The dry cells are all of the LeClanche type, the liquid electrolyte of that type being replaced by a semi-solid substance that is capable of retaining moisture for a considerable period.

As in the ordinary wet LeClanche cell, the electrodes are of carbon and zinc, the zinc element being in the form of a cylindrical cup and forming the retaining vessel of the cell, while the carbon element is in the form of a rod or plate and occupies a central position with regard to the zinc, being held out of contact with the zinc, however, at all points.

A cross-section of an excellent form of dry cell is shown in Fig. 63.  The outer casing is of zinc, formed in the shape of a cylindrical cup, and serves not only as the retaining vessel, but as the negatively charged electrode.  The outer surface of the zinc is completely covered on its sides and bottom with heavy pasteboard so as to insulate it from bodies with which it may come in contact, and particularly from the zinc cups of other cells used in the same battery.  The positively charged electrode is a carbon rod corrugated longitudinally, as shown, in order to obtain greater surface.  This rod is held in the center of the zinc cup out of contact therewith, and the intervening space is filled with a mixture of peroxide of manganese, powdered carbon, and sal ammoniac.  Several thicknesses of blotting paper constitute a lining for the inner portion of the zinc electrode and serve to prevent the manganese mixture from coming directly into contact therewith.  The cell is sealed with pitch, which is placed on a layer of sand and sawdust mixed in about equal parts.

[Illustration:  Fig. 63.  Dry Cell]

The electrolyte in such cells varies largely as to quantities and proportions of the materials employed in various types of cells, and also varies in the method in which the elements are introduced into the container.

The following list and approximate proportions of material will serve as a fair example of the filling mixture in well-known types of cells.

Manganese dioxide             45 per cent
Carbon or graphite, or both   45 per cent
Sal ammoniac                   7 per cent
Zinc chloride                  3 per cent

Water is added to the above and a sufficient amount of mixture is taken for each cell to fill the zinc cup about seven-eighths full when the carbon is in place.  The most suitable quantity of water depends upon the original dryness and fineness of material and upon the quality of the paper lining.

In some forms of dry batteries, starch or other paste is added to improve the contact of the electrolyte with the zinc and promote a more even distribution of action throughout the electrolyte.  Mercury, too, is often added to effect amalgamation of the zinc.

As in the ordinary wet type of LeClanche cell, the purpose of the manganese is to act as a depolarizer; the carbon or graphite being added to give conductivity to the manganese and to form a large electrode surface.  It is important that the sal ammoniac, which is the active agent of the cell, should be free from lumps in order to mix properly with the manganese and carbon.