530 BELL SYSTEM TECHNICAL JOURNAL 



be asked and answers given. The speed with which the connection is 

 estabHshed is comparable to that experienced in long distance tele- 

 phone service, the average being about 1.3 minutes from the time a 

 subscriber calls the operator until the conversation between subscribers 

 begins. The service has grown until at the present time there are 

 over 8,500 * subscriber stations which may be connected together in 

 pairs or in groups for teletypewriter communication. The switching 

 is done at about 150 switching centers scattered throughout the 

 United States as shown in Fig. 1 and connected by over 500,000 miles 

 of telegraph circuit. 



This paper deals primarily with the transmission system used for 

 passing the teletypewriter code signals between the customers. The 

 details of the switchboards and signaling facilities, and the methods of 

 handling customers' connections are described in another paper. ^ 

 With the exception of the switchboards, the equipment used in TWX 

 service is similar to that used in other telegraph services. 



The teletypewriters are provided with a keyboard similar to that 

 of a typewriter for sending, and the typing is done in capital letters 

 either on a narrow tape or on a page, the page being used in the large 

 majority of the stations. Printed forms may be used on the page 

 machines if desired. The speed of operation is set for a maximum of 

 60 words per minute. The teletypewriters are of the start-stop type, 

 using a 5-unit selecting code, each group of selecting impulses being 

 preceded by a start impulse and followed by a stop impulse. The 

 teletypewriter mechanism is operated from a local source of power, 

 and in general all signaling current is furnished from central ofifice 

 power plants. 



The line circuits may be any of the well known types utilizing 

 2 current values or line conditions of variable duration for the trans- 

 mission of signals. Actually about 90 per cent of the circuit mileage 

 used in the TWX service is of the carrier type, since this is the most 

 economical type of facility for large groups over the longer distances. 

 The line circuits will be discussed in more detail in another section of 

 the paper. 



Elements of Teletypewriter Signal Transmitting and 



Receiving Mechanism 



To translate intelligence which is received in the form of a code the 



receiving mechanism must be capable of doing two things. First, it 



must identify the unit time intervals, and second, it must determine 



which of the 2 line conditions should be recorded for each time interval. 



* Since this paper was prepared the number of subscriber stations has increased 

 to over 9,500. 



^ For all numbered references see list at end of paper. 



