Fundamentals of Teletypewriters Used in the Bell System 



By E. F. WATSON 



During the past few years the use of teletypewriters has become 

 quite extensive in the Bell System. Simpler and cheaper machines 

 have recently been made available for meeting the simpler service 

 requirements and attachments have been designed to provide 

 additional features for meeting more complex service requirements. 

 This article discusses the fundamental principles and various 

 features of the teletypewriter machines now in common use and 

 explains the more important factors which have been controlling in 

 their development. 



WITH the growth of Teletypewriter Exchange Service and the 

 general increase in the use of teletypewriters in private line 

 services of various types, questions frequently asked are: How do 

 teletypewriters operate? What is the "start-stop" system? Why is 

 it used? What is a regenerative repeater? 



This article will attempt to answer some of these questions and 

 explain also the fundamental principles and features of teletype- 

 writers and their auxiliary arrangements as now employed in the Bell 

 System. These have been developed to meet the needs of customers 

 for a typed or similar record form of communication and at the same 

 time be suitable for operation in connection with the Bell System plant. 



Code 



For economical transmission over long distances it is fundamental 

 that only a single wire or transmission channel be required to carry 

 the signals. Furthermore, long experience with manual telegraphy on 

 land lines has proved that reliable and efficient operation is secured by 

 using not more than two conditions on the line, such as current and no 

 current or positive impulses and negative impulses, as contrasted with 

 the use of three or more conditions, or current values. The entire 

 telegraph plant of the Bell System as well as practically all other land 

 line telegraph systems have been built on this two-condition basis. 



The familiar Morse code uses sequences of dots and dashes to 

 represent the different characters of the alphabet and meet the above 

 conditions. This code is not well adapted for teletypewriter control, 

 however, since the signals for different characters vary widely in the 

 time they require, from a single dot for the letter E to a combination of 



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