The dry cell is not, as its name implies, "dry," but the exciting agent or electrolyte, instead of being a liquid, is a wet paste which cannot spill or run over. The top of the cell is poured full of molten pitch, thus effectively sealing it and making it possible to place the cell in any position.
Dry cells can be purchased from almost any electrical house or garage for twenty-five cents each. It will therefore hardly pay the young experimenter to make his own dry cells. For the sake of those who may care to do so, however, directions for building a simple but efficient dry cell of the type used for door-bells and ignition work, will be found below.
Fig. 57.—A Dry Cell.
The principle of a dry cell is the same as that of a Leclanche cell of the disk type. The exciting solution is ammonium chloride, the electrodes or elements are zinc and carbon, and the carbon is surrounded by manganese dioxide as a depolarizing agent.
Obtain some sheet zinc from a plumbing shop or a hardware store and cut out as many rectangles, 8 x 6 inches, as it is desired to make cells. Also cut out an equal number of circles 2 3/8 inches in diameter.
Roll the sheets up into cylinders 2 3/8 inches in diameter inside and 6 inches long. The edges are lapped and soldered. Fit one of the round circles in one end of each of the cylinders and solder them securely into place, taking care to close up all seams or joints which might permit the electrolyte to escape or evaporate.
Secure some old carbon rods or plates by breaking open some old dry cells. The carbons will be in the form of a flat plate, a round rod, or a star-shaped corrugated rod, depending upon the manufacture of the cell. Any of these types of carbons will serve the purpose well, provided that they are fitted with a thumb-screw or a small bolt and nut at the top so as to make wire connections with the carbon.
Make a wooden plunger of the same shape as the carbon which you may select, but make it slightly larger. Smooth it with sandpaper and give it a coat of shellac to prevent it from absorbing moisture.
This wooden plunger is temporarily inserted in the center of one of the zinc cups and supported so that it will be about one-half inch above the bottom.