=288. Dry Cells= are for open circuit work. Sheet zinc forms at the same time the active plate and the outside cylinder or case. A carbon plate acts as the inactive or - plate. The exciting fluid is kept from spilling by its being absorbed by one of the various substances used for that purpose. =289. The Bichromate of Potash Cell= is a very common one for laboratory use. It gives a strong current, and although a single fluid cell, it does not readily polarize. Zinc and carbon plates are used. In the sulphuric acid, which is the exciting fluid, is dissolved bichromate of potash. This cell is used for running small motors, induction coils, etc. It has, with fresh solutions, an E. M. F. of about 2 volts. (See Apparatus Book, Chapter I., for Home-Made Batteries.) =290. The Daniell Cell=, of which the two-fluid cell used in Exp. 112 is a form, is noted for its constant current. The E. M. F. is a little over 1 volt, and it should be kept working through a resistance when not in regular use; it should not be left in open circuit. The porous cup keeps the two fluids from mixing, but it does not stop the current. =291. The Gravity Cell= is a form of the above. As one of the fluids is heavier than the other, no porous cup is needed. Gravity, together with the action of the current, tends to keep the fluids separated. A copper plate is placed in the bottom of the jar, and upon this is put the copper sulphate solution. The zinc plate is supported by the top of the jar and rests in a solution of zinc sulphate, which is lighter than the blue solution below. An insulated wire extends from the copper through the liquids. This cell is used for telegraph and similar work. (See Apparatus Book for Home-Made Gravity Cell, its Regulation, etc.) CHAPTER XVI. THE ELECTRIC CIRCUIT. =EXPERIMENT 114. To see what is meant by "divided circuits" or "shunts."= _Apparatus._ The galvanoscope, G V (No. 58); astatic galvanoscope, A G (No. 59); two-fluid cell, 2-F C (see § 281); 6 wires with connectors; small thin pieces of tin or other metal, M P, for rapidly making connections (§ 226). _Arrange_ as in Fig. 88. The wires, 1 and 4, from 2-F C, lead to the metal plates M P-A and M P-B, for convenience. The wires, 2 and 3, from G V, are also connected with these plates. The wires, 5 and 6 (dotted lines), lead from A G, to be used as directed in part (B) of the experiment. See that G V is properly placed. See that A G is adjusted. [Illustration: Fig. 88.] =292. Directions.= (A) Without A G in place, take the reading of G V. The current now passes from Cu through 1, M P-B, 2, G V, 3, M P-A, 4 to Zn. (B) Connect wires 5 and 6 to the plates, as shown by the dotted lines. Again take reading of G V, and compare it with the first reading. Does some of the current pass through A G?