+------------------+-------------------+---------------

-----+
|  CAUSTIC POTASH  | HYDROCHLORIC ACID | POTASSIUM SULPHIDE |
+------------------+-------------------+--------------------

+
|    + Zinc        |    + Zinc         |     + Zinc         |
|      Tin         |      Cadmium      |       Copper       |
|      Cadmium     |      Tin          |       Cadmium      |
|      Antimony    |      Lead         |       Tin          |
|      Lead        |      Iron         |       Silver       |
|      Bismuth     |      Copper       |       Antimony     |
|      Iron        |      Bismuth      |       Lead         |
|      Copper      |      Nickel       |       Bismuth      |
|      Nickel      |      Silver       |       Nickel       |
|    - Silver      |    - Antimony     |     - Iron         |
+------------------+-------------------+--------------------

+

It is important to remember that in all cells, no matter what elements or what electrolyte are used, the electrode which is consumed is the one that becomes negatively charged and its terminal, therefore, becomes the negative terminal or pole, while the electrode which is not consumed is the one that becomes positively charged, and its terminal is, therefore, the positive terminal or pole of the cell.  However, because the current in the electrolyte flows from the consumed plate to the unconsumed plate, the consumed plate is called the positive plate and the unconsumed, the negative.  This is likely to become confusing, but if one remembers that the active plate is the positive plate, because it sends forth positive ions in the electrolyte, and, therefore, itself becomes negatively charged, one will have the proper basis always to determine the direction of the current flow, which is the important thing.

Polarization. If the simple cell already described have its terminals connected by a wire for some time, it will be found that the current rapidly weakens until it ceases to be manifest.  This weakening results from two causes:  first, the hydrogen gas which is liberated in the action of the cell is deposited in a layer on the copper plate, thereby covering the plate and reducing the area of contact with the liquid.  This increases the internal resistance of the cell, since hydrogen is a non-conductor.  Second, the plate so covered becomes in effect a hydrogen electrode, and hydrogen stands high as an electro-positive element.  There is, therefore, actual reduction in the electromotive force of the cell, as well as an increase in internal resistance.  This phenomenon is known as polarization, and in commercial cells means must be taken to prevent such action as far as possible.

The means by which polarization of cells is prevented or reduced in practice may be divided into three general classes: