NEUTRALIZATION OF TELEGRAPH CROSSFIRE 419 



closely parallel circuits extending between two stations, consider- 

 able interference is generally experienced both at the station from 

 which the disturbing signal is transmitted and also at the distant 

 station. In this paper, the crossfire current (noted in the interfered — 

 with circuit) at the station from which the interfering signal is sent 

 will be referred to as " sending-end crossfire " and that at the distant 

 station as "receiving-end crossfire". For example, assume two 

 parallel wires from A to B; if a signal be sent on wire No. 1 from A 

 to B, sending-end and receiving-end crossfire will appear in the receiv- 

 ing apparatus of wire No. 2 at A and B, respectively. This may 

 mutilate incoming signals, or in extreme cases cause false signals. 



The type of line circuit and the kind of apparatus employed have 

 a considerable effect upon the amount of crossfire between circuits. 

 It has been found that it depends chiefly upon the amount of mutual 

 capacitance and, to a lesser extent, upon the natural mutual in- 

 ductance of the wires; mutual conductance or leakage is responsible 

 for some d-c. crossfire during periods of low insulation resistance 

 but this increment is in general comparatively unimportant. As 

 will be brought out later, loading^ of circuits has a large efifect on 

 crossfire. Such factors as the gauge of wire, separation between 

 wires, length of circuit and the presence of other wires on the same 

 pole line have, of course, considerable influence. 



In the Bell System plant, crossfire is in general of little consequence 

 except among the four wires of a "phantom" group, the reasons 

 for which will be discussed later. It is of interest to note that receiv- 

 ing-end crossfire is comparatively much more serious between wires 

 in cable than between those of open-wire lines. Entrance cable, 

 that is, cable employed to bring open-wire circuits into large cities, 

 has comparatively little effect, as the length is generally short. Such 

 apparatus as the composite sets which are used to derive d-c. tele- 

 graph circuits from telephone wires, and in certain cases filters used 

 in connection with superposed carrier-current systems, contribute 

 to crossfire inasmuch as they introduce some coupling, chiefly mutual 

 capacitance. 



Fig. 1 shows schematically the circuit arrangement of the polar- 

 duplex telegraph apparatus in conjunction with a pair of wires com- 

 posited for simultaneous telephone and telegraph operation. These 

 types of apparatus are well known and will therefore be described 

 only briefly. Independent two-way telegraph transmission is pos- 

 sible on each wire since the receiving relay occupies a position in a 



1 See "Development and Application of Loading for Telephone Circuits", Shaw 

 and Fondiller, A. I. E. E. Jour. March, 1926. 



