Direct Current Power Systems 159
time. After this period of time its chemicals can no longer produce elec-
trical energy. Secondary cells can be renewed after they are used, by reacti-
vating the chemical process that is used to produce electrical energy. This
reactivation is known as charging. Both primary cells and secondary cells
have many applications. When two or more cells are connected in series,
they form a battery.CHARACTERISTICS OF PRIMARY CELLS
The operational principle of a primary cell involves the placing of
two unlike conductive materials called electrodes into a conductive solu-
tion called an electrolyte. When the chemicals that compose the cell are
brought together, their molecular structures are altered. During this al-
teration, their atoms may either gain additional electrons or lose some of
their electrons. These atoms then have either a positive or a negative elec-
trical charge, and are referred to as ions. The ionization process thus de-
velops a chemical solution capable of conducting an electrical current. A
carbon-zinc primary cell is shown in Figure 7-1.
When an external load device, such as a lamp, is connected to a cell,
a current will flow from one electrode to the other through the electro-
lyte material. Current leaves the cell through its negative electrode, flows
through the load device, then reenters the cell through its positive electrode,
as shown in Figure 7-2. Thus, a complete circuit is established between the
cell (source) and the lamp (load).
The voltage developed by a primary cell is dependent upon the elec-
trode materials and the type of electrolyte used. The familiar carbon-zinc
cell of Figure 7-1 produces approximately 1.5 volts. The negative electrode
of the cell is the zinc container, and the positive electrode is a carbon rod.
A sal ammoniac paste, which acts as the electrolyte, is placed between the
electrodes. This type of cell is usually called a dry cell.Internal Resistance of Cells
An important characteristic of a chemical cell is its internal resistance.
Since a cell conducts electrical current, its resistance depends on its cross-
sectional area, the length of its current path, the type of materials used,
and the operational temperature. The amount of current that a cell will de-
liver to a load is expressed as: