If the meter is allowed to remain connected to the cell for a short time it will be noticed that the pointer will commence to slowly drop back towards zero.
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.