The following article on making pocket, storage batteries or accumulators, is in reply to a correspondent who has asked for information on the subject.
Pocket accumulators are small cells measuring 4 in. from top to bottom, 3| in. from side to side, and 1 3/4 in. thick. The cases are made of sheet gutta-percha 1/2 in. thick, or of sheet ebonite No. 14 gauge. They may be rectangular in shape with sharp corners, or curved to fit the pocket with rounded corners, the two forms being shown in full and dotted lines in Fig. 1. As guttapercha is a material more easily worked than the ebonite, and the rectangular form easiest to make, this may first be tried.
First cut a wood block to the shape and size of the inside of the proposed coil, and smooth its surface. Next, get a sheet of gutta percha and cut from it a strip 9 1/2 in. long by 3 3/4 in. wide, to form the two sides. Put this over the outside of a vessel containing hot water, until the strip is soft and pliable, bend it over the woodblock to the required shape, and put both under pressure until cold and hard. Then cut two strips of the same material, 3 7/8 in. long by 1 in. wide, to form the ends of the coil, and have them quite flat and straight with true edges. Fit three into the spaces at the end, press them close to the inside block, and melt some gutta-percha well into the seams to make the joints water-tight. This can be done with a hot iron.
. If the cell is to be a 4-volt one, it must now be divided into two equal compartments by moans of a vertical partition in the center.
To make this partition, cut a strip of the gutta-percha sheet, 3 3/8 in. long and 1 in. wide, to fit exactly the inside of the cell to within 1/2 in. of the top, and then make all the seams tight with melted gutta percha. A cover, Fig. 2, must also be cut to fit in the top of the cell, and this must be perforated with four holes, two for the vent tubes A, Fig. 3, and two smaller end ones for the tangs B of the terminal.
The vent tubes are made of 1-in. lengths of ebonite tube. The top part of each tube is screwed to fit the inside of an ebonite cap C as shown in section at Fig. 3, and the top of this cap is pierced with a fine pin hole D. The lower part of the vent tube goes down into the top part of the cell, and is used to convey the acid charge into the cell while the cap is off. When the cell is charged and the cap screwed on, the pin hole in the top serves as a vent for the gas generated in the cell while working.
If 'the accumulator is to be of the curved form, it maybe constructed as follows: Having first made a wooden curved core of the required size and shape, cut a strip of gutta percha 10 1/2 in- long and 4 in. wide, scarf the two ends, warm the strip as before, bend it round the wood core to the right shape, and close the edges with a hot iron to -make a sound joint. Then put the whole under pressure to keep the shape until cold. Then cut curved pieces for the bottom and the top, with the same arrangement for vent tubes and terminals as before.
The bottom piece must be fitted in and made water tight with gutta-percha, and the partition fitted in the center in the rectangular form. If an ebonite cell is preferred, the dimensions are as for a gutta percha cell, but ebonite will need more heat than gutta-percha to render it pliable. Powdered shellac is used in making the joints water-tight, these being kept hot and under pressure to force out excess melted shellac and to make a good joint. Ebonite makes a stiffer cell which does not alter its form when worn as guttapercha does, but it is not so easily worked, and the joints present some difficulties in being made water-tight, even when melted shellac is used as a cement.
The lead plates forming the elements of the cells are of two kinds, positive and negative, the first being coated with peroxide of lead, and the latter with finely divided lead. These plates are really grids of lead, the holes being filled with the required pastes. The grids may be made by hand as follows: Procure some sheet lead 1-16 in. thick, and cut from it four strips 7 1/2 in. long by 1 1/4 in. wide. Mark off 6 1/2 in. on each of these strips, and rule the surface with 1/8-in. squares. Then punch or drill a 1/8 in. hole in each square, and counter-sink both sides of the holes.
"Now take some red lead in a saucer and make it into a stiff paste with equal parts of sulphuric acid and water, using a spatula for mixing. Lay two of the lead strips on a sheet of thick glass and cover all the holes with a layer of the lead paste well pressed in with the spatula. Then turn up 3 1/4 in. of each plate and bend over the other 3 1/4 in. to form a double plate with the paste inside. Press the two sides close together, scrape off the exuding paste, and coat the outsides with a layer of the paste, again pressing it well into the countersunk mouths of the holes. Then set the pasted plates in a warm place for fifteen hours to dry and harden. Meanwhile, make a strong solution of chloride of lime in rainwater and set aside to settle and clear. When the plates are dry and hard they will have a grayish-brown appearance and must now be immersed in the clear chloride of lime solution until the color changes from brown to puce, that is, until the sulphate of lead and red-lead have changed to peroxide of lead.
The plates must now be gently rinsed in clean water and they are then ready to put into the cells. The two other plates should be coated with a paste made of finely divided lead in water in a similar manner, but will not require the process of " forming" in chloride of lime. The finely divided lead is prepared from acetate of lead (sugar of lead) in the following manner. Make a strong solution of sugar of lead in distilled water, and in it suspend some pieces of clean zinc, which will separate the lead in small flakes. When all action ceases and all the lead is extracted, pour away the liquid and well wash the lead flakes in water. Drain off all excess water, leaving enough to form a stiff paste, which is then used to paste the negative plates, and they are placed in the coils while still wet.
It will have been noticed that 1 1/4 in. of single blank lead strip is left on each plate. These blanks are left to form the lugs or connections between the plates of adjoining cells and connections to the terminals. These connections need only be 1/4 in. wide; the remainder can be cut off, making each plate of the form shown at Fig. 1. The plates are to be put in the cells with a positive on one side and a negative opposite, as at E and F, the positive in one cell being on the same side as the negative in the next cell, to which it must be connected by soldering together their two lugs G over the partition dividing the two cells.
Before doing this, the plates should be placed together, with a thin strip of ebonite II, Fig, 5, separating each pair, a rubber band holding all firmly. Each pair can then be slipped into their compartments and connected. Then cut an inside cover, similar to the outside cover, for the cells from a thin strip of ebonite. Lay this ou top of the plates, put in the vent tubes C, Fig. 6, and fill up the space between the outer and inner covers with melted marine glue to seal the cells nearly water tight.
The top cover K must now be fixed. First note the marks on the five Iugs of the plates on each side, and make corresponding marks on the ends of the cell, to distinguish the position of the positive and negative plates inside the cell. Bring the lugs G up through narrow slits in the cover, and solder them to small thin brass nuts, through which the tangs or terminas M will pass into the small holes in the cover. Then, on top of these nuts place ebonite collars, and screw down the terminals firmly. Next press the cover down into its place, flush with the top of the cell, and secure it with melted gutta-percha round the edges. The completed arrangement is shown in the section by Fig. 6.
The cells must now be filled and charged. They are filled through the vent tubes with a solution of sul-phuric acid in water, made by adding in a thin stream 5 parts by measure of strong sulphuric acid to 31 parts of water. This mixture should be put into the cells when cold. They are charged by sending a continu-ous current of 1/2 ampere through the two cells at a pressure of 5 volts during a period of four hours. If a primary battery is used to furnish the charging current, three bichromate cells should be employed and the carbon plates connected to the positive terminal of the accumulator. It is advisable to have an ammeter in circuit with the cell while being charged, and also-to employ resistance to keep the current down to a 1/4 ampere, since injury may be wrought to the plates by a higher rate of charge. This should also be done if the current is obtained from a dynamo.
The following hints may be useful in making and working pocket accumulators: The holes in the gridsmay be of any shape. When many grids are required, it is advisable to cast them in plaster-of-Paris moulds, as the hand punch method is tedious. In charging the cells, no injury can follow prolonged charing a low low rate, but much damage can be done from a short charging at a high rate. Cells should never be run down entirely, nor put aside uncharged. They should always be fully charged before they are placed in store, and frequently recharged while in store. Jolting and shaking tend to injure the plates, so also does short-circuiting the cells, as by spanning the terminal with a short wire. The E. M. P. of each cell should be 2 volts, and if it falls below this the cells should be recharged.-"Work," London.