542 BELL SYSTEM TECHNICAL JOURNAL 



economical to provide facilities having coefficients as low as 1.5 the 

 traffic routing instructions call for the use of additional regenerative 

 repeaters at suitable points. 



The types of telegraph facilities that are used for these various classes 

 of loll link and subscriber line are discussed farther on. 



Facilities Used in TWX Network— Teletypewriter Stations 

 AND Subscriber Lines 



A typical teletypewriter station, illustrated in Fig. 6, includes the 

 sending and receiving equipment, together with power supply, and 

 supervisory equipment for initiating a call, informing the attendant 

 of an incoming call, or recalling the switchboard operator during the 

 progress of a connection if desired. These features are described in 

 detail in the paper on switchboards and signaling facilities referred to 

 previously.^ 



Any one of several types of teletypewriter subscriber lines may be 

 used to connect a station with the switchboard from which it is served, 

 the type chosen depending upon conditions in the particular case. 

 A large majority of subscriber lines, however, consist of cable pairs 

 used exclusively for that purpose. In these the telegraph method 

 employed is one in which polar signals (a positive potential for spacing 

 and a negative potential for marking pulses or vice versa) are impressed 

 on the subscriber line by a telegraph repeater in the cord circuit at 

 the central office, and neutral signals (the circuit closed for marking 

 and opened for spacing) are transmitted by the sending contacts of 

 the teletypewriter at the subscriber station. 



The polar signals transmitted from the central office are symmetrical 

 and the transmission quality of these signals is not affected seriously 

 by the capacitance of the cable loop. As is ordinarily the case in 

 duplex transmission, the current impulses are transmitted ditTerentially 

 through two windings of a relay in the cord circuit repeater which 

 responds to incoming signals but not to the outgoing differentially 

 transmitted signals. To prevent the undesired response of this relay 

 to the outgoing signals, it is necessary that the differential winding 

 not connected to the subscriber line be terminated to ground through 

 an impedance similar to that of the subscriber line. Since subscriber 

 lines from a given type of switchboard are all arranged to use the same 

 current value the resistance component of the station line impedance 

 may be balanced by fixed resistance. 



With cable circuits of appreciable length, however, the capacitance 

 becomes of importance. Up to a certain length the effect of the 

 capacitance on balance can be minimized by locating a substantial 



