198 BELL SYSTEM TECHNICAL JOURNAL 



telegraph circuits equipped with level compensators must be subjected 

 to a series of specialized tests and adjustments whenever placed in 

 service and at periodic intervals thereafter. To provide for this, a 

 special testing set comprising in readily available form all the test 

 equipment required for this purpose, has recently been introduced. 

 It includes the following features: 



1. Bias measuring circuit. 



2. Filament-activity test-circuit and filament-current measuring 



circuit. 



3. Drift measuring circuit. 



4. Test amplifier. 



5. Adjustable attenuator. 



With this set all terminal tests may be made from one end of the 

 circuit without the use of a line or external line-simulating repeater as 



Fig. 22 — Carrier-telegraph test set. Instrument panel. 



was done in the past. It is mounted on a small wagon which may be 

 wheeled into position adjacent to the terminals to be tested, then 

 connected by means of a long cord and multiple contact plug to the 

 necessary battery supplies, grounds, etc. The general appearance of 

 the set is shown in Fig. 21, while a more detailed view of the face 

 equipment may be obtained from Fig. 22. 



The bias measuring circuit is shown in simplified form in Fig. 23. 

 It uses a 215 type polar relay ,^^ or its equivalent, with a meter con- 

 nected in the armature circuit. This meter, which permits measuring 

 to an accuracy better than 1 per cent, is specially designed to have the 

 proper ballistic and damping characteristics to permit measuring dot 



