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BELL SYSTEM TECHNICAL JOURNAL 



To obtain a zero reading on the meter for each undistorted test 

 character the ratio of the marking to spacing currents is made inversely 

 proportional to the ratio of the marking to spacing time intervals. 

 This is accomplished by means of taps on a potentiometer as indicated 

 in Fig. 11. With the connection made to the middle or Reversals 



im't l pH receiving 



RELAY 



IhAA- 



Fig. 11 — Outlying-station test set. Schematic circuit for measurement of systematic 



distortion. 



tap, equal and opposite currents flow through the meter when the 

 relay armature rests first on its marking and then on its spacing 

 contact. If the relay is repeating undistorted reversals, the time bias 

 in the signals is zero and the average meter indication will be at zero. 

 With a given undistorted recurring character such as Blank, and with 

 the potentiometer set at the Blank tap, the meter indication will again 

 average at zero. The meter circuit is arranged so that the systematic 

 distortion is indicated directly in percentage based on the duration of 

 a unit signal element of a teletypewriter character. 



The circuit used in the measurement of interference is indicated by 

 Fig. 12. The interfering effect is measured by noting on the meter M 

 the biasing current which will just prevent the armature of the receiving 

 relay from responding to the interfering currents. Movement of the 

 armature from its contact is indicated by a response in a telephone 

 receiver connected in the armature circuit when the switch is operated 

 as indicated. The biasing current variation is effected by means of 

 potentiometer P and the biasing current may be reversed by means 

 of a switch (not shown). 



The circuit of Fig. 12 may also be used to give an indication as to 



