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 crystals of blue-stone or copper sulphate.  Frequently, in later forms of cells, the copper electrode consists merely of a straight, thick, rectangular bar of copper laid horizontally, directly on top of the blue-stone crystals.  In all cases a rubber-insulated wire is attached by riveting to the copper electrode, and passes up through the electrolyte to form the positive terminal.

[Illustration:  Fig. 64.  Gravity Cell]

The zinc is, as a rule, of crowfoot form, as shown, whence this cell derives the commonly applied name of crowfoot cell.  This is essentially a two-fluid cell, for in its action zinc sulphate is formed, and this being lighter than copper sulphate rises to the top of the jar and surrounds the zinc.  Gravity, therefore, serves to keep the two fluids separate.

[Illustration:  INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION MATERIAL]

In the action of the cell, when the external circuit is closed, sulphuric acid is formed which attacks the zinc to form sulphate of zinc and to liberate hydrogen, which follows its tendency to attach itself to the copper plate.  But in so doing the hydrogen necessarily passes through the solution of sulphate of copper surrounding the copper plate.  The hydrogen immediately combines with the SO_{4} radical, forming therewith sulphuric acid, and liberating metallic copper.  This sulphuric acid, being lighter than the copper sulphate, rises to the surface of the zinc and attacks the zinc, thus forming more sulphate of zinc.  The metallic copper so formed is deposited on the copper plate, thereby keeping the surface bright and clean.  Since hydrogen is thus diverted from the copper plate, polarization does not ensue.