IDEA OF POTENTIAL. 



189 



where e is the intensity of the uniform field. The locus of the 

 points where the two fields would give the same potential is 

 shown by the dotted curve in Fig. 128, and the equation to this 

 curve may be found by equating the two expressions for potential 

 given above. 



In order that the two fields as indicated by the coarse and fine 

 lines of force in Fig. 128 may actually be obtained, it is sufficient 

 to have the cylinder of radius r and the end disks at zero 

 potential; to have the middle disk at potential el\ and to pro- 



plate 



C .-t 



cylinders 

 insulation 



Fig. 129. 



vide a number of insulated rings of tinfoil along the common 

 boundary of the two fields (as represented in section by the sepa- 

 rate dots of the dotted curve), these tinfoil rings being brought 

 to potentials which conform to both fields. The arrangement 

 shown in Fig. 128 is such as would be used around one leg of the 

 core of a core-type transformer, and the various parts of the 

 boundary (tinfoil rings, disks, etc.) are brought to the desired 

 potentials by being connected to tap-points on the high-voltage 

 coil of the transformer. 



