166 BELL SYSTEM TECHNICAL JOURNAL 



frequency Fi. This switching of the transmitting and receiving cir- 

 cuits from one frequency to another is necessary where more than 

 two stations arc operated on the same system and it is desirable for 

 every station to \k> able to call every other station without routing 

 the call through a central point. 



If station .4i is connected with station A by means of two or more 

 pairs of telephone wires which are not exposed to high voltage power 



fREQUENCY IN KILOCVCUS 



on 20 30 40 50 60 70 eO 90 WO 110 IfO 130 KO ISO 100 170 180 190 



Fig. 14 — Transmission Characteristic of Coupling Band Pass Filter 



lines, a simple D.C. remote control circuit may be emplo\cd. How- 

 ever, if only two wires are available or if tiie telephone lines to be used 

 arc exposed to high voltage power lines and must therefore be equipped 

 with insulating transformers and drainage coils, it is necessary to 

 emi)lo\- a somewhat more complex alternating current control circuit, 

 in this circuit the 135 cycle interru|itcrs and rcla\s familiar to the 

 telephone plant are employed. 



The voice frequency circuits used in connection with this carrier 

 equipment are the standard two wire and four wire circuits used in 

 commercial tele|)hone practices. 



ColI'I.INfi HV Co.NDF.NSFRS AND BY DlSTRIHUTED CAPACITY 



Fig. 13 shows two of the 120 K.V. coupling condensers developed 

 by the Ohio Brass Co. Each, of these condensers has a capacity of 

 .003 fii although similar condensers ha\ing a capacity of .007 /if are 

 also available. These condensers are ajjproximately 5 ft. in diameter 

 and 12 ft. high over the bushing and weigh about 8,000 pounds. The 



