FAST AND HEAVY RAILWAY SERVICE. 257 
34 in. in diameter, while the track rails are bonded to the 
main structure at their middle points. 
The feeders on this road are of copper cable, bare on 
the structure, but boxed over. They are supported every 
ten feet by vitrified clay insulators arranged as shown in 
Fig. 132. Every hundred feet this clay insulator is replaced 
by an iron clamp provided with insulating bushing. The 
cables are of 1,000,000 and 1,500,000 ¢. m. section. 
The contact rail is well guarded in this construction, 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
FIG. 132, 
a precaution that should be carriea out on every such road 
and particularly when a contact rail is used as at Nan- 
tasket for a surface road. 
It is highly probable that copper feeders are in the 
long run more economical than feeders composed of rails. 
When freshly bonded the rail feeders just described had 
about one-tenth the net conductivity of the same weight 
of copper. At present prices of new rails and copper the 
total cost of the feeding system is about the same by 
either method, with the maintenance and depreciation