The copper oxide acts both as the positive element and as a depolarizer, for the oxygen of the oxide immediately combines with any hydrogen tending to form on the plate. This type of cell has some advantages but also many disadvantages, chief among which is the fact that the E. M. F. is very low. It is used principally for railway signal work, slot-machines, etc. *A Tomato-Can Battery* using caustic soda as the exciting liquid is a simple form of home-made battery whose only disadvantage is the low voltage that it delivers. [Illustration: Fig. 67.—A Tomato-Can Cell; Sectional View.] [Illustration: Fig. 68.—The Tomato-Can Cell Complete.] The cell is liable to polarization, but the large surface of its positive elements protects it to some extent. The positive element and the outer vessel is a tomato can. Within it is a porous cup made out of blotting paper or unglazed earthenware such as a flower pot. The space between the can and the porous cup is filled with fine scrap-iron such as borings and turnings. A zinc plate is placed in the porous cup. The cell is filled with a ten-per-cent solution of caustic soda. The following table gives the names, elements, fluids, voltage, etc., of the most useful batteries, all of which may be easily constructed by the experimenter. Secondary or Storage Batteries The storage battery is a very convenient means of taking energy at one time or place and using it at some other time or place. Small storage batteries are used in automobiles to supply current for the headlights and spark-coils. Many automobiles are now equipped with "electric starters," consisting of a dynamo-motor and a storage battery. Throwing a switch will cause the current from the storage battery to drive the motor and "crank" the engine. After the engine is started, the motor acts as a dynamo and generates a current for recharging the storage battery. Storage batteries are also used to drive electric vehicles and cars. Many central lighting and power stations employ storage batteries to supply the extra current demanded during rush hours. In the middle of the day, when the "load" is light, the surplus current of the dynamos is used to recharge the storage batteries. What is really effected in the storage battery is the electrical storage of _energy_, not the storage of electricity. Properly speaking, the energy is put into the form of chemical energy, and there is really _no more electricity in the cell_ when it is charged than after it is discharged. [Illustration: Fig. 69.—Two Methods of Connecting Cells so as to obtain Different Voltage and Amperage Values.] Storage batteries are made up of plates of lead (the electrodes) or an alloy of lead cast into a "grid" or framework. The framework may be one of a large number of patterns, but usually consists of a set of bars crossing one another at right angles, leaving a space between.