242 



VOLTAIC ELECTRICITY. 



deposited upon the copper plate. The current from this 

 battery is especially constant. 



(a.) In the figure, the copper plate is represented as a cleft 

 cylinder within the porous cup, the crystals being piled up around 

 it. It is common to interchange the plates, the zinc being in dilute 

 sulphuric acid within the porous cup, and the copper plate in the 

 saturated acid outside the porous cup. Sometimes the outer vessel 

 itself is made of copper instead of glass, the vessel then becoming 

 the negative plate. The internal resistance of a Daniell's cell is as 

 great as that of a quarter of a mile of ordinary telegraph wire. 



3S2. Smee's Battery. An element of Smee's 

 battery is represented by Fig. 186. It 

 consists of a silver plate coated with 

 platinum powder placed between two zinc 

 plates, the plates being hung in dilate 

 sulphuric acid. The use of the pbtiimm 

 powder is to free the plate from the 

 liberated hydrogen. 



FIG. 186. 



383. Potassium Bi-chromate 

 Battery. The potassium bi-chromate 

 battery differs from Smee's in the sub- 

 stitution of a carbon plate for the silver 

 plate, and of a solution of potassium 

 bi-chromate in dilute sulphuric acid 

 for the liquid there used. Here the 

 hydrogen is given an opportunity for 

 chemical union as fast as it is liberated. 



(<z.) The bottle form of this battery, repre- 

 sented in Fig. 187, is the most convenient for 

 the laboratory or lecture table. By means of 

 the sliding rod, the zinc plate can be raised 

 out of the solution when not in use ; and thus 

 adjusted, the cell can remain for months with- 

 out any action, if desired, and be ready at a 

 moment's notice. One of the best proper - 



FIG. 187. 



