Dry-Cell Construction.
As a matter of practice, there is no really dry cell; all so-called cells contain liquid held in suspension, and their output is limited to the amount of fluid. One of this type can easily be made in the following manner: A containing jar is made up of first-quality sheet zinc, the edges being joined by a turned seam and then soldered, the bottom of zinc being also soldered in. In soldering here, as actually in all such operations, be absolutely sure the edges of the metal are clean. The jar is partially filled with the following composition: Oxide of zinc, 1 part; sal ammoniac, 1 part; plaster of paris, 3 parts; chloride of zinc, 1 part; water, 2 parts, all by weight; or sal ammoniac, 1 part; chloride of calcium, 5 parts; calcined magnesia, 5 parts; water, 2 parts, or enough water to make a thin paste. A brass binding post is soldered to the zinc case and a carbon plate having a binding post is inserted in the centre of the cell, care being taken that it does not touch the zinc. A small disc of wood laid in the bottom of the cell will prevent contact at the bottom. Molten pitch or a composition of pitch and rosin in the proportion of 6 to 1 is poured on top, so as to seal the cell. As gas is generated in the cell, a safety valve should be provided, either a piece of porous cane or a short length of hard rubber tube, inside of which have been placed a few strands of woollen thread. This class of cell is so cheap and so many forms are available for choice that it is rarely desirable to make one's own. They will not do for steady current, but only for intermittent work. The large sizes being of low internal resistance, can be used for signalling in wireless telegraphy, where it is not possible to use wet (or free fluid) cells. The principal dry cells on the market are the Mesco, the O. K., the Nungesser, and the Samson semi-dry cell.
CHAPTER XI.
STORAGE OR SECONDARY CELL.
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.
Fig. 62. Fig. 63.
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.
In modern commercial cells this operation is no longer pursued, the plates are variously constructed of lead frameworks holding plugs of litharge or lead oxide, which is "formed" with great facility. For many purposes other than operating Ruhmkorff coils, a few simple cells made, as described, are handy to have and easy to make. In sealing the cells up for portability, care must always be taken to leave a small hole in the cover for the escape of the sulphurous acid gas.