WIRE COMMUNICATION AIDS TO AIR TRANSPORTATION 471 



route located approximately every 50 miles afford convenient points 

 from which branch circuits are extended to the intermediate airway 

 stations. The several branch circuits are of the grounded open-wire 



PITTSBURGH. 



ALTOONA. 



HARRISBURG 



- KEY- 



z:^=^= Metallic Telegraph Circuit in. Cable. 



'>/N<vw\A. Carrier Current O'rcuit in Cable. 



Grounded Circuit on Open Wire. 



■•■••••— Airway. 



O Dept.of Ccmm. Intermediate Field. 



@ Commercial or Municipal Airport ■ 



► Telegraph Repeater 



Fig. 4 — Layout of typical teletypewriter circuit along airway. 



type and total 331 miles. A total circuit mileage, therefore, of 846 

 miles is required in this case for connecting all stations along an air 

 route a little over 400 miles long. 



The Newark-Pittsburgh section of the main circuit is operated on 

 metallic telegraph cable facilities, a type particularly adapted to use 

 where stations to be connected to the circuit are spaced at frequent 

 intervals. Between Pittsburgh and Cleveland a channel of a voice 

 frequency carrier telegraph system on cable facilities is used. This 

 type of facility is generally used where stations are located 150 or more 

 miles apart. The longer branch circuits on open wire employ polar 

 transmission with repeaters at both the repeater station and terminal 

 and use two wires, one for each direction of transmission. The shorter 

 branch circuits use one wire with a grounded duplex repeater at the 

 repeater station and a constant d.c. potential at the outlying terminal. 

 Detail descriptions of these various telegraph systems have been given 

 in previous papers. 



Cable circuits are less susceptible than open-wire circuits to inter- 



