622 BELL SYSTEM TECHNICAL JOURNAL 



key is also operated they type the corresponding capital letters. The 

 fourth or top row of keys carries the numerals and certain punctuation 

 marks. The teletypewriter types capital letters but not small letters 

 so that by using a shift or Figures key the upper case position of the 

 letter keys is available for the usual punctuation marks and numerals. 

 Thus only three rows of keys are required on the teletypewriter key- 

 board. The operation of the Figures key sends a signal causing the 

 receiving machine to shift to upper case so that numerals and punctua- 

 tion marks will be printed until a Letters or Space signal is sent which 

 restores the machine to lower case. 



Start-Stop System 



For transmitting the signals of the five-unit code over a telegraph 

 line, it is necessary to have some system of timing so that each of the 

 five impulses may be properly received, identified and interpreted at 

 each receiving station. The start-stop system is used for this purpose. 

 One arrangement of this system using segmented distributors with 

 revolving brushes, is illustrated in Fig. 3. 



In this system both sending and receiving brush arms are normally 

 at rest but are maintained under constant torque, tending to rotate 

 them in the direction of the arrows, by constantly running motors 

 driving them through friction clutches. Normally the line circuit is 

 closed and carries current. When a key of the keyboard is operated 

 to send a signal, the start magnet of the sending distributor is ener- 

 gised releasing the sending brush arm and allowing it to rotate. As 

 this brush passes from the stop to the start segment, the line circuit 

 is opened and this open signal transmitted to the receiving station 

 where it causes energization of a start magnet which releases the 

 receiving brush and allows it to rotate. 



Both sending and receiving brush arms rotate at approximately the 

 same speeds since they are driven from motors running at approxi- 

 mately the same speeds. These motors are either small synchronous 

 motors driven from constant frequency commercial 60-cycle 110-volt 

 power supply or by commutator type motors, equipped with centrifugal 

 governors to hold them at approximately a constant speed, for use on 

 other commercial a-c. or d-c. supplies. 



Now as the sending brush arm sweeps over the sending face, the 

 impulses of the five-unit code, as set up by the particular key de- 

 pressed, will be transmitted over the line as shown in Fig. 4 for the letter 

 A, and through the action of the rotating receiving brush, Nos. 1 and 2 

 current impulses will cause the energization of Nos. 1 and 2 selecting 



