The development of the storage or secondary cell has been one of the most important electrical advances of the century. For purposes of experiment or work, where a large or steady current is required from compact and readily tended apparatus, the storage cell proves its utility. The simplest form was that used by the early experimenters, and as it is easy to make, a form of it may very well be described.
From a sheet of lead ⅛ inch thick two or more pieces are cut of the requisite size, say, 5 inches square. In making these plates, they should be cut so as to leave a strip 1 inch wide and 3 inches long, projecting from one corner, _A_ (Fig. 62), for the purpose of connection. This is for the reason that the fumes of the sulphuric acid solution would quickly corrode any wires or screws in the plates, and also to give a better connection. The number of plates cut must be an odd one, as it is general to make the two outside plates of the same polarity—viz., negative. These plates are then scored with a steel point across and across on both sides to perhaps a depth of 1∕64 of an inch. This scoring is not absolutely necessary; it somewhat hastens the formation of the plates. The plates are then laid face to face, being separated by pieces of wood, rubber, or, still better, by a piece of grooved wood, Fig. 63 having a thin piece of asbestos on each side. These grooves are to carry off the gas, and should run up and down the board, as in the figure. The wood is ⅛ of an inch thick or thereabouts, and preferably perforated with holes ¼ of an inch or larger. When laid together, a few strong rubber bands hold the plates from coming apart. To prevent lateral motion, a few rubber pins may be thrust through the plates. The alternate strips are to be connected together in two series, as in a condenser, and the complete series placed in a jar containing a mixture of seven parts of water to one of sulphuric acid. The terminal of the strips connected to the smallest number of plates is to be marked _P_ or +, for positive.
This terminal is now to be connected to a charging current (not over 1 ampere), as described in the directions for charging batteries, for eight hours, and then discharged at a rate not over 1 ampere for six hours. Then the connections are to be reversed and the cell charged backward, as it were, and discharged. This has to be repeated for a long period, perhaps a month, before the cell is in good condition; on the final charge it is to be connected positive to positive of charging source. This operation is called "forming," and the result is to change the metallic lead of the positive plate into red-brown peroxide of lead, and the lead negative plates into spongy lead.