Galvanism
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.
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 [914] 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.
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.
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.
Large illustration (433 kB)
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.