The Dry Cell.
The Leclanche cell being a cell with much free liquid is liable to dry up if not watched. The dry cell ([Fig. 3]) is a modern form of the Leclanche where the liquid is held by an absorbent material, such as blotting paper, or plaster.
Fig. 3
A typical dry cell[A]is shown in the figure. An outside case of zinc is lined with blotting paper dampened with chloride of zinc and sal ammoniac. A carbon rod is then inserted in the centre and packed around with carbon dust and peroxide of manganese. The latter mixture is also somewhat dampened.
[A] For full description of this class of battery see No. 3 Book on “Dry Batteries.”
Molten wax, or a suitable composition, is then poured on top of the contents of the cell to seal it up and prevent the evaporation of the fluid. A terminal on the carbon rod and another on the zinc case complete the cell.
Fig. 4
The voltage of both the Leclanche and the dry cell is about 1.45, when it goes below this it indicates that the cell is worked out.
The two cells described are known as open-circuit cells and are only intended for intermittent working.
When a current is needed for a long period at a time a closed circuit cell should be used, such as the gravity Daniell cell.