But when the circuit is of too low resistance, the oxidizing action of the peroxide of manganese is not rapid enough, and a film of hydrogen, which is a poor conductor, forms over the negative plate, increasing the internal resistance of the cell and setting up local action. In the best class of these open circuit cells, this hydrogen is absorbed after a rest, and the battery recuperates and is ready for work again. The circuit of the Ruhmkorff coil is low, and this polarization always occurs a few minutes after the contact-breaker is started.
Fig. 59.
In the class of closed circuit cells, chosen for the present purpose, the Grenet or bottle bichromate is one of the handiest for occasional use. A glass bottle-shaped jar, J, Fig. 59, is provided with a hard rubber cap, G, on which are mounted the binding posts A B. To the underside of this cap are attached two carbon plates C C, which reach nearly to the bottom of the jar, being connected together on the cap by a varnished copper strip, the latter being in turn connected to one binding post. Through the centre of the cap passes a brass rod, R, having attached to its lower end a piece of sheet zinc, Z, well amalgamated with mercury. This process of amalgamation consists in cleaning the zinc, then rubbing its surface with a rag dipped in dilute sulphuric acid, and pouring a few drops of mercury on the wet zinc. The mercury will spread readily over the zinc, provided it has been well cleaned, and if properly done should give the zinc plate a bright, shining appearance.
When the cell is not in use, the zinc is drawn up into the neck of the bottle and clamped by a set screw against the brass rod. A copper spring pressing on the rod serves to carry the current to the second binding post.
This cell originated in France, whence its name, but a cheaper form is now made in the United States known as the Novelty Grenet. The shape of the jar is somewhat different, and the carbon is moulded, whereas the French carbon is sawed from the carbon deposited in the gas retort; but the American form is practically of as great utility as the French, and the cost recommends it.
The bichromate solutions are affected by light, and deteriorate less it kept in stoneware jugs. The Grenet battery can very well be fitted into a neat wood case, which will serve the further purpose of preventing chance knocks from fracturing the glass jar.
Carbons which are used in batteries containing the foregoing solution should be well washed in warm water whenever the solution is changed, and especially when it is intended to put the battery out of active service. When the solution acquires a decidedly green hue it should be replaced with fresh. The electromotive force of this cell varies from 1.90 to 2 volts, and the amperage is dependent on the size of the plates, running from 5 amperes upward.
The glass jar is filled up to the commencement of the neck with a solution of bichromate of potash or sodium, called electropoion fluid, and prepared as follows: To 1 gallon of water add 1 pound of bichromate of sodium, mixing in a stoneware vessel. When dissolved add 3 pounds of sulphuric acid in a thin stream, stirring slowly. As the mixture heats on the introduction of the acid, care must be used to pour in the latter slowly. This solution should not be used until quite cold.