The cell is then becoming *polarized*, which is to say that small bubbles of hydrogen which are liberated by the chemical action, collect on the carbon and cause the strength of the battery to fall off. If the battery is agitated or the carbon is lifted out and scraped it will be found that the current will immediately rise again to its first strength. It would be a nuisance if it were continually necessary to scrape the carbon or shake the battery so as to avoid *polarization* and so another means is employed to secure the desired result. This is accomplished by introducing certain chemicals into the solution which will give forth *oxygen*. When oxygen and hydrogen meet under proper conditions they combine and form ordinary *water*. *Bichromate of potash* or as it is also often called *potassium bichromate* is the chemical most commonly employed for this purpose. [Illustration: FIG. 27.—Ordinary Jelly Glasses, Tumblers, Fruit Jars, etc, make good Jars for small cells by cutting off the tops.] Homemade Batteries. *The materials* required for making batteries, suitable as a source of current for the experimenter, will not be found expensive in most cases. *Carbon rods* and *plates* may be purchased from an electrical supply house but they can also be easily and cheaply obtained from old dry cells. Dry cells may be split open with a cold chisel and a hammer. Care should be exercised not to break the carbon in removing it. The round carbon rods used in arc lamps may be used for making batteries provided that if they are copper plated, the copper is first removed by immersing the rod in a bath of nitric acid. If this precaution is not taken there will be a "local action" set up between the copper and the carbon and the battery will not be as efficient as it will be if the copper is removed. Carbon rods and plates are easily drilled with an ordinary hand drill. Carbon is quite brittle and breaks easily, therefore only very light pressure should be used. While zinc rods and plates may also be purchased they are easily made by the experimenter who possesses a little ingenuity. The melting point of zinc is quite low. It can be melted in a small iron pot and cast into the form of rods or plates in plaster-of-Paris moulds. Plates may also be cut out of heavy sheet zinc. [Illustration: FIG. 28.—A Simple Home-made Cell.] Ordinary jelly-glasses, tumblers, fruit jars, etc., make good jars for small cells. The tops of fruit jars and batteries can be cut off so as to make the opening larger. The cutting can be done with an ordinary glass cutter or by filling a scratch completely around the jar or bottle, at the place it is desired to cut it off, with a three cornered file. If a hot poker or wire is then held against the scratch it will commence to crack along the line and follow the hot poker as it is drawn around. [Illustration: FIG. 29.—A Home-made Battery having two Carbon Plates with a Zinc Rod between.]