Blcctric <3enerator6. 63 



another name for pressure; the symbol for 

 which is E. M. F. 



If we were to put two zinc plates in the 

 hatu-ry fluid and connect them in the ordinary 

 way there would be no electricity evolved (as- 

 suming that they were perfectly homoge- 

 neous), because they are both of the same po- 

 tential, or have the same possible amount of 

 stored electrical energy measured by its work- 

 ing power. If one of the zinc plates were softer 

 than the other, a feeble current would be de- 

 veloped, for one would be more readily acted 

 upon by the acids than the other. The bat- 

 tery that has been most used in America for 

 telegraphic purposes is called the gravity-bat- 

 tery. It is constructed by putting a copper 

 pinto in some form at the bottom of a jar, 

 usually of glass, and filling it partly full of the 

 crystals of sulphate of copper, commonly called 

 " bluestone." Zinc, usually cast in some open 

 form, so as to expose a large surface to the 

 solution, is suspended in the upper part of the 

 jar, which is then filled with water till it 

 covers the zinc. The zinc is the positive 

 im-t.-il. lut it i> called the negative pole. The 

 energy developed by the zincpasses from zinc to 

 copper and out on the circuit from the copper 

 pole. Hence the copper came to be called the 

 v<- p.lr. although in relation to zinc it is 

 negative. Copper would, however, be positive 

 to some other metal whose potential was less. 



