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.
The Gravity Daniell Cell.
The gravity cell, [Fig. 4], has a zinc block Z suspended from the side of the jar and a number of copper leaves C standing on edge at the bottom. A quantity of bluestone (sulphate of copper) is poured over the copper leaves and the jar filled with water.
During the working of this cell, copper is deposited on the copper plate, and sulphate of zinc formed at the zinc. To hasten the action a small quantity of zinc sulphate can be added to the solution when setting up the cell.
The name of this cell comes from the fact that the copper solution being heavier remains at the bottom of the jar. If the cell is not worked enough, all the solution will become blue and the zinc will blacken. If very dirty from this cause, remove the zinc, scrape and wash it thoroughly. Throw out all the solution, add new sulphate and water and replacing the zinc, then put the cell on short circuit by connecting the copper and zinc together for a few hours.