If a simple cell is put in circuit with a galvanometer, it is observed that the current gradually diminishes in strength, due to the film of hydrogen bubbles which adhere to the copper. This condition is called "polarization.° If the bubbles are brushed away, the current resumes (nearly) its former strength. It be comes necessary then to establish some mechan ical means of removing the hydrogen or to use some chemical substance in the cell which will combine with it and so remove it as fast as it forms. Mercuric chloride is sometimes used for this purpose. In the bichromate cell the oxygen of the b:chromate seizes upon the hydro gen and combines with it to form water. In the Leclanche cell the depolarizer is manganese dioxide. Another method of avoiding polariza tion is a cell construction which admits of using two separate liquids, the metal on which the hydrogen collects being placed in a solution of some chemical which combines with the hydro gen as it forms.
Another phenomenon which affects the strength of current passing through a cell and thence through the conductor which connects the two dry poles of the battery is what is called °resistance.° This is of two k:nds or divisions: internal and external. The former is that within the cell itself — the metals and the liquids; the latter in the outside conductor.
If this conductor is of some substance wh:ch has a low degree of electric conductivity, like lead; or even if of high conductivity, like copper, but is very long, or of very small cir cumference, or both, the electric current will move along it very sluggishly, as if being held back by some obstacle —a resistance. This has the effect of slowing down the chemical action in the cell, and the result is what is termed a °weak current.° With a short and comparatively large conductor of a high degree of conductivity .the external resistance is re duced to a minimum. The internal resistance of a cell is increased by polarization, as already mentioned, and this is remedied by using a depolarizer. The internal resistance may also be further reduced by giving the metallic com ponents large areas and placing them quite close together, malting the travel of the current through the electrolyte as short as possible.
Primary electric batteries are cla§sified as wet batteries and dry batteries. In the first group liquids are used as electrolytes; in the second, chemicals which retain moisture for a long time take the place of electrolytes.