ELECTRIC POTENTIAL. 13 



tive terms to designate different degrees of potential and 

 not different kinds of electricity. 



The sign ( + ) is used to denote positive potential, and 

 ( ) to denote negative potential. 



The earth's potential, then, is the electric zero, just 

 as the freezing point is the zero of temperature in the 

 centigrade thermometer, and all uninsulated bodies are 

 said to be connected with the earth, and to have zero 

 potential when not under special influence from insu- 

 lated, electrified bodies in their vicinity. 



When the electric potential of a body is changed 

 from zero by an increase of its electricity, it is said to 

 be positively electrified ; and when its potential is 

 changed from zero by a decrease, it is said to be nega- 

 tively electrified. 



ELECTRIC MOVEMENT. When a difference of electric 

 potential exists between different bodies, or different 

 parts of the same body, there is a constant tendency to 

 equalization. 



A state of equilibrium seems to be the natural condi- 

 tion of bodies, and to produce difference of potential 

 requires, as we have seen, the exercise of force in the 

 performance of work, by which this equilibrium is dis- 

 turbed. 



We find in other forms of energy, as gravity and heat, 

 the same tendency to equilibrium, requiring the exercise 

 of force to overcome it, as in the illustrations already 

 given. 



The restoration of equilibrium is always effected by 

 a transfer of energy from the body having the greater 

 to the one having the less energy ; that is, from higher 

 to lower potential. 



In the case of gravity this transfer of energy carries 



