VOLTAIC COMBINATIONS. 



porcelain , containing a solution of the sulphate of zinc or the 

 chloride of sodium (common salt). A hollow cylinder of zinc A, 

 split down the side so as to be capable of being enlarged, or con- 

 tracted at pleasure, is immersed in this solution surrounding the 

 bladder. The poles are indicated by the conductors p and n, the 

 positive proceeding from the copper, and the negative from 

 the zinc. 



M. Pouillet states that the action of this apparatus is sustained 

 without sensible variation for entire days, provided the cup above 

 the cone d is kept supplied with the salt, so as to maintain the 

 solution in the saturated state. 



In the batteries used for the telegraphs on the French railways, 

 the liquid in which the zinc cylinder is immersed is pure water, 

 and this is found to answer in a very satisfactory manner. 



The current flows from the copper cylinder and returns as usual 

 to the zinc. 



34. Grove's battery consists of two liquids, sulphuric and nitric 

 acids, and two metals, zinc and platinum, arranged in the following 

 manner : 



A hollow cylinder of zinc z z, fig. 5, open at both ends as already 

 described, is placed in a vessel of glazed porcelain v v. "Within 

 this is placed a cylindrical vessel v v, of unglazed 

 porcelain, a little less in diameter than the zinc 

 z z, so tliat a space of about a quarter of an inch 

 may separate their surfaces. 

 In this vessel v v, is inserted a 

 cylinder c c of platinum, open 

 at the ends, and a little less 

 than v v t so that their surfaces 

 may be about a quarter of an 

 inch asunder. Dilute sulphuric 

 acid is then poured into the 

 vessel v v, and concentrated 

 nitric acid into vv; p proceed- 

 ing from the platinum will then 

 be the positive, and N proceeding from the zinc the negative pole. 



Bunsen contrived a battery which has taken his name, and 

 which, while it retains all the efficiency of Grove's, can be con- 

 structed at much less expense, the platinum element being replaced 

 by the cheaper material of charcoal. 



In the vessel v v is inserted, instead of a hollow cylinder of 

 platinum, a solid cylindrical rod of charcoal, made from the 

 residuum taken from the retorts of gas-works. A strong porous 

 mass is produced by repeatedly baking the pulverised coke, to 

 ^ liieh the required form is easily imparted. Dilute sulphuric acid 



127 



Fig. 5. 



N ' 



