Fig. 2.—Copper spiral surrounded by broken coke in rag or flannel bag.
The next operation is to twist an 8-inch piece of copper wire for about 6 inches of its length round a pencil, thus forming a spiral (X), round which a flannel bag (A) filled with small coke (B) is tied ([Fig. 2]). At least two thicknesses of flannel are advantageous, or if this be found rather expensive, flannel and rag combined, or odd pieces of rag alone may be utilized.
The chief considerations are to construct a porous wall of appreciable thickness round the coke, and to avoid colored rags if possible. The bag is to stand upright in the middle of the jar, leaving about 1⁄2-inch space all round to be packed with zinc scrap, which for convenience may well be “granulated.” To make this, melt up as much waste zinc as can be collected in a ladle and pour it in a thin stream into a large bowl of cold water, moving the ladle over the surface of the water meanwhile, in order to cool the zinc stream as suddenly as possible. The zinc which forms in a heap at the bottom of the bowl should be breakable into very small pieces, and is termed “granulated” ([Fig. 3]).
Fig. 3.—Granulating zinc.
Fig. 4.—Section of the complete cell.
When the cell has thus been assembled with curved tin sheet, bag of coke and broken zinc in place, it is nearly filled with strong salt solution, and above this, in order to prevent evaporation, a thin layer of melted tallow may well be poured. [Fig. 4] represents a section of the complete cell, A being the flannel bag containing coke; B², wire from coke; C, wire from tin; D, layer of tallow; E, level of solution; F, the jar; G, the tin; H, the zinc.
