426 BELL SYSTEM TECHNICAL JOURNAL 



the four wires of a phantom group only. Then arrangements for 

 use with a group of four wires employing the principles explained 

 above in connection with the case of two parallel wires, will be covered. 



It is of interest to consider the effect of the loading coils which 

 are employed in conjunction with many telephone lines.^ In Fig. 6, 

 coils D and F represent side-circuit loading coils on pairs 1-2 and 3-4, 

 respectively, and P a phantom-circuit loading coil. Such coils are 

 connected into telephone circuits at intervals to introduce inductance 

 into the two telephone side circuits and the phantom telephone cir- 

 cuit, respectively. 



The action of the side-circuit coil, (D), will first be considered. 

 If a positive telegraph impulse is sent from A to B over wire 1, as 

 indicated by the heavy arrow, it is evident that the coil acting as 

 a transformer will set up a crossfire current in wire 2 in the same geo- 

 graphical direction, as indicated by the feathered arrow. The rela- 

 tion of this impulse to those due to capacity coupling is of interest 

 since the capacity effect predominates. Comparison with Fig. 5, 

 will show that at the transmitting station the impulse due to coil D 

 will oppose the sending-end crossfire which is due to capacity coupling 

 between circuits, while at the distant end it will augment the receiv- 

 ing-end crossfire due to capacity coupling. Coil F functions sim- 

 ilarly in pair 3-4. 



In the case of the phantom loading coil (P) sending an impulse 

 from A to B on wire 1 results in disturbing currents in the same 

 geographical direction in wires 3 and 4 and in the opposite direction 

 in wire 2, since the coil is connected so that two windings are series- 

 opposed in each side circuit and parallel-aiding in the phantom cir- 

 cuit. Comparing with Fig. 5, as before, it will be seen that for wires 

 of a group but not of the same pair these coils tend to neutralize the 

 sending-end crossfire which is due to mutual capacitance and augment 

 the receiving-end crossfire due to capacity coupling; the conditions 

 will be reversed however for wires of the same pair. 



In the case of loaded circuits, crossfire, therefore, is due to loading 

 as well as to the mutual capacitance and inductance of the wires 

 and the coupling which exists in office apparatus, so that the final 

 result is difficult to predict. Work with loaded circuits, which has 

 been largely confined to cables indicates that on such circuits receiv- 

 ing-end crossfire is generally greater than sending-end crossfire, and 

 sending-end crossfire between wires which are in the same phantom 

 group but not in the same pair is so small as to be almost negligible. 



Line transposition of telephone circuits has been discussed at 



' Shaw and Fondiller, loc. cit. 



