Chapter IV: Various Voltaic Cells

=49. Single-Fluid and Two-Fluid Cells.= The simple cell (§ 45) is a single-fluid cell. The liquid is called the _electrolyte_, and this must act upon one of the plates; that is, chemical action must take place in order to produce a current. The simple cell polarizes rapidly, so something must be used with the dilute sulphuric acid to destroy the hydrogen bubbles. This is done in the _bichromate of potash cell_.

In order to get complete depolarization--that is, to keep the carbon plate almost perfectly free from hydrogen, it is necessary to use _two-fluid cells_, or those to which some solid depolarizer is added to the one fluid.

=50. Open and Closed Circuit Cells.= If we consider a voltaic cell, the wires attached to it, and perhaps some instrument through which the current passes, we have an _electric circuit_. When the current passes, the circuit is _closed_, but when the wire is cut, or in any way disconnected so that the current can not pass, the circuit is _open_ or _broken_. (See "Study," § 266.)

_Open Circuit Cells_ are those which can give momentary currents at intervals, such as are needed for bells, telephones, etc. These must have plenty of time to rest, as they polarize when the circuit is closed for a long time. The _Leclanché_ and _dry_ cells are the most common open circuit cells.

_Closed Circuit Cells._ For telegraph lines, motors, etc., where a current is needed for some time, the cell must be of such a nature that it will not polarize quickly; it must give a strong and constant current. The _bichromate_ and _gravity cells_ are examples of this variety. (See "Study," § 286.)

=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.

=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.

=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.

=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.)

=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.

=57. The Leclanché Cell= is an open circuit cell. Sal ammoniac is used as the exciting fluid, carbon and zinc being used for plates. Manganese dioxide is used as the depolarizer; this surrounds the carbon plate, the two being either packed together in a porous cup or held together in the form of cakes. The porous cup, or pressed cake, stands in the exciting fluid. The E. M. F. is about 1.5 volts.

Fig. 46 shows a form with porous cup. The binding-post at the top of the carbon plate forms the + electrode, the current leaving the cell at this point.

_The Gonda Prism Cell_ (Fig. 47), is a form of Leclanché in which the depolarizer is in the form of a cake.

=58. Dry Cells= are open circuit cells, and can be carried about, although they are moist inside. The + pole is the end of the carbon plate. Zinc is used as the outside case and + plate. Fig. 48 shows the ordinary forms.

Fig. 49 shows a number of dry cells arranged in a box with switch in front, so that the current can be regulated at will.

=59. The Edison-Lelande Cells=, Fig. 50, are made in several sizes and types. Zinc and copper oxide, which is pressed into plates, form the elements. The exciting fluid consists of a 25 per cent. solution of caustic potash in water. They are designed for both open and closed circuit work.