The Story of Electricity | Page 7

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THE ELECTRICITY OF CHEMISTRY.
A more tractable kind of electricity than that of friction was discovered
at the beginning of the present century. The story goes that some edible
frogs were skinned to make a soup for Madame Galvani, wife of the
professor of anatomy in the University of Bologna, who was in delicate
health. As the frogs were lying in the laboratory of the professor they
were observed to twitch each time a spark was drawn from an electrical
machine that stood by. A similar twitching was also noticed when the
limbs were hung by copper skewers from an iron rail. Galvani thought
the spasms were due to electricity in the animal, and produced them at
will by touching the nerve of a limb with a rod of zinc, and the muscle

with a rod of copper in contact with the zinc. It was proved, however,
by Alessanjra Volta, professor of physics in the University of Pavia,
that the electricity was not in the animal but generated by the contact of
the two dissimilar metals and the moisture of the flesh. Going a step
further, in the year 1800 he invented a new source of electricity on this
principle, which is known as "Volta's pile." It consists of plates or discs
of zinc and copper separated by a wafer of cloth moistened with
acidulated water. When the zinc and copper are joined externally by a
wire, a CURRENT of electricity is found in the wire One pair of plates
with the liquid between makes a "couple" or element; and two or more,
built one above another in the same order of zinc, copper, zinc, copper,
make the pile. The extreme zinc and copper plates, when joined by a
wire, are found to deliver a current.
This form of the voltaic, or, as it is sometimes called, galvanic battery,
has given place to the "cell" shown in figure II, where the two plates Z
C are immersed in acidulated water within the vessel, and connected
outside by the wire W. The zinc plate has a positive and the copper a
negative charge. The positive current flows from the zinc to the copper
inside the cell and from the copper to the zinc outside the cell, as
shown by the arrows. It thus makes a complete round, which is called
the voltaic "circuit," and if the circuit is broken anywhere it will not
flow at all. The positive electricity of the zinc appears to traverse the
liquid to the copper, from which it flows through the wire to the zinc.
The effect is that the end of the wire attached to the copper is positive
(+), and called the positive "pole" or electrode, while the end attached
to the zinc is negative (-), and called the negative pole or electrode. "A
simple and easy way to avoid confusion as to the direction of the
current, is to remember that the POSITIVE current flows FROM the
COPPER TO the ZINC at the point of METALLIC contact." The
generation of this current is accompanied by chemical action in the cell.
Experiment shows that the mere CONTACT of dissimilar materials,
such as copper and zinc, electrifies them--zinc being positive and
copper negative; but contact alone does not yield a continuous current
of electricity. When we plunge the two metals, still in contact, either
directly or through a wire, into water preferably acidulated, a chemical
action is set up, the water is decomposed, and the zinc is consumed.
Water, as is well known, consists of oxygen and hydrogen. The oxygen

combines with the zinc to form oxide of zinc, and the hydrogen is set
free as gas at the surface of the copper plate. So long as this process
goes on, that is to say, as long as there is zinc and water left, we get an
electric current in the circuit. The existence of such a current may be
proved by a very simple experiment. Place a penny above and a dime
below the tip of the tongue, then bring their edges into contact, and you
will feel an acid taste in the mouth.
Figure 12 illustrates the supposed chemical action in the cell. On the
left hand are the zinc and copper plates (Z C) disconnected in the liquid.
The atoms of zinc are shown by small circles; the molecules of water,
that is, oxygen, and hydrogen (H2O) by lozenges of unequal size. On
the right hand the plates are connected by a wire outside the cell; the
current starts, and the chemical action begins. An atom of zinc unites
with an atom of oxygen, leaving two atoms of hydrogen thus set free to
combine with another atom of oxygen, which in turn frees two atoms of
hydrogen. This interchange of atoms goes on