The Story of Electricity | Page 8

until the two atoms of
hydrogen which are freed last abide on the surface of the copper. The
"contact electricity" of the zinc and copper probably begins the process,
and the chemical action keeps it up. Oxygen, being an
"electro-negative" element in chemistry, is attracted to the zinc, and
hydrogen, being "electro-positive," is attracted to the copper.
The difference of electrical condition or "potential" between the plates
by which the current is started has been called the electromotive force,
or force which puts the electricity in motion. The obstruction or
hindrance which the electricity overcomes in passing through its
conductor is known as the RESISTANCE. Obviously the higher the
electromotive force and the lower the resistance, the stronger will be
the current in the conductor. Hence it is desirable to have a cell which
will give a high electromotive force and a low internal resistance.
Voltaic cells are grouped together in the mode of Leyden jars. Figure
13 shows how they are joined "in series," the zinc or negative pole of
one being connected by wire to the copper or positive pole of the next.
This arrangement multiplies alike the electromotive force and the
resistance. The electromotive force of the battery is the sum of the
electromotive forces of all the cells, and the resistance of the battery is
the sum of the resistances of all the cells. High electromotive forces or
"pressures" capable of overcoming high resistances outside the battery

can be obtained in this way.
Figure 14 shows how the zincs are joined "in parallel," the zinc or
negative pole of one being connected by wire to the zinc or negative
pole of the rest, and all the copper or positive poles together. This
arrangement does not increase the electromotive force, but diminishes
the resistance. In fact, the battery is equivalent to a single cell having
plates equal in area to the total area of all the plates. Although unable to
overcome a high resistance, it can produce a large volume or quantity
of electricity.
Numerous voltaic combinations and varieties of cell have been found
out. In general, where-ever two metals in contact are placed in a liquid
which acts with more chemical energy on one than on the other, as
sulphuric acid does on zinc in preference to copper, there is a
development of electricity. Readers may have seen how an iron fence
post corrodes at its junction with the lead that fixes it in the stone. This
decay is owing to the wet forming a voltaic couple with the two
dissimilar metals and rusting the iron. In the following list of materials,
when any two in contact are plunged in dilute acid, that which is higher
in the order becomes the positive plate or negative pole to that which is
lower:--
POSITIVE Iron Silver Zinc Nickel Gold Cadmium Bismuth Platinum
Tin Antimony Graphite Lead Copper NEGATIVE
There being no chemical union between the hydrogen and copper in the
zinc and copper couple, that gas accumulates on the surface of the
copper plate, or is liberated in bubbles. Now, hydrogen is positive
compared with copper, hence they tend to oppose each other in the
combination. The hydrogen diminishes the value of the copper, the
current grows weaker, and the cell is said to "polarise." It follows that a
simple water cell is not a good arrangement for the supply of a steady
current.
The Daniell cell is one of the best, and gives a very constant current. In
this battery the copper plate is surrounded by a solution of sulphate of
copper (Cu SO4), which the hydrogen decomposes, forming sulphuric
acid (H2SO4), thus taking itself out of the way, and leaving pure
copper (Cu) to be deposited as a fresh surface on the copper plate. A
further improvement is made in the cell by surrounding the zinc plate
with a solution of sulphate of zinc (Zn SO4), which is a good conductor.

Now, when the oxide of zinc is formed by the oxygen uniting with the
zinc, the free sulphuric acid combines with it, forming more sulphate of
zinc, and maintaining the CONDUCTIVITY of the cell. It is only
necessary to keep up the supply of zinc, water, and sulphate of copper
to procure a steady current of electricity.
The Daniell cell is constructed in various ways. In the earlier models
the two plates with their solutions were separated by a porous jar or
partition, which allowed the solutions to meet without mixing, and the
current to pass. Sawdust moistened with the solutions is sometimes
used for this porous separator, for instance, on board ships for laying
submarine cables, where the rolling of the waves would blend the
liquids.
In the "gravity" Daniell the solutions are kept apart by their specific
gravities, yet mingle by slow diffusion. Figure 15