TF.IJ'.TyPEWRITEK liX CHANGE SYSTEM 



517 



30| inches. It may be seen that, with a ratio of 7.5 subscriber lines to 

 one trunk, a capacity of about 1,300 Hues is obtainable. Because the 

 ratio of trunks to subscriber lines is somewhat ^^reater on small switch- 

 boards than on the larger boards, due to the relative inefficiency 

 of smaller trunk groups, the multiple is designed on the basis of 1,200 

 subscriber lines and 240 inter-loll trunks which gives a ratio of five 

 subscriber lines to one trunk. 



2000 



m^ 

 cq£ 

 5-" 

 z>a: 1600 



ZuJ 



z5 

 — a. 

 >u 

 t<fi 1200 



2: 10 



<u. 



"^O 800 



4 6 8 10 12 14 16 



RATIO OF SUBSCRIBER LINES TO INTER-TOLL TRUNKS 



20 



Fig. 8 — Curve showing variation of subscriber line capacity for No. 3A Teletype- 

 writer Switchboard. 



Circuit Functions 



The foregoing paragraphs have given a picture of the physical 

 arrangement of the more important switchboards, and an idea of the 

 number of subscriber lines and inter-toll trunks that can be accom- 

 modated by each. Some idea must also be given of the circuit methods 

 by means of which connections are established between the various 

 subscribers and supervised by the operators. 



Subscriber Station and Station Circuit 



The basic instrument by means of which the subscriber sends and 

 receives his message is the teletypewriter. It is not proposed to give 

 here a description of the teletypewriter as this is discussed in other 

 papers. Other equipment, however, is required in addition to the 

 teletypewriter to provide for the necessary signaling facilities for the 

 exchange service. 



A typical installation in a subscriber's office is shown in Fig. 9. 

 The arrangement shown provides for the No. 15 (page) tele- 

 typewriter, used predominantly in the TWX service, mounted on a 

 table which has been designed to provide adequate mounting facilities 

 for the signaling equipment. This table is arranged with a removable 

 panel known as a control panel, which is mounted in an opening in the 

 top of the table to the right of the teletypewriter to make the key 



