15. Gravity Batteries of two more cells are needed when used on telegraph lines. You will need 1 cell to each sounder; that is, for a short line in the house with two sounders, use 2 cells. If you use a few hundred feet of wire running to a friend's house, use 3 cells. They must be joined in series; that is, the copper of one to the zinc of the other. (See diagram of complete telegraph line.) Do not use ground connections for short lines and home-made sounders; use a return wire. Do not use different kinds of cells upon the same line.
16. Storage Battery. To show the principle of storage batteries it is only necessary to use two plates of lead dipped in the battery fluid of [App. 14]. The cell may be made as in [App. 5], Fig. 5, the only difference being that both plates are of sheet-lead. It will be an advantage to make the plates rough by hammering against them a coarse file. (See explanations and experiments with this form of cell in text-book.)
Fig. 10.
17. Porous Cups for Two-fluid Cells. Fig. 10. Very good porous cups can be made from ordinary blotting-papers, the average ones measuring 9½ × 4 in. White ones should be used, so that you will not be bothered with the color coming out. Soak the edge along one end of the blotter in paraffine (Index) for about ¼ in. When this is cold, roll the blotter into the form of a cylinder that is a little over 1 in. inside diameter, and have the paraffined end on the outside. This will make 2 thicknesses of paper all around, and a little to spare. Rub a hot nail over the paraffine to melt it, and stick the end to the cylinder. By putting on a little more paraffine along the edge where the end laps over, a good solid cylinder can be made. The cylinder should be strengthened still more by dipping each end into melted paraffine for about ⅛ in. The dark stripes around the ends and down the front of the cylinder (Fig. 10) are to represent the paraffine. Cut out a bottom about ¼ in. larger all around than the cylinder. This may be paraffined to make it stiff. It should be fastened to the cylinder with paraffine. Paraffine is not acted upon or softened by water or acid, as is the case with glue.
18. Porous Cups for Two-fluid Cells. Instead of the blotters of [App. 11], you can use short lengths of mailing-tubes, which are used to protect pictures, etc., when sent by mail. If you find that the particular tube tends to unwind when soaked, you can use a little paraffine along the edges of the spiral, as suggested in [App. 11]. Bottoms can be made for the cups as before.