Publication: Western Electrician
Chicago, IL, United States
Bell Hangers' Hand-Book.
By F. B. BADT.
Two dissimilar metals brought in contact produce opposite kinds of electricity on the two surfaces, one becoming positively (+) and the other negatively ( -) electrified. In the following table metals are arranged in such a series that each becomes positively electrified when placed in contact with one below it:
The Voltaic or Galvanic Cell.
To make a simple voltaic or galvanic cell, place in a glass jar water, acidulated with a few drops of sulphuric acid, a strip of zinc, and a strip of copper. This cell is capable of generating a continuous flow of electricity through a wire, whose ends are connected to the two metal strips. See Fig. I.
Generation of Current.
The current in such a cell, as shown in Fig. 1, starts from the positive zinc plate, flows through the fluid to the copper plate, out through the external circuit and back to the zinc plate. The copper strip, whence the current starts on its journey through the external circuit, is called the positive pole (+) and the zinc strip is called the negative pole (—). When the external circuit is broken no current flows, but still the wire connected to the positive pole (copper plate) is called the positive wire, and the wire connected to the zinc pole the negative wire. As in almost all commercial batteries zinc is used as one pole, it may be well to remember that the zinc pole is always the negative pole.
When the current flows the zinc strip is observed to waste away. Its consumption furnishes the energy required to drive the current through the fluid from zinc to copper, and through the external circuit. At the same time it will be noticed that a few bubbles of hydrogen gas appear on the surface of the copper plate. Both these actions go on as long as the wires are joined to form a complete circuit. Thus the production of an electric current by a voltaic cell is always, accompanied by chemical action in the cell. Zinc and the other metals which stand at the electro-positive end of the contact-series will be dissolved, while the electro-negative substances — copper, silver, gold, platinum and graphite — will not be attacked.
A piece of quite pure zinc when dipped, alone into dilute sulphuric acid is not attacked by the liquid. The ordinary commercial zinc, however, is not pure and will be dissolved, a large quantity of hydrogen bubbles being given off from the surface of the metal.
As shown before, when the current flows through the cell and chemical action commences, the bubbles of hydrogen are evolved not at the zinc plate, nor throughout the liquid, but at the surface of the copper plate. This apparent transfer of the hydrogen gas through the liquid from the zinc to the surface of the copper plate must be borne in mind to understand the action of the different voltaic cells.
When the circuit is not closed, the current cannot flow and there should be no chemical action. The impure zinc of commerce, however, will continuously dissolve in the acid and give off hydrogen bubbles. This is called local action. It is caused by impurities in the zinc, such as particles of iron or other metals which behave in contact with the particles of zinc and the acid like miniature voltaic cells, and thus cause a constant waste of the zinc even if the battery circuit should be open.
To do away with this local action the zinc plates are amalgamated. The iron particles do not dissolve in the mercury, but are carried off from the surface of the zinc plate by the hydrogen bubbles. As the zinc in the amalgam dissolves the film of mercury unites with fresh portions of zinc, and consequently always presents a clean, bright surface to the liquid. The amalgamation of the zinc plates may be very well done by first immersing the zincs in a solution of dilute sulphuric acid and then in a bath of mercury. A brush or cloth may be used to rub them, so as to reach all points of the surface. Where a large number of zincs is to be amalgamated, the following will be found to be a good method: Dissolve eight ounces of mercury in a mixture consisting of two pounds of hydrochloric and one pound of nitric acid; when the solution is complete, add three pounds of hydrochloric acid. The zincs are amalgamated by immersing them in this solution for a few seconds; they should then be removed to a vat of clear water and rubbed as in the first case with a brush or cloth. If the solution is kept in a covered vessel it may be used a number of times.
The bubbles of hydrogen liberated at the surface of the copper electrode stick to it in great numbers, and form a film over its surface; hence the effective amount of service of the plate is very much diminished in a short time. This will cause an immediate falling off in the strength of the current, sometimes even stopping it entirely. A battery in this condition is said to be polarized. The effects of polarization are: First, it weakens the current by the increased resistance which it offers to the flow, for bubbles of gas are bad conductors; and secondly it weakens the current by setting up an opposing electromotive force, for hydrogen is nearly as oxydizable as zinc, and is electro positive.
It is, of course, very important to prevent this polarization, as otherwise the current would not be constant.
Various remedies are employed. These may be classed as mechanical, chemical and electro chemical.
1. Mechanical Means. — The liquid may be agitated or air may be blown through it, thus preventing the hydrogen bubbles from sticking to the positive pole.
The surface of the latter may be roughened so the bubbles will collect at the points and be quickly carried off. Example: Smee's cell.
2. Chemical Means. — If a highly oxidizing substance be added to the acid, it will prevent the formation of hydrogen bubbles, as the oxygen of this substance will combine with the hydrogen and from water. Such substances are bichromate of potash, binoxide of manganese, nitric acid, and chloride of lime. These substances, however, would attack the copper, and they can only be used in zinc-carbon or zinc-platinum cells. Nitric acid also attacks zinc when the circuit is open and cannot be employed in the same single cell with the zinc plate. Examples of cells: Grenet, Grove, Bunsen, carbon, nickel plating, Fuller, Leclanche, diamond carbon.
3. Electro Chemical Means. — Double cells can be arranged in such manner that a solid metal such as copper shall be liberated instead of hydrogen bubbles. This entirely prevents polarization. Examples of cells: Daniell, gravity.
A battery to be really good should fulfill the following conditions:
Its electromotive force should be high and constant.
Its internal resistance should be small.
It should give a constant current and must therefore be free from polarization, and not liable to rapid exhaustion requiring frequent renewal of material.
It should consume no material when the circuit is open.
It should be cheap, and of durable materials.
It should be manageable and if possible, should not emit corrosive fumes.
No single battery fulfills all these conditions, however; some batteries are better for one purpose and some for another. Thus, for telegraphing through a long line of wire, a considerable internal resistance is of no great consequence, as it is but a small fraction of the total resistance in circuit.
For electric gas lighting or other low resistance circuits, on the other hand, much internal resistance would be, if not absolutely