To set up the cell, place the copper at the bottom and drop in enough of the crystals to generously cover the bottom, but do not try to imbed the metallic copper in the crystals; then fill the jar half full of clear water. In another jar dissolve two ounces of sulphate of zinc in enough water to complete the filling of the jar to within two inches of the top; then hang the zinc crow-foot on the edge of the jar so that it is immersed in the liquid and is suspended about three inches above the top of the copper strip. The wire that leads up from the copper should be insulated with a water-proof coating and well covered with paraffine. A number of these cells may be connected in series to increase the power of the current, and for a working-battery this will show a high efficiency. Note that at first the solutions will mingle. To separate them, join the two wires and start the action; then, in a few hours, a dividing line will be seen between the white, or clear, and the blue solutions, and the action of the cell will be stronger. After long-continued use it may be necessary to draw off some of the clear zinc sulphate, or top solution, and replace it with pure water. The action of the acids reduces the metallic zinc to zinc sulphate and deposits metallic copper on the thin copper strips, and in this process an electrical current is generated. A Plunge-battery When two or more cells (in which sulphuric acid, bi-chromate of potash, or other strong electropoions are employed) are coupled in series, it would be well to arrange the copper and zinc, or the zinc and carbon, poles on a board, so that all of them may be lowered together into the solutions contained in the several jars. A simple arrangement of this kind is shown in Fig. 9, where a rack is built for the jars and at the top of the end boards a projecting piece of wood, supported by a bracket, is made fast. A narrow piece of board nearly the length of the jar-rack is fitted with the battery-poles, as shown at Fig. 9 A. The carbon and zinc, or copper and zinc, poles are attached to small blocks of wood (as described for Fig. 5), and this block in turn is fastened to the under side of the board with brass screws. The poles of the cells are to be connected (as explained in Fig. 6), and when the battery is in use the poles are immersed in the solution contained in the jars. When the battery is at rest the narrow board should be lifted up and placed on the projecting arms of the rack, so that the liquid on the poles may drain into the jars directly underneath. One or more of these battery-racks may be constructed, but they cannot be made to hold conveniently more than four or six cells each; if more cells are required, those contained in each rack must be coupled up in series. [Illustration: FIG. 9] [Illustration: FIG. 10]