228 THE ELECTRIC TELEGRAPH. 



zinc, fall upon the oxide of zinc and remain there unaltered ; 

 and the sulphate of copper which, by diffusion, makes its 

 way through the porous pot, can only reach the zinc ring 

 after traversing the whole body of the oxide of zinc ; but 

 the acid of the sulphate of copper, on entering the upper 

 compartment, combines with the oxide, forming sulphate 

 of zinc, which is dissolved in the liquid, and releasing 

 the oxide of copper, which remains in the form of a black 

 powder. So long, therefore, as there is oxide of zinc in the 

 diaphragm, through which the sulphate of copper must pass, 

 the zinc will retain its original lustre and its electro-posi- 

 tiveness unimpaired. Of course such a diaphragm gives rise 

 to a great resistance perhaps twenty times as much as an 

 ordinary porous pot but this is of no consequence when 

 the resistance of the circuit outside is great. 



9. Sand Batteries. For the use of the Needle telegraphs, 

 the element mostly in use consists of plates of amalgamated 

 zinc and copper, the spaces between them being filled up 

 with sand moistened with sulphuric acid and water. 



Such batteries are mostly made up in boxes containing 

 each ten or a dozen elements; they have a considerable 

 resistance, and after being set up a short time, a very in- 

 considerable electromotive force. Such as it is, however, 

 it lasts pretty constant for many weeks without requiring 

 attention, and is by no means a " dirty " battery. 



10. Alum Battery. Stohrer says that the most constant 

 pile he has met with is that composed of carbon and un- 

 amalgamated zinc, when both plates are immersed in sand 

 moistened with a saturated solution of alum in water, and 

 separated by a porous diaphragm. Nine such elements 

 which he used as a battery for telegraphic purposes, 

 retained for two years very nearly their original strength. 

 A disadvantage is found with these batteries which is 

 common^ to all those which liberate hydrogen gas, that after 

 the circuit has been closed for a time without much resist- 

 ance, the battery exhausts itself. On breaking the circuit 

 again, however, it recovers very speedily ; and as the cir- 

 cuit in most telegraphic systems is open the greater part 



