CHARACTERISTICS OF TOLL TELEPHONE CABLES 



303 



R. P. Booth^° who made measurements on 18-gauge and 20-gauge pairs in 

 various arrangements and on a 55-foot length of 19-gauge quadded cable. 



Overall inductance variations of 19-gauge pairs with frequency and 

 temperature are shown by the curves of Figs. 6 and 7. 



The magnitude of inductance variations from pair to pair in reel lengths 

 of cable is about ±3 per cent from the mean with a standard deviation of 

 about 1.5 per cent. 



.00112 



.00110 



LfJ .00106 



Z .00104 



.00102 



jOOIOQ 



10 



20 



70 



80 



100 



30 40 50 60 



FREQUENCY - KILOCYCLES 



Fig. 6 — Inductance per mile vs. frequency — 19 gauge pairs 



Capacitance 



The usual formula for capacitance of two parallel wires in space, separated 

 by a distance negligible compared with their length, is 



0.019415 



C = 



1 2Z) 



logio^ 



X 10 farads per loop mile 



(6) 



Conditions in a cable are vastly different from those assumed in this formula 

 which assumes that the two wires are at a great distance from other wires 

 and from the ground. In the cable, pairs are twisted and, in addition, other 

 wires are very near and the sheath is effectively at ground potential, resulting 

 in a considerable modification of the capacitance. Moreover, the formula 

 assumes that the wires are in air, which has a dielectric constant almost 

 equal to unity. (1.00059 at 0° Centigrade) The dielectric constant of 



1" R. N. Hunter and R. P. Booth, "Cable Crosstalk—Effect of Non-Uniform Current 

 Distribution in the Wires," B.S.T.J., XIV (1935) pp. 179-194. 



