2. Inconstancy,
3. Increase of internal resistance,
are remedied in the double fluid battery, of which the Daniell's cell was the first invented, and is a good example. Of this kind of cell many forms are in use, but the principle is the same throughout. There is a positive and negative element, and the cell is divided into two receptacles for the two fluids. In the most constant form of Daniell cell, the zinc is plunged into a semi-saturated solution of sulphate of zinc, the copper in a saturated solution of sulphate of copper, and these two solutions are separated either by a porous barrier, or by taking advantage of the different specific gravities of the two solutions. By a saturated solution is meant a liquid which has dissolved as much of the substance as it possibly can.
The Chemical Action of a Daniell Cell.—The chemical action of this form of Daniell cell is as follows:—
The zinc electrode combines with oxygen; the oxide thus formed combines with sulphuric acid and forms sulphate of zinc. Oxide of copper is separate from the sulphate; and the copper in this oxide is separated from the oxygen. The oxygen of the water is separated at the zinc electrode from the hydrogen, and at the other electrode this hydrogen recombines with the oxygen from the oxide of copper. This alternate decomposition and recombination of the elements of water can neither increase nor decrease the E.M.F. of the cell, the actions being equal and opposite. The result of the series of actions above described is that the sulphuric acid and oxygen of the sulphate of zinc are transmitted to the zinc, combine with it, and form fresh sulphate of zinc; the sulphuric acid and oxygen of the sulphate of copper are transmitted to the zinc set free by the above process, and reconvert it into sulphate of zinc; the copper of the sulphate of copper is transmitted to the copper electrode, and remains adhering to it. The whole result is therefore the substitution of a certain quantity of sulphate of zinc for an equivalent quantity of sulphate of copper, together with a deposition of copper on the copper or negative electrode.[X] The following is a plan of the process:—
| Zinc | Oxide of Zinc | . · | Sulphate of Zinc found at positive plate. | ||||||
| Water | Oxygen | ||||||||
| Hydrogen | Water. | ||||||||
| Sulphate of Copper | Sulphuric Acid | ||||||||
| Oxide of Copper | Oxygen | ||||||||
| Copper | Copper at negative plate. | ||||||||
Description of the "Callaud" and "Marié-Davy" Batteries.—The Voltaic batteries in general use for the different purposes of torpedo warfare have been fully described in [Chapter IV]., and therefore it will be only necessary here to explain the construction of the "Callaud" and "Marié-Davy" batteries, these being much used abroad in connection with telegraphy.
The Callaud cell, named from the inventor, is a modification of the Daniell cell, and is also called a gravity battery, the liquids being simply prevented from mixing by the law of gravity forbidding the heavier of the two from rising through the lighter. It consists of a thin plate of copper, which is laid on the bottom of a good insulating jar having an insulated wire leading up the side, and on this plate are placed crystals of sulphate of copper. A solution of sulphate of zinc is then poured in, and on the top is fitted a zinc plate, which forms the positive element. The vessel must not be shaken, or the sulphate of copper when dissolving will mix with the solution above it.
The Marié-Davy cell consists of a carbon electrode in a paste of proto-sulphate of mercury and water contained in a porous pot, and a zinc electrode in dilute sulphuric acid, or in sulphate of zinc.