NEUTRAIJZATION OF TELEGRAPH CROSSFIRE 



423 



An additional serious effect of crossfire is that it interferes with 

 the obtaining of accurate duplex balance adjustment, since cross- 

 fire currents mask the effect of small changes in the balancing ar- 

 tificial line. 



Principles of Neutralizing Arrangements 



The principles involved in neutralizing the crossfire will first be 

 discussed for the simplest case, that is, with only two parallel wires, 

 reserving the case of four wires for the next section of this paper. 



Sending-end Crossfire 



An arrangement suitable for neutralizing the sending-end cross- 

 fire between two polar-duplex circuits is illustrated in Fig. 4. The 



Sldtion A Stalion B 



III II h --1 



Fig. 4 — Method of neutralizing sending-end crossfire between two telegraph circuits 



heavy arrows indicate the disturbing line current which flows when 

 the tongue of the pole-changer of circuit Y at station A moves from 

 the negative to the positive pole. The feathered arrows show the 

 direction of the resulting crossfire currents w^hich tend to flow through 

 the polar relays of circuit X. It will be apparent that the sudden 

 increase, in a positive direction, of the potential applied to circuit Y 

 would cause an impulse of current in the relay of X at the sending 

 station A in the direction shown if the circuits were coupled by cap- 

 acity only or by the natural mutual inductance of the two parallel 

 ground-return circuits. Neutralization is effected by providing a 

 mutual admittance between the two balancing artificial lines to 

 simulate that existing between the real lines. It will be clear that 

 upon the operation of the pole-changer of circuit Y, an impulse will 

 pass through the neutralizing circuit C, R, and through the relay 

 of circuit X at A in such direction as to oppose the crossfire current. 



