(C) After the needle has remained in one position, without change, for 2 or 3 minutes, take hold of the wooden cross-piece, move the plates back and forth in the acid to dislodge the hydrogen bubbles, and note carefully the action of the needle. Does the current seem stronger when the plates are moved? Can you get the needle back to the first reading? (D) Remove the plates from the acid, dry and clean the copper, let them stand in the air for 15 minutes, then take another quick reading and compare it with the first. _=278. Polarization of Cells.=_ The acid gets a little weaker, of course, as it is decomposed by the zinc (§ 270), but the chief cause of the weakening of the current is hydrogen, which forms a filmy coating upon the copper plate. This coating even seems to soak into the copper, and it takes some time for it to be thoroughly removed. The zinc plate is kept comparatively free from hydrogen by amalgamation. _=279. Effects of Polarization.=_ The hydrogen bubbles weaken the current in at least two ways. In the _first_ place, hydrogen is not a conductor of electricity; so it holds the current back, as any other resistance would. _Secondly_, acid acts upon hydrogen as it would upon another metal. When the copper plate becomes well covered with hydrogen, the acid cannot touch it; so we really have a _hydrogen plate_ in the cell. Hydrogen acts very much like zinc in the acid; we say that it is more electro-positive than copper. The result is, then, that a new current starts up, and as this is towards the zinc, in the acid, it partially destroys or neutralizes the main zinc-to-copper current. Practical use is made of the principles of polarization (see Secondary Batteries). _=280. Remedies for Polarization; Depolarizers.=_ Any scheme by which the hydrogen may be destroyed and kept from the inactive, or negative plate, will prevent polarization. _Mechanical_ means have been employed to brush away the hydrogen by keeping up a constant circulation of the liquid. _Chemical action_ is another means by which the hydrogen may be side-tracked before it gets to the - plate in single-fluid cells. Substances like nitric acid and bichromate of potash, called _depolarizers_, contain large quantities of oxygen, and, during the chemical changes that take place, this oxygen unites with the hydrogen. These substances are used in zinc-carbon cells. (See § 286, etc. for various forms of cells.) There is another form of cell, the _two-fluid_ type, in which _electro-chemical_ means are employed, and in which a metal is deposited upon the - plate instead of hydrogen. The - plate is usually copper, copper being deposited upon it. =EXPERIMENT 113. To study the "two-fluid" Galvanic cell.=