In the action of cell, the salammoniac attacks the zinc in which chemical action electricity is evolved. The electricity is conducted to the carbon pole through the carbon and the salammoniac solution which in this case is the electrolite. In the dissolution of the zinc, hydrogen gas is liberated which adds to the resistance of the cell and thus reduces the current. The presence of the hydrogen is increased when the action of the cell is rapid and the decrease in current is said to be due to _polarization_. The manganese dioxide is mixed with the filler in order that the free hydrogen may combine with the oxide and thus reduce the resistance. This process is known as _depolarization_. The combination between the hydrogen and the oxide is slow and for this reason the depolarization of batteries sometimes require several hours. Dry cells are usually contained in paper cartons to prevent the surfaces from coming into contact and thus destroying their electrical action. The best cell is that which gives the greatest amount of current for the longest time. Under any condition the working value of a cell is determined by the number of amperes of current it can furnish. The current is measured by a battery tester such as Fig. 257. The + connection of the tester is placed in contact with the + pole of the cell or battery and the other connection placed on the-pole. The pointer will immediately indicate the current given out by the battery. A new dry cell will give 20 or more amperes of current for a short time but if used continuously the quantity of current will be reduced by polarizing until but a very small amount is generated. A cell that indicates less than 5 amperes should be replaced. If short-circuited, that is if the poles are connected without any intervening resistance, a large amount of current will be given but the cell will soon wear out and possibly be ruined. A cell should, therefore, never be allowed to become short-circuited. The voltage of a cell is practically continuous and should be from 1.5 to 1 volt. It is quite possible that a cell may possess its normal voltage and yet deliver little current; the voltage of a cell does not indicate its working property. In order to be assured of active cells they should be tested at the time of purchase with an ammeter. The moisture in the paste of a cell is that which forms the circuit between the zinc and the carbon elements. If the paste has dried out its resistance is increased and the cell generates little current. The voltage of such a cell may be normal while the amperage is very low. Cells in this condition may be revived by adding moisture to the paste as a temporary remedy. This may be accomplished by puncturing the can with a nail and adding water. A solution of salammoniac may be used instead of water and the cell soaked to accomplish the same purpose; this, however, is only a temporary expedient.