158 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



bring in alarms and necessitate manual readjustment. In this connection, 

 the voltmeters which indicate the drop through the series regulating 

 tubes provide a very convenient magnification of any drift in cable cur- 

 rent. The multiplying factor is the effective dc impedance of the regu- 

 lated system, that is, more than 100,000 ohms. Thus 25 volts, which is 

 an appreciable fraction of the nominal 300 volts across the series regu- 

 lating tubes, is equivalent to less than 0.25 ma., which is of the order of 

 0.1 per cent of normal cable current. As a matter of fact, the behavior of 

 this voltage provides the final criterion for precise adjustment of cable 

 current to assure long-term stability. 



EQUIPMENT DESIGN 



Description 



Fig. 10 shows the complete dc equipment for supplying one polarity 

 of power to one cable. Similar equipment provides the opposite po- 

 larity to the other cable. The tw^o equipments are located facing each 

 other across a common aisle with their conmion control bays directly 

 opposite. Regulator 1 on the right is normally operated in parallel with 

 Regulator 2. Regulator 3 on the left is the spare regulator, normally off. 

 The common bay, between Regulators 1 and 2, includes the cable 



Fig. 10 — DC regulator and common control bays. 



