180 ADVANCED ELECTRICITY AND MAGNETISM. 



107. The method of electric images. Any equipotential surface 

 in an electric field may be wholly or in part replaced by a thin 

 metal shell without altering the electric field distribution ; and if an 

 equipotential surface is wholly replaced by a thin metal shell, the 

 electric field on one side of the shell may be obliterated without 

 altering the field on the other side. 



Thus one or both of the equipotential surfaces GH and EF 

 in Fig. 118 may be replaced by metal, and the electric field be- 

 tween them will remain unchanged. In this way it is known 

 that the distribution of electric field between two parallel metal 



Fig. 120. 



cylinders of equal or unequal diameters is identical with a certain part 

 of the electric field between parallel line charges properly spaced. 

 Thus Fig. 1 20 shows accurately the trend of the electric lines of 

 force between two parallel metal cylinders of the same diameter. 

 The very small circles show the positions of the equivalent line 

 charges. Simi'arly Fig. 121 shows accurately the trend of the 

 lines of force between two parallel metal cylinders one of which 

 surrounds but is not co-axial with the other ; the small circles 



