22 BELL SYSTEM TECHNICAL JOURNAL 



equal and opposite currents at any point. These currents would 

 produce a magnetic field as indicated on the figure. If circuit 3-4 

 parallels 1-2 a certain amount of this magnetic flux would thread 

 between wires 3 and 4 and induce a voltage in circuit 3-4 which would 

 result in a crosstalk current in this circuit. This induced voltage is, 

 of course, due to the difference between the two magnetic fields set 

 up by the opposite directional currents in wires 1 and 2. Since wires 

 1 and 2 are not very far apart, the resultant field is much weaker than 

 if transmission over wire 1 with ground return were attempted. It is 

 important, therefore, that the wires of a circuit be placed as close 

 together as practicable and that these wires be similar in material and 

 gauge in order to keep the currents practically equal and opposite. 



Equal and opposite charges accompany the equal and opposite 

 currents in wires 1 and 2. The equipotential lines of the resultant 

 electric field set up by the two charges are also indicated by Fig, 1. 

 This field will cause different potentials at the surfaces of wires 3 and 

 4 and this potential difference will cause a crosstalk current in circuit 

 3-4. As in the case of magnetic induction this current may be 

 minimized by close spacing and electrical similarity between the two 

 wires of a pair. 



Calculations of crosstalk coupling must, in general, consider both 

 the electric and magnetic components of the electromagnetic field of 

 the disturbing circuit. 



The exact computation of crosstalk coupling between communication 

 circuits is very complex.^ Approximate computations are sufficient 

 for transposition design. In such computations, it is convenient to 

 divide the total coupling into components of several general types. 

 In calculations of coupling of these types it is assumed that the two 

 wires of a circuit are similar in material and gauge. If there is any 

 slight dissimilarity, such as extra resistance in one wire due to a poor 

 joint, the effect on the crosstalk may be computed separately. The 

 general types of crosstalk coupling are : 



1. Transverse crosstalk coupling. 



la. Direct. 

 \h. Indirect. 



2. Interaction crosstalk coupling. 



A multi-wire pole line involves many circuits all mutually coupled. 

 In explaining the above terms, it is convenient to start with the 

 simple conception of but two paralleling coupled circuits; Fig. 2 A 



^ The general mathematical theory is given in the Carson-Hoyt paper listed under 

 "Bibliography." 



