ELECTROSTATICS 



35 



(5) If a sphere of dielectric constant K is placed in the field, as 

 shown in Fig. 28, the permeance of the path between the plates is 

 increased, but the increase of flux is not confined to the sphere. 

 Where the sphere approaches nearest to the plates the density is 

 very great, since a large number of lines take the short path 

 through the air in order to follow the long path through the 

 sphere. The stress in the air in these regions is very largely in- 

 creased. 



(6) In Fig. 29 A is a small piece of material with a high dielectric 

 constant such as a drop of oil. It offers a local path of high 



FIG. 29. 



FIG. 30. 



permeability, and a greater flux density is produced in it than in 

 other parts of the field, and therefore the stress in the air at its 

 surface will be greater than the average stress throughout the 

 field. 



(7) In Fig. 30 the drop of oil is replaced by a knob on the con- 

 ductor forming one boundary of the field. Since the dielectric 

 constant of the conducting knob is infinity the stress in the air at 

 its surface will be greater than in the case of the oil. 



(8) Fig. 31 shows the field at the edge of the condenser in case 

 (1). In this region the dielectric flux density is not uniform, and 

 just at the edges of the plates it is greater than in the main body 



