ELECTRICAL APPLIANCES AND MACHINES 253 



The potential or electrical pressure of the zinc is about 

 1.86 volts higher than that of the liquid, while the copper 

 has a potential of only 0.81 volts higher than the sur- 

 rounding liquid, because the copper is oxidized less easily 

 than the zinc. So the zinc has a potential of about 1.05 

 volts higher than the copper, but still there will be no 

 flow of electricity that can be detected until the copper 

 is connected to the zinc by a conductor. If the zinc and 

 copper are made to touch at the top or are connected by 

 a long conductor, there will be a rush of electricity through 

 the acid from the zinc to the copper and from the copper 

 to the zinc through the external conductor, as indicated 

 by the arrows in the illustration. A small portion of the 

 zinc is at the same time dissolved, the zinc parting with 

 its stored energy as its atoms combine with the acid. 

 This energy is spent in forcing electricity through the 

 acid to the copper strip and thence through the external 

 circuit back to the zinc strip. 



Electricity flows from high potential to lower potential, 

 or from positive to negative, represented by plus (+) 

 and minus ( ) respectively. Since the current flows 

 from the zinc to the copper in the liquid, the zinc is the 

 positive plate and the copper the negative plate when 

 the current in the cell is under consideration. Since the 

 current leaves the cell by way of the copper strip and 

 flows through the wire to the zinc, the copper is the 

 positive pole and the zinc is the negative pole, when the 

 external circuit is under consideration. 



If two copper wires are united to the tops of the two 

 strips, one to the zinc and one to the copper, though no 

 current flows as long as the two wires are kept separate, 

 the wire attached to the copper will have a positive 

 charge and the one attached to the zinc will have a 



