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BELL SYSTEM TECHNICAL JOURNAL 



circuit c will, however, be negligible compared with the series e.m.f. 

 proportional to current in circuit a. This is evident since the current 

 in circuit c is a crosstalk current which approaches zero as dx ap- 

 proaches zero while the current in circuit a does not vary with dx. 





ICL 



Ic 



dx 



Fig. 34 — Schematic used in deriving formulas for indirect crosstalk coefficients. 



The shunt e.m.f. in circuit h dependent on the charges of circuit c is 

 not, however, negligible compared with the shunt e.m.f. in circuit h 

 due to charges in circuit a since the charges in both a and c approach 

 zero as dx decreases. In other words the magnetic field of circuit c 

 may be neglected but the electric field must be considered. (Both 

 fields must be considered in computing interaction crosstalk.) 



To determine the equivalent shunt e.m.f. in circuit h which depends 

 upon the electric field of circuit c the voltage between the wires of 

 circuit c must be determined. If circuit c did not exist, the electric 

 field of circuit a would cause a difference of potential between the 

 points actually occupied by wires 5 and 6 at the left-hand end of dx 

 in Fig. 34. This difference of potential would be: 



V ac ^ V a^a yac ' a-l ac 



With circuit c present, this difference of potential is changed to 

 Vc, the actual voltage across circuit c. The voltage could not change 

 from Vac to Vc without charges on circuit c and the charge per wire 

 per unit length is proportional to the change in voltage from V,,,- to 

 Vc which may be designated Uc. The equivalent shunt e.m.f. in 



