INDUCTIVE COORDINATION 



85 



cuits either directly or through intervening trunk circuits and pro- 

 viding supervision during the call. 



The line signaling equipment with its associated relay is either 

 bridged across the line or arranged so that, when two subscriber cir- 

 cuits are interconnected, any ground connections on the line relays are 

 automatically opened. The line signaling equipment is not, therefore, 

 ordinarily a factor in noise considerations. Occasionally, however, the 

 effect on noise of the ground connection on the line signaling equip- 

 ment requires specific treatment when the longitudinal-circuit induc- 

 tion is sufficiently high. The noise in such instances occurs either dur- 

 ing the pre-answering period before the line relay is "cut-off" or, in 

 certain types of switchboards, on conversations between two persons 

 on the same line (party-line) where the use of a switching circuit in the 

 office is unnecessary. 



The linking or switching equipment in the central office may consist 

 of a pair of wires with bridged supervisory relays as in the case of a 

 magneto office or may be a complicated arrangement of relays, re- 

 peating coils, condensers, etc., as in the case of common-battery offices 

 of the manual or dial type. The necessary ground connections of the 

 latter type of apparatus introduce the possibility of the unbalances in 

 the equipment contributing to the overall noise when the longitudinal- 

 circuit induction on the outside conductors is impressed on the switch- 

 ing circuits. Ordinarily in urban areas, due either to the frequency 

 make-up of the longitudinal-circuit induction or to the relationships of 

 the various impedances-to-ground, the amount of noise contributed 

 by the central office equipment is relatively low. This is readily 

 evident from Table B which shows, at 500 and 1000 cycles, the relative 

 proportions of overall noise due to the action of induced voltages on 

 station, cable and central office unbalances: 



TABLE B * 

 Relative Importance of Circuit Unbalances 



See p. 72 of Vol. I of Eng'g Reports. 



