In most houses electricity is used for operating the door bell, table bell and perhaps the electric gas lighters. We have learned how stored up chemical energy is changed into heat and force in the stove and in the human body; but in the electric cell, chemical energy is changed into electrical energy.

If a strip of pure zinc be placed in a weak solution of acid, no chemical action takes place. Place in the same solution a strip of sheet copper and again no action takes place; but let the copper and the zinc be brought in contact, or connected by a copper wire, and immediately vigorous chemical action will begin at the surface of the copper plate; bubbles of hydrogen collecting there. This action is as follows: the zinc dissolves in the acid and hydrogen is set free. This hydrogen travels with an electric current set up in the liquid, passing from particle to particle through the liquid until it reaches the copper. Here the hydrogen stops, but the electric current passes up the copper plate and over the wire to the zinc and down that to the liquid and so on. This arrangement of acid and metals is called a simple voltaic cell. Fig. 27.

Other cells are arranged with different liquids and solids to gain various ends, and several cells may be united by wires between the plates to gain additional strength of current. The form of cell often employed to work electric bells is the Leclanche cell. Fig. 28. This consists of a plate of carbon (or a porous cell containing carbon), in place of. the copper, a strip or rod of zinc, and a solution of ammonium chloride which takes the place of the acid. The zinc is not affected by the ammonium chloride unless it is connected with the carbon, but when there is a circuit for the electricity, a current is generated. The common conductors of the electric current are the metals and carbons.

The zinc is gradually changed to zinc chloride, at the expense of the ammonium chloride, and after a time both the zinc and the ammonium chloride must be renewed. In renewing the battery, the jars should be cleaned out carefully and the zincs renewed if they are completely eaten through. A quarter of a pound of pure ammonium chloride (sal-ammoniac) is dissolved in enough water to about half fill a jar. When the carbon and the zinc are replaced, this will bring the liquid up to two inches from the top. The jar should not be filled too full. The wares which have been disconnected should be reconnected as before.

For bell work the cells are usually connected up 'in series," that is, the zinc of one cell is connected to the carbon of the next, the outside circuit being established between the end carbon and end zinc. Fig. 29.

If there is a short circuit anywhere in the line, that is, if the current has a chance in any way to flow from one wire to the other without going through the bell or other apparatus, the batteries are very quickly exhausted.

A modification of this cell has been made in which the spaces inside it are filled with some spongy mass in the pores of which the ammonium chloride is held. These may easily be carried about without danger of spilling solutions. They are called dry cells and when exhausted cannot readily be renewed.

Fig. 27. A Simple Voltaic Cell

Fig. 27. A Simple Voltaic Cell

Fig. 28. A Leclanche Cell

Fig. 28. A Leclanche Cell