FUNDAMENTALS OF TELETYPEWRITERS 629 



a motor. A photograph of such a recently developed regenerative 

 repeater unit is reproduced in Fig. 6. 



By means of these regenerative repeaters rehable teletypewriter 

 service may be extended to any desired distances so long as the signals 

 in any one regenerative repeater section are not too badly distorted to 

 permit reliable operation of that section alone. Several regenerative 

 repeaters may be operated in tandem on a single very long circuit if 

 required and in fact a number of very difficult long circuits are operat- 

 ing satisfactorily under these conditions at the present time. A point 

 worthy of note and which has not previously been mentioned is that 

 the stop impulse in the code adopted for Bell System apparatus is 

 slightly longer than the other impulses, which facilitates the use of 

 regenerative repeaters in tandem without requiring complex speed 

 control arrangements. 



General Features of Teletypewriters 



Teletypewriters are widely used for high speed written communi- 

 cations. Generally speaking, written communications are desired for 

 purposes of accuracy. Therefore, high speed, accuracy and reliability 

 are basic requirements for teletypewriter service. 



In choosing an operating speed at which distributors of teletype- 

 writers are to be set, several factors must be considered. These are the 

 capabilities of the mechanisms of the machines, the average capabilities 

 of operators for continuous sending at high speeds, the commercial 

 need for high speeds, and the capabilities of the line circuits for 

 transmitting the signals reliably over long periods without excessive 

 distortions or excessive attention for maintenance and adjustment. 

 A satisfactory compromise among these different factors seems at the 

 present time to be about 60 words per minute, or 368 machine opera- 

 tions per minute, which is the speed usually employed in the Bell 

 System. The machines themselves may be arranged and adjusted 

 to be capable of higher speeds up to about 75 words per minute, and it 

 may be that, in the future, service at these higher speeds will be 

 justified under certain circumstances. 



Accuracy and freedom from breakdown troubles are necessarily 

 inter-related and both required to a very high degree for machines 

 handling important written communications over long distances. To 

 give some idea of the severity of these requirements, we have found 

 from long experience that to produce a good machine we can not be 

 satisfied in our laboratory tests unless the machine is capable of typing 

 at least 1,000,000 consecutive words (6,000,000 operations) without 



