MEASUREMENT OF TELEGRAPH TRANSMISSION 



161 



the amount of transmission degradation to be expected due to the 

 interference measured and to given changes in operating currents. 

 In this case the switch is operated to connect meter Ml in circuit 

 for the purpose of measuring bias. The effect on the bias of changes 

 in the biasing current for a given operating condition may then be 



LINE 



Fig. 12 — Outlying-station test set. Schematic circuit for interference measurement. 



noted on this meter when receiving reversals or any of the selected 

 teletypewriter characters. The biasing current is varied by means of 

 potentiometer P as before. 



This set has several interesting operating features. By inspection 

 of Fig. 13, which shows a view of the set, it will be seen that it contains 

 its own receiving relay located in the lower left corner. This relay 

 may be connected in series in a line circuit or in the local circuit of 

 the subscriber set using the line jacks 1 and 2 (located in the upper 

 right-hand corner of the set) or if desired convenient connection may 

 be made by means of the splecial plug and adapter shown in the lower 

 part of the figure. In the latter case the subscriber set relay is trans- 

 ferred to the measuring set and the special plug, and adapter if required, 

 is inserted into the relay connecting block of the subscriber set in place 

 of the relay. By operating the proper keys on the test set the currents 

 in different circuits of the subscriber set may then be measured conven- 

 iently. Distortion may also be measured with this connection but in 

 most cases it is desirable to measure with the receiving relay in its nor- 

 mal position in the subscriber set to obtain representative conditions. 



Because of the advantages of the set, namely, the accuracy possible 

 in the measurement of distortion, the convenience afforded in the 



