51. Bichromate of Potash Cells are very useful for general laboratory work. They are especially useful for operating induction coils, small motors, small incandescent lamps, for heating platinum wires, etc. These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is used as the exciting fluid, and in this is dissolved the bichromate of potash which keeps the hydrogen bubbles from the carbon plate. (See "Apparatus Book," § 26.) Zinc and carbon are used for the plates, the + pole being the wire attached to the carbon.

Fig. 39 shows one form of bichromate cell. It furnishes a large quantity of current, and as the zinc can be raised from the fluid, it may be kept charged ready for use for many months, and can be set in action any time when required by lowering the zinc into the liquid. Two of these cells will burn a one candle-power miniature incandescent lamp several hours. The carbon is indestructible.

Note. For various forms of home-made cells, see "Apparatus Book," Chapter I., and for battery fluids see Chapter II.

52. The Grenet Cell. Fig. 40 is another form of bichromate cell. The carbon plates are left in the fluid constantly. The zinc plate should be raised when the cell is not in use, to keep it from being uselessly dissolved.

Fig. 40. Fig. 41.

53. Plunge Batteries. Two or more cells are often arranged so that their elements can be quickly lowered into the acid solution. Such a combination, Fig. 41, is called a plunge battery. The binding-posts are so arranged that currents of different strengths can be taken from the combination. The two binding-posts on the right of the battery will give the current of one cell; the two binding-posts on the left of the battery will give the current of two cells, and the two end binding-posts will give the current of all three cells. When not in use the elements must always be hung on the hooks and kept out of the solution.

54. Large Plunge Batteries. Fig. 42, are arranged with a winch and a bar above the cells; these afford a ready and convenient means of lifting or lowering the elements and avoiding waste. In the battery shown, Fig. 42, the zincs are 4×6 inches; the carbons have the same dimensions, but there are two carbon plates to each zinc, thus giving double the carbon surface.