240 PLEASANT WA YS IN SCIENCE. 



more for a complete system of communication such as we 

 now have, was hopeless, so long as frictional electricity was 

 employed, or considerable electrical intensity required. 



We have now to consider how galvanic electricity, dis- 

 covered in 1790, was rendered available for telegraphic 

 communication. In the first place, let us consider what 

 galvanic or voltaic electricity is. 



I have said that electricity can be generated in many 

 ways. It may be said, indeed, that every change in the 

 condition of a substance, whether from mechanical causes, 

 as, for instance, a blow, a series of small blows, friction, and 

 so forth, or from change of temperature, moisture, and the 

 like, or from the action of light, or from chemical processes, 

 results in the development of more or less electricity. 



When a plate of metal is placed in a vessel containing 

 some acid (diluted) which acts chemically on the metal, this 

 action generates negative electricity, which passes away as 

 it is generated. But if a plate of a different metal, either 

 not chemically affected by the acid or less affected than the 

 former, be placed in the dilute acid, the two plates being 

 only partially immersed and not in contact, then, when a 

 wire is carried from one plate to the other, the excess of 

 positive electricity in the plate least affected by the acid is 

 conveyed to the other, or, in effect, discharged ; the chemical 

 action, however, continues, or rather is markedly increased, 

 fresh electricity is generated, and the excess of positive 

 electricity in the plate least affected is constantly discharged. 

 Thus, along the wire connecting the two metals a current of 

 electricity passes from the metal least affected to the metal 

 most affected ; a current of negative electricity passes in a 

 contrary direction in the dilute acid. 



I have spoken here of currents passing along the wire 

 and in the acid, and shall have occasion hereafter to speak 

 of the plate of metal least affected as the positive pole, this 

 plate being regarded, in this case, as a source whence a 

 current of positive electricity flows along the wire connection 

 to the other plate, which is called the negative pole. But I 



