PULP INSULATION FOR TELEPHONE CABLES 



17 



density covering on the wire, the more rapid drying out of the moisture 

 from the pulp resulting in less shrinkage of the insulation on the con- 

 ductors and the producing of more nearly round and better centered 

 insulation. Of these factors perhaps the one having the greatest 

 effect on lowering the mutual capacitance was that of improving the 

 out of roundness of the insulated conductors. In studying this phase 

 of the problem, advantage was taken of the effect of flatness of the 

 insulation, on the component parts which make up the mutual 

 capacitance. The mutual capacitance of a pair of wires is composed 

 of the direct capacitance between the two wires augmented by a 

 series arrangement of two other direct capacitances, one from each of 

 the two wires to the grounded group consisting of all other wires and 

 sheath. As two wires with oval shape insulation are twisted, there is a 



1.0 I.I 1.2 1.3 1.4 1.5 1.6 1.7 1.1 



DG 

 DM 



.9 2.0 2.1 2.2 



Fig. 9 — Curve showing mutual capacitance versus direct capacitance to ground 

 divided by direct capacitance to mate. 



decided tendency for two flat sides to stay together resulting in the 

 average separation of wire and mate being less than where circular 

 sections are involved. To determine accurately the degree of out of 

 roundness representing the average condition throughout a length of 

 cable by mechanical means is next to impossible, whereas the direct 

 capacitance between wire and mate automatically integrates this 

 condition. Measurements therefore are made of the component direct 

 capacitances and their ratio used as a sensitive indicator of the effect 

 of flatness of the insulation on the mutual capacitance. By using 

 the ratio of capacitances the cable length error is eliminated and 

 accurate determination can readily be made on short lengths of 



cable. 



As illustrative of the above relation there are given in the following 

 table and curve. Fig. 9, data which were obtained on four short 



