IDEA OF POTENTIAL. l8l 



C and C' show the positions of the equivalent line charges. 

 This method of building up particular cases where the electric field 

 distribution (or potential distribution) is known is called the 



Fig. 121. 



method of electrical images. The origin of this term may be illus- 

 trated by discussing Fig. 122. A line charge A near and parallel 

 to a metal cylinder BB having an equal and opposite amount of 



Fig. 122. 



charge per unit length gives in the region RRR an electric field 

 distribution which is exactly the same as would be produced by A 

 and an equal and opposite line charge at B' with the metal cylinder 

 removed. Therefore one may think of the field RRR as being 



