TRANS^fISSION OF PO WER BY ALTERNA TING CURRENT 403 



renditions, the insulator string acts as a flexible support for the 

 conductor and offers little or no resistance to horizontal forces. 

 Hence, rli, stresses in adjacent spans should be nearly balanced 

 or the string will be pulled out of the vertical line. When a 

 span breaks, the string is thrown temporarily into the adjacent 

 unbroken span as a strain or dead-end insulator. Suspension 

 insulators are also used as strain 

 insulators at deadends, railroad 

 . Fi mi re 367 shows 

 a section of a link-type suspen- 

 sion insulator in which the sus- 

 pension loops link each other. 

 I iuure 368 shows a string of 

 such insulators arcing overundei 

 high voltage. 



.'.07. Section of link-type 



in insulator. 



368. Arc-over, nt fi() cycles, of :i 

 (.-unit string of link-type insulators. 



172. Transmission Structures. There are three umeral types 

 of transmission structures employed in this country, wooden poles. 



Steel pi I steel tOWOTB. < 'oncrele poles a re use< 1 occasionally . 



Wooden poles are used on the lighter lines, especially where the 

 voltage i- low. Wooden poles have the advantage of bring 

 cheap, particularly when n.-ed near wooded -eetions. They are 

 also light, easily fitted and effected On the other hand, their 

 comparatively short H> that they require frequent renew- 

 al-. They are not MiHicie M tl\ for heavy li- -iting 

 at hiiih \ limited height of wooden poles, 

 the spans must be sh 



