FUNDAMENTALS OF TELETYPEWRITERS 637 



and interruptions to the operator result in direct losses of circuit time 

 and effectively slower transmission. 



Another important advantage of the perforated tape method is that 

 errors may be corrected in the tape before transmission with the result 

 that only errorless copy is transmitted on the circuit. This is done in 

 the following manner and is illustrated in the section of perforated 

 tape shown below. If the operator in attempting to write the word 

 THE should strike the keys T and J (in error), realizing her error she 

 back spaces the tape one division, strikes the "letters" key and then 

 the correct keys H and E. The transmission of the "letters" signal 

 will cause no operation in the recording teletypewriters since they are 

 already in the "letters" case, and the word will be recorded correctly 

 as though no error had been made. Similarly, entire words or groups 

 of characters may be erased from the tape if desired. 



LETTERS FEED 



SPACE — ^ I ^ SPACE HOLES 



• • • • • 



• • •••• 



• •• •• • 



•••• • •• 



•• ••• • • 



FROM T HE LARGE 



Fig. 9 — Sample of strip of perforated tape. 



For TWX service a further advantage of the perforated tape method 

 is that the entire message may be punched in tape and checked by 

 printing, if desired, before a call is placed and a connection established. 

 Then, when the connection is established, the message can be auto- 

 matically transmitted at maximum speed requiring a minimum time 

 for the connection and giving a minimum charge for the call. 



It is true, of course, that in this method there is some delay between 

 perforation and transmission. For this reason short to and fro mes- 

 sages, as required in setting up a connection, may be better handled 

 by direct keyboard. To facilitate such working, the perforator key- 

 board is normally arranged so that by throwing a switch this same 

 keyboard may be used for direct keyboard sending without perforating. 

 This switch also has an intermediate position in which the keyboard is 

 connected for simultaneous direct sending and perforating. This 

 provides for meeting the needs of certain TWX subscribers who wish 

 to simultaneously type and punch the message so that the typed copy 

 may be checked as it is perforated. 



The complete page printing set arranged for tape transmission is 



