162 ADVANCED ELECTRICITY AND MAGNETISM. 



^ glass plate 

 copper 



Fig. 110. 



Figure no is the same as 

 Fig. 1 08 except that a soft bit 

 of copper is placed under the 

 steel point. In this case the 

 copper gives way, and the 

 stress is not so greatly con- 

 centrated. In fact it is the 

 same as if one were pushing a 

 flat ended tool against the 

 glass, when a very great force 

 is required to break the glass. 



^dielectric 

 air 



air 



Fig. 111. 



Figure in is the same as 

 Fig. 109 except that the point is 

 surrounded by air instead of oil. 

 The electricstrengthof the air is 

 less than that of the layer of 

 dielectric, and therefore the air 

 breaks down electrically and 

 prevents an extreme concen- 

 tration of electrical stress near 

 the point. The result is very 

 much as if the plate of dielec- 

 tric were placed between two 

 flat metal plates, when a very 

 arge voltage is required to 

 puncture the dielectric. 



