THE PHENOMENA OF ELECTROSTATICS. 



215 



other plate. Figures 148 to 151 show the successive aspects of 

 the electric field while the ball is moving once across from plate 

 to plate. 



(c) The electromotive force between the two metal balls A 

 and B, Fig. 1 10, may be increased until the intervening dielec- 



Fig. 148- 



Fig. 150. 



Fig. 149. 



Fig. 151. 



trie breaks down, causing the formation of an electric spark. An 

 electric spark is a conducting path, like a wire, and its effect is to 

 completely discharge the two balls A and B. 



122. Progress of the electric spark. Let A and B, Fig. 

 152, be two metal balls upon which electric charge has accumu- 

 lated until the intensity of the electric field has reached the 



