The first storage battery was made of two sheets of lead rolled together and kept apart by a strip of flannel. The lead strips thus separated were immersed in dilute sulphuric acid. A current from another battery was passed through this cell for a long time--first in one direction, then in the other. This roughened the surface of the lead plates, so that the battery would hold a greater charge. The battery was then charged by passing a current through it in one direction only for a considerable length of time. Feeble cells were used for charging. It took days, and sometimes weeks, to charge the first storage batteries. Then the storage battery would give out a strong current lasting for a few hours. It slowly accumulated energy while being charged, and then gave out this energy rapidly in the form of a strong electric current. For this reason the storage battery was called an "accumulator." While charging the storage cell there was formed on the negative plate a coating of soft lead, and on the positive plate a coating of dark-brown oxide of lead. It was found better to apply these coatings to the lead plates before making up the battery. Later it was found that the battery would hold a still greater charge if the plates were made in the form of "grids" (Fig. 40), and the cavities filled with the active material--the negative with spongy lead, and the positive with dark-brown lead oxide. Some excellent commercial storage batteries are made from lead plates by the action of an electric current, very much as Planté made his batteries. Fig. 41 shows one of these plates. [Illustration: FIG. 40--A STORAGE BATTERY, SHOWING THE "GRIDS"] [Illustration: FIG. 41--A STORAGE-BATTERY PLATE MADE FROM A SHEET OF LEAD] The storage battery does not store up electricity. It produces a current in exactly the same way as any other battery--by the action of the acid on the plates. When this action ceases it is no longer a battery, though it may be made one again by passing a current through it in the opposite direction from that which it gives out. In this it differs from the voltaic battery, for when such a battery is run down it can be restored only by adding new solution or new plates. The storage battery is especially useful for "sparking" in gas or gasolene motors. Edison has invented a storage battery that will do as much work as a lead battery of twice its weight. Edison's battery is intended especially for use in electric automobiles. By reducing the weight of the battery which the machine must carry the weight of the truck may also be reduced. In the Edison battery the positive plates are made of a grid of nickel-plated steel containing tubes filled with pure nickel. The negative plate consists of a nickel-plated steel grid containing an oxide of iron similar to common iron-rust.