The spaces are filled with a paste of _lead oxide_. They are then "formed" by placing in a tank of acid solution and connected to a source of electric current. [Illustration: Fig. 70.—Small Storage Cells.] The plate connected to the positive wire gradually turns dark-brown in color, due to the changes in the paste, which gradually turns into _lead peroxide_. The paste in the negative plate becomes gray in color and changes into a form of metallic lead called _spongy lead_. The positive and negative plates are placed in a bundle after the forming process has been completed. They are kept apart by strips of wood or rubber called separators. The negative plates of one cell are all connected in parallel at one end of the cell. The positive plates are connected at the other end. The liquid surrounding the plates is diluted sulphuric acid. When the battery has been exhausted, it is charged by connecting a dynamo with the terminals of the battery and sending a current through it. This current reverses the chemical action, which goes on during the discharge of the battery. *A Storage Battery* furnishes the most convenient source of current for performing all sorts of electrical experiments. It is capable of giving more current for a longer period than dry cells and is not expensive, for it merely requires recharging and does not have to be thrown away each time the current is used up. The storage cell described below is made in a very simple manner and will well repay any time or expense spent in its construction. [Illustration: Fig. 71.—How to make the Plates for a Storage Cell.] The plates are cut out of a large sheet of lead, one-quarter of an inch thick. They may be made any convenient size to fit the jars which the experimenter may have at hand. We will assume that they are to be made two and seven-eighths inches wide and three and one-half inches long. They will then fit the rectangular glass storage cell which is already on the market and can be procured from dealers in electrical supplies. A long terminal or lug is left projecting from the plate as shown in Figure 71. Any number of plates may be placed in a single cell, depending of course upon the size of the glass jar. We will suppose that three will just fit the jar nicely. An odd number of plates should always be used, so that a positive plate may come between two negatives. Each cell will give two volts regardless of the number of plates. Increasing the number of plates, however, will give the cell a greater amperage capacity and make the charge last longer. Three cells (six volts) will form a convenient set for running small fan-motors, miniature lights, etc. Cut out nine plates and pile them up in sets of three with a piece of thin wood (cigar-box wood) between each pair of plates. Clamp them together in a vise and bore full of one-quarter-inch holes.