so that finally we have the electrodes and
electrolyte brought back to their original composition and condition,
we have the cell just as it was before we used it for the production
of an electrical pressure. The cell can now again be used as a source
of electricity as long as the electrolyte acts upon the electrodes, or
until it is "discharged" and incapable of any further production of
electrical pressure. Sending a current through a discharged cell, so
as to reverse the chemical actions which brought about the discharged
conditions, is called "charging" the cell.

[Fig. 2 A complete cell; Negative group; Positive group]

Cells in which an electrical pressure is produced as soon as the
electrodes are immersed in the electrolyte are called it "primary"
Cells. In these cells it is often impossible, and always
unsatisfactory to reverse the chemical action as explained above.
Cells whose chemical actions are reversible are called "storage" or
"secondary" cells. In the "storage" cells used today, a current must
first be sent through the cell in order to cause the chemical changes
which result in putting the electrodes and electrolyte, in such a
condition that they will be capable of producing an electrical
pressure when the chemical changes caused by the current are complete.
The cell now possesses all the characteristics of a primary cell, and
may be used as a source of electricity until "discharged." It may then
be "charged" again, and so on, the chemical action in one case causing
a flow of current, and a reversed flow of current causing reversed
chemical actions.

We see from the above that the "storage" battery does not "store"
electricity at all, but changes chemical into electrical energy when
"discharging," and changes electrical into chemical energy when
"charging," the two actions being entirely reversible. The idea of
"storing" electricity comes from the fact that if we send a current of
electricity through the cell for a certain length of time, we can at a
later time draw a current from the cell for almost the same length of
time.

[Fig. 3 Complete Element]

Fig. 3. A complete element, consisting of a positive and negative
group of plates and separators ready for placing in the hard rubber
jars.

Three things are therefore required in a storage cell, the liquid or
"electrolyte" and two unlike substances or electrodes, through which a
current of electricity can pass and which are acted upon b