A different proceeding obtains with another type of cell. This is called a _storage battery_, or an accumulator. In these cells, the same _plates_ and electrolyte are _used_ without change _for extended periods_, sometimes for a number of years. For this reason storage batteries have displaced many other types of cells, and they are now used (a) to operate many telephone, telegraph, and fire-alarm circuits, (b) to work the spark coils of gas and gasoline engines, (c) to help carry the "peak" load upon lighting and power circuits and (d) to furnish power for electric automobiles. Since a storage battery can deliver an electric current only after an electric current from an outside source has first been sent through it, they are often called _secondary batteries_. =288. Construction and Action of a Storage Cell.=--The common type of storage cells consists of a _number of perforated_ plates made of an alloy of lead and a little antimony. (See Figs. 265, 266, 267.) Into the perforations is pressed a paste of red lead and litharge mixed with sulphuric acid. The plates are placed in a strong solution (20 to 25 per cent.) of sulphuric acid. The plates are now ready to be charged. This is accomplished by sending a direct current from an electric generator through the cell. The hydrogen ions are moved by the current to one set of plates and change the paste to _spongy_ metallic lead. The sulphions move to the other set of plates and change the paste to lead oxide. This electrolytic action causes the two plates to become quite different chemically so that when the cell is fully charged it is like a voltaic cell, in having plates that are different chemically. It has, when fully charged, an E.M.F. of about 2.2 volts. The several plates of a cell being in parallel and close together, the cell has but small internal resistance. Consequently a large current is available. [Illustration: FIG. 265.--The positive plate of a storage cell.] [Illustration: FIG. 266.--The negative plate of a storage cell.] [Illustration: FIG. 267.--A complete storage cell.] [Illustration: FIG. 268.]