THE ELECTEIC TELEGRAPH. 



lated solution in which they are immersed. The production of 

 the current depends on the chemical action of the solution on the 

 zinc. That metal being very susceptible of oxydation, decom- 

 poses the water which is in contact with it. One constituent of 

 the water combining with the zinc, produces a compound called 

 the oxyde of zinc, and this oxyde entering again into combination 

 with the acid which the water holds in solution, forms a soluble 

 salt. If the acid, for example, be sulphuric acid, this salt will be 

 the sulphate of the oxyde of zinc, and as fast as it is produced it will 

 be dissolved in the water in which the slips of metal are immersed. 



Meanwhile, the copper not being as susceptible of chemical 

 action as the zinc, remains comparatively unaffected by the solu- 

 tion ; but the hydrogen evolved in the decomposition of the water 

 collects upon its surface, after which it rises and escapes in 

 bubbles at the surface of the solution. 



It is to this chemical action upon the zinc that the production 

 of the electric current is due. If a like action had taken place in 

 the same degree on the copper, a similar and equally intense 

 electric current would be produced in the opposite direction ; and 

 in that case the two currents would neutralise each other, and no 

 electric effect would ensue. 



From this it will be seen that the efficacy of the combination 

 must be ascribed to the fact, that one of the two metals immersed 

 in the solution is more oxydable than the other, and that the 

 energy of the effect and the intensity of the current will be so 

 much the greater as the susceptibility of oxydation of the one 

 metal exceeds that of the other. 



28. It appears, therefore, that the principle may be generalised, 

 and that electricity will be developed, and a current produced by 

 any two metals similarly placed, which are oxydable in different 

 degrees. 



Zinc being one of the most oxydable metals, and being also 

 sufficiently cheap and abundant, is generally used by preference 

 for voltaic combinations. Silver, gold, and platinum are severally 

 less susceptible of oxydation, and of chemical action generally, 

 than copper, and would therefore answer voltaic purposes better, 

 but are excluded by their greater cost, and by the fact that copper 

 is found sufficient for all practical purposes. 



29. It is not, however, absolutely necessary that the inoxydable 

 element c c of the combination should be a metal at all. It is 

 only necessary that it be a good conductor of electricity. In 

 certain voltaic combinations, charcoal properly solidified has 

 therefore been substituted for copper, the solution being such as 

 would produce a strong chemical action on copper. 



30. In the above illustration, we have supposed that the 

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