the previous case, and the electromotive force is

the same.

The Dry Cell, Fig. 15, is a convenient modification of the Leclanché cell and is widely used in telephone work. The outer case z is of zinc and forms the positive plate, connection with which is

made at the binding screw a. Within this is packed the negative carbon plate c in a mixture usually composed of sal-ammoniac, chloride of zinc, dioxide of manganese, plaster, flour, and water. The top of the cell is sealed with bitumen. Although the mixture is not dry, it is non-spillable,

and in this respect is preferable to cells employing corrosive chemicals. The maintenance cost is practically nothing, and the first cost is low in comparison with other cells. On the other hand, the dry cell has a greater internal resistance than the wet cell, and its useful life is much shorter. The internal resistance is about 0.3 ohm, and the electromotive force 1.4 volts.

FIG. 15a.-Internal Arrangement of Dry Cells.

Arrangement of Parts. The interior arrangement of the regular American design of dry cell is clearly shown at Fig. 15a, which depicts a sectional view of the type commonly used in telephone service and in gas engine ignition systems. The main container is a zinc shell or can, and

is lined with several layers of blotting paper that have been saturated with the exciting solution. The space between the blotting paper and the carbon rod in the center of the cell is filled with a depolarizer, or oxygen liberator that is depended on to liberate sufficient oxygen to combine with the hydrogen evolved by the chemical action between the zinc can and the alkaline excitant. If these hydrogen bubbles were not combined with oxygen to form water, they would collect on the carbon rod and increase the resistance of the cell to such a point that it would not be capable of giving forth a current of any value. The depolarizer usually employed is black oxide of manganese and powdered carbon which is mixed with it to increase its electrical conductivity. The depolarizer is saturated with electrolyte and is tightly packed in the zinc can around the carbon rod. As these dry cells are widely used on rural circuits, it is well to notify the subscribers of the importance of keeping the receivers on the hooks when not in use and not to talk unnecessarily long, because while such a cell is suited for open circuit work it soon becomes depleted if left on a closed circuit. Dry cells have the important advantage of not requiring any maintenance and of being very low in initial cost, so they are widely used on telephone lines where the instruments receive somewhat limited use.

Storage or Secondary Batteries.-The secondary battery operates on practically the same

principles as the primary batteries, as far as current generation is concerned, but differs in an important respect in that it can be recharged when depleted by passing a current of electricity through it and without adding any new material, such as new plates or entirely changing the electrolyte. The plates of a storage battery, such as shown at Fig. 15b, are composed of lead; the negative is a spongy lead, while the positive plate is covered with lead peroxide after charging.

FIG. 15b.-Plates of Storage Battery.

The positive plate is chocolate in color, the negative is leaden gray. The electrolyte is sulphuric acid and distilled water and the potential of the lead cell is about 2 volts when charged. The plate assembly is composed of several plates in order to increase their capacity, because, while the voltage of a cell remains independent of the number of plates used, the amperage or current output is directly proportional to the relative areas of the positive and negative members. A typical set of elements for an American storage

The assem

cell is clearly shown at Fig. 15c. bly is installed in the glass container jar to form the complete cell.

These cells do not need

FIG. 15c.-Elements or Plates of Storage Battery. to be described at length, because complete descriptions and instructions for maintenance are furnished by the manufacturers of this product.

The amount of charging current, the length of the charge, and the charging rate all vary according to the size of the battery, but it is important that the charging current be always passed through the battery in the same direction and that after the charge is once started it should be continued until complete. Only direct current should be used for charging storage batteries. though alternating current can be used if passed through a rectifier before it flows through the battery. The usual method of charging a small