[Illustration: 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. [Illustration: Fig. 42.] =55. The Fuller Cell=, Fig. 43, is another type of bichromate cell, used largely for long-distance telephone service, for telephone exchange and switch service, for running small motors, etc. It consists of a glass jar, a carbon plate, with proper connections, a clay porous cup, containing the zinc, which is made in the form of a cone. A little mercury is placed in the porous cup to keep the zinc well amalgamated. Either bichromate of potash or bichromate of soda can be used as a depolarizer. [Illustration: Fig. 43.] [Illustration: Fig. 44.] =56. The Gravity Cell=, sometimes called the _bluestone_ or _crowfoot_ cell, is used largely for telegraph, police, and fire-alarm signal service, laboratory and experimental work, or whenever a closed circuit cell is required. The E.M.F. is about one volt. This is a modified form of the Daniell cell. Fig. 44 shows a home-made gravity cell. A copper plate is placed at the bottom of the glass jar, and upon this rests a solution of copper sulphate (bluestone). The zinc plate is supported about four inches above the copper, and is surrounded by a solution of zinc sulphate which floats upon the top of the blue solution. An insulated wire reaches from the copper to the top of the cell and forms the positive pole. (See "Apparatus Book," § 11 to 15, for home-made gravity cell, its regulation, etc. For experiments with two-fluid Daniell cell, see "Study," Exp. 113, § 281 to 286.) [Illustration: Fig. 45.] =56a. Bunsen Cells,= Fig. 45, are used for motors, small incandescent lamps, etc. A carbon rod is inclosed in a porous cup, on the outside of which is a cylinder of zinc that stands in dilute sulphuric acid, the carbon being in nitric acid.