ELECTRIC FIELD. 



147 



the greater part of the magnetomotive force of a dynamo field- 

 winding upon the air gap in the magnetic circuit; a small portion 

 only of the magnetomotive force is required to force the magnetic 

 flux through the highly permeable iron, and a large portion of the 

 magnetomotive force is required to force the magnetic flux 

 through the less permeable air layer. A small portion of the 

 total voltage is required to force the electric flux through the 

 highly inductive glass in Fig. 93 and a large portion of the 

 voltage is required to force the electric flux through the less 

 inductive air. 



Mechanical analog of the above. The glass and the air layers 

 are in series in Fig. 93 and the electric flux density or electrical 

 strain, or yield, is the same in 

 both ; whereas two mechanical 

 elements have the same stress 

 when they are in series; to get 

 the same strain, or yield in two 

 mechanical elements they must be 

 in parallel. Thus Fig. 95 shows 

 a column of steel and a column 

 of rubber equally compressed 

 between two bars A and B 

 (the steel and rubber columns 

 are in parallel); the easily yielding rubber (high inductivity) 

 supports only a small part of the compressing force, and the stiff 

 steel (low inductivity) supports a large part of the compress- 

 ing force. 



Best shape of insulator. When the electric flux passes from 

 one insulating substance into another, an excessive concentration 

 of electrical stress is likely to be brought about in the insulating 

 substance of smaller inductivity, as above mentioned. There- 

 fore, whenever possible, the surface of an insulator should be 

 everywhere parallel to the lines of force of the electric field. 



Fig. 95. 



