176 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



gerated in Fig. 109. In this figure a floating particle and a 

 minute projecting point on the plate B are represented. The 

 intensity of the electric field near the point and near the ends of 

 the particle is much greater than the average intensity Ejx 

 between the plates, and the dielectric begins to give way at these 

 places when the field intensity there reaches the breaking value. 



. TABLE.* 



Dielectric strengths. 



Lines of force of the electric field between two oppositely 

 charged metal spheres are shown in Fig. no. In this case the 



electric field is not uniform, 

 and the intensity of the elec- 

 tric field in volts per centi- 

 meter near the surface of one 

 of the spheres may be suffi- 

 cient to start a rupture, al- 

 though the intensity of the 

 field at a distance from the 

 surface of one of the spheres may be much less than that which 

 corresponds to the rupture of the dielectric. Furthermore, the 

 electric field intensity in Fig. 1 10 is not, of course, equal to the 

 electromotive force between the spheres divided by their distance 

 apart, because the field is non-uniform. Therefore the electro- 

 motive force required to rupture a dielectric between two metal 

 spheres is not proportional to their distance apart. 



*From the measurements of Macfarlane and Pierce, Physical Review, Vol. I, 

 page 165. 



Fig. 110. 



