42 



BELL SYSTEM TECHNICAL JOURNAL 



The telephone and telegraph carrier systems although alike in their 

 essentials differ very materially in the details of their operation. With 

 the present equipment the frequencies employed in carrier telephony 

 are much higher than in carrier telegraphy, thereby requiring more 

 frequent repeater stations. In both telephone and telegraph systems 

 it is necessary to provide for two way operation. This may be ac- 

 complished by using different carrier frequencies in the two directions 

 or by using the same frequency in each direction with directional 

 selectivity obtained by the three-winding coil (hybrid coil) used in 

 repeater work. In this latter case it is necessary to provide networks 



Fig. 1 



to balance the lines over which the carrier system is operated. In 

 the past both of these methods have been used but the tendency is 

 now in the direction of eliminating balance entirely on account of its 

 attendant maintenance difficulties and of providing for directional 

 selectivity entirely by means of different frequencies in the two 

 directions. 



In order to show the variations in equipment arrangements which 

 have been used in carrier systems, Figs. 1, 2 and 3 have been included. 

 Fig. 1 shows one terminal of the original Pittsburgh-Baltimore carrier 



