630 BELL SYSTEM TECHNICAL JOURNAL 



error or trouble of any kind, and without requiring service attention 

 of any kind other than normal replacement of paper and inking 

 ribbons. 



For rendering service economically with teletypewriters on sub- 

 scribers' premises, an important requirement in order that the expense 

 of maintenance be not prohibitive is that the machine should not 

 require maintenance attention except at very infrequent intervals. 

 Bell System machines are designed to require routine maintenance 

 attention not oftener than once in two months where the machine is 

 used continuously over periods of eight hours each day. To accomplish 

 this the problem of lubrication has required very careful attention. It 

 has necessitated the provision of oil reservoirs in certain places and the 

 careful selection and specification of oils and greases. Another feature 

 making for economical ma.n c : nance is interchangeable parts. In other 

 words, if a part breaks or wears, it is replaceable by another part of the 

 same type without requiring fitting and usually without readjustment. 



At times customers wish to use teletypewriters on tables especially 

 designed and arranged to suit the convenience of their offices. For 

 this reason teletypewriters are designed as far as feasible to be self- 

 contained units which can be mounted on any desk or table. 



All present Bell System teletypewriters employ the start-stop system 

 of synchronizing and are well adapted for the connection of any 

 number of machines to one circuit with facilities for rapid to and fro 

 intercommunication among the various stations. To permit optimum 

 control of intercommunication and interruption of the sending station 

 when desired, a device known as the "break lock" is incorporated in 

 many machines. This device, together with a "break" key located on 

 each machine, provides facilities whereby any station may interrupt a 

 station which is sending, take control of the circuit and send. The 

 operation of the "break" key opens the line transmitting a signal which 

 causes the "break lock" device to function at the station which is 

 sending and automatically stop any further sending from that station 

 until the device is manually restored. This device is very important 

 in the case of transmission from a perforated tape, which is described 

 later. 



Motor control devices are of importance for stations which are not 

 in continuous use but which may wish to receive messages from time 

 to time from distant stations without requiring an attendant to turn on 

 the machine. Such devices are used both in private line and in TWX 

 services. In the case of a private line it is often desired to have the 

 machine normally idle with the motor stopped but so arranged that. 



