684 



CONSTANT ELEMENTS. 



Nitric acid, however, cannot be used in copper-zinc 

 elements for preventing the deposition of hydrogen, 

 because copper is strongly attacked by nitric acid and 

 rapidly dissolved. Constant elements with plates of 

 copper and zinc may be constructed by placing the 

 copper plate in a solution of cupric sulphate. It has 

 already been shown that the action of the current upon 

 a solution of cupric sulphate is to deposit the copper 

 upon the cathode, and since within such an element 

 the copper plate forms the cathode, it gradually be- 

 comes thicker by receiving new deposits of the metal 

 of which it consists, and its action therefore remains 

 unchanged. This action was taken advantage of by 

 Daniell in the construction of the first form of a con- 

 stant element which came into use, known as Daniell 's 

 cell, and represented in fig. 347. V is a glass or porce- 

 lain vessel containing a saturated solution of cupric 

 sulphate, hT which is immersed a copper cylinder, C, 

 open at both ends and perforated by holes. At the 

 upper part of this cylinder there is an annular shelf, G, 

 also perforated by small holes, and 

 below the upper surface of the 

 solution. The shelf is intended to 

 support crystals of cupric sulphate 

 to replace that decomposed as 

 the electrical action proceeds. In- 

 side the cylinder is a thin porous 

 vessel, P, of unglazed earthen- 

 ware, which contains either sale 

 water or dilute sulphuric acid, 

 in which is placed the cylinder of 

 amalgamated zinc, Z. Two thin strips of copper, p and 



FIG. 347. 



