from one pole of a battery to the other. But they always said that it flowed from the carbon to the zinc; and they called the carbon the positive pole and the zinc the negative. Although we now know that the electrons flow from the zinc to the carbon, it is much more convenient to use the old way of speaking, as was explained on page 199. Practically, it makes no difference which way the electrons are going as long as a current of electricity is flowing through the wire from one pole of the battery to the other pole. So every one speaks of electricity as flowing from the positive pole of a battery (usually the carbon or copper) to the negative pole (usually the zinc), although the electrons actually move in the other direction. [Illustration: FIG. 113. A storage battery.]