As this type of battery will work efficiently on _either_ open or
closed circuit, we have thought best to describe it separately at this
place, in order not to confuse your ideas while reading about batteries
generally.
The type of cell we will now describe was originated by an inventor
named Lalande, and was known by that name; but it has been greatly
improved and rendered more efficient by Edison, and is now manufactured
and sold by him under the name of the Edison Primary Battery.
Before describing the cell itself, let us consider the action that
takes place in a battery of this kind.
If certain metals are placed in a suitable solution, and are connected
together, outside of the solution, by wires, vigorous chemical action
will take place at the surfaces of the metals, and electrical energy
will be produced. The plates must be of different metals, and the
solution should be one that will dissolve neither of them except when
an electric current is allowed to flow.
One of the metals is usually zinc, which is gradually eaten away or
dissolved by the solution while the battery is delivering electrical
energy. It is the chemical combination of the zinc and the solution
that produces this energy, which leaves the zinc in the form of an
electric current, and passes through the solution to the other metal,
out of the cell to the wire, and thence back by another wire to the
zinc, where it is once more started on its circuit.
At the surface of the other metal, which may be, and frequently is,
copper, small bubbles of the gas called hydrogen are produced. This gas
rises to the surface of the liquid and gradually passes off into the
air. But its presence offers resistance to the passage of the current;
so that generally there is associated with the copper a supply of the
gas oxygen. Oxygen and hydrogen are always very eager to mix with
each other, and, therefore, when the hydrogen bubbles appear they are
quickly taken up by the oxygen near by. The mixture of these two gases
forms water, which becomes part of the solution. All of this happens so
quickly that the hydrogen cannot be perceived so long as there is any
oxygen left in the copper-oxide plate.
[Illustration: Fig. 34]
In the Edison Primary Battery (Fig. 34) the plates are zinc, known as
the negative, and copper oxide (copper and oxygen), or the positive.
These are suspended in a solution of caustic soda and water, the plates
and solution being contained in jars of glass or porcelain. The plates
are provided with suitable wires for connecting the cells with one
another and with the lamps, motors, or other devices which they are to
operate. There are usually two zinc plates and one copper-oxide plate,
or multiples thereof. The quantity of current that may be withdrawn
depends on the size and number of the plates, as well as upon their
construction and arrangement.
The voltage of these cells is low, being about 0.65 volt each; but this
is more than compensated for by the fact that the internal resistance
of the battery is so low that the voltage is not perceptibly affected
even at continuous high-discharge rates, and that the voltage remains
practically constant throughout the life of the cell.
Furthermore, when the battery is not in use there is practically no
local action. Consequently, the cells may remain on open circuit (that
is, doing no work) for years and there will be no loss of energy. The
cell will then operate with the same practical efficiency as if it were
new. In some classes of work this battery remains in service from four
to six years without attention.
Another peculiar advantage of this battery lies in the fact that the
plates and the electrolyte are so well proportioned that they are all
exhausted at the same time, and then new plates and solution can be put
in the jar, restoring it to its original condition. These batteries are
used in great numbers for railway signal work and for other purposes,
such as fire and burglar alarm systems, various telephone functions,
operation of electric self-winding and programme clock systems, small
electric-motor work, for low candle-power electric lamps, gas-engine
ignition, electro-plating, telegraph systems, chemical analysis, and
other experimental work where batteries are required that will remain
in use for long periods of time without requiring any attention or
renewal.
The remarks that have been made on previous pages about connecting up
batteries in series, multiple, and multiple series apply also to these
Edison Primary Cells. Fig. 35 shows a battery of four of these cells
connected in series.