ELECTRIC FIELD. 



at the edge of this crack, and 

 the crack extends farther and 

 farther until the bar is broken 

 in two. 



glass plate 



Fig. 108. 



Figure 108 shows a hardened 

 steel point pressed against a 

 small glass plate. The con- 

 centration of the mechanical 

 stress at the point is so great 

 that a comparatively small 

 force is sufficient to break the 

 plate. 



ductor, as if it were an exten- 

 sion of the metal point. There- 

 fore the electrical stress is con- 

 centrated at the end of the 

 incipient rupture so that the 

 rupture extends farther and 

 farther until the spark reaches 

 from A to B.* 



dielectric 



oil 



oil 



Fig. 109. 



Figure 109 shows a plate of 

 glass or other dielectric be- 

 tween a flat metal plate and 

 a metal point. The concentra- 

 tion of the electrical stress near 

 the point is so great that a 

 comparatively small electro- 

 motive force between metal 

 plate and metal point is suffi- 

 cient to puncture the dielectric. 



* This purely mechanical view of the formation of a spark is inadequate when 

 one goes into a minute study of the phenomena of electric discharge. The point 

 of view which has shown itself to be the most useful is the point of view which is 

 involved in the electron theory which is briefly discussed in Chapter X. 



12 



