The sodium salt is preferable to the potassium, owing to its not forming the crystals of chrome alum, and also on account of its lower cost and greater solubility, the latter being four times greater than that of the potassium salt. The commercial acid used should contain at least 90 per cent pure acid and should be free from impurities. On filling the battery use utmost care not to splash the solution on any of the metal work, or it will cause corrosion. Although the salts in the solution will most likely make a stain, the corrosive action of the acid can be arrested if the solution be splashed on the clothes by the prompt application of ammonia solution.
Fig. 60.
The "Fuller" cell, Fig. 60, which is another type of the bichromate cell, is one from which a steady current can be obtained for a longer interval than from the Grenet, but the current is less. The electromotive force is the same, but the current is only 3 amperes, except in certain modifications.
In the porous cup is a cone-shaped zinc having a stout copper wire cast in. This wire is occasionally covered with rubber insulation, but, as a rule, is bare. The porous cup is of unglazed porcelain, thick, but very porous. This sets in the glass jar, a wooden cover fitting loosely over the whole to exclude dust. Through this cover passes the wire leading from the zinc, and also the carbon plate carrying a machine screw and check nuts for connection. The cover is dipped in melted paraffin, as is also the upper end of the carbon and the rim of the glass jar. This is to prevent the creeping of the salts in the solutions and the corrosion of the brass work.
Into the porous cup is poured a solution composed of 18 parts by weight of common salt and 72 parts by weight of water. Electropoion fluid is held by the glass jar, the two solutions reaching a level of two thirds the height of the jar. One ounce of mercury is added to the porous cup solution to ensure the complete and continuous amalgamation of the zinc. The salt can be more readily dissolved in warm water, but all solutions must be used cold. It is not always necessary to renew the solutions when the battery fails to give out its accustomed strength, but several ounces of water can be substituted for a similar amount of fluid in the porous cup. Stir the solution by moving the zinc up and down, and a temporary improvement will be noticed.
To obtain a greater current from this cell, use a larger zinc, such as a well-amalgamated zinc plate, and add a teaspoonful of sulphuric acid to clean water for the porous cup solution. Additional carbon plates connected together and placed round the porous cup will lower the resistance of the cell and increase the current, and also tend to keep down the polarization.
A new form of this battery was described by M. Morisot a short time ago.
The positive pole is of retort carbon in the outer cell in a depolarizing mixture made of 1 part sulphuric acid, 3 parts saturated solution bichromate of potash, crystals of the latter salt being suspended in the cell to keep up the saturation. A porous cup contains a solution of caustic soda. The zinc is in a second porous cup placed within the first, which holds a caustic soda solution of greater density. The electromotive force is 2½ volts when the cell is first placed in circuit, and will remain at 2.4 for some hours. The internal resistance is low, but varies with the thickness of the porous cups. This cell is not suitable for any but use for a few hours at one time.