MEASUREMENTS IN MULTIPAIRED CABLES 663 



pendix II. e^/emut ratios obtained for three pulp insulated cables do not 

 differ from unity by more than d=2%. These results, although not con- 

 clusive, tend to substantiate the above qualitative comparisons. It is be- 

 lieved, therefore, that e values obtained for paper ribbon or pulp insulated 

 cables are probably accurate to about 1 or 2%. 



Table III shows results of the use of the methods herein described to obtain 

 the average S, e and effective D for examples of some cable designs now 

 in use or under experimental investigation. These data are included pri- 

 marily to illustrate the method for a variety of cases, and are not compre- 

 hensive enough to serve as the basis for a study of the various cable designs 

 and insulations. 



c. Dielectric Constant from L X C at High Freqttencies 



For the ideal pair structure the mutual capacitance and inductance are 

 inversely related when the frequency is so high that current does not pene- 

 trate either conductors or shield. The relation between inductance and 

 capacitance, in practical units, is then: 



€= 34.70 CL«, (5) 



Loo = limiting value approached by inductance as the fre- 

 quency increases indefinitely — mh/mi. 

 C = capacitance — /zf/mi. 



It is known that Loo of individually shielded pairs, that is, pairs having 

 metallic tape shields applied with an overlapped longitudinal or helical 

 seam, can be accurately approximated from a series of inductance tests using 

 the relation: 



Lf = L^ + ^y (6) 



Where Lf = distributed inductance at frequency F. 



M = a constant 



F = frequency 

 The tests are usually made in the range from 2 to 5 mc. Application of 

 formulas (5) and (6) to individually shielded pairs is known to be an ac- 

 curate technique for evaluating the dielectric constant of non-homogeneous 

 combinations of air and solid materials and has been in use for many years. 

 This same method has sometimes been used with cable pairs. However, 

 for the filled cable having a dielectric constant of about 2.20, these techniques 

 gave a dielectric constant value of 2.58, which is about 17% too high. The 

 reason for this large discrepancy is apparent when it is considered that the 

 inverse relationship of L and C obtains only when the static and magnetic 



