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BELL SYSTEM TECHNICAL JOURNAL 



easily repaired. Care must be exercised, however, to prevent conden- 

 sation of metal and fluxes in the pores of the insulator. 



In open wire construction it is customary to accommodate line 

 variations brought about by temperature changes by adjusting the 

 sag of the conductors. Provisions for temperature variations in con- 

 centric tube lines are not so simple. The first obvious remedy is to 

 employ lines buried at sufficient depth so that temperature changes 

 are reduced to a slow seasonal variation. At the present time a buried 

 3/8-inch line has been in service for more than one year without de- 

 veloping faults. Without longer experience with concentric lines we 

 would question the advisability of burying larger lines which are to be 



Fig. 18 — ^A short section of three-quarter inch diameter line showing support for 

 holding line in a sinuous form. 



employed at high voltages due to the difficulty of finding faults should 

 these occur. 



A very simple scheme, suitable for small lines, is to reduce the 

 effects of temperature variations by laying the line in a sinuous path as 

 shown in Fig. 18. This construction permits the line to buckle slightly 

 at the curves as the length varies and cumulative changes in length do 

 not appear at the line terminals. The inner conductor is held loosely 

 within the sheath so that it may buckle independently of the sheath. 

 The outer conductor changes its length both at a different rate and at 

 a different time from the inner conductor. With increasing tempera- 

 ture the sheath is at a higher temperature than the inner conductor. 

 There is an appreciable time lag in heating of the inner conductor due 



