First—­mechanical means.  If the hydrogen bubbles be simply brushed away from the surface of the electrode the resistance and the counter polarity which they cause will be diminished.  The same result may be secured if air be blown into the solution through a tube, or if the liquid be kept agitated.  If the surface of the electrode be roughened or covered with points, the bubbles collect more freely at the points and are more quickly carried away to the surface of the liquid.  These means are, however, hardly practical except in cells for laboratory use.

Second—­chemical means.  If a highly oxidizing substance be added to the electrolyte, it will destroy the hydrogen bubbles by combining with them while they are in a nascent state, and this will prevent the increase in internal resistance and the opposing electromotive force.  Such substances are bichromate of potash, nitric acid, and chlorine, and are largely used.

Third—­electro-chemical means.  Double cells, arranged to separate the elements and liquids by means of porous partitions or by gravity, may be so arranged that solid copper is liberated instead of hydrogen at a point where the current leaves the liquid, thereby entirely obviating polarization.  This method also is largely used.

Local Action. When a simple cell stands idle, i.e., with its circuit open, small hydrogen bubbles may be noticed rising from the zinc electrode instead of from copper, as is the case where the circuit is closed.  This is due to impurities in the zinc plate, such as particles of iron, tin, arsenic, carbon, etc.  Each of these particles acts with the surrounding zinc just as might be expected of any pair of dissimilar elements opposed to each other in an electrolyte; in other words, they constitute small voltaic cells.  Local currents, therefore, are generated, circulating between the two adjacent metals, and, as a result, the zinc plate and the electrolyte are needlessly wasted and the general condition of the cell is impaired.  This is called local action.

Amalgamated Zincs. Local action might be prevented by the use of chemically pure zinc, but this, on account of its expense, cannot be employed commercially.  Local action, however, may be overcome to a great extent by amalgamating the zinc, i.e., coating it with mercury.  The iron particles or other impurities do not dissolve in the mercury, as does the zinc, but they float to the surface, whence the hydrogen bubbles which may form speedily carry them off, and, in other cases, the impurities fall to the bottom of the cell.  As the zinc in the pasty amalgam dissolves in the acid, the film of mercury unites with fresh zinc, and so always presents a clear, bright, homogeneous surface to the action of the electrolyte.

The process of amalgamating the zinc may be performed by dipping it in a solution composed of