650 



BELL SYSTEM TECHNICAL JOURNAL 



Signaling over the carrier channels is effected by means of 1,000- 

 cycle ringers which are connected to the circuits at the two terminals. 

 As the ringing current is within the voice range, it is transmitted over 

 the regular carrier channel so that no additional signaling equipment 

 is necessary. 



In order to insure satisfactory operation, all necessary testing 

 facilities are included. Meters and keys are provided for measuring 

 the voltages of the plate, grid and filament batteries as well as the 



Fig. 13 



plate and filament currents of all tubes. Individual rheostats are 

 inserted in all filament circuits for making any adjustments that may 

 be necessary. Alarms are provided to indicate any abnormal condi- 

 tion which might develop on any tube. Thermocouples and artificial 

 lines have been conveniently arranged for checking the efficiency of 

 all units such as the modulators and demodulators. Jacks are located 

 at suitable points so that any changes which may be necessary can 

 be quickly made. 



