We shall now describe an entirely different type of storage battery,
which contains neither lead nor acid. It is one of the many inventions
of Thomas A. Edison.
In the alkaline storage battery the gas called oxygen plays a very
important part, and we will try to make it clear to you what this part
is.
You are well aware of the fact that if you leave your pocket-knife
out in the air it will get rusty. The reason for this is that iron or
steel quickly tends to combine with the oxygen of the air, and this
combination of oxygen and iron is rust, otherwise called oxide of iron,
or iron oxide.
This iron oxide, or rust, is therefore the result of a chemical action
between the iron and the oxygen.
Now as all chemical actions require the expenditure of energy, there
has been developed either heat or electricity in the process. The
oxygen may be taken away from the iron oxide, chemically; but here
again would be another chemical action which would require energy to be
once more expended.
Iron oxide may be made chemically in many different ways. It is
frequently made in the form of a powder. Therefore, we do not have to
depend upon iron rust for a supply of this material.
Before going further we must consider another oxide--namely, nickel
oxide. It is characteristic of nickel that when it is combined with
oxygen to a certain degree so as to form the compound known as nickel
oxide, it will receive still more oxygen.
Now, if under proper conditions we compel iron oxide to give up its
oxygen to some other kind of chemical compound, such as nickel oxide,
we must expend energy. But, on the other hand, if this nickel oxide
gives back the oxygen to the iron--which it will do if opportunity is
given--there is energy produced again in receiving the oxygen. In other
words, the energy previously expended, or part of it, is now returned.
This action and reaction are practically those that take place in the
Edison alkaline storage battery. For simplicity of illustration we
will consider a cell containing only two plates, one positive and one
negative.
The negative plate is made up of a number of small, flat, perforated
pockets containing iron oxide in the form of a fine powder. The
positive plate is made up of small, perforated tubes containing
nickel oxide mixed with very thin flakes of metallic nickel. (Fig. 37
illustrates these plates, the positive being in front.)
[Illustration: Fig. 37]
These two elements, positive and negative, having wires or conductors
attached, are placed in a nickeled-steel can containing the
electrolyte, which consists of a potash solution. You will see that
this differs from a lead storage battery, in which the electrolyte is
sulphuric acid and water. If we were to put this acid solution into a
metallic can (except one made of lead) the can would not last long, as
the acid would quickly eat holes through it.
Now let us see what takes place in the Edison alkaline storage
battery. If an electric current from a dynamo or other source of
electricity is caused to pass through the positive to the negative
plate the oxygen present in the iron oxide passes to and remains with
the nickel oxide. During all the time this is going on the battery is
said to be "charging," and when all the oxygen has been removed from
the iron oxide and is taken up by the nickel oxide, then the battery
is said to be "charged," and the flow of current into the battery is
stopped.
A change has now taken place. The powder in the negative plate is no
longer iron oxide, but has been reduced to metallic iron, because the
oxygen has been removed. The powder in the positive plate is now raised
to a higher or super oxide of nickel, because it has taken the oxygen
that was in the iron.
But the nickel oxide will readily give up its excess of oxygen, and the
iron will receive it back freely if permitted. If the proper conditions
are established, this transfer of oxygen will take place, but the iron
cannot receive it without delivering energy.
[Illustration: Fig. 38]
The proper conditions are established by providing a conducting
circuit between the two elements, in which lamps, motors, or other
electrical devices are placed. As soon as this circuit is provided, the
opportunity is given to the iron to receive the oxygen. This it does,
and in so doing develops electrical energy.
This energy is in the form of electric current which is then delivered
by the battery on what is called the "discharge," and this current may
be used for lighting lamps or for operating motors or other electrical
devices.
The battery is said to be discharging as long as the iron is receiving
oxygen from the nickel oxide. As soon as it becomes iron oxide once
more, the giving out of energy ceases and the battery is said to be
"discharged," and must again be charged to obtain further work from it.
Such a battery can be charged and discharged an indefinite number of
times.
This type of battery is very rugged, and its combinations are not
self-destructive. It is very simple, as it provides chiefly for the
movement of the oxygen back and forth; besides, it gives much more
current for its weight than the lead type of storage battery. (Fig. 38
shows the plates of a standard Edison cell removed from container.)