Chapter IX: The Storage Battery, and How It Works

=84. Polarization.= It has been stated that a simple cell polarizes rapidly on account of hydrogen bubbles that form upon the copper plate. They tend to send a current in the opposite direction to that of the main current, which is thereby weakened.

=85. Electromotive Force of Polarization.= It has been shown, Fig. 71, that water can be decomposed by the electric current. Hydrogen and oxygen have a strong attraction or chemical affinity for each other, or they would not unite to form water. This attraction has to be overcome before the water can be decomposed. As soon as the decomposing current ceases to flow, the gases formed try to rush together again; in fact, if the water voltameter be disconnected from the cells and connected with a galvanoscope, the presence of a current will be shown. This voltameter will give a current with an E. M. F. of nearly 1.5 volts; so it is evident that we must have a current with a higher voltage than this to decompose water. This E. M. F., due to polarization, is called the E. M. F. of polarization.

=86. Secondary or Storage Batteries=, also called _accumulators_, do not really store electricity. They must be charged by a current before they can give out any electricity. Chemical changes are produced in the storage cells by the charging current just as they are in voltameters, electroplating solutions, etc.; so it is potential chemical energy that is really stored. When the new products are allowed to go back to their original state, by joining the electrodes of the charged cell, a current is produced.

Fig. 81 shows two lead plates, A and B, immersed in dilute sulphuric acid, and connected with two ordinary cells. A strong current will pass through the liquid between A and B at first, but it will quickly become weaker, as chemical changes take place in the liquid. This may be shown by a galvanometer put in the circuit before beginning the experiment. By disconnecting the wires from the cells and joining them to the galvanometer, it will be shown that a current comes from the lead plates. This arrangement may be called a simple storage cell. Regular storage cells are charged with the current from a dynamo. (See "Study," Exp. 151.)

The first storage cells were made of plain lead plates, rolled up in such a way that they were close to each other, but did not touch. These were placed in dilute sulphuric acid. They were charged in alternate directions several times, until the lead became properly acted upon, at which time the cell would furnish a current.

A great improvement was made in 1881, by Faure, who coated the plates with red lead.

The method now generally practiced is to cast a frame of lead, with raised right-angled ribs on each side, thus forming little depressed squares, or to punch a lead plate full of holes, which squares or holes are then filled with a pasty mixture of red oxide of lead in positive plates, and with litharge in negatives. In a form called the chloride battery, instead of cementing lead oxide paste into or against a lead framing in order to obtain the necessary active material, the latter is obtained by a strictly chemical process.

Fig. 82 shows a storage cell with plates, etc., contained in a glass jar. Fig. 83 shows a cell of 41 plates, set up in a lead-lined wood tank. Fig. 84 shows three cells joined in series. Many storage cells are used in central electric light stations to help the dynamos during the "rush" hours at night. They are charged during the day when the load on the dynamos is not heavy.

Fig. 85 shows another form of storage cell containing a number of plates.

=87. The Uses of Storage Batteries= are almost numberless. The current can be used for nearly everything for which a constant current is adapted, the following being some of its applications: Carriage propulsion; electric launch propulsion; train lighting; yacht lighting; carriage lighting; bicycle lighting; miners' lamps; dental, medical, surgical, and laboratory work; phonographs; kinetoscopes; automaton pianos; sewing-machine motors; fan motors; telegraph; telephone; electric bell; electric fire-alarm; heat regulating; railroad switch and signal apparatus.

By the installing of a storage plant many natural but small sources of power may be utilized in furnishing light and power; sources which otherwise are not available, because not large enough to supply maximum demands. The force of the tides, of small water powers from irrigating ditches, and even of the wind, come under this heading.

As a regulator of pressure, in case of fluctuations in the load, the value of a storage plant is inestimable. These fluctuations of load are particularly noticeable in electric railway plants, where the demand is constantly rising and falling, sometimes jumping from almost nothing to the maximum, and _vice versa_, in a few seconds. If for no other reason than the prevention of severe strain on the engines and generators, caused by these fluctuations of demand, a storage plant will be valuable.