Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/47

Rh mould, where it is cast into short hexagonal prisms or pastilles. These are arranged in the bottom half of a plate mould, the top half of which is permanently fixed

to a hydraulic press ; the press is closed and molten lead is injected under a pressure of 150 H> per square inch, to fill in the spaces between them. They are thus embedded in a lead plate whose thickness is equal to their own length, their ends being flush with the face of the plate. The chlorine has now to be extracted, which is done by putting the plate between sheets of zinc in chloride of zinc solution, when the zinc takes chlorine from the lead chloride, and leaves the metallic lead in a finely-divided, porous condition in the space previously occupied by the chloride. After several washings, the plate is made a negative pole in sulphuric acid for a little while, to ensure absolute freedom from chlorine, and is then ready for use. Fig. 2 shows the negative plate in position in a cell.

The holes in the hexagonal plugs are to facilitate diffusion of acid. For the positive plates, lead grids are first cast under pressure, the mould being arranged to give a plate of lead 0‘4 inch thick, pierced by a number of circular holes about half an inch in diameter with slight countersinks on the faces. Into each of these holes is thrust a roll or rosette of lead ribbon, which has first been cut to the right breadth (equal to the thickness of plate), then ribbed or gimped, and finally coiled into a rosette. The rosettes have sufficient spring to fix themselves in. the circular holes of the lead plate, but are keyed into position by putting the plate through a hydraulic press. The plates are then put into tanks with temporary negatives, and a current is passed for a long time, whereby the rosettes become coated with a fine adherent hard crystalline peroxide. The cell can be discharged at very high rates; one square foot of plate will give a current of 40 amperes when necessary.

A second cell of the Plante type is that known as the “d.p.” Its construction is very simple (Fig. 3). A number of corrugated lead strips, about one quarter of “ D.P. ’ an inch wide, are piled to a height of 7 or 8 cell. inches, and their ends are burned together.

Thus a plate is formed through which the acid can easily pass. A number of such plates are joined together, to form the positives, the negatives being formed by pasting a somewhat similar framework with litharge. Having put the two sets of plates into the cells, a prolonged charging current is sent through; the litharge is reduced to spongy lead, and the surface of the corrugated strips which form the positive plate becomes covered with peroxide.

In the “ Monobloc ” cell, the general appearance and arrangement of which are shown in Fig. 4, the positive block is built up of corrugated sheets of lead, perforated by rectangular holes. When these perforated sheets have been laid on each other to a height of about 9 inches, and the four corners burned to stout lead fillets, a very compact block is obtained, to all parts of which acid is able easily to penetrate. The negative plate consists of a series of rods, each having a lead core on which lies spongy lead. These are burned to a lug at the top, and each rod being enclosed in a perforated ebonite sheath the whole is put into the perforations of the positive blocks. These cells have been used on the tramway cars at Ghent and other places on the Continent.

Pasted Types.—The best-known cell of this type is the e.p.s. cell, made by the Electric Power Storage Company. The paste for the positive plates is a mixture °f £ p s ceI1 red lead with sulphuric acid, and for the negative plates litharge is used instead of red lead. Different forms of grids are adopted, according to the special purpose for which the cells are intended. For ordinary isolated installations, the grid wall has a section shown in Fig. 5, the holes A, as seen from the direction of the arrow, being square and slightly larger on the negative than on the positive grid. For rapid discharges (up to two hours) the negative grid is much the same, except that the holes