Figure 28 shows a simple arrangement consisting of a carbon and a zinc plate mounted upon a wooden strip. The strip is used to support the plates and rests across the top of the jar so that the plates hang below in the solution. Most chemicals attack wood and for that reason it is well to dip the strip in some hot paraffin. The carbon and zinc plates are fastened on opposite sides of the wooden strip by means of a round headed screw and a washer. A wire lead should be placed under the washer on each plate. If the screw and washer are then smeared with some hot paraffin or vaseline they will be protected from corrosion. Care should be used so that the two screws employed to fasten the plates to the strip do not touch each other in the wood. If they should, the battery will be "short circuited" and the current will flow through the screws instead of the wires. Figure 29 shows an arrangement consisting of two carbon plates mounted upon a wooden strip. The zinc element consists of a rod set in a whole in the strip between the two carbon plates. It will be found that two carbon plates will form a better cell than one with only one plate or rod. The arrangement illustrated in Figure 30 shows two carbon rods and one zinc rod clamped between two wooden strips. The zinc rod is placed in the center and the carbons to either side. [Illustration: FIG. 30.—The Elements for a Simple Home-made Cell composed of two Carbon Rods and one Zinc Rod clamped between two Wooden Strips.] The wooden strips are cut away a bit at the points where they clamp the rods so as to form sort of a groove into which the rods fit without slipping or twisting. The strips are drawn together tightly at the ends by two wood screws. When more than one carbon rod or plate is used in a cell, the carbons should all be connected together so as to form a single unit. The drawing in Figure 30 shows a wire twisted around the carbons so as to connect them together but it would be a far better connection if the wire was clamped between the carbons and the wood so that it is held firmly. Four carbon rods may be utilized by following the suggestion shown by the drawing in Figure 31. This consists of a square piece of wood about 4 x 4 inches and one-half of an inch thick. [Illustration: FIG. 31.—Four Carbon Rods and one Zinc Rod arranged to form the Elements of a Cell.] A zinc rod is set in a hole in the center. Four carbon rods are set in a circle around the zinc and held in place by screws. All the carbon rods should be connected together. The wooden top not only serves to support the carbon and zinc rods, but will also act as a cover for the cell and prevent the solution from evaporating. Battery Solutions or Electrolytes.