There is a great variety of ways in which closed-circuit batteries
are made, but, as the main principles are very much alike, we will
only describe two general kinds, those with and those without a porous
cup.[2]
In the first place, we must state that closed-circuit batteries proper
usually consist of a glass jar and two elements--carbon and zinc.
Sometimes a porous cup is used; for what reason you will soon learn.
The chemicals that are used are usually different from those used in
the open-circuit batteries and are much stronger. These chemicals are
usually sulphuric acid and bichromate of potash (or chromic acid),
which are mixed with water.
We will now examine two of the types of closed-circuit batteries,
taking first the one without the porous cup, of which the Grenet is a
good example.
[Illustration: Fig. 30]
This battery, as you see, consists of a glass jar, in which are placed
two plates of carbon and one of zinc. (Fig. 30.) The latter is between
the two carbon plates and is movable up and down, so that it may be
drawn up out of the solution when it is not desired to use the battery.
When the zinc is in the solution there is a steady and continuous
current of electricity developed, which can be taken away by wires from
the connections on top of the battery.
If the zinc were left in the solution when the battery was not in use,
the acid would act upon it almost as much as though the electricity
were not being used, and thus the zinc would be eaten away and the acid
would be neutralized, so that no more action could be had when we
wanted more electricity.
Now, in the Grenet battery we can light a lamp or run a motor for
several hours continuously, but at the end of that time the solution
would become black and it would do no more work. Then we must throw out
that solution and put in fresh, and the battery will do the same work
again, and so on.
If you should only want to light your lamp or run your motor for a few
minutes, you could pull the zinc up from the solution and put it down
again when you wanted the electricity once more. The carbon element in
the battery is not consumed by the acid, although the zinc is.
[Illustration: Fig. 31]
Now you will see the use of the porous cup. We will take as an
illustration of this type an ordinary battery in which a porous cup is
used. (Fig. 31.)
Here, you will see, the carbon is placed in the porous cup, while the
zinc is outside in the glass jar. In the glass cell with the zinc
is usually used water made slightly acid, and the strong solution of
sulphuric acid and bichromate of potash (or chromic acid) is poured in
the porous cup, where the carbon is placed.
The strong solution penetrates the porous cup very slowly and gets to
the zinc, when it immediately produces a current of electricity. But
the acid does not get at the zinc so freely as it does in the battery
without a porous cup, and, consequently, neither the acid nor the zinc
is so rapidly used up.
Where porous cups are used, the batteries will give a continuous
current for a very much longer time than without them, and will,
sometimes, give many hours' work every day for several months without
requiring any change of solution.
_Polarization._--There is one other reason why a longer working time
can be had from a battery with a porous cup, and that is, in a battery
without a porous cup the action of the acid upon the zinc is so rapid
that the carbon plates become covered with gas, and, therefore, the
proper action by the acid cannot take place upon them. Thus, the
battery ceases to work, and is said to be "polarized." When a porous
cup is used, the action of the acid upon the zinc is slow enough to
give off only a small amount of gas, and thus the acid has a chance to
act upon the carbon plates and develop a steady current of electricity.