CHAP, iv.] ELECTRICITY AND MAGNETISM. 



199 



of one erg see the Note on Units at the end of this 

 lesson) ; there will be then unit difference of potential 



\ 



s , 



? 



Fig. 97. 



between the surface drawn through Q and that drawn 

 through P, and it will require one erg of work to carry 

 a + unit from any point on the one surface to any point 

 on the other. In like manner we might construct a 

 whole system of equipotential surfaces about the point A, 

 choosing them at such distances that there should be 

 unit difference of potential between each one and the 

 next. The widths between them would get wider and 

 wider, for, since the force falls off as you go further from 

 A, you must, in doing one erg of work, bring up the 

 + unit through a longer distance against the weaker 

 opposing force. 



The form of the equipotential surfaces about two small 

 electrified bodies placed near to one another would not 

 be spherical ; and around a number of electrified bodies 

 placed near to one another the equipotential surfaces 

 would be highly irregular in form. 



243. Lines of Force. The electric force, whether 

 of attraction or repulsion, always acts across the equi- 

 potential surfaces in a direction normal to the surface. 

 The lines which mark the direction of the resultant 

 electric forces are sometimes called Lines of Electric 



