If it is desirable that they be portable and arranged so that the acid will not easily spill, it will be necessary to seal them at the top. The sealing is accomplished by cutting a "cover" strip out of thin wood which will slip down over the lugs into the jar so that it comes about one-half an inch below the top. A small hole should be bored in the centre of the cover strip to receive a short piece of hard rubber or lead tubing, which will act as a vent and permit the gases formed during charging to escape or the electrolyte to be emptied at will. [Illustration: FIG. 44.—The Elements of a Storage Cell composed of two Negative Plates and one Positive Plate in their proper position.] The cover strip should fit into the jar tightly so that when the sealing mixture is poured in it will not run down around the plates or into the jar. The top of the battery is then poured full of a molten compound of asphaltum and pitch. No attempt should be made to seal the batteries when they contain acid. The inside of the jar should be clean and dry. After the cells are sealed and filled with electrolyte they are ready for either forming or charging, depending upon whether the empty grids were purchased and pasted by the experimenter or the plates were bought already pasted and formed. If they require forming, they must be put through the same forming process which has already been described. The finished cells when sealed will appear like those shown in Figure 45, according to the sizes of plates and jars used. [Illustration: FIG. 45.—Three different sizes of Storage Cells which may be purchased ready made or built by the experimenter out of prepared materials as explained.] Recharging and Caring for Storage Cells. Storage cells are especially affected by the usage given them. If they are mistreated they will quickly go to pieces, whereas, on the other hand if well treated they will last indefinitely. It is important that the electrolyte used in the cells should always be of the proper strength. The only accurate method of preparing the electrolyte is with the aid of a *hydrometer*. A hydrometer is an instrument for determining the specific gravity of solutions. It is a little device which looks somewhat like a thermometer. It is placed in the solution and allowed to float. There is a numbered scale along the upper part of the hydrometer and the *specific gravity* or strength of the solution is indicated on the scale by the point which is level with the surface of the solution. The normal specific gravity for a storage battery solution should be about 1.250. The strength can be increased by adding more acid and decreased by adding water.