THE STORAGE BATTERY is an attempt in this last direction. The name is misleading, since even in this attempt electricity is in no sense "stored," but a chemical action producing a current takes place in the machine. The arrangement is in its infancy. Instances occur in which, under given circumstances, it is more or less efficient, and has been improved into greater efficiency. But many difficulties intervene, one of which is the great weight of the appliances used, and another, considerable cost. The term "storage battery" is now infrequently used, and the name "secondary" battery is usually substituted. The principle of its action is the decomposing of combined chemicals by the action of a current applied from a stationary generator or dynamo, and that these chemicals again unite as soon as they are allowed to do so by the completing of a circuit, _and in re-combining give off nearly as much electricity as was first used in separating them._ The action of the secondary, "storage," battery, once charged, is like that of a primary battery. The current is produced by chemical action. Two metals outside of the solution contained in a primary battery cell, but under differing physical conditions from each other, will yield a current. A piece of polished iron and a piece of rusty iron, connected by a wire, will yield a small current. Rusty lead, so to speak, so connected with bright lead, has a high electromotive force. Oxygen makes lead rusty, and hydrogen makes it bright. Oxygen and hydrogen are the two gases cast off when water is subjected to a current. (See _ante_ under _Electrolysis_) So Augustin Planté, the inventor of as much as we yet have of what is called a storage or secondary battery, suspended two plates of lead in water, and when a current of electricity was passed through it hydrogen was thrown off at one plate, making it bright, and oxygen at the other plate, peroxydizing its surface. When the current was removed the altered plates, connected by a wire, would send off a current which was in the opposite direction from the first, and this would continue until the plates were again in their original condition. This is the principle and mode of action of the storage battery. So far it has assumed many forms. Scores of modifications have been invented and patented. The leaden plates have taken a variety of forms, yet have remained leaden plates, one cleaned and the other fouled by the electrolytic action of a current, and giving off an almost equivalent current again by the return process. The arrangement endures for several repetitions of the process, but is finally expensive and always inconvenient. The secondary battery, in its infancy, as stated, presents now much the same obstacles to commercial use the galvanic, or primary, battery did before the induced current had become the servant of man. CHAPTER IV. ELECTRICAL INVENTION IN THE UNITED STATES.