260 



EIGHTEENTH CENTURY. 



FT. III. 



a galvanometer^ p. 365), by which the passage of the 

 faintest electric current can be detected, he proved that 

 such a current does exist in the frog, and it has since been 

 found to be common to all animals. 



Meanwhile, however, Volta had also made a very re- 

 markable discovery, namely, that two different metals when 

 joined together in contact with moisture, and separated from 

 other substances, produce a current of electricity. This 

 may easily be tried in its very simplest form. If you take 

 a. halfpenny and a half-crown and put one above your 

 tongue and one below it, you will- feel nothing remarkable 



FIG. 46. 

 Volta's Crown of Cups (Fownes). 



z, Zinc. C, Copper, a, a, I, Connecting wires. The arrows show the coui 

 the positive currents. 



of 



so long as the two metals are kept separate, but directly 

 you let them touch each other at the ends, a tingling sensa- 

 tion will pass through your tongue, proving to you that an 

 electrical current is passing between the metals. If you 

 put the half-crown under your lip, so that the halfpenny 

 may remain outside your mouth, you may, perhaps, even 

 see a slight flash when the two metals meet. 



Volta found not only that it was necessary to have mois- 



** ture between the two metals, but that some acid put in the 



water greatly increased the strength of the electricity. Fig. 



46 shows the first electric battery which he made, and 



