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. [Illustration: 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. The electrolyte is prepared by mixing together the following ingredients in the proportions shown: Sal Ammoniac. 1 part Zinc Chloride. 1 part Plaster of Paris. 3 parts Flour. 3/4 part Water. 2 parts [Illustration: Fig. 58.—The Different Operations involved in Making a Dry Cell.] The above paste is then firmly packed into the zinc shell around the wooden plunger, leaving a space of about 3/4 of an inch at the top. The paste can be poured in very readily when first mixed but sets and hardens after standing a short while. After it has set, withdraw the wooden plunger, thus leaving a space inside of the dry cell a little larger than the carbon. The carbon is now inserted in this hole and the surrounding space is filled with a mixture composed of: Sal Ammoniac. 1 part Zinc Chloride. 1 part Manganese Dioxide. 1 part Granulated Carbon. 1 part Flour. 1 part Plaster of Paris. 3 parts Water. 2 parts