LEAD-COVERED PAPER-INSULATED TELEPHONE CABLE 449 



placed in position over the moisture proofing membriine. The floor 

 was prepared in a similar manner. 



The concrete ceiling of the room was covered with a layer of alumi- 

 num foil suitably overlapped and held in place by varnish. 



As an added protection, all entrances are vestibuled and all cable 

 ports are equipped with air tight cable tubes leading to the presses. 

 When the press is not in use an air tight door is closed over the inner 

 end of the cable port. 



Air Conditioning Equipment 

 The primary object in drying toll cable is to obtain as low con- 

 ductance and capacitance values and as high insulation resistance 

 as possible. This has a very important effect on the transmission 

 quality of the cable, and consequently justifies considerable expense. 



20 40 60 80 100 120 140 



HOURS OF STORAGE 



Fig. 18 — Effect of moisture regain on conductance of vacuum dried cable. 



A large amount of experimental work has been done to determine 

 the best methods of obtaining and retaining dry cables. At the end 

 of the vacuum drying cycle the cable paper is in such a dry condition 

 that its moisture regain when exposed to higher humidities is exceed- 

 ingly rapid. This is indicated by Fig. 18, showing the increase in 

 conductance over a period of hours when dry cable is exposed to 

 approximately 6-7 per cent relative humidity. 



Working from these data and an estimate of the manufacturing ad- 



