The electric battery. A battery consists of two pieces of different kinds of metal, or a metal and some carbon, in a chemical solution. If you hang a piece of zinc and a carbon, such as comes from an arc light, in some water, and then dissolve sal ammoniac in the water, you will have a battery. Some of the molecules of the sal ammoniac divide into two parts when the sal ammoniac gets into the water, and the molecules continue to divide as long as the battery is in use or until it "wears out." One part of each molecule has an unusually large number of electrons; the other part has unusually few. The parts with unusually large numbers of electrons gather around the zinc; so the zinc is negatively charged,—it has more than the ordinary number of electrons. The part of the sal ammoniac with unusually few electrons goes over to the carbon; so the carbon is positively charged,—it has fewer than the ordinary number of electrons.
Making the current flow. Now if we can make some kind of bridge between the carbon and the zinc, the electrons will flow from the place where there are many to the place where there are few. Electrons can flow through copper wire very easily. So if we fasten one end of the copper wire to the carbon and the other end to the zinc, the electrons will flow from the zinc to the carbon as long as there are more electrons on the zinc; that is, until the battery wears out. Therefore we have a steady flow of electricity through the wire. While the electricity is flowing from one pole to the other, we can make it do work.
Experiment 64. Set up two or three Samson cells. They consist of a glass jar, an open zinc cylinder, and a smaller carbon cylinder. Dissolve a little over half a cup of sal ammoniac in water and put it into the glass jar; then fill the jar with water up to the line that is marked on it. Put the carbon and zinc which are attached to the black jar cover into the jar. Be careful not to let the carbon touch the zinc. One of these cells will probably not be strong enough to ring a doorbell for you; so connect two or three together in series as follows:
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.
Fig. 111. A wet battery of three cells connected to ring a bell.
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.
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.
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.
Fig. 113. A storage battery.
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.