50 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



tration of the particular side of the Atlantic on which they are located. 

 They will not be discussed here, although it might be well to point out 

 that the equipment supply for the Bell equipment is direct current, ob- 

 tained from floated storage batteries, while the supply for the Post Office 

 equipment is alternating current from rotary machines driven normally 

 from the station ac supply, with storage battery back-up. 



The cable current supplies for the North Atlantic Hnk^" are complex 

 and highly special. It is pertinent to discuss here briefly the requierments 

 which beget this complexity. To assist in this, an elementary schematic 

 of the cable power loop is shown in Fig. 10. 



The following main requirements governed the design of these plants: 



1. Constant maintenance of uniform and known current in the power 

 loop. 



2. Protection of HF line against faults induced by failures in power 

 bays. 



3. Protection of HF line against damage from power voltage surges 

 caused by faults in the line itself. 



Maintenance of constant and known operating current in the line is 

 very important from the standpoint of system life because of the de- 

 pendence of the rate of electron tube aging on the power dissipated in the 

 heaters. ^^ 



The principal factor which tends to cause variations in the line current 

 is the earth potential which may appear between the terminals of the 

 system. This potential may be of varying magnitude and of either po- 



20 



40 60 80 100 120 140 160 



FREQUENCY IN KILOCYCLES PER SECOND 



180 



Fig. 9 — Receiving fixed equalizer — loss-frequency characteristic. 



