336 BELL SYSTEM TECHNICAL JOURNAL 



at a temperature of about 160 to 180° F. with a resultant relative 

 humidity of not over 10 per cent. Cables maintained at this humidity 

 would pick up very little moisture but in transit from the vacuum drier 

 to the storage oven some moisture might be absorbed ; also working in 

 and out of these hot ovens was not particularly pleasant. Therefore, 

 a method was developed for installing the vacuum driers in such a 

 way that one end opens into an enclosed storage area in which the air 

 is maintained at a temperature of about 100° F. and a relative humidity 

 of less than 10 per cent until the cables are covered with lead. This 

 temperature and humidity are obtained by cooling the incoming air 

 to a dew point corresponding to the temperature and relative humidity 

 desired and then passing it into the oven. A considerable engineering 

 problem was involved in determining the heat given off by the vacuum 

 driers and the hot cables and the additional moisture introduced by 

 infiltration through walls, doors, etc. ; also the relation between relative 

 humidity, moisture content of paper and electrical characteristics 

 presented a most interesting field for study. 



The method outlined above has proved very satisfactory as the 

 cables do not absorb enough moisture to affect their electrical properties 

 and the conditions in the storage area are not unpleasant; in fact, 

 during the summer time they are somewhat more agreeable than the 

 outside air during periods of high humidity. 



The process of applying lead sheath to cable is one which has not 

 undergone any change in principle since sheath was first applied di- 

 rectly to the cable instead of cable being pulled into it. There have 

 been, however, a number of developments tending to improve the 

 quality or increase the output. 



In covering a large cable something more than half of the total time 

 of one cycle of operation is taken up by filling the cylinder with lead 

 and cooling under pressure to the point where it can be extruded. 

 The tendency, therefore, has been to build presses with larger lead con- 

 tainers in order to increase the time of extrusion relative to the total 

 cycle. 



The diagram (Fig. 12) shows schematically an early type of press, 

 one which was considered standard a few years ago, and one of the 

 presses designed and built recently. Underneath each press is a figure 

 showing the lead content per charge and the relative amount of lead 

 extruded per hour by each of the three presses. 



As will have been noted from the diagram, the stroke of the newest 

 type of presses is about one foot longer than that of the former presses 

 although the diameter of the lead container and the diameter of the 

 water ram are the same. 



