472 BELL SYSTEM TECHNICAL JOURNAL 



ference and storm trouble, and where they are available they have been 

 used generally for establishhig teletypewriter circuits furnished both 

 the Department of Commerce and the transport companies. At 

 present, over one third of the mileage of these circuits is in cable, 

 [''acilities on alternate routes are available to be substituted for the 

 regular circuit in the event circuit trouble develops. 



Teletypewriter Equipment 



The theory of teletypewriter operation and descriptions of the ma- 

 chines generally used in this country have been given in other papers 

 but are briefly reviewed here in order to describe some of the specific 

 equipment arrangements used in airways service. 



The teletypewriter is designed to perform the functions of an or- 

 dinary typewriter with added features that permit the typing units of 

 a number of similar machines located at distant points to be controlled 

 by the operation of the keyboard of any one of them. This is accom- 

 plished by the translation of the mechanical action of any key in the 

 keyboard unit to electrical impulses arranged in a code and transmitting 

 them over an electrical circuit to the distant machines where the 

 impulses are translated back to the mechanical action of a type bar in 

 the typing unit corresponding to the key struck in the distant key- 

 board. Electric motors and electro-magnets provide the mechanical 

 power and the means of translation of electrical impulses to mechanical 

 action. It is necessary, of course, that the mechanical action of all of 

 the machines be synchronized. This is provided for by the use of 

 synchronous motors or governed motors regulated to the same speed 

 and a start-stop rather than a continuously rotating system. By the 

 use of the start-stop system the effect of variations in motor speeds is 

 minimized, accurate synchronization being required only during the 

 interval of typing of one letter after which a clutch releases and stops 

 the receiving mechanism momentarily to permit it again to start in 

 synchronism with the sending mechanism. To provide the start-stop 

 feature and sufficient code combinations for the letters and symbols 

 required a seven-impulse code is used consisting of a start pulse, five 

 selecting pulses, and a stop pulse. 



Teletypewriters are available to print on an ordinary page or on a 

 narrow strip of tape. Tape machines are generally used in airways 

 service because they are particularly adapted to the handling of short 

 messages and weather sequences where it is generally desirable to 

 rearrange the messages received by cutting and pasting the tape on 

 separate pages to form a continuous weather record for each point. 

 This is preferable to a chronological message record requiring a search 



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