_What is galvanism?_
It is the production of _electrical disturbance_ by chemical action.
_What is the most simple manner of illustrating the production of
this electricity?_
If we place a piece of silver on the tongue, and a piece of zinc
underneath it, no effect will be produced as long as the two metals are
kept asunder; but when their ends are brought together, a _distinct
thrill_ will pass through the tongue, a metallic taste will diffuse
itself, and, if the eyes are closed, a sensation of _light_ will be
evident at the same moment.
_To what is this result owing?_
To a _chemical action_ developed the moment the two metals touched each
other.
The _saliva_ of the tongue _oxidizes_ a portion of the _zinc_, which
excites _electricity_, for no chemical action ever takes place without
producing electricity. Upon bringing the ends of the two metals
together, a slight current passes from one to the other.
_By whom was the production of galvanic electricity first noticed?_
By _Galvani_, professor of anatomy at Bologna, Italy, in 1790.
Having occasion to dissect several frogs, he hung up their hind legs on
some _copper hooks_, until he might find it necessary to use them for
illustration. In this manner he happened to suspend a number of the
copper hooks on an iron balcony, when, to his great astonishment, the
limbs were thrown into violent convulsions.
_On investigating the phenomena what did Galvani discover?_
He found that whenever the nerves of a frog’s leg were touched by one
metal and the muscles by another, convulsions took place on bringing the
two different metals in contact.
_What is the simplest way of exciting a current of galvanic
electricity?_
By arranging a _series of metal plates in a pile_, placing them in
pairs, with a wet cloth between them, it being necessary that one of
each pair should be more easily oxidized than the other. The simple
contact of these plates will produce a feeble and continued galvanic
current.
_What is such an arrangement of plates for producing electrical
currents called?_
A _galvanic_ or _voltaic battery_.
_Why are the terms “galvanic” and “voltaic” applied?_
They originated in honor of _Galvani_ and _Volta_, the Italian
philosophers who first developed these phenomena of chemical
electricity, and the means of producing them.
[Illustration: =HIGH-RESISTANCE GALVANOMETER FOR VERY SMALL CURRENTS=]
_Are there many metals or other substances which, when brought
together, are capable of producing galvanic action?_
The number is _quite large_; among them we may enumerate the following:
_zinc_, _lead_, _tin_, _antimony_, _iron_, _brass_, _copper_, _silver_,
_gold_, _platinum_, _black lead_ or _graphite_, and _charcoal_.
_Will any two of these brought together produce a galvanic current?_
They _will_; but they possess the power in _different degrees_; and the
more remote they stand from each other in the order above given, the
more decidedly will the chemical electricity be developed.
Thus zinc and lead will produce a voltaic battery, but it will be much
less active than zinc and iron, or the same metal and copper, and this
last less active than zinc and platinum, or zinc and charcoal.
_Does galvanic or voltaic electricity appear to consist of two
kinds, positive and negative, as in ordinary electricity?_
It does; positive electricity always flows _from the metal which is
acted upon_ most powerfully, and negative electricity _from the other_.
_What do we mean when we speak of a galvanic circuit?_
The connection of the two metals in the battery, so that the positive
and negative electricities can _meet, and flow in opposite directions_.
_At what point in the circuit will the manifestations of electricity
be most apparent?_
At the point where the _two currents meet_.
_What is meant by the poles of the battery?_
The two metals forming the elements of the battery are generally
connected by copper wires; the _ends_ of these wires, or the _terminal
points_ of any other connecting medium used, are called the poles of the
battery.
Thus, when zinc and copper poles are used, the end of the wire conveying
positive electricity from the zinc would be the positive pole, and the
end of the wire conveying negative electricity from the copper plate
would be the negative pole. Faraday describes the poles of the battery
as the doors by which electricity enters into or passes out of the
substance suffering decomposition.
A very simple, and at the same time an active, galvanic circuit may be
formed by an arrangement as represented in the accompanying
illustration. The current of positive electricity, when the circuit is
closed, passes from the zinc, through the liquid, to the copper, and
from the copper, along the conductors to the zinc. A current of negative
electricity traverses the circuit also, from the copper to the zinc, in
a direction precisely reversed.
_By what chemical action can the greatest abundance of galvanic
electricity be developed?_
By the _oxidation of metallic zinc_ by weak sulphuric acid.
[Illustration: =TYPES OF ELECTRIC CELLS OR “BATTERIES”=
(1) _Grove’s Cell._--Z. Zinc plate in dilute sulphuric acid; P. platinum
plate in strong nitric acid. (2) _Daniell’s Cell._--Z. Zinc rod in
porous pot P containing dilute sulphuric acid; C. copper plate in outer
vessel containing copper sulphate solution. (3) _Leclanche Cell._--Zinc
in sal-ammoniac solution; carbon slab in charcoal and manganese
dioxide.]
_The electricity developed by the action of a single pair of plates
immersed in acid water is very feeble: how can it be increased?_
By increasing the _number of the plates_ and the quantity of the liquid,
we increase the intensity of the electricity developed.
ACTION WITHIN A VOLTAIC CELL.--Let us try to see now how an electric
current is set up in a simple voltaic cell, consisting of a zinc plate
and a copper plate immersed in dilute acid. First we must understand the
meaning of the word _ion_.
If we place a small quantity of salt in a vessel containing water, the
salt dissolves, and the water becomes salt, not only at the bottom where
the salt was placed, but throughout the whole vessel. This means that
the particles of salt must be able to move through the water. Salt is a
chemical compound of sodium and chlorine, and its molecules consist of
atoms of both these substances. It is supposed that each salt molecule
breaks up into two parts, one part being a sodium atom, and the other a
chlorine atom, and further, that the sodium atom loses an electron,
while the chlorine atom gains one. These atoms have the power of
traveling about through the solution, and they are called ions, which
means “wanderers.”
An ordinary atom is unable to wander about in this way, but it gains
traveling power as soon as it is converted into an ion, by losing
electrons if it be an atom of a metal, and by gaining electrons if it be
an atom of a non-metal.
Returning to the voltaic cell, we may imagine that the atoms of the zinc
which are immersed in the acid are trying to turn themselves into ions,
so that they can travel through the solution. In order to do this each
atom parts with two electrons, and these electrons try to attach
themselves to the next atom. This atom, however, already has two
electrons, and so in order to accept the newcomers it must pass on its
own two. In this way electrons are passed on from atom to atom of the
zinc, then along the connecting wire, and so to the copper plate. The
atoms of zinc which have lost their electrons thus become ions, with
power of movement. They leave the zinc plate immediately, and so the
plate wastes away or dissolves. So we get a constant stream of electrons
traveling along the wire connecting the two plates, and this constitutes
an electric current.
_What are the most ordinary effects produced by the developed
electricity of a large galvanic battery?_
The _production of sparks_ and _brilliant flashes of light_, the heating
and fusing of metals, the deflagration of gunpowder and other
inflammable substances, and the decomposition of water, saline
compounds, and metallic oxides.
_How may the most splendid artificial light known be produced?_
By fixing pieces of pointed charcoal or carbon to the wires connected
with opposite poles of a powerful galvanic battery, and bringing them
into contact.
_What does this produce?_
Electric light.
_Can intense heat be developed by the action of the galvanic battery
as well as intense light?_
The _greatest artificial heat_ man has yet succeeded in producing has
been through the agency of the _galvanic battery_.
_What refractory substances can be fused by the aid of the galvanic
battery?_
All the metals, including platinum, can be _readily melted_; quartz,
sulphur, magnesia, slate and lime are liquefied; and the diamond fuses,
boils, and becomes converted into coal.
[Illustration: =HOW AN ELECTRIC BATTERY GENERATES ELECTRICITY=
The above simple voltaic battery, or cell, consists of a plate of copper
and one of zinc dipping into a vessel containing dilute sulphuric acid
to twenty of water by volume. When these plates are joined externally by
a wire or other conductor a current flows from the copper plates, called
the positive pole of the battery, to the zinc plate, called the negative
pole of the battery. This is due to the fact that a difference of
_potential_ is set up between the plates on immersion in the acid, in
consequence of which an electro-motive force is generated that drives
the current round the circuit. The potential between the plates is
maintained by the chemical action now going on in the cell. This action
results in the gradual consumption of the zinc plate with formation of
zinc sulphate and evolution of hydrogen at the copper plate. In a short
time the current in the circuit falls off in consequence of _local
action_ and _polarization_.]
_What is electrotyping, or electro-metallurgy?_
It is the art or process of _depositing_, from a _metallic solution_,
through the agency of galvanic electricity, a _coating_ or _film_ of
metal upon some other substance.
_Upon what principles is it accomplished?_
The process is based on the fact, that when a galvanic current is passed
through a solution of some metal, as a solution of sulphate of copper
(sulphuric acid and copper), _decomposition takes place_; the metal is
separated in a metallic state, and attaches itself to the negative pole,
or to any substance that may be attached to the negative pole; while the
acid or other substance before in combination with the metal, goes to,
and is deposited on the positive pole.
In this way a medal, a wood-engraving, or a plaster cast, if attached to
the negative pole, may be covered with a coating of copper; if the
solution had been one containing silver or gold, the substance would
have been covered with a coating of silver or gold instead of copper.
_How can the thickness of the deposits be regulated?_
The thickness of the deposit, providing the supply of the metallic
solution be kept constant, will depend on the _length of time the object
is exposed to the influence of the battery_.