12 ELEMENTS OF STATIC ELECTRICITY. 



are simply different forms of expression for difference of 

 potential. 



To produce this difference work must first be ex- 

 pended, and this work is the measure of the potential 

 acquired. 



The lifting of the pound weight ten feet against the 

 force of gravity gave it the ten foot-pounds of gravity 

 potential. The work of heating the metal, whether 

 represented by combustion, by friction, or by concus- 

 sion, gave it the one thousand degrees of heat potential. 

 And the rubbing of the sealing-wax gave it the one- 

 sixteenth of a foot-pound of electric potential. 



As there is ordinarily no practical difference of elec- 

 tric potential between different points on the -earth, 

 within a limited area, its potential is considered zero, 

 and taken as the base of all measurements of electric 

 potential. 



The qualification of this statement, as above, becomes 

 necessaiy, since there are often great differences of po- 

 tential over widely separated areas. 



POSITIVE AND NEGATIVE. Bodies whose potential 

 is higher than that of the earth are said to have positive 

 potential, while those whose potential is lower are said 

 to have negative. 



The potential of bodies is also considered positive or 

 negative with reference to each other. If a body has 

 a higher potential than the earth, but lower than that 

 of another body, it is said to be positive with reference 

 to the earth, but negative with reference to the other 

 body. In like manner a body may have negative 

 potential with reference to the earth, but positive with 

 reference to another body of lower potential. 



Hence, positive and negative are merely convenient rela- 



