DRY BATTERIES.
The ordinary commercial dry cell is vir tually a Leclanche cell in which the electrolyte is in the form of paste instead of a liquid. It is, therefore, not accurately a dry cell but a moist cell. It is made inside of a cup or cylinder of sheet zinc which forms the anode. This zinc cylinder is lined generally with ab sorbent pulpboard or layers of blotting paper which are saturated with the electrolyte, a con ccntrated solution of sal-ammoniac. Through the centre of the cell runs a carbon bar consti tuting the cathode, and around this is packed the depolarizing paste. The composition of this paste is a trade secret, each manufacturer hav ing his own formula. It is pretty well under stood that the absolutely essential ingredients and their usual proportions are as follows : Manganese peroxide, 100 parts ; powdered coke, 80 parts ; vitrified graphite, 20 parts ; sal-am moniac, 20 parts ; zinc chloride (30° Baurne), 7 parts. Other ingredients which are known sometimes to enter the composition are glucose, dextrine, common salt, lime, arsenic, mercury bichloride, hydrochloric acid and plaster of Paris.
After the paste is firmly packed in nearly to the top, the lining is folded down upon it, a thin layer of sawdust is laid in, a snug collar of corrugated pulpboard is fitted, a layer of sand is spread on ; and upon this is melted in the asphalt cover or seal. Some makers of dry cells place next to the zinc, instead of the pulpboard lining, a prepared paste of flour, dextrine and gum tragacanth, with possibly other ingredients.
A strictly dry cell is made in the same way of dry materials without moisture. This cell is inert until it is wet, and provision is made for the wetting by making the carbon bar hollow and perforating its sides. The end is closed with a rubber cork. When the battery is wanted for use, the cork is removed and water poured in These absolutely dry batteries are made par titularly for shipping long distances across the ocean, as in the Egyptian and African trade