e. The
positively charged electrode must not be considered as merely the
carbon plate or rod alone, but rather the carbon rod with its
surrounding mixture of peroxide of manganese and broken carbon. Such
being the case, it is obvious that the separation between the
electrodes is very small, while the surface presented by both
electrodes is very large. As a result, the internal resistance of the
cell is small and the current which it will give on a short circuit is
correspondingly large. A good cell of the two and three-quarters by
six-inch size will give eighteen or twenty amperes on short-circuit,
when new.

As the action of the cell proceeds, zinc chloride and ammonia are
formed, and there being insufficient water to dissolve the ammonia,
there results the formation of double chlorides of zinc and ammonium.
These double chlorides are less soluble than the chlorides and finally
occupy the pores of the paper lining between the electrolyte and the
zinc and greatly increase the internal resistance of the cell. This
increase of resistance is further contributed to by the gradual drying
out of the cell as its age increases.

Within the last few years dry batteries have been so perfected
mechanically, chemically, and electrically that they have far greater
outputs and better recuperative power than any of the other types of
LeClanche batteries, while in point of convenience and economy,
resulting from their small size and non-breakable, non-spillable
features and low cost, they leave no room for comparison.

_Closed-Circuit Cells_. Gravity-Cell:--Coming now to the consideration
of closed-circuit or constant-current cells, the most important is the
well-known gravity, or blue-stone, cell, devised by Daniell. It is
largely used in telegraphy, and often in telephony in such cases as
require a constantly flowing current of small quantity. Such a cell is
shown in Fig. 64.

The elements of the gravity cell are electrodes of copper and zinc.
The solution in which the copper plate is immersed is primarily a
solution of copper sulphate, commonly known as blue-stone, in water.
The zinc plate after the cell is in action is immersed in a solution
of sulphate of zinc which is formed around it.

The glass jar is usually cylindrical, the standard sizes being 5
inches diameter and 7 inches deep; and also 6 inches diameter and 8
inches deep. The copper electrode is of sheet copper of the form
shown, and it is partly covered with cryst