The Fuller, or Bichromate Cell.
Although not often necessary in gas-lighting work, there is at times a demand for a heavy current, such as in lighting a big building, where a large coil must be operated. At such times a bichromate of potash cell becomes of service. One of the types is shown in Fig. 56. J is a jar containing electropoion fluid described below. C is a carbon plate immersed in this fluid. P is a porous cup holding the zinc, Z, and being filled with a solution of 18 parts common salt, 72 parts water, and one ounce mercury.
Fig. 56.
The electropoion fluid for the outer jar is made by one pound bichromate of potash or soda to one gallon of water, mixing in a stone vessel. When dissolved, add three pounds commercial sulphuric acid carefully, a little at a time, and stir the mixture constantly as it gets hot. Always add the acid to the mixture; never attempt to pour the mixture into the acid, or trouble will result. The sodium salt is preferable to the potassium, owing to its greater solubility and its not forming chrome alum—a hard precipitate which sticks to jars, elements, etc., to their detriment.
Fig. 57.
Fig. 57, a form of battery known as the Grenet battery, is used where there is no porous cup. The zinc element, Z, is mounted on a rod R passing through the cap G of a glass jar, J, and can be raised or lowered into the electropoion fluid which the jar contains. This is a good scheme where the battery is liable to stand idle for a long period.