524 BELL SYSTEM TECHNICAL JOURNAL 



That section of the receiving selector between terminals 1 to 5 

 inclusive is used for the call signal which is actuated manually by the 

 originating operator. 



Cord Circuits 

 In order to provide each operator with sufficient traffic for operating 

 efficiency, especially in the smaller offices and during light load periods, 

 the cord circuits in the TWX switchboards are made universal, that is, 

 adapted to handling all types of calls. This universal feature is 

 obtained by equipping them with a simple type of repeater. By 

 means of this repeater it is possible to provide for the maximum 

 length of station loop and at the same time establish the following 

 connections: 



1. Subscriber line to subscriber line, known as a local to local connection. 



2. Inter-toll trunk to subscriber line, or vice versa, known as a toll to local connection, 



or local to toll connection. 



3. Inter-toll trunk to inter-toll trunk, known as a through connection. 



In a local-to-local connection the two loops could not be connected 

 together directly unless the repeater were provided in the cord circuit 

 for two reasons: first, each loop may be maximum in length so that the 

 two loops in tandem would result in the operating current being halved ; 

 and second, each loop is normally terminated on the negative side of 

 the telegraph battery. Because it is essential, in TWX service, to 

 make interconnections without requiring adjustments, all loops are 

 padded or "built out" to the same value as the resistance of a maxi- 

 mum loop and each side of the cord circuit repeater is arranged to 

 operate with each loop. 



With the provision of the repeater in the cord circuit to permit 

 interconnecting two subscriber lines, the same cord may be used for 

 toll-to-local and toll-to-toll connections because the loop circuits of 

 the inter-toll trunk repeaters are all terminated on the negative side 

 of the telegraph battery and the loop resistance of each is built out to 

 equal that of the longest station loop. 



A very simplified form of the essential elements of a TWX cord 

 circuit is shown in Fig. 13. The cord circuit basically consists of a 

 repeater of the type before mentioned, a key known as the typing 

 key, by means of which the operator may cut her teletypewriter in 

 and out of the circuit for monitoring purposes, and facilities for 

 receiving the recall and disconnect signals both from the subscriber 

 lines and the inter-toll trunks. 



The method of receiving these recall and disconnect signals was 

 explained in the section on inter-toll trunk supervision, and the method 

 used to receive those from the subscriber was pointed out in the 



