624 BELL SYSTEM TECHNICAL JOURNAL 



magnets while Nos. 3, 4 and 5 no-current impulses will not energize 

 selecting magnets 3, 4 and 5. 



Similarly other signals for other characters may be sent and received 

 as permutations on the selecting magnets whose operation may in turn 

 control the selection of an individual type bar to be operated to type 

 the desired character. 



While brush and segment distributors are shown above for purposes 

 of illustration, in modern machines these are replaced by simple 

 mechanical devices, sending contacts and a single receiving magnet, 

 which function in the same manner but are cheaper, more reliable and 

 easier to maintain. 



Advantages of the start-stop system over other systems of timing 

 include its simplicity, the fact that highly accurate speed regulation 

 of the motors is not required, that to start a station it is only necessary 

 to turn on the power, and that lag in the line signals due to time for 

 propagation is automatically compensated for. The lag compensation 



NO CURRENT 



V.VsTOP':::;:>. 



-COMPLETE SIGNAL- 



START 



^ — T 

 I 

 I -1. 



I ' 1 



ST op; ••::•; 



TIME 



Fig. 4 — Chart of line signal for letter A. 



is automatic because the receiving distributor does not start until the 

 start signal has been propagated and received and then the other 

 signals always follow in a fixed definite time relation. This makes it 

 possible to connect any number of stations to a single circuit and have 

 them intercommunicating; that is, signals sent from any station will 

 be received and printed at all other stations without requiring read- 

 justments, regardless of the distances or circuit complications due to 

 repeaters and the like intervening. 



To illustrate one of these points further, it may be mentioned that 

 with the start-stop system, if other distortions were not present, it 

 would be theoretically possible to secure perfect operation without 

 errors between two ideal distributors one of which was running either 

 faster or slower than the other by as much as 7 or 8 per cent. This is 

 because a correction is automatically made after each character is 

 transmitted. Since some distortion of the signals is usually ex- 

 perienced in transmission, in order to maintain a large tolerance for 

 such distortions, it is the usual practice to attempt to maintain the 

 speed of each distributor within ± .75 per cent although a much 



