210 BELL SYSTEM TECHNICAL JOURNAL 



sheathing operation. This was accompHshed by passing calcium 

 chloride dried air at a temperature of 270 degrees Fahrenheit through 

 the heated cable for a period of twenty-four hours. About 1917 the 

 brick ovens were replaced by steam heated vacuum tanks which 

 reduced the drying period to about one-third that used for the brick 

 ovens and very greatly improved the dryness of all types of telephone 

 cable. 



Calcium chloride drying of toll cable was continued until 1927. It 

 then became necessary to provide a means of keeping the cable from 

 regaining appreciable amounts of moisture between the vacuum dry- 

 ing operation and lead covering. After considerable investigation a 

 dry core storage room was developed where the dry cable could be 

 held at .3 per cent to .5 per cent relative humidity until ready for 

 covering.^ With the improvement in vacuum tank drying and with 

 cable stored under such a dry condition until the protective sheath 

 could be applied, transmission quality of the shorter lengths of cable 

 approached the level of the calcium chloride drying while there was 

 some improvement in drying the longer lengths. This change in 

 handling cable resulted in a large reduction in drying cost. 



Still further improvements in drying methods have been obtained 

 by heating the cable electrically rather than by radiation from steam 

 coils in the drying chamber. Considerable thought had been given 

 to this method over a number of years and the first unit of equipment 

 was installed experimentally at the Baltimore Works in 1931. This 

 unit has been in successful operation since that time for drying a part 

 of the toll cable output and has furnished most of the data used in 

 engineering the Kearny installation. The choice of the Kearny Plant 

 for the first large scale installation was due largely to reduced oper- 

 ating and maintenance costs. The following discussion covers the 

 type of equipment used in this installation and points out in closing 

 some of the advantages gained in reduced cost and better drying 

 of cable. 



In preliminary experimental work on electrical drying a low voltage 

 transformer was used to supply alternating current and the cable 

 rendered non-inductive on its core truck by short-circuiting one end 

 and dividing the other end and attaching to the source of power. 

 However, such a set-up places the full voltage between a considerable 

 portion of the conductors near the clamped ends, a condition not 

 suited for telephone cable. Since, in telephone cable, the conductors 

 are insulated with a thin tube of pulp or ribbon paper, the insulation 



* Drying and Air Conditioning in Cable Manufacture, J. Wells and L. G. Wade, 



Chemical and Metallurgical Engineering, March 1932. 



