148 



ADVANCED ELECTRICITY AND MAGNETISM. 



Thus Fig. 96 shows a wire lying in a groove in a glass or porce- 

 lain support, and the surface of the porcelain is everywhere par- 

 allel to the lines of force of the electric field. 



lines of force 



wire 



end view 



side view 



Fig. 96. 



89. Condenser type of insulating bushing. An insulating 

 bushing is a sleeve of insulating material which surrounds a wire 

 where it passes through a conducting wall. Thus when a trans- 

 former is placed in an iron case, the wires which lead out from 

 the transformer pass through insulating bushings in the cover of 

 the transformer case. One difficulty in the design of such a 

 bushing may be appreciated in terms of a mechanical analog as 

 follows: When a very thick walled steel tube is subjected to 

 internal pressure, the metal next to the bore is strained excessively 

 before the outside metal comes into action to any great extent. 

 Thus the full strength of the metal wall is not available for the 



withstanding of internal pressure. In 

 the case of the bushing, the electrical 

 stress in the insulating material next 

 to the wire may be excessive, while 

 the electrical stress in the outer por- 

 tion of the bushing is very low, thus 

 the full electrical strength of the bush- 

 ing is not utilized. 



The condenser type of insulating 

 bushing is a bushing designed so as to equalize the electrical 

 stress throughout the material of the bushing, and the principle 



wire 



wire 



Fig. 97. 



