An electrical battery is a device in which one or more chemical
substances act upon a metal and a carbon, or upon two different metals,
producing thereby a current of electricity, which will continue as long
as there is any action of the chemicals upon the metal and carbon, or
upon the two metals.
Batteries for _producing_ electricity may be divided into two classes,
called "open circuit" batteries and "closed circuit" batteries.
Open-circuit batteries are those which are used where the electricity
is _not_ required constantly without intermission--for instance, in
telephones, electric bells, burglar alarms, gas-lighting, annunciators,
etc.
Closed-circuit batteries are those which are used where the effect
produced must be continuous every moment, as, for instance, in electric
lights and motors.
The open-circuit battery is made in many different ways, so we only
describe two of the principal ones.
As we told you in an early part of this book, we do not know just what
electricity is, nor why it is produced under the conditions existing
in a battery. But we do know that by following certain processes and
making certain chemical combinations we can make as much electricity
and in such proportions as we want.
The two metals, or the metal and carbon, in a battery are called the
"elements," and to these are connected the wires which lead from the
battery to the instruments to be worked by it.
_The Leclanché Battery._--This form of open-circuit battery consists of
a glass jar in which is placed the elements. One element consists of a
rod of zinc, and the other element is carbon and powdered black oxide
of manganese. These two (the carbon and black oxide of manganese) are
placed in an earthenware vessel called a "porous cup." This is simply a
small jar made of clay which is not glazed. Thus, the liquid which is
in the glass jar penetrates through the porous cup to the carbon and
manganese which it contains, and so the chemicals affect both these
and the zinc at once, for, in order to obtain electricity, you will
remember that the chemical action must take place at the same time upon
both the elements in the same vessel. (Fig. 29.)
The chemical substance used in this battery is sal-ammoniac, or salts
of ammonia. A certain quantity of this salt is dissolved in water,
and this solution is poured into the glass jar. When this is done the
battery will generate electricity at once.
[Illustration: Fig. 29]
It should be remembered that the proper term for the chemical mixture
which acts upon the elements in any battery is "electrolyte."
_The Dry Battery._--The cleanliness, convenience, high efficiency, and
comparatively low internal resistance of the dry cell has brought it
into great favor in the last few years. It is now extensively used in
preference to the Leclanché and other open-circuit batteries having
liquid electrolyte for light work, such as bells, gas-lighting, burglar
alarms, ignition on motor-boats, automobiles, etc.
The dry cell is also used in great numbers for pocket flash-lamps,
and in other ways where it would be impossible to employ batteries
containing liquids.
A dry cell consists of zinc, carbon, and the electrolyte, which is a
mixture so made that it is in the form of a gelatinous or semi-solid
mass, so that it will not run or slop over.
A piece of sheet zinc is formed into a long tube, and a round, flat
piece of zinc is soldered at one end, thus making a cup open at one
end. This forms the cell itself, and at the same time becomes one of
the elements. The other element is a piece of battery carbon which is
long enough to project out of the top of the cell about half an inch or
more. While the cell is being filled with the electrolyte the carbon is
held up by a support so that it does not touch the zinc at the bottom
of the cup. Of course, the zinc cup and the carbon are provided with
proper binding-posts or other attachments, so that conducting wires can
be connected.
The electrolyte is packed into the cup and around the carbon in such
a way that the cup is entirely filled within about half an inch from
the top, and then some melted tar or pitch is poured over the top of
the electrolyte. This seals the cell and binds the contents solidly
together. Just before the sealing compound hardens, one or two holes
are made in it so that the gases may escape.
The composition of the electrolyte itself is not exactly alike in all
dry cells, as the various manufacturers follow their own particular
formulas. However, as you may be curious to know something about it, we
would state that one formula embraces flour, water, plaster of Paris,
granulated carbon, zinc chloride, ammonium chloride, and manganese
binoxide.
You will remember that the Leclanché and the dry batteries are purely
open-circuit cells, and that they can be used to advantage for electric
bells, annunciators, burglar alarms, gas ignition, etc., where _the
current of electricity is not doing_ continuous work, but only for a
few seconds at a time. Consequently, the batteries have a little rest
in between, if only for a few seconds.
Now, if we were to attempt to use open-circuit batteries for electric
lights or motors, where the electricity must work constantly every
second, the batteries would "polarize"--that is to say, they would only
work a few minutes and then stop, because the chemicals used in them
are of that kind that they will only allow the battery to do a little
work at a time.
The batteries we have been describing will do the ordinary work for
which they are intended for sometimes a year without requiring any
attention, but if we try to make them do work for which they were not
intended, they would only last a few days.
If we should want to operate electric lights or motors continuously
from a battery we must, therefore, use