A prominent factor in the electrical art is the _Storage Battery_, Secondary Battery, or Accumulator, as it is variously called. A storage battery acts upon the same general principle as the ordinary galvanic or voltaic battery in giving forth electrical current as the correlated equivalent of the chemical force, but differs from it in this respect, that when the elements of a primary battery are used up, the battery is exhausted beyond repair. With the storage battery, it may be regenerated at will by simply subjecting it to an electric current from a dynamo. The dynamo stores up in this battery its electric force by converting it into chemical force, which is imprisoned in chemical compounds that are formed while the power of the dynamo is being applied. These chemical compounds are, however, in a condition of unstable chemical equilibrium, which is undisturbed so long as the poles of the storage battery are not connected, but when connected through a circuit, the instability of the chemical compounds asserts itself, and in passing back to a condition of normal equilibrium the disruption gives off the correlative equivalent of electric current stored up in it by the dynamo. Probably the earliest suggestion of a storage battery is by Ritter in 1812, in his “secondary pile.” This device consisted of alternate discs of copper and moistened card, and was capable of receiving a charge from a voltaic pile and of then producing the physical, chemical, and physiological effects obtained from the ordinary pile. The first storage battery of importance, however, was made by Gaston Planté in 1860, which consisted of leaden plates immersed in a 10 per cent. solution of sulphuric acid in water. In Fig. 64 is shown a modification of the Planté type of storage battery, composed of a series of plates shown on the left. Each of these plates is built up, as shown in detail in Fig. 65, of lead strips corrugated and arranged in layers alternately with flat strips, within perforated leaden cases. The corrugation of the leaden laminæ gives greater superficial area, and the alternation of flat and corrugated strips keeps them properly spaced, so the sulphuric acid solution may penetrate and act upon the same. Each plate section has a rod to connect it with its proper terminal. When the charging current is applied, the positive lead plate becomes covered with lead peroxide (PbO₂) and finely divided metallic lead is deposited on the negative plate. When the battery is being discharged the peroxide of lead gives up one of its atoms of oxygen to the spongy metallic lead deposited on the other plate, and both plates remain coated with lead monoxide (PbO). [Illustration: FIG. 64.--PLANTÉ STORAGE BATTERY.] [Illustration: FIG. 65.--ENLARGED DETAIL OF PLANTÉ PLATE.]